HVDC AND FACTS UNIT-I INTRODUCTION

UNIT I

INTRODUCTION
Topics:
Electrical Transmission Networks, Conventional Control Mechanisms-Automatic Generation
Control, Excitation Control, Transformer Tap-Changer Control, Phase-Shifting Transformers;
Advances in Power-Electronic Switching Devices, Principles and Applications of
Semiconductor Switches; Limitations of Conventional Transmission Systems, Emerging
Transmission Networks, HVDC and FACTS.

ELECTRICAL TRANSMISSION NETWORKS:

• A transmission network is a high-voltage system that transfers electric power.

• Transmission networks are often compared to a "highway system" that allows electricity to be
transmitted over long distances to large users or to more localized electric distribution systems.
• Transmission lines are made up of various components, including poles, lattice structures,
conductors, cables, insulators, foundations, and earthing systems.
• Transmission lines carry electricity at voltages of over 200 kV to maximize efficiency.
• Transmission lines can carry alternating current or direct current, or a system can be a combination
of both.
• Electric current can be carried by either overhead or underground lines.
The main function of transmission network is to transfer power from power generating station to
distribution sub station.

ELECTRICAL TRANSMISSION NETWORKS

The rapid growth in electrical energy use, combined with the demand for low cost energy,

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has gradually led to the development of generation sites remotely located from the load centers.
In particular, the remote generating stations include hydroelectric stations, which exploit sites
with higher heads and significant water flows; fossil fuel stations, located close to coal mines;
geothermal stations and tidal-power plants, which are site bound; and, sometimes, nuclear power
plants purposely built distant from urban centers. The generation of bulk power at remote
locations necessitates the use of transmission lines to connect generation sites to load centers.
Furthermore, to enhance system reliability, multiple lines that connect load centers to several
sources, interlink neighboring utilities, and build the needed levels of redundancy have gradually
led to the evolution of complex interconnected electrical transmission networks. These networks
now exist on all continents.

An electrical power transmission network comprises mostly 3-phase alternating-current

(ac) transmission lines operating at different transmission voltages (generally at 230 kV and
higher). With increasing requirement of power-transmission capacity and/ or longer transmission
distances, the transmission voltages continue to increase; indeed, increases in transmission
voltages are linked closely to decreasing transmission losses. Transmission voltages have
gradually increased to 765 kV in North America,with power transmission reaching 1500 MVA
on a line limited largely by the risk that a power utility may be willing to accept because of
losing a line.

An ac power transmission network comprises 3-phase overhead lines, which, although

cheaper to build and maintain, require expensive right-of- ways. However, in densely populated
areas where right-of-ways incur a premium price, underground cable transmission is used.
Increasing pressures arising from ecological and aesthetic considerations, as well as improved
reliability, favor underground transmission for future expansion.

In a complex interconnected ac transmission network, the source-to-a- load power flow

finds multiple transmission paths. For a system comprising multiple sources and numerous loads,
a load-flow study must be performed to determine the levels of active- and reactive-power flows
on all lines. Its impedance and the voltages at its terminals determine the flow of active and
reactive powers on a line. The result is that whereas interconnected ac transmission networks
provide reliability of power supply, no control exists on line loading except to modify them by
changing line impedances by adding series and/ or shunt-circuit elements (capacitors and

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reactors).

The long-distance separation of a generating station from a load center requiring long
transmission lines of high capacity and, in some cases in which a transmission line must cross a
body of water, the use of ac/ dc and dc/ ac converters at the terminals of an HVDC line, became
a viable alternative many years ago. Consequently, beginning in 1954, HVDC transmission has
grown steadily to the current ±600 kV lines with about 4000 A capacity. Also, direct current (dc)
transmission networks, including multi terminal configurations, are already embedded in ac
transmission networks. The most significant feature of an HVDC transmission network is its full
controllability with respect to power transmission.

Until recently, active- and reactive-power control in ac transmission networks was

exercised by carefully adjusting transmission line impedances,

as well as regulating terminal voltages by generator excitation control and by transformer tap
changers. At times, series and shunt impedances were employed to effectively change line
impedances.

Conventional Control Mechanisms.

In the foregoing discussion, a lack of control on active- and reactive- power flow on a
given line, embedded in an interconnected ac transmission network, was stated. Also, to maintain
steady-state voltages and, in selected cases, to alter the power-transmission capacity of lines,
traditional use of shunt and series impedances was hinted.

In a conventional ac power system, however, most of the controllability exists at

generating stations. For example, generators called spinning reserves maintain an instantaneous
balance between power demand and power supply. These generators, in fact, are purposely
operated at reduced power. Also, to regulate the system frequency and for maintaining the
system at the rated voltage, controls are exercised on selected generators.

Automatic Generation Control (AGC)

Automatic generation control (AGC) is a system that adjusts the power output of multiple
generators in response to load changes. It is a secondary control loop that helps the governing
system maintain system frequency stability and tie-line power interchange after a disturbance.

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AGC's main objectives are to:

• Maintain system frequency
• Keep energy balanced within each control area
• Maintain the scheduled net interchanges between control areas
• Track load variations
• Maintain optimal generation levels close to scheduled values
• Balance the generation and load in power systems at a minimum cost

AGC has three operating modes defined according to the Area Control Error (ACE):
• Tie-Line-Bias (TLB)
• Flat-Frequency (FF)
• Flat-Tie-Line (FTL)

The megawatt (MW) output of a generator is regulated by controlling the driving torque,
Tm, provided by a prime-mover turbine. In a conventional electromechanical system, it could be
a steam or a hydraulic turbine. The needed change in the turbine-output torque is achieved by
controlling the steam/water input into the turbine. Therefore, in situations where the output
exceeds or falls below the input, a speed-governing system senses the deviation in the
generator speed because of the load- generation mismatch, adjusts the mechanical driving torque
to restore the power balance, and returns the operating speed to its rated value. The speed-
governor output is invariably taken through several stages of mechanical amplification for
controlling the inlet (steam/water) valve/ gate of the driving turbine. Figure 1.1 shows the basic
speed-governing system of a generator supplying an isolated load. The operation of this basic
feedback- control system is enhanced by adding further control inputs to help control the
frequency of a large interconnection. In that role, the control system becomes an automatic
generation control (AGC) with supplementary signals.

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To avoid competing control actions, in a multi generator unit station each speed-governor
system is provided with droop (R) characteristics through a proportional feedback loop (R,
Hz/ MW). Figure 1.2 shows an AGC on the principal generating unit with supplementary
control. In contrast, the second, third, and remaining generating units in a multiunit station
operate with their basic AGCs. In a complex interconnected system, the supplementary
control signal may be determined by a load-dispatch center

Excitation Control

The basic function of an exciter is to provide a dc source for field excitation of a

synchronous generator. A control on exciter voltage results in controlling the field current,
which, in turn, controls the generated voltage. When a synchronous generator is connected to
a large system where the operating frequency and the terminal voltages are largely unaffected
by a generator, its excitation control causes its reactive power output to change.

In older power plants, a dc generator, also called an exciter, was mounted on the main
generator shaft. A control of the field excitation of the dc generator provided a controlled
excitation source for the main generator. In contrast, modern stations employ either a
brushless exciter (an inverted 3-phase alternator with a solid-state rectifier connecting the
resulting dc source directly through the shaft to the field windings of the main generator)or a
static exciter (the use of a station supply with static rectifiers).

An excitation-control system employs a voltage controller to control the excitation

voltage. This operation is typically recognized as an automatic Voltage regulator (AVR).
However, because an excitation control operatesquickly, several stabilizing and protective
signals are invariably added to thebasic voltage regulator. A power-system stabilizer (PSS)
is implemented byadding auxiliary damping signals derived from the shaft speed, or the

terminal frequency, or the power—an effective and frequently used technique for enhancing
small-signal stability of the connected system. Figure 1.3 shows the functionality of an
excitation-control system.

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Model of Excitation control.

Transformer Tap-Changer Control:

Next to the generating units, transformers constitute the second familyof major power-
transmission-system apparatuses. In addition to increasing and decreasing nominal voltages,
many transformers are equipped with tap- changers to realize a limited range of voltage
control. This tap control can be carried out manually or automatically. Two types of tap
changers are usually available: offload tap changers, which perform adjustments when de-
energized, and on-load tap changers, which are equipped with current- commutation capacity
and are operated under load. Tap changers may be provided on one of the two transformer
windings as well as on autotransformers.

Because tap-changing transformers vary voltages and, therefore, the reactive power
flow, these transformers may be used as reactive-power- control devices. On-load tap-

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changing transformers are usually employed to correct voltage profiles on an hourly or

daily basis to accommodate load variations. Their speed of operation is generally slow, and
frequent operations result in electrical and mechanical wear and tear.

Figure of tap changer transformer

Phase-Shifting Transformers:

A special form of a 3-phase–regulating transformer is realized by combining a

transformer that is connected in series with a line to a voltage transformer equipped with a tap
changer. The windings of the voltage transformer are so connected that on its secondary side,
phase-quadrature voltages are generated and fed into the secondary windings of the series
transformer. Thus the addition of small, phase-quadrature voltage components to the phase
voltages of the line creates phase-shifted output voltages without any appreciable change in
magnitude.
A phase-shifting transformer is therefore able to introduce a phase shift in a line.

Figure 1.4 shows such an arrangement together with a phasor diagram. The phasor
diagram shows the phase shift realized without an appreciable change in magnitude by the
injection of phase-quadrature voltage components in a 3-phase system. When a phase-

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shifting transformer employs an on-load tap changer, controllable phase-shifting is achieved.

The interesting aspect of such phase shifters is that despite their low MVA capacity, by
controlling the phase shift they exercise a significant real-power control. Therefore, they are
used to mitigate circulating power flows in interconnected utilities. A promising application of
these devices is in creating active-power regulation on selected lines and securing active-
power damping through the incorporation of auxiliary signals in their feedback controllers.
From this description, it is easy to visualize that an incremental in-phase component can also
be added in lines to alter only their voltage magnitudes, not their phase.

Advances in Power-Electronics Switching Devices

Generally speaking, advancements in power electronic switches are primarily

to increase the switching speed, that is to reduce the time it takes for the switch to turn-on and
turn-off, and to increase the power the switch is capable of handling.
Power electronic switching devices have several advances, including:
• Increased switching speed: The time it takes for a switch to turn on and off is reduced.
• Increased power handling: The power a switch can handle is increased.
• Improved efficiency: Silicon carbide (SiC) and gallium nitride (GaN) are materials that
can increase efficiency. These materials are ideal for applications that require fast
switching and low consumption.
• New applications: Power electronic converters have a new application in zero-emission

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electric vehicles. They have high efficiency, low switching losses, high reliability, and
lower acoustic noise.
• Improved performance: Power MOSFETs, bipolar junction transistors (BJTs), and
isolated gate bipolar transistors (IGBTs) have improved performance. Examples include
lower on-state resistance, increased blocking voltage, and higher drive currents.

What are the new trends in power electronics devices?

With regard to the technology incorporated into power electronics devices, a more central role
is being played by materials that can provide increased efficiency, such as silicon carbide
(SiC) and gallium nitride (GaN), which are ideal for applications that require fast switching
and low consumption.
As we know that , the full potential of ac/ dc converter technology was better
realized once mercury-arc valves were replaced by solid-stateswitching
devices called thyristors. Thyristors offered controlled turn-on ofcurrents but not their
interruption. The rapid growth in thyristor voltage andcurrent ratings accelerated their
application, and the inclusion of internallight triggering simplified the converter
controls and their configurationseven more. Most applications, however, were
based on the naturalcommutation of currents. In special cases where forced
commutation wasrequired,elaborate circuitry using discharging Capacitors to reate
temporary current zeroes were employed.

Thyristors are now available in large sizes, eliminating the need for paralleling them
for high-current applications. Their voltage ratings have also increased so that relatively few
are required to be connected in series to yield switches or converters for power-transmission
applications. Actually, the present trend is to produce high-power electronic building blocks
(HPEBBs) to configure high-power switches and converters, thus eliminating the custom-

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design needs at the device level. Availability of HPEBBs should accelerate development of
new FACTS devices. The HPEBB thyristors are available in compact packaging and in
sufficiently large sizes (e.g., 125-mm thyristors: 5.5 kV, 4 kA or 4.5 kV, 5.8 kA) for most
applications. For switching applications, such as that for tap changers or static phase shifters,
anti-parallel–connected thyristor modules, complete with snubber circuits, are available.
These switches provide sufficiently high transient-current capacity to endure fault currents.

The GTO semiconductor devices facilitate current turn-on as well asturnoff by

using control signals. This technology has grown very rapidly; consequently, high-power
GTOs are now available (100 mm, 6 kV or 150mm, 9kV). Full on–off control offered by
GTOs has made pulse width– modulated (PWM) inverters easy to realize.

Advances in semiconductor technology are yielding new efficient, simple to-operate

devices. The insulated gate bipolar transistor (IGBT) and the metal oxide semiconductor
(MOS)–controlled thyristor (MCT) control electric power using low levels of energy from
their high-impedance MOS gates, as compared to high-current pulses needed for thyristors or
GTOs. Unfortunately, the available voltage ratings of these devices are still limited.

The MOS turn-off (MTO) thyristor combines the advantages of both thyristors and
MOS devices by using a current-controlled turn-on (thyristor) and a voltage-controlled turn-
off having a high-impedance MOS structure.Hybrid MTOs are being proposed that show
substantially low device losses relative to GTOs. Because MTOs use nearly half the parts
of GTOs, theirapplication promises significant reliability improvement

Principles and Applications of Semiconductor Switches

• Diode as a switch

A diode works as a switch by forward and reverse biasing. When the forward voltage
is higher than the cut-in voltage of the PN junction diode, current flows through the
junction. This makes the diode junction a short circuit.
• Power semiconductor devices
These devices are used as switches in power electronics applications. They can block
large forward and reverse voltages.

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• Silicon controlled rectifiers

These devices, also known as thyristors, are power semiconductor devices that can act
as fast solid state AC switches. They can control large AC voltages and currents.
• MOSFETs
These devices are used for digital switching. The oxide layer between the gate and the
channel prevents DC current from flowing through.
• Bipolar junction transistors (BJTs)
These transistors have terminals labeled base, collector, and emitter. For switching
applications, the transistor must be biased so that it operates between its cut-off region in the
off-state.
• Rectifier diodes
These are discrete semiconductor components that rectify alternating current. They are
commonly found in AC adapters.
• Optoelectronics
These devices are based on semiconductor materials that can absorb near-infrared and
visible l.ight.
Applications:-
Semiconductors are used in almost every sector of electronics. Consumer
electronics: Mobile phones, laptops, games consoles, microwaves and refrigerators all operate
with the use of semiconductor components such as integrated chips, diodes and transistors
LIMITATIONS of CONVENTIONAL TRANSMISSION SYSTEMS:
Conventional transmission systems have several limitations, including:
• Stability limits
The maximum power transfer is limited by steady state and transient stability
considerations.
• Voltage limit
The ability to transfer AC power is limited by voltage limit.
• Spur gears
Spur gears used in conventional transmission systems produce a lot of noise and
provide less comfort to the operator.
• Transmission distance
The power carrying capability of an AC line is inversely proportional to transmission
distance.
• Shunt inductors
To compensate for the receiving end voltage (Vr) becoming double the sending end

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voltage (Vs), shunt inductors are connected across the transmission line.
Other limitations of conventional AC transmission systems include:
• Thermal limits
• Short circuit current limit
• Dynamic stability
• Steady-state stability
• Frequency collapse
• Voltage collapse
• Sub-synchronous resonance

EMERGING TRANSMISSION NETWORKS

Transmission voltage in India (highest) is 765 kV AC and these lines are erected by
Power Grid Corporation for interstate connections throughout India.

Wardha – Aurangabad Line is a 1200kV overhead line with a length of 696km from
Wardha, Maharashtra, India, to Aurangabad, Maharashtra, India

Which is the first HVDC transmission line in India?

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HVDC links in India The first HVDC link to be commissioned in the country
was Rihand-Dadri [1] in 1991 connecting Thermal power plant in Rihand, Uttar Pradesh
(Eastern Part of Northern Grid) with Dadri (Western Part of Northern Grid). It has a line
length of about 816 km.

2023

1. First time in India 6.6 GW power has been transferred through ±800kV Raigarh -
Pugalur HVDC link.

2022

• 400KV, D/C, Jeerat (New) - Subhasgram (PM-JTL-TBCB) (214 CKM) implemented by

PGCIL, commissioned in Aug’22

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2021

• Bipole-I (3000 MW) of ±800 kV Raigarh - Pugalur HVDC transmission line

commissioned.

• First time in India, Voltage Source Convertor (VSC) Technology: Monopole 1 & 2 of
±320kV VSC based HVDC terminals and associated ± 320 kV HVDC Pugalur - North
Thrissur transmission line.

• The Raigarh – Pugalur – Thrissur 6000MW Mega HVDC Project has been completed.

• POWERGRID has put 250th substation (765/400 kV Khetri substation) in remote

operation from NTAMC.

2020

• Commissioned ± 800 kV Champa – Kurukshetra HVDC station Bipole-II

• Commissioned Pole-I of the 6000 MW Raigarh- Pugalur HVDC

• Completed commissioning of 11 Renewable Energy Management Centres (REMCs) for

renewable energy integration for Govt. of India

• Commissioned India’s first indigenously developed 400 kV Optical Current

Transformer in collaboration with BHEL

A flexible alternating current transmission system (FACTS) is a system composed of static

equipment used for the alternating current (AC) transmission of electrical energy. It is meant to
enhance controllability and increase power transfer capability of the network. It is generally
a power electronics-based system.

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Examples of series compensation

Examples of FACTS for series compensation (schematic)

• Static synchronous series compensator (SSSC)

• Thyristor-controlled series capacitor (TCSC): a series capacitor bank is shunted by
a thyristor-controlled inductor reactor
• Thyristor-controlled series reactor (TCSR): a series reactor bank is shunted by a thyristor-
controlled reactor
• Thyristor-switched series capacitor (TSSC): a series capacitor bank is shunted by a thyristor-
switched reactor
• Thyristor-switched series reactor (TSSR): a series reactor bank is shunted by a thyristor-
switched reactor

Examples of shunt compensation

Examples of FACTS for shunt compensation (schematic)

• Static synchronous compensator (STATCOM); previously known as a static condenser

(STATCON)
• Static VAR compensator (SVC). Most common SVCs are:
o Thyristor-controlled reactor (TCR): reactor is connected in series with a bidirectional
thyristor valve. The thyristor valve is phase-controlled. Equivalent reactance is varied
continuously.

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o Thyristor-switched reactor (TSR): Same as TCR but thyristor is either in zero- or full-
conduction. Equivalent reactance is varied in stepwise manner.
o Thyristor-switched capacitor (TSC): capacitor is connected in series with a bidirectional
thyristor valve. Thyristor is either in zero- or full- conduction. Equivalent reactance is
varied in stepwise manner.
o Mechanically-switched capacitor (MSC): capacitor is switched by circuit-breaker. It
aims at compensating steady state reactive power. It is switched only a few times a day.

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UNIT – II
HIGH VOLTAGE DC TRANSMISSION – I

Syllabus: Types of HVDC links - Monopolar, Homopolar, Bipolar and Back-to-

Back, Advantages and disadvantages of HVDC Transmission, Analysis of Greatz
circuit, Analysis of bridge circuit without overlap, Analysis of bridge with overlap
less than 600, Rectifier and inverter characteristics, complete characteristics of
rectifier and inverter, Equivalent circuit of HVDC Link.

About HVDC:
Electric power transmission was originally developed with direct current.
The availability of transformers and the development and improvement of
induction motors at the beginning of the 20th century, led to the use of AC
transmission.

DC Transmission now became practical when long distances were to be

covered or where cables were required. Thyristors were applied to DC
transmission and solid state valves became areality.

With the fast development of converters (rectifiers and inverters) at higher

voltages and larger currents, DC transmission has become a major factor in the
planning of the power transmission. In the beginning all HVDC schemes used
mercury arc valves, invariably single phase in construction, in contrast to the low
voltage poly phase units used for industrial application. About 1960 control
electrodes were added to silicon diodes, giving silicon- controlled-rectifiers (SCRs
or Thyristors).

Today, the highest functional DC voltage for DC transmission is +/-

600kV. D.C transmission is now an integral part of the delivery of electricity
in many countries through out the world.
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Comparison of AC and DC Transmission

The merits of two modes of transmission (AC & DC) should be compared
based on the following factors.
1) Economics of transmission
2) Technical Performance
3) Reliability
Economics of Power Transmission:

In DC transmission, inductance and capacitance of the line has no

effect on the power transfer capability of the line and the line drop. Also,
there is no leakage or charging current of the line under steady conditions.

A DC line requires only 2 conductors whereas AC line requires 3 conductors in

3-phase AC systems. The cost of the terminal equipment is more in DC lines
than in AC line. Break-even distance is one at which the cost of the two
systems is the same. It is understood from the below figure that a DC line is
economical for long distances which are greater than the break-even
distance.

Figure: Relative costs of AC and DC transmission lines vs distance

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Types of HVDC Links

Three types of HVDC Links are considered in HVDC applications which are
1. Monopolar Link:

A monopolar link as shown in the above figure has one conductor and
uses either ground and/or sea return. A metallic return can also be used
where concerns for harmonic interference and/or corrosion exist. In
applications with DC cables (i.e., HVDC Light), a cable return is used. Since
the corona effects in a DC line are substantially less with negative polarity of
the conductor as compared to the positive polarity, a monopolar link is
normally operated with negative polarity.

2. Bipolar Link:

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A bipolar link as shown in the above figure has two conductors, one
positive and the other negative. Each terminal has two sets of converters of
equal rating, in series on the DC side. The junction between the two sets of
converters is grounded at one or both ends by the use of a short electrode
line. Since both poles operate with equal currents under normal operation,
there is zero ground current flowing under these conditions. Monopolar
operation can also be used in the first stages of the development of a bipolar
link. Alternatively, under faulty converter conditions, one DC line may be
temporarily used as a metallic return with the use of suitable switching.

3. Homopolar Link:

In this type of link as shown in the above figure two conductors having
the same polarity (usually negative) can be operated with ground or metallic
return.
HVDC & FACTS Unit-II

Due to the undesirability of operating a DC link with ground return,

bipolar links are mostly used. A homopolar link has the advantage of reduced
insulation costs, but the disadvantages of earth return outweigh the
advantages.

4. Back-to-Back HVDC Link

It is called back to back system because in electronics and power electronics if
two bipolar components are connected in series with opposite polarity then this
pair is known as back to back system. Here also rectifier and inverter are
identical and connected in series operating on High Voltage DC so this is known
as HVDC back to back System.

An HVDC back-to-back station can be used to create an asynchronous

interconnection between two AC networks. An HVDC Light back-to-back station
consists of two converters located in the same building. An HVDC back-to-back
station can be used to create an asynchronous interconnection between two AC
networks.
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List of HVDC Links in INDIA:

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Advantages of high voltage DC transmission :

1) Economical transmission of the bulk power

In a conventional transmission line, the distance cannot be more than the
breakeven distance. But in the HVDC transmission line, the distance can be more
than the breakeven distance.

2) Decrease in the number of conductors

In the HVAC system, the power transmitted in the form of three-phase AC power.
Therefore, three or four conductors need as per the type of transmission line.

But in the case of HVDC transmission lines, only two conductors required. Hence,
the cost of the conductor decreased.

3) Corona
Corona effect appears in both HVAC and HVDC systems. But, in an HVDC system,
the effect of the corona is very less compared to the HVAC system. And there is
no disturbance to the nearby communication line.

4) Size of tower
In the HVDC transmission line, phase-phase and phase-ground clearance required
is less compared to the HVAC line. Therefore, the height and width of the tower
required is less.

The number of conductors required in this system is less. So, the size of the tower
is less which results in less cost of the tower.

5) Earth return
For the monopolar HVDC transmission system, earth return can be used. That
means, only one conductor required to transmit the power. This is not possible in
the HVAC transmission line.

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HVDC & FACTS Unit-II

6) Charging current
In the DC transmission line, the capacitance is not produced between two phases
or between the phase and ground. Therefore, the charging current is absent in
the HVDC system.

7) Skin effect
The current density is uniform throughout the line. Hence, there is no skin effect
in the HVDC system.

And it utilizes an entire cross-section area of the conductor. So, the resistance of
the line is not increasing and the power loss is less.

8) Reduction in line loss

The line loss reduced due to the absence of the reactive power in the HVDC
transmission line. This increases the efficiency of the system.

9) Reduction in size of the conductor

When equal power transmitted for the same distance, less volume of conductor
required for the HVDC two-wire system compared to the HVAC three-phase three-
wire system.

10) Underground cable

The underground system can be established for the HVDC system because of the
absence of the charging current.
In the HVAC system, the distance of underground cables is a constraint. For
example, 145 kV line the distance is 60 km, for 245 kV it is 40 km and for 400 kV
it is 25 km.

Disadvantages/Limitations
1) Cost of terminal equipment
In the HVDC transmission line, the rectifier used at the sending end and the
inverter used at the receiving end. The smoothing filters need at receiving end.

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HVDC & FACTS Unit-II

The cost of this equipment is very high.

2) DC circuit breaker
The DC circuit breaker is still under development and the cost is high compared to
the AC circuit breaker.

3) Additional equipment
This system needs some additional equipment like converter transformer,
electrical and mechanical auxiliaries, pole control, valve control, and many more.
All this equipment is of high technology and the cost of this equipment is high.

4) Complicated control
The converter used to control the transmission line. But it is difficult to control the
converter under certain abnormal conditions.

5) Change the voltage level

In the AC system, with the help of a transformer, the voltage can be easily
stepped up and stepped down. Therefore, this system cannot use for low voltage
transmission.
6) System failure
There is some abnormal operating condition in which the system may fail to
operate.
7) Harmonic filter
In the input side, the AC supply is given to the rectifiers. To mitigate these
harmonics, a large amount of filter required. And the cost of this equipment is
high.
8) Complicated cooling
The converter used power electronics switches. When this is in operation, a
very high amount of heat produced in the thyristor.
9) Overload capacity
The converters cannot operate on overload conditions. Therefore, it is not
permissible.

10
HVDC & FACTS Unit-II

10) Multi-terminal network

HVDC transmission line is not suitable for a multi-terminal network.

11) Power loss

The losses occur in the converters and other auxiliaries, which nullify the reduced
loss in the line.

HVDC advantages and disadvantages

HVDC links are being used worldwide at power levels of several gigawatts with the
use of thyristor valve. So here this article gives information about the advantages
and disadvantages of HVDC to know more details about it.

Advantages of high voltage DC transmission :

1. Fault clearance in HVDC is faster, therefore the DC transmission system
possesses improved transient stability

2. Size of the conductor in DC transmission can be reduced as there is no skin

effect

3. Cost is less as compared to the AC transmission

4. HVDC tower is less costly

5. No requirement of reactive power

6. No system stability problem

7. HVDC require less phase to phase and ground to ground clearance

8. Require less number of conductor for same power transfer

9. Improve line loading capacity

10. To ac system at different frequencies can be interconnected through HVDC

transmission lines

11. HVDC is preferred as it requires no charging current

12. Power loss is reduced with DC just because of fewer numbers of lines are
required for power transmissionHVDC is a more flexible system

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 HVDC & FACTS Unit-II

Disadvantages of high voltage DC transmission:

1. Expenses inverters with limited overload capacity

2. HVDC is less reliable

3. IN HVDC very accurate and lossless power flows through DC link

4. The disadvantages of HVDC are in conversion, switching, control, maintenance

5. Lower availability than the AC system

6. HVDC is very complicated

7. The circuit breaker is used in HVDC for circuit breaking, and Inverter and
rectifier terminals will generate harmonics which can be reduced by using active
filters, which are also very expensive

8. HVDC does not have transformers for changing the voltage levels

9. Heat loss occurs in converters substation

Analysis of Graetz circuit (6-pulseconverter bridge):
A Graetz bridge converter is a six pulse converter with a 12 pulse converter made up of two
bridges in series. The configuration of the bridge is also called a Graetz circuit.

During normal operating conditions, the overlap angle is between 0 and 60 degrees. In this
range, two or three valves are conducting. If the overlap angle is between 60 and 120
degrees, then three to four valves are conducting, which is known as abnormal operation
mode.

The leakage inductance of the converter transformers and the impedance in the supply
network cause the current in a valve to not change suddenly. This means that commutation
from one valve to the next cannot be instantaneous.

Here are some things to consider when buying a Graetz bridge converter with overlap:

• Commutation voltage: The overlap angle is a function of the commutation voltage and
the dc current. A decrease in commutation voltage or an increase in dc current can
cause an increase in μ.

• Valve utilization factor: The valve utilization factor (VUF) is given by VUF = P IV Vd0.

• Firing angle: In an HVDC transmission, the firing angle is between 0° to 90° in rectifier
while in the inverter the firing angle is between 90° to 180°

A Graetz bridge converter with overlap has a current in a valve that cannot change
12
 HVDC & FACTS Unit-II

suddenly. This means that the commutation from one valve to the next cannot be
instantaneous.

In a Graetz bridge converter without overlap, two valves are conducting at any given
time. One valve is from the upper commutation group and the other is from the lower
commutation group. When the next valve in a commutation group fires, the preceding valve
turns off.

During normal operating conditions, the overlap angle is between 0 and 60 degrees. If the
overlap angle is between 60 and 120 degrees, then three to four valves are in a conducting
state. This is known as abnormal operation mode.

The schematic diagram of a six-pulse Graetz circuit is shown in the fig.

❖ This Graetz circuit utilizes the transformer and the converter unit to at most level
and it maintains low voltage across the valve when not in conduction.
❖ Due to this quality present in Graetz circuit, it dominates all other alternative
circuits from beingimplemented in HVDC converter.
❖ The low voltage across the valves is nothing but the peak inverse voltage which
the valve should withstand.
❖ The six-pulse Graetz circuit consists of 6 valves arranged in bridge type and the
converter transformer having tapings on the AC side for voltage control.
❖ AC supply is given for the three winding of the converter transformer connected in
star with grounded neutral.
❖ The windings on the valve side are either connected in star or delta with
ungrounded neutral.
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 HVDC & FACTS Unit-II

❖ The six valves of the circuit are fired in a definite and fixed order and the DC
output obtained contains six DC pulses per one cycle of AC voltage wave.

a) Operation without overlap:

❖ The six pulse converter without over lapping valve construction sequence
are 1-2, 2-3, 3-4, 4-5, 5-6, 6-1.

❖ At any instant two valves are conducting in the bridge. One from the
upper group and other from the lower group.Each valve arm
conducts for a period of one third of half cycle i.e., 60 degrees.

❖ In one full cycle of AC supply there are six pulses in the DC waveform.
Hence the bridge is called as six pulse converter.

For simple analysis following assumptions are much:

i) AC voltage at the converter input is sinusoidal and constant

ii) DC current is constant

iii) Valves are assumed as ideal switches with zero impedance when
on(conducting) and with infinite impedance when off(not conducting)
In one full cycle of AC supply we will get 6-pulses in the output.
Each pair of the devices will conduct 60 degrees. The dc output voltage
waveform repeats every 60 degrees interval.
Therefore for calculation of average output voltages only
consider one pulse and multiply with six for one full cycle. In this case
each device will fire for 120 deg.

Firing angle delay:

Delay angle is the time required for firing the pulses in a converter for its conduction.
➢ It is generally expressed in electrical degrees.

➢ In other words, it is the time between zero crossing of commutation

voltage and starting point of forward current conduction.

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HVDC & FACTS Unit-II

➢ The mean value of DC voltage can be reduced by decreasing the

conduction duration, which can be achieved by delaying the pulses ie., by
increasing the delay angle we can reduce the DC voltage and also the
power transmission form one valve to another valve can also be reduced.
✓ when α = 0, the commutation takes place naturally and the converter acts as a
rectifier.
✓ when α > 60 deg, the voltage with negative spikes appears and the
direction of power flow is from AC to DC system without change in

magnitude of current.
✓ when α = 90 deg, the negative and positive portions of the voltage are
equal and because of above fact, the DC voltage per cycle is zero. Hence
the energy transferred is zero.
✓ when α > 90 deg, the converter acts as an inverter and the flow of
power is from DC system to AC system.
Let valve 3 is fired at an
angle of α. the DC output
voltage is given by Vdc =
Vdo Cos α

From above equation we can say that if firing angle varies, the DC output voltage
varies

DC Voltage waveform:
The dc voltage waveform contains a ripple whose frequency is six
times the supply frequency. This can be analysed in Fourier series
and contains harmonics of the order h=np
Where p is the pulse number and n is an integer.

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HVDC & FACTS Unit-II

The r.m.s value of the hth order harmonic in dc voltage is given by

❖ Although α can vary from 0 to 180 degrees, the full range cannot be
utilized. In order to ensure the firing of all the series connected
thyristors,it is necessary to provide a minimum limit of α greater than zero, say
5 deg.

❖ Also in order to allow for the turn off time of a valve, it is necessary to
provide an upper limit less than 180 deg.
❖ The delay angle α is not allowed to go beyond 180-γ where γ is called the
extinction angle (sometimes also called the marginal angle).
❖ The minimum value of the extinction angle is typically 10 deg, although
in normal operation as an inverter, it is not allowed to go below 15deg
or 18deg.

AC current waveform:
16
HVDC & FACTS Unit-II

It is assumed that the direct current has no ripple (or harmonics) because of
the smoothing reactor provided in series with the bridge circuit.
The AC currents flowing through the valve (secondary) and primary windings of
the converter transformer contain harmonics.

The waveform of the current in a valve winding is shown in fig.

By Fourier analysis, the peak value of a line current of fundamental frequency

component is given by,

Now the rms value of line current of fundamental frequency component is given by

Generally, the RMS value of nth harmonic is

given by,
17
HVDC & FACTS Unit-II

where I = Fundamental current

n = nth order harmonic.

The harmonics contained in the current waveform are

of the order given by h = np + 1
where n is an integer, p is the pulse number.
For a 6 pulse bridge converter, the order of AC harmonics are 5, 7, 11, 13 and higher
order.
They are filtered out by using tuned filters for each one of the first four
harmonics and a high pass filter for the rest.
The Power Factor:
The AC power supplied to the converter is given by

where cosФ is the power factor.

The DC power must match the AC power ignoring the losses in the converter. Thus, we
get

Substituting for Vdc = Vdo Cos α, and I1=

6L ,


we obtain cos Ф = cos α

The reactive power requirements are increased as α is increased from 0
When α = 90 deg, the power factor is zero and only reactive power is consumed.

ii) With overlap:

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HVDC & FACTS Unit-II

Lc indicates leakage inductance of transformer

Vd, Id = DC voltage and current flowing

in the line Ld = DC side reactance

V1 = voltage across the thyristors

p,n = positive and negative pole on the line

Due to the leakage inductance of the converter transformers and the

impedance in the supply network, the current in a valve cannot change
suddenly and thus commutation from one valve to the next cannot be
instantaneous.

For example, when valve 3 is fired, the current transformer from

valve 1 to valve 3, takes a finite period during which both valves are
conducting. This is called overlap and its duration is measured by the
overlap (commutation) angle ‘μ’.

Commutation delay:

The process of transfer of current from one path to another path with
both paths carrying current simultaneously is known overlap.
The time required for commutation or overlapping which is expressed
in electrical degrees is done with commutation angle, denoted by μ.

During normal operating conditions the overlap angle is in the range

of 0 to 60 degrees, in which two (or) three valves are conducting.

However, if the overlap angle is the range of 60 to 120 degrees,

then three to four valves are in conducting state which is known as
abnormal operation mode.

During commutation period, the current increases from 0 to Id in

the incoming valve and reduce to zero from Id in the outgoing valve.

The commutation process begins with delay angle and ends with extinctionangle ie., it

starts when ωt = α and ends when ωt = α+μ = δ, where δ is known as an

extinction angle.
There are three modes of the converter as follows:

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HVDC & FACTS Unit-II

1. Mode-1 - Two and three valve conduction (μ<60 deg)

2. Mode-2 - Three valve conduction (μ=60 deg)
3. Mode-3 - Three and four valve conduction (μ>60 deg)
Depending upon the delay angle α, the mode 2
must be just a point on the boundary of modes 1
and 3.

i) Analysis of Two and Three valve conduction mode:

Generally overlap angle will be less than 60 deg, so let us analyse thismode.
Timing diagram

In this mode each interval of the period of supply can be divided into twosubintervals.

In the first subinterval, three valves are conducting and in the

second subinterval, two valves are conducting.

20
 HVDC & FACTS Unit-II

Each valve will conduct for 120 degrees and each pair will conduct for60 degrees, if there is

no overlap.
Let us consider non-overlap of only valve 1,2 conducting followed
by overlap of 3 with 1. Ie., 1,2 and 3 conducting.
When only valve 1 and 2 conducting

21
HVDC & FACTS Unit-II

When valve 3 is fired then 3 will overlap with 1 and it will be 3 valve
conduction periods ie., 1, 2 and 3.
For this period the emanation for the voltage and current are
different and thus can be obtained as follows:

Consider that valve 3 is ignited at angle ‘α’ and for overlap angle both 1 and
3 conduct
together.
The duration of overlap 1 and 3 will conduct top with 2 at the bottom as
shown in the fig.
Just at the beginning, ωt = α
At ωt = α

22
HVDC & FACTS Unit-II

The angle (α+μ) is called extinction angle During overlap a loop

is formed as N-3-1-N For this loop,

Assuming the dc current either i1 alone conduct, i3 alone when 3 alone

conducts should be equal to Id So both 1 and 3 conduct overlapSo
At ωt = (α+μ);

23
HVDC & FACTS Unit-II

DC voltage and valve voltage waveforms for rectifier when α=15 deg,
µ = 15 deg, δ = 30 deg

24
HVDC & FACTS Unit-II

Converter Control Characteristics

Basic Characteristics:

Here are some characteristics of rectifiers and inverters in high voltage

direct current (HVDC) links:

• Rectifier

The rectifier station converts AC power into DC power. In rectifier

mode, the voltage across the valve is negative immediately after the arc
extinction. Rectifiers typically operate within a range of 150–200 so that the
rectifier voltage can increase to control DC power flow.

• Inverter

The inverter station converts DC power into AC power. In inverter

mode, the voltage across the valve is negative for a much shorter duration
than in the rectifier mode. Inverter characteristics are similar to those of
rectifiers, but the operation as an inverter requires a minimum commutation
margin angle. Inverter mode requires maintaining a certain minimum
extinction angle to avoid commutation failure.

A complete HVDC system always includes at least one converter

operating as a rectifier and at least one operating as an inverter.

The intersection of the two characteristics (point A) determines the

mode of operation- Station I operating as rectifier with constant current
control and station II operating at constant (minimum) extinction angle.
There can be three modes of operation of the link (for the same
direction of power flow) depending on the ceiling voltage of the rectifier which
determines the point of intersection of the two characteristics which are
defined below
1) CC at rectifier and CEA at inverter (operating point A) which is the

25
HVDC & FACTS Unit-II

normal mode of operation.

2) With slight dip in the AC voltage, the point of intersection drifts to
C which implies minimum α at rectifier and minimum γ at the
inverter.
3) With lower AC voltage at the rectifier, the mode of operation shifts
to point B whichimplies CC at the inverter with minimum α at the
rectifier.

26
HVDC & FACTS Unit-II

The characteristic AB has generally more negative slope than

characteristic FE because

the slope of AB is due to the combined resistance of (Rd + Rcr ) while is

the slope of FE is due to Rci .

The above figure shows the control characteristics for negative

current margin Im (or where the current reference of station II is larger
than that of station I). The operating point shifts now to D which implies
power reversal with station I (now acting as inverter) operating with
minimum CEA control while station II operating with CC control.
This shows the importance of maintaining the correct sign of the

27
HVDC & FACTS Unit-II

current margin to avoid inadvertent power reversal. The maintenance of

proper current margin requires adequate telecommunication channel for
rapid transmission of the current or power order.

Equivalent circuit of HVDC Link:

The major advantage of a HVDC link is rapid controllability of transmitted
power through the control of firing angles of the converters. Modern converter
controls are not only fast, but also very reliable and they are used for protection
against line and converter faults.

A DC link is a connection which connects a rectifier and an

inverter. These links are found in converter circuits and in VFD circuits.
The AC supply of a specific frequency is converted into DC. This DC, in

turn, is converted into AC voltage.

The DC link is the connection between these two circuits. The DC link
usually has a capacitor known as the DC link Capacitor. This capacitor is
connected in parallel between the positive and the negative conductors.
The DC capacitor helps prevent the transients from the load side from
going back to the distributor side. It also serves to smoothen the pulses
in the rectified DC.

28
HVDC & FACTS Unit-II

Equivalent Circuit of HVDC link

29
 HVDC & FACTS Unit-II

Applications of HVDC transmission:

High-voltage direct current (HVDC) lines have many applications, including:
• Long-distance transmission: HVDC lines are commonly used for long-distance
power transmission because they require fewer conductors and fewer power
losses than AC lines.
• Interconnection: HVDC systems can be interconnected with adjacent power
systems to increase the intermittency of new generation sources.
• Grid reinforcement: HVDC can be used for grid reinforcement.
• Bulk power transmission: HVDC can be used for long-distance bulk power
transmission, including access to remote renewables.
• Submarine cable transmission: HVDC can be used for submarine cable transmission.
• Asynchronous ties: HVDC can be used for asynchronous ties, such as transmission
in areas with severely restricted ROWs.
• Offshore wind power plant transmission: HVDC can be used for transmission of
offshore wind power plants.
• Multi-terminal HVDC systems: HVDC can be used for multi-terminal HVDC systems.

30
HVDC & FACTS Unit-II

31
 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II
UNIT III

HVDC TRANSMISSION-II
Topics:
Desired features and means of control, control of the direct current transmission link, Constant
current control, Constant ignition angle control, Constant extinction angle control, Converter
firing- angle control-IPC and EPC, frequency control and Tap changer control, Starting,
Stopping and Reversal of power flow in HVDC links.

DESIRED FEATURES OF THE CONTROL:

DESIRED FEATURES OF THE CONTROLLER:

1. Control system should not be sensitive to normal variations in voltage and frequency of
the AC supply system
2. Control should be fast reliable and easy to implement.
3. There should be continuous operating range of full Rectification to full Inversion.
4. Control should be such that it should require less reactive power.
5. Under at steady state conditions the valves should be fired symmetrically.
6. Control should be such that it must control the maximum current in the DC link and
limit the fluctuations of the current.
7. Power should be controlled independently and smoothly which can be done by
controlling the current or voltage or both.
8. Control should be such that it can be used for protection of the line and the converter.
9. DC filter
A DC filter is connected between the pole bus and neutral bus to divert the DC harmonics to
earth and prevent them from entering DC lines.
10. Protection against faults
Modern converter controls should be used for protection against line and converter faults.
11. VSC-based HVDC systems

VSC-based HVDC systems offer a faster active power flow control with respect to the more
mature CSC-HVDC

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

Here are some ways (means) to control HVDC lines:

• VSC based HVDC
Power can be controlled by changing the phase angle of the converter AC voltage with respect to
the filter bus voltage. Reactive power can be controlled by changing the magnitude of the
fundamental component of the converter AC voltage with respect to the filter bus voltage.
• Individual phase control (IPC) method
This method was frequently used as a turn-on method for thyristors in the earlier days when
HVDC systems were first introduced but it has been replaced with the equidistant pulse control
(EPC) method.
• Firing circuits of the thyristors
These can be used to control the current on the HVDC side.
• Converter valves
These can be controlled by providing signals to operate the converter valve semiconductors.

An HVDC control system is a hierarchically structured control system:

• System Control
• Master Control
• Station Control
• Pole or Converter Control
• Valve base Control (VBC) / Valve Unit Control (VUC)

Control of the direct current transmission link:

The power in a DC link can be controlled by adjusting the current or voltage. To minimize loss,
a constant voltage should be maintained in the link while adjusting the current to meet the
required power.

Here are some other ways to control the DC link:

• Change the polarity of the DC voltage to achieve power reversal
• Use AC and DC filters to reduce the harmonic effects of the DC links
• Change the phase angle of the converter AC voltage with respect to the filter bus voltage
• Change the magnitude of the fundamental component of the converter AC voltage with respect
to the filter bus voltage.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

Basic means of control:

OVERALL EQUIVALENT CIRCUIT OF HVDC SYSTEM:

From the equivalent circuit of the HVDC System we can write:

Vdr cos −Vdi cos

Id =
Rcr + Rd − Rci
The DC voltage and current in the DC link can be controlled by controlling rectifier
voltages using two methods.
GRID CONTROL
MANUAL CONTROL
• GRID CONTROL: It is done by varying ignition angle of the valves. It is rapid or
instantaneous control .

• MANUAL CONTROL: It is done changing the taps ratio of the converter transformer. It
is slow and done in steps Power reversal can be done by changing the polarity of the DC
voltage at both ends.

BASIS FOR SELECTION OF THE CONTROL:

• Prevention of large fluctuating current due to variations of AC voltages
• Maintaining the DC voltage near it’s rated.
• Maintaining the power factor at the sending and receiving end as high as possible
• Prevention of various faults in the valve

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

CONSTANT VOLTAGE CONSTANT CURRENT

Voltage is constant Current is constant
Current is varied to change power Voltage is varied to change power
Loads and power sources are connected in Loads and power sources are connected in
parallel in order to turnoff a load or a source series in order to turn off a load or source it
respective branch is opened should be bypassed
AC transmission and DC Distribution Street lighting in DC
DC system the fault current can be greater limited Short circuit current is ideally limited by
by circuit resistance load current and it is twice of the rated
current and Accidental open circuits give
rise to huge voltages

Power loss is α (power transmitted)2 Power loss is α full load value

Note: Rectifier will take care of the current and the Inverter will take care of the voltage.

Rectifier - Constant Current Control (CCC)

Inverter-Constant Extinction Angle Control (CEA)

Let us examine how AC voltages changes reflect in the DC current and which controller
has to be exercised to make DC link current at rated value.

INCREASE IN THE RECTIFIER VOLTAGE:

Current in the DC link will increase to control the current in the rectifier end, controller will
increase delay angle α while at the inverter end controller will maintain CEA. Increase in
the delay angle worsens the power factor. Generally it is controlled in steps thereafter tap
change is done

INCREASE IN THE INVERTER VOLTAGE : Current in the DC link will decrease to control the
current in the rectifier end, controller will decrease delay angle α up to α min while at the
inverter end controller will maintain CEA. decrease in the delay angle improves the power
factor. Generally it is controlled in steps thereafter tap change is done

DECREASE IN THE RECTIFIER VOLTAGE: : Current in the DC link will decrease to control the
current in the rectifier end, controller will decrease delay angle α up to α min while at the
inverter end controller will maintain CEA. Decrease in the delay angle improves the power
factor. Generally it is controlled in steps thereafter tap change is done. If the further
decrease in the rectifier voltage characteristics falls below and CEA characteristics does not

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II
intersect then Dc link current will be zero. Therefore inverter also should be equipped with
constant current controller

DECREASE IN THE INVERTER VOLTAGE: Current in the DC link will increase to control the
current in the rectifier end, controller will increase delay angle α while at the inverter end
controller will maintain CEA. Increase in the delay angle worsens the power factor. Generally
it is controlled in steps thereafter tap change is done

Control of HVDC Links.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

Ideal HVDC Link Characteristics:

The rectifier characteristics can be shifted horizontally by adjusting the current command or
current order. If the measured current is less than the command the regulator advances the
firing by decreasing α.
The inverter characteristics can be raised or lowered by means of the transformer tap changer.
When the tap is moved the CEA regulator quickly restores the desired gama. As a result
the DC current changes which is then quickly restored by current regulator of the rectifier.

The rectifier maintains constant current in the DC link by changing α however α• cannot be
less than αmin. Once αmin is hit no further increase in voltage is possible. This is called
Constant Ignition Angle Control(CIA).
Negative slope due to finite gain of the controller.
In practice as current controller will have a proportional controller it has high negative slope
due to finite gain of the controller.

Constant current control (CC) is a technique used to control the DC link of high voltage
direct current (HVDC) bridges. In CC control, power is varied by changing the
voltage.
CC control involves the following steps:

1. Measuring the DC current

2. Comparison of Id with the set value Ids or Iord (called as Reference/Current Order/Current

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II
Command)
3. Amplification to the differences called error.
4. Application of the output signal of the amplifier to the phase shift circuit that alters the
ignition angle α of the valves in the proper direction for reducing the error.
5. The dynamic performance of CC control is determined by the charging and discharging
process of the output capacitor.
6. The rectifier subsystem of the HVDC system operates in CC control mode, while the inverter
subsystem operates in constant extinction angle (CEA) control mode

Constant Current Control Scheme:

FCO: Final Change order

CPG: Central pattern Generator

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

Constant Ignition Angle control:

The Natural Voltage (NV) Characteristic corresponds to zero delay angle α=0. The Constant
Ignition Angle control is a similar characteristic that is parallel to the NV characteristic
with a controllable intercept V0 cosα. The Inverter is usually operated at a constant
extinction angle(CEA).
To control the firing angle of a converter, it is necessary to synchronize the firing pulses
emanating from the ring counter to the AC commutation voltage that has a frequency of
60 Hz in steady state.

The current or extinction angle controller generates a control signal Vc which is related to the
firing angle required. The firing angle controller generates gate pulses in response to the
control signal Vc . The selector picks the smaller of the α determined by the current and
CEA controllers.

Firing Angle Control

The operation of CC and CEA controllers is closely linked with the method of generation of
gate pulsesforthevalvesinaconverter.TherequirementsforthefiringpulsegenerationofHVDC
valves are

1. The firing instant for all the valves are determined at ground potential and the firing signals
sent to individual thyristors by light signals through fibre-optic cables. The required gate
power is made available at the potential of individual thyristor.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II
2. While a single pulse is adequate to turn-on a thyristor, the gate pulse generated must send a
pulse whenever required, if the particular valve is to be kept in a conducting state.
The two basic firing schemes are
1. Individual Phase Control(IPC)
2. Equidistant Pulse Control(EPC) and these are further classified as:

Firing Angle Control

Individual Phase Control (IPC) Equidistant Pulse Control (EPC)

Constant α Control Inverse Cosine Control

Pulse Frequency Control Pulse Period Control Pulse Phase Control

Individual Phase Control(IPC)

This was used in the early HVDC projects. The main feature of this scheme is that the firing pulse
generation for each phase (or valve) is independent of each other and the firing pulses are rigidly
synchronized with commutation voltages.

There are two ways in which this can be achieved

1. Constant α Control
2. Inverse Cosine Control

Constant α Control
Six timing (commutation) voltages are derived from the converter AC bus via voltage
transformers and the six gate pulses are generated at nominally identical delay times
subsequent to the respective voltage zero crossings. The instant of zero crossing of a
particular commutation voltage corresponds to α = 0o for that valve.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

The delays are produced by independent delay circuits and controlled by a common
control voltage V derived from the current controllers.

Inverse Cosine Control

The six timing voltages (obtained as in constant α control) are each phase shifted by 900
and added separately to a common control voltage V.

The zero crossing of the sum of the two voltages initiates the firing pulse for the
particular valve is considered. The delay angle α is nominally proportional to the inverse cosine
of the control voltage. It also depends on the AC system voltage amplitude and shape.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

The main advantage of this scheme is that the average DC voltage across the bridge
varies linearly with the control voltage Vc.

Drawbacks of IPC Scheme

The major drawback of IPC scheme is the aggravation of the harmonic stability problem
that was encountered particularly in systems with low short circuit ratios (less than 4). The
harmonic instability, unlike instability in control systems, is a problem that is characterized by
magnification of non characteristic harmonics in steady-state.

This is mainly due to the fact that any distortion in the system voltage leads to
perturbations in the zero crossings which affect the instants of firing pulses in IPC scheme. This
implies that even when the fundamental frequency voltage components are balanced, the firing
pulses are not equidistant in steady-state. This in turn leads to the generation of non characteristic
harmonics (harmonics of order h ≠ np ±1)in the AC current which can amplify the harmonic
content of the AC voltage at the converter bus. The problem of harmonic instability can be
overcome by the following measures

1. Through the provision of synchronous condensers or additional filters for filtering out non
characteristic harmonics.

2. Use of filters in control circuit to filter out noncharacteristic harmonics in the commutation
voltages.

3. The use of firing angle control independent of the zero crossings of the AC voltages. This is
the most attractive solution and leads to the Equidistant Pulse Firing scheme.

Equidistant Pulse Control (EPC)

The firing pulses are generated in steady-state at equal intervals of 1/pf , through a ring
counter. This control scheme uses a phase locked oscillator to generate the firing pulses.
There are three variations of the EPC scheme
1. Pulse Frequency Control(PFC)

2. Pulse Period Control

3. Pulse Phase Control(PPC)

Pulse Frequency Control (PFC)
A Voltage Controlled Oscillator (VCO) is used, the frequency of which is determined by the
control voltage Vc which is related to the error in the quantity (current, extinction angle or

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II
DC voltage) being regulated. The frequency in steady-state operation is equal to pfo where fo
is the nominal frequency of the AC system. PFC system has an integral characteristic and
has to be used along with a feedback control system for stabilization.
The Voltage Controlled Oscillator(VCO)consistsofanintegrator,comparatorandapulsegenerator.

PFC

Pulse Period Control:

It is similar to PFC except for the way in which the control voltage Vc is handled. The structure
of the controller is the same, however, Vc is now summed with V3 instead of V1 . Thus, the
instant tn of the pulse generation is

WithVc =0,the interval between consecutive pulses, in steady-state, is exactly equal to1/pfo.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II
The frequency correction in this scheme is obtained by either updating V1 in response to the
system frequency variation or including another integrator in the CC or CEA controller.

Pulse Phase Control(PPC)

Ananalogcircuitisconfiguredtogeneratefiringpulsesaccordingtothefollowingequation

where Vcn and Vc(n-1) are the control voltages at the

instants tn and tn-1 respectively.
For proportional current control, the steady-state can be reached when the error of Vc is
constant.

The major advantages claimed for PPC over PFC are

(i) easy inclusion of α limits by limiting Vc as in IPC and

(ii) linearization of control characteristic by including an inverse cosine function block after the
current controller. Limits can also be incorporated into PFC or pulse period control system.

Drawbacks of EPC Scheme:

EPC Scheme has replaced IPC Scheme in modern HVDC projects; it has certain limitations
which are:

1. Under balanced voltage conditions, EPC results in less DC voltage compared to IPC.
Unbalance in the voltage results from single phase to ground fault in the AC system
which may persist for over 10 cycles due to stuck breakers. Under such conditions, it is
desirable to maximize DC power transfer in the link which calls for IPC.
EPC Scheme also results in higher negative damping contribution to torsional oscillations
when HVDC is the major transmission link from a thermal station.

Constant extinction angle control: (CEA)

The current controller is in variably of feed back type which is of PI type.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

The extinction angle controller can be of predictive type or feedback type with IPC control.
The predictive controller is considered to be less prone to commutation failure and was used
in early schemes. The feedback control with PFC type of Equidistant Pulse Control
overcomes the problems associated with IPC.
The extinction angle, as opposed to current, is a discrete variable and it was felt the feedback
control of gamma is slower than the predictive type. The firing pulse generation is based on
the following equation.

Where ecj is the commutation voltage across valve j and tn is the instant of
its firing. In general, the prediction of firing angle is based on the equation

Under large disturbances such as a sudden dip in the AC voltage, signals derived from the
derivative of voltage or DC current aid the advancing of delay angle for fast recovery from
commutation failures.
Tap Changer Control:

A tap changer is a part of a power transformer that can be used to get different voltage levels and
various turns ratios. Tap changers are used in HVDC lines to:
• Change the transformer winding ratio to control the firing angle
• Compensate voltage variations
• Control the DC output voltage when the converter is used as a rectifier

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II
• Control voltage drops due to HVDC conversions
• Switch until the DC voltage returns to the normal range (17.5–21.5°) when the DC voltage
exceeds the allowable range
• Provide the required valve side voltage at every load point
• Compensate voltage drops of the converter or compensation of deviations of the AC voltage
from the design value.
Note: Tap changer is an essential part of any power transformer for obtaining various turns ratios
to get different voltage levels.

Starting and Stopping of DC Link:

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

Energization and Deenergization of a Bridge:

Consider N series connected bridges at a converter station. If one of the bridges is to be
taken out of service, there is need to not only block, but bypass the bridge. This is because
of the fact that just blocking the pulses does not extinguish the current in the pair of valves
that are left conducting at the time of blocking. The continued conduction of this pair injects
AC voltage into the link which can give rise to current and voltage oscillations due to lightly
damped oscillatory circuit in the link formed by smoothing reactor and the line capacitance.
The transformer feeding the bridge is also subjected to DC magnetization when DC current
continues to flow through the secondary windings. The bypassing of the bridge can be done
with the help of a separate bypass valve or by activating a bypass pair in the bridge (two
valves in the same arm of the bridge). The bypass valve was used with mercury arc valves
where the possibility of arc backs makes it impractical to use bypass pairs. With thyristor
valves, the use of bypass pair is the practice as it saves the cost of an extra valve.

With the selection of bypass pair 1 and 4, the commutation from valve 2 to4 is there,
but the commutation from valve 3 to valve 5 is prevented. In the case of a predetermined
choice of the bypass path, the time lapse between the blocking command and the current
transfer to bypass path can vary from 600 and 1800 for a rectifier bridge. In the inverter,
there is no time lag involved in the activation of the bypass pair. The voltage waveforms for
the rectifier and inverter during de- energisation are shown below where the overlap is
neglected.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

The current from bypass pair is shunted to a mechanical switch S1. With the aid of the
isolators S, the bridge can be isolated. The isolator pair S and switch S1 are interlocked such
that one or both are always closed.

The energisation of a blocked bridge is done in two stages. The current is first diverted from
S1 to the bypass pair. For this to happen S1must generate the required arc voltage and to
minimize this voltage, the circuit inductance must be small. In case the bypass pair fails to
take over the current, S1must close automatically if the current in that does not become zero
after a predetermined time interval. AC breakers with sufficient arc voltage, but with
reduced breaking capacity are used as switch S1 .
In the second stage of energisation, the current is diverted from the bypass pair. For the
rectifier, this can take place instantaneously neglecting overlap. The voltage waveforms for
this case are shown below.

Start-Up of DC Link:

There are two different start-up procedures depending upon whether the converter firing
controller provides a short gate pulse or long gate pulse. The long gate pulse lasts nearly 1200 ,
the average conduction period of a valve.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II
Start-up with long pulse firing:
1. De block inverter at about γ=900
2. De block rectifier at α=850toestablishlowdirectcurrent
3. Ramp up voltage by inverter control and the current by rectifier control.
Start-up with short pulse firing:
1. Open by pass switch at one terminal.
2. De block that terminal and load to minimum current in the rectifier mode
3. Open bypass switch at the second terminal and commutate current to the bypass pair
4. Start the second terminal also in the rectifier mode
5. The inverter terminal is put into the inversion mode
6. Ramp up voltage and current.
The voltage is raised before raising the current. This permits the insulation of the line to be
checked before raising the power. The ramping of power avoids stresses on the generator shaft.
The switching surges in the line are also reduced.
The required power ramping rate depends on the strength of the AC system. Weaker systems
require fast restoration of DC power for maintaining transient stability.

Power Control:
The current order is obtained as the quantity derived from the power order by dividing it by
the direct voltage. The limits on the current order are modified by the voltage dependent
current order limiter (VDCOL). The objective of VDCOL is to prevent individual thyristors
from carrying full current for long periods during commutation failures.
By providing both converter stations with dividing circuits and transmitting the power order
from the leading station in which the power order is set to the trailing station, the fastest
response to the DC line voltage changes is obtained without undue communication
requirement.
The figure below shows the basic power controller used.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

When the DC line resistance is large and varies considerably e.g., when the overhead line is
very long and exposed to large temperature variations, the DC line voltage drop cannot be
compensated individually in the two stations. This problem can be solved by using a current
order calculated in one substation only and transmitting its output to the other substation.
Power Reversal in HVDC Lines:

Power reversal in hybrid HVDC lines is achieved by changing the polarity of the DC voltage.

Here are some strategies for power reversal in hybrid HVDC systems:
• LCC link: The DC polarity of the LCCs can be reversed by changing their control modes. This is
because the LCC operates as a current source, and the DC current in the LCC always remains in
one direction.
• LCCHMMC: A sequence of power reversal after stopping operation is proposed.
• LCC-FMMC: A new modulation strategy is proposed to maintain the sub-module voltage during
the power reversal without blocking the converters.
A power flow reversal control strategy is designed with integrating DC voltage control and
active current control
Note: During the period of power reversal, there is no need to stop the operation of either LCC
or FB-MMC.

FB-MMC stands for full-bridge modular multilevel converter. Modular multilevel converters
(MMCs) are made up of identical sub modules that are individually controllable. They have a
modular structure, transformer less operation, and are fault-tolerant.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II
The Modular Multilevel Converter (MMC) represents an emerging topology with a scalable
technology making high voltage and power capability possible. The MMC is built up by
identical, but individually controllable sub modules.

Power reversal characteristics in HVDC Lines.

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 HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

Example of a Sub Module.

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HVDC AND FACTS UNIT-III HVDC TRANSMISSION-II

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 DEPARTMENT OF ELECTRICAL AND ELECTRONICS
ENGINEERING
UNIT IV
FLEXIBLE AC TRANSMISSION SYSTEMS-I
Topics:
Types of FACTS Controllers, brief description about various types of FACTS controllers,
Operation of 6-pulse converter, Transformer Connections for 12-pulse, 24-pulse and 48-
pulse operation, principle of operation of various types of Controllable shunt Var
Generation, Principle of switching converter type shunt compensator, principles of operation
of various types of Controllable Series Var Generation, Principle of Switching Converter
type series compensator.

The FACTS controllers can be classified as

1. Shunt connected controllers
2. Series connected controllers
3. Combined series-series controllers
4. Combined shunt-series controllers
Depending on the power electronic devices used in the control,
The FACTS controllers can be classified as
(A) Variable impedance type based.
(B) Voltage Source Converter (VSC) based.
The variable impedance type controllers includes.
(i) Static Var Compensator (SVC), (shunt connected)
(ii) Thyristor Controlled Series Capacitor or compensator (TCSC), (series connected)
(iii) Thyristor Controlled Phase Shifting Transformer (TCPST) of Static PST (combined shunt
and series)
The VSC based FACTS controllers are:
(i) Static synchronous Compensator (STATCOM) (shunt connected)
(ii) Static Synchronous Series Compensator (SSSC) (series connected)
(iii) Interline Power Flow Controller (IPFC) (combined series-series)
(iv) Unifed Power Flow Controller (UPFC) (combined shunt-series)
Some of the special purpose FACTS controllers are
(a) Thyristor Controlled Braking Resistor (TCBR)
(b) Thyristor Controlled Voltage Limiter (TCVL)
(c) Thyristor Controlled Voltage Regulator (TCVR)
(d) Interphase Power Controller (IPC)
(e) NGH-SSR damping.

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 Schematic diagrams of FACTS Controllers
Brief description about various types of FACTS controllers:
Fig (a) shows the general symbol for FACTS controller; with a thyristor arrow inside a box.

Fig (b) shows the series controller could be variable impedance, such as capacitor, reactor etc. or
it is a power electronics based variable source of main frequency subsynchronous frequency and
harmonics frequencies or combination of all to serve the desired need. The principle of series
controller is to inject the voltage in series with the line. Even variable impedance multiplied by
the current flow through it, represents an injected series voltage in the line. So long as the
voltage is in phase quadrature with the line current, the series controller supplies or consumes
variable reactive power. If any other phase relation involves it will handle the real power also.

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Fig (c) shows the shunt controllers. As series controller, the shunt controller also has variable
impedance, variable source, or a combination of all. The principle of shunt controller is to inject
current into the system at the point of connection. Even variable shunt impedance connected to
the line voltage causes a variable current flow and hence represents injection of current into the
line. As long as the injected current is in phase quadrature with the line voltage. The shunt
controller supplies or consumes variable reactive power. If any other phase relationship involves,
it will also handle real power.

Fig (d) shows the combination of two separate series controllers, which are controlled in a
coordinated manner, in a multi line transmission system. Otherwise it could be unified controller.
As shown in Fig (d) the series controllers provide independent series reactive compensation for
each line and also transfer the real power among the lines via the unified series-series controller,
referred to as inter-line power flow controller, which makes it possible to balance both the real
and reactive power flow in the lines and thereby maximizing the utilization of transmission
system. Note that the term “unified” here means that the D.C terminals of all controller
converters are connected together for real power transfer.

Fig’s (e & f) shows the combined series-shunt controllers. This could be a combination of
separate shunt and series controllers, which are controlled in coordinated manner in Fig (e) or a
unified power flow controller with series and shunt elements in Fig (f). The principle of
combined shunt and series controllers is, it injects current into the system with the shunt part of
the controller and voltage through series part. However, when the shunt and series controllers are
unified, there can be a real power exchange between the series and shunt controllers via the
power link.

Basic types of FACTS Controllers

Series controllers: The series controller could be a variable impedance or a variable source
both are power electronics based. In principle, all series controllers inject voltage in series
with the line.
Shunt controllers: The shunt controllers may be variable impedance connected to the line
voltage causes a variable current flow hence represents injection of current into the line.
Combined series-series controllers: The combination could be separate series controllers or
unified seriesseries controller--- Interline Power Flow Controller.
Combined series-shunt controllers: The combination could be separated series and shunt
controllers or a unified power flow controller.

Relative Importance of Different Types of Controllers

For a given MVA size, the series controller is several times more powerful than the shunt
controller in application of controlling the power/current flow.
Drawing from or injecting current into the line, the shunt controller is a good way to control
voltage at and around the point of connection.
The shunt controller serves the bus node independently of the individual lines connected to the
bus.
Series connected controllers have to be designed to ride through contingency and dynamic
overloads, and ride through or bypass short circuit currents. A combination of series and

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 shunt controllers can provide the best of effective power/current flow and line voltage.
FACTS controllers may be based on thyristor devices with no gate turnoff or with power
devices with gate turn-off capability.
The principle controllers are based on the dc to ac converters with bidirectional power flow
capability.
Energy storage systems are needed when active power is involved in the power flow.
Battery, capacitor, superconducting magnet, or any other source of energy can be added in
parallel through an electronic interface to replenish the converter’s dc storage.
A controller with storage is more effective for controlling the system dynamics.
A converter-based controller can be designed with high pulse order or pulse width modulation
to reduce the low order harmonic generation to a very low level.
A converter can be designed to generate the correct waveform in order to act as an active filter.
A converter can also be controlled and operated in a way that it balances the unbalanced
voltages, involving transfer of energy between phases.
A converter can do all of these beneficial things simultaneously I the converter is so designed.

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Fig shows a three phase wave converter with six valves, i.e. (1-1') to (6-6') they are designated
in the order. 1 to 6 represents the sequence of valve operation in time.It consists of three
legs, 120º apart. The three legs operate in a square wave mode; each valve alternately
closes for 180º as in the wave form of Fig (b), Va, Vb and VC.

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 These three square-wave waveform are the voltages of A.C. buses a, b and c with respect to a
D.C. capacitor mid point „N‟ with peak voltages of +Vd/2 and -Vd/2. The three phase
legs have their timing 120º apart with respect to each other to a 6-phase converter
operation phase leg (3- 6) switches 120º after phase leg (1-4) and phase leg (5-2) switches
120º after phase (3-6), thus completing the cycle as shown by the valve close-open
sequence.
Fig (c) shows the three phase-to-phase voltages Vab, Vbc and Vca, where VAB = Va-Vb, Vbc
= Vb-Vc and Vca = Vc-Va. These phase-to-phase voltages have 120º pulse width with
peak voltage magnitude of Vd. The periods of 60, º when the phase-to-phase voltages are
zero, represents the condition when two valves on the same order of the D.C. bus.
For example the waveform for Vab shows voltage Vd when device „1‟ connects A.C. bus „a‟
to the D.C. + Vd/2, and device 6 connects A.C. bus „b‟ to the D.C. bus -Vd/2, giving a
total voltage Vab = Va-Vb = Vd. It is seen 120º later, when device „6‟ is turned OFF and
device „3‟ is turned ON both A.C. buses „a‟ and „b‟ become connected to the same D.C.
bus +Vd/2, giving zero voltage between buses „a‟ and „b‟. After another 60º later. When
device 1 turns OFF and device „4‟ connects bus „a‟ to -Vd/2, Vab becomes -Vd. Another
120º later, device „3‟ turns OFF and device „6‟, connects bus „b‟ to -Vd/2, giving Vab =
0 the cycle is completed, after another 60º. device „4‟ turns OFF and device „1‟ turns ON,
the other two voltages Vab and Vca have the same sequence 120º a part.
The turn ON and turn OFF of the devices establish the wave forms of the A.C. bus voltages in
relation to the D.C. voltage, the current flows itself, is the result of the interaction of the
A.C. voltage with the D.C. system. Each converter phase-leg can handle resultant current
flow in either direction.
In fig (d) A.C. current „Ia‟ in phase „a‟ with +ve current representing current from A.C. to
D.C. side for simplicity, the current is assumed to have fundamental frequency only. From
point t1 to t2. For example phase „a‟ current is -ve and has to flow through either valve
(1-1') or valve (4-4'). It is seen, when comparing the phase „a‟ voltage with the form of
the phase „a‟ current that when device 4 is ON and device „1‟ is OFF and the current is -
ve, the current would actually flow through diode 4'. But later say from point t2, t3, when
device „1‟ is ON, the -Ve current flows through device „1‟, i.e., the current is transferred
from diode 4' to device „1‟ the current covering out of phase „b‟ flows through device „6‟
but then part of this current returns back through diode 4' into the D.C. bus. The D.C.
current returns via device „5‟ into phase „e‟. At any time three valves are conducting in a
three phase converter system. In fact only the active power part of A.C. current and part of
the harmonics flow into the D.C. side, as shown in Fig.

TRANSFORMER CONNECTION FOR 12-PULSE OPERATION :

The harmonics content of the phase to phase voltage and phase to neutral voltage are 30º out of
phase. If this phase shift is corrected, then the phase to neutral voltage (Van) other then
that of the harmonics order 12n±1 would be in phase opposition to those of the phase to
phase voltage (Vab) and with 1/√3 times the amplitude.
In Fig (a) if the phase to phase voltages of a second converter were connected to a
deltaconnected secondary of a second transformer, with √3 times the turns compared to
the star connected secondary, and the pulse train of one converter was shifted by 30º with
respect to the other “in order to bring „Vab‟ and „Van‟ to be in phase”, the combined out
put voltage would have a 12-phase wave form, with harmonics of the order of 12n±1, i.e.

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11th , 13th , 23rd , 25th …. And with amplitudes of 1/11th, 1/13th , 1/23rd 1/25th.
respectively, compared to the fundamental.

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Fig 2.5 (b): shows the two wave forms Van and Vab, adjusted for the transformer ratio and one
of them phase displaced by 30º. These two wave forms are then added to give the third
wave form, which is a 12-pulse wave form, closer to being a sine wave than each of the
six-phase wave form.
In the arrangement of Fig 2.5 (a), the two six-pulse converters, involving a total of six-phase
legs are connected in parallel on the same D.C. bus, and work together as a 12-pulse
converter. It is necessary to have two separate transformers, otherwise phase shift in the
non 12-pulse harmonics i.e. 5th, 7th, 17th, 19th …. In the secondary’s it will result in a
large circulating current due to common core flux. To the non 12-pulse voltage
harmonics, common core flux will represent a near short circuit. Also for the same reason,
the two primary side windings should not be directly connected in parallel to the same
three phase A.C. bus bars on the primary side. Again this side becomes the non 12-pulse
voltage harmonics i.e. 5th, 7th, 17th, 19th …. while they cancel out looking into the A.C.
system would be in phase for the closed loop. At the Vd - - same time harmonics will also
flow in this loop, which is essentially the leakage inductance of the transformers. The
circulating current of each non 12-pulse harmonics is given by:
In
= 100( X T * n2 ) Percent
I1
Where I1 is the nominal fundamental current, n is the relevant harmonic number, and XT is the
per unit transformer impedance of each transformer at the fundamental frequency. For
example, if XT is 0.15 per unit at fundamental frequency, then the circulating current for
the fifth harmonic will be 26.6%, seventh, 14.9%, eleventh, 5.5%, thirteenth, 3.9%, of the
rated fundamental current, and so on. Clearly this is not acceptable for practical voltage
sourced converters. Therefore, it is necessary to connect the transformer primaries of two
separate transformers in series and connect the combination to the A.C. bus as shown in

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 Fig 2.5 (a), with the arrangement shown in Fig 2.5 (a), the 5th, 7th, 17th, 19th….
harmonics voltages cancel out, and the two fundamental voltages add up, as shown in Fig
2.5 (b), and the combined unit becomes a true 12-pulse converter.

TRANSFORMER CONNECTIONS FOR 24-PULSE AND 48-PULSE OPERATION :

Two 12-pulse converters phase shifted by 15º from each other can provide a 24-pulse
converter, with much lower harmonics on both A.C. and D.C. sides. It‟s A.C. out put
voltage would have 24n±1 order of harmonics i.e. 23rd, 25th, 47th, 49th …. , with
magnitudes of 1/23rd , 1/25th, 1/47th, 1/49th …. respectively, of the fundamental A.C.
voltage. The question now is, how to arrange this phase shift. One approach is to provide
15º phase shift windings on the two transformers of one of the two 12-pulse converters.
Another approach is to provide phase shift windings for (+7.5º) phase shift on the two
transformers of one 12-pulse converter and (- 7.5º) on the two transformers of the other
12-pulse converter, as shown in Fig, the later - is preferred because it requires
transformer of the same design and leakage inductances. It is also necessary to shift the
firing pulses of one 12-pulse converter by 15º with respect to the other. All four six-pulse
converters can be connected on the D.C. side in parallel, i.e. 12-pulse legs in parallel.
Alternately all four six-pulse converters can be connected in series for high voltage or two
pair of 12-pulse series converters may then be connected will have a separate transformer,
two with star connected secondaries, and the other two with delta-connected secondaries.

Transformer connections in Series and Parallel.

Primaries of all four transformers can be connected in series as shown in Fig 2.6 (b) in order to
avoid harmonic circulation current corresponding the 12-pulse order i.e. 11th, 13th, and
23rd, 24th . It may be worth while to consider two 12-pulse converters connected in
parallel on the A.C. system bus bars, with inter phase reactors as shown in Fig 2.6 (b) for
a penalty of small harmonic circulation inside the converter loop. While this may be

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 manageable from the point - - of view of converter rating. Care has to be taken in the
design of converter controls, particularly during light load when the harmonic currents
could become the significant part of the A.C. current flowing through the converter. As
increase in the transformer impedance to say 0.2 per unit may be appropriate when
connecting two 12-pulse transformers to the A.C. bus directly and less than that when
connected through inter phase reactors. For high power FACTS Controllers, from the
point of view of the A.C. system, even a 24-pulse converter with out A.C. filters could
have voltage harmonics, which are higher then the acceptable level in this case, a single
high pass filter turned to the 23rd - 25th harmonics located on the system side of the
converter transformers should be adequate. The alternative of course, is go to 48-pulse
operation with eight six pulse groups, with one set of transformers of one 24-pulse
converter phase shifted from the other by 7.5º, or one set shifted (+7.5º) and the other by
(-3.7º). Logically, all eight transformer primaries may be connected in series, but because
of the small phase shift (i.e. 7.5º) the primaries of the two 24- pulse converters each with
four primaries in series may be connected in parallel, if the consequent circulating current
is accepted. This should not be much of a problem, because the higher the order of a
harmonic, the lower would be the circulating current. For 0.1 per unit transformer
impedance and the 23rd harmonic, the circulating current can be further limited by higher
transformer inductance or by inter phase reactor at the point of parallel connection of the
two 24-pulse converters, with 48-pulse operation A.C. filters are not necessary.

OBJECTIVES OF SHUNT COMPENSATION:

Shunt compensation is used to influence the natural characteristics of the transmission line to “
steady-state transmittable power and to control voltage profile along the line” shunt
connected fixed or mechanically switched reactors are used to minimize line over-voltage
under light load conditions. Shunt connected fixed or mechanically switched capacitors
are applied to maintain voltage levels under heavy load conditions.
Var compensation is used for voltage regulation.
i. At the midpoint to segment the transmission line and
ii. At the end of the line
To prevent “voltage intangibility as well as for dynamic voltage control to increase transient
stability and to damp out power oscillations”.
MID-POINT VOLTAGE REGULATION FOR LINE SEGMENTATION:
Consider simple two-machine(two-bus)transmission model in which an ideal var compensator
is shunt connected at the midpoint of the transmission line

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NOTE: i. The midpoint of the transmission line is the best location for compensator because the voltage
sage along the uncompensated transmission line is the longest at the midpoint
ii. The concept of transmission line segmentation can be expanded to use of multiple compensators,
located at equal segments of the transmission line as shown in fig.

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END OF LINE VOLTAGE TO SUPPORT TO PREVENT VOLTAGE INSTABILITY:

A simple radial system with feeder line reactance X and load impedance Z is shown

NOTE:

1. For a radial line , the end of the line, where the largest voltage variation is experienced, is
the best location for the compensator.

2. Reactive shunt compensation is often used too regulate voltage support for the load when
capacity of sending –end system becomes impaired.

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IMPROVEMENT OF TRANSIENT STABILITY:

The shunt compensation will be able to change the power flow in the system during and
following disturbances. So as to increase the transient stability limit. The potential
effectiveness of shunt on transient stability improvement can be conveniently evaluated by
“EQUAL AREA CRITERION”.

Assume that both the uncompensated and compensated systems are subjected to the same
fault for the same period of time. The dynamic behavior of these systems is illustrated in the
following figures.

METHODS OF CONTROLLABLE VAR GENERATION:

Capacitors generate and inductors (reactors)absorb reactive power when connected to an ac

power source. They have been used with mechanical switches for controlled var generation
and absorption. Continuously variable var generation or absorption for dynamic system
compensation as originally provided by

1. over or under-excited rotating synchronous machines•

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2. saturating reactors in conjunction with fixed capacitors

Using appropriate switch control, the var output can be controlled continuously from
maximum capacitive to maximum inductive output at a given bus voltage.

More recently gate turn-off thyristors and other power semiconductors with internal turn off
capacity have been use of ac capacitors or reactors.

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It is evident that the magnitude of current in the reactor can be varied continuously by the
method of delay angle control from maximum (α=0) to zero (α=90).

In practice, the maximum magnitude of the applied voltage and that of the corresponding
current will be limited by the ratings of the power components (reactor and thyristor valve)
used. Thus, a practical TCR can be operated anywhere in a defined V-I area ,the boundaries
of which are determined by its maximum attainable admittance, voltage and current ratings
are shown in fig.

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Fig.shows Static Var Compensator(SVC).

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 SERIES COMPENSATOR
Series compensation, if properly controlled, provides voltage stability and transient
stability improvements significantly for post-fault systems. It is also very effective in damping
out power oscillations and mitigation of sub-synchronous resonance.
Voltage Stability :
Series capacitive compensation reduces the series reactive impedance to minimize the
receiving end voltage variation and the possibility of voltage collapse. Figure 3.1 (a) shows a

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simple radial system with feeder line reactance X, series compensating reactance Xc and load
impedance Z. The corresponding normalized terminal voltage Vr versus power P plots, with
unity power factor load and 0, 50, and 75% series capacitive compensation, are shown in Figure
3.1(b). The “nose point” at each plot for a specific compensation level represents the
corresponding voltage instability. So by cancelling a portion of the line reactance, a “stiff”
voltage source for the load is given by the compensator.

Fig: Transmittable power and voltage stability limit of a radial transmission line as a
function of series capacitive compensation.
Transient Stability Enhancement:
The transient stability limit is increased with series compensation. The equal area
criterion is used to investigate the capability of the ideal series compensator to improve the
transient stability.

Fig: Two machine system with series capacitive compensation

Assumptions that are made here are as follows:
• The pre-fault and post-fault systems remain the same for the series compensated system.
• The system, with and without series capacitive compensation, transmits the same power Pm.
• Both the uncompensated and the series compensated systems are subjected to the same fault for
the same period of time.
Figures (a) and (b) show the equal area criterion for a simple two machine system
without and with series compensator for a three phase to ground fault in the transmission line.
From the figures, the dynamic behaviour of these systems are discussed. Prior to the fault, both
of them transmit power Pm at angles 61 and 6s1 respectively. During the fault, the transmitted

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electric power becomes zero, while the mechanical input power to the generators remains
constant (Pm). Hence, the sending end generator accelerates from the steady-state angles 61 and
6s1 to 62 and 6s2 respectively, when the fault clears. In the figures, the accelerating energies are
represented by areas A1 and As1. After fault clearing, the transmitted electric power exceeds the
mechanical input power and therefore the sending end machine decelerates. However, the
accumulated kinetic energy further increases until a balance between the accelerating and
decelerating energies, represented by the areas A1, As1 and A2, As2, respectively, are reached at
the maximum angular swings, 63 and 6s3 respectively. The areas between the P versus 6 curve
and the constant Pm line over the intervals defined by angles 63 and 6crit, and 6s1 and 6scrit,
respectively, determine the margin of transient stability represented by areas Amargin and As
margin for the system without and with compensation.

Fig: Equal area criterion to illustrate the transient stability margin for a simple two-
machine system (a) without compensation and (b) with a series capacitor.
Comparing above figures (a) and (b), it is clear that there is an increase in the transient
stability margin with the series capacitive compensation by partial cancellation of the series
impedance of the transmission line. The increase of transient stability margin is proportional to
the degree of series compensation.
Variable impedance type static Var generators: (list as follows)
1. Thyristor Controlled and Thyristor Switched Reactor(TCR and TSR),
2. Thyristor Switched Capacitor(TSC)
3. Fixed Capacitor Thyristor Controlled Reactor Var Generator:
4. Thyristor Switched Capacitor-Thyristor Controlled Reactor
5. Switching converter type var generators,
6. Hybrid Var generators

The performance and operating characteristics of the impedance type var generators are
determined by their major thyristor-controlled constituents:

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 ▪ The thyristor controlled reactor and the Thyristor-Switched Capacitor.

TCR and TSR

Thyristor - Controlled Reactor Consists of a reactor of Inductance L and a bidirectional
thyristor valve (or switch) sw.
A thyristor valve can be brought into conduction by application of a gate pulse to
thyristor. The valve will automatically block immediately after the ac current crosses zero, unless
the gate signal is reapplied.
The current in the reactor can be controlled from maximum to zero by the method of
firing delay angle control. When the gating of the valve is delayed by an angle α (0<=α<=90)
with respect to the crest of the voltage, the current in the reactor can be expressed with V(t)=V
cosωt as follows.

The thyristor valve opens as the current reaches zero, is valid for the interval (α <= ωt <=
π-α). Fig a shows basic thyristor controlled reactor, b shows firing delay angle control and c
shows the operating waveforms.

Since the valve automatically turns off at the instant of current zero crossing this process
actually controls the conduction interval (or angle) of the thyristor, the delay angle σ defines the
prevailing conduction angle σ. Thus, as the delay angle α increases, the correspondingly it results
in the reduction of the conduction angle the valve σ.
here, σ = π-2α

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 At the maximum delay of α = π/2, the offset also reaches its maximum of V/ωL, at which
both the conduction angle and the reactor current become zero, the magnitude of the current in
the reactor can be varied continuously by this method of delay angle control from maximum (α =
0) to zero (α = π/2).
Adjustment of current in the reactor can takes place for every half cycle only.
The amplitude of the fundamental reactor current Ilf(α) can be expressed as a function of
angle α

TCR can control the fundamental current continuously from to maximum as if it was a
variable reactive admittance. Thus, an effective reactive admittance, BL(α), for the TCR can
be defined as.

Amplitude variation of fundamental current component with delay angle is shown in figure
below.

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If the TCR switching is restricted to a fixed delay angle, usually α = 0, then it becomes a
thyristor-switched reactor (TSR).
TCR can be operated anywhere in a defined V-I area, the boundaries of which are
determined by its maximum attainable admittance, voltage, and current ratings.

THYRISTOR - SWITCHED CAPACITOR.

It consists of a capacitor, a bidirectional thyristor valve, and a relatively small surge current
limiting reactor.
Under steady-state conditions, when the thyristor valve is closed and the TSC branch is
connected to a sinusoidal ac voltage source, the current in the branch is given by

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The TSC branch can be disconnected at any current zero by prior removal of the gate drive
to the thyristor valve. At the current zero crossing, the capacitor voltage is shown in
equation below.
n2
VC = V
n 2 −1
The disconnected capacitor stays charged to this voltage and consequently, the voltage
across the non conducting thyristor valve varies between zero and the peakto-peak value of
the applied ac voltage as shown in the waveform.
If voltage across the disconnected capacitor remains unchanged, the TSC bank could be
switched in again, without any transient, at the appropriate peak of the applied ac voltage, as
illustrated for a positively and negatively charged capacitor in figure (a)and (b) below.
Normally, the capacitor bank is discharged after disconnection. Thus, the reconnection of
the capacitor may have to be executed at some residual capacitor voltage.
The transient disturbance can be minimized by switching in TSC when capacitor residual
voltage and ac voltage are equal.

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The operating V-I area of single TSC is shown below.

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HVDC AND FACTS UNIT-V FACTS-II
UNIT V

FLEXIBLE AC TRANSMISSION SYSTEMS-II

Topics:
Unified Power Flow Controller (UPFC) – Principle of operation, Transmission Control
Capabilities, Independent Real and Reactive Power Flow Control; Interline Power Flow
Controller (IPFC) – Principle of operation and Characteristics, UPFC and IPFC control
structures (only block diagram description), objectives and approaches of voltage and phase
angle regulators.

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THE UNIFIED POWER FLOW CONTROLLER

The Unified Power Flow Controller (UPFC) concept was proposed by Gyugyi in 1991. The
UPFC was devised for the real-time control and dynamic compensation of ac transmission
systems, providing multifunctional flexibility required to solve many of the problems facing the
power delivery industry. Within the framework of traditional power transmission concepts, the
UPFC is able to control, simultaneously or selectively, all the parameters affecting power flow in
the transmission line (i.e., voltage, impedance, and phase angle), and this unique capability is
signified by the adjective "unified" in its name.
Alternatively, it can independently control both the real and .reactive power flow in the line. The
reader should recall that, for all the Controllers discussed in the previous chapters, the control of
real power is associated with similar change in reactive power, i.e., increased real power flow
also resulted in increased reactive line power source. The transmission line current flows through
this voltage source resulting in reactive and real power exchange between it and the ac system.
The reactive power exchanged at the ac terminal (Le., at the terminal of the series insertion
transformer) is generated internally by the converter. The real power exchanged at the ac
terminal is converted into de power which appears at the de link as a positive or negative real
power demand. The basic function of Converter 1 is to supply or absorb the real power
demanded by Converter 2 at the common de link to support the real power exchange resulting
from the series voltage injection. This de link power demand of Converter 2 is converted back to
ac by Converter 1 and coupled to the transmission line bus via a shuntconnected transformer. In
addition to the real power need of Converter 2, Converter 1 can also generate or absorb
controllable reactive power, if it is desired, and thereby provide independent shunt reactive
compensation for the line. It is important to note that whereas there is a closed direct path for the
real power negotiated by the action of series voltage injection through Converters 1 and 2 back to
the line, the corresponding reactive power exchanged is supplied or absorbed locally by
Converter 2 and therefore does not have to be transmitted by the line. Thus, Converter 1 can be
operated at a unity power factor or be controlled to have a reactive power exchange with the line
independent of the reactive power exchanged by Converter 2. Obviously, there can be no
reactive power flow through the UPFC de link

Conceptual representation of UPFC in a two machine power system.

UPFC is a combination of STATCOM and SSSC coupled via a common DC voltage link.

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 HVDC AND FACTS UNIT-V FACTS-II
Principle of Operation :

The UPFC is the most versatile FACTS controller developed so far, with all encompassing
capabilities of voltage regulation, series compensation, and phase shifting.
It can independently and very rapidly control both real- and reactive power flows in a
transmission.
It is configured as shown in Fig. and comprises two VSCs coupled through a common dc
terminal.
One VSC converter 1 is connected in shunt with the line through a coupling transformer; the
other VSC converter 2 is inserted in series with the transmission line through an interface
transformer.
The dc voltage for both converters is provided by a common capacitor bank.
The series converter is controlled to inject a voltage phasor, Vpq, in series with the line, which
can be varied from 0 to Vpq max. Moreover, the phase angle of Vpq can be independently varied
from 00 to 3600 .
In this process, the series converter exchanges both real and reactive power with the transmission
line.
Although the reactive power is internally generated/absorbed by the series converter, the real-
power generation/ absorption is made feasible by the dc-energy–storage device that is, the
capacitor.
The shunt-connected converter 1 is used mainly to supply the real-power demand of converter 2,
which it derives from the transmission line itself. The shunt converter maintains constant voltage
of the dc bus.
Thus the net real power drawn from the ac system is equal to the losses of the two converters and
their coupling transformers.
In addition, the shunt converter functions like a STATCOM and independently regulates the
terminal voltage of the interconnected bus by generating/ absorbing a requisite amount of
reactive power.

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 HVDC AND FACTS UNIT-V FACTS-II

Modes of Operation

The concepts of various power-flow control functions by use of the UPFC are illustrated in
Figs. (a)–(d). Part (a) depicts the addition of the general voltage phasor Vpq to the existing
bus voltage, V0, at an angle that varies from 00 to 3600 .

Voltage regulation is effected if Vpq =ΔV0 is generated in phase with V0, as shown in part (b).
A combination of voltage regulation and series compensation is implemented in part (c),
where Vpq is the sum of a voltage regulating component ΔV0 and a series compensation
providing voltage component Vc that lags behind the line current by 900. In the phase-
shifting process shown in part (d), the UPFC-generated voltage Vpq is a combination of
voltageregulating component ΔV0 and phase-shifting voltage component Va.
The function of Va is to change the phase angle of the regulated voltage phasor, V0 + ΔV, by an
angle α. A simultaneous attainment of all three foregoing power-flow control functions is
depicted in Fig.
The controller of the UPFC can select either one or a combination of the three functions as its
control objective, depending on the system requirements.
The UPFC operates with constraints on the following variables :
1. The series-injected voltage magnitude
2. The line current through series converter
3. The shunt-converter current
4. The minimum line-side voltage of the UPFC
5. The maximum line-side voltage of the UPFC and

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 HVDC AND FACTS UNIT-V FACTS-II
6. The real-power transfer between the series converter and the shunt converter.
Transmission Control Capabilities:

The implementation of UPFC models in power flow is a controlled power flow problem. The
UPFC has control over all the parameters affecting power flow on the transmission line. UPFC
is capable of controlling the real and reactive power on the transmission line and the bus
voltage simultaneously and independently.

A Unified Power Flow Controller (UPFC) controls power flow in transmission lines by adjusting
line parameters like phase angle, line impedance, and node voltages. It can control the real and
reactive power on the transmission line, as well as the bus voltage simultaneously and
independently. It can also control the transmission line voltage, impedance, and angle, either
concurrently or selectively.

The UPFC is capable of the following:

• Stability control

Suppresses power system oscillations to improve the transient stability of the power system

• Static and dynamic operation

Offers advantages in terms of static and dynamic operation of the power system

The UPFC can increase or decrease the amount of active power transmitted over the line,
increase the magnitude of the voltages at either end (i.e. voltage support), and/or reduce or
increase the reactance of the line (i.e. line compensation).

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Independent Real and Reactive Power Flow Control:

Inorder to investigate the capability of UPSC to control real and reactive power flow in the
transmission line .

Let first assumed that the injected compensated voltage Vpq is zero.

The the original elementary two machine system with VS ,Vr , transmission angle δ and line
reactance X is restored.

V2
P0(δ) = { } sin δ =sin δ
X

V2
Q0(δ) = Q0s(δ) = -Q0r(δ) = { }{1-cosδ} = {1 – cosδ}
X

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Interline Power Flow Controller (IPFC):

Basic structure of IPFC

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 HVDC AND FACTS UNIT-V FACTS-II
The schematic representation of an IPFC shown in Figure .There are two back-to-back
voltagesource converters (VSCs), based on the use of gate-turnoff (GTO) thyristor valves. The
VSCs produce voltages of variable magnitude and phase angle. These voltages are injected in
series with the managed transmission lines via series transformers. The injected voltages are
represented by the voltage phasors .The converters labeled VSC1 and VSC2 are coupled together
through a common dc link. Illustrates the IPFC phasor diagram. With respect to the
transmissionline current, in phase and quadrature phase components of injected voltage,
respectively, determine the negotiated real and reactive powers of the respective transmission
lines. The real power exchanged at the ac terminal is converted by the corresponding VSC into
dc power which appears at the dc link as a negative or a positive demand. Consequently, the real
power negotiated by each VSC must be equal to the real power negotiated by the other VSC
through the dc lines.VSC1 is operated at point A. Therefore, VSC2 must be operated along the
complementary voltage compensation line, such as point B, to satisfy the real power demand of
VSC1. This is given by Psc1+Psc2=V1pI1+V2pI2=0 (5.1) In the IPFC structure, each converter
has the capability to operate as a stand-alone SSSC under contingency conditions, such as the
outage of another ac line, other converter, or opening the dc link between the two converters.
The protective actions can be divided into two levels in each converter station. In case a failure
occurs and affects all components, the protection system The IPFC has the in-built capacity to
bypass the rest of the compensation that, when a failure occurs at a series transformer, the
associated SSSC is bypasses using the bypass breaker. For instance, when a failure takes place in
a valve of the VSC, built within the GTO thyristor module, the GTO module is bypassed. This
means that, where a number of failures occur affecting a single component, the protective actions
is specifically employed on that particular component, bypassing the failure and setting it right.

Principle of operation: IPFC is designed as a power flow controller with two or more
independently controllable static synchronous series compensators (SSSC) who are solid state
voltage source converters injecting an almost sinusoidal voltage variable magnitude and are
linked via a common DC capacitor.

1. Consider an elementary IPFC scheme consisting of two back-to-back dc-to-ac

converters, each compensating a transmission line by series voltage injection. This arrangement
is shown functionally in Figure, where two synchronous voltage sources, with phasors V1pq and

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 HVDC AND FACTS UNIT-V FACTS-II
V2pq in series with transmission Lines 1 and 2, represent the two back-to-back dcto-ac
converters. The common dc link is represented by a bidirectional link for real power exchange
between the two voltage sources. Transmission Line 1, represented by reactance X1 has a
sending-end bus with voltage phasor V1s and a receiving-end bus with voltage phasor V1r. The
sending-end voltage phasor of Line 2, represented by reactance X2, is V2s" and the receiving-
end voltage phasor is V2r. all the sending-end and receiving-end voltages are assumed to be
constant with fixed amplitudes, V1s= V1r= V2s= V2r=1.0 p.u and with fixed angles resulting
in identical transmission angles, δ1= δ2=30o for the two systems. In order to establish the
transmission relationships between the two systems, System 1 is arbitrarily selected to be the
prime system for which free controllability of both real and reactive line power flow is
stipulated. The reason for this stipulation is to derive the constraints which the free
controllability of System 1 imposes upon the power flow control of System.

Figure 2 illustrates which can function individually as conventional (voltage, impedance

or angle) Controllers, but can also be converted from one functional use to another, and, more
importantly, can be connected to a common dc link to provide comprehensive transmission
control capabilities. As can be observed two voltagesourced converter modules, used to control
the power flow between bus 1 and bus 2, can be used individually as a STATCOM and an SSSC,
or can be combined to function as a UPFC for the comprehensive control of both real and
reactive power flow between bus 1 and bus 2. With the addition of the third converter module,
the second line receives an independent series reactive compensator (SSSC). However by

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 HVDC AND FACTS UNIT-V FACTS-II
connecting this converter to the common dc bus, a generalized IPFC is established which can
control and optimize under the prevailing system condition the real and reactive power in both
lines, from bus L to bus 2, and also from bus 1 to bus 3. This simple example shows the
capability of the voltage-sourced converter-based approach to maintain full convertibility and
individual functionality while also providing a powerful potential for an integrated transmission
management system with capacity of real and reactive power flow control and handling of
dynamic disturbances In the multifunctional FACTS Controller arrangements discussed above,
each Controller in a line can independently carry out limited compensation and control functions
and thus the common dc connection does not represent a significant single point failure.

Control Structure (only Block diagram) of UPFC:

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Based on park transformation, it designs decoupled control scheme of UPFC shunt

branch and cross-coupling control scheme of UPFC series branch in d and q-axis which
eliminates the dynamic interaction between active and reactive power flow through the line. The
control system of UPFC series branch consists of outer power loop, middle voltage loop and
inner current loop. The response of the inner current loop is much faster than response of the
outer voltage loop, which ensures that input current follows input voltage and improves dynamic
performance of UPFC.

Control Structure (only Block diagram) of IPFC:

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 HVDC AND FACTS UNIT-V FACTS-II

Fig: Basic control scheme for two converter IPFC.

The basic control structure of an Interline Power Flow Controller (IPFC) is that the series
converter of the UPFC controls the transmission line real/reactive power flow, and the shunt
converter of the IPFC controls the bus voltage/shunt reactive power and the DC link capacitor
voltage.

Objectives and approaches of voltage and phase angle regulators:

Voltage regulators control voltage levels to a specific value, keeping it consistent and stable,
regardless of any fluctuations or changes in the input voltage. This helps to prevent voltage
spikes and dips, and maintain a steady flow of power to devices.

Phase angle regulators are used in electrical power distribution systems to correct the phase angle
difference between two parallel connected electrical transmission systems. This controls the
power flow between the two systems so that each can be loaded to its maximum capacity.

A phase angle regulator (PAR) is a circuit that consists of two transformers. The first transformer
is called the regulating transformer and is connected in shunt with the line. The PAR can vary
between zero and the full-winding voltage with desired steps in between.

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Some approaches of voltage regulators include:

• Step-Down (Buck) Regulators

• µModule Buck Regulators

• External Power Switch Buck Controllers

• High Input Voltage Buck Regulators

• Internal Power Switch Buck Regulators

• Multiple Output Buck Regulators

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Phase angle regulators (PARs) are used to control power flow in electrical power distribution
systems. They are placed between the sending-end generator and the transmission line. The
objectives of PARs are to:

• Correct phase angle differences: PARs correct the phase angle difference between two
parallel connected electrical transmission systems.

• Control power flow: PARs control the power flow between the two systems so that each
can be loaded to its maximum capacity.

• Improve transient stability: PARs improve transient stability.

• Provide power oscillation damping: PARs provide power oscillation damping.

• Minimize post-disturbance overloads: PARs minimize post-disturbance overloads and the

corresponding voltage dips.

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Example figures of Voltage and Phase Angle regulators.

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