UNIT – III:

HIGH VOLTAGE DC TRANSMISSION – II

Syllabus: 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.

3.1 Basic means of control:

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 OVERALL EQUIVALENT CIRCUIT OF HVDC SYSTEM

From the overall equivalent circuit of HVDC system

 With the objectives of providing efficient and stable operation and

maximizing flexibility of power control without compromising the safety of
equipment, various levels of control are used in a hierarchical manner.

 Consider the HVDC link shown in Figure 1(a). It represents a monopolar

link or one pole of a bipolar link.

 The corresponding equivalent circuit and voltage profile are shown in

Figures 1(b) and (c), respectively.

 The direct current flowing from the rectifier to the inverter is

and at the inverter terminal is

The DC voltage and current in the DC link can be controlled by controlling

rectifier voltages and inverter voltages using two methods

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  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:

The following considerations influence the selection of control

characteristics:

1. Prevention of large fluctuations in direct current due to variations in ac

system voltage.
2. Maintaining direct voltage near rated value.
3. Maintaining power factors at the sending and receiving end that is as
high as possible.

4. Prevention of commutation failure in inverters and arc-back in

rectifiers using mercury-arc valves. Rapid control of the converters to prevent
large fluctuations in direct current is an important requirement for satisfactory
operation of the HVDC link.

Referring to the equation

the line and converter resistances are small; hence, a small change in
Vdor or Vdoi causes a large change in Id.

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 For example, a 25% change in the voltage at either the rectifier or the
inverter could cause direct current to change by as much as 100%. This
implies that, if both αr and yi are kept constant, the direct current can vary
over a wide range for small changes in the alternating voltage magnitude
at either end.

What is the Need for power factor high?

 There are several reasons for maintaining the power factor high:

a. To keep the rated power of the converter as high as possible for

given current and voltage ratings of transformer and valve;
b. To reduce stresses in the valves;
c. To minimize losses and current rating of equipment in the ac
system to which the converter is connected;
d. To minimize voltage drops at the ac terminals as loading
increases; and
e. To minimize cost of reactive power supply to the converters

 From the equation of the power factor,

We get,

Therefore, to achieve high power factor, α for a rectifier and y for an

inverter should be kept as low as possible.

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3.1.1 DESIRED FEATURES OF THE CONTROLLER:

• Control system should not be sensitive to normal variations in

voltage and frequency of the AC supply system.
• Control should be fast reliable and easy to implement.
• There should be continuous operating range of full
Rectification to full Inversion.
• Control should be such that it should require less reactive
power.
• Under at steady state conditions the valves should be fired
symmetrically.
• Control should be such that it must control the maximum
current in the DC link and limit the fluctuations of the current.
• Power should be controlled independently and smoothly which
can be done by controlling the current or voltage or both.
• Control should be such that it can be used for protection of
the line and the converter

3.2.1 CHARACTERISTICS OF THE HVDC SYSTEM:

In order to satisfy basic requirements for better voltage

regulation and current regulation it is always be advisable to assign
these parameters for the converters. Under normal operations
Rectifier will take care of the current and the Inverter will take care
of the voltage.

Rectifier - Constant Current Control (CC)

Inverter - Constant Extinction Angle Control (CEA)

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 IDEAL CHARACTERISTICS:

Fig: Equivalent circuit HVDC transmission link

 In satisfying the basic requirements identified above, the responsibilities

for voltage regulation and current regulation are kept distinct and are
assigned to separate terminals.

 Under normal operation, the rectifier maintains constant current (CC), and
the inverter operates with constant extinction angle (CEA), maintaining
adequate commutation margin.

 The basis for this control philosophy is best explained by using the
steady-state voltage-current (V-I) characteristics, shown in Figure 1. The
voltage Vd and the current Id forming the coordinates may be measured at
some common point on the dc line.

 In Figure 1, we have chosen this to be at the rectifier terminal. The

rectifier and inverter characteristics are both measured at the rectifier;
the inverter characteristic thus includes the voltage drop across the line.
Mr. G.DILLI BABU, ASSISTANT PROFESSOR 6
 With the rectifier maintaining constant current, its V-I characteristic,
shown as line AB in Figure 1, is a vertical line. From Figure

 This gives the inverter characteristic, with y maintained at a fixed value. If

the commutating resistance Rci is slightly larger than the line resistance R L,
the characteristic of the inverter, shown as line CD in Figure 1, has a small
negative slope.

 Constant extinction angle control mode is essentially the same as

constant margin angle control. Under normal operation, the commutation
margin angle and the extinction angles are equal. The distinction arises
during conditions such as operation with a large overlap angle; the valve
voltage may become positive earlier than when the sinusoidal portion of
the voltage would have crossed zero under normal conditions.

ACTUAL CHARACTERISTICS:

 The rectifier maintains constant current by changing α. However, α cannot

be less than its minimum value (αmin). Once αmin is reached, no further

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 voltage increase is possible, and the rectifier will operate at constant
ignition angle (CIA). Therefore, the rectifier characteristic has really two
segments (AB and FA) as shown in Figure 1.
 The segment FA corresponds to minimum ignition angle and represents
the CIA control mode; the segment AB represents the normal constant
current (CC) control mode.

 In practice, the constant current characteristic may not be truly vertical,

depending on the current regulator. With a proportional controller, it has a
high negative slope due to the finite gain of the current regulator, as
shown below.

 With a regulator gain of k, we have

 In the terms of perturbed values,

Or,

 With a proportional plus integral regulator, the CC characteristic is quite

vertical. The complete rectifier characteristic at normal voltage is defined
by FAB. At a reduced voltage it shifts, as indicated by F'A'B. The CEA
characteristic of the inverter intersects the rectifier characteristic at E for
normal voltage. However, the inverter CEA characteristic (CD) does not

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 intersect the rectifier characteristic at a reduced voltage represented by
F'A'B. Therefore, a big reduction in rectifier voltage would cause the
current and power to be reduced to zero after a short time depending on
the dc reactors.

3.2.2 Constant Current Control (CC)

In a d.c. link it is common practice to operate the link at constant current

rather than at constant voltage. [Of course, constant current means that
current is held nearly constant and not exactly constant]. In constant
current control, the power is varied by varying the voltage. There is an
allowed range of current settings within which the current varies.

3.2.3 CONSTANT EXTINCTION ANGLE CONTROL:

Maximum utilization of an inverter's capacity and minimum consumption of

reactive power demands an accurate constant extinction angle control. ...
The control is extended to cover the converter operation over its full range

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from a minimum permissible angle of delay to the minimum permissible
extinction angle

3.2.4 COMBINED RECTIFIER AND INVERTER CHARACTERISTICS:

Power reversal can be done by changing the current settings of the

converter and inverter which is shown in the dotted line above

3.3 Firing Angle Control

The operation of CC and CEA controllers is closely linked with the

method of generation of gate pulses for the valves in a converter. The
requirements for the firing pulse generation of HVDC 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.
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)

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3.3.1 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.

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 90o and added separately to a common control
voltage V.

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 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.

The main advantage of this scheme is that the average DC voltage

across the bridge varies linearly with the control voltage Vc .

ADVANTAGES OF IPC:

 The output voltage will be high

DISADVANTAGES OF IPC:
 Harmonic instability with less SCR.

 Non characteristics harmonics introduction in the system.

 Parallel resonance with filter impedance and system

impedance

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3.3.2 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. Thre are three variations
of the EPC scheme
1. Pulse Frequency Control (PFC)
2. Pulse Period Control
3. Pulse Phase Control (PPC)

1.Pulse Frequency Control (PFC):

 The basic components of the system are Voltage Controlled Oscillator

(VCO) and a ring counter. The VCO delivers pulses at a frequency
directly proportional to the input control voltage.

 The train of pulses is fed to a ring counter which has six or twelve
stages.

 One stage is on at a time with the pulse train output of the VCO
changing the on stage of the ring counter.

 As each stage turns on it produces a short output pulse once per

cycle.

 Over one cycle a complete set of 6 or 12 output pulses are produced

by the ring counter at equal intervals.

 These pulses are transferred to the firing pulse generator to the

appropriate valves of the converter bridge

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  Under steady state conditions V2=0 and the voltage V1 is proportional
to the AC line frequency ω1.

 This generates pulses at the line frequency and constant firing delay
angles α.

 If there is a change in the current order , margin settings or line

frequency., a change in V3 occurs which in turn results in change in
the frequency of the firing pulses.

 A change in the firing delay angle results from the time integral of the
differences between the line and firing pulse frequencies.

 It is apparent that this equidistant pulse control firing scheme is based

on pulse frequency control.

2. Pulse Period Control:

In this scheme a step change in control signals causes a spacing of

the only pulse to change these results in a shift of phase only.

ADVANTAGES:

 Equal delay for all the devices.

 Non characteristics harmonics are not introduced

DISADVANTAGES:
 Less DC output voltage than IPC

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.

Mr. G.DILLI BABU, ASSISTANT PROFESSOR 14

3. Pulse Phase Control (PPC)
An analog circuit is configured to generate firing pulses according to the
following equation

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.

(iii) 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.
2. EPC Scheme also results in higher negative damping contribution
to torsional oscillations when HVDC is the major transmission link
from a thermal station.

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Current and Extinction Angle Control
The current controller is invariably of feedback type which is of PI
type.

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.
3.4 Tap changer control:
High voltage DC transmission is gaining more and more
importance due to various advantages over high voltage AC
transmission. In High voltage DC converter transformer is the main
component which is used in HVDC transmission. Tap changer is an
essential part of any power transformer for obtaining various turns
ratios to get different voltage levels. Conventional mechanical tap
changers are commonly employed for this purpose.

The conventional mechanical tap changers available in market

uses mechanical contacts for switching. The main disadvantage of
mechanical tap changer is the slow response and arc formation in the
contacts while switching. The arc formation in the contacts reduces the

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life of tap changers. In addition to this, the arc formation deteriorates
the quality of oil used in oil immersed tap changers. So, regular check-
ups and maintenance are required for mechanical tap changers. The
problems caused by the mechanical tap changers can be overcome by
using solid state devices for switching. The advantages of solid-state tap
changers are its low maintenance cost, high performance and high
operating speed.

High voltage power electronic converter circuits are used to

convert high voltage AC to high voltage DC. A transformer that is used
for HVDC conversion is known as HVDC converter transformer. Tap
changers are included in this type of transformer to obtain various turns
ratio, so that the secondary voltage can be controlled. The reactive
voltage drops due to HVDC conversions and voltage drop due to other
reasons can be compensated using tap changers.

Mainly two topologies are evaluated in High voltage power

electronic converter circuits. The power quality issues due to power
electronic tap changer can be minimized by using suitable topologies for
the tap changer. The topology has also a main role in determining the
cost of tap changer.

Topology-1: The first topology consists of two-anti parallel thyristors

connected to each tap positions. The configuration of this tap changer is
similar to that of AC voltage controller. These anti parallel thyristors
form a bidirectional switch which ensures the current flow in both
directions.

This configuration consists of larger number of thyristors so the

cost of tap changer is more. The advantage of this topology is harmonic
reduction. A separate HVDC converter is required to convert high
voltage AC supply to High voltage DC supply.

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Topology -2: The second topology consists of only one thyristor
connected to each tap positions. The configuration is similar to that of a
half wave diode rectifier circuit. This configuration allows current flow in
one direction only. Which means only one-half cycle of an AC supply is
conducted through the thyristors.

The cost of tap changer is also reduced due to a smaller number

of thyristors are used in this configuration. The drawback of this
configuration is the harmonic distortion and discontinuity in supply
voltage waveform

3.5 Starting and Stopping of DC Link

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.

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 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.

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 S1 must 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, S1

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must 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.

Start-up with long pulse firing:

1. Deblock inverter at about γ = 900

2. Deblock rectifier at α = 850 to establish low direct current

3. Ramp up voltage by inverter control and the current by rectifier control.

Start-up with short pulse firing:

1. Open bypass switch at one terminal

2. Deblock that terminal and load to minimum current in the rectifier mode

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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.

Power Reversal in HVDC links:

In contrast to line-commutated HVDC converters, voltage-source converters

maintain a constant polarity of DC voltage and power reversal is achieved
instead by reversing the direction of current. This makes voltage-source
converters much easier to connect into a Multi-terminal HVDC system or “DC
Grid”.

Power reversal in the LCC link can be achieved by reversing LCCs' DC

polarity through changing their control modes

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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Mr. G.DILLI BABU, ASSISTANT PROFESSOR 22