Electric power transmission was initially established using direct

current. The emergence of transformers and the advancement of

induction motors in the early 20th century facilitated the adoption
of AC transmission.

DC transmission has now become feasible for long-distance

applications or where cables are necessary. Thyristors were
utilized in direct current transmission, and solid-state valves were
actualized.

The rapid advancement of converters (rectifiers and inverters)

operating at elevated voltages and substantial currents has become
DC transmission a significant consideration in power transmission
planning. Initially, all HVDC systems employed mercury arc valves,
which were predominantly single-phase in design, unlike the low-
voltage polyphase devices utilized in industrial applications. In
around 1960, control electrodes were included into silicon diodes,
resulting in the development of silicon-controlled rectifiers (SCRs
or Thyristors).

The maximum operational DC voltage for direct current

transmission is +/- 600 kV. Direct current transmission has become
a fundamental component of power distribution in numerous
countries globally.

Comparison Between AC & DC

Transmission
Feature AC Transmission DC Transmission
Power Higher due to inductive & Lower due to absence of
Losses capacitive effects reactive power
Cost-effective for long
Cost Lower for short distances
distances
Prone to oscillations & More stable with no
Stability
stability issues reactive power issues
Control Limited power flow control Easy and flexible control

Economics of Electrical Transmission:

In DC transmission, the inductance and capacitance of the line do

not influence the power transfer capacity or the line drop.
Furthermore, there is an absence of leakage or charging current in
the line under steady-state conditions.

A direct current (DC) line necessitates merely two conductors, but

an alternating current (AC) line demands three conductors in
three-phase AC systems. The expense of terminal equipment is
greater in DC lines compared to AC lines. Break-even point
Figure: Comparative expenses of AC and DC transmission lines in
relation to distance

The distance at which the costs of the two systems are equivalent.
The figure below indicates that a DC line is cost-effective at
distances beyond the break-even point.

Technical Efficacy:

A DC transmission system offers rapid controllability, complete

regulation of transmitted power, the capacity to improve transient
and dynamic stability in connected AC networks, and the capability
to restrict fault currents in the DC lines. Moreover, DC
transmission mitigates some issues related to AC transmission.

Boundaries of Stability:

The power transfer in an AC line relies on the angular disparity

between the voltage phasors at both ends of the line. For a
specified power transmission level, this angle escalates with
distance. The greatest power transfer is constrained by the factors
of steady-state and transient stability. The power transmission
capacity of an AC line is inversely related to the distance, while the
power transmission capacity of DC lines remains unaffected by
distance.

Voltage Regulation

Voltage regulation in alternating current lines is hampered by line

capacitance and voltage attenuation. The voltage profile in an AC
line remains essentially constant only at a specific power transfer
level corresponding to its Surge Impedance Loading (SIL). The
voltage profile fluctuates with the line loading. For a constant
voltage at the line terminals, the midpoint voltage diminishes for
line loadings exceeding the Surge Impedance Loading (SIL) and
increases for loadings below the SIL.

Maintaining a steady voltage at both ends necessitates reactive

power management as line loading increases. The demand for
reactive power escalates with the length of the line. While DC
converter stations necessitate reactive power in relation to the
transmitted power, the DC line itself does not demand any reactive
power. The steady-state charging currents in AC cables provide
significant challenges, resulting in a break-even distance for cable
transmission of approximately 50 kilometers.

Line Remuneration:

Line compensation is essential for long-distance AC transmission to

address issues related to line charging and stability constraints.
The enhancement of power transfer and voltage regulation is
achievable with the implementation of shunt inductors, series
capacitors, Static Var Compensators (SVCs), and the recent
advancements in Static Compensators (STATCOMs). Compensation
is unnecessary for DC lines.

Challenges of AC Interconnection:

The integration of two power systems via AC ties necessitates the

coordination of the automatic generation controllers of both
systems via tie line power and frequency signals. The functioning of
AC ties can be troublesome, even with coordinated control of
interconnected systems, due to:

The occurrence of substantial power oscillations that may result in

frequent tripping.

Elevation in fault level, and

Transmission of perturbations between systems.

The rapid regulation of power flow in DC lines resolves all

aforementioned issues. Moreover, the asynchronous connecting of
two power systems can alone be accomplished through the
utilization of DC lines.

Soil impedance:
In AC transmission, the presence of ground (zero sequence)
current is inadmissible in steady-state conditions because to the
substantial ground impedance, which adversely impacts efficient
power transfer and causes telephonic interference. The ground
impedance is insignificant for direct current, allowing a DC link to
function with a single conductor and ground return (monopolar
operation).

The ground return is problematic solely in the presence of

underground metallic infrastructure, such as pipelines, which are
susceptible to corrosion due to direct current flow. In monopolar
mode, the AC network supplying the DC converter station functions
with balanced voltages and currents. Consequently, single pole
operation of direct current transmission systems may be sustained
for prolonged durations, whereas alternating current transmission
cannot support single phase operation (or any imbalanced activity)
for more than a second.

Drawbacks of Direct Current Transmission:

The applicability of DC transmission is constrained by

Elevated expenses associated with conversion apparatus.

Incapability to employ transformers for voltage level modification.

Harmonic generation.

Demand for reactive power and

Intricacy of regulations.

Significant advancements in DC technology have occurred

throughout the years, aiming to address the aforementioned
shortcomings, with the exception of (2). These represent
Augmentation of the ratings of a thyristor cell constituting a valve.
Modular fabrication of thyristor valves.

Twelve-pulse (and higher) converter operation.

Implementation of compelled commutation.

Utilization of digital electronics and fiber optics in converter

control.

Dependability:
The reliability of DC transmission systems is commendable and
comparable to that of AC systems. The dependability of DC links
has been notably high.
Overall system reliability is assessed by two metrics: energy
availability and transient reliability.

Current time
Equivalent outage time is defined as the product of real outage
time and the proportion of system capacity diminished due to the
outage.

Temporary dependability:

This factor delineates the performance of HVDC systems during

documentable faults in the corresponding AC systems.

Transient reliability = 100 × (Number of times HVDC systems

functioned as intended) / (Number of recordable AC faults)

Recordable AC system faults are defined as faults that result in one

or more AC bus phase voltages falling below 90% of their pre-fault
levels.

The energy availability and transient dependability of current DC

systems with thyristor valves is 95% or higher.

Utilization of Direct Current Transmission

Owing to their expenses and unique characteristics, the majority of

DC transmission applications typically belong to one of the
following three classifications.

Cables located underneath or underwater:

For extensive cable connections above the breakeven distance of

around 40-50 km, a DC cable transmission system demonstrates a
significant benefit over AC cable connections. Instances of this
category of applications include the Gotland (1954) and Sardinia
(1967) initiatives. The current advancement of Voltage Source
Converters (VSC) and the utilization of durable polymer DC cables,
known as the “HVDC Light” alternative, are gaining increasing
attention. An instance of this application is the 180 MW Direct Link
connection established in 2000 in Australia.

Long-distance bulk electricity transmission:

Long-distance bulk power transmission is optimally suited for DC

transmission and is more cost-effective than AC transmission when
the breakeven distance is surpassed. Numerous instances of this
use exist, ranging from the earlier Pacific Intertie to the newer
connections in China and India.
The breakeven distance is being significantly lowered due to lower
costs of new compact converter stations, facilitated by recent
advancements in power electronics.

Stabilization of power flows in an integrated power system:

In extensive interconnected systems, power flow in AC ties,

especially during disturbance situations, can become
uncontrollable, resulting in overloads and stability issues that
jeopardize system security. Strategically positioned DC lines can
mitigate this issue owing to the rapid controllability of DC power,
hence offering essential damping and prompt overload capacity.
The planning of DC transmission in these applications necessitates
a comprehensive analysis to assess the advantages. An such is the
Chandrapur-Padghe connection in India.

The current quantity of DC lines in a power system is significantly

lower than that of AC lines. This signifies that DC transmission is
warranted just for particular purposes. Despite technological
advancements and the implementation of Multi-Terminal DC
(MTDC) systems, it seems unlikely that the AC grid will be
supplanted by a DC power grid in the future. Two principal
explanations account for this:

The management and safeguarding of MTDC systems is intricate,

and the lack of voltage transition in DC networks incurs financial
drawbacks.

Secondly, advancements in power electronics technology have

enhanced the performance of AC transmissions utilizing FACTS
devices, such as static VAR systems and static phase shifters.

Types of HVDC Links

Three types of HVDC Links are considered in HVDC applications

which are 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.

Bipolar Link:

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.

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.

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.

HVDC Converter Station

The major components of a HVDC transmission system are

converter stations where conversions from AC to DC (Rectifier
station) and from DC to AC (Inverter station) are performed. A
point to point transmission requires two converter stations. The
role of rectifier and inverter stations can be reversed (resulting in
power reversals) by suitable converter control.

A typical converter station with two 12 pulse converter units per

pole is shown in figure below.

The block diagram of converter station is given above.

Converter Unit:

This usually consists of two three phase converter bridges

connected in series to form a 12 pulse converter unit as shown in
above figure. The total number of valves in such a unit is twelve.
The valves can be packaged as single valve, double valve or
quadrivalve arrangements. Each valve is used to switch in segment
of an AC voltage waveform. The converter is fed by converter
transformers connected in star/star and star/delta arrangements.

The valves are cooled by air, oil, water of freon. Liquid cooling
using deionized water is more efficient and results in the reduction
of station losses. The design of valves is based on the modular
concept where each module contains a limited number of series
connected thyristor levels.

Valve firing signals are generated in the converter control at

ground potential and are transmitted to each thyristor in the valve
through a fiber optic light guide system.

The valves are protected using snubber circuits, protective firing

and gapless surge arrestors.

Converter Transformer:

The converter transformer has three different configurations-

(i) three phase, two winding,

(ii) single phase, three winding and (iii)sing1e phase, two winding
The valve side windings are connected in parallel with neutral
grounded. The leakage reactance of the transformer is chosen to
limit the short circuit currents through any valves.

The converter transformers are designed to withstand DC voltage

stresses and increased eddy current losses due to harmonic
currents. One problem that can arise is due to the DC
magnetization of the core due to unsymmetrical firing of valves.

Filters:

There are three types of filters used which are

AC Filters:

These are passive circuits used to provide how impedance, shunt

paths for AC harmonic currents. Both tuned and damped filter
arrangements are used.

DC Filters:

These are similar to AC filters and are used for the filtering of DC
harmonics.

High Frequency Filters:

These are connected between the converter transformer and the

station AC bus to suppress any high frequency currents. Sometimes
such filters are provided on high-voltage DC bus connected
between the DC filter and DC line and also on the neutral side.

Reactive power source:

Converter stations require reactive power supply that is dependent

on the active power loading. But part of the reactive power
requirement is provided by AC filters. In addition, shunt capacitors,
synchronous condensors and static VAR systems are used
depending on the speed of control desired.

Smoothing Reactor:

A sufficiently large series reactor is used on DC side to smooth DC

current and also for protection. The reactor is designed as a linear
reactor and is connected on the line side, neutral side or at
intermediate location.

DC Switchgear:
It is modified AC equipment used to intemipt small DC currents. DC
breakers or Metallic Return Transfer Breakers (MRTB) are used, if
required for intemiption of rated load currents.

In addition to the DC switchgear, AC switchgear and associated

equipment for protection and measurement are also part of the
converter station.

Modern Trends in DC Transmission

To overcome the losses and faults in AC transmission, HVDC

transmission is preferred.

The trends which are being introduced are for the effective
development to reduce the cost of the converters and to improve
the performance of the transmission system.

Power semiconductors and valves:

The IGBTs or GTOs employed required huge amount of current to

turn it ON which was a big problem. GTOs are available at 2500V
and 2100A. As the disadvantage of GTOs is the large gate current
needed to turn them OFF, so MCT which can be switched OFF by a
small current is preferred as valves.

The power rating of thyristors is also increased by better cooling

methods. Deionized water cooling has now become a standard and
results in reduced losses in cooling.

Converter Control:

The development of micro-computer based converter control

equipment has made possible to design systems with completely
redundant converter control with automatic transfer between
systems in the case of a problem.

The micro-computer based control also has the flexibility to

implement adaptive control algorithms or even the use of expert
systems for fault diagnosis and protection.

DC Breakers:

Parallel rather than series operation of converters is likely as it

allows certain flexibility in the planned growth of a system. The DC
breaker ratings are not likely to exceed the full load ratings as the
control intervention is expected to limit the fault current.

Conversion of existing AC lines:

There are some operational problems due to electromagnetic
induction from AC circuits where an experimental project of
converting a single circuit of a double circuit is under process.

Operation with weak AC systems:

The strength of AC systems connected to the terminals of a DC link

is measured in terms of Short Circuit Ratio (SCR) which is defined
as

SCR = Short circuit level at the converter bus Rated DC Power

If SCR is less than 3, the AC system is said to be weak. The

conventional constant extinction angle control may not be suitable
for weak AC systems.

Constant reactive current control or AC voltage control may

overcome some of the problems of weak AC systems.

The power modulation techniques used to improve dynamic

stability of power systems will have to be modified in the presence
of weak AC systems.