HVDC transmission allows for more efficient long distance transmission of electricity compared to AC transmission. It became commercially viable with the development of mercury arc rectifiers and thyristor valves in the 1950s. HVDC transmission has advantages over HVAC such as lower transmission losses over long distances, ability to interconnect AC systems of different frequencies, and easier control of active and reactive power. Modern HVDC systems use voltage source converters to efficiently convert AC to DC and back with independent control of active and reactive power. They require converter stations on both ends but allow efficient long distance transmission of high voltage DC between the stations.

1. HVDC TRANSMISSION
INTRODUCTION
Transmission and distribution of electricity started with direct current but was inefficient due
to the power loss in conductors compared to alternating current transmission. Large
quantities of electrical energy are transmitted using three phase alternating current but this
method of transmission has a constraint in that there’s a limit to the distance that bulk
amount of alternating current unless some form of reactive compensation is used.
Alternating current or A.C can be used for overhead transmission for over a distance of 50km
with reactive compensation. [1] [2]
However, for very high power transmission, AC transmission is disadvantageous for example
undersea transmission. But with the invention of mercury arc rectifiers and thyristors valves
HVDC transmission became feasible. The world's first commercial HVDC transmission link,
was built in 1954 between the Swedish mainland and the island of Gotland, with a rating of
20 MW, 200 A and 100 kV. [2]
Limitations of AC interconnection.
1. Losses are high due to corona discharge and skin effect.
2. Cost of the transmission lines is high.
3. Towers are bigger therefore more expensive as compared to HVDC.
4. It is not reliable.
5. More insulation is required.
6. Transmitter less power as compared to HVDC.
7. Voltage regulation and control ability is limited.
Advantages of HVDC transmission/DC interconnection. [1] [2]
1. Allows the interconnection of two large a.c systems without having to ensure
synchronism and over stability between them.
2. Allows transmission of high power over long distances where the a.c transmission
towers, insulators and conductors are more expensive than those of HVDC.
3. Allows interconnection between systems of different frequency e.g. connecting two
systems of 50Hz and 60Hz frequency.
4. HVDC can carry more power for a given size of conductor.
5. Direction of flow of power can be changed swiftly i.e. it is bi-directional.
6. HVDC has a low effect on the environment hence doesn’t need a right of way.
7. VSC technology is an economical and efficient technology for other energy sources.

2. Advantages of HVDC compared to HVAC. [2]
1. HVDC transmission has no limitation to the distance of transmission for both
overhead lines and underground cables unlike HVAC which experiences more power
losses.
2. Two conductors, positive and negative to ground, are required instead of three,
thereby reducing tower or cable costs.
3. The direct voltage can be designed equivalent to the peak of the alternating voltage
for the same amount of insulation to ground (i.e. � . =√2� .
4. The voltage stress at the conductor surface can be reduced with d.c, thereby
reducing corona loss, audible emissions, and radio interference.
5. Fast control of converters can be used to damp out oscillations in the a.c. system it’s
connected to.
Disadvantages of HVDC
1. The higher cost of converter stations compared with an a.c. transformer substation.
2. The need to provide filters and associated equipment to ensure acceptable waveform
and power factor on the a.c. networks.
3. Limited ability to form multi-terminal d.c. networks because of the need for
coordinated controls.
4. High cost of switching because HVDC circuit breakers are difficult to build.
5. High maintenance costs.
6. It is less reliable and has a lower availability than AC systems due to extra conversion
equipment.
Components of HVDC transmission [2] [3]
AC power is generated at the substation it can then be converted into DC by using a rectifier
therefore DC flows through the overhead lines and at the user end again DC is converted
into AC by using inverters. Components include:
The converter station at the transmission and receiving ends, the transmission medium, and
the electrodes.

3. Figure 1: HVDC system components
The converter station: The converter stations at each end are replicas of each other and
therefore consists of all the needed equipment for going from AC to DC or vice versa. [3]
Thyristor valves:
They perform the function of conversion from AC to DC. Each single thyristor valve consists
of series connected thyristors with their auxiliary circuits. All communication between the
control equipment at earth potential and each thyristor is done with fibre optics because they
are at a high potential. [2]
Converter Transformers:
Transform the voltage level of the AC bus-bar to the required level of the converter.
Smoothing Reactor:
It performs the following functions;
· Prevent discontinuous current.
· Limit DC fault current.
· Prevent resonance in the DC circuits.
Harmonic Filters/AC filters:
They are on the AC side of the HVDC converter station. The perform the following
functions;
· Absorbing harmonic currents generated by the HVDC converter.
· Supplying reactive power.
DC filter Circuits:
A.C Filter
Transmission Cable
DC Filter
Transformer Converter
Smoothing
Reactor
Control
System
AC bus

4. These together with active harmonic filters act as a supplement to passive filters due to the
better performance.
Surge arrestors: To protect the equipment from over-voltages.
DC transmission circuit: Include the DC line, cable, high speed DC switches and earth
electrode.
Control System: For operation management and supervision of the whole HVDC system
equipment.
VSC Valves:
The VSC Converter consists of two level or multilevel converter, phase-reactors and AC-
filters. They control equipment and cooling equipment.
Ac Circuit Breakers:
All faults in the transformer are cleared by the circuit breakers. These circuit breakers are
also used to disconnect the DC links.
CONVERTING AND INVERTING TECHNOLOGIES. [3]
Conversion of electrical current from AC to Dc at the transmitter end and from DC to AC
at the receiving end is the most important process in HVDC transmission. Three ways are
usually used.
1. Natural Commutated Converters(NCC):
They are the most used in HVDC systems. It consists of the thyristor as the converting
component, which is a controllable semiconductor that can carry current up to around
4000A and can block very high voltages of up to 10kV. By means of connecting the
thyristors, thyristor valves which is able to operate at very high voltages are made. The
thyristor valve is operated at net frequency i.e. 50 Hz or 60 Hz and by means of a control
angle it is possible to change the DC voltage level of the bridge hence transmitted power
can be controlled rapidly and efficiently.
On average construction of the NCC based HVDC system takes 3 years from contract
date to commissioning.
2. Capacitor Commutated Converters (CCC).
The CCC concept is characterized by the use of commutation capacitors inserted in series
between the converter transformers and the thyristor valves. The commutation capacitors
improve the commutation failure performance of the converters when connected to weak
networks. It is an improvement to the thyristor-based commutation.
On average construction of the CCC based HVDC system takes 2 years from contract
date to commissioning.
3. Forced Commutated Converters(FCC):

5. The valves of these converters are built up with semiconductors with the ability to turn-
on and turn-off. They are known as VSC (Voltage Source Converters). The VSC
commutates with high frequency.
Two types of semiconductors are normally used in the voltage source converters: the
GTO (Gate Turn-Off Thyristor) or the IGBT (Insulated Gate Bipolar Transistor). The
operation of the converter is achieved by Pulse Width Modulation (PWM) to create any
phase angle and amplitude by changing the PWM pattern.
On average construction of the VSC based HVDC system takes 1 year from contract date
to commissioning.
Advantages of the above conversion system include;
· Ability to feed to passive networks without generation.
· It allows independent control of active and reactive power almost instantaneously
using PWM.
· Has high power quality.
HVDC SYSTEMS
The HVDC system can be mono-polar or bipolar.
The mono-polar system
The mono-polar system uses a high voltage conductor and a ground return. Its economically
advantageous but causes corrosion to pipes and other buried metallic material. [2]
Figure 2 Mono-Polar System

6. The bipolar system
The bipolar system uses two conductors, with opposite polarities and the mid-point is
grounded.
Figure 3 Bipolar System
Construction Cost
The cost of an HVDC transmission system depends on factors, such as:
· Power capacity to be transmitted.
· Type of transmission medium.
· Environmental conditions and other safety, regulatory requirements etc.
Even when these are available, the options available for optimal design for example the
different commutation techniques, variety of filters, transformers etc. render it is difficult to
give a cost figure for an HVDC system. [1]

7. References
[1] N. B.M Weedy B.J Cory, Electric Power Systems, UK: John Wiley and Sons Ltd, 2012.
[2] I. D.M. Larruskain, Transmission and Distribution Networks:AC versus DC.
[3] J. C. S. Roberto Rudervall, High Voltage Direct Current (HVDC)Transmission Systems Technology
Review Paper, vol. Energy Week 2000, Washington, 2000.