Haramaya Institute of Technology
School of Electrical and Computer Engineering
Power Systems (ECEG4231)
Chapter Five
HVDC Transmission
By: Mr. Lidetu A.
1/30/2022
January 2022
1
Chapter Five
HVDC Transmission
1. Introduction
▪ The first Central Electric Station was installed by Edison in New York in 1882.
▪ The development of HVDC transmission system dates back to the 1930s when mercury arc
rectifiers were invented.
▪ In 1941, the first HVDC transmission system contract for a commercial HVDC system was placed:
60 MW were to be supplied to the city of Berlin through an underground cable of 115 km in length.
- HVDC transmission systems combine high reliability with a long useful life.
- Their core component is the power converter, which serves as the interface to the AC transmission
system.
- At the sending end a.c. is converted into d.c. and at the receiving end it is converted back to a.c. for
use1/30/2022
2
Largest HVDC transmission lines in the world
1. Belo Monte-Rio de Janeiro transmission line, Brazil
- 800kV UHVDC; 2,543km long; 4GW capacity from 11.GW HPP; completed 2019
2. Rio Madeira transmission link
- 600kV HVDC; 7.1GW; commisioned 2013
3. Belo Monte-Estreito transmission line, Brazil
- the first 800kV UHVDC; 2,092km; 4GW; commissioned 2017
4. Jinping-Sunan transmission line, China
- 800kV UHVDC; 7.2GW; commisioned December 2012
5. Xiangjiaba-Shanghai transmission line, China
- 800kV UHVDC; 1,980km; 7.2GW;
1/30/2022
3
HVDC In Ethiopia
500 kV HVDC Ethiopia–Kenya Link
- It is the first HVDC line in East Africa and a major channel of outbound power
transmission for Ethiopia.
- The interconnector entails the construction of a 1,045-km, ±500 kV HVDC bipole
overhead transmission line.
- includes two 2,000 MW, ±500 kV, HVDC bipolar converter stations, one at
Wolaita Sodo in Ethiopia and the other at Suswa in Kenya.
1/30/2022
4
Principles of AC/DC conversion
Rectification:- Transformer secondary can be connected to give 3-phase supply to the
rectifier valves.
- But 6-phase connection is found to be sufficiently good from all practical
viewpoints.
- The 3-phase system is the simplest convertor circuit but is not practical
because the direct current in the secondary windings saturates the
transformer core.
- 1/30/2022
This could be avoided by using zig-zag connections.
5
Cont’d…
- Figure below shows the current and voltage wave-form in the three
phases of the supply transformer.
- When grid control is not used, conduction will take place between the
cathode and the anode of highest potential and, therefore, the output
voltage is indicated by the thick line and the current output will be
continuous.
- From the voltage wave-form it is clear that the change-over from one
anode to the other takes place at an electrical angle calculated as follows:
1/30/2022
6
1/30/2022
7
- For an n-phase or n-anode system the changeover takes place at (π/2 – π/n)
and conduction continues up to (π /2 + π /n).
- Now since conduction takes place only during the positive half cycle, the
average value of the d.c. voltage will be:
- The wave of anode current is a rectangular pulse of height Id and length
120°. Its average value is Id/3 and the r.m.s. value Id/ 3 = 0.577Id.
1/30/2022
8
The Three Phase Bridge Rectifier
- The bridge rectifier is the most practical circuit used
for converting a.c. into d.c. for HVDC transmission.
- For a given alternating voltage the output direct
voltage is doubled as the two anodes conduct
simultaneously and hence the power is doubled.
- Let Va be the most positive at the beginning of the
sequence say point O in Fig. (b).
- Corresponding to this point Vb is most negative;
therefore, the conduction will take place between
phase a and b from a to b.
- The complete sequence of the diodes conducting is,
therefore, 1 and 6, 1 and 2, 3 and 2, 3 and 4, 5 and
1/30/2022
4,
5 and 6 and 1 and 6 again
9
Inversion
- For inverter operation it is required to transfer power from the direct current to
the alternating current system which can only be obtained by the reversal of
the average direct voltage.
- Therefore, for inversion, an alternating voltage system must exist on the
primary side of the transformer and grid control of the converter is essential.
1/30/2022
10
5.2 Reactive Power Demand
- A d.c. line itself does not require reactive power and voltage drop on the line is only the IR drop
where I is the d.c. current.
- The converters at both ends of the line, however, draw reactive power from the a.c. system.
- It varies with the transmitted power and is approximately half of the power at each end.
- It is independent of the length of the line. Filters are provided on the a.c. side of the converters
for HVDC transmission to reduce harmonic currents and voltages on the a.c. side which would
otherwise produce interference with other equipment or with communication lines.
- These filters are of large size and involve considerable cost.
- But fortunately these can also provide a major part of the leading reactive power required by
the converter.
1/30/2022
11
- While active power is the same on both sides of the DC link, the, reactive power flowing
between one HVDC terminal and its associated AC network can differ and substantially
determines the AC–DC interface action.
- This reactive power flow may be controlled independently on both sides of an HVDC twopoint system, or at each terminal of an HVDC multi-terminal system.
- This independence also applies whether or not the AC systems of the terminals are
synchronously coupled to each other.
- Two correcting variables are available for influencing the flow of reactive power:
(1) generation of reactive power in filter circuits and capacitor banks (more rarely, by means
of continuously controllable static compensators or synchronous condensers) and
(2) reactive power consumption of the converter, which can be influenced by changing the
nominal control angle.
1/30/2022
12
5.3 Economic Considerations
- The cost per unit length of a d.c. line is much less as compared to a.c. line for the
same power capacity and comparable reliability whereas the cost of terminal
equipment is much more in case of d.c. (converters and inverters) than in case of
a.c. (Transformers).
- DC of 400 km of distance the power to be transmitted should be at least 100 MW
1/30/2022
Comparative costs of ac and dc overhead lines Vs distance
13
Kinds of DC links
- Monopolar: As the name suggests
monopolar lines are those in which the line
has one conductor only and the earth is
used as the return conductor in fig (a).
- Bipolar: have two conductors—one operating
with +ve polarity and the other negative
polarity. There are two converters of equal
voltage rating and connected in series at each
end of the d.c. line refer to fig. (b)
- Homopolar: The homopolar lines have two or
more conductors having the same polarity
usually negative for the reason of corona and
radio interference fig (c).
1/30/2022
14
5.4 Applications of HVDC
Figure below shows example applications of HVDC transmission systems.
1/30/2022
15
1. Power transmission of bulk energy through long distance overhead line.
2. Power transmission of bulk energy through sea cable.
3. Fast and precise control of the flow of energy over an HVDC link to create a positive
damping of electromechanical oscillations and enhance the stability of the network by
modulation of the transmission power by using a Back-to-Back.
4. Since an HVDC link has no constraints with respect to frequency or to phase angle
between the two AC systems, it can be used to link systems with different frequencies
using an Asynchronous Back-to-Back.
5. When power is to be transmitted from a remote generation location across different
countries or different areas within one country, it may be strategically and politically
necessary to offer a connection to potential partners in the areas traversed by using a
multiterminal DC link.
1/30/2022
16
6. An HVDC transmission system can also be used to link renewable energy sources,
such as wind power, when it is located far away from the consumer.
7. VSC (Voltage-Source Converter) based HVDC technology is gaining more and more
attention. This new technology has become possible as a result of important advances
in the development of Insulated Gate Bipolar Transistors (IGBT).
In this system, Pulse-Width Modulation (PWM) can be used for the VSC as opposed
to the thyristor based conventional HVDC. This technology is well suited for wind
power connection to the grid.
8. Since reactive power does not get transmitted over a DC link, two AC systems can
be connected through an HVDC link without increasing the short circuit power; this
technique can be useful in generator connections.
1/30/2022
17
5.5 Advantages and Disadvantages of HVDC
Advantages
- Sending and receiving end frequencies are independent.
- Transmission distance using DC is not affected by cable charging current.
- Offshore installation is isolated from mainland disturbances, and vice versa.
- Power flow fully defined and rapidly controllable.
- Low cable power losses.
- Higher power transmission capability
- No technical limit to the length of a submarine cable connection.
- No increase to the short circuit capacity imposed on AC switchgear.
- Immunity from impedance, phase angle, frequency or voltage fluctuations.
- Improves both the AC system’s stability and, therefore, improves the internal power carrying
capacity, by modulation of power in response to frequency, power swing or line rating.
1/30/2022
18
Disadvantages
• Expensive Converters: The converters required at both ends of the line
have proved to be reliable but they are much more expensive than the
conventional a.c. equipments.
- Produce lot of harmonics both on d.c and a.c. sides
- Filters are required on the a.c. side of each converter
- Therefore, Cost increases.
• Voltage Transformation: The power transmitted can be used at lower
voltage only. Voltage transformation is not easier in case of d.c. and hence
it has to be done on the a.c. side of the system.
• Circuit breaking for multi-terminal lines is difficult
1/30/2022
19
The End!
1/30/2022
20