Basics Of HVDC Transmission Prepared by Demsew Mitiku Addis Ababa Science and Technology University.

1. Addis Ababa Science and Technology University
Electrical and Computer Engineering Department
HVDC FUNDAMENTALS
Prepared by: DemsewM.
July 2017

2. HVDC
• Introduction
• Principle of AC/DC conversion
• HVDC Link Configuration
• Economic considerations, applications of HVDC
• Advantages and disadvantages of HVDC systems.

3. Introduction
In the past ...
At the beginning of 20th century, DC (Direct Current) was superseded by AC
(Alternating Current) for large-scale electrification. DC power did nevertheless
survive, in applications like electric traction and drives.
Today ...
Today, bulk power systems are 3-phase AC, while utilization is either 1-phase or 3-
phase AC.
In our country ...
….is actually a single AC power system, running synchronously at 50 Hz,
spanning from N_S_E_W!

4. Introduction
• Power Transmission was initially carried out in the early
1880s using Direct Current (d.c.).
• So why dominated by AC-Transmission?
o With the availability of transformers
o The development of robust induction motor
o The availability of the superior synchronous
generator,
o The facilities of converting a.c. to d.c.

5. History of Events
1880s
Power Transmission was initially carried out using Direct Current
1930s
Efficient static AC/DC conversion (mercury arc valves) was made
possible.
1940s
High Voltage DC (HVDC) bulk power transmission was studied in
Germany.
1954
First commercial application in Sweden: submarine link between
mainland and Gotland island (100 kV-20 MW-90 km).
1970s
Today ...
Thyristors (SCRs) took over; today, HVDC operation voltages attain 600
kV, transmitted power over 3000 MW.
DC made its way back into bulk power systems!

9. ADVANTAGES
Advantages of dc
transmission
Technical
Advantages
Economic
Advantages
Environmental
Benefits

10. Technical Merits and Demerits of HVDC
Advantages of DC:
• More power can be transmitted per conductor per
circuit how?
o For the same insulation level and conductor size
the per phase power analysis give us:

11. Cont’d…
• Higher Capacity available for cables
o The same power transmitted P, same losses PL and
same insulation level, we can determine the reduction
of conductor cross-section Ad over Aa.

12. Cont’d…
• Smaller Tower Size
o Since HVDC allows small size of
insulator and conductor to
transmit power over long
distance
o And also due to lack of a time
varying magnetic flux in dc
system so conductor spacing
and clearance is not that much
high due to the above reason
smaller tower size is required
for HVDC

13. AC

14. DC

15. DC

16. Cont’d…
• No skin effect
o Under a.c. conditions, the current is not uniformly
distributed over the cross section of the conductor. The
current density is higher in the outer region (skin effect)
and result in under utilisation of the conductor cross
section.
o Skin effect under conditions of smooth d.c. is completely
absent and hence there is a uniform current in the
conductor, and the conductor metal is better utilised.

17. Cont’d…
• Less corona and radio interference
o Since corona loss increases with frequency (in fact
it is known to be proportional to f),
o For a given conductor diameter and applied
voltage, there is much lower corona loss and hence
more importantly less radio interference with d.c.

18. Cont’d…
• No Stability Problem
o There is no voltage phase angle in dc
o There is no frequency in dc
o There is no need of synchronization
o But, In a.c. links the phase angle between sending
end and receiving end should not exceed 30o at full-
load for transient stability (maximum theoretical
steady state limit is 90o).

19. Cont’d…
• Asynchronous interconnection possible
o With a.c. links, interconnections between power systems
must be synchronous.
o Thus different frequency systems or different voltage
systems cannot be interconnected.
o Such systems can be easily interconnected through hvdc
links.
o For different frequency interconnections both convertors
can be confined to the same station.

20. Problems Associated with HVDC
1. Expensive convertors (More than double of transformer station)
2. Reactive power requirement (50% of the DC link active power capacity)
3. Generation of harmonics (Need extra investment for harmonic filter)
4. Difficulty of voltage transformation
5. Difficulty of high power generation (this also the challenge of AC)
6. Absence of overload capacity (Convertors have very little overload
capacity unlike transformers)

21. Economic Comparison
• The hvdc system has a lower line cost per unit length as
compared to an equally reliable a.c. system due to the
lesser number of conductors and smaller tower size.
• However, the d.c. system needs two expensive convertor
stations which may cost around two to three times the
corresponding a.c. transformer stations.
• Thus hvdc transmission is not generally economical for
short distances, unless other factors dictate otherwise.

22. Cont’d…
• Estimates for the break even
distance of overhead lines are
around 500 km to 800km.
• This value depending on the
magnitude of power transfer
and the range of costs of lines
and equipment.
• The breakeven distances are
reducing with the progress
made in the development of
converting devices.

23. Cont’d…
• For cables, the break-even distance is much
smaller than for overhead lines and is of the order
of 25 km for submarine cables and 50 km for
underground cables.
• For a long cable connection, e.g. beyond 40 km,
HVDC will in most cases offer the only technical
solution because of the high charging current of
an AC cable.

25. Environmental Concern
• The land coverage and the associated right-of-
way for an HVDC overhead transmission line is
not as high as that of an AC line.
• This reduces the visual impact and saves plant
deforestation and impact on cultural heritages.
• It is also possible to increase the power
transmission capacity for existing rights of way.

26. AC/DC Conversion Fundamental
• Basically for practical Application of HVDC
transmission the following converter bridges
are used:
o12-pulse converter
o18-pulse converter and,
o24-Pulse Converter
• But those converter are fundamental blocks of
6-pulse converter

27. Six-pulse Converter Bridge and Their Input-Output

28. DC-Output Voltage of 6-Pulse Bridge
• If E is the r.m.s, line-to-line voltage, then the dc voltage output is :
𝑉𝑎𝑣𝑒 = 𝑉𝑑𝑐 =
1
𝑇 0
𝑇
2 ∗ 𝐸 sin( 𝑤𝑡)𝑑(𝑤𝑡)
𝑉𝑎𝑣𝑒 = 𝑉𝑑𝑐 =
6
2𝜋 𝜋
3
2𝜋
3
2 ∗ 𝐸 sin( 𝑤𝑡)𝑑 𝑤𝑡
𝑉𝑑𝑐 =
3 2
𝜋
𝐸 = 1.35𝐸 =
3 3 ∗ 2
𝜋
∗ 𝐸 𝑝ℎ𝑎𝑠𝑒 = 2.34𝐸 𝑝ℎ𝑎𝑠𝑒

29. HVDC links can be broadly classified into:
Monopolar links
Bipolar links
Homopolar Link
HVDC System Configurations
29
29

30. Monopolar Link
 It uses only one conductor.
 Due to negative polarity, no corona effect occurs.
 For low transmission capacity,
 May have ground electrode or dedicated metallic
return path

31.  A bipolar links has two conductors, one positive and the other negative with
respect to earth.
 The mid-points of converters at each terminal station are earthed via electrode
lines and earth electrodes.
 The voltages between the conductors is equal to two times the voltage between
either of the conductors and earth.
 Since one conductor is at positive polarity with respect to earth and the other is
at negative polarity with respect to earth, a bipolar HVDC system is described
as say .500kV
 A bipolar system is advantageous in the sense that when one pole goes out of
operation, the system may be changed to monopolar mode with ground return.
Thus, the other pole continues to supply half the rated power through ground
return.
 Bipolar links are most commonly used in all high power HVDC systems.
Bipolar Links

32. Bipolar Long-Distance Transmission Schemes
• A Bipole is a combination of
two poles in such a way that a
common low voltage return
path, if available, will only
carry a small unbalance current
during normal operation.
• This configuration is used if the required transmission
capacity exceeds that of a single pole.

33. Cont’d…
• During maintenance or outages of one pole, it is still
possible to transmit part of the power.
• More than 50% of the transmission capacity can be
utilized, limited by the actual overload capacity of the
remaining pole.
• The advantages of a bipolar solution over a solution
with two monopoles are reduced cost due to one
common or no return path and lower losses.

34. Bipole with Ground Return Path

35. Bipole with Dedicated Metallic Return Path
• If there are restrictions even
to temporary use of
electrodes, or if the
transmission distance is
relatively short, a dedicated
LVDC metallic return
conductor can be considered
as an alternative to a ground
return path with electrodes.

36. • It has two or more conductors all having the same polarity, usually
negative.
• Since the corona effect in DC transmission lines is less for negative
polarity, Homopolar link is usually operated with negative polarity.
• The return path for such a system is through ground.
Homopolar Links
36
36
 A homopolar link is shown in Fig. It has two conductors of the same
polarity (usually negative), and always operates with ground or metallic
return. This system is not used presently.