1. A REPORT
HIGH VOLTAGE DC
TRNSMISSTION
SYSTEM
Submitted in the partial fulfillment of
Bachelor of Technology
Jodhpur National University, Jodhpur.
Session (2011-2012)
Guided By: - Submitted By:-
Mrs sidhika gupta
Asst.prof. (EE) Suthar Vishnu (09ET403231)
DEPARTMENT OF ELECTRICAL ENGINEERING
FACULTY OF ENGINEERING & TECHNOLOGY
JODHPUR NATIONAL UNIVERSITY, JODHPUR (RAJ).

2. HIGH VOLTAGE DC TRANSMISSION
INTRODUCTION:-
The first commercially used HVDC link (20mv, 100kv) in the
world was built in 1954 between the mainland of Sweden and the
inland of Gotland. Since then the technique of power transmission
by HVDC has been continuously developed. In 1970 thyristor
valves replaced the valves based on the mercury-arc technique. To
date biggest HVDC transmission is ITAIPU in Brazil (two-
bioples, 6300 MV and 300kv). DC transmission is an effective
means to improve system performance. It is mainly used to
complement AC systems rather than to displace these. In India, the
first HVDC line is Rihand-Delhi (500kv, 800mv). Global HVDC
transmission capacity has increased from 20mv in 1954 to 17.9 GV
in 1984. The largest device rating is now in the range of 5kv, 3 ka.
The highest transmission voltage reached is 600kv. Now the
growth of transmission capacity has reached an average of 2500
MW/YEAR.
PRINCIPLES OF AC/DC CONVERSION:-
HVDC transmission consists of two converter stations
which are connected to each other by a DC cable or an overhead
DC line. A typical arrangement of main components of an HVDC
transmission.
Two series connected 6-pulse converter (12-pulse bridge)
consisting of valves and converter transformers are used. The
valves convert AC to DC, and the transformer provide a suitable
voltage ratio to achieve the desired direct voltage and galvanic
separation of the AC and the DC systems. A smoothing reactor in
DC line, and possible transient overcurrents. Filters are used to
take care of harmonic generated at the conversion. Thus we see

3. that in an HVDC transmission, power is taken from one point in an
AC network, where it is converted to DC in a converter to DC in a
converter station (rectifier), transmitted to another converter station
(inverter) via line or a cable and injected into an AC system.
HARMONICS:-
The AC/DC converter is a source of harmonics on AC as well AS
dc sides. In order to reduce harmful effects of harmonics on AC
side, shunt filters are installed. At fundamental frequency, the
filters act as shunt capacitors supplying reactive power to the
converter. A smoothing reactor is installed to limit harmonics on
the DC side.
In addition to reactive power consumption by converters,
converter transformers also consume reactive power. Considering
normal values of rectifier and inverter, the reactive power demand
usually is the range of 50-60% of the transmitted active power.
This fig. is for each converter station.
The reactive power may be supplied from:
(1) AC filters
(2) Shunt capacitors
(3) Excessive reactive power from AC network
(4) Static compensators(for fast voltage regulation) &
(5) Synchronous condensers(if AC network is weak)
PRINCIPLES OF HVDC CONTROL
One of the most important aspects of HVDC systems
is its fast and stable controllability. In DC transmission, the
transmitted power is proportional only to the difference in terminal
DC voltage between the two ends, and hence the transmitted power
can be rapidly controlled by changing the DC voltages. The current

4. in the system can only flow in one direction in fig.1 for a given
setting. Power is transported from rectifier to inverter and by
altering the voltages; the power flow direction is reversed.
In an HVDC transmission, one of converter stations,
generally is inverter station, is so controlled that the direct voltage
of system is fixed and has rigid relation to the voltage on the AC
side. Tap changers take care of the slow variations on the AC side.
The other terminal station(rectifier) adjusts the direct voltage on its
terminal so that current is controlled to the desired transmitted
power.
TYPES OF HVDC LINK SYSTEM
(1) Mono polar system
(2) Bi-polar system
(3) Homo polar system
(4) Back to back system

5. (1) MONO POLAR SYSTEM
 This link has only one conductor usually of negative polarity and
uses ground or Sea water as the return conductor.
 The negative polarity is preferred on over head linkes due to lesser
redio interference,
In fig shows a mono polar link
(2) BI- POLAR SYSTEM:-

6.  This link has two conductors one positive and the other negative at
each terminal two convertors of equal rated voltage are connected in
series on DC side.
 The neutral point (I.E- the junctions between converter) are
grounded.at One or both end. If both the neutrals are grounded. The
two poles operated independently.If the current in the two conductors
are equal. The ground conductor is zero.
 If a fault develops on one conductor the other conductor(along with
ground return can supply half of the rated load . the rated voltage of
aBi-polar link is
Expressed as ±500 KV
(3)HOMO POLAR SYSTEM:-
 A Homo polar link has two (or More) conductors all having the same
polarity (usually negative) and always operates with ground as
the return conductor. If a fault develops on one conductor.
 The converter equipment can be reconnected so that the healthy
conductor(having some overload capacity) can supply more than
half the rated power. Such a reconnection is very complicated in a bi-
polar scheme and therefore, a homo polar scheme provided
continuous ground return does not pose additional problems. Fig
shows a homo polar link.

7. (4) Back to back
 This does not require any d.c transmission and a.c lines terminate
on the Rectifier and Inverter which are connected back to back.
 This system used to couple a.c system of different friquency or
Different system controls.

8. ECONOMIC CONSIDERATIONS:-
Consider an AC line and a DC line employing the same number of
conductors and insulators. Let us compare the power per conductor
on the two lines. If in each case the current equals the RMS
alternating current. Assume also that the insulators withstand the
same peak voltage is 1.414 times the RMS AC voltage.
HVDC APPLICATION:-
The following modes of implanting a DC link in a predominant
AC system may be used:
(1) Interconnection of systems of the same frequency through a
zero length DC (back to back connection): This does not require
any DC transmission line and AC lines terminate on the rectifier
and inverter which are connected back to back fig.3 A typical
example is the Eel river scheme in Canada connecting the Quebec
hydro system with that of new Brunswick.
(2) HVDC links are used to evacuate power from the remote super
power stations to the load centre several hundred kilometers away.
If there are faults in the AC network, this will not trip the units at
the power station since the asynchronous DC link insulates the
power station from the AC system.
(3) Interconnection between power systems or pools: For smooth
interchange of power between neighbouring grids irrespective of
voltage and frequency fluctuations, such link ensure retention of
the tie under the most stringent conditions of the constituent grids.

9. (4) High power underground distribution system feeders: Here it is
found that DC may be cheaper at distances greater than
approximately 50 km with a power level of 1000-2000 MV. With
AC we need forced cooling due to the higher amount of heat
produced. Also there are increased dielectric losses at EHV AC.
(5) Stabilizing AC system by modulating DC power flow.
ADVANTAGES OF HVDC SYSTEMS:-
The advantages of the HVDC systems are as under:
(1) These systems are economical for long distance bulk
power transmission by overhead lines.
(2) There is greater power per conductor and simpler line
construction.
(3) Ground return is possible.
(4) There is no charging current and skin effect.
(5) The voltage regulation problem is much less serious for
DC, since only the IR drop is involved. For the same
reason steady state stability is no longer a major problem.
(6) The DC line is an asynchronous flexible link and it can
interconnect two rigid systems operating at different
frequencies.
(7) There is easy reversibility and controllability of power
flow through a DC link.
(8) For a singe DC line between two converter stations, circuit
breakers are unnecessary since control of the converters
can be used to block current flow during faulted
conditions.
(9) Each conductor can be operated as an independent circuit.
(10) Smaller amount of right of way is required. The distance
between two outside conductors of a 400kv AC line is

10. normally 20m, whereas the same between a corresponding
DC line is roughly half, i.e. 10m only.
(11) There is considerable insulation economy. The peak
voltage of the 400kv AC line is 1.41*400=564kv. So the
AC line requires more insulation between the tower and
conductors, as well as greater clearance above the earth as
compared to corresponding 400kv DC line.
(12) There is no technical limit to the distance over which
power may be transmitted by lines or cables because of
absence of both charging current and stability limitations.
(13) Line losses are smaller.
(14) It is possible to bring more power into an AC system via a
DC link without raising the fault level and circuit breaker
ratings.
(15) No reactive compensation of DC line is required.
(16) Corona loss and ratio interference are less as compared to
AC.
(17) HVDC line and HVDC link can be used in parallel as an
AC-DC system.
(18) The contribution of HVDC link to SCC of AC system is
considerably less as compared to that of an alternative AC
link.
(19) DC cables can be worked at higher voltage gradient.
(20) Low AC current is required on DC line.
DISADVANTAGES OF HVDC SYSTEMS:-
(1) The systems are costly installation of
complicated converters and DC switchgear is expensive.
(2) Converters require considerable reactive power.
(3) Harmonics are generated which require filters.
(4) Converters do not have overload capability.

11. (5) There is nothing like DC transformer which can change
the voltage level in a simple way. Voltage transformation
has to be provided on the AC sides of the system.
(6) Reactive power required by the load is to be supplied
locally as no reactive power can be transmitted over a DC
link.
FUTURE TRENDS:-
Considerable research and development work is under way to
provide a better understanding of performance of HVDC links to
achieve more efficient and economic designs of the thyristor
valves and related equipment and to justify the use of alternative
AC/DC system configurations.
Future power systems would include a transmission mix of AC and
DC. Future controllers would be more and microprocessor based,
which can be modified or upgraded without requiring hardware
changes, and without bringing the entire system down. While one
controller is in action the duplicate controller is there as ‘hot
standby’ in case of a sudden need.