High Voltage Direct Current-HVDC

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The first commercially used HVDC link in the world was built in 1954 between the mainland of Sweden and island of Gotland. Since the technique of power transmission by HVDC has been continuously developed.In India, the first HVDC line in Rihand-Delhi in 1991 i.e. I 500 KV, 800 Mkl, 1000 KM. In Maharashtra in between Chandrapur & Padaghe at 1500 KV & 1000 MV. Global HVDC transmission capacity has increase from 20 MW in 1954 to 17.9 GW in 1984. Now the growth of DC transmission capacity has reached an average of 2500 MW/year.

ADVANTAGES OF HVDC SYSTEM

1. The cost of d.c. transmission line is less than 3 - phase a.c. line because only two
conductors are necessary for D.C. line.
2. Tower designs are simple.
3. The dielectric strength of cable is high .
4. The dielectric loss is low.
5. For D.C. overhead transmission lines length is unlimited.
6. Power transmission capacity is higher than a.c.
7. Corona & radio frequency interference losses are less.
8. HVDC link has accurate & quick control of power in the required direction.

LIMITATION OF HVDC TRANSMISSION

1. Transformer for step up ? step down voltages are not available in case of HVDC.
2. The terminal equipment is costly.
3. Reliable d.c. ckt. Breakers for higher ratings are not available.
4. Earth current may cause some side effects.
5. Reactive MVA cannot be transferred over a HVDC link.
6. Although inverters are used, the wave farm of output a.c. is not exactly sinusoidal and it contains harmonic distertion.

HVDC TRANSMISSION SYSTEM
In case of HVDC transmission, following systems are used :-
(i) Two pole one wire.
(ii) Two pole two wire.
(iii) Three pole two wire.
(iv) Three pole three wire.
The standard voltages used are :-
100 , 200, 300, 400, 600 & 800 KV.

The HVDC system is accepted for transmission of power for following reasons :
(i) for long distance high power transmission.
(ii) for interconnection between two a.c. systems having their own load frequency control.
(iii) for back to back a synchronous tie substations.
(iv) for under ground or submarine cable transmission over long distance at high voltage.
At present, HVDC links have been installed in the world upto the year 2001, 100 links are expected with a total transfer capacity of 75000 MW. The choice between 400 KV a.c. 705 KV a.c., 1100 KV a.c. and HVDC transmission alternatives is made on the basis technical and economic studies for each particular line and associated a.c. system although, alternating current system continuous to be used for generation, transmission, distribution & utilization of electrical energy.

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PRINCIPLES AC/DC CONVERSION

HVDC transmission consist of two converter stations which are connected to each other by a DC cable or DC line. A typical arrangement of main components of an HVDC transmission is shown in fig.

Two series connected 6 pulse converters (12-pulse bridge) consisting of valves & converters transformer 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 & DC systems. A smoothing reactor in the DC ckt reduces the harmonic currents in the DC line, & possible transient over currents. Filters are used to take care of harmonics generated at the conversion. Thus we see that in an HVDC in an HVDC transmission, power is taken from one point in an AC network, where it is converted to DC in a converter station ( rectifier ), transmitted to another converter station (inverter) via line or cable and injected in to an ac system.

By varying the firing angle & ( point on the voltage wave when the gating pulse is applied & conduction starts ) the DC output voltage can be controlled between two limits, +ve and negative. When a is varied, we get,
maximum DC voltage when ? = 0
0
.
Rectifier operation when 0< infinity < 90
0
Inverter operation When 90
0
< infinity < 180
0
While discussion inverter operation, it is common to define extinction
angle ? = 180
0
- infinity.

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