H.V.D.C TRANSMISSION

1. The HVDC converter

1. Does not consume reactive power
2. Consumes as much reactive power as real power
3. Consumes 50% of the real power

Ans. c

2. A back to back HVDC link can be advantageous compared to AC primarily because

1. It is cheaper
2. Of stability considerations
3. Of controlled power glow

Ans. c

2. A 12-pulse bridge is preferred in HVDC because

1. It eliminates certain harmonics
2. It results in better power factor
3. Series connection of converters on D.C. side is better

Ans. a

2. Fault on a two terminal DC link is removed by

1. Breakers on DC side
2. Breakers on AC side
3. Current control of converters

Ans. c

Earliest inventions/discoveries and the first practical applications were with DC. AC however gained complete ascendancy very fast in the beginning of the 20 th century with development of transformer, synchronous generator, and induction motor.

Revival of interest in DC was generated because of the following advantages of DC:

• No stability limitations with long distance DC transmission
• No charging current-much longer distance underground cable transmission possible with DC

Although these advantages were known, the spread of HVDC was limited due to non-availability of reliable, maintenance free, high voltage, high current, controlled rectification device. After the advent of high power thyristors in the seventies, the spread of HVDC has been phenomenal

• Monopolar
• Bipolar
• Homopolar
• Multiterminal

Negative polarity in monopolar and homopolar is used to reduce communication interference. Bipolar is most widely used due to its higher reliability and absence of ground currents.

Almost all applications so far have been with two terminal DC links. These do not require HVDC circuit breakers. Multi-terminal systems require HVDC circuit breakers, which have become commercially available

Sketch and show the major components

• 12-pulse converter
• transformer
• smoothing reactors
• DC filters
• Ac filters

12-pulse, 3-phase, Graetz Bridge supplied from two transformers connected in Y-Y and Y-delta. Parallel connection on AC side and series connection on DC side.

Special air or liquid cooling is used. De-ionised water-cooling is quite common. Ratings are limited by short-circuit current rather than steady-state loading.

Valve firing uses fibre-optic light guide system. Valves are protected using snubber circuits, protective firing and gapless surge converters. Microprocessor based control & monitoring system is used.

Different configurations. Connections. Valve-side star or delta with ungrounded neutral, AC side star with grounded neutral.

Transformer must withstand DC voltage stresses and higher eddy current losses due to harmonics. Dc core magnetisation due to unsymmetrical firing is also a problem.

Converter control results in harmonic generation on AC and DC sides. AC filters remove AC harmonics while supplying much needed reactive power. Converter control results in substantial reactive consumption at both ends (50-60 % of active power).

Large series reactor maintains smooth rectangular current waveform and also protects against transient current fluctuations.

The Dc switchgear is modified AC equipment for interruption of small DC currents

1. Current limit

 

Long overhead line -other limits are reached first.

Long distance underground cable transmission is difficult due to excessive line charging, leaving little margin for the normal load current.

2. Voltage limit

 

With DC, switching surges are lower. Corona under foul weather conditions is a limiting factor in AC transmission. Losses and radio interference associated with corona are a matter of concern. In DC foul weather reduces RI associated with corona

3. Reactive power & voltage regulation

 

Excessive reactive power consumption and drop in voltage is associated with long distance AC transmission when loading is above he surge impedance loading, (Economic loading on overhead lines is always above SIL). Ferranti effect is another problem associated with long AC lines. Hence, long distance AC transmission is feasible only with the use of series & shunt compensation.

4. Stability

This is the most serious limitation of long distance Ac transmission. Series compensation (35-50%) is always necessary. Even then transmission distances are limited. No such problem with Dc since it is an asynchronous link.

5. Short circuit currents

AC interconnections result in tremendous increase in the fault level. DC links do not increase the fault level. Fault currents on DC line are restricted due to fast thyristor control.

6. Power per conductor per circuit

3-phase, 3-conductor Ac line carries the same power as a bipolar DC link9for the same current and insulation level0. Reliability of the DCline is equal tothat of double circuit AC line. DC line is simpler, cheaper, and more efficient and requires lesser right of way. With underground cables, the ratio of DC to AC power per cable is 5-10.

7. Ground return

This is feasible with DC. It is not feasible with Ac due to higher impedance and interference with communication circuits

 

8. Harmonics

Converters in Dc link generate harmonics on AC and DC sides. Elaborate filtering is required.

9. Circuit breakers

These are not required with two-terminal Dc links. Fast current control limits the fault current. However multi-terminal links require breakers. The problem with DC breakers is the absence of current zero. Currently, high voltage DC breakers are available.

10. Control of line power

This is easy because very simple and fast control of DC links is possible through communications and firing angle control. In AC, it is difficult.

11. Pollution

This affects DC link more than AC link.

• Submarine crossings
• Interconnection of systems with different frequency
• Long distance overhead lines
• Underground transmission
• Back to back ties for interconnection without affecting the fault level and independence of operation