Advantages and Disadvantages of HVDC Transmission

HVDC Power Transmission Line
Fig: Schematic Line Diagram of HVDC Power Transmission

What is High Voltage DC Transmission?

Generally, HVDC (high-voltage direct current) transmission requires converter station at both sending end and receiving end. The converter stations are transformers and thyristor valves. At sending end thyristor valves to act as a rectifier to convert ac into dc which is transmitted over the line, whereas at the receiving end thyristor valves act as an inverter to convert dc into ac which is utilized at receiving end. Each converter can function as rectifier or inverter. Thus power can be transmitted in either direction. The figure above shows the schematic line diagram of a typical HVDC power transmission line.

They also have some advantages compared with ac systems:

1. they can link two ac systems operating unsynchronized or with different nominal frequencies, that is 50 ↔ 60 Hz;

2. they can help instability problems related to sub-synchronous resonance in long ac lines;

3. they have very good dynamic behaviour and can interrupt short-circuit problems very quickly;

4. if the transmission is by submarine or underground cable, it is not practical to consider ac cable systems exceeding 50 km, but dc cable transmission systems are in service whose length is in hundreds of kilometres and even distances of 600 km or greater have been considered feasible;

5. reversal of power can be controlled electronically by means of the delay firing angles α; and

6. some existing overhead ac transmission lines cannot be increased. If overbuilt with or upgraded to dc transmission can substantially increase the power transfer capability on the existing right-of-way.

Why Choose the HVDC Rather Than HVAC for Power Transmission Line?

There are different major problems associated with EHV transmission line as follows:

Corona loss and Radio Interference : 
Generally, corona appears in High voltage transmission line which not only a source of power loss but it is also a source of the interference with radio and television.

Heavy supporting structure and erection difficulties:- 
Generally EHV –AC transmission uses bundle conductor, Which results in large mechanical loading on the tower. Similarly, large air and ground clearances, dynamic forces due to broken conducts etc makes large mechanical load. Hence the strength of the tower should be heavy.

Insulation requirement:- 
Generally, EHV – AC transmission requires high insulation to withstand the voltage surges due to internal sources i.e. switching operation or due to external sources i.e lightning etc. which produces very high voltage generally 2-3 times of normal voltage. Hence insulation level depends upon switching over-voltages, temporary over-voltages and atmospheric overvoltages.


Advantages of HVDC Power Transmission: 

HVDC transmission has many technical and Economics advantages over ac transmission as follows:

Cheaper in cost: Bipolar HVDC Transmission lines require two-pole conductors which is very cheap.

No skin effect: As there is uniform current distribution in dc so no skin effect in HVDC.

Lower transmission losses: HVDC Transmission system needs only two conductors and therefore the power losses in a dc line is less.

Voltage regulation: Due to absence of inductance it has better voltage regulation.

Line loading: Generally loading on AC line limited by transient stability limit and line reactance but no such limit in B. It also has no SIL with greater reliability.

Low short circuit current: It has no short circuit.

Lesser corona loss and radio interference: It has lesser corona loss due to zero frequency.

Higher operating voltages and no reactive power compensation: Generally B has high operating voltage with the absence of reactive power compensation.

No stability limit: There is no stability limit in HVDC transmission system.

Limitation of HVDC Power Transmission : 

HVDC Transmission system has the following limitations as follows:

Costly terminal equipment: The converters are used in HVDC are very costly along with the converters produce a lot of harmonics both on dc and ac sides, which requires filtering and smoothing equipment resulting in extra added expense. It also requires a complex cooling system and circuit breaker, which again adds cost.

More maintenance of line insulators: It requires more maintenance for in insulation. 

Circuit breaking in multi-terminal dc system is difficult and costlier.

Voltage Transformer: Voltage transformation is not easier in case of dc and hence it has to be accomplished on the ac side of the system. DC System can‟t be employed for distribution sub-transmission and backbone transmission. 

Interruptions in HVDC system transmission still occur rather frequently, although a slight decrease can be detected over the decades. Most of the HVDC system faults that cause outages do occur on converter stations, and although HVDC cable faults are quite rare, any such can be challenging to locate and time-consuming to repair. 

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