Although DC transmission has so many advantages over AC, but it will never fully overtake AC transmission. The problems and Limitations of DC transmission are given below:
MAINTENANCE OF LINE INSULATORS: HVDC
Maintenance of insulators in HVDC line is more than AC line.
VOLTAGE TRANSFORMATION: HVDC
Voltage transformation is not so easy in case of Dc. Hence is has to be accomplished on the AC side of the system. DC system cannot be employed for Sub Transmission and Distribution.
VERY COSTLY TERMINAL EQUIPMENT AND INSTRUMENT: HVDC
- The converter required at both end of the line are much more expensive than the conventional AC equipment.
- The converters have very little overload capacity and absorb considerable reactive power . The reactive power can be as high as 25% of true power.
- The converter produce lot of harmonics both on DC and AC side which may cause interference with the audio frequency communication lines. Filtering and Smoothing equipment are provide at the converter stations in order to remove ripples from the DC output. It is also possible to provide filter on AC side to absorb the harmonic currents. Thus there is considerable increase of cost of the converter substation.
- No reactive power can be transmitted by HVDC transmission system and therefore, equipment such as static or synchronous capacitors are installed at the receiving end to generate reactive power ( MVAR ) according to the demand of the load and converting apparatus.
- HVDC converters need complex cooling system.
CIRCUIT BREAKER SYSTEM: HVDC
Circuit Breaker system comprises several components including the main circuit breaker capacitors, reactors etc, and the total cost is likely to be several times of an AC circuit breaker for equivalent rating.
Circuit breaking in multi terminal Dc system is difficult and costly.
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