High voltage DC transmission

It is well known that electric power generated in power plants is transmitted to the load centre on three phase ac transmission lines. However, for bulk power transmission over long distance, High voltage DC transmission line are preferred. First let we talk about full form of HVDC.

The additional cost of converting and inverting equipment makes HVDC transmission uneconomical for low power supply over short distance. However for large power transmission over long distance, HVDC turn out be economical. As a result, HVDC links are being used worldwide at power levels of several gigawatts with the use of thyristor valve.

How does HVDC system work ?

As we know that when AC power is generated which can be converted into DC by using the rectifier. so in end of the line there are two terminals inverter and rectifier. rectifier convert AC to DC while the inverter convert DC to AC.

The power transmitted remains same at the receiving end of the line. In substation system having more then two converter stations and one transmission line is called as a two terminal DC system. DC is transmitted over long distance because  it decreases the losses and improve the overall efficiency. If more than two converter and interconnecting DC terminal lines called as multi-terminal DC substation.
HVDC transmission possesses the following advantages over AC transmission system

Advantages of high voltage DC transmission :
  • Fault clearance in HVDC is faster, therefore DC transmission system possesses improved transient stability
  • Size of conductor in DC transmission can be reduced as there is no skin effect
  • Cost is less as compare to the AC transmission
  • HVDC tower are less costly 
  • No requirement of reactive power
  • No system stability problem
  • HVDC require less phase to phase and ground to ground clearance
  • Require less number of conductor for same power transfer
  • Improve line loading capacity
  • HVDC is preferred as it requires no charging current 
  • Power loss is reduced  with DC just because of fewer numbers of lines are required for power transmission
  • HVDC is more flexible system

Disadvantages of high voltage DC transmission :
  • Expenses inverters with limited overload capacity
  • HVDC is less relaible
  • IN HVDC very accurate and lossless power flows through DC link
  • The disadvantages of HVDC are in conversion, switching, control, mainatance
  • Lower availability than AC system
  • HVDC  is very complicted 
  • circuit breaker are used in HVDC for circuit breaking, and Inverter and rectifier terminals will generate harmonics which can be reduced by using active filters, which are also very expensive
  • HVDC does not have transformers for changing the voltage levels
  • Heat loss occurs in converters substation

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