DC transmission is an effective means to improve dynamic system performance. HVDC transmission consists of two converter stations which are connected to each other by a DC cable or an overhead DC line.
By varying the firing angle (α), the DC output voltage can be controlled thereby controlling the power flow.
In DC transmission, the transmitted power is proportional only to the difference in terminal DC voltage between the two ends. Hence, the transmitted power can be rapidly controlled by changing the DC voltages.
Advantages of HVDC Transmission
- Charging current is absent.
- There is no skin effect.
- No proximity effect.
- A DC line requires no reactive compensation.
- Synchronous operation is not required.
- There is no stability problem with dc line.
- A DC tie line provides loose coupling between two AC systems to be interconnected.
- A DC link may interconnect two AC systems at different frequencies.
- There is no transfer of fault energy from one AC system to another if they are connected by a DC tie line.
- The current carrying capacity of HVDC cable is considerably large due to reduced dielectric losses.
- The corona power loss and radio interference are less as compared to AC transmission.
- Tie line power control is simpler for tie lines operating on DC.
- Insulation level of a DC line is lower than that of AC line and a DC line has two connectors instead of three for AC lines.
- Lighter supporting structure and lesser insulating devices are required, thereby reducing the right of way requirements. Hence, DC line construction is simpler and cheaper than AC line construction.
- Switching surges are lower than AC lines.
- Ground return can be used and therefore each conductor can be operated as an independent circuit.
Disadvantages of HVDC Transmission
- The cost of terminal equipment increases. Conversion & inversion equipment is to be provided at both the ends. At the same time, the reversible operation of these devices required conversion and inversion equipments add the cost of terminal station and this cost become a governing factor in the economic comparison of AC and DC systems.
- The terminal equipments require large MVAR. It generates harmonics which produces interference with communication lines.
- Filters are required to minimise the harmonics, thereby increasing the cost of terminal stations.