HVDC Transmission System

HVDC Transmission

In recent trends in power system power transmission through HVDC (High Voltage DC) transmission line are also used. Dc transmission is an effective means to improve system performance. In Dc Transmission power transferred from one station to other station is governed only by the magnitude of terminal dc voltage of two ends due to this reason controllability of HVDC power transmission system is fast and sable. In India, the first 500 kV HVDC overhead transmission line is 800 km long in between Rihand and Delhi. In India, the highest transmission voltage level is reached 800kV. Dc transmission requires converter at each end of the line. The sending end converter acts as a rectifier converting AC into DC and the receiving end converter acts as an inverter converting DC to AC.

 

Basic Principle of Operation of HVDC Transmission System

As we know that the generation of power universally is AC in nature. So for Dc transmission it requires to convert in DC. So we can say that DC Transmission requires converter at each end of the line. The Sending end converter acts as a rectifier converting Ac to Dc for transmission and the receiving end converter acts as inverter converting Dc to AC for distribution. The sending end converter (rectifier) is fed from an AC source through the Step-Up Transformer and receiving end converter (inverter) feeds AC load through Step-down Transformer. The output voltages of each end converters are controlled by varying the firing angle of the thyristor valves of converter.

Comparison of HVAC and HVDC Transmission

HVAC Transmission	HVDC Transmission
Three conductors are required for 3-phase power	Only two conductors are sufficient
Number of conductors requires is more and also number of towers required to carry three conductors is more. So over long distance it is economically expensive.	HVDC transmission system is economically cheaper for high power transmission above 800 MW as compared to HVAC transmission.
In HVAC Transmission voltage is constant current is variable depending on the load.	In HVDC Transmission current flowing the line is constant voltage is variable.   For maintaining constant current through the line current source converter are used.
In AC transmission system power transferred is governed by magnitude of voltages as well as phase difference between voltages of each end.	In HVDC transmission system power transferred is governed only by the voltages of each end.
Skin effect is present in HVAC transmission	There is no skin effect in HVDC transmission
Corona power loss is more because of frequency is 50Hz	Corona power loss is less
In HVAC transmission system, transmission line has inductive and capacitive components so it is required to supply reactive power to line.	In HVDC transmission system, transmission line has only resistive component so that there is no need of reactive power supply.
In HVAC transmission system, only possible to connect two systems operating at same frequency	In HVDC transmission system by using back to back connection it is possible to connect two different frequencies system.
Ferranti effect occurs.	There is on Ferranti Effect.
In HVAC transmission system, if fault occurs in one of the phase of three phase line then all the circuit breakers have to be open and isolate the faulty part.	In HVDC transmission system if fault occurs in one conductor then other conductor can supply half of the rated power by using bipolar link.

Advantages of HVDC Transmission System over HVAC Transmission System

There are many advantages of DC transmission system over AC transmission which are listed below.

• HVDC Transmission Systems are economical cheaper for long distance bulk power transmission over ac transmission because it requires only two conductors for power supply.

• In Dc transmission system ground return is possible.

• Losses are less in case of DC transmission system as compared to ac transmission system.

• There is no charging in case of DC transmission system because there is no reactive component in DC transmission.