A high-voltage direct current (HVDC) is a transmission system which uses direct electric current (DC) to transmit electricity. It is more efficient over long distances distances than the usual high-voltage alternating current (HVAC).
The converter in HVDC system performs the task of converting AC electricity to DC electricity or vice versa.
Types of HVDC converters:
- Line-commutated converters (LCC)
- Voltage-source converters (VSC)
The transformer at the converter station ensures that the station is isolated from the AC system connected to the HVDC system. It also provides the correct voltage level which will be converted to DC voltage.
Because harmonics are always produced in electronic equipment, harmonic filters are used at the AC terminal of the converter. The DC harmonic filters can also be used at the DC line after the AC/DC conversion.
The electrode provides a ground return path for line current in monopolar configuration and unbalanced current in bipolar configuration.
In HVDC monopole configuration, one of the terminals of the rectifier is connected to earth ground. The other terminal is connected to the transmission line. The earthed terminal may be connected to the corresponding connection at the inverting station by means of a second conductor.
In HVDC bipolar configuration, a pair of conductors are used, each at a high potential with respect to ground in opposite polarity. The DC current flows via the positive pole and returns via the negative pole. The connection point between the positive and negative poles is grounded to the earth.
In this configuration, virtually no earth or neutral current is flowing. When one pole is broken, half of its transfer capability is still available.
A back-to-back HVDC station is a HVDC station in which both converters are located within the same building area where the length of the DC line is being kept as short as possible.
Advantages and disadvantages
- HVDC is more economic than HVAC to transmit large power over long distance
- HVDC line needs fewer conductors than HVAC line
- HVDC line uses thinner conductor for the same amount of power transmitted in HVAC line because HVDC line doesn't have skin effect
- HVDC system can transfer power between different AC frequency system
- HVDC underground or submarine line has no line capacitance like HVAC, hence needs no load coils
- HVDC system has lower availability than HVAC system
- The required converter stations for HVDC are expensive and have limited overload capacity
- At a shorter transmission line distance, HVDC converter station will have more loss than HVAC system for the same distance
- HVDC circuit breakers are harder to build than HVAC circuit breakers
- HVDC system operations require more spare parts to be kept than HVAC system
- HVDC technology changes faster and less standardized than HVAC technology
- Australia: Mainland Australia-Tasmania
- Canada: New England-Quebec
- China: Shanghai-Xiangjiaba
- Russia: Moscow-Kashira
- Sweden: Mainland Sweden-Gotland
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