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Technologies > HVDC Converter TypesHVDC systems use electrical converters in order to switch from DC to AC and vice-versa. There are two main types of energy converters used, the Current Source Converters (CSC) and the Voltage Source Converters (VSC). Both of these converter types will be analyzed and ultimately we will conclude that VSC-HVDC is not only a competitor to CSC-HVDC, but it is the only possible solution for multi-terminal systems. Up until recently multi-terminal systems were not attractive for renewable energy interconnects or for energy distribution, but nowadays their use is being explored.
During the 20th century, HVDC systems used exclusively current source converters, while at that point VSC technology did not exist. Things changed in the 90s when insulated gate bipolar transistors (IGBT) which were a new type of self-commutation high power switches became available. IGBTs are three-terminal power semiconductor devices. They offer both high efficiency and fast switching, two factors which are of great importance for HVDC power systems. In addition, they allow the system to be much smaller compared to the use of other more conventional devices. CSC systems could not benefit from the new IGBTS and hence became inferior to VSC-HVDC.
Both converter
types act differently on the AC and the DC side. More specifically, CSC act as voltage
sources and requires major AC filtering in order to eliminate the harmonics which
exists in the AC side of the system. VSC acts as a constant current source with
no need for large filters and reactive power supply. Their roles are reversed when
we move on to the DC side. VSC uses a capacitor to store energy and due to this
capacitor no additional DC filtering is required. This is not the case for CSC.
Another advantage of VSC-HVDC systems is that the power flow in VSC-HVDC can simply be changed by changing the direction of the current, while in the case of CSC the DC voltage polarity had to be altered which is hard to achieve. Therefore it is clear that multi-terminal systems are only feasible with the use of VSC technology.
CSC point to point transmission does not require HVDC circuit breakers, since AC circuit breakers can be used instead. Multi-terminal systems though, which use VSC technology, will require HVDC circuit breakers so that the whole system does not have to shut down.
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