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IGBT with Power Transmission Systems

In a typical HVDC power transmission system, the power is transmitted at very high voltages (above 100-kV) in order to reduce the current on the cables. Large currents in cables require more copper which adds to the cost and weight. Since power semiconductor devices are unable to withstand such high voltages, it is necessary to connect many devices in series to satisfy the system requirements. In addition, for higher power levels, many devices may have to be connected in parallel as well. The series and parallel combination of power devices comprises an HVDC valve. The most common configuration for modern overhead HVDC transmission lines is bipolar because it provides two independent DC circuits each capable of operating at half capacity. Two basic converters topologies are used in modern HVDC transmission systems: conventional line-commutated, current-source converters (CSC) based up on thyristor-valves and self-commutated, voltage-sourced converters (VSC) based up on IGBT-valves. Each valve consists of a large number of series connected thyristors or IGBTs to sustain the desired DC voltage rating. In the case of current source converters with thyristor valves, a Graetz bridge configuration is used allowing six commutations or switching operations per period. Self-commutated, voltage-source converters using IGBTs are preferred because they allow independent rapid control of both active and reactive power. Reactive power can also be controlled at each end of the transmission line providing total flexibility in network design. The self-commutated, voltage source converters can be constructed using IGBTs without the snubbers required for GTOs. The rate of rise of the current in the IGBT can be controlled by tailoring the gate drive voltage waveform without any ancillary components. This allows controlling the reverse recovery of the anti-parallel rectifiers without the snubbers. The reduced passive components in the IGBT-based VSC inverters reduce system cost.