The power system is dependent of a stable and reliable control of active and reactive power to keep its integrity. Loosing this control may lead to a system collapse. Voltage Source Converter transmission system technology such as HVDC Light™ has the advantage of being able to almost instantly change its working point within its capability curve. This can be used to support the grid with the best mixture of active and reactive power during stressed conditions. In many cases is a mix of active and reactive power the best solution compared to active or reactive power only. VSC transmission systems can therefore give added support to the grid. Some small ‘text-book’ grid examples are used to show that for asynchronous infeed, active power modulation damp ~4 times better than reactive power modulation and that local loadability can increase with ~2 times installed converter MVA size. In a parallel case where the VSC transmission system is connected in parallel with the AC system, the VSC transmission system can damp ~2-3 times better than reactive shunt compensation and increase loadability ~1.5 times installed MVA converter size. The benefits with a VSC transmission system during a grid restoration can be considerable since it can control voltage and stabilize frequency when active power is available in the remote end. The frequency control is then not limited in the same way as a conventional power plant where boiler dynamics may limit the operation during a grid restoration. Since ABB introduced its Voltage Source Converter DC-transmission system ‘HVDC Light’ in Hellsjön 1997, the rating has increased 100 times to 330 MW presently operating in the Cross Sound cable connection. A number of projects has now been commissioned and are showing good operating experience. The transmission capacity and converter sizes of a VSC transmission system are becoming large enough to also play a role in system stability improvement. Different applications will make good use of the high controllability of active and reactive power considerably improving system stability. Three basic grid configurations will be discussed, parallel AC and DC systems, series connected AC and DC systems and asynchronous connection of a DC system between two AC systems.

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