Abnormal DC Overvoltage in STATCOM during Gating-Pulse Blocking: Its Mechanism and Countermeasures

DC overvoltage after blocking the gating-pulse of all GTOs in multipulse converter during system faults is observed in the development of STATCOM. Such an abnormal DC voltage, which may be as high as three times of rating DC voltage in the worst cases, results in serious damage to the devices, in particular, the DC capacitor bank. In order to research into its mechanism and explore ways of solving the problem, the authors carry out a thorough investigation. Analysis shows that such a steady-state overvoltage is due to the nonlinearity of the magnetic core of transformer. Based on the investigation, an effective and easily implemented approach is proposed as countermeasures. The validity of this method is verified by both simulation and field-test results.