The presence of negative-sequence current in a generator, due to system imbalance, induces double-frequency currents in the rotor iron. These currents may cause rotor overheating and serious damage if permitted to persist. Negative-sequence current relaying practices that alarm or trip the generator before damage can occur are well established in the power industry. However, tripping major generation facilities may create social, security, and economic problems. In addition, for an isolated system with relatively poor frequency regulation, the combination of negative-sequence current and frequency perturbation may cause supersynchronous resonance in turbine blades if one of the mechanical resonance frequencies of the turbine blade is close to double the system frequency (60 Hz). This paper presents a method for correcting the problem at source by reducing the negative-sequence current through a specially designed delta-connected static var compensator (SVC). The derivation of the mathematical model and computer simulations verify and explain the capability of this approach. In addition, a scaled-down model is implemented in a physical simulation laboratory. Some laboratory test results are also presented. © 1996 Elsevier Science S.A.
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