| With the increase of voltage level, CVT is more and more widely used in power grid recent years. As one of the most important characteristics, ferroresonance will cause great damage to CVT. Therefore, it is important to study the ferroresonance performance of CVT as well as the suppression methods.Earlier studies on CVT ferroresonance and its suppression methods are mainly conducted through theoretical and experimental methods. With the development of computer and electromagnetic transient computation software, simulation study is now becoming the widest and most efficient method. But most of the simulation studies of CVT are based on the simplified circuit model in which the hysteresis of intermediate transformer and saturated reactor damper and MOA's capacitive leakage current are ignored. Therefore, to reveal the most realistic ferroresonance performance, a circuit model is established considering the hysteresis of intermediate transformer and saturated reactor damper and the capacitive leakage current of MOA. Meanwhile, the simulation model is built using ATP-EMTP. The simulation waveform obtained from the model is proved to be credible by our experiment.Based on the simulation model, a series of simulation study and partial experimental study of ferroresonance performance of CVT are conducted. The result of the study on considering or neglecting the hysteresis and capacitive leakage current shows: hysteresis effect causes great impacts to ferroresonance of CVT, and it will increase the overvoltage and short circuit current and lead to worse ferroresonance; if considering the capacitive leakage current of MOA, the simulation waveform of secondary short circuit current of ferroresonance comes out with a small ripple which is proved to be highly coincident with the experiment.The result of the study on causes leading to ferroresonance and ferroresonance performance under different voltage shows: switch closing and secondary recovering after short circuit will lead to strong ferroresonance while primary recovering after short circuit will not; with the decreasing of voltage, the frequency of resonance decrease. For example, under the 1.2 times normal voltage, the frequency appears to be 1/3 of power frequency while it is 1/4 of power frequency under 0.8 times the normal voltage.For the most widely used ferroresonance suppression method now in which the saturated reactor damper and MOA is used, simulation and experimental study on selecting the resistance of damper are conducted and the results demonstrate that: a best resistance value(or within a small range) that can suppress the ferroresonance effectively is existed, not every value calculated theoretically; during the process of CVT designing and manufacturing, the resistance value range can be calculated theoretically first, then narrowed down by simulation study and finally determined by experiment.The results of the study on flux density of intermediate transformer and saturated reactor damper indicate that: increasing the flux density of intermediate transformer and decreasing the flux density of saturated reactor damper are helpful to suppress ferroresonance. Decreasing the flux density of intermediate transformer can make the transformer less saturated under the same overvoltage, which means a decreasing of ferroresonance frequency as well as ferroresonance energy. But in order to guarantee the accuracy class and fast suppression, the flux density of the damper is usually kept 0.9 times the saturated value under 1.2 times overvoltage.The work of this paper supplement and perfect the studies on circuit model of CVT, as well as the ferroresonance performance. Besides, the work of improvement on selecting the resistance value of damper is helpful to short the design cycle of CVT and save cost. |
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