Analysis And Countermeasure Study On Ferro-resonance Of CVT In Isolated Neutral System Of Shiping Substation

In the neutral node ungrounded power distribution system, ferroresonance due to the nonlinear excitation inductance of three-phase inductive potential transformer once happened, has a serious impact on the safety operation of power system. The inductive potential transformer is replaced with Capacitor Voltage Transformer (CVT) in some distribution system in order to avoid ferroresonance. But the resonance over-voltage phenomena and high side fuse of CVT often fusing still occurred when operating the system. For example, more than ten times unexpected high side fuse of CVT fuse in Shiping substation, which is result from ferroresonance through preliminary analysis. Therefore, researching on ferroresonance of CVT and resonance between the systems to find the reasons of high side fuse often fusing and giving some suggestions about the solutions are the key issue of the Shiping substation.Aimed at 500kV Shiping substation, the CVT transient model and the system model are established according to the working principle of CVT and the practical operation situation of Shiping substation. The MATLAB simulation software is used to research the ferroresonance of the system. The simulation results show the reasons why the high side fuse often fuses and some suggestions about this issue.At first, the operation features of Shiping substation and the records of high side fuse of CVT often fusing are introduced. The CVT transient equivalent circuit is built based on the principle of the CVT and from the point of view of input impedance phase-frequency characteristic, and the main reasons of CVT resonance are analyzed. Through the test data, the conversion method and the partition linearization method are adopted to show the nonlinear flux-current relations of intermediate transformer, compensation reactor and damper. The transient simulation model of CVT is established based on MATLAB software and simulations on several different excited operations are performed based on the simulation model, including: primary switching at different time, different initial capacitor voltage, secondary short and sudden elimination at different time.Secondly, the equivalent circuits of main transformer, bus bar, overhead line, reactor banks, capacitor banks and substation-used transformer are established based on the Shiping system structure and the system simulation model is built with the MATLAB. According to the records of high side fuse often fusing, simulation on the whole system is performed when 220kV transfer bus is connected to system. Simulation results show that this operation will result in series resonance of CVT branch and overflow of the primary side.Finally, the main reasons of unexpected fusing of CVT high side fuse are summarized and rational suggestions are presented to avoid the resonance based on the simulation results of CVT and Shiping substation system.