Research On Potential Transformer Damaged And Influence Factors In Distribution Network

Potential transformer is the electrical equipment connecting the primary systemand the secondary system. It installed in every grid nodes and provides voltageinformation of the primary system to the secondary system for monitoring, control andregulating. In China, it doesn’t trip immediately and can be allowed to continue to runtwo hours in isolated neutral system when single-phase grounding fault happens. Fromcurrent research, the cause of potential transformer damage is the L-C seriesferro-resonance over current of potential transformer inductance and the capacitance ofthe power grid. And some methods of resonance suppression are taken such as installinga harmonic eliminator at the potential transformer. However, in recent years, thisaccident still occurs frequently with the development of the grid, even the potentialtransformer and its harmonic eliminator are damaged together. So researching thedamage mechanism and suppression measures in isolated neutral power system whensingle-phase grounding fault happens is an important research subject.In this paper, theoretical analysis, dynamic simulation experiment andmathematical simulation are used to research potential transformer abnormal damage inisolated neutral power system on the basis of the present structure of distribution system.The main factors influencing the potential transformer heating are found, and themeasures suppressing potential transformer damaged are studied. Specific researchcontent is as follows:①The series model and parallel model of potential transformer is comparativelyanalyzed, which points out that the series model derivation has fault and is hard toestablish. The L-C series ferro-resonance over current is not the main reason causingpotential transformer abnormal damage. So it proposed parallel model to analyze thetransient state to seek the mechanism of the potential transformer damage, proposedtransient over current leading to the potential transformer thermal stability destruction,and found the main factors influencing the potential transformer heating: the mainreason of potential transformer damage in isolated neutral power system is system stateswitching frequently when intermittent single-phase grounding fault. Due to theelectromagnetic potential transformer is nolinear, the intermittent single-phasegrounding fault makes core into saturation zone. The magnetizing rush current in themoment grounding fault occurring and the capacitance current after grounding fault disappearing act on the potential transformer winding, bringing huge heat may make thepotential transformer damage.②The static characteristic experience and transient characteristics experience oftwo kinds of potential transformer in production site show: The stable single-phasegrounding won’t produce a lot of steady current, won’t make potential transformerdamage. However, potential transformer transient impulse current can reach more thanhundred times of the rated current. With the inrush current surging repeatedly, it leads tothe fuse even potential transformer thermal stability destruction.③According to the results of dynamic simulation experiment, the factorsinfluencing the transient inrush current are analyzed, and the simulating calculation ofpotential transformer is made. The potential transformer heating is calculated duringsingle-phase intermittent grounding fault occurring at different kind of situationaccording to change the value of each factor. The intuitive analysis and varianceanalysis of calorific value are studied using the method of orthogonal design, and foundthe influential factor of heat——the length of system line. The longer the system line is,the greater the inrush current is after fault disappearing, the harder the influence forpotential transformer is.④The theoretical analysis and dynamic simulation experiment are made to studythe measures of restraining the damage of potential transformer: it can avoid thepotential transformer abnormal damage effectively by improving the power gridoperation mode, reducing the partition capacitance and installing arc suppression coil atgrid neutral point. The potential transformer manufacturing specification in isolatedneutral power system should be completed: checking the potential transformer thermalstability according to single-phase intermittent grounding fault in isolated neutral powersystem. It can reduce the probability of potential transformer damage.