10kV Potential Transformer Damage Mechanism Research Caused By Single Phase Grounding

Potential transformer(PT), providing primary system voltage information tosecondary system to measure, monitor, and protect the power grid, is a very importantequipment to ensure the safe operation of power grid. It can be allowed to continue torun two hours in isolated neutral system when single phase grounding occurs. It is easyto cause the potential transformer damage in the meantime, so looking for the damagemechanism is an important research topic in power system.Most existing research shows that the cause of potential transformer damage is theL-C series ferroresonance of potential transformer and neutral point capacitance in therole of neutral-to-ground voltage. The smaller neutral point capacitance is, the moreresonance occurs. Therefore, take the installation of harmonic eliminator on thepotential transformer and other measures to eliminate resonance. However, in recentyears, the power grid operation shows that it still often occurs fusing high voltage fuseand even more PT burned frequently when the single-phase grounding occurs in powersystem with the development of the grid and line.According to the information obtained by the production site, the potentialtransformer damage is more accompanied by intermittent grounding phenomena. Basedon the parallel relationship of potential transformer and the grid capacitance, parallelmodel and preventive measures are proposed in this paper according to the transientanalysis to seek the mechanism of the voltage transformer damage.The dynamic simulation experiments and the potential transformer of production siteexcitation characteristic experiments show that the potential transformer excitationcurrent grows rapidly when the potential transformer port voltage crosses the linevoltage. The arc grounding simulation circuit of isolated neutral system is established inMATLAB to study the operating conditions of the potential transformer and currentimpact process. Analysis and calculations show that the operating point of the potentialtransformer core runs into the depth of saturated zone in the unstable single-phasetransient process. Thus produce a high multiple of over-current, resulting in a thermalstability of destruction and electrical breakdown owing to the square growth of heat.Simulation calculation and experiments prove that single-phase grounding currenthas nonlinear growth characteristics in isolated neutral system and with increase of thegrid capacitance, the current grows rapidly, which make ungrounded phase steady-state voltage higher than the line voltage, and the over-voltage multiples increases with theincrease of grid capacitance, which breaks the current linear growth and ungroundedphase under the line voltage in the traditional concept. Owning to ungrounded phasesteady-state voltage higher than line voltage, it will increase a significant over-currentmultiple in potential transformer of the depth of saturated zone during the transientprocess. Compared with the series resonant model, the parallel transient model showsthe opposite conclusion that the larger neutral point capacitance is, the more resonanceoccurs.Obtained by simulation and dynamic simulation experiments, arc suppression coilcompensation in isolated neutral system, can effectively reduce the potentialtransformer over-current multiples after single-phase grounding. Choosing betterexcitation characteristics potential-transformer to reduce the normal operation point ofthe potential transformer core and the transient over-current, and checking the potentialtransformer thermal stability according to the sustained transient over-current of arcgrounding is most fundamental measures to reduce the potential-transformer damagedby arc grounding in isolated neutral system.