Analysis And Restrain Measure Of Potential Transformer Transient Over-current In Ungrounded-neutral Power System

PT is an equipment realizing power grid measurement, monitoring, protection and control, its normal operation is the guarantee of power grid security and stability. But with the continuous development of the power system, abnormal damages of PT are serious in ungrounded-neutral power system, especially when unstable single-phase-to-earth fault occurs. Some research suggested that the abnormal damages of PT are caused by its transient over-current, which is created by PT saturation characteristics and system capacitance when single-phase-to-earth fault occurs, but perfect theoretical derivation and effective inhibition measures are not given. Therefore, it is necessary to study PT transient over-current and seek measures to restrain its abnormal damages.In this paper, PT transient over-current and its main influence factors in ungrounded-neutral power system are studied by theoretical analysis and digital simulation method, the restrain measures are also studied based on that. The main research contents are as follows:(1) PT transient over-current digital simulation system model is built in MATLAB/Simulink, by introducing PT nonlinear model with losses and intermittent arc-earth model. Considering PT losses and temperature rises, JDZX16-10 RG PT nonlinear model with losses is built, compared to JDZX16-10 RG PT no-load experiment results to prove the validity of the model. Using power frequency extinguishing arcing theory, only considering external volt-ampere characteristics of arc resistance, intermittent arc-earth digital simulation model is built.(2)Introducing PT nonlinear model with losses, PT transient over-current math analytic expression is deduced based on instantaneous symmetrical component method during single-phase-to-earth fault and the fault disappears in ungrounded-neutral power system, according to each sequence voltage change when single-phase-to-earth fault occurs and disappears. PT transient over-current obtained by digital simulation and the analytic expression is compared to prove the validity of system simulation model and theoretical analysis.(3)PT transient over-current influence factors during the single-phase-to-earth fault and the fault disappears are researched by digital simulation, including line length, system positive and zero sequence capacitances, PT saturation characteristics, the moment of single-phase-to-earth fault occurs and disappears. It's considered that more PT abnormal damages occur when system intermittent fault happens, so PT transient over-current is simulated when intermittent arc-earth fault occurs, by introducing intermittent arc-earth model. The influence of the system faults to PT is studied by comparison to get the characteristic parameter which can effectively reflect PT and system fault states.(4)According to theoretical analysis and numerical simulation results, PT transient over-current can be restrained by changing system zero sequence parameters and diminishing intermittent arc-earth fault. There are two methods to change zero sequence parameters: reducing system zero equivalent capacitances and consuming electric field energy stored by system zero capacitance. The research results indicate that, reducing system zero equivalent capacitances can obtain better suppression effect.