Study Of Ferroresonance Caused By Electromagnetic Transformer In Medium Voltage Distribution Network

In medium voltage distribution network,in order to monitor the voltage of bus to ground,electromagnetic voltage transformer(PT)with star primary winding and directly grounded neutral point is usually installed on the bus.Due to the nonlinear characteristics of the excitation inductance of electromagnetic voltage transformer,under the disturbance of external voltage,it is easy to cause the core saturation,resulting in the decrease of the excitation inductance.It works together with the parameters of the ground capacitance in the medium voltage distribution network,resulting in ferromagnetic resonance and overvoltage in the distribution network.This kind of over-voltage may cause not only power supply interruption and some economic losses,but also serious equipment damage and personal injury.In the thesis,the basic principle of fundamental frequency ferroresonance is analyzed by graphic method for series ferroresonance circuit composed of nonlinear inductors and capacitors.The parameters range of ferroresonance of various types are summarized from the aspects of the frequency shift overvoltage,harmonic resonance overvoltage,frequency division and high frequency resonance caused by Pt saturation.Secondly,the nonlinear dynamics is used to analyze the simple system with the value of grounding capacitance as the variable,the occurrence of ferroresonance overvoltage in different cases is obtained,and the spectrum and phase trajectory in various cases are analyzed.Then,in the Simulink simulation environment,in order to study the resonance characteristics of multiple groups of Pt parallel operation,the simulation model of 10 k V neutral ungrounded system is built,the calculation method of transformer excitation characteristics and the program compiled according to the calculation are given,and the excitation characteristics of transformer are calculated.Taking the transient disappearance of single-phase ground fault as the trigger condition,the characteristics of different types of ferroresonance overvoltage and overcurrent are simulated and analyzed,and the law of electrical quantity when ferroresonance occurs is summarized.Finally,based on the electrical characteristics of fundamental frequency resonance and fractional frequency resonance,the recognition algorithm of fundamental frequency resonance and fractional frequency resonance is established.Through the simulation of different resonant open triangle voltage signals,using windowed FFT algorithm for spectrum analysis,the frequency of resonance can be obtained from the spectrum,and the fundamental frequency resonance and single-phase grounding can be distinguished by the change of three-phase voltage and phase angle jump.The simulation results show that the proposed method can effectively identify the resonance.