Research On Ferromagnetic Resonance Caused By Potential Transformer In Medium Voltage Distribution Networks

Potential Transformer(referred to as PT)are commonly used in medium voltage distribution networks that operate without a grounded neutral.One of the common abnormal operating conditions in such networks is the iron core saturation-induced ferromagnetic resonance overvoltage of PT.Due to the nonlinear characteristics of the excitation inductance of PT,disturbances such as faults and operations in the distribution network may cause the iron core of the transformer to become saturated,resulting in a decrease in the excitation inductance.Coupled with the capacitive effect between the distribution network and ground,this can lead to ferromagnetic resonance and sustained overvoltage and overcurrent.Ferromagnetic resonance overvoltage and overcurrent in medium voltage distribution networks may cause power outages,equipment damage,and even injuries or fatalities.Accurately identifying ferromagnetic resonance is a prerequisite for implementing measures to mitigate it.Due to the complexity of the electrical characteristics of the distribution network and the nonlinear nature of the resonance process,the methods for identifying ferromagnetic resonance are not yet perfect and may lead to misjudgments.Therefore,it is of great engineering significance to conduct further research on ferromagnetic resonance based on a detailed analysis of the resonance process.Starting from the non-linear characteristics of PT reactors,this paper summarizes the series resonant circuit with a single non-linear reactor,analyzes the basic properties of fundamental frequency resonance overvoltage,as well as the principles of subharmonic and high-frequency resonance,and then summarizes the principle and characteristics of fundamental frequency and harmonic resonance in three-phase distribution networks.By establishing a simulation model similar to the structure of the medium-voltage distribution network with multiple transformers and actual ungrounded operation,this paper analyzes the ferromagnetic resonance characteristics of the distribution network,studies the variation law of electrical quantities when ferromagnetic resonance occurs under different influencing factors such as line length,single-phase grounding fault elimination time,different excitation methods,and PT excitation characteristics.Based on the summary and analysis of the electrical characteristics of fundamental frequency and harmonic frequency ferromagnetic resonance,identification criteria were proposed.For harmonic frequency resonance,identification criteria were established based on abnormal phase voltage and the presence of frequency components as the subharmonic frequency.For fundamental frequency resonance,voltage changes and phase reversal of the three-phase voltage were used as identification criteria.The proposed identification criteria and method for fundamental frequency resonance can effectively distinguish virtual ground faults.An implementation method for ferromagnetic resonance identification process was presented.The effectiveness and feasibility of the proposed identification criteria and method were verified through simulation,which showed that they can accurately identify both subharmonic and fundamental frequency resonances.