Ferromagnetic Resonance Studies Of The Voltage

In the neutral node ungrounded power distribution system, ferroresonance due to the nonlinear excitation inductance of three-phase inductive potential transformer(PT) once happened, has a serious impact on the safety operation of power system.Foreign and domestic exports have done many experiments and studies on ferroresonance and have proposed a number of prevention measures in the past fifty years. However, the operation parameter is random due to the complexity of system configuration and the variability of operation style. In addition, various prevention measures all have disadvantages, leading inefficacy in certain cases. Therefore, based on the analysis of mechanism and characteristic of ferroresonance of neutral node ungrounded system, it is necessary to discuss more effective preventive methods.This paper models on Xieqiao substation 10kV power distribution system in Jiaxing. Simulations on several prevention measures are performed with EMTP, including: connecting nonlinear resistance to the neutral point of three-phase PT high voltage side, connecting single-phase PT to the neutral point of three-phase PT high voltage side, connecting an arc-extinguishing winding to the neutral point of power system, connecting damping resistance or subharmonic ferroresonance eliminator to the broken delta winding of three-phase PT et al. Then detailed analysis and comparative study on the above prevention measures are presented.Results show that ferroresonance is effectively eliminated when nonlinear resistance is connected to the neutral point of three-phase PT high voltage side, which not only keep the phase voltages steady but also restrain the inrush currents flowing in the high voltage windings of three-phase PT in the period of earth faults and then at the instant of disappearance. Moreover, zero sequence voltage on the broken delta windings can reach 100% during earth faults. In this case, the voltage relay can operate correctly and the system can increase security. Positive temperature coefficient (PTC) thermistor and triple frequency harmonic filter can reduce zerosequence voltage when nonlinear resistance is used to prevent ferroresonance. Connecting nonlinear resistance is better than connecting single-phase PT in capability and entirety of ferroresonance elimination. In addition, quantitative research has been done on effective prevention coverage for arc-extinguishing winding. Reasons why the subharmonic ferroresonance eliminator can't prevent ferroresonance effectively are analyzed. Moreover, validity of some other prevention measures is discussed. The results of this paper are proved to be of great use in ferroresonance elimination for electric power department and greatly improving the reliability of power distribution system.