Research On PT Ferromagnetic Resonance Overvoltage Protection Measures For 35kV Power Grid

The operation mode of the 35 k V power grid in China mostly uses the neutral point ungrounded mode.In this mode,because the bus is connected to the three-phase electromagnetic voltage transformer(PT),when the system is electromagnetically disturbed,the PT excitation inductance may be different from the saturation of its core.The system matches the capacitance of the ground to form parameters,which causes ferromagnetic resonance overvoltage,which is likely to cause PT high-voltage fuse blowing and PT burnout and other failures.In severe cases,it often affects other equipment in the PT cabinet or even the entire bus,which seriously affects the safety of the system run.In order to meet the user's requirements for power supply reliability and power quality,it is necessary to further study and elucidate the generation mechanism of ferromagnetic resonance overvoltage,especially the study of the PT ferromagnetic resonance overvoltage simulation model and harmonic elimination measures for 35 k V power grids.In view of the above problems,this paper first studies and clarifies the mechanism,characteristics,classification and harm of ferromagnetic resonance overvoltage caused by PT excitation saturation,and expounds H.A.Peterson's experimental conclusion and ferromagnetic resonance parameter interval.The second is to use ATP-EMTP software to establish an accurate and effective 35 k V power grid PT ferromagnetic resonance overvoltage simulation calculation model,analyze the PT core and coil magnetization characteristics and conversion methods,establish PT model,harmonic elimination model,power supply model,etc.Clarify various excitation conditions of ferromagnetic resonance.The third is to establish a fault simulation model by analyzing two accident cases,and analyze the actual cause of the accident according to the accident process and simulation results,propose corresponding harmonic elimination schemes and preventive measures,and verify the application effect by simulation.The fourth is to use the established simulation model to connect the non-linear resistance type resonance eliminator to the neutral point of the fully insulated PT primary side,the PT opening triangle to the microcomputer harmonic elimination device,and the zero sequence PT to the neutral point of the fully insulated PT primary side.Simulation analysis of harmonic elimination measures such as PT with good excitation characteristics and increased grid-to-earth capacitance is carried out,and targeted preventive measures and resonance control schemes are proposed.The research in this paper shows that measures should be taken as soon as possible toprevent the system from maintaining the resonance state for a long time after the ferromagnetic resonance occurs.When the ferromagnetic resonance phenomenon exists on the line,PT operation must be prohibited.In order to avoid the expansion of the accident,the training of the operation and maintenance personnel should be strengthened It can correctly identify and handle system anomalies.Since the design and test methods of D-parameter discharge tubes are not strict enough at present,it is preferred to use fully insulated PT when conditions permit,to maintain the voltage resistance of each terminal of PT to the relative insulation level,and fundamentally solve Improper coordination of PT insulation.The use of fully insulated PT at the primary side neutral point resonance suppressor can quickly eliminate resonance.The smaller the damping resistance of the secondary harmonic elimination device,the more obvious the harmonic elimination effect,but the structure of this type of device is more complicated,and the phenomenon of harmonic elimination is prone to occur.The 4PT method has a long harmonic elimination time,and there is a risk of abnormal fuse blowing and PT burning.Improving the PT excitation characteristics cannot be used as the main harmonic elimination measure,but it has a certain effect on reducing the probability of ferromagnetic resonance.Increasing the capacitance of the power grid to the ground cannot be used as a harmonic elimination measure.The single-phase grounding arc caused by the excessive capacitance does not extinguish itself,which has a great hidden danger to the safe operation of the system.This article proposes a targeted resonant overvoltage protection and treatment plan,which requires strengthening planning and design,and the first and second harmonic elimination measures are taken simultaneously.The semi-insulated PT in the power grid must be gradually replaced by the fully insulated type In the neutral point,an automatic arc suppression coil is installed to compensate the grounding capacitor current.The subject research provides theoretical and technical support for the prevention and control of on-site ferromagnetic resonance.