Ferromagnetic Resonance Of Neutral Ungrounded System And Its Suppression Measures

In 10 kV distribution network,electromagnetic voltage transformer(PT)is often used to monitor bus-to-ground voltage.Because of its non-linear character istics,under some faults(such as single-phase grounding fault elimination)or a bnormal operating conditions,it may lead to core saturation matching with syst em capacitance to ground,induce ferroresonance,generate overvoltage and over current,and even fuse fuse fuse in serious cases.Break,or even burn down th e voltage transformer.This threatens the stable and safe operation of power sy stem.Therefore,it is very important to study this phenomenon.Firstly,the basic principle of ferroresonance is described.From the point o f view of the non-linear excitation characteristics of PT,the mechanism of ferr oresonance is discussed by graphic method and mathematical function fitting m ethod.The parameters range of ferroresonance and the harm of ferroresonance are summarized.It is analyzed that when the parameters matching of the syste m exceeds the parameters range of ferroresonance,similar ferroresonance overv oltage will still appear.The reason of overcurrent-low frequency non-linear o scillation.Then,combined with and simplified the main wiring diagram and paramet ers of substation,based on ATP-EMTP simulation platform,a single-phase thre e-winding PT model considering PT leakage reactance and iron loss is construc ted,and a simulation model of 10 kV neutral point ungrounded system is buil t.Taking the instantaneous disappearance of single-phase grounding fault as ex citation condition,the characteristics of over-voltage and over-current of differe nt types of ferroresonance are simulated and analyzed,and the effects of four factors on Ferroresonance over-voltage and over-current are studied.Finally,according to the simulation model,the existing restraining measure s are simulated and analyzed,including the restraining measures of PT open tri angle side damping resistance,PT primary side neutral point non-linear resistan ce or linear resistance,as well as the system side increasing ground capacitanc e,system neutral point small resistance grounding and system neutral point arc suppression coil grounding restraining measures,comprehensive comparative ana lysis.The advantages and disadvantages of each harmonic elimination measure and its application scope.In view of the limitations of the existing harmonic elimination measures,a new harmonic elimination measure is proposed,which h as obvious suppression effect and can improve the accuracy.The neutral point of primary side of PT is grounded by zero sequence PT parallel harmonic eli mination or linear resistance.On this basis,the influence of different suppressi on measures on harmonic elimination is analyzed for the parallel operation of multi-bus PT.