The Study On Ferroresonance Overvoltage And Its Suppressive Measures For35kV Neutral Point Ungrounded System

Potential transformers has been widely used in35kV neutral ungrounded system.PT can work in saturation state when the circuit breaker operating or the systemfaulting. If the inductance parameter of PT and the capacitance(such as the distributecapacitance of the network) parameter matched, the ferroresonance will be excited.The safe operation of the distribution network has been seriously threatened by theferroresonance. Although there are also several measures which have often been usedto avoid ferroresonance at the present time. The ferroresonance can’t be eliminatedcompletely because of the complexity and randomicity caused by different parameterand operation modes of different system, which make those suppressive measureshave some limitations. So it is necessary to do an further study on ferroresonancewhich aims at neutral ungrounded system. Through analysis and comparison onvalidity of diversiform measures that are Often used to eliminate ferroresonance, itcan summarize the law of suppression of overvoltage and overcurrent caused byferroresonance by measures used to eliminate ferroresonance, which providemeaningful evidence for choosing suitable measures to eliminate ferroresonace inpractical projects.In this thesis, the no-load voltage-current characteristic of three35kV PTs havebeen tested at first. Then the Progressive Deduction Method have been used to turnvoltage-current characteristic curve into magnetic flow-current charateristic curve. Asimulation model has been built based on a typical35kV neutral ungrounded systemprimary sides devices map by using Electromagnetic Transient Program(ATP-EMTP).Many influencing factors that affect ferroresonance have been summarized on thebasis of theory of Ferroresonance. Calculation and analysis have also been done toanalyze the impact of these factors. For example, operation modes of system, modesof outside excitation, grounding resistance at single-phase ground fault point, and soon. After that, calculation and analysis have also been done to summarize the law ofthe impact of those factors on ferroresonance. In order to conclude the law offerroresonance suppression by measures used to eliminate ferroresonance, theinstance of ferroresonance suppression by measures used to eliminate ferroresonancehave been quantitatively analysed and compared in this thesis. The simulation results show that most of the ferroresonance caused by saturationof PT in distribution network happen affter the disappearance of single-phase groundfault. The disappearance of single-phase ground fault can easily excite subharmonicferroresonance. Diversiform measures used to eliminate ferroresonance now caneliminate or suppress ferroresonance in a certain extent, but all of them have certainmerit, demerit and applicable scope. Connecting a nonlinear resistance eliminatorbetween the neutral point and high voltage side of PT and ground is a effectivemeasure to eliminate or suppress ferroresonance, the heat capacity of nonlinearresistance resonance eliminator and the corrtect action of the protection groundingsignal device should meet the requirement when this measure is used to suppressferroresonance that is especially caused by single-phase ground fault. A biggishovercurrent in closed delta winding may be engendered when4PT is used to suppressferroresonance. When connecting damping resistance to the broken delta winding ofhost PT is used to suppress ferroresonance caused by the disappearance ofsingle-phase ground fault, the resistance should be connected as soon as thesingle-phase ground fault disappears. To the primary sides devices map and operationmodel for the study system of this paper, These methods including connectingPetersen coil to the neutral point of system, connecting nonlinear resistance resonanceeliminator between the neutral point on high voltage side of PT and ground and4PTcan be choosed in turn to restrain the overcurrent and the overvoltage of the PTcaused by lower frequency subharmonic ferroresonance, the effect of connectingPetersen coil to the neutral point of network is the best. Connecting resistance orresonance device to the broken delta winding of host PT can’t effectively restrain theovercurrent of the PT caused by lower frequency subharmonic ferroresonance.