| The paper describes that 35 k V system of Daqing Oilfield power grid, in which the neutral point ungrounding was originally adopted. In order to detect whether the system is insulated against ground and provide voltage for the secondary equipment, the bus of power plant and substation widely uses electromagnetic potential transformer(PT for short) with the core. When operation mode changed or fault appeared, as a result, the original steady operation parameter relationship is broken and PT changes into saturated state, then ferroresonance occurs. The ferroresonance generate overvoltage which is the most common in power system can easily cause damage to the primary equipment which seriously affects the safe and stable operation of power system equipment. Among all these effects, the burning fuses of PT primary terminal which is caused by PT ferroresonance overvoltage is the frequent trouble in the resonance process. The virtual ground phenomenon often makes operators’ error in judgments and the safety fuse produced by resonance often causes metering losses.In this thesis, the formation and development of PT ferromagnetic resonance, character and harm of resonance, the range of ferroresonance parameters and the normal measures to restrain ferroresonance are elaborated, the necessary conditions for ferroresonance are summarized, and then a statistical analysis for fuse melting in the PT primary terminal of Daqing oil-field power grid 35 k V system in recent years has been taken. In order to accurately simulate calculations of the ferroresonnace, a magnetic saturation type transformer PT model is adopted as PT simulation model, fully taking into account PT iron loss and the leakage reactance influence on its nonlinear working characteristic. According to the typical primary substation main wiring, a simulation model of 35 k V ungrounded power system is set up with the PT model. Matlab program is written to convert the valid values of the filed test PT volt-ampere characteristic data j(i) and u( i) characteristic of nonlinear resistor with point by point method.In this thesis, the factors which influence ferroresonance including system operation mode change, equipment parameters change, different grounding fault, etc. are studied with the above simulation model. The results of simulation graphics and data show that, as the length of operation lines is different, the system will generate different characteristic resonance and frequency dividing resonance is easy to occur and is the most harmful to the system. The change of system single-phase grounding fault point resistance and primary fuse direct current resistance will not cause substantial influence to the ferroresonance. In the same system parameter condition, different time of fault disappearance will generate different overvoltage ferroresonnance, but will not change the ferroresonnance property. Also, the simulation effect of several common ferroresonance suppression measures are tested and verified, and the advantage, disadvantage and applicable conditions of each measure are objectively analyzed. Finally, the resonance suppression measurement of Daqing Oilfield present 35 k V power grid and the neutral point grounding mode of arc suppression coil parallel resistance after renovation are introduced. And the actual effect before and after application is analyzed and compared.According to this paper, we expect to provide a reference of fault diagnosis, PT engineering selection and selection of harmonic elimination measures for maintenance and operation personnel in the actual operation. |
PDF Full Text Request