Research On The Nonlinear Characteristics And Flexible Suppression Method On Ferroresonance Overvoltage

Smart grid is the direction for the world’s power grid in the future, an importantfeature of which is "self-healing grid". This feature means that the power grid shouldhave the following abilities: real-time, online, continuous safe assessment and analysis;early warning, prevention and control for power accidents; and automatic faultdiagnosis, fault isolation and self-recovery. With such abilities, the security, stabilityand reliability of power grid can be significantly enhanced. The various defence lines ofpower grid are coordinated well so that the power grid owns the ability to defend someunexpected events and serious failures, which can effectively prevent the occurrence ofa wide range of cascading failures and significantly improve power supply reliabilityand reduce power losses.Ferroresonance can occur in nonlinear inductance and capacitance circuit, it causesovervoltage and over-current that lasts for a long time and can even exist stably.Overvoltage and over-current may cause equipment failure and damage in powersystem. Therefore, the mechanism, the basic characteristics, the development law andthe suppression method of the ferroresonance are studied by a lot of scholars at homeand abroad. However, owning to the extremely various ferroresonance circuits and thevarious ferroresonance types, the effect of the present suppression method on theferroresonance under some certain condition may be limited. For this reason, powersystem key equipments may endure a long time ferroresonance overvoltage, makes itmay cause insulation damage and equipment damage. So, ferroresonance problem isstill a complex problem for the safe operation of the power system for a long time. Withthe development of the power grid, the ferroresonance problem in power system isincreasingly prominent. On the one hand, the rapid development of power distributionnetwork makes the distribution system parameter variation range increase very sharpand the power distribution network operation is more frequent, which leads to a higherincidence of ferroresonance. On the other hand, the UHV system is very strict withinternal overvoltage, and ferroresonance overvoltage may exceed its internalovervoltage level and cause key equipment damage and even a wide range of accident.Therefore, it is urgent needed to study the nonlinear characteristics of ferroresonanceand then put forward a more feasible ferroresonance overvoltage suppression strategy.At present, most of the study on the basic ferroresonance nonlinear characteristics analysis and its suppression method are based a basic ferroresonance circuit model. Themodel is simplified by a typical ferroresonance circuit. To a certain extent, the simplifymodel can explain the basic principle of ferroresonance and lay a solid foundation forthe study of the ferroresonance. However, the traditional ferroresonance model can onlybe established before acquiring the ferroresonance circuit and precise system parameters.In the actual power system, the ferroresonance circuit and its parameters change withthe change of the operation mode. Therefore, the characteristics and the suppressionmethod obtained based on the traditional ferroresonance model cannot satisfy the fieldapplication.Based on the present research, this thesis proposes a new method to analyze thenonlinear characteristics of ferroresonance and then study the suppression method forferroresonance. The study is directly based the ferroresonance overvoltage time seriesand not relying on the accurate ferroresonance model and precise parameters. Firstly,aiming at the characteristics of the ferroresonance overvoltage, appropriate phase spacereconstruction methods are chosen to reconstruct the ferroresonance overvoltage timeseries. Base on the reconstruction, the hidden information of the overvoltage time seriescan be dig for ferroresonance nonlinear characteristics analysis. The nonlinearcharacteristics analysis can be used for extracting the nonlinear characteristics quantitiesof the ferroresonance, including quasi-periodic and chaotic ferroresonance, andidentifying their types. All of the study can be supported to establish an online real-timemonitoring, analysis and active suppression system. Secondly, based on the phase spacereconstruction of the ferroresonance overvoltage time series, a lower order polynomialmodel for ferroresonance is identified using the key parameters determined in phasespace reconstruction. Based on the model, an active suppression strategy based onovervoltage time series is studied to demonstrate that it is feasible and effective. Finally,based on the above research, a flexible suppression method by combining with thepower electronic technology for ferroresonance overvoltage is proposed. High-speedfully controllable switches are adopted in this method. Electromagnetic transientsimulation model considering the hysteresis characteristic of the PT and scale-downlaboratory test platform are established individually, based on that, the suppressioneffect of the changeable parameters on the ferroresonance overvoltage is analyzed indetail, and the suppressions on the different types and different resonance degrees offerroresonance overvoltage are investigated.In this thesis, the nonlinear characteristics of the ferroresonance overvoltage time series and its flexible suppression method are carried out in detail though theoreticalanalysis, simulation and experiment research. The results are as follows:Mutual information method and Cao method are adopted to reconstruct the pahsespace of the ferroresonance overvoltage. Based on the phase space reconstruction, thereconstructed attractor in sense of topological equivalent can be acquired, which can beused to analyze the nonlinear characteristics of the ferroresonance. The method used inthis thesis overcomes the traditional analysis method, which depends on the accurateferroresonance model and its precise parameters. The results of the method used in thisthesis are agreed with that of the traditional method. Based on the phase spacereconstruction, two important nonlinear characteristics quantities can be furtherobtained: the average gray value of the reconstructed attractor and the largest Lyapunovexponent of the overvoltage time series. The two nonlinear characteristics quantities ofthe ferroresonance overvoltage time series can be to classify the non-periodicferroresonance. A two-order polynomial model is identified using the reconstructionparameters and the ferroresonance overvoltage time series rather than extracting fromthe ferroresonance circuit. The model can represent the basic characteristics of theoriginal ferroresonance system. A feedback control method proposed based on theidentified model can suppress different types of ferroresonance overvoltage successfully.Fully controlled power electronic switches are adopted in this thesis to adjust the highfrequency damping resistance characteristics to realize ferroresonance suppression. Thesimulation and experiment research show that the damping resistance and duty cycle forputting into the damping resistor influence the suppression effect significantly.Comparison results show that the he proposed flexible ferroresonance overvoltagesuppression method can effectively restrain different types and different core saturationdegrees of ferroresonance overvoltage. The method can make up for the inadequacy oftraditional suppression method and not increase the burden to equipment. Moreover, themethod will not cause harmonic load for the system.