Research On Harmonic Resonance Overvoltage Of Reactive Power Compensation Shunt Capacitor And The Switching Strategy In Low Voltage System

Shunt capacitor,the important reactive compensation equipment is widely used in low voltage distribution network.It can help to maintain the system voltage and power factor as well as minimize the power losses.However the random and uncertainty characteristic of the complicated load harmonic sources in power system,makes the harmonic amplification or even harmonic resonance phenomenon of the parallel capacitor inevitably in practical application.If the capacitor works in unrated environment,the deceleration of the capacitor lifetime may probably occur.The influence of harmonic on the capacitor lifetime in the traditional capacitor switching strategy is generally ignored.The harmonic resonance model of the parallel capacitor and the system impedance is established.The overvoltage criterion is proposed based on the capacitor lifetime.According to the actual load harmonic condition,the overloading condition of the shunt capacitor can be predicted.The parallel capacitor switching strategy can be improved,and the capacitor operational lifetime can be enlarged.The harmonic resonance phenomenon can be avoided,and the adverse effect of harmonic on the compensation capacitor can be reduced.The main work of this paper is as follows.(1)Through three kinds of representative loads monitoring such as medical,offical,and residential load,the harmonic characteristic of different loads can be obtained by the monitoring voltage and current data.(2)As the parallel capacitor has three possible connection forms,the three corresponding equivalent harmonic models are established,and the parameters of the components is calculated respectively.Based on these models,the resonant relationship between the parallel capacitor and the system impedance and the harmonic overvoltage theory are studied,and the key parameters which can affect the system resonance are determined.The simulation results are presented to verify the conclusion.(3)Based on the harmonic resonance model established in this paper and the measured harmonic source data,the symmetrical component method is used to solve the capacitor overvoltage prediction under the actual asymmetrical load condition after the capacitor is switched.The response of the capacitor voltage and current under different load harmonic conditions are comparatively analyzed.(4)The characteristics of the resonant overvoltage waveform are discussed considering the harmonic influences on the capacitor lifetime.The capacitor loading index under time-varying stress is evaluated by the Weibull probability model.Then,the application of the capacitor lifetime index on the switching strategy of the parallel capacitor to avoid the harmonic resonance is proposed.Based on the above researches,the optimized parallel compensation capacitors switching strategy developed in this paper can predict the overvoltage and avoid the harmonic resonance where the capacitor life index is taken into account the different load situations in low voltage distribution system.The Simulink simulation results of the different load situations in three typical buildings show the validity of the avoidance strategy.