Suppression Method Against DC And Harmonic Earth Current At Transformer Neutral Point And Its Field Application

With the deep integration of urban power system and transportation system,the earth current generated by urban rail transit in economically developed areas frequently invades the AC system through the neutral point of transformer for a long time,resulting in serious problems such as DC bias of transformer,amplification of harmonic current,and misoperation of relay protection,which poses a serious threat to the safe and stable operation of power equipment and grid system.Meanwhile,the abnormal vibration of transformer caused by the earth current results in serious noise near the substation,which seriously affects the life of the surrounding residents.With the development of urban rail transit in cites,these problems will be exacerbated.To eliminate the adverse effects of the DC and harmonic earth current on the power system,the dissertation focuses on the suppression method against DC and harmonic earth current at transformer neutral point and its field application.Aiming at the problem of abnormal noise generated by urban substations,the actual field measurement for the neutral current of 220 k V substation in power system is carried out,and the variation characteristics of DC and harmonic components in the earth current are analyzed.Based on the analytical model of the earth current under the bilateral power supply mode of urban rail transit,the distribution of track potential and traction current in traction power supply system is simulated.On this basis,a coupling model of the distribution of the earth current generated by urban rail transit in AC system network is established,and the variation law of the earth current at the neutral point and the coupling constraints of the system parameters affecting the earth current distribution are deeply analyzed.To effectively suppress the DC and harmonic earth current at the neutral point of transformer,a hybrid suppression method combining active converter and capacitor is proposed.The capacitor is used to block the DC current,and the active converter is used to suppress the harmonic current.When a short-circuit grounding fault in power system leads to the large current flowing through the neutral point,to ensure the reliable grounding of the transformer neutral point and the bypass protection of the equipment connected in series with the neutral point,the corresponding high-speed bypass protection scheme is proposed.A real-time control strategy for fundamental voltage and neutral harmonic current is proposed.The system model is simulated to verify the ability of the proposed method to suppress the DC and harmonic earth current and the effectiveness of the bypass protection in case of the short-circuit grounding fault.The laboratory tests simulating the field working conditions realize the performance verification of the suppression method.Aiming at the problem of regional distribution of the earth current in power system,an earth current governance strategy based on the DC and harmonic earth current suppression system is proposed.Taking the suppression devices configured in substations as decision variables and designing a reasonable optimization objective,an optimization mathematical model for the global configuration of the suppression devices is established to solve the problem.The constructed cases prove that the strategy can effectively avoid the local earth current out of limit owing to the application of single suppression device.An earth current governance strategy of connecting the neutral point with a virtual impedance in series is proposed.The mathematical description of the global optimal allocation problem for virtual impedance is established,and the alternating direction multiplier method is used to solve the mathematical model efficiently.The numerical simulation results indicate the effectiveness of the strategy in controlling the DC and harmonic earth current.An industrial prototype of the DC and harmonic earth current suppression system is developed.The topology of the prototype and the electrical parameter of each component are designed,and the control strategy of the whole prototype and its functional modules are introduced in detail.The bypass protection of the prototype adopts a multiple redundant trigger mechanism combining digital detection and hardware circuit detection,so that when a short-circuit grounding fault occurs in power system,the neutral point of the transformer can be reliably grounded and the security of the transformer and the functional modules of the DC and harmonic earth current suppression system is effectively guaranteed.The high current impulse and withstand tests have fully verified the rapidity and reliability of the bypass protection for the prototype.The prototype has been tested in a 220 k V substation and put into practical application,and the field test results have exhibited that the proposed DC and harmonic earth current suppression method has a good ability to suppress the DC and harmonic earth current.