Research On Power Grid Response Strategy To Deal With Adverse Effects Of DC Bias Current On Primary/Secondary Equipment

In order to solve the natural problem of inverse distribution between energy resources and load demand in China,a large number of long-distance UHVDC projects have been constructed and put into operation in recent years.The coupling between AC and DC system has become tighter and the interaction has become more complex.The DC bias induced by the monopolar operation of DC system seriously threatens the normal operation of primary/secondary equipment in power grid based on transformers,and even endangers the safe and stable operation of the entire AC power grid.Therefore,aiming at the shortcomings of current suppression schemes and corresponding strategies for DC bias,this paper studies the optimal configuration strategies of multiple DC bias suppression devices and the cooperative operation strategies of multiple DC bias suppression strategies in order to minimize the DC bias current flowing into the transformer as much as possible from the perspective of primary equipment.Besides,a new transformer protection improvement scheme is studied to achieve an effective response to DC bias current of different sizes and duration from the perspective of secondary equipment.The above research guarantees the safety and stable operation of the power grid to the maximum extent.The specific contents are as follows:First,based on the global optimization configuration of DC bias control equipment which is generally ignored by current research,a global optimization configuration method of DC bias control equipment that takes into account the coordination of multiple devices is proposed.The method aims to optimize the governance effect and investment cost,and obtains the feasible optimal configuration scheme set.Furthermore,a new evaluation system is designed to compare the advantages and disadvantages of different DC bias suppression device configuration schemes.The feasible scheme set is checked,and the optimal configuration scheme with the most comprehensive performance is selected objectively and reasonably.The simulation results show that the new method can effectively improve the governance effect and reduce the investment cost,which provides a more reasonable choice for the future DC bias current governance.Then,a DC bias suppression method with a coordination of active interconnection of multiple grounding electrodes and DC blocking devices is proposed based on the reasonable introduction of DC bias control methods with different principles.Different evaluation indexes are selected for the scenario that the selected scheme set meets or does not meet the governance requirement.By using the quantum genetic algorithm and the combination weighting method,the optimal governance scheme which meets the evaluation indexes is selected without introducing new DC bias suppression device.Compared with the traditional method,simulation results show that the proposed method has better DC bias governance effects and AC topology adaptability.Finally,in order to solve the problem of transformer protection misoperation in secondary side caused by DC bias,which is difficult to solve in current DC bias governance method,a transformer protection improvement scheme is proposed to eliminate the adverse effects of DC bias.Based on the detailed analysis of the protection misoperation mechanism caused by DC bias,this paper selects the transformer protection principle with superior performance based on phase space trajectory identification,analyzes its action performance in the DC bias scene,and finally improves its protection logic and setting method combined with the actual working principle of DC blocking device so as to realize the effective response to different DC bias scene.Simulation results based on PSCAD/EMTDC verify the effectiveness of the proposed method.