Study On The Security And Stability Of Weak Sending-end Power Grid And Control Measures Under Strong DC Disturbance Impacting

With a series of large-scale UHV DC gradually put into operation in recent years,power grid in our country has formed a nationwide regional interconnection power system at present,and the sending end of dc interconnection has shown the network frame structure of "strong DC and weak AC".When the DC system is disturbed,there is a threat to the stable operation of the weak sending end.Therefore,it is necessary to study the possible problems about safety and stability of the weak sending end power grid under strong direct current disturbance.Based on the typical data of the northwest UHV AC / DC interconnected power grid in the target year,this paper studies the security and stability of weak sending-end power grid and the prevention and control measures under the impact of strong dc disturbance by the PSD simulation software.According to the structure characteristics of weak sending-end power grid,the power angle instability mechanism of strong DC fault impact on the weak AC channel at the sending end power grid is revealed,and the characteristics and risks of the strong dc wide-area impact weak sending end power grid are analyzed.Based on the extended equal area criterion(EEAC)principle,starting from the coupling between AC and DC transmission lines,the power angle instability mechanism of wide-area impact on the weak AC channel of the sending end power grid under the typical fault disturbances such as DC blocking and commutation failure is studied.And the impact characteristics and risks of single DC locking and commutation failure and multiple DC simultaneous locking and commutation failure are simulated and analyzed.The study reveals the coupling characteristics and risks of the weak AC channel and the fault DC under different transmission modes.In this paper,a multi resource coordinated control strategy is proposed to deal with the wide-area impact weak sending end power grid by DC fault.Aiming at the impact of DC commutation failure,the emergency control strategy of active locking DC after reaching the bearable maximum commutation failure times is proposed is proposed.In this way,the prevention and control of DC commutation failure will be converted to the prevention and control of DC locking.For the transient instability problems by DC blocking fault impact weak sending end power grid,the resources of multiple safety control measures have been analyzed,including multiple DC modulation and wind turbine cutting,etc.This paper established the optimization model of multi resource coordination control,and it determines the amount of control resources in considering network security constraints and the northwest power grid.At the same time,a heuristic method is used to determine the action sequence of multiple resources involved in coordinated control.The simulation results show that the control cost can be effectively reduced while improving the stability of the system based on the multi resource control strategy.The characteristics of partial transient voltage rise in the weak sending end power grid and the new energy turbines disconnection from grid caused by the commutation failure of the UHV DC transmission of large scale new energy power are analyzed.Based on the fluctuation of DC power and the coupling between the DC and the AC network of weak sending end power grid during DC commutation failure,a qualitative formula for the transient voltage rise of the unit is derived.For the problem of new energy turbines disconnection from grid caused by DC commutation failure,the relevant laws and characteristics are explored,and the main factors are summarized.A multi measure prevention and control strategy is proposed to slow down the transient voltage rise of the weak sending end power grid caused by DC commutation failure and the risk of large scale new energy turbines disconnection from grid.From the main factors of causing the transient voltage rise,measures should be put forward to deal with the problem of high pressure,such as DC increasing the configuration of conventional turbines,DC rectifier station configuring phase modifier,new energy station configuring dynamic reactive power compensator,etc.Based on varieties of measures,this paper further optimizes and adjusts the high voltage crossing capacity of the new energy unit.The actual system simulation verifies the effectiveness of multi measures control strategy to slow down transient voltage rise and wind turbines disconnection from grid for high voltage.