Study On The Power Grid Security And Stability Characteristics And Control Strategy Under Large DC Access

Because of the unbalanced distribution of primary energy and economic development in China,it is necessary to give full play to the role of power grid in optimizing the allocation of energy resources,reduce coal consumption in load center,improve the environment and promote the harmonious development of economy and society.Under this background,many UHVDC projects have been put into operation in China.While the scale of power transmission among regions has been greatly increased,the safety and stability of UHVDC systems are facing new pressures and challenges due to the large capacity of UHVDC projects,the complex coupling relationship between UHVDC projects and AC systems,and the impact of fault disturbances on the system.This paper focuses on the security and stability of UHVDC feed-in receiving power grid,including the support capability index of AC system to DC system,the risk and control strategy of weak AC channel impacted by DC fault,the risk and control strategy of voltage instability caused by AC and DC fault.By consulting the relevant literature on power transfer and voltage change caused by DC and AC faults,this paper clarifies the research status inside and outside.The multi-infeed effective short-circuit ratio index considering DC capacitor compensation can reflect the strength of AC system more accurately than the ex isting multi-infeed effective short-circuit ratio index.The influence of the distance between the compensation point and the DC drop point,the distance between the DC drop point and the compensation capacity on the effective short-circuit ratio of multi-feed capacitor compensation is studied by formula derivation and simulation calculation.This paper studies the characteristics of weak AC tie-line impacted by power transfer after DC fault,which results in power angle instability on both sides of tie-line.A multi-resource optimal control strategy is proposed,which utilizes cutting pumps,DC power modulation and load shedding.In order to evaluate the effect of different DC power-up modulation,based on the instability attributes and fundamental characteristics,the phase difference between buses on both sides of tie-line is proposed as the main evaluation index,and the modulation effect of three-circuit DC fed into Shandong Province is evaluated.In order to cope with the risk of DC fault impacting weak AC channel,a multi-resource optimal control strategy with cutting pumps,DC power modulation and load shedding as priorities.Multi-resource optimal control strategy with the objective of minimizing load shedding.The feasibility and effectiveness of the strategy are verified by an example of North China Power Grid.This paper studies the risk of voltage instability in multi-DC feed-in grid,and puts forward two countermeasures: pre-control mode and improvement of DC control link.Taking Shandong power grid with high AC/DC ratio as an example,the characteristics of voltage instability caused by different faults such as multi-DC blocking faults and three-phase permanent N-2 faults in AC system are analyzed.The main causes of voltage instability in AC system with serious faults are analyzed.It is concluded that the dynamic reactive power support capability of AC system after faults cannot meet the reactive power demand of DC system to restore normal state.The key factors include restricting DC power,increasing the rotating reserve capacity of the receiving system units and increasing the number of start-ups of the condenser.The control strategy of improving DC control link is put forward to reduce reactive power demand and prevent voltage instability by optimizing DC VDCOL parameters.