Research On Static Voltage Stability Analysis And Preventive Control For AC/DC Hybrid Power Grid

With the asynchronous interconnection between large regional power grids using long-distance HVDC and back-to-back HVDC,China has formed a complex large-scale AC/DC hybrid power grid.When HVDCs are connected to weak AC grids,the interaction between HVDCs and AC grids has a great impact on voltage stability,so the problem of static voltage stability becomes increasingly prominent.Considering the operation point is gradually approaching the stability limit,it is more possible to occur voltage instability after a contingency;on the other hand,the integration of largescale new energy makes the thermal power unit adjustable capacity decrease gradually therefore the voltage control methods are reduced,so it is necessary to further tap the control potential of HVDC for voltage stability.Therefore,in order to maintain the safe and stable operation of the system,it is of great significance to study the static voltage stability and its preventive control strategy of AC/DC hybrid power grid.The main research contents are as follows:(1)The influence of DC system on the voltage stability of the receiving-end bus is studied from the relationship between reactive power and voltage.Based on the equivalent model of receiving-end grid,the power formula of receiving-end bus is derived and the reactive power supply/demand models of receiving-end bus under different control strategies of DC transmission line are established,therefore the relationship between DC variable and system voltage stability is established.The differences of static voltage stability of receiving-end bus under different receiving-end strength,different control strategies and different control parameters are compared and analyzed,and the coupling relationship between state variables and receiving-end voltage amplitude under different control strategies is studied.Based on the method of U-Q sensitivity,the static voltage stability index of receiving-end bus is proposed,which lays a theoretical foundation for coordinating DC and AC power grids to enhance the static voltage stability.(2)A converter station operation mode adjustment strategy is proposed to solve the problem that the DC system limit is not considered in the traditional AC/DC continuation power flow algorithm.Firstly,the converter transformer ratio is relaxed and the AC/DC power coupling term is taken into account in the Jacobian matrix,so as to enhance the robustness of traditional alternating iteration power flow algorithm;secondly,regarding continuation power flow algorithm,the improved parameterization method and the improved step control strategy are adopted to make the algorithm track the SNB reliably;then,in the process of load increase,considering the DC system overlimit,three-layer coordinated control strategy of converter station is proposed,including pole control layer,converter transformer ratio layer and reactive power control layer,so as to maintain that the DC system will not exceed the limit under heavy load conditions,therefore the obtained critical point has practical significance.(3)An on-line evaluation and preventive control architecture of static voltage stability considering multi-scale is proposed.Considering the flexibility of DC control and the diversity of disturbances suffered by power grid,a multi-scale static voltage stability region model is established and a multi-scale preventive control strategy is proposed based on the static voltage stability region theory.The control measures of DC system are divided into three scales according to the three-scale disturbances.The cost of each control measure is comprehensively evaluated from four aspects:response speed,regulation range,control cost and the impact on the system after action,and the preventive control optimization model is established to maintain the voltage stability margin at the minimum cost.(4)A fitting method for quickly solving the load margin of AC/DC hybrid power grid is proposed,and on this basis,a multi-contingency preventive control strategy based on comprehensive sensitivity is proposed.The proposed fitting method considers the DC system limit to quickly screen the multi-contingency and calculate the sensitivity with respect to control parameters.Based on a comprehensive sensitivity considering the feasibility,effectiveness and cost of control measures,a two-stage preventive control strategy for multi-contingency static voltage stability is proposed.In the first stage,the multi-contingency static voltage stability margin is enhanced according to the order of comprehensive sensitivity from high to low until the load margin under each contingency reaches the thresholds;in the second stage,according to the order of comprehensive sensitivity from low to high,check whether the load margin after removing each control measure reaches the thresholds,so as to remove redundant control measures.The proposed method considers the limits of DC system,and can meet the SVSM thresholds at a low cost without solving complex nonlinear programming problems.