Research On VSC-HVDC Power Control And Integration Strategy To Improve System Stability

Power network in China is undergoing an important stage of rapid development to highquality development.In order to improve the stability and the flexibility of the transmission grid,the Voltage Source Converter based High Voltage Direct Current Transmission(VSCHVDC)become more widespread,and the voltage level and the transmission capacity continue to increase.But it still has the following problems:(1)The weak AC power grid has poor stability,the impact of VSC-HVDC on the stability characteristics of the weak AC power grid and the research on control measures to improve the stability of the power grid is insufficient.(2)The safe and economic operation of the AC-DC hybrid power grid requires more flexible active power control,but the control methods of the AC power grid are limited,and the VSCHVDC control strategy needs to be further studied to improve the steady-state and transient control capabilities of the power grid.(3)There are few studies on the mutual constraints and evaluation methods between the stable operation of AC power grids and the VSC-HVDC.(4)The demand for the VSC-HVDC regulation ability is very urgent in the development of power grids.However,there is still a lack of quantification and standardized VSC-HVDC configuration method for each typical application scenario,and the wide application of the VSC-HVDC is limited.In response to the above problems,in order to improve the stability of the power grid,this paper has carried out a series of studies on the VSC-HVDC integration strategy and power control.The main work is summarized as follows:(1)Due to the poor anti-disturbance capability of the weak AC grid,this paper proposes the VSC-HVDC control strategy to improve the stability of the weak AC grid.First,a mechanical and electrical transient simulation model of the VSC-HVDC is built,the model has the function of limit simulation and AC fault ride-through strategy simulation;Secondly,for the weak AC power grid with high impedance characteristics after fault disturbance,the instability phenomenon and behavior mechanism of AC / DC hybrid power grids under large disturbances are revealed,and the flexible control strategy for the VSC-HVDC active power is proposed including real-time parameter optimization,perturbation control,and active power emergency control.Then,for the case of low inertia AC power grids on both sides of the VSCHVDC,this paper proposes an energy compensation control strategy based on a modular multilevel converter.The energy comes from the bridge arm capacitance of MMC,it can decouple the dynamic energy connection of the DC system for a short time,provide virtual inertia for the low-inertia and weak AC grid on the local side,and avoid affecting the stability of the AC grid on the contralateral side.(2)In order to meet the demand for flexible regulation of active power in AC grids,the VSC-HVDC active power control strategy is proposed.First,the VSC-HVDC active power steady-state optimization control strategy is proposed.On the premise of ensuring the safety and stability of the transmission grid,the second-order cone relaxation algorithm is introduced to achieve power flow optimization,minimize the grid loss,and improve the grid’s ability to resist fault disturbance.Secondly,the relationship between power grid shortage,load level and spinning reserve is analyzed.In order to improve the frequency stability,based on the least square method,the coordinated control strategy of the VSC-HVDC and spinning reserve for asynchronous interconnected power grids is proposed.Then,in order to improve the flexibility,safety and economy of power grid operation,based on the above research results,the synthetical control strategy for the VSC-HVDC active power is proposed,incuding the steady-state power flow optimization in normal mode,transient power control to deal with component overload after fault,and transient frequency control to deal with active power shortage after failure.(3)To solve the problem of the interaction between the VSC-HVDC and AC power grids,the evaluation method and varieties of constraints are proposed.First,the dynamic voltage stability margin index is proposed.The index considers the influence of various factors,such as: conventional and new energy units,dynamic reactive power compensation,motor load and other dynamic components.It can be used to accurately assess the power supply or consumption capacity,locate the constraints of the power supply or consumption capacity,and provide quantitative evaluation indicators for the VSC-HVDC transmission capacity and converter station site selection.Secondly,in some scenarios,the short-circuit current level exceeds the standard after the VSC-HVDC connection.This paper analyzes the generation mechanism of short-circuit current supplied by VSC-HVDC when the near-region AC system is shortcircuited,and proposes a quantitative evaluation method of short-circuit current considering the influence of VSC-HVDC equipment.Then,the impact of AC grid strength on the VSC-HVDC stable operation is discussed,the critical short-circuit ratio of the safe and stable operation with the VSC-HVDC applied to AC power grid is proposed.(4)At present,during the development of the AC/DC hybrid power grid in China,there are problems such as excessive power flow transfer after failure,limited transmission capacity due to uneven distribution of cross-section power flow,simultaneous commutation failure of multi-feed DC.In view of the above problems,the configuration method for the VSC-HVDC applied to AC power grid in multiple scenarios is proposed.First,for the interconnection scenario of district grids,the method and process for the VSC-HVDC locating and sizing are proposed.In this method varieties of target constraints are considered,including power supply and receive capacity,short-circuit current,AC grid strength,transmission corridor and system stability.Secondly,the configuration method for the VSC-HVDC applied to AC power grid is proposed.This method can maximize the use of VSC-HVDC to improve the transmission capacity of AC section by analyzing the line load rate and the post-fault transfer ratio.Then,the electrical distance between traditional DC inverter stations can be increased after the integration of the VSC-HVDC.Taking advantage of this feature,the method for using the VSC-HVDC to reduce the risk of simultaneous DC commutation failure caused by AC faults is proposed.Finally,the effectiveness of the above method is verified by simulation analysis in the actual power grid.