Optimal Operation And Control For VSC-HVDC Grid

Compared with line commutated converter(LCC),Voltage Source Converter(VSC)has salient advantages of without reactive power compensation,without commutation failure,supplying power for passive system,independently controlling active/reactive power,readily reversing the direction of power flow,etc.It is easily to form the system which constitutes of multi-power supplies and multi-infeed.It is widely used in grid interconnection,large-scale wind,solar and other clean energies integration,with broad prospects for development.With the large-scale access of flexible DC network to AC transmission network,its impact on AC network becomes more significant.Therefore,this paper makes use of the flexibility and rapidity response of VSC and deeply studied the security and economy of AC/DC transmission system with MTDC grid.The research has important engineering reference value and theoretical analysis significance.This paper mainly studies the steady-state operation characteristics of MTDC grid,including the calculation of power flow and optimal power flow for AC/DC system,optimal corrective control for MTDC grid,the calculation of security constrained optimal power flow(SCOPF)for AC/DC system,etc.The main work is as follows:(1)Research on the calculation of power flow(PF)and optimal power flow(OPF)for AC/DC system.First,aiming at the shortcomings of unified iterative algorithm and sequential iterative algorithm,propose the PF algorithm based on bi-directional iteration and design study case to verify the correctness and effectiveness of the method.Then,build the mathematical model of the OPF for AC/DC system,utilize the interior point algorithm tracing the center track to calculate the OPF and compare the results with power flow to verify the economy of OPF.(2)Aiming at the possible risk of security operation after N-1 outage of MTDC gird,study the optimal corrective control method.First,propose optimal corrective control strategy covering different load levels by utilizing the flexible controllability of VSC-HVDC,Then,build an optimal corrective control model of AC/DC hybrid system in accordance with the power shift strategy.After the N-1 outage of MTDC grid,fully utilize regulation ability of converter station to minimize the impact of DC faults on AC grid.Finally,the proposed method is applied to"Zhangbei±500 kV four-terminal bipolar HVDC grid".Numerical results and electromagnetic transient simulation verify the effectiveness of the presented method.A modified 10-machine 39-bus system with inclusion of a five-terminal bipolar HVDC grid is employed to further verify the generality of the method.(3)Research on the SCOPF considered rapid controllability of MTDC grid.First,build the mathematical model of SCOPF for AC/DC hybrid system,including short-term post-contingency OPF and long-term post-contingency.Then,to avoid getting the local optimal results,a hybrid algorithm combing the interior point algorithm and the differential evolution(DE)algorithm is developed to solve the SCOPF problem.Finally,the effectiveness of the presented method is validated by comparing with the method without power shifts as the corrective control means.Parallel computing is applied to effectively speed up the computation.