Optimal Power Flow And Self-healing Reconfiguration For AC/DC Power Systems With VSC-Based MTDC

The future power system will gradually transform into a coordinated development mode which transmission network and distribution network complement each other.The VSC-HVDC technology will cause great significant impact on both transmission network and distribution network.At the transmission level,the VSC-MTDC and DC grid technology provide effective technical means for large-scale renewable energy consumption,energy complementary optimization in a wide range,interconnection of asynchronous regional grid.At the distribution level,the VSC-HVDC technology provides a new solution for partition interconnection of HV distribution networks,and closed-loop operation of MV distribution networks.Therefore,the AC/DC power grid based on VSC-MTDC will become important pattern and key component of the next-generation power system.The development stage and application level of VSC-HVDC technology in transmission network and distribution network are different.The concerns and emphases for research are also different:In transmission networks,the VSC-HVDC technology is developing towards the direction of multi-terminal,multi-infeed and DC grid.The wide interconnection and deep mixing of AC grid and DC grid will make the structure of power system changed profoundly,also make the operation control strategies of power system more complex.This is leading to the traditional operational analysis method is not fully applicable.In distribution networks,the application of VSC-HVDC technology is still in the demonstration test stage,still need to study the role and impact of its introduction on the operation and control of distribution networks.According to the current situation of the research and application of VSC-HVDC technology in transmission and distribution network,this paper studies different optimization problems of AC/DC power system,such as optimal power flow,stochastic optimal power flow,and the self-healing reconfiguration.The main research work is as follows:(1)The original power flow algorithm is no longer completely appropriate due to the diversified control modes of DC grid.An AC/DC power flow calculation method considering DC system operation control strategies is proposed.The steady-state model of AC/DC system with VSC-based MTDC is established.The control modes of VSC and DC/DC converter,as well as the control strategies of DC grid,are comprehensively analyzed.The power flow variables,Jacobian matrix and network parameters are derived according to different control modes,which is suitable for the power flow calculation of DC grid considering various control modes.Finally,the DC gird test system suggested by CIGRE and the modified IEEE RTS-96 case are carried out,verifying the feasibility and effectiveness of the proposed method.This algorithm in this paper could provide reference for the steady-state analysis and power flow calculation of large scale AC/DC power system.(2)An optimal power flow algorithm of AC/DC system with VSC-MTDC networks considering the precise converter power losses and the voltage-droop control strategy is presented.Based on the generalized model of the VSC station accounting for the converter power losses,the optimal power flow mathematical model of AC/DC system with VSC-MTDC networks is developed.The effects of different control strategies of VSC station on the setting of optimal variables and equality constraints are deduced in detail.Aiming at the "coupling" problem between the AC-side power injection and the converter loss of droop-control converters,an iterative calculation method for the AC-side active power of voltage-droop converters is proposed.Moreover,analyzing the constructions of Jacobian matrix considering different control strategies,the step-controlled primal-dual interior point method(PDIPM)is used to solve the optimal power flow model.Finally,the modified IEEE 30-bus test system is carried out for case studies on the introduced optimal power flow algorithm,in which the validity,accuracy,and versatility are demonstrated.This paper offers a reference for the operation analysis and benefit evaluation of large scale complex AC/DC system with VSC-MTDC networks.(3)Based on the study of AC/DC optimal power flow,a stochastic optimal power flow(S-OPF)analysis method for the distribution network with VSC-based interconnection equipment(VSC-IE)is proposed,considering the randomness and uncertainty of DG power output.The S-OPF model of distribution network with VSC-IE is established.The probabilistic models of DG output are discretized to construct multiple conditional probability problems with the certain power injection of DG.The S-OPF model is solved in each conditional event based on PDIPM and cumulant-based analytic method.The full probability distributions of stochastic variables in optimized operation are obtained by accumulating all Gauss distribution functions of conditional events according to the total probability formula.A case study based on IEEE 3 3-bus test system is carried out in different scenarios.Results show that VSC-IE plays an active role on the optimal operation of distribution network,especially with high penetration of DG.The proposed method has theoretical and practical significance for the access planning of VSC-IE in the distribution network.(4)The introduction of VSC-IE makes the self-healing reconfiguration problem of distribution networks become a mixed integer nonlinear programming(MINLP)problem involving both the continuous variable and the 0-1 discrete variable.Combining the fast non-dominated sorting genetic algorithm(NSGA-?)with PDIPM,this paper proposed a hybrid optimization algorithm,which uses NSGA-? to realize the fast solution of the switch state under multi-objective optimization,and uses AC/DC optimal power flow based on PDIPM to calculate the transmission power of VSC-IE.The hybrid optimization algorithm can solve large-scale MINLP problem accurately and rapidly,realizing the simultaneous optimization of tie switches and VSC-IE in the self-healing reconfiguration of distribution network.The simulation results show that the application of VSC-IEs can help to reduce the power loss in the process of fault recovery,reduce the switching operation times and reduce the network losses of the system after recovery.