提升低压直流配电稳定性的时滞模型预测附加控制

Time-Delay Model Predictive Additional Control Strategy to Improve the Stability of Low-Voltage DC Distribution System

针对由低压多端直流系统中接入的可再生能源和直流负荷不确定性波动引起的系统直流电压波动、越限甚至跌落、振荡发散等电压稳定问题,提出一种主从控制下提高直流配电系统稳定性的时滞模型预测附加控制策略。首先,针对系统通信网络存在延时的情况,搭建了相应的系统时滞预测模型;然后,在此预测模型基础上,附加控制策略以系统直流电压波动最小和最小附加调节指令为目标,实现以最小出力平抑不确定扰动下系统直流电压的波动,进一步分析了通信延时对系统稳定性及附加控制策略性能的影响;最后,通过三端低压直流系统进行仿真和算例分析,结果表明,所提出的时滞模型预测附加控制策略能很好地平抑多端直流系统受不确定扰动带来的电压波动问题,提高系统稳定性,具有较好的鲁棒性。

Aiming at the voltage stability problems such as DC voltage fluctuation,voltage over-limit or even voltage drop,oscillation instability caused by uncertain fluctuation of distributed generation and DC loads connected to the low-voltage multi-terminal DC(MTDC)system,additional control strategy has been widely studied because of its simple structure and no need to change the system existing control loop structure.However,most of the current additional control strategy relies on the experience of designers in terms of parameter design,which leads to the problem that the control cost is high and the optimal control effect cannot be guaranteed.In addition,current method does not take into account of the communication time-delay.Since for centralized additional control mostly depends on the scheduling of the upper central controller and certain physical distance exists between the local controller of the converter station and the upper central controller,the problem of communication delay cannot be avoided,which will have a certain impact on the additional control effect that needs to be studied.In order to solve the above problems,considering the characteristics of MTDC system under master-slave control,a time-delay model predictive additional control strategy is proposed which uses the model predictive control(MPC)algorithm to build the additional controller.Different from the traditional MPC method,the proposed method establishes a time-delay prediction model,which can more accurately describe the dynamic characteristics of system under communication time-delay,and improve the robustness of the control strategy.Firstly,the structure of a centralized MTDC system under master-slave control is studied.Secondly,a time-delay prediction model based on Pade approximation is constructed.Based on the time-delay prediction model,an additional controller which takes the minimum voltage fluctuation and the minimum output of system as the optimization goal is constructed,and through rolling optimization and feedback correction,the additional power control commands are generated in real-time,which are sent to the outer power loop of the local controller of slave VSC thus can stabilize the system voltage fluctuation and ensure the safe operation of the system under uncertain disturbance.Then,the influence of power disturbance and the communication time-delay on the stability and the additional control strategy performance is analyzed by studying the root locus diagram of the system.Finally,the simulation and case analysis of a three-terminal low-voltage DC system are carried out using Matlab/Simulink plate-form,which proves the effectiveness and robustness of the proposed method.The following conclusions can be drawn from the theoretical and simulation analysis:(1)The proposed timedelay model predictive additional control strategy can solve the voltage instability problem when the MTDC system is subject to uncertain disturbance with minimum output,improve the system voltage stability and power quality,and has good economy.(2)When the system communication network delay is less than a certain range,the optimal control strategy proposed in this paper can well describe the dynamic characteristics of the time-delay system compared to the traditional additional control strategy,better suppress the system voltage fluctuation under the uncertain disturbance,ensure the safe operation of the system,and has good robustness.