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| Time-Delay Model Predictive Additional Control Strategy to Improve the Stability of Low-Voltage DC Distribution System |
| Zhuang Ying1, Pei Wei1, Liu Ziqi2, Deng Wei1 |
1. Institute of Electrical Engineering of Chinese Academy of Sciences Beijing 100190 China;
2. University of Southern California Los Angeles 90007 USA |
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Abstract 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 time-delay 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.
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Received: 29 March 2022
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2023, Vol. 38 
