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| An Autonomous and Smooth Mode Switching Control Strategy Based on DC Bus Signaling for Small-Scale Stand-Alone DC Microgrid |
| Zhai Fan, Li Guidan, Wang Yifeng, Chen Bo, Wang Zhongjie |
| Key Laboratory of Smart Grid of Ministry of Education Tianjin University Tianjin 300072 China |
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Abstract In a stand-alone DC microgrid system, energy storage systems (ESSs) and photovoltaic (PV) power sources are the main energy sources. However, the battery’s DC bus voltage regulation capability is limited and decreases with increasing photovoltaic penetration. In this scenario, there is an increasing demand for PV sources to participate in DC bus voltage regulating, especially in small-sized stand-alone DC microgrids. In the current control method, the control for PV generators needs to be switched to different modes according to the corresponding voltage level. During the switching, the output power of PV modules fluctuates greatly, which influences the performance of the system. Therefore, transients are particularly challenging in stand-alone DC microgrids and may cause erroneous switching between control sets. The switching control strategies must be carefully designed, especially when a time delay exists in detecting the switching situation.
This paper introduces an autonomous and smooth mode-switching control strategy based on DC Bus Signaling (DBS) to cooperate with multiple PV sources and ESS. The proposed control strategy utilizes the alternation between saturation and activation of the PI controller with a back-calculation mechanism. Some outer closed loops of the PI compensator reach the actuator limits, and the feedback loop is broken during mode switching. This method can solve the chattering and deadlock problems during mode switching, shorten the mode switching time, and reduce the overshoot of DC bus voltage. The communication between the converters is realized through the DC bus voltage, which reduces the overall cost of the system. Based on the DBS method, different operation modes of the system are divided, and the five representative cases in the DC microgrid system are indicated. In addition, the control methods for PV converters and battery energy storage (BES) converters are discussed. Particularly, to fully use PV energy, the proposed control strategy considers the state of charge (SOC) of ESS and enables PV sources to regulate the DC bus voltage.
Besides, the mode-switching time is obtained by analyzing the back-calculation PI compensators to optimize the mode-switching process. The results show that the mode switching time is related to IC, voltage difference, and the anti-windup gain. The errors between simulation results and theoretical results are less than 5%. The design method for controller parameters is also provided.
Finally, a small-scale stand-alone DC microgrid experimental system on a 375 V/2 kW prototype is set up to demonstrate the feasibility of the proposed method for DC microgrid controls. The steady-state and dynamic performances have been discussed to verify the feasibility of the proposed control scheme. The mode switching performance under different control parameters Ka is given. Compared with the traditional method, the proposed control method can achieve autonomous and smooth mode switching without chattering and deadlock problems, shorten the mode switching time, and reduce the overshoot of the DC bus voltage during the mode-switching process. The proposed control scheme can autonomously alter the control objectives without the central controller and enhance the reliability of the DC microgrid.
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Received: 17 March 2023
Published: 07 June 2024
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2024, Vol. 39 
