An Adaptive Discrete Piecewise Droop Control in DC Microgrids

In DC microgrids, the discrete piecewise droop control is an attractive decentralized control strategy due to its ability to better achieve load distribution targets within a preset bus voltage deviation range. However, the practical application of this control strategy is challenged by the difficulty of implementing switching among different segments of droop curves. The conventional hysteresis-based switching approach suffers from serious undesired droop curve switching issues that parallel distributed generators extremely easily switch to different segments of droop curves as loads vary. Consequently, the power distribution significantly deviates from its intended control objective, leading to suboptimal system performance and adversely affecting the durability and longevity of energy storage units that are costly to maintain. Even worse, certain units may abruptly overload, posing a significant safety threat to the entire system. In this paper, first, the negative effect and corresponding cause of undesired droop curve switching that occurs in the hysteresis-based discrete piecewise droop control are both comprehensively investigated. Accordingly, a decentralized adaptive discrete piecewise droop control featuring a distinct control structure and control logic is proposed. Compared to the hysteresis-based switch approach, the control structure enables continuous command regulation rather discretely. The control logic restricts the command change to a small value during a step cycle instead of a significant change. Furthermore, the proposed controller is comprehensively designed based on the transient analysis. Results from both simulations and experiments demonstrate the effectiveness and advantages of the proposed control method.