Coordinated control of renewable energy power generators in microgrids

Abstract

Microgrids, offering a feasible solution for assimilating a diverse array of renewable energies, obtain widespread implementation in recent decades. To guarantee the normal operation of microgrids, the elaborate control strategies are necessitated. Although the existing methods can yield good performance, some challenges (e.g., circulating currents among distributed generators (DGs) and the parallel converters) still exist and higher system performances (e.g., faster system dynamics and less communication burden among DGs) are widely anticipated. To solve the above issues, advanced decentralized and distributed control methods are investigated from the converter level and the microgrid level, respectively, in this thesis. Firstly, the common ground circulating currents emerge when grounding faults occur in photovoltaic (PV) power plants, which undermine the system's stability. In response, utilizing the equivalent model of circulating currents (CCs), this thesis devises a decentralized strategy to diminish these CCs. Moreover, to preserve the stable currents and DC-link voltages during the transience of grounding faults, a delicate control scheme is introduced, utilizing a three-level PV interfacing converter to curb the surges in currents and voltages. Experiments validate the effectiveness of these methods. Secondly, single-phase three-level neutral-point clamped inverters have been extensively used to interface the renewable energy and the grid. To accommodate high-power scenarios, multiple inverters are typically connected in parallel, which will cause circulating currents (CCs) when the hardware parameters are asymmetric, threatening the reliability of the system. Aiming at this problem, this thesis proposed a decentralized strategy for CC suppression, leveraging an enhanced 3LNPCI topology with expanded modulation freedoms. This strategy can concurrently address CC suppression and achieve the desired current sharing. The efficacy of this strategy is substantiated through experiments. Thirdly, to meet the heightened performance criteria set for DC microgrids, a distributed fixed-time secondary control strategy is proposed to simultaneously regulate DC bus voltage and ensure proportional current distribution among DGs within a fixed time, demonstrating a fast dynamic response. Besides, only one variable needs to be transmitted in the proposed control strategy, significantly reducing the communication traffic. Moreover, this method can eliminate the need for bus voltage monitoring, which simplifies the system architecture and enhances reliability. Simulations tests validate the viability of this scheme. Lastly, to further reduce data exchanges between DGs within DC microgrids, this thesis designs an event-triggered fixed-time secondary control strategy. Besides the ability to concurrently manage DC bus voltage and achieve proportional current distribution among DGs within a fixed time, by leveraging the event-triggered communication protocol, each DG is required to update its status to adjacent DGs only upon meeting predefined triggering conditions, thereby substantially reducing communication overhead. Experimental tests are carried out to verify the efficacy of this strategy. In summary, this thesis proposes a series of control strategies to address some existing challenges in the large-scale application of renewable energy sources, and satisfactory results have been confirmed through both simulation and experimental verifications. This study lays the foundation to some extent for the efficient utilization of renewable energy sources and achieving decarbonization goals.