The increasing penetration of distributed renewable energy resources causes voltage fluctuations in distribution networks. The controllable active and reactive power resources such as energy storage (ES) systems and electric vehicles (EVs) in active distribution networks play an important role in mitigating the voltage excursions. This paper proposes a two-timescale hybrid voltage control strategy based on a mixed-integer optimization method and multi-agent reinforcement learning (MARL) to reduce power loss and mitigate voltage violations. In the slow-timescale, the active and reactive power optimization problem involving capacitor banks (CBs), on-load tap changers (OLTC), and ES systems is formulated as a mixed-integer second-order cone programming problem. In the fast-timescale, the reactive power of smart inverters connected to solar photovoltaic systems and active power of EVs are adjusted to mitigate short-term voltage fluctuations with a MARL algorithm. Specifically, we propose an experience augmented multi-agent actor-critic (EA-MAAC) algorithm with an attention mechanism to learn high-quality control policies. The control policies are executed online in a decentralized manner. The proposed hybrid voltage control strategy is validated on an IEEE testing distribution feeder. The numerical results show that our proposed control strategy is not only sample-efficient and robust but also effective in mitigating voltage fluctuations.

This paper proposes a new topology of a hybrid ac/dc microgrid, where there are multiple subgrids connected to the common bus by bidirectional ac/dc converters (BADCs) and bidirectional dc/dc converters (BDDCs) in the system. The rated ac frequencies in different ac subgrids and the rated dc voltages in different dc subgrids can be different for different applications. All the storages in the system are concentrated to form a storage subgrid for convenient management and high efficiency, which maintains the common bus voltage. To ensure the hybrid microgrid operates well, a proper power management method is necessary to control the power flow among multiple subgrids. This paper proposes a decentralized power management method for the hybrid microgrid to make the interacted subgrids operate in coordination and support each other. First, taking the characteristics of the common bus configuration into consideration, a P _dc - v _dc ² droop control strategy is proposed to maintain the common bus voltage and to realize power sharing among storages in the storage subgrid. Second, as the interaction among multiple subgrids is more complicated compared to the conventional hybrid, a coordinated power control strategy based on the common bus voltage, ac subgrid frequency, and dc subgrid voltage is designed for the BADCs and BDDCs to realize the power interaction among different subgrids. Furthermore, the proposed strategy takes the capacities and load types of each subgrid into consideration; hence, it is still suitable when the capacities of subgrids are mismatched and it can ensure the power quality of the subgrids with high proportion of critical loads. The effectiveness of the proposed power management method is verified by the real-time hardware-in-loop tests.

This paper proposes a distributed coordination control method for multiple bidirectional power converters (BPCs) in a hybrid ac/dc microgrid. The researched hybrid ac/dc microgrid is composed of both ac and dc subgrids connected by multiple parallel BPCs, and only the ac subgrid contains storages for the sake of economy. The proposed control method can realize the appropriate power interaction between the two subgrids and make the two subgrids to support each other, so the hybrid microgrid can operate well in both the grid-connected and islanded modes. Concretely, there are three improvements in this proposed control methods. First, a d-q-0 three-axis control strategy instead of the conventional d-q two-axis control strategy is adopted to suppress the circulating current on the ac side generated by the multiple parallel BPCs. And then the square of the dc voltage rather than the dc voltage itself is controlled, which can make the control of the dc voltage linearized and can make the BPCs provide well voltage support for the dc subgrid. Second, the dc droop control is adopted for the dc current sharing. Also, in order to relieve the storages from generating too much reactive power if the ac load reactive power is very large in the ac subgrid, the reactive power droop control is adopted to make the BPCs share some reactive power. Third, taking the capacities and load types of the two subgrids into consideration, a coordinated control strategy based on the dc voltage versus the frequency droop is designed to realize the power interaction between the two subgrids. Furthermore, the stability analysis of the proposed control method is conducted. All the conclusions are verified by the real-time hardware-in-loop tests.

For the islanded dc microgrids, the photovoltaic (PV) generators and storages are the main components to provide power and maintain the system stability. To avoid the dc bus voltage rise, the overcharge and overdischarge of storages, the coordination between generations and storages is critically important. In this paper, a decentralized control method is proposed to realize the generation-storage coordination based on the dc bus voltage signaling. First, a novel state-of-charge (SOC) based droop control strategy is designed for storages to provide dc bus voltage support and balance their SOCs, which can prevent the overuse of a certain storage. Second, an adaptive power control strategy is applied to PV generators, which can seamlessly regulate the output power of PV generators off maximum power point tracking mode according to the dc bus voltage. Combining these two strategies through the dc bus voltage signaling, the generation-storage coordination can be realized. Furthermore, the stability of the proposed control method is also analyzed. All the conclusions are verified by the real-time hardware-in-loop (HIL) tests.

In the hybrid ac-dc microgrid, the ac and dc subgrids are connected by bidirectional power converters (BPCs) that play an important role in the load power sharing and power interaction between the ac and dc subgrids. The coordination control and circulating current suppression for the parallel BPC system are very challenging. In this paper, a decentralized coordination control method is proposed for parallel three-phase BPCs, which can suppress the circulating currents, realize proper power interaction, and achieve overall load power sharing in both the grid-connected mode and the islanded mode. The performance of the proposed control methods is verified by the real-time hardware-in-loop tests.

In the grid-connected mode, the dc microgrid is connected to the utility by parallel bidirectional power converters (BPCs), which provide voltage support for the dc microgrid. The coordination control for these parallel BPCs is very challenging. First, for the parallel BPCs, there is circulating current among them, which limits the total system capacity and may damage switching devices. Second, taking the constant power load into consideration, the performance especially the stability of the dc microgrid is degraded if the conventional i _dc - v _dc droop is adopted to share the power. Third, the voltage restoration should be faster for the better quality of dc voltage, and the restoration strategy should be scalable to the changes of the dc microgrid topology for the better flexibility. For these problems, this paper proposes a decentralized control method for the parallel BPCs, which can make the BPCs operate in coordination and provide voltage support for the dc microgrid well. Furthermore, the reliability of the proposed control method is illustrated by the stability analysis. All the conclusions are verified by the real-time hardware-in-loop tests.