Zhanqiang Zhang

MAS-Based Hierarchical Distributed Coordinate Control Strategy of Virtual Power Source Voltage in Low-Voltage Microgrid

To settle reactive power sharing inaccuracy among distributed generations (DGs) associated with mismatched lines impedance, a hierarchical control strategy in the framework of multi-agent system (MAS) is proposed. Replace DG by virtual power source (VPS) in droop control and synchronize VPSs voltages by a hierarchical control. Initially, in primary control, improve line feature by defining virtual impedance value, so that VPSs voltages are roughly consistent. Then, design a VPS voltage evaluation index based on reactive power outputs, whose trigger determines whether secondary control is activated. Finally, in secondary control, consensus protocol is used to strictly synchronize actual VPSs voltages with limited voltages information exchange by the sparse communication network. Therefore, MAS is used to provide an appropriate DGs interaction manner and hierarchical control frame. Each DG is associated with a first-level distributed agent to execute primary control. Coordination part is regarded as secondary-level agents to realize secondary control. Hierarchical control provides double safeguards of VPSs voltages synchronization. To verify the effects of control strategy, simulations are carried out on RTLAB and MATLAB/Simulink.

Hierarchical Delay-Dependent Distributed Coordinated Control for DC Ring-Bus Microgrids

In this paper, a hierarchical distributed coordinated control method is proposed based on the multi-agent system for dc ring-bus microgrids to improve the bus voltage performance. First, a two-level multi-agent system is built, where each first-level unit control agent is associated with a distributed energy resource to implement local decentralized control, and the second-level control agent is associated with the first-level agent to implement distributed coordination control together with the first-level agent. Afterward, the assessment index of each distributed energy resource subsystem is established. By the assessment index, the multi-agent system can judge whether the subsystem should implement the local decentralized control or the distributed coordinated control. Furthermore, by means of the assessment index, both the local controller and distributed coordinated controller are designed, respectively, based on two kinds of dynamic models of DER unit. To reduce the communication pressure, the distributed coordinated controller is built by local controller combined with the coordinated control laws. The coordinated control laws are synthesized by using the states from only neighboring subsystems. Considering the effect of communication delays on control performance, a delay-dependent

MAS-Based Distributed Cooperative Control for DC Microgrid Through Switching Topology Communication Network with Time-Varying Delays

\text{H}\infty

A decentralized control method for frequency restoration and accurate reactive power sharing in islanded microgrids

robust control method is proposed to design the distributed coordinated controller. Finally, the validity of the proposed control scheme is testified by simulation results.

Improved droop control based on virtual impedance and virtual power source in low-voltage microgrid

A multiagent system based distributed cooperative control is proposed for a dc microgrid. First, the multiagent system based control scheme is built, where each distributed energy resource unit is associated with a first-level unit control agent to deal with primary control and a secondary-level distributed cooperative control agents to implement secondary control. For the secondary control purposes of global voltage regulation and proportional current sharing, both voltage and current regulators are installed in each secondary-level agent. Their consensus secondary control protocols are designed by using the state information exchanged among only adjacent secondary-level control agents based on a switching topology communication network with time-vary delays. Then these protocols are applied into the associated first-level agent to structure an improved outer-loop droop controller. According to the reference set points corrected by the improved droop controller, an inner-loop voltage/current robust controller is designed in place of the conventional PI control to regulate the voltage and current of distributed energy resource unit. Finally, the validity of the proposed control is testified by means of simulation results.

An Improved Droop Control Strategy Based on Changeable Reference in Low-Voltage Microgrids

Abstract To achieve the frequency restoration (FR) and accurate reactive power sharing (RPS) in islanded microgrids (MGs), an improved P-f droop control is proposed. Firstly, the inverter impedance, whose value is set by the virtual impedance method, is used to minimize the impact of line resistance on powers coupling and RPS. Then, in order to restore the frequency of distributed generations (DGs) to the rated value, the reference is changed for compensating the frequency deviation (FD) caused by loads change. And the fast FR rate is achieved under a large constant k. Besides, in order to eliminate the inaccuracy of RPS caused by voltages difference (VD), the line voltage drop (LVD) is used to compensate the voltage droop characteristics. The use of voltage feedback ensures that the obtained voltage is desired after the LVD compensation. Finally, the simulation in RT-LAB indicates the effectiveness of proposed method in an islanded MG model.