Distributed Cooperative Control Of Microgrids In Islanded Operating Mode

Micro-grids (MGs), as a core component of the smart grid distribution network, is a small power generation, distribution and utilization system linked by Distributed Energy Resources (DERs), energy storage devices, energy conversion devices, control, monitoring, protection, energy transmission devices and loads. Micro-grids can operate in network operating mode or islanded operating mode when the main power grid break down or need disconnected operation. The voltage and frequency are controlled by the power grid in network operating mode, while MGs must maintain a stable state and well power qualities in islanded operating mode. Micro-grids must achieve by control of their own to meet the following requirements: voltage and frequency stability and deviations meet the load demand, power allocation between the distributed units, less harmonic and small loop currents, etc. So the hierarchical control strategy is used in this paper. That is, the whole micro-grids control will be made up of the primary control, secondary control and tertiary control. The primary control is to maintain voltage and frequency stability in islanded operating mode, distribute of the powers and reduce the unexpected loop currents etc. Traditional droop control strategy can realize decoupling control of active reactive powers, and reach the "plug and play" function, but it requires a high inductive out impedance of MGs. In the paper, Virtual impedances are designed to improve the inductive part and reduce the resistant part, so that, the out impedance presents high inductive features.The secondary control is to compensate the voltage and frequency offset caused in primary control. Compared with the centralized control with low reliability and flexibility, this paper adopts the distributed multi-agent system (MAS) distributed cooperative control theory to realize secondary control. Cooperative control can reach output voltage and frequency of the distributed generation units consistently. In this paper, we establish the mathematical model of MGs, design the distributed cooperative controller and analysis the stability of closed-loop system. Finally, Simulation results are presented to validate those methods.