Research On Distributed Secondary Control Strategy For Microgrids Based On Multi-Agent System

In recent years,problems such as global warming,environmental pollution and energy crisis have led to increased use of distributed generation(DG).As an effective way to absorb DGs nearby,microgrids have been developing rapidly.However,with the increase of DGs penetration rate,the problems of coordinated control and low power quality among distributed power sources in islanded microgrids have become increasingly prominent.Microgrid contains a large number of power generation units with power electronic interfaces(such as photovoltaic arrays connected by inverters),and its low inertia characteristics make it different from traditional large-scale power system control.Therefore,the design of appropriate control strategies to ensure the safe and reliable operation of micro-grid has become the focus of current research.Separate local control is difficult to achieve global regulation.In order to achieve system-level control,the method of microgrid hierarchical control is proposed.In hierarchical control,the primary control quickly maintains the stability of the system,but the frequency and voltage deviate from their respective set-points.The secondary control mainly focuses on the recovery frequency and voltage.In order to overcome the shortcomings of centralized and decentralized secondary control,this paper proposes a distributed control method based on multi-agent system(MAS).Since the communication delay is unavoidable in the actual communication network,and the communication delay may adversely affect the stable operation of the microgrid.Therefore,this paper focuses on the impact of communication delay on the performance of microgrid control systems.The main research contents of this paper are as follows:(1)Starting from the hierarchical control architecture of the microgrid and the mathematical model of the three-phase grid-connected inverter,the existing four control methods of the grid-connected inverter are studied: PQ control,V/f control,droop control and control of virtual synchronous generator.At the same time,their advantages and disadvantages as well as their application occasions are also discussed.(2)In view of the shortcomings of the primary control to make the system frequency deviate,a distributed secondary frequency control architecture based on MAS of islanded microgrids is proposed.In the secondary control,the event triggering mechanism is adopted,that is,the distributed control strategy is started only when the system frequency deviation is greater than the set security threshold.The proposed strategy allows the active power set-point of each DG unit to be determined in a distributed manner in the presence of a communication delay in the network,so it has the function of replicating the discrete time proportional integrator in the traditional Automatic Generation Control(AGC).(3)In order to solve the problem of voltage deviation caused by primary control and uneven distribution of reactive power(affected by line impedance and other factors),a distributed secondary global average voltage recovery control strategy is proposed based on primary control.The consensus algorithm obtains an estimate of the global total reactive power demand in a distributed manner,adjusts the reactive power set-points of each generator unit,thereby realizing automatic adjustment of the node voltage.The simulation verifies that the proposed strategy can restore the average voltage to a certain safety range and ensure accurate load reactive power sharing in the case of line impedance mismatch.