Research On Hierarchical Frequency Regulation Control Strategy In Islanded Microgrids

Microgrid,as an effective carrier of distributed generation technology,is a key component of smart grid.However,between microgrid and traditional power system,due to the differences in operation modes,microsource types and control strategies,some new problems restricting the utilization and development of new energy arise in the operation process of microgrid.In order to improve the coordination,control and operational capabilities of distributed generation units,the direct method is used to derive the nonlinear hierarchical frequency regulation control strategy that satisfies the global asymptotic stability condition when the transient energy function of the island microgrid meets the Lyapunov criterion.The hierarchical frequency regulation control strategy not only eliminate the island microgrid frequency deviation under the global asymptotic stability condition but also achieve active power sharing among the distributed generation(DG)units.Given the complexity of the island microgrid,it is very difficult to construct its transient energy function directly.Therefore,taking advantages of the conservation of the potential energy of the spring add the kinetic energy of the vibrator in the Kuramoto model,the Kuramoto spring oscillator model of the microgrid system is used as a breakthrough to solve the structural problem of the transient energy function.Based on the operating characteristics of synchronous generators,this paper proposes a simplified modeling method from island microgrid to its kuramoto model.Aiming at the different operating characteristics of each DG unit in the microgrid,the first and second order hybrid Kuramoto spring oscillator models of the microgrid system are formed.The simulation results verify the validity of the model.To achieve the goal of power sharing,primary droop control is used to complete the power allocation of the microgrid system.By studying the inertial characteristics of each DG unit in the microgrid system,the nonlinear characteristics of the droop control is restored,a second-order Kuramoto model of the microgrid system including the underlying equipment and primary droop control is established.Based on this model,the transient energy function construction method of the microgrid system is proposed.The core idea of this method is to use the energy of the simple subsystem to characterize the total energy of the complex system composed of subsystems.According to the above idea,the transient energy function of the microgrid system can be characterized by the energy weighted sum of the "spring system" and the "vibration subsystem" in the Kuramoto spring oscillator model.Futhermore,based on the constructed transient energy function of the microgrid system,the secondary frequency regulation control strategy of the microgrid system that satisfies the global asymptotic stability criterion is derived.However,the derived controller can reduce the frequency deviation caused by the primary droop control,but due to the DG decentralization,it is difficult to obtain the optimal value of the adjustment,the system frequency deviation still exists.To solve this problem,a communication mechanism of nonlinear hierarchical frequency regulation control strategy is designed.This mechanism obtains a consistent and optimal adjustment amount through information interaction.Therefore,nonlinear hierarchical frequency regulation control strategies restores the power sharing between DG units while eliminating the frequency deviation of the microgrid system,which achieves distributed control of the microgrid system.The effectiveness of the distributed nonlinear hierarchical frequency regulation control strategy is verified by the 3-machine 5-node microgrid built by Matlab/Simulink software.