Research On The Parallel Operations Of Power Electronics Converters Based On Hierarchy Control Scheme Of Microgird

Recently,the share of the distributed renewable energy in the power generation has been increasing fast.Traits of such power source such as randomness have challenged the operation of the power system.Microgrid,one of important part of the smart grid,can provide ideal interface for renewable energy.Therefore,the control and optimization of the microgrid have been attractive research field in the recent research.In this paper,a series of issues about hierarchical control of mircogrid have been discussed based on the existing research.Switching circulating current problem in the primary control level and power flow optimization problem in the tertiary control level are addressed in this paper.A set of the control algorithms for these problems has been proposed to enhance the performance of the microgrid.Firstly,a method to synchronize the carrier wave of parallel converters is proposed.This method is based on the digital-based virtual dead-zone oscillators coupled with each other though electrical connections between parallel inverters.These oscillators are synchronized based on the phenomenon of synchronization in networks of coupled oscillators.Thus,the carrier waves of different converters are confirmed to be synchronized since they are generated by the oscillators.Compared to the existing solutions to attenuate the switching frequency circulating current,this methods is free of communication and extra devices and is more suitable in microgrid.In this paper,the parallel three-phase system is used as an example.The model of the circulating current and the sufficient condition of global synchronization of oscillators are derived in this paper.The methodology for controller design is outlined based on the aforementioned analysis.Experimental results for a system with three phase inverters are given to illustrate the effectiveness of the proposed control.Secondly,based on the existing power management algorithm,a multi-agent based algorithm for economy power dispatch for microgrid is introduced in this paper.This algorithm combined the droop control which maintains the power balance in microrid,and a consensus algorithm based economy regulator which turn the output power of each generator to reach the optimal power dispatch for the microgrid.This algorithm is fully distributed and free of central coordinator.In addition,with the help of droop control,the communication only happens between the traditional generators which will greatly reduce the communication nodes in the system.In this paper,the dynamic model and convergence of the system is analyzed,and a guideline of parameter design is given based on the model.The effectiveness of the control is validated by the experiments.At last,a consensus based fully distributed hierarchy control scheme for DC-microgrid is developed in this paper.This method utilizes the droop control as primary control to maintain the power balance of the system.A consensus based compensator to cancel out the effectiveness of the line impedance on the algorithm is employed as the secondary control.The economic power dispatch algorithm works as tertiary control to distribute the power in the microgrid economically.This algorithm can achieve the goals of the traditional hierarchical control with a fully distributed manner,and only with communication between controllable devices.Thus it can enhance the reliance and dynamic performance of the system.The model is established in the paper and parameter design procedure is given in the paper.The effectiveness of the algorithm is validated with experiments in a scaled down DC-microgrid system.