Research On The Key Technology Of Power Coordination Control In Microgrid

Microgrid is an emerging frontier technology based on renewable energy power generation.As an important supporting technology of “Internet +” smart energy,it can improve the security and reliability of power systems,promote the access of renewable energy and local consumption,and enhance the utilization efficiency of renewable energy.However,a single microgrid has some disadvantages such as limited working capacity,the weak anti-disturbance capability,output power intermittency of renewable energy and load variability.Under the high penetration of renewable energy resource,there are still many technical problems that need to be resolved urgently about how to achieve the power coordinated control for mi crogrid,improve the power quality,and enhance reliability of power supply.This paper is supported by the Key Program of National Natural Science Foundation “Research on multi-inverters parallel operation and power quality control method for microgrids”,the General Program of National Natural Science Foundation “The theory research on multi-inverters parallel resonance and circulation suppression for microgrids”,and the Hunan Provincial Innovation Foundation for Postgraduate “The key technology study on microgrid cluster system”.This article conducts the research on the secondary ripple current equalization suppression technique for bidirectional storage converter,power coordinated control technology for single-phase/three-phase hybrid microgrid,the autonomous power coordinated control technology for hybrid AC/DC microgrid,and the autonomous energy coordinated control technology for microgrid cluster.It not only solves the key problems of power coordination control technology for multi-microgrid groups,but also realizes the safe and stable operation of microgrid clusters.Besides,it can provide theoretical basis and practical guidance for the demonstration and promotion of microgrid cluster operation.And the main technical innovations are as follow s:(1)As a large scale of AC load connects to the DC microgrid,it will introduce lots of secondary ripple current to the DC bus via the inverter.And the secondary ripple current will be transferred to the storage system through bidirectional energy storage converter,which seriously affects the life of energy storage battery.For this,a secondary ripple current equalization suppression technique of bidirectional storage converter is proposed.The virtual impedance is introduced at twice fundamental frequency through the band pass filter to improve the equalization control of the secondary ripple current.Due to the increase of the equivalent output impedance of the bidirectional storage converter at twice fundamental frequency,most of the secondary ripple current would reflux through the filter capacitor,so that none or few secondary ripple current would be transmitted to the energy storage system,which can effectively avoid adverse effects of the secondary ripple current on the storage system.Simulation and experimental results show that the control strategy is effective and reasonable.(2)For the voltage fluctuation and three-phase power unbalance in traditional single-phase/three-phase hybrid microgrids operating in the islanded mode,a hybrid single-phase/three-phase microgrid structure and its power coordinated control method are proposed.The proposed structure is composed of power sharing unit,energy storage unit,single-phase microgrid and three-phase microgrid.When three-phase power is unbalanced,the proposed energy coordinated control method for power sharing unit is used to coordinate power exchanges among three phases to restrain three-phase power unbalance.Meanwhile,it can also offer fast support for active and reactive power to suppress the voltage fluctuation of microgrid at the renewable energy output or the load mutation.The energy storage unit is used to stabilize the dc-bus voltage of the power sharing unit and achieve the buffer for the power fluctuation in the hybrid microgrid.Simulation and experimental results indicate the rationality and effectiveness of the proposed method.(3)Aiming at the problems of power sharing control and power coordinated control for multiple bidirectional interfacing converters in parallel,this pa per presents an autonomous power coordination control strategy for hybrid AC/DC microgrid.This method introduces the adjustment amount relating to the bus voltage of DC microgrid into the traditional droop control method,which achieves effectively power coordination control on both sides of the hybrid microgrid and power sharing between bidirectional interfacing converters.Furthermore,in order to suppress the power fluctuation of hybrid AC/DC microgrid,energy storage systems are added to the DC microgrid.Meanwhile,an autonomous power split coordinated control strategy is proposed for energy storage systems.By adding the low-pass or high-pass filter in the power exchange equation of energy storage systems,the proposed control method can make the battery and super-capacitor respond to the power fluctuations at different frequencies,respectively.The simulation and experimental results verify the effectiveness and rationality of the proposed autonomous power coordination control strategy.(4)To address the issues of energy mutual aid and power coordinated exchange control among various subgrids in microgrid cluster,a microgrid cluster structure and its autonomous coordination control strategy are proposed.The proposed structure is composed of AC subgrids,DC subgrids,power exchange units and energy pool.Power exchange units are mainly used to coordinate power exchange among the subgrids and energy pool,which can not only realize energy mutual aid among each subgrid,but also maintain the stability of the bus voltage and frequency in each subgrid.Energy pool is used to stabilize the dc-link voltage,and reasonably allocate the net exchange power that power exchange units' need.Moreover,the power coordinated control method based on adaptive power exchange coefficients for power exchange units and the hierarchical coordinated control method for EP are proposed,respectively.With the application of the proposed control strategies,autonomous coordinated control for the microgrid cluster is effectivel y realized.The validity of the proposed microgrid cluster structure and its autonomous coordination control strategy has been verified by simulations and experiments.