Research On Multi-agent Theory-based Distributed Power Sharing Control And Power Quality Management Strategy For Multiple-inverters In Microgrids

Nowadays,distributed generations give priority to with clean energy and renewable energy,the technology on them has received great attention and developed worldwide.Microgrid is becoming important concept to integrate large amount of distributed generations(DG)and to connect DG to power grid.The concept has been developed to cope with the penetration of renewable energy system,and to improve the stability and reliability of power supply and power quality.Compared with traditional power distribution network,the deficiencies of the DG such as intermittent,randomness and change of power flow can bring profound influence to the microgid.Also,there are many technical limitations in the DG.Especially,one of the most innovative and challenging research are the coordinated operation among DGs,accurate power sharing,stability of voltage and frequency and power quality management.On other hand,because of the penetration of renewable energy systems in the microgrids and various operation modes of the microgrid,it is difficult to collect global information in the microgids and to control them.Therefore,the controls of the microgrid containing multible DGs are impossible to be realized in the conventional centralized control strategy.Moreover,multi-agent based distributed control strategy for solving this kind of distributed problem has obvious advantage.Therefore,in dissertation,some key technical problems in distributed control were studied with multi-agent based control theory for multiple-inverters in the microgids,and some innovative research results are obtained with including multi-agent based virtual impedance control method,the accurate power sharing control among DGs in islanding microgird,secondary voltage and frequency restoration control,and power quality management control.The concrete research contents are as follows:1.On the problem of voltage and frequency deviations due to conventional droop control,the model of microgrid and DG are established with considering the mismatched line impedance,the limitations of exsiting secondary controller are analyzed and then consensus-based secondary control methods are proposed for the voltage and frequency restoration.In the secondary frequency restoration control,the distributed controller is proposed with considering accurate active power sharing;in the secondary voltage restoration control,the dynamic consensus-based average voltage controller is proposed with considering the mismatched line impedance.The simulation results are shown to verify that the proposed distributed secondary control method can be restored the voltage and frequency in the microgrid.2.On the problem of inaccurate reactive power sharing resulted from the conventional droop control,with considering the actual situation of the mismatched line impedance,the reactive power sharing and the suppressing circulating current method are analyzed through the conventional voltage-reactive power droop control and line impedance.Then the virtual impedance control approach is proposed to realize decoupling between the active power and reactive power,and accurate reactive power sharing.The consensus algorithm is used to control the virtual impedance to realize accurate reactive power sharing.This approach discovers reactive power sharing error to regulate virtual impedance with using the consensus algorithm.The load reactive power among DGs is shared through the all DG virtual impedance which is regulated in inverse proportion to its DG power rating.3.The single phase DG and single phase load connected in the microgrid result in the voltage unbalance at point of common coupling(PCC).The DG can be employed as distributed compensators for improving the voltage unbalance at PCC in islanded microgrids.With analyzing of the unbalanced microgrid system,the voltage unbalance compensation and the accurate imbalance power sharing approach is proposed.On this basis,the distributed imbalance power sharing strategy is proposed.This strategy is realized with using consensus algorithm and the fundamental negative-sequence virtual impedance control.In contrast with the previously proposed methods,this control approach does not require a dedicated central controller,and the communication links are only required between neighboring DGs.With proposed control strategy,the load imbalance power is accurately shared;meanwhile the voltage unbalance at PCC is satisfied IEEE standard on power quality of distribution networks.4.The inverter that connects DG to the microgrid and the nonlinear load at PCC will cause the voltage and current harmonic distortion.The microgrid including the nonlinear loads is analyzed according to the superposition theorem.On this basis,a harmonic droop controller is proposed that the right amount of harmonic voltage is added to the inverter reference voltage to compensate the PCC harmonic voltage.The PCC voltage at the individual harmonic frequency is closed to zero by using the proposed harmonic droop controller to improve the total harmonic distortion(THD)of the PCC voltage considerably.Also,a consensus-based distributed harmonic virtual impedance control approach is proposed to address harmonic power sharing problem.The proposed method can realize accurate regulation of DG unit equivalent impedance at harmonic frequencies.By using this proposed method,the accurate harmonic power sharing can be realized with compensating of the impact of the mismatched line impedance.