Characteristics Analysis And Control Strategy Of MMC-UPQC Under Abnormal Operation Conditions

Unified power quality conditioner(UPQC)is a customized power quality manage equipment which can compensate voltage quality and current quality problems.UPQC utilizes series converter to solve voltage quality problem on the grid side,uses shunt converter to manage current quality problem on the customer side.It can meet the needs of managing multiple power quality problems that appear in the same distribution network,and can solve the difficult problem of coordination among various power quality control equipment and devices at the same time.Due to the limitation of power electronic devices withstanding voltage,the existing UPQC structure is difficult to apply to high voltage and large capacity distribution grid system.The emergence of Modular Multilevel Converter(MMC)has brought new changes to the development of UPQC converter technology.MMC adopts highly modular design and can be linearly expanded to suit different voltage levels.Compared with traditional cascaded multilevel topology,MMC requires fewer switching devices and simpler control,and it has the advantages of high output waveform quality,low switching frequency and strong fault handling ability.Series and shunt converter of UPQC adopt MMC structure to form MMC-UPQC,which provides an effective way for UPQC to be applied in high voltage and large capacity distribution grid system.Considering the complexity of internal structure and external environment of MMC-UPQC,the efficient and reliable operation of MMC-UPQC under abnormal operation is an important condition for effectively controlling power quality problems and ensuring stable operation of the system.In order to improve the operation performance of MMC-UPQC under abnormal operation state,Research on the operation characteristics and corresponding control methods of MMC-UPQC under internal sub-module(SM)fault and DC single-pole-grounding fault,and the power distribution characteristics and improvement methods of MMC-UPQC under non-ideal grid has important guiding significance for practical engineeringIn this paper,the mathematical model and basic control method of MMC-UPQC are studied firstly.Then,three main problems of MMC-UPQC fault tolerant control strategy under SM fault,characteristic analysis and energy rebalance control strategy of MMC-UPQC under DC single-pole-grounding,power control strategy of MMC-UPQC with energy storage unit under non-ideal grid are studied in depth.The main contents of this paper are as follows:(1)SM fault is a common fault type in MMC-UPQC.Focusing on the asymmetric operation of bridge arm caused by SM fault.A fault tolerant control strategy for MMC-UPQC is proposed.Firstly,a mathematic model of arms under asymmetric operation based on the switch function is established.It is revealed that the inconsistency of upper and lower arms SMs switching functions is the cause of asymmetric operation.Then,by means of compensating impedance to counteract the asymmetry component caused by the unequal number of arm SMs,the coupling equivalent circuit among AC current,DC current and AC circulation is established under asymmetric conditions.Finally,adopting current decoupling control method at different frequencies,the arms current control strategy with SM fault-tolerant ability is proposed.The unbalance and circulation problems caused by asymmetric operation of bridge arm are solved(2)The DC pole-to-ground fault is one of the most serious faults in MMC-UPQC station.Focusing on a series of problems caused by DC single-pole grounding fault,this paper proposes the fault characteristic analysis method and energy rebalancing control strategy.Firstly,this paper establishes fault current calculation model,and analyses the mechanism of fault current generation.Secondly,making use of this characteristics that the hybrid MMC arm can output negative voltage,this paper adjusts the operation mode of hybrid MMC to eliminate AC voltage DC bias and fault current.By analyzing the power flow characteristics of arm,Energy imbalance occurs between upper and lower arms.The SM capacitance voltage of the fault arm keeps rising,which affects the safe operation of switching devices.Finally,an energy rebalance strategy based on fundamental frequency circulating current injection is proposed in this paper.The proposed control strategy ensures that the normal pole can still transmit half of the rated power under DC pole-to-ground fault,and realize uninterrupted operation of MMC-UPQC system.(3)Focusing on the unreasonable power distribution between series and shunt converter with conventional compensation strategy,the coordinated power control strategy of the UPQC with energy storage unit is proposed.By reasonably allocating the output power of the series and shunt converter,the function of the series converter is fully developed.In this scheme,the work voltage and current of the series converter is fixed at the rated value and thus it can share the burden of the shunt converter inverter furthest within the limitation of the capacity.Besides,the overcurrent risk of the feeder is eliminated since the source current is limited to the active component of the load current under normal operating conditions with the utilization of energy storage unit.When the source voltage drops to zero,the load active power is supplied by the energy storage unit and the UPS function is realized.the proposed control strategy realizes the coordinated allocation of the series/shunt converter and the energy storage unit of UPQC.The above control strategies are validated in PSCAD/EMTDC simulation platform,and the simulation results prove the effectiveness and feasibility of the control strategy.