| Modular Multilevel Converter(MMC)series structure microgrid is designed to solve some problems existing in ordinary microgrid structurally.And when the MMC series structured microgrid system operates in isolation,the output of each phase of photovoltaic,wind and other micro-sources fluctuates or the load is switched on,making the problem of power imbalance between phases more serious than traditional microgrids.Therefore,this dissertation studies the power balance control of MMC series microgrid in island mode.First,the topological structure and operating mechanism of t he MMC series microgrid system are introduced in detail.According to the characteristics of the system island operation control,the DC link voltage of the power generation unit can be stabilized by controlling the energy storage device,at the same time,build a random micro-source simulation model to verify the DC link voltage stability control strategy;the generation mechanism of the interphase circulating current is derived from the island equivalent circuit;the modulation strategy applicable to the system is analyzed in detail,and passed Simulation verification,Compare the advantages and disadvantages of modulation strategies.Then,a mathematical model of the output power of the MMC-MG system is established from the equivalent circuit,and its pha se-to-phase power imbalance is analyzed,and three phase-to-phase power flow modes are determined.under load balancing conditions,a phase-to-phase power balance control scheme based on active power correction is proposed.Considering the phase-to-phase SOC balance,the power that needs to be corrected is determined according to the micro-source power and load power,calculate the DC circulating current reference value.At the same time,a circulating current controller is designed to use the control of th e DC circulating current to achieve mutual power between phases and achieve the purpose o f power balance between phases.Finally,in the case of unbalanced load,the output voltage control strategy is adopted to reduce the unbalance of the output voltage,and to ensure that the output voltage is symmetrical in the island operation mode.On this premise,the control strategy of interphase power balance under the condition of load imbalance is studied,and the power that needs to be corrected is determined by allocating the proportion of each phase load power to the total load power.Using the control of the DC circulating current,the modified modulation signal obtained from it is superimposed on the original modulation signal of the upper and lower bridge ar ms for control,which in turn allows the power to flow between the three phases.And built a system simulation model for verification. |
