Research On The Control Of Three-phase Four-wire Three-level Neutral-point-clamped Converters

Three-phase four-wire structure is widely used in the factory power supply and city power supply systems.With the advantages of simple structure,little stress of switch,low devices loss and EMI,three-level neutral-point-clamped(NPC)structure has become an important topology of multi-level three-phase four-wire system.However,as one of the essential drawbacks of this structure,the problem of unbalance of voltage of neutral-point(NP)of DC side capacitors needs to be solved.The paper mainly researches on the balance control of NP voltage.Firstly,two methods to keep the balance of NP voltage in three-phase four-wire three-level NPC converter are analyzed:the control strategy based on the optimized 3D space vector modulation(03DSVM)under the ?-?-0 coordinates and the orthorhombic a-b-c coordinates.The basic control principles of the two stategies are analyzed.And then,the imperfections of the two strategies are analyzed respectively.In allusion to the limitation of the control under the orthorhombic a-b-c coordinates that cannot compensate the NP voltage differential quantitatively,a new control strategy is proposed based on the carrier modulation.By this control strategy,the offset of NP voltage differential can be compensated quantitatively through decomposing the action time of zero level to positive and negative level equivalently in one phase selected based on the theory of neutral-point control margin(NPCM),and it will not change the average value of the output voltage in this way.So,the NP voltage control can be realized.The strategy can suppress the offset and fluctuation of NP voltage under different operation conditions.Although it cannot fully eliminate the fluctuation of NP voltage,it improves the stability of the NP voltage as much as possible without adding too much switching frequency.The idea of disassembly of zero level is extended to the NP voltage control of three-phase three-wire NPC structure.A strategy based on disassembly of zero is proposed.In this strategy,one proper phase is selected on the basis of the comparison of the region of the NP current before and after the disassembly of zero levels of different phases.In this way,choose the correct phase to disassemble its zero level,and the offset of NP voltage can be compensated quantitatively.Moreover,another control strategy under zero-sequence voltage injected into the modulation signals is proposed,which directly selects the phase whose value is the middle of the modulation voltages to disassemble zero level on the basis of analysis of the rules of zero level after zero-sequence voltage injection.So,the NP voltage balance control can be realized by this control strategy.The experimental platform is set up,and the validity of the strategies mentioned above are proved by experiments.