Research On Neutral Point Voltage Stability Control Of Three-phase Four-wire Emergency Power Supply

The situation for three-phase emergency power supply to feed unbalanced load is common.In case of independent work off power grid,the output power quality can only be guaranteed by the system itself.The presence of three-phase four-wire inverter neutral-line make the neutral-line current can be controlled,it is easy to achieve three-phase voltage symmetrical output under unbanced load and phase voltage can be directly outputted.So this inverter fits three phase emergency power supply system.The neutral-line current flow into split DC-link capacitors will cause neutral point voltage fluctuation when traditional three-phase four-wire inverter feed unbalanced load.The neutral point voltage fluctuation can be suppressed by split DC-link capacitors if the mid line current is small.However,with the increase of load unbalance degree,the midline current will be increased,a larger capacitance is required to ensure that the neutral point is stable.The increase in capacitance will put great cost and volume on the inverter and the capacitance difference between the two capacitors also affects the stability of the neutral point voltage.In view of problems and needs above,this paper first analyses the working principle of traditional three-phase four-wire inverter,as well as the influence of neutral voltage fluctuation on the output voltage amplitude and phase.The necessity of three-phase four-wire inverter neutral point stability to symmetrical three phase voltage output is determined.This paper points out the shortcomings of the independent neutral-line neutral point stability control topology control system.A kind of auxiliary winding structure in DC/DC converter is applied to design a half-bridge neutral point stability control topology,The energy between the split capacitors is balanced by the forward principle,as well,its working mode is analyzed in detail and the corresponding control strategy is designed.According to its possible core saturation risk,an improved full-bridge neutral point stability control topology is designed,and detailed working modal analysis is carried out,meanwhile the control strategy is improved.The SIMULINK simulation results are used to verify the correctness of the theoretical analysis.It is shown that the half-bridge neutral point stability control topology and the neutral point voltage can be stabilized by the full-bridge neutral point stability control topology under the condition of unbalanced load.The inverter can output three-phase voltage symmetrically.In order to verify the advantages and effectiveness of the proposed three-phase four-wire neutral point stable inverter topology,a prototype of novel three-phase four-wire neutral point stability control system is designed and manufactured.The experimental results show that the half-bridge and full-bridge neutral point stability control three-phase four-wire inverter can guarantee the stable control of the neutral voltage under the condition of unbalanced load,further to realize the three-phase voltage symmetrical output.Meanwhile the three-phase inverter system and neutral point stability control module can work in independent control,so that a modular design is achieved in this design.