Research On Three-phase Four-wire Inverter With Medium-frequency Output

Aircraft ground power requires not only the output fundamental frequency to reach 400 Hz,but also the ability to operate with unbalanced load.The power quality of the power supply is an important prerequisite to promise a safe and reliable operation for the aviation equipment.Therefore,it is of great significance to research and design a three-phase four-wire inverter power supply with the medium-frequency output.The related technology for this kind of inverters can also be applied in the field of motor speed regulation such as high-speed centrifuge and flywheel energy storage.In this thesis,a three-phase four-wire inverter prototype with medium-frequency output is developed,meanwhile,the working principle and control strategy are analyzed.The multi-level topology can improve the output fundamental frequency on the premise of having a good output power quality when working at a constant switching frequency.In order to improve the output fundamental frequency and power quality of the inverter,the switched capacitor based active clamp seven-level inverter topology is adopted.The voltage and current double closed-loop control strategy is carried out under the αβ static coordinate system to simplify the control strategy and shorten the control period.Aiming at further improving the output power quality at a high fundamental frequency of the inverter under a limited calculating speed of the controller,a control method combining open-loop and closed-loop is proposed,which interpolates the modulation wave of the closed-loop output and makes full use of the advantages of switching frequency.Load unbalance under three-phase three-wire system may lead to neutral potential deviation on AC side.The problems in three-phase three-leg inverters with unbalanced loads are analyzed in this thesis,a scheme of using neutral point regulator to control neutral point potential is proposed,and the output voltage law and power characteristics of the neutral point regulator are obtained.For seven-level three-phase inverters,a neutral point regulator topology using NPC is presented.Since the DC voltage needed to regulate the deviation of neutral point potential is very small and the output power of the regulator is only reactive power,the DC power supply only needs to support the power of the semiconductor loss for the regulator.Thus,the low voltage and large current leg can be adopted as the regulator.Focusing on the problem that the neutral point regulator can not compensate for high harmonics only by using proportional resonance control,a control strategy of repetitive multi-resonance of the neutral point regulator is proposed in this thesis.The parameters of the proportional resonance controller and the repetitive controller are designed to realize the completely independent control of the regulator.Finally,a simulation model and an experiment platform are established.The simulation and experiment are carried out under three conditions:balanced load,unbalanced load and non-linear unbalanced load.The results show that the three-phase output voltage of the inverter can be balanced under different load conditions,which proves the correctness and feasibility of the proposed regulator and control strategy.