Research On Main Circuit Topology And Control Strategy Of Large Capacity 400Hz Medium Frequency Inverter

Compared with the traditional rotary unit type medium frequency power supply,the large capacity 400 Hz medium frequency inverter have great advantages,they have gradually replaced the traditional rotary unit type medium frequency power supply,they are widely used in vessel system,aviation power supply system,high performance Uninterruptible Power Supply and other important occasions.However,due to the high fundamental frequency,low amplitude and high output current of 400 Hz medium frequency inverters with large capacity,this leads to the contradiction between switching frequency and control bandwidth;The load of medium frequency inverters is usually non-linear and unbalanced,in order to obtain high performance medium frequency inverters,the traditional inverters topology can not be directly applied,and appropriate control strategies need to be selected.In the digital system of large capacity medium frequency inverters,due to the limitation of switching frequency and the delay caused by traditional digital Sinusoidal Pulse-Width Modulation method at low switching frequency,the performance of medium-frequency inverters is greatly affected,so,it is necessary to further study how to reduce the digital delay.In this paper,starting from the inverting topology,by choosing the appropriate inverting topology and modulation strategy,improving the equivalent switching frequency of inverter,and satisfy unbalanced and non-linear load demand at the same time.Combined with the control strategy proposed in the current research,and improved on this basis,and strives to propose a better solution to the key problems existing in the medium frequency inverter,realize quality control of inverter output waveform,high dynamic,static characteristics and high stability of large-capacity 400 Hz medium frequency inverter power supply are obtained.Based on the multi-level inverter topology,the advantages and disadvantages of several traditional multi-level topology structures are analyzed,and an improved multi-level topology structure,two-unit five-switch cascaded H-bridge inverter,is proposed.The improved topology can output more levels under the condition of less switching devices,improve the output voltage waveform quality of the inverter,and increase the equivalent switching frequency of the inverter.Then,the principle of two traditional SPWM,carrier phase shift and carrier amplitude shift,is introduced,and an improved carrier shift modulation strategy is proposed.The quality of the output voltage waveform and its harmonic characteristics under three modulation strategies are analyzed by simulation experiments,and the advantages of theimproved carrier amplitude shift modulation strategy are explained.In order to improve the multilevel topology,the corresponding improved carrier phase shift modulation strategy is presented.Finally,the correctness of the theoretical analysis is verified by simulation and experiments.Taking the single-phase two-unit five-switch cascade H-bridge inverter topology as an example,establishes the equivalent mathematical model of it,and introduces the double-loop control principle of the medium frequency inverter power supply system,in which the proportional resonant control strategy is chosen.The principle of proportional resonance controller and its application in intermediate frequency inverter system are analyzed in detail.An improved resonant controller is derived by analyzing the relation between input and output of closed loop system.The improved resonant controller has the same control function as the traditional resonant controller,and the it has a higher phase margin than the traditional resonant controller,which is more conducive to the stability of the system.Finally,the effectiveness of the improved resonant controller is illustrated by simulation experiments under different loads and different working conditions.Aiming at the time delay problem introduced in digital control process,this paper first introduces the effect of time delay on medium frequency inverter system.After that,the principle of two traditional digital SPWM methods,symmetric regular sampling and asymmetric regular sampling,and the mechanism and size of introducing delay are analyzed in detail.The working principle and the time delay of multiple sampling immediate updating method which can effectively reduce delay are introduced.It is shown that this method is suitable for digital control of large capacity medium frequency inverter power supply.