Research On Waveform Control And Parallel Operation Technique For Three-Level Inverter

Uninterrupted Power Supply's (UPS) paralleling operation is the main technique to enhance the reliability and enlarge the capacity of the power supply; it satisfies the development trend of the future UPS. The multilevel inverter is widely used in high-voltage high-power application field, and now its advantage is introduced into mid/low-power inverter equipment and parallel operation. Based on the research and design for multi-format modularization UPS, several contents about inverter are studied in detail, such as SPWM modulation strategy of Diode-clamped three-level inverter, instantaneous voltage PID controller, parallel sharing current technology based on output power regulation and instantaneous circulating current feedback, etc.The multi-level converter technology and its control strategy are introduced simply, at the same time the SPWM modulation strategy about three-level inverter and its traditional realization are discussed mainly, the difficulty of digital realization included. Based on the traditional single polarity pulse width modulation method, the traditional SPWM modulation strategy for single sinusoidal and two triangles is improved, and an improved modulation method and the analog and digital realization are presented.A linear mathematical model about an open-loop PWM inverter is established and the main results which influence output waveform are analyzed. The waveform control method about instantaneous voltage PID feedback is introduced, and then the mechanism that how this control method restrain the load current disturbance is analyzed from the aspect of equivalent output impedance. The digital control technology about instantaneous PID is studied, then the simulation and experiment results indicate that the stability of inverter using digital PID controller becomes worse and the steady area gets smaller.Based on the grid-connection theory of synchronous generator in electric power system, the traditional parallel control strategy which adopts output power regulation is presented, and then the power transfer characteristics and circulating current characteristics in parallel system are analyzed in detail. According to two main control variables, active power and reactive power, the formula of average power is introduced and simplified based on Fourier series theory. Using the traditional parallel control strategy and simplified power numeration, the experiment has been carried out and the results are presented.A parallel equivalent model with output impedance is established and circulating current characteristics about this model are analyzed in detail. For the parallel system without reactance, the instantaneous circulating current feedback is introduced based on PID closed-loop feedback control and it regulates with the output voltage at the same time, so that the system can share instantly and the impedance between two inverters is enlarged without losing the dynamic characteristic of the single inverter, and then the circulating current is reduced. The transfer function of the circulating current is brought out, and by which the circulating current characteristic is analyzed. In order to explain the correctness and feasibility of this method, the simulation has been carried out, and the simulation results are coincident with the analysis above.