Research On The Control And Parallel Operation Control For PWM Inverters

Digital control and parallel operation control are among the key techniques for high performance inverters with the features of excellent regulation, modular, high reliability, and redundancy etc, and the corresponding research activities have been implementing since the development of digital signal processing and wide application of inverter power supply systems.Methods and techniques for digital control PWM (Pulse Width Modulation) inverters are researched systematically. Equivalent mathematical model of inverter is built, and digital regulators are designed and implemented. The control strategy for real-time regulation of digital voltage reference signal with mean value of digital modulating signal is proposed to eliminate the flux imbalance in output transformer or output filter inductor effectively. A compound digital control strategy combining output voltage and inductor current dual-loop control with repetitive control is proposed for inverter to improve its voltage regulation and nonlinear load performance.A novel system configuration and parallel control method for inverter parallel system based on power line communication (PLC) are porposed, in which the parallel operation control is decoupled to voltage reference synchronization control and output current distribution control to simplify the parallel realization, especially the module hot-swap operation realization. The frequency and phase information of voltage reference signal in each module is delivered to all the other modules in the parallel system, and the frequency of each voltage reference signal is adjusted accordingly to realize synchronization control. Based on pre-synchronized voltage reference signals, output current or output power information is shared among the parallel system, and current distribution control is implemented by modified SPWM (Sinusoidal PWM) output regulation.The voltage reference synchronization control based on synchronization bus is researched and realized by means of interconnecting line and wired-and logic in uninterruptible power supply (UPS) parallel system. The voltage reference synchronization control based on PLC is proposed, in which the phase angle of each voltage reference signal is calculated with the same phase reference, and phase difference information is exchanged by PLC among the parallel modules. With the same synchronization regulation algorithm as that of the former method, all the voltage reference signals share the identical frequency and phase.A load distribution control based on output power difference is proposed, in which the active power and reactive power are calculated in each module to obtain the active power difference and reactive power difference among the modules by PLC, and the phase and amplitude of each voltage reference signal are regulated in accordance with output power difference. The power regulation accumulative algorithm is proposed to realize load power distribution.In inverter system, output voltage root-mean-square (RMS) regulation is a common solution to improving good load regulation. The circulating current characteristics of the distributed inverter parallel system with analog control are researched to reveal the interference between current distribution and load regulation. A novel control method for compensating the impact on load distribution by output voltage RMS regulator is proposed, in which the amplitude of output current or circulating current in each moduled in each module is introduced into the output voltage RMS regulator to give attention to both good load regulation and precise current distribution.The application scope of decoupled parallel control for PWM inverter parallel system is extended to general-purpose three-phase inverter system with conventional U/f control to realize voltage reference synchronization control within the frequency range of zero to 50 Hz. By detecting the circulating current signals and adding them into the corresponding digital modulating signals of each phase in the slave module, the master-slave current distribution control is realized with three-phase asynchronous machine load.Different experimental platforms are built to verify the control methods mentioned previously, including single-phase full-bridge inverter, single-phase diode-clamped three level half-bridge inverter, and three-phase inverter. Theoretical analysis, simulation evaluation, and experimental results are provided to demonstrate the validity and feasibility of the proposed control methods. Furthermore, the main control methods in this dissertation have been transfered successfully to both civil and military parallel inverter and UPS products.