Control For Three Phase Inverters In Redundant Parallel Operation

Paralleled control for inverters is one of most key and challenging techniques to achieve power supply systems featuring high reliability, high capacity as well as high expansibility. Paralleled control technique has been for many decades and is still a hot topic and one technique challenging in the power electronics field.Based on the paralleled control strategy for constant voltage constant frequency inverters (CVCF), a distributed control strategy with wired connection, implemented by a digital signal processor (DSP), is proposed for variable voltage variable frequency paralleled inverters feeding an asynchronous motor. The system is decoupled into two independent subsystems for synchronization regulation and current sharing control respectively by introducing the local feedback technique. A square waveform, containing information of both frequency and phase angle, is used to generate a synchronization signal for all the inverter in parallel operation, which is shared by all the inverters to keep them in phase. Then the voltage references for current sharing control are regulated through sensing the circulating currents, aiming at equally sharing the load currents. The stability issue for synchronization control and current sharing control is analyzed and the control strategy is verified both in simulation and experimentally.Referring to the well-known hierarchical control applied to power dispatching in large AC power system, a multilayer control for inverters in parallel operation with wireless connection is proposed in this paper to enhance the stability and reliability of the paralleled system. A controller, embedded in each inverter, consists of three layers:The first layer is based on an improved droop method, based on which the paralleled operation can be achieved under conditions with small phase angle difference through regulating reactive power and active power to fine adjust system frequency and initial phase angle. As a result, good steady state performance and dynamic response are obtained. The second layer is designed for compensating the droop control and thus improving the load regulation performance of the inverter. The third layer, called quasi-synchronization control, aims to limit the phase deviation between the inverter and the shared ac bus. If the phase difference is out of limit, the phase of the inverter is regulated immediately in order to ensure the phase difference to be kept within the appropriate range within which the first layer can take effect. And the paralleled inverters can keep in phase with each other and the inrush currents among the inverters are suppressed at hot plug-in. The coordinated control for all layers is proposed and analyzed in detail, and the stability of the control is analyzed as well.A quasi-synchronization control scheme is proposed for the inverters in wireless connection parallel operation. Combining the features of the inverters with the space vector control and the phase angle of the shared ac bus, the phase differences of the reference voltage and the shared ac bus are regulated within a certain small range to keep the frequency and phase angle of the inverters as same as those of the shared ac bus, which will effectively suppress the inrush currents and improve the performance of the hot swap when the inverters start to put into parallel operation. During the transients in parallel operation, the control scheme makes sure that the phase angle differences of the inverters and the shared ac bus are kept within a certain range to avoid the out-of-synchronization. The relationship between the quasi-synchronization control and the droop synchronization control is studied and the stability between them is analyzed as well.The proposed wireless parallel control technique is applied to high power and modularized train's auxiliary power system. A system architecture and its operation mode are proposed for train's auxiliary power system and the logic in the establishment of auxiliary power system is set according to the principle of the multilayer control. A prototype, configured by two paralleled35kVA inverters, is set up. And the experimental results verify the validility and feasibility of the proposed control. The theoretical and technical bases for realizing the train's auxiliary power system with high reliability, high maintainability and high redundancy are provided.