Sistema de comunicação e controle para múltiplos conversores aplicados em transformador de estado sólido

Abstract

This PhD Dissertation proposes a communication and control structure for multiple converters systems, with emphasis on modular structures applied in solid state transformers (SST) with three power stages. The SST under study uses series converters in the highvoltage side, independent isolated dc-dc converters for each module, and parallel converters at the low-voltage side. A review of data communication and synchronization structures in multiple converter systems is developed. The SST control structure is defined to determine the flow of variables between the control loops and the power modules. From this definition and the modular configuration of SST the ways of implementation of signal processors are evaluate, considering the use of a central unit. A communication structure that meets the modularity feature is proposed and its characteristics, functionalities and influence in the control are discussed. In order to meet the control demands, a customized protocol is proposed, operating on a double bus, and its performance is compared to the CAN protocol performance. The proposed protocol is implemented experimentally and its performance characteristics and constraints, imposed by the digital processors, are obtained. The control loops are designed considering the implementation delays. Simulation and experimental results are presented for the independent operation of the converters and in the SST. Finally, stability and controllability are presented and discussed for the isolated DC-DC converter connected to the DC-AC converter. The DAB has good dynamic response to DC voltage control when operate isolated, but connected to the DC-AC converter, it may present loss of controllability. This is due to the constant power behavior at the DC-AC converter input operating in closed loop. Thus, it is proposed a closed-loop model for the voltage perturbation as a function of the constant power step. The power transfer characteristics of the DAB converter are used to determine the minimum voltage required for power transfer. Simulation results for constant power loads are presented and a minimum gain crossing frequency is defined, which guarantees controllability. Experimental results are obtained for validation of voltage sag curve.