System control and simulation of a novel three-port isolated PV inverter
Through the investigation of PV inverter, its market is growing, but further levelized cost of energy (LCOE) reduction is still required to complete with traditional fossil-fuel generation. This thesis proposes a novel three-port single-stage PV inverter with high power density and grid service capability, for which the LCOE can be reduced dramatically. The DC-AC stage, bidirectional battery port, and MPPT of the PV inverter system are studied respectively, with a focus on modeling, control, real-time digital simulation. For the DC-AC stage, the modulation and ZVS constrains of dual-active-bridge (DAB) converter are analyzed in depth. A hybrid control strategy with both single-phase-shift (SPS) and dual-phase-shift (DPS) is utilized to accomplish the DC-AC conversion. This control strategy can guarantee ZVS, therefore, a high efficiency over a line cycle can be achieved. The offline simulation is conducted to test the THD performance of the control algorithm. To facilitate fast responding regulation service (FRRS), a battery energy storage system (BESS) is integrated with the PV system to provide fast grid service as well as regulating the reactive power. The bidirectional battery charger is designed with CC-CV control. Through both offline simulation and hardware experiment, the performance and stability of the controller is validated. In addition, the MPPT control algorithm for the PV inverter is designed, implemented, and verified. As the whole control system must be implemented digitally, controller-hardware-in-loop (cHIL) simulation can de-risk the complicated digital control system development. cHIL provides a virtual environment for the digital controller test and enables feedback control test without a power hardware setup. The control algorithms proposed above have been validated in cHIL environment successfully. At last, a detailed power loss model of DAB converter is built. Specified methods for calculating switching losses and magnetics losses are applied. The theoretical losses calculation and circulating-power hardware test result are compared in details and the results indicate that the proposed power loss model can predict the inverter efficiency accurately.