The next generation grid-connected PV inverters for high penetration applications

The increasing consumer demand and government incentives are driving the rapid growth of renewable energy generation. In particularly, the number of distributed photovoltaic (PV) system installations is increasing quickly. However, the high cost of the PV systems and the potential impacts on the safe operation of utility grid could become barriers to their future expansion. To enable the high penetration of distributed PV systems, it is necessary to bring down the PV system cost and mitigate its adverse impacts on utility grid operation. As the interface between the renewable sources and the utility grid, advanced power electronics technologies will play important roles in realizing the above goals.;The transformerless cascaded multilevel inverter (CMI) is considered to be a promising topology alternative for low-cost and high-efficiency PV systems. This research work presents a single-phase transformerless PV system based on cascaded quasi-Z-source inverters (qZSI). In this system, each qZSI module is connected to a single PV panel and serves as a PV module-integrated converter (MIC). The advantages of the proposed MIC structure include low voltage gain requirement, single-stage energy conversion, enhanced reliability and good output power quality. The innovative structure can reduce the cost and increase the efficiency of the power conversion stage. Furthermore, the enhancement mode gallium-nitride field effect transistors (eGaN FETs) are employed in the qZSI module for efficiency improvement at higher switching frequency. Optimized module design is developed based on the derived qZSI ac equivalent model and power loss analytical model to achieve high efficiency and high power density. A design example of qZSI module is presented for a 250 W PV panel with 25 V ~ 50 V output voltage. The simulation and experimental results prove the validity of the analytical models. The final module design achieves up to 98.06% efficiency with 100 kHz switching frequency. The peak efficiency can be further improved to 98.66% with synchronous rectification.;Though the tranformerless CMI-based PV systems can achieve high performance and low cost, the leakage current issue resulted from the parasitic capacitors between the PV panels and the earth remains a challenging. In this research work, the leakage current paths in PV CMI are analyzed and the unique features are discussed. Two filter-based suppression solutions are then presented to tackle the leakage current issue in different PV CMI applications. The first method is more suitable for the CMIs operated at high switching frequency. The second method extends the application to the CMIs operated with lower switching frequency by bringing in extra wire connections among the cascaded modules and the grid output. Simplified leakage current analytical models are derived to study the suppression mechanisms and design the suppression filters. Study cases are demonstrated for each of the solutions. The first solution is applied to the above presented PV system based on cascaded qZSIs. The second solution is executed in a PV system with two cascaded H-bridges where each switching device is operated at 10 kHz. Simulation and experimental results are provided to validate the effectiveness of the proposed solutions.;To mitigate the adverse impacts on utility grid operation associated with high penetration level of PV systems, an autonomous unified var controller is proposed to address the system voltage issues and unintentional islanding problems. The proposed controller features integration of both voltage regulation (VR) and islanding detection (ID) functions in a PV inverter based on reactive power control. Compared with the individual VR or ID methods, the function integration exhibits several advantages in high PV penetration applications: 1) fast voltage regulation due to the autonomous control; 2) enhanced system reliability because of the capability to distinguish between temporary grid disturbances and islanding events; 3) negligible non-detection zone (NDZ) and no adverse impact on system power quality for ID and 4) no interferences among multiple PV systems during ID. As the VR and ID functions are integrated in one controller, the controller is designed to fulfill the requirement of VR dynamic performance and ensure small ID NDZ simultaneously. The interaction among multiple PV systems during VR is also considered in the design procedure. The feasibility of the proposed controller and the controller design method is validated with simulation using a real time digital simulator (RTDS) and a power hardware-in-the-loop (PHIL) testbed. Finally, conclusions are given and the scope of future work is discussed.