Active Power Decoupling (APD) Converter for PV Microinverter Applications

Abstract

Under global challenges in climate change, the demand for renewable energy is continuously growing. Photovoltaic (PV) power and its integration into the utility grid are gaining increasing traction. To lower the levelized cost of energy (LCOE) of PV systems, enhance the adoption of PV applications, and ensure the delivery of high-quality power to the utility grid, there is a growing need for reliable, cost-effective, efficient, and compact PV inverters. One key challenge in single-phase PV systems is the short lifetime and poor reliability of electrolytic capacitors used for decoupling the double line frequency (DLF) power. To eliminate the less reliable electrolytic capacitor, the active power decoupling (APD) technique is widely adopted. Various topologies can be used for APD, but the selection of proper topology, modulation scheme, and circuit components, along with the control strategy, will enhance the efficiency, power density, reliability, and cost of the overall PV microinverter. This Ph.D. dissertation proposes an APD converter circuit suitable for PV microinverters, designed for optimized efficiency, power density, and cost. The proposed APD converter is controlled to achieve good power decoupling performance and to optimize the system's maximum power point tracking (MPPT) efficiency.

The proposed APD converter circuit is analyzed in the low-frequency domain for power flow and in the high-frequency domain for modulation strategy, where different topologies are considered, taking into account the voltage and current ratings of active devices and decoupling capacitors. Two modulation approaches, continuous conduction mode (CCM) and critical conduction mode (CRM), are compared, considering detailed zero voltage switching (ZVS) operation and different loss mechanisms. Parametric design and multi-objective optimization are performed for CCM and CRM to select circuit components and switching frequency for each modulation strategy to minimize power loss, volume, and costs. With the results of multi-objective optimization, Pareto-optimal designs for CCM and CRM are analyzed in terms of the impact of various circuit elements, namely: switching device output capacitance and on-state resistance, inductor winding turns and core geometries, as well as capacitor dimensions and capacitance.

With the optimal CCM- and CRM-operated APD realizations, closed-loop control algorithms are designed, and the corresponding system characteristics are compared. A simple pulse width modulation (PWM) based control strategy that does not rely on zero-crossing detection (ZCD) is proposed to implement closed-loop CRM modulation. In addition, advanced control technologies, including double sampling-based average current control, current observer-based reduced sensor control, and sensorless predictive control, are proposed to improve APD converter performance, reduce system complexity, and lower circuit cost. The proposed APD converter operation is extended to different application scenarios, including burst-mode operation and non-sinusoidal power delivery, including systems with non-linear circuit components, non-linear local loads, or non-ideal grids. A feed-forward control solution is proposed to enable power decoupling for non-sinusoidal power with improved control accuracy and reduced closed-loop design burden. The circuit design, associated analyses, and control approaches are validated by the design, development, and testing of 400 VA APD hardware prototypes.