Neutral-point-clamped circuits of single-phase PV inverters: Generalized principle and implementation

The common-mode leakage current should be carefully considered when designing a transformer-less photovoltaic (PV) inverter since the leakage current can cause the output current distortion and increase the operational risk. The unipolar SPWM of traditional H-bridge inverter can produce superior output performance but will cause a high-frequency common-mode voltage and consequently the non-negligible leakage current. To attenuate the fluctuation of common-mode voltage, few neutral-point-clamped circuits have been designed to clamp the neutral point voltage and maintain the common mode voltage constant. This paper reviews the already published single-phase inverters with constant common mode voltage in theory and proposes a generalized design principle of multi-terminal neutral-point-clamped circuits, whose unidirectional and bidirectional variations are fully analyzed respectively. Subsequently, two families of new single-phase PV inverters with neutral-point-clamped circuits are proposed. Also, the efficiency, cost and control complexity are compared between the proposed topologies and the traditional NPC inverters to prove the superiority of the proposed topologies. The simulation and experimental results verified the theoretical findings.