A Variable Switching Frequency Multimode Control Scheme for Single-Phase Grid-Tied Multilevel PV Microinverters

Abstract

Single-phase grid-tied microinverters are required to operate over a wide power range in photovoltaic (PV) applications. It is desirable to improve the efficiency over the entire power range without extra cost. This paper proposes a variable switching frequency (VSF) multimode control scheme to optimize the device losses at each operating point for a single-phase T-type hybrid-bridge inverter. The proposed optimal control scheme is devised based on the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V-I</i> plane analysis and can therefore be applied in both unity power factor (PF) and non-unity power factor operations. With reasonable simplifications and curve fitting, neither a multi-dimension lookup table nor excessive computational burden is required for the controller. A 1kW prototype is built to verify the proposed optimal VSF scheme and compare it with conventional constant switching frequency (CSF) scheme. The experimental results show that the proposed optimal VSF scheme can achieve higher efficiency than the CSF scheme over a wide power range. The weighted average efficiency of the proposed scheme is improved by a maximum of 1.3% in unity PF operation and 0.63% in non-unity PF operation.