A Capacitance-Minimized, Doubly Grounded Transformer less Photovoltaic Inverter With Inherent Active-Power Decoupling

Two major challenges in the transformerless photovoltaic (PV) inverters are the presence of common-mode leakage currents, and as in most single-phase converters the need for reliable and compact double-line-frequency power decoupling. In the proposed doubly grounded inverter topology with innovative active-power-decoupling approach, both of these issues are simultaneously addressed. The topology allows the PV negative terminal to be directly connected to the neutral, thereby eliminating the capacitive-coupled common-mode ground currents. The decoupling capacitance requirement is minimized by a dynamically variable dc-link with large voltage swing, allowing an all-film-capacitor implementation. Furthermore, the use of wide bandgap devices enables the converter operation at higher switching frequency, resulting in smaller magnetic components. The topology uses only four switches and potentially enables a high power density solution. The operating principles, design and optimization, and control methods are explained in detail, and compared with other transformer-less, active-decoupling topologies. A 3 kVA, 100/75 kHz single-phase hardware prototype at 400 V dc nominal input and 240 V ac output with a wide range of power factor has been developed using SiC MOSFETs with only 45 μF/1100 V dc-link capacitance. Extensive experimental results from the prototype are presented to validate the concept, design, and superior performance of the proposed topology.

I. Introduction

Solar energy is among the fastest growing renewable energy resources accounting for an increasing and significant share of new generation capacity additions each year [1], [2]. In the U.S., new solar installations have exceeded 1 GW dc in each quarter since 2014, reaching a total capacity 29 GW dc in March 2016. In Q1 of 2016, solar photovoltaic (PV) systems accounted for 64% of new electric generation added in the U.S. making it the largest source of capacity addition across all fuel types [1]. The distributed PV systems are garnering the interest of both the utility providers and residential consumers with the reducing solar panel costs, government incentive programs, and regulatory policies [3]– [5]. The power electronic converter is a key component of the grid-connected PV systems, extracting maximum power from PV and interfacing with the grid. The transformerless PV inverters are increasingly more attractive due to their lower cost, reduced footprint, and improved efficiency compared to inverters with transformer isolation. However, a major challenge with the transformerless inverters is the presence of common-mode leakage currents [6], which can increase the system loss, distort the grid current, and induce severe electromagnetic interference. In addition, similar to most single-phase converters, another main challenge is the presence of double-line-frequency power ripple [7], which is the difference between the instantaneous grid-injected power and the constant dc power corresponding to the maximum power point (MPP) from the PV panels, necessitating the use of large filters in conventional topologies.