Dual polarity DC–DC converter integrated grid‐tied single‐phase transformer less inverter for solar application

Abstract

This study proposes a novel single-phase transformer-less inverter, using the principle of combined Ćuk SEPIC (CCS) converter for grid-connected photovoltaic (PV) systems. The new inverter has a common ground between the grid and the PV source, which helps to eliminate the leakage current for the grid-connected PV application. Unlike common ground-type charge-pump-based transformer-less inverter, this topology eliminates inrush current and hence reduces the current stress on the components. The CCS allows voltage control with both step-up and step-down abilities, along with more robustness against solar panel side fault. Further, application of wide band-gap devices, such as SiC MOSFETs allows higher switching frequency to be achieved, and thus reduction of the passive components. A novel switching strategy, proposed here allows current in both directions, positive and negative (to the load/grid or from the load/grid, for reactive loads), making the converter suitable for grid connection (unity power factor), as well as stand-alone operation. The proposed concept has been discussed in detail, along with simulation results. Finally, a prototype hardware has been fabricated and the experimental results are reported. 11Introduction Grid-connected photovoltaic systems (PVs), particularly low power single-phase systems (up to 5 kW) are becoming more common worldwide, due to declining PV price, government incentives and advancement in power electronics and technology [1]. To date, the PFC boost converter, coupled with H-bridge and transformer has been the most popular topology for PV systems. Besides providing the additional capability to adjusting voltage level, the inherent galvanic isolation blocks common mode (CM) current and limits the fault current. However, due to weight and volume of the line frequency transformer, different topologies [2, 3] have been proposed to avoid them without losing the main benefit of blocking CM current. It has been previously reported, by using a virtual DC bus [4], the common ground connection can be achieved, which diverts CM current. In this paper, a novel converter topology has been proposed based on dual polarity DC-DC converter, which creates separate positive and negative virtual DC bus; thus allowing the common ground connection of the solar panel with the grid. The dual polarity DC-DC converter allows more flexibilities for voltage control with both step-up and step-down abilities, along with more robustness against solar panel side fault. The next section details the converter topology and operating principles, along with pulse width modulation (PWM) strategy of different switches in the converter. The proposed converter has been investigated with simulation studies, using MATLAB-Simulink and PLECS, and the results are reported. Finally, a laboratory prototype has been developed and experimental results are presented to validate the concept. 22Converter topology and operation The circuit schematic of the converter is shown in Fig. 1. The converter can be analysed as a two-part system, consisting of a combined Ćuk SEPIC (CCS) converter [5] along with a bipolar DC to single phase AC converter. The CCS converter generates a bipolar DC output, which is referred as virtual DC bus. The switching node of the Ćuk and SEPIC converter is unified by the switch S 1. The Ćuk converter generates a negative voltage with respect to the common ground node G, at node N. Similarly the SEPIC converter creates the positive voltage output at node P with respect to the ground. The output of the Ćuk and the SEPIC converter is related to input. DC voltage as V Cuk = − V in d 1 − d (1) V SEPIC = V in d 1 − d (2) where V in is the input DC voltage, and d is the duty ratio of the switch. The dual polarity output generated by this CCS converter, between nodes P, N and G, is fed to the bipolar DC to single phase AC converter consisting four switches S p , S n , S zp , and S zn , which produces an AC output. It can be observed, the load neutral point, node G is common to the source ground, thus allowing a common ground connection which eliminates the CM leakage current. Switch S p of the bipolar DC-AC converter is switched in sine PWM during the positive half cycle. During switch-on period-positive-active, the circuit path is shown in Fig. 2-positive voltage active. The anti-parallel diode with the MOSFET allows the current path for both the directions. When the switch S p is turned off, the zero state switches − S zp and S zn , need to be turned on, to allow the current path for the load. Depending on the direction of the current, it can take either of the two paths. For the current to the load (positive direction) the path is completed by the