Basics

Inverters are commonly used circuits for converting DC waveforms to AC waveforms, typically for transmission. Within these inverters, there is often a transformer, which is used for energy conversion. Inverters that do not use a transformer lack galvanic isolation between the grid and the photovoltaic system. This can lead to dangerous leakage currents if varying common mode voltages exist. Despite this, a few transformerless inverter approaches exist, the two most common of which are the half bridge and the full bridge (with unipolar and bipolar PWM control). A drawback of the half bridge is that it requires a large input voltage. A drawback of the full bridge with unipolar control is a varying common mode which generates leakage currents. Lastly, although the bipolar full bridge only requires half the input voltage of half bridge, it is largely inefficient because there is twice the number of devices that switch the entire input voltage. 

Concept

​Grid-connected photovoltaic (PV) systems often use a line transformer to perform energy conversion and to enable galvanic isolation. Unfortunately, most of these PV systems are single-phase and operate at low-power and low-frequency. As a result, the line transformer is typically large, heavy and expensive. To reduce the size and cost of these inverters for owners of private systems, but without impacting the inverters performance or creating dangerous leakage currents, transformerless inverter topologies have been investigated in the recent past. We will perform an analysis and modeling of one such topology. Additionally, we want to see if there are any improvements we can make to the efficiency of the topology.

A Transformerless Inverter Topology

Below, we see the inverter topology that is to be modeled in SPICE. There are four main stages - the solar array, the voltage blocking and bypass stage, the full bridge stage, and the output stage (from left to right). The common-mode voltage is the value VAB/2, given the voltage at node A and node B. The idea is that if this voltage stays constant during the four commutation periods of operation, there won't be any leakage current generated in the inverter.

Eliminating Common-Mode Voltage

Above, we see that the voltage VAB is a PWM with a positive and negative half-cycle and the reference load current is a sinusoid. The common-mode voltage of the topology we our analyzing is expected to be constant, eliminating any and all leakage currents, as seen above (from the research paper we analyzed).