Single-phase transformerless unipolar switched inverters for utility-connected photovoltaic applications

Single-phase transformerless unipolar switched inverters for utility-connected photovoltaic applications

[Abstract]: The disadvantages of using solar energy are its capital cost (which is about A$6/W), in comparison to that of conventional sources of energy (which is about A$1.80/W), and its conversion efficiency, which in commercially available Photovoltaic (PV) systems is less than 20%. Consequently, for utility connected PV generation to become a viable alternative energy source, its efficiency needs to be improved, its cost reduced, and the quality of power supplied by the inverters must meet stringent standards.

This dissertation describes the research work carried out to optimise the conversion efficiency and to minimise the cost of a single-phase, hysteretic current control
unipolar switched inverter system, for use as an interface between solar panels and the grid network. The 1 kW (peak power) PV system being considered does not use
energy storage batteries and the inverter output is connected to the grid supply without the use of a power transformer. Improvements in the efficiency of such an
inverter system often come at the expense of the quality of its output power and an increase in cost. However, in the proposed inverter system the harmonics of the
output current has been improved without compromising its overall efficiency or its cost. An improvement in power quality has been achieved using a novel AC splitinductor
filter network that reduces electromagnetic interference, prevents unwanted operation of the inverter switches, attenuates switching frequency harmonics,
minimises low frequency harmonics and provides an average value of the inverter output current necessary for the removal of DC offset currents.

An improvement in inverter efficiency and a reduction in cost has been achieved by omitting the 50 Hz power transformer (transformerless) and by optimising the
inverter current control strategies. In Australia, some power supply authorities permit transformerless PV inverters of less than 10 kW rating to be connected to their supply system. However, avoiding the use of transformers can lead to magnitudes of DC offset current outside the limits specified by Australian Standard 4777.2, 2005 being injected into the grid supply. In this project a new cost effective DC offset current controller that removes DC offset current from the output of the inverter has been realised. This result translates into two primary benefits; firstly, a saving of about
20% in the cost of the power transformer and in the cost of providing additional solar panels to overcome transformer power losses, and secondly the DC offset controller can also be utilised in inverter applications where power transformers are used, to prevent distortion of the magnetising current.

The novel design procedure proposed in this thesis for a current controller takes into account intentional and unintentional switching circuit delays, and yields higher
efficiencies without sacrificing power quality or increasing the cost of the inverter system. The inclusion of the effect of circuit delays in the design procedure is
significant as it is shown that delay not only has an adverse effect on the performance of the current controller but also on the efficiency and the power quality of the inverter system.

Of paramount importance for the successful completion of this project was the relationship between switching circuit delays and the level of low frequency
harmonics generated by unipolar switched inverters. Theoretical analysis is developed to show why circuit delays, inverter DC input voltage and the inductance
of the current loop, are responsible for low frequency harmonics in unipolar switched and not in bipolar switched inverters. It has also been established that unipolar
switched inverters can be designed to operate within the limits specified by the Australian Standard 4777.2, 2005 and that the low frequency harmonics can be maintained at acceptable levels.

For a current controller using unipolar switching, the choice of only one of four equivalent switching combinations of the inverter switches leads to suppression of switching noise, and prevents unwanted switching without the need for additional filters. Results are presented to demonstrate the unique advantage of unipolar
switching over bipolar switching.