Comparison and Design of High Efficiency Microinverters for Photovoltaic Applications

With the decrease in availability of non-renewable energy sources coupled with the increase in the amount of energy required for the operation of personal electronic devices there has been an increased focus on developing systems that take advantage of renewable energy sources. Renewal energy sources such as photovoltaic (PV) panels have become more popular due to recent developments in PV panel manufacturing that decreases material costs and improves energy harvesting efficiency. Since PV sources are DC sources power conversion stages have to be used in order to interface this power to the existing electrical utility system.

The structure of large scale PV systems usually consists of several PV panels connected in series to achieve a high input source voltage that can be fed into a high power centralized DC-AC inverter. The drawback to this approach is that when the PV panels are subjected to less than ideal conditions. If a single PV panel is subjected to drastically less solar irradiation during cloud conditions, then its output power will drop dramatically. Since this panel is series connected with the other PV panels, their current output is also dragged low decreasing the power output of the system. Algorithms that have the power converter operate at different input conditions allow the system to operate at a maximum power point (MPP), however this only allows the system to operate at a higher power point and not the true MPP.

To get around this limitation a new PV system implementation was created by giving each panel its own DC-AC power conversion system. This configuration gives each panel the ability to operate at its own MPP increasing the total system energy harvest. Another advantage of the single panel DC-AC microinverter power conversion stage is that the outputs are parallel connected to the utility grid easily allowing the ability to expand the system without having to shut down the entire system.

The most prevalent implementation of the microinverter consists of a single power converter that uses the PV low voltage DC and outputs high voltage AC. In order to ensure that the double line AC ripple does not propagate to the PV panel a large bank of electrolytic capacitors are used to buffer the ripple. There is concern that the electrolytic capacitor will degrade over time and affect the system efficiency. To get around having to use electrolytic capacitors a two stage microinverter has been proposed. The two stage microinverter consists of a DC-DC converter that steps up the low DC voltage of the PV panel to high voltage DC and the second stage is a DC-AC inverter that takes the high voltage DC and converts it to high voltage AC. There is a capacitor that connects the two power converter stages called the DC link capacitor which can buffer the double line energy ripple without using electrolytic capacitors.

This thesis focuses on the review of several DC-AC inverter topologies suitable for use in PV microinverter systems. Operation capabilities such as common mode noise and efficiency are compared. The main focus of the review is to determine the optimal DC-AC inverter using the performance metrics of cost, efficiency and common mode performance. A 250 W prototype is built for each inverter topology to verify its performance and operation.