Control of a single phase three level voltage source inverter

Power Conversion POWER CONVERSION SYSTEMS DC-DC; AC-DC; AC-AC; UPS

Control of a single phase three level voltage source inverter for

grid connected photovoltaic systems

Hinz H.*, Mutschler P.*, Calais M.**

Darmstadt University of Technology, Institute for Power Electronics and Drives

Landgraf-Georg Straße 4 D-64283 Darmstadt Tel.:+49-6151-162166 Fax:+49-6151-162613

Email: hhinz@srt.tu-darmstadt.de pmu@srt.tu-darmstadt.de

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Curtin University of Technology, Centre for Renewable Energy Systems Technology

Australia (CRESTA), GPO Box U 1987, Perth 6845, Western Australia

Email: pcalaism@cc.curtin.edu.auAn improved approach for grid connected photovoltaic systems is the application of inverterwithout transformers. The single phase three level half bridge is a reasonable solution forsystems in the lower power range. To achieve a steady state operation the control of the dc-side (solar array) and the ac-side (utility grid) is required. This paper describes the controlstrategy of the transformerless photovoltaic inverter.

The maximum available power of the solar modules depends on insolation and temperature. Tooperate the system at the maximum power point, a suited tracking method is applied. Thismethod depends on the fact, that in a single phase system the instant power oscillates withtwice the line frequency. This oscillation in ac-power also leads to a 100 Hz ripple in the dc-voltage and dc-power. The maximum power tracking is based on the analyses of the phaserelationship between the oscillation in the dc-voltage and dc-power.

To supply a sinusoidal line current with low distortion the connection of the inverter to theutility grid is made via an ac-filter which consists of an L-C-L combination. For the inverteroutput current a hysteresis controller with a variable hysteresis width is used. Additionally thestate variables of the filter are fed back by a superimposed state variable controller to achievean actively damped filter.

The final paper explains the control method and discusses simulation and experimental results.

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