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Comparison of AC, DC, and AC/DC Bus Configurations for PV Hybrid Systems

Michael M.D. Ross
RER Renewable Energy Research

Full Text Report
Link to CETC-Varennes

Acknowledgements:

Research conducted on behalf of the Photovoltaics and Hybrid Systems Program at the CETC-Varennes (Natural Resources Canada) with partial funding from the Panel on Energy Research and Development (PERD).

Citation:

Ross, Michael M. D. Comparison of AC, DC, and AC/DC Bus Configurations for PV Hybrid Systems. Report to CETC-Varennes (Natural Resources Canada). Montreal, Qc: RER Renewable Energy Research, 2004.

Summary:

The PVToolbox was used to compare the operation of three bus configurations for photovoltaic-genset hybrid systems: a conventional AC/DC bus, an AC bus, and a DC bus. The simulations used one year of synthesized weather data for Freiburg, Germany.

The AC/DC bus configuration, commonly used in North America, connects the photovoltaic array to the battery by a DC connection, whereas the AC bus configuration, recently introduced to Europe, connects the array to the battery through an AC connection with the module or string inverter at one end and the system’s principal bi-directional converter at the other. The DC bus configuration employs a variable-speed genset with DC output to charge the battery directly

This study investigates the findings of a 1998 paper, “Modelling of Stand-Alone PV-Hybrid Systems and Comparison of System Concepts”, by Gabler and Wiemkin of the Fraunhofer Institute [Gabler & Wiemken, 1998]. First, Gabler and Wiemken’s simulation results are recreated using the PVToolbox. Then performance of the AC bus and the AC/DC bus are compared over a range of conditions. The annual load is kept constant, but the pattern of variation over the course of a day as well as over the seasons is altered. The size of the array is also varied. Finally, the above two bus configurations are compared to the DC bus. The investigation includes DC loads, unlike Gabler and Wiemken, who restrict their study to AC loads.

Gabler and Wiemken find that that the AC bus configuration is comparable in performance to a conventional AC/DC bus system. Here it is shown that if maximum power point tracking is included in both systems, the AC bus system requires about 10 to 18% more electricity from the engine-driven generator. In contrast, the DC bus system requires at most 2 to 3% more electricity from the engine-driven generator. These conclusions were relatively insensitive to load patterns and array size.

Because a variable speed genset can be used with the DC bus system, the fuel consumption of the DC bus system has the potential to be lower than that of the AC/DC bus system. This study found that the fuel consumption will be 7 to 10% lower, but for a variety of reasons, this probably overestimates the gains from the variable speed genset.

The conventional AC/DC bus system offers some compelling advantages compared with either AC or DC bus systems. It can satisfy AC loads directly, unlike a DC bus system, and its fuel consumption suffers only during part load operation. If intelligent dispatch can minimize the time spent during part load operation, the fuel consumption may even be lower than that of the DC bus system.