Cost Savings of 1500 Vdc Photovoltaic Systems
By increasing panel numbers in strings to deliver 1500 Vdc to the combiners for the same 15 MW of power outlined on the previous slide, current drops to 66.6% of the 1000 Vdc value and resistive cable losses to 44.4% because of the “I2” in I2R, resulting in higher system efficiency and lower cost of installation with smaller cables and connectors. With fewer strings to achieve 15 MW, 31% fewer combiner boxes are needed (94 compared to 137 assuming each box can handle 20 strings in this example). This means that the associated combiner cabling, connector, and maintenance costs are lowered as well. Broken down in the table below, research has analyzed the cost per watt of 1000 Vdc and 1500 Vdc systems in a 10 MW plant to show a potential deployment savings of $400K in 1500 Vdc installations. But while certainly advantageous from a cost perspective, higher string voltages do have some challenges that need to be taken into account.
||System Cost (per W)
|Cables, Conduits, Trenching
Auxiliary Circuits in PV Systems
The combiners and inverters in a PV system need low voltage isolated power for monitoring and control derived from the 1500 Vdc line, but finding small dc-dc converters that operate at these levels is not easy. The lower voltage end could also dip under 200 Vdc under particular conditions, meaning the converter needs to provide at least a 7.5:1 input range to support these wide fluctuating voltages. This wide input range is difficult to achieve with standard flyback or forward converter topologies, especially with high maximum input voltages. The variation in pulse width to regulate the output, internal peak voltages and extreme currents, further necessitates a more complex topology. Operating under different illumination conditions means the dc-dc converters require protection from frequent “brown-outs” as the input drops below the minimum threshold, while also needing to deal with over current, over voltage, and short circuit fault conditions typically seen in remote applications. As PV systems also require direct sunlight, temperatures in control cabinets can rise dramatically, making the operating temperature range of the dc-dc converter another key consideration. All of these challenges combine to make the selection of a dc-dc converter for PV applications no simple task.