Optimizing Array-to-Inverter Power Ratio: Page 3 of 3

The basis of comparison when optimizing dc loading ratio is typically a financial metric like cost per kilowatt-hour, levelized cost of energy, net present value or internal rate of return. Several project-specific factors determine the optimal level of dc loading, including location, system design and inverter topology. The project’s cost structure and financial goals also drive design decisions. On average, designers and developers reach a point of diminishing economic returns at array-to-inverter ratios of about 1.5. Beyond that, they eventually reach a tipping point above which the incremental cost to increase array capacity outweighs financial gains from the additional energy production. This tipping point is unique for every project. To ensure that a project meets energy production and financial performance goals, it is important for designers to optimize dc-to-ac power ratios on a project-by-project basis.

For example, Figure 2 shows the modeled internal rate of return (IRR) at three different load ratios for a project in Ontario, where the available solar resource is modest but the value of PV-generated energy is relatively high. The model inputs assume that installed system costs are relatively low, as might be the case with aggressive module pricing, and that the system is deployed using high-efficiency transformerless string inverters. The results confirm that higher dc load ratios can increase a project’s IRR, even though initial installation costs are greater. Of course, the optimal dc load ratio is very different in Ontario than in New Mexico. Similarly, the dc load ratio sweet spot might be different for a project that uses central inverters rather than distributed string inverters.

Inverter Operational Limits

To deploy PV systems with high dc load ratios, designers and developers need to account for the effects of array oversizing and observe any operational limits that the inverter manufacturer imposes.

Common-sense limits. When the available dc power from a PV array exceeds the inverter maximum power rating, the control logic in the inverter responds by moving the PV array off its maximum power point. Limiting power in this manner ensures that excess power is not dissipated as waste heat in the inverter. In effect, the inverter components are not exposed to this excess power under normal operating conditions. However, an inverter with a high dc load ratio is still exposed to higher internal operating temperatures compared to an inverter with a low dc load ratio, simply because an overloaded inverter operates at its maximum rated power more often and for longer periods of time. Further, the inverter may operate less efficiently when limiting array power, with an increase in internal waste heat.

Designers can account for this effect by designing the system to promote optimal inverter cooling. Additionally, system owners and O&M providers should ensure that inverter cooling system components—like fans and filters—are maintained properly.

Hard design limits. Designers must use inverters in accordance with the manufacturer’s installation instructions. As part of UL 1741, the product safety standard for “Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources,” inverters are subjected to a series of abnormal-condition tests. During the output-overload test, a technician applies twice the rated input current to the inverter, which must maintain its rated output power. While this test proves that a listed inverter can limit power under normal operating conditions, it is only one of several factors that determine the maximum equipment rating.

Another critical factor is the amount of short-circuit current that internal components such as busbars and disconnect switches can withstand during a fault on the dc side of the inverter. In the unlikely event that the inverter’s firmware fails to limit the input current from the PV array, the inverter components must be able to withstand the full short-circuit current of the dc source for the duration of the fault without breaking or compromising safety. For this reason, designers should follow inverter manufacturers’ recommendations for maximum allowable array-to-inverter ratios. Most importantly, designers must ensure that the available PV array short-circuit current never exceeds the manufacturer’s published maximum dc input short-circuit current value.

Verena Sheldon / AE Solar Energy / Sacramento, CA / solarenergy.advanced-energy.com

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