Inverter Sizing and Production Curtailments

Design engineering of solar power system includes inverter sizing.  Optimal inverter sizing must consider how much DC power will be produced by the solar array and how much AC power the inverter is able to output (its power rating).  Appropriate sizing will typically allow DC production greater than the AC power rating.  This article explains why inverter clipping or production curtailments help to maximize total power output

Inverter Sizing and the DC-to-AC Ratio

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The DC-to-AC ratio is defined as the ratio of installed DC capacity to the AC power rating of the inverter. In practice, it makes sense to oversize a solar array, such that the output of the solar modules is greater than the inverter power rating (e.g. the DC-to-AC ratio is greater than 1). The result can be greater energy output in total, notably when production is below the inverter’s power rating most of the day.

For example, consider the graph of energy production as a function of time of day in the figure above. The lower blue line shows a typical bell curve of AC output with power output peaking at noon, just below the rating of the inverter indicated by the dashed line. If we increase the size of the solar array by adding more panels, thereby increasing the DC-to-AC ratio of the system (as illustrated by the green curve), we can get more energy throughout the day. The area between the green and blue curves is the energy that is gained by increasing the DC-to-AC ratio.

Inverter Clipping

Oversizing the solar array relative to the inverter’s rating will capture more energy throughout the day, but this approach is not without costs. What Figure 1 also shows is that inverter clipping cause a flatline in the green curve, and production curtailments, during the peak production hours. The inverter effectively prevents the system from reaching its maximum power point, capping the power at the inverter’s nameplate power rating.

It is crucial to model inverter clipping in order to properly design a system with a DC-to-AC ratio greater than 1, especially in regions that frequently see an irradiance larger than the standard test conditions assumption of 1000 W/m2 (because higher levels of irradiance lead to higher power output). Meanwhile, if the system design has a DC-to-AC ratio of less than 1, it will never clip; however, we will also not fully utilize the AC capacity of the inverter and total power output will be less. The inherent design trade-off is between the cost of purchasing and installing a new inverter that is over-sized and the value of the energy lost due to inverter clipping.

Key Takeaways for Inverter Sizing

  • Oversizing a solar array relative to an inverter’s rating (DC-to-AC ratio greater than one) allows for increased energy harvest throughout most of the day, especially in the morning and late afternoon.
  • When a DC array produces more energy than the inverter is rated to handle, the inverter clips the excess power and caps its output at its rated power (an effect known as inverter clipping).
  • An alternate approach to increase energy production while avoiding inverter clipping would be to include another inverter. When deciding what approach to take, designers must consider the trade-off between the cost of purchasing and installing an additional inverter compared to the value of the energy that will be lost due to inverter clipping if they oversize the solar array.
  • When estimating the energy production of a solar project design, it’s important that your performance simulations take inverter clipping into account in order to ensure production results accurately reflect the system size of the design.

 

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