In response to growing interest and direct enquiries, GSES recently released a technical paper on oversizing PV arrays. A sample of this paper is below; to read the full article, visit the Resources and Information page.
Oversizing PV Arrays
‘Array oversizing’ refers to solar photovoltaic (PV) systems that have been designed so that the solar array has a higher peak capacity than the inverter.
A solar system of a given installed capacity under irradiance consistent with Standard Test Conditions, may actually deliver up to 20% less than the nameplate rating of the solar array. This is because of the system’s and the solar modules’ losses. Oversizing the solar array has the potential to increase the system yield, as it results in a higher PV array power output during times of low irradiance (e.g. the morning and evening, and cloudy days). The recent trend of reduced cost of PV modules compared with the cost of inverters has introduced the concept of oversizing the PV array as a cost-effective method for increasing system output. The metric used to measure oversizing is known as the DC:AC ratio, array-to-inverter ratio, or simply the oversizing ratio. For example, a 3 kW inverter with an oversizing ratio of two will use a 6 kWp array.
To connect an inverter with an oversized array, it is important to understand how the inverter limits the input DC current. For an inverter to be suitable for use with an oversized array, it must be capable of limiting the current flowing from the array. This means the inverter will function as follows:
When the array current (IMP) increases to greater than the inverter’s maximum current rating, the inverter will increase the operating voltage of the array, thus reducing the current (to a value below IMP). In effect, the array will be operating at a decreased efficiency and a sub-optimal voltage (above VMP). No additional heat is therefore required to be dissipated from the inverter, the array is simply operated at a lower efficiency. However, it is important to ensure that the array’s open circuit voltage does not exceed the inverter input ratings. If the inverter is exposed to a voltage greater than that for which it is rated, it will be damaged. Some inverters can protect themselves from currents greater than their specified range, but all inverters will be damaged from open circuit voltages above their specified range.