PV System Commissioning: Page 9 of 11

After you have calculated the expected performance and measured the actual performance, a simple comparison helps you determine whether the system has been successfully commissioned. Depending on the certainty of the assumed and measured factors, the actual performance should be within about 5% of the expected performance.

Some of the necessary measuring and reporting can be automated or accomplished more easily by using an installed monitoring system, also known as a data acquisition system (DAS). Often, these systems report inverter ac power, irradiance and module cell temperature. Some even measure and report individual string or module outputs. During initial system commissioning, however, the DAS may not be properly calibrated or the network it relies on may not be set up. Further, the DAS reporting should be verified by the on-site field measurements previously described.

CASE STUDY: 50 KWP COMMERCIAL SYSTEM, MULTIPLE INVERTERS

PV array capacity: 50,310 W STC; 234 SunPower SPR-215-WHT-U modules
Inverters: Six SunPower SPR-7000m and one SunPower SPR-4000m
Array installation: Thirty-six of the modules are on a much steeper roof plane than the others and are dedicated to their own inverter.

The system was originally commissioned, or partially commissioned, just after construction was completed in the middle of the winter. There was some midafternoon shading on parts of the array that resulted in overall system performance of about 5% below the expected, unshaded value. Because of the winter shading, the system was recommissioned 3 months later, when the weather was clear and there was no shade. However, there was slight module soiling after 3 months (module soiling factor set to 0.99). Multiple irradiance and power readings were taken for each inverter, with results summarized in Table 1.

Inverter 1 is the 4,000 W inverter with 18 modules, whereas the other six are 7,000 W inverters with 36 modules each. The 36 modules on Inverter 7 are installed on a steeper roof, which is clearly a great angle for the early March sun, as seen by the higher irradiance in that module plane and higher production on that inverter. In addition, the 36 modules on the steeper roof plane receiving more irradiance were operating at a higher cell temperature (42°C) than the other, lower-angle modules (35°C).

Temperature factor. The temperature factor, KT, is calculated as follows: KT = 1 + (CT × (TC − TSTC)). In this case, the temperature coefficient of power, CT, is −0.38 %/°C , as supplied by the module manufacturer.

The measured cell temperature, CT, is 35°C for Inverters 1 through 6 and 42°C for Inverter 7. The STC reference temperature, TSTC, is 25°C. The temperature factor for Inverters 1 through 6 is therefore:

KT = 1 + (CT × (TC − TSTC))
KT = 1 + (−0.38 %/°C × (35°C − 25°C))
KT = 1 + (−0.38 %/°C × 10°C)
KT = 1 + (−0.038)
KT = 0.962

Using the same methodology, the temperature factor, KT, for Inverter 7 is calculated as 0.935.

System derating factor. Table 2 illustrates how this is calculated using the appropriate subfactors. The system derating factor, KS, for this system is calculated at 0.89.

Irradiance factor. The average recorded irradiance values, divided by the STC reference value of 1000 W/m2, provide the system’s irradiance factor, KI. For example, the average irradiance during testing of Inverter 1 is listed in Table 1 as 842 W/m2. The irradiance factor for Inverter 1 is therefore 0.842 (842 W/m2 ÷ 1,000 W/m2).

The performance test results for this system are provided in Table 3. Clearly, the system was performing quite well. Ongoing performance verification is simplified since the data acquisition system at the site monitors individual inverter production, as well as irradiance and cell temperature.

Article Discussion