Proposal/Contract Language for Array Commissioning

PV Array Performance Tests

String I-V curve tracing tests shall be performed in accordance with procedures detailed in this section, and test data shall be recorded.

String I-V Curve Test

Purpose: Measure string-level I-V curves and extract key performance parameters. Compare measurement results across the population of strings in the sub-array or system. Also compare the measurement results with the predictions of a PV performance model, accounting for irradiance and temperature variations. Evaluate the shapes of the I-V curves and the values of key operating parameters derived from the I-V curves, including short circuit current, open circuit voltage, maximum power current and voltage, open circuit voltage, maximum power, and fill factor. Repair any non-conforming strings and retest them. Demonstrate via the resulting data set that the array meets performance expectations.

Scope: Measure all strings or specified strings [Measurement of all strings provides the most complete performance verification. This is a best practice for commissioning new systems. However, for periodic checkups, a sampling basis may be suitable. For repair situations, the repaired strings and a number of neighboring strings should be measured and compared.]

Party: Tests shall be performed by installer, EPC technical personnel, or contracted testing service. [Performance testing for commissioning purposes is often performed by a third party. Alternatively, the testing may be performed by the installer and reviewed by a third party such as the customer’s engineer. ]

Schedule: Tests shall be performed prior to substantial completion, during startup testing, during commissioning, and during O&M.

Conditions

The characteristics of an I-V curve change with irradiance and temperature. At low irradiance, the shape of the curve changes to an extent that data analysis is less accurate. These limitations apply for all types of performance measuring equipment (I-V curve tracing and conventional measurement methods). To allow for accurate data analysis, the preferred conditions for PV array performance measurements are clear skies, an irradiance value of at least 700 W/m2 in the plane of the array, and low wind. In addition, the measurements should be performed within a four hour time window centered on solar noon. The shape of an I-V curve changes as a function of irradiance, and the change becomes severe below 300W/m2. For this reason, testing at irradiance values of less than 300 W/m2 shall be avoided. If satisfactory measurement conditions are not available, the tests shall be postponed until the earlier of:  the date on which such conditions exist, and  5 business days following the date on which the test would otherwise have been conducted. If after 5 business days such conditions are still not available, then the test will be conducted within 5 more days. Any anomalous data apparently due to changing sky conditions shall be re-measured under stable sky conditions.

Criteria

I-V Curves

Record the measured I-V curves and display them together at the level of each combiner box and array. [Overlaying of I-V curves is an excellent way to demonstrate performance uniformity and to identify any non- conforming strings. The comparison is most useful when the irradiance and temperature are relatively constant across the family of measurements. For a large system, the conditions are usually stable for the time period required to measure the strings in a single combiner box. For a small system, the conditions may be stable enough to compare the I-V curves of the entire system.] Identify and troubleshoot any substantial deviations from normal I-V curve shape. Look for relative differences in slope in the horizontal and vertical legs of the I-V curve, and for any steps or notches in the shape of the curve.

Documentation

A report describing the test results and issue resolutions shall be provided. The data for each string shall be identified by string, combiner box and inverter numbers.