After doing the initial inspections and documentation of plant, testing is the next most important step in commissioning. The testing includes but not limited to PV module, earthing, inverter, wiring, etc. One of the first tests performed is to check the insulation resistance of conductors. This test measures insulation of conductors ensuring that there is no leakage current via conductor (shown in Figure 2). With the plant being expected to last for 25 years, the conductors shall be exposed continuously to altering weather conditions. Such alterations could lead to degradation of insulation which in the long term could compromise the safety of power plant. Thus recording the resistance value at the beginning and tracking it over the course of plant’s life would give significant information over conductor’s deterioration and hence its quality.

Figure 2: Ongoing insulation resistance test (Source: Google images)

At module level, random or all (in case of small rooftop plants) strings are selected and open circuit voltage (Voc) of each of these strings is measured (as shown in Figure 3). For plants with smaller string inverters, Voc measurement happens at line side of DC disconnect whereas for larger inverters, such measurements happens at line side of combiner box with its fuse open. Obtaining a similar voltage in all the strings means that all the strings have similar number of modules with correct polarity. A similar exercise to measure the maximum current of string (Imp) is carried out by a DC clamp meter. The output for different strings if within range of ± 0.1A signifies that the orientation and tilt of the strings is same.

Figure 3: Voltage and current measurement of PV string (Source: Google images)

While the above testing happens when the plant is de-energized (energizing refers to condition when plant is generating power and is connected to the grid), the testing of inverter is a little different. After checking and testing all the AC & DC interconnections in addition to testing conductors and modules, the inverter is energized. However, a day before energizing the inverter the grid side/AC side transformer is energized (i.e. it is connected to the grid) while the inverter and module remain isolated (i.e. AC breaker of inverter is kept open) (top side of Figure 4). This is to ensure that the transformer generates ingress current (which is the current generated via magnetic coupling) in a way that there is no problem during its further operations. On the next day, the entire plant is energized and its output voltage, current and frequency is monitored. In case of inverter tripping, ground faults in the plants are checked for. Further both DC and AC power are monitored.

Figure 4: Energizing of transformer (top) and inverter (in bottom)