PV system commissioning - Perfection is the key!
24th September 2018
Effective management of a solar PV plant could be divided into 3 distinctive steps i.e. Engineering Procurement & Construction (EPC), Commissioning and Operations & Management (O&M). EPC deals with the pre-construction and construction activities of the PV power plant. Commissioning is the process that starts along with the construction of plants and proceeds through PV system acceptance. It also deals with the necessary documentation processes required for system acceptance. O&M deals with processes and/or actions required for maintaining and operating the power plant to its maximum capacity. While a lot is known and talked about EPC & O&M, a little is known about commissioning, which is a mandatory and significant process. For a novice in the solar field, it is important for him to know what the important process/checks while commissioning are. Additionally commissioning also ensures that the plant performs as expected/planned which in-turn drives its ROI. This blog aims to educate its readers on basics of commissioning along with few important tips.
A solar PV plant may have been built to fulfill goals for a customer say reduction in electric bill, maximized return on investment, maximized energy generation to name a few. Commissioning ensures that such goals are achieved to its maximum. Let us understand the crucial steps in commissioning.
Verification and documentation of installation
Amongst the first of the steps would be to verify the entire installation and ensure that it is complete and matching with the power plant which was initially designed without any deviations. This includes checking of any bends or dents in structure which may reduce its strength. It also includes checking the module alignment which should look perfect in-line with other modules (as shown in Figure 1). This would ensure that the installation is robust and aesthetically acceptable.
Figure 1: Checking of structure (on left) &a rooftop plant (on right) commissioned by Waaree
While important to match the ideal characteristics / design of the power plant, there are always certain deviations from it. Not only is it important to exactly locate such deviations (at micro level) but it is also important note such deviations for a power plant. One of the best solutions is to document the entire plant along with the deviations or document as-build conditions. This document shall preliminarily help the O&M team to closely monitor the deviations for any potential problems. In the longer run it would help the team to understand any fault if arising due to such deviations and rectify it(as and how needed).
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)
The final check of commissioning is to gauge the performance of the plant. After the energizing of the plant is success, a bright clear & sunny day with moderate temperature and appropriate time is selected. At appropriate time of day, both DC and AC power is monitored. The performance ratio (PR) of the power plant is calculated. The practically obtained data on field is compared to the expected performance which was calculated while designing the plant. Such monitoring may be undertaken from mere few hours to as long as few weeks. Such monitoring also checks for any erroneous behavior of power plant which may cause unexpected loss of power and hence its revenue. Before final handing over of plant to the end consumer, it is expected that the plant is performing at an optimum level with the power losses below the threshold level.
Thus commissioning is one of the significant steps to ensure that the solar PV plant operate smoothly. Waaree has an experience of executing and commissioning more than 500 MWs of EPC projects. We ensure to follow all the safety standards and necessary guidelines in order to erect a power plant. An expertise like such enables end customer to be assured that their plant would be up and running for 25 years.
Let us all pledge to make solar energy the primary source of energy in the near future.
RAHE ROSHAN HAMARA NATION