Light Talk: Solar Plant Commissioning

June 23, 2021

A comprehensive solar facility commissioning plan ensures a solar plant transitions gracefully from construction to continuous operation. A good commissioning team has a full construction understanding coupled with experience in engineering, operations, and maintenance. These are some of the steps we take when commissioning solar facilities prior to transferring the facility to a new owner:

Third Party Review

A third-party review from an independent engineer (IE) is often a necessary initial step. Prior to any site activities, the IE reviews the test procedures and identifies any deficiencies or short comings in the plan. They provide an independent perspective on how the plant should be commissioned compared to typical industry practices across a wide variety of geographies and technologies. The IE also monitors progress during commissioning and ensures compliance with manufacturer recommendations for some often-overlooked details such as proper grounding, sequencing, signage, and safety practices.

Alternating Current Equipment

At the grid interconnection Alternating Current (AC) side of the project, we will typically validate the main switchgear, medium voltage cables, generation step-up transformers, reclosers, and air switches as a single, significant portion of the commissioning process. The AC equipment is checked for proper mechanical and electrical connections, grounding, programming, and operation. The conductors and transformer are checked with a Megohmmeter (or “megger” for short), or insulation resistance tester, which is a special type of ohmmeter used to measure the integrity of conductor insulation. If any conductors were damaged during installation, the megger will help to locate the problem. All relays and protective devices are tested to ensure they operate when abnormal conditions are present. The grid operator or utility company also participates in most site commissioning efforts by opening a single phase on the air switch to validate that the site properly shuts down in the event of a partial outage.

Direct Current Equipment          

The Direct Current (DC) portion of the site usually takes the most time and effort because the solar array makes up majority of the site’s area. In addition to checking a portion of the mechanical and electrical connections across the solar array, the solar modules should be checked for their electrical performance. A curve tracer (or “I-V tracer”) is a special tool which measures the solar array’s current and voltage characteristics. Each “string” of solar modules, usually consisting of 15 to 25 modules, should be checked with a calibrated curve tracer. The I-V curve provides a quick and effective means of assessing the solar array’s true performance characteristics. It can be used to identify and locate module or wiring issues and compare power generation performance against previous performance data or product warranty data. Curve tracing should be conducted in specific irradiance levels, above 500 W/m², to validate the solar array is operating effectively. Curve tracers are important tools in the commissioning process because they save time, storing hundreds of solar module string data which can be downloaded and compiled for analysis. Additionally, this DC measurement at commissioning is important to establish the site’s baseline electrical capacity and can be used for comparison to validate expected performance as the solar modules age. While most of the DC commissioning is about the solar panels, the DC conductors must also be checked with a megger testing unit, and the overcurrent protection checked for proper installation, including signage required.


The inverters are fed Direct Current from the solar array and Alternating Current from the utility company. One might call the inverter the heart of the system. This is where the Direct Current is electrically chopped-up, reassembled into Alternating Current, and exported. The inverters should be commissioned according to the manufacturer’s instructions which varies somewhat by manufacturer. The inverter mechanical installation should be verified to be structurally sound along with proper grounding of the cabinets including hinged doors. The electrical installation should be checked for proper overcurrent protection, torqued terminations, wire management, and phase rotation. In addition, the incoming voltage should be verified at the AC and DC terminals prior to applying power to the inverters. Upon proper power-up sequencing the anti-islanding system should be verified which includes a 5-minute countdown timer, as required by the IEEE1547 standard, from initial power-up until the inverters begin operating.

Data Acquisition System

The site data is collected and presented by a Data Acquisition System (DAS) or Supervisory Control and Data Acquisition (SCADA) system. This system collects information from the inverters, meters, temperature sensors, wind speed, irradiance sensors, and more. These types of data systems vary quite a lot depending on system size and utility/owner requirements. Overall, all data points, data sources, internet access, and IP addresses need to be checked for accuracy.

Capacity Testing

The word “capacity” refers to the peak power (measured in MW or kW) capabilities of the solar power plant. Most capacity testing complies with ASTM E2848-13 which is the “Standard Test Method for Reporting Photovoltaic Non-Concentrator System Performance” and guides how to perform the system AC capacity test. This is a detailed testing method and requires a certain amount of operational data to be collected over time when the system is operational. Then a regression analysis is performed to compare the data taken with the expected computed performance. In the end the actual Capacity Rating is compared to the computed Capacity Rating to determine whether the project meets minimum peak power production targets.


It is important to measure the temperature of all equipment after it has been operating in a steady-state for at least 1 hour. A thermal camera should be used to detect potential anomalies in all overcurrent devices, switches, cabinets, and other gear. In addition, there is an increasing momentum to perform aerial thermography of solar arrays to comprehensively assess the solar modules while in operation to detect cell-level problems or complete module failures. Once aerial thermographic images are processed and stitched together many anomalies can be caught early and dealt with while the photovoltaic system is still being perfected.

Final Report

The final output from the site commissioning effort should be a professional report which documents the tests, presents photographs for all major equipment interior and exterior, and describes all actions taken to validate the site. The solar plant is intended to operate for its 35-year life and a full analysis prior to continuous commercial operation is an important quality control effort. The commissioning report is essential to close financing and transfer ownership because it establishes the performance baseline and ensures a sound investment.

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