Engineering, Procurement & Construction Best Practice Guidelines (Version 2.0)

System commissioning is one of the most important stages of the EPC service provider’s work as it closes the construction period and prepares the PV plant for commercial operation. This crucial step of the project includes performance and reliability tests. These make sure that the PV plant is built according to the international standards and industry best practices, and that it complies with the requirements as agreed with the Owner, grid specifications and guaranteed performance levels. Tests are undertaken for all individual components from checking that components function to more detailed measurements and verifications of the overall system. Successful commissioning and timely achievement of the Commercial Operation Date (COD) is linked to the release of a milestone payment as defined in the contract as well as the release of the performance bond. It is, therefore, very important that the contract clearly describes the requirements, criteria, documentation, and reporting required to complete the EPC service provider’s scope of work and handover to the Asset Owner and the O&M service provider’s team.

9.1. Pre-Commissioning

Mechanical completion happens the final construction stage (see section 8.2.7. Mechanical completion) meaning that all principal components that are part of the PV plant have been erected or installed. At this point, the EPC service provider will usually conduct a detailed inspection of the works, possibly accompanied by the Owner or any third-party representative (such as a technical advisor). This option should be clearly stated in the EPC contract clause referring to commissioning (if the Owner intends to apply it). Activities carried out under pre-commissioning should be detailed and agreed in advance with the Asset Owner in a specific document.

The pre-commissioning activities fall within the construction phase and are mostly undertaken in parallel to the last steps of electro-mechanical works. In large scale projects, the first blocs are ready under pre-commissioning while other parts are still being erected.

The pre-commissioning phase includes the following main activities:

• Systematic compliance checks performed on each component of each system, performed in a non-energised state

Testing of appliances, energisation of cables, testing of instrumental circuits, testing of circuit breakers, etc During the pre-commissioning phase, the following tests should be performed, as a minimum requirement:

• Mechanical integrity of the modules with visual inspection and the correct wiring. Thermographic analysis (via drones) can be added at this stage as a best practice

• Verification of the nominal power of the installed system carried out as the sum of the nominal power at STC of all the installed modules

• Verification of the correct operation of all auxiliary services (fire system, rodent protection, forced ventilation of transformers, temperature sensors, UPS systems and related storage systems, lighting systems, etc.)

• Control of all input signals to the SCADA system

• Verification of all power supplies of the auxiliary services of the cabins

• Commissioning of UPS systems and related storage systems, SCADA system and of weather stations and environmental sensors

• Verification of IP addresses on all equipment

• Setting of all alarm thresholds on the equipment

• Verification of the correct polarity and electrical continuity of all the strings

• Check all electrical connections

• Completion and functional verification of the earthing system.

After execution of pre-commissioning activities, the plant will be ready for energisation and for the commissioning activities.

Usually, a detailed checklist covering all components and parts is used to make sure that nothing is missing or incomplete. The works are thoroughly checked through the following items:

• Inverters

• Modules

• Foundations

• PV Module Mounting Structures

• LV and MV Cabling

• Transformers

• Protection, distribution centres and switch gear at the substation

• Combiner boxes

• Civil works

• Low and medium voltage installation works

• Monitoring and security systems

Finally, the checklist should be provided to the Owner and their advisors, together with the compilation of an initial list of construction defects (commonly referred to as a “punch list” or “snagging list”). Counterchecking the EPC service provider checklist and providing own observations and items to add, as defined by the Owner or their advisors, is recommended. This punch list should include only minor finishing works, the cost of which usually equates to a small percentage of the overall contract value. The contract also needs to specify the timeframe for correcting punch list items, and what the conditions are for granting Provisional Acceptance if punch list items remain unfinished. Once the punch list has been issued by the Owner’s representative a meeting is required between them and the EPC service provider to agree specific resolution for each item and determining if any items are disputed.

Mechanical completion, as described in section 8.2.7., allows for further testing activities to commence. In large scale projects, this is often undertaken by batch and delayed over time, as different parts of the plant are in different stages of construction.

9.2. Commissioning, off-grid and on-grid tests

9.2.1. Commissioning activities

Commissioning activities include operational checks and tests executed on energised electrical systems. The Test Protocol must be agreed between the parties before the start of the tests as part of a Start Up Plan, defined before the start of the Mechanical Completion and Pre-Commissioning activities.

The Test Protocol must respect all the requirements contained in the contract and its basic content should include:

• Results of the visual inspection and related checklist

• Test methodologies

• Instrumentation used for testing

• Test program

• Test conditions

• Test data

• Results of the Pre-Commissioning and Commissioning tests

• The start-up protocols issued for the key components (inverters, transformers, etc.)

This testing aims to verify and certify that the plant has been constructed professionally, according to the pre-established technical prescriptions, and in accordance with the project and any approved variants.

Before the plant is energised, a series of functional tests and measurements should be undertaken as per the reference norm IEC 62446: Grid connected photovoltaic systems. Minimum requirements for system documentation, commissioning tests and inspection for all electrical commissioning.

The testing procedure should be handed over to the Owner prior to commencing the tests, as is usually defined in the EPC contract. This allows the Owner or advisors to review and comment on the testing procedure before implementation. At the end of the commissioning phase, the EPC service provider submits a Test Protocol to the Asset Owner, summarising the results of the Pre-Commissioning and Commissioning tests.

The following test regime shall be performed on all systems. Any test indicating a fault should lead to default rectification and re-testing of the components.

On the AC side, all AC circuits, including AC cables from inverters to transformers, transformers themselves, and main MV switchgear should be tested according to the requirements of IEC 60364-6.

On the DC side, the following tests shall be carried out on the DC circuits and components forming the PV array:

• Continuity of earthing and/or equipotential bonding conductors, where fitted

• Polarity test

• Combiner box test

• String open circuit voltage test

• String circuit current test (short circuit or operational)

• Functional tests

• Insulation resistance of the DC circuits

Some expanded test, not mandatory but often included in the EPC service provider contractor scope, can also be carried out to ensure the best system performance and reliability:

• String I-V curve measurements on a selected sample (10% of the plant at 500W/m²)

It is a best practice to take a pragmatic approach to tests which require minimum levels of irradiance. String tests and thermography should be carried out above certain irradiance minimums. Conducting them at lower levels will provide reduced value from the results. If necessary, some tests may need to be deferred until high season to be valid.

In addition to the above electrical tests, all other equipment should be tested according to the manufacturer’s guidelines and industry best practices to ensure that it functions properly before the energisation of the PV plant. All other equipment and materials include:

• Meteorological stations and monitoring system

• Low voltage installation, civil works, and medium voltage installation

• Security system as well as cyber-security system

• Sanitary system

• Firefighting system

9.2.2. Off-grid testing

The first tests to be conducted are the polarity and combiner tests which need to be undertaken while all strings are still disconnected.

The off-grid tests should include measuring 100% of the open circuit voltage (V_oc) and the short circuit current (I_sc) of the module strings according to IEC 62446. Prior to starting testing, the Owner must confirm the adequacy of the measurement devices to be used by the EPC service provider (measurement uncertainty, calibration, etc.). A report with measurement results from all the strings will be presented by the EPC service provider in digital form, as an Excel file.

The V_OC test is passed if all the V_{OC, string} on the tested strings is within 5% of the expected value derived from the module datasheet. Note that most of the time, the theoretical value should be adjusted with the actual temperature recorded at the time of the measurements as it may be far from STC (25°C).

A commonly used formula is:

Where V_th is the theoretical open circuit voltage for the strings and calculated as follows:

The I_sc test is passed if all the I_sc,string on the tested strings satisfy the following condition:

It should be noted that the short circuit current test is not intended to detect system underperformance but only used for fault detection in string cabling.

Once the commissioning phase of all the plant sections has been completed and the protocol test issued, the Ready For Start Up (RFSU) certificate of the plant is released by the Asset Owner and then the On-Grid performance and functional tests can be started.

9.2.3. On-grid testing

Once the above off-grid tests have been successfully performed, the PV plant can be energised at inverter level and main switchgear level at the point of interconnection with the grid. The EPC service provider shall demonstrate that the overall system and equipment operates in accordance with the:

• Equipment manufacturer specifications especially for inverters, transformers and MV equipment

• Grid Connection Agreement which should be annexed to the EPC contract, or at least its technical annexes regarding testing and commissioning specifications

• Specifications set out in the EPC Contract

• Any relevant Applicable Standard, mainly IEC 61727 and local grid code

Inverters and transformers shall be commissioned by their manufacturer or an authorised representative of the manufacturer, using the manufacturer’s specified procedures. Commissioning reports shall be issued in a format provided by the manufacturer.

All SCADA system equipment shall be commissioned and tested using the manufacturer’s specified procedures. Tests shall verify the correct operation of the SCADA system, meters, sensors, weather station instruments, and all inverters, while verifying the correct data input logging from trackers (if any), breakers, and other components monitored by the system. The SCADA system shall be fully remotely accessible. A SCADA system commissioning protocol or report shall be provided.

Before energisation, the EPC service provider shall verify the completeness of the substation and the correct installation of all components. A detailed inspection of the substation shall be executed. The testing and commissioning of the PV plant substation connection to the grid system should be performed, including but not limited to:

• MV equipment

• Control and Monitoring System

• Protection system

• Telecommunication system

• Metering devices

• Auxiliary supply equipment and back-up (UPS, diesel, etc.)

In some countries, compliance with the grid code and local safety standards need to be validated by an independent body, and a certificate provided to the grid operator to allow power injection. These compliance tests may also be carried out by the grid operator themselves.

Prior to achieving Provisional Acceptance, it is common practice to carry out module thermography, using aerial inspections as best practice. 100% module thermography should be carried out at this stage according to IEC 62446-3:2017. Issues identified from this inspection will need to be resolved to pass PAC. These inspections and the reports generated should form part of the handover documentation.

9.3. Provisional Acceptance Certificate

The Provisional Acceptance stage marks the end of the construction works and obligations of the EPC service provider. It means the Asset Owner is giving their conditional acceptance of the works. This triggers the two-year standard warranty period, across which the EPC service provider must prove a minimum level of performance from the PV plant, as defined in the contract. At this stage, the plant is also handed over to the Owner and the O&M service provider which may be the same company as the EPC service provider or a third-party.

The conditions for issuing the PAC may differ from contract to contract but the key elements are as follows:

• All commission tests have been successfully completed, including Mechanical Completion, grid connection and energisation of the plant

• The noncritical punch list items have been identified and signature have been provided for corrections. The value of this remaining work does not exceed a certain proportion of the contract price (typically 2-5%)

• The Provisional Acceptance performance tests have been passed (PR but also functional and capacity tests in some cases)

• All equipment and sub-contractor warranties are transferred to the Owner

• The EPC service provider has provided the Owner with the initial or minimum stock of spare parts, as defined in the contract (see also section 10.6. Setup of strategic spare parts warehouse).

• All as-built documentation has been provided to the Owner (see also section 6.4. As-built design)

• Training of the O&M service provider’s teams has been performed and relevant O&M manuals issued

• Liquidated damages (LDs) related to performance or delays have been paid by the EPC service provider

• Any performance security or warranty bond required during the EPC warranty period has been delivered to the Owner

The PAC is signed off by the Asset Owner and, if stipulated in the contract, can also be validated and signed by an independent advisor.

9.3.1. Performance Ratio test

After the functional test, the PV system’s performance, in terms of energy and power, is evaluated in the Start-Up phase. To validate the PV plant performance at Provisional Acceptance phase, the PR test is conducted over a limited period and compared to the guaranteed PR, set based on simulations. The usual duration of PR tests is 7 to 15 days, depending on the contract. From an Owner’s perspective, having the longest testing period possible is recommended, as this helps to check performance in a wide range of climatic conditions, and facilitates comparisons with simulated values.

Usually, the testing period needs to fulfil minimum requirements regarding weather conditions and plant availability such as:

• Minimum irradiance threshold in daily values on a certain number of days (e.g., 8 days over a 15-day period with irradiance greater than 5kWh/m²/day) which should be adapted depending on the season of the test and specific conditions of the project location

• Minimum irradiance threshold on a single day for consecutive hours (e.g., irradiance over 500W/m² during at least 3 consecutive hours in 8 days over a 15-day testing period), also to be adapted to the season and project location

• Total number of testing hours with irradiance above a certain threshold (e.g., 500W/m² for at least 20 hours in a 15-day period)

• Availability should be 100% during the testing period at least at inverter level. Grid availability should also be 100%. The SCADA and the environmental monitoring system must also guarantee 100% availability of data throughout the test period

If the above conditions are not fulfilled within the testing period, it is generally extended until they are. Conditions should be set pragmatically and potentially adjusted to avoid delaying the PAC and leading to difficult negotiations and distrust between parties. The time of year should be considered so that unrealistic thresholds are avoided. The performance tests should ideally be performed during spring as this is usually when performance is at its peak due to better weather conditions. Poor weather conditions can penalise performance compared to simulated values (high summer temperatures, winter shadows or low irradiance).

If the continuity of the test is interrupted due to faults or events related to the malfunction of the plant or one of its parts, the test will be suspended and repeated from the beginning.

If the causes of the interruption are not attributable to the EPC service provider, the test will be suspended and will resume at the end of the interruption.

The PR calculations are based on the mathematical definition formula, but each parameter can differ and have its own specifications from contract to contract. It is important to check the consistency of the formula and the input values definitions and measurement rules.

These definitions are based on (Woyte et al. 2014) in line with IEC 61724-1:2017 and are common practice.

For projects located in regions with high temperatures and temperature variability, a temperature-corrected PR methodology needs to be implemented to account for the weather effects.

Finally, the measured PR is compared to the guaranteed value based on the pre-construction yield assessment simulations. A buffer between the simulated value and the guaranteed one is generally used by the EPC service provider. It is important to ensure that the design reference yield has been updated to reflect any changes made during the project. More specifically, the internal and self-shading factor should be checked for accuracy. The guaranteed PR at Provisional Acceptance should be presented as a monthly breakdown of the yearly simulation to ensure accurate comparison with the measured PR for the testing period. Given the short duration of the test, guaranteed PR at Provisional Acceptance is only used as a validation criterion for the Owner’s “take over”. It does not usually trigger performance liquidated damages as they are linked to the results of annual PR tests. If PR is below the guaranteed threshold, corrective action might be undertaken, and testing should be repeated.

Once the PR criteria and any other requirements have been met, the PAC is issued. The project reaches the handover phase, which is the start of the operational phase and O&M activities.

9.3.2. Other tests

In some contracts, complementary tests can be performed at the Provisional Acceptance stage. These tests can reflect the requirements of the energy off taker with the Power Purchase Agreement (PPA), whether or not the system functions, or simply be used as additional quality assurance measures.

To prove the project’s ability to perform to its maximum capacity, a Reliability Test can be undertaken. This means the project must go a certain period (e.g., 7 consecutive days, or 100 consecutive hours) without significant system failure or malfunction. Furthermore, the project must prove that it can run for a certain amount of time without inverter failures or shutdowns, with full availability of AC and DC equipment, and less than a certain threshold (typically 2%) of string or tracking system failure (if any). If a system failure or malfunction occurs, corrective action shall be taken by the EPC service provider and the Reliability Test is restarted the following day.

Additionally, a Capacity Test may be required to prove that the installed capacity can reach the level promised to the off taker. This is usually based on the DC capacity of the plant, calculated based on the peak powers of the installed PV modules, as stated on the manufacturer’s data sheets. Alternatively, this is calculated from the sum of the peak powers of the Flash Test of the PV modules, provided by the manufacturer at shipment. These values must be signed off by an independent third-party.

9.3.3. Start of commercial operation

Once all performance tests described in the above sections have been completed, the Asset Owner issues the PAC and commercial production starts (Commercial Operation Date, COD).

To ensure a smooth and efficient handover to operation activities, the Asset Owner should be involved well in advance and participate in the commissioning phase and performance tests. It is also a best practice to involve the operations function of the Asset Owner during the development and engineering phase, so that an O&M perspective can also be taken into consideration.

Comprehensive and detailed as-built documentations (Annex F), manuals and procedures (Annex C “Documentation set accompanying the solar PV plant” of the O&M Best Practice Guidelines) should be part of the training activities. For more information on the Handover to a specialised O&M service provider, please refer to Chapter 10 on Handover to O&M.

9.4. Intermediate and Final Acceptance Certificate

There is a standard duration of 24 months (depending on the EPC contract) between the start of the Taking-Over phase to the Defects Notification Period. The EPC service provider is usually responsible for O&M and rectifying any defects that may be identified during this period. However, this may vary from market to market. During this period, a performance warranty based on a guaranteed PR is still in place and can be reviewed on a yearly basis. Annual PR tests are crucial for checking the PV plant performance, as they do not include seasonal bias. For smaller scale projects, this Defects Notification Period can be reduced to 12 months. It is always recommended to carry out PR verifications for at least one full year.

The calculation methodology is different to Provisional Acceptance and should be based on long-term PR tests. The guaranteed performance ratio should be adjusted to account for module degradation over the first and second years of operations. Should the measured PR be above the expected threshold of guaranteed value, then Intermediate and Final Acceptance certificates are issued accordingly. The Owner can then issue a performance certificate and release the performance warranty bond of the EPC service provider. This performance certificate constitutes the full acceptance of the PV plant by the Owner and the release of the Contractor’s obligations.

The guaranteed PR (and therefore the guaranteed energy) takes into account any event causing non-production due to periods of plant downtime. Owner and EPC service provider may agree, and provide for this in the EPC contract, not considering certain special events. In general, it is reasonable to exclude certain events that are outside the control of the EPC service provider (e.g., vandalism, plant stop imposed by the Transmission System Operator) and Force Majeure events.

The EPC contract shall include provisions on how to deal with cases where actual performance is lower than guaranteed performance. These provisions in general are included in the penalty clause.

Where actual performance is lower than guaranteed performance, EPC service provider shall:

• Make all interventions necessary to ensure that guaranteed process parameters are achieved

• Liquidate both the production lost (difference between actual and theoretical production during the period from PAC to the Final Acceptance Test) and the estimate of the lost production expected for the remaining useful life of the plant

If the measured PR is below the guaranteed levels, the EPC service provider is required to pay performance Liquidates Damages (LDs) up to a certain amount (see chapter 12.5 Limitation of liability and Liquidated Damages) to the Owner for the compensation of revenue losses. During the Intermediate Acceptance phase, the LDs are based on the annual production shortfall and the electricity selling price of the PV plant. During the Final Acceptance phase, the LDs are also calibrated to reflect the loss of revenues that are expected for the full project lifetime or duration of the Power Purchase Agreement. This is usually calculated as the Net Present Value of future revenues shortfall linked to the PR shortfall. Below is an example formula for additional LDs at Final Acceptance:

Other requirements at Final Acceptance stage should include an inspection of the whole plant, including the civil works, electrical infrastructure, every piece of equipment and device installed, and the auxiliary systems, to verify that the EPC service provider is leaving the plant in optimum condition. This should ideally be done in the presence of the Owner and an independent third-party (technical advisor). All existing defects must be solved as a condition for acquiring the Final Acceptance Certificate (FAC). Spare parts can also be replenished in accordance with the O&M contract requirements to ensure a smooth transition between both service providers. Additionally, further testing such as repeated module thermography, across all modules, should be performed as a best practice, preferably using aerial inspections during the period between PAC and FAC. This is to ensure that any issues identified can be resolved before the date for Final Acceptance.  It will enable the identification of any early-stage degenerative issues. These activities can be included within the EPC service provider’s scope or under the responsibility of the Owner at their own costs.

After the Final Acceptance Test the Owner shall issue the FAC and shall take over the full responsibility of the plant.