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

The Procurement phase covers purchasing components such as PV modules and inverters, as well as identifying and mitigating risks. It involves supplier selection and onboarding, and conducting inspections, and tests to qualify materials to be used in construction throughout the procurement process.

This chapter will help stakeholders to identify risks in the procurement process of components (such as PV modules, inverters) and to mitigate them through suitable inspection, testing and qualification mechanisms for individual projects. The procedures shall be underlined with definitions of acceptance level and criteria.

7.1. General procurement guidelines

This section addresses general guidelines applicable to the procurement of any component of a system and provides guidance on how to integrate quality aspects into the procurement process. It is recommended to apply the general requirements, where relevant, to subcontracted activities such as engineering, construction, or quality management activities. This section follows the different steps of procurement, from supplier onboarding to inspection and tests until completion of the procurement process. The guidelines are independent from the procurement process itself and remain applicable whether the EPC service provider decides to work through recurring orders, single purpose contracts or project-based procurement.

7.1.1. Use of third parties

Involving third parties in the procurement process can lead to the delivery of better products as they can bring quality expertise and objectivity to the judgement of products and suppliers. The contractual agreement on the Quality Monitoring measures, be it by third-party or other means, often leads to more quality awareness on the supplier’s side and to more stringent application of quality standards. A contractual agreement should address the scope of the responsibility assigned to the third party and their authority to make decisions on quality. In general, involving third parties in the procurement process is best practice but not an outright necessity.

Third party technical assessments vary significantly in terms of thoroughness, accurateness, completeness, reliability, validity, and transparency. A good guide to identifying a credible third-party service provider may be the proof of an accreditation according to ISO 17020, ISO 17025 or acceptance by IECRE.

7.1.2. Integration of Quality Management into the procurement process

Regardless of the procurement process they have defined, the EPC service provider is ultimately responsible for providing the required quality level for the activities and components provided by its suppliers. The EPC service provider must ensure that the quality of the components and services procured from external sources fulfil their internal quality standards, and that risks related to procurement activities are identified and mitigated.

Therefore, the EPC service provider shall define and deploy the appropriate procedures for:

• The selection, evaluation and monitoring of its suppliers

• The monitoring it intends to apply on the products and services procured from external sources, such as quality requirements and evaluation criteria, products release procedures, auditing processes

7.2. Management of suppliers

7.2.1. Selection of suppliers

Prior to signing a contract with a supplier, the EPC service provider should determine the ability of the supplier to consistently deliver products and services that can meet the requirements in sufficient quantities. Alongside this, suppliers’ performance on technical, financial, legal, and social regulation and standards should be assessed. This requires cooperation from suppliers along all these lines. There is a severe risk of reputational damage linked to a lack of supply chain transparency. For more information on building sustainable and transparent supply chains, please refer to SolarPower Europe’s Sustainability Best Practices Benchmark. In addition, SolarPower Europe is currently (as of December 2021) developing a supply chain monitoring programme that will further this effort.

When selecting a supplier, TIER-ratings give an overview of the track record of the manufacturer, but only provide limited information on the quality of a product. Therefore, selecting products just on their TIER rating is insufficient. Consequently, the selection must be based on product testing accompanied by factory audits and a documentation review.

A technical rating of products can be based on accessible product data and quality assurance information provided by the manufacturer. It can be used as preselection criteria as a part of an overall quality review process for PV power plants. The rating or scoring system of suppliers should start before the sourcing phase. A rating may be based on a questionnaire, which should include the product-related data as well as quality assurance information, including:

• Technical Specifications

• Bill of Materials

• Certificates

• Warranties

• Any beyond standard quality assurance/quality control measures (e. g. extended reliability test programmes)

• Manuals, labels, and data sheets

• Quality management in the production

7.2.2. Qualification of suppliers

While all products are usually qualified and manufactured under a valid quality management system, variations in production lines, bills of material, and general fluctuations in quality are still common in the solar industry. Quality measures should be monitored during manufacturing and shipping to get a full assessment of the quality of the procured goods. It is recommended that one carries out quality review measures for production supervision that are in line with international conformity assessment standards. Alternatively, on-site assessments, and product testing should be done before signing a contract with a supplier.

Documentation Review

A general document review, submitted by the supplier, should contain:

• Product certificates and associated reports (all relevant market access documents)

• Factory certificates (management system, laboratory accreditations)

• Warranty condition

• Review of recalls / claim handling

Factory Inspection

Before production starts, a pre-production factory inspection is recommended. The aim is to identify issues in the manufacturing and quality assurance processes that can have a negative impact on the quality of the components. The inspection should consist of verification and evaluation of the following processes and procedures:

• Incoming inspections and preparation of materials – warehouse

• Production process assessment

• Electrical safety tests

• Outgoing performance / output power verification

• Evaluation of equipment and procedures for quality control tests (such as solar simulators, visual inspection tools, electroluminescence (EL), insulation test)

• Quality assurance/control (storage and handling of materials, production areas, staff training, claim handling)

• Handling of test and calibration equipment

• Documentation of process data

• Process for handling faulty products

• Conditioning of the finished product

• Review and comment on the warranty claim list

• Product traceability

Product qualification testing

While type approval and safety certification are the minimum requirements to market any product, series production might show fluctuations in production quality based on production lines or material variations. 

It is therefore recommended to fix a particular bill of materials and factories / production lines in the purchasing agreements and pre-test products accordingly. The product qualification testing shall be based on standards and be product / component specific. Depending on the size of the project, extensive factory inspections may be performed, as well as a conformity testing, to ensure that the components arriving on site have been manufactured using the correct bill of materials, and the manufacturing process has delivered the specified quality requirements. Further details are listed in section 7.5 Specific requirements per key component.

7.3. Supply review

When large quantities are procured with a specific deadline, it is important to assess the supplier’s ability to meet it by checking material supply as well as actual production capacity.

7.3.1. Pre-production review

Prior to production, it is advisable to assess the factory’s readiness to supply the products ordered within the agreed lead-time and at the right quality level.

Focus shall be put on:

• The availability of agreed components (bill of materials)

• The status of maintenance and calibration of the production and testing equipment

• The communication of specific quality requirements of the project and availability of related documentation

• The qualification of the production manager to apply the specific quality requirements

7.3.2. During production inspections

During production inspections are performed after production of the components for the PV project (e.g., modules) has started.

The inspection shall focus on the following topics:

• Verification of production on the agreed manufacturing lines

• Use of material in accordance with the agreed bill of materials

• Quality Monitoring during the manufacturing process

• Verification (spot-check basis) through in-line tests

• Verification of performance determination test on a spot-check basis

• Verification that contractually agreed specifications are met

Sampling plans and the acceptance criteria for required verification tests and inspections shall be agreed upon in advance. Any production inspection process is a compromise between cost and thoroughness. Ideally, manufacturers should be monitored closely enough to ensure that no significant unobserved material deviation from the agreed features goes unnoticed.

7.4. Delivery

7.4.1. Post-production monitoring

Contractually agreed upon monitoring for post-production (before dispatch, after receipt, during construction) is an important tool for assessing the consistency of quality and thus the degree of fulfilment of the contract. Sound statistical sampling at this early stage of the project helps avoid long-term failures. For example, if the components have already been installed and show early faults, the cost of handling complaints and, if necessary, subsequent replacement is more expensive. Comprehensive testing would add significant costs to the project thus a standard like ISO 2859 should be applied. The case to be considered in testing should be agreed with the component suppliers in the contracts. The level of a batch conformity should also be agreed with the component supplier in the contracts as this may influence the financial risk assessment and thus affect the overall cost of funding. Typically testing will be in the production line, but should be witnessed and verified through sample third party testing (see 7.4.3 Pre-shipment testing, factory acceptance testing). Typically General Inspection Level I (visual and electric properties) and Special Inspection Level 2 (insulation and dimension checks) with AQL Major 1.5 and Minor 2.5 is used for sample size definition.

7.4.2. Pre-shipment inspection

Pre-shipment inspections are carried out on a sample basis and used to release finished goods for shipment if they meet the agreed requirements. The inspections include:

• Visual inspection

• Power verification

• Electric insulation

• Label verification

• Verification of packaging and fit for shipping

7.4.3. Pre-shipment testing, Factory Acceptance Testing

Critical tests that determine the conformity to agreed ratings as well as quick quality monitoring tests are performed on random samples taken from packages ready for shipment. It is important to agree on pass/fail criteria and clear criteria for shipment rejection. Testing / inspection can be done at the factory site (Factory Acceptance Testing, FAT) or at a warehouse, depending on access and availability of testing equipment.

A FAT shall include the following aspects:

• Assessment of quality standards of production line / manufacturing site

• Verifying the quality system in place in the production line, considering procedures, compliance of all staff and processes, traceability, and problem mitigation

• Mechanical specifications of the product

• Electrical specification of the product

• Documentation, including manuals, SLDs and warranty

• Service and support quality

• Data management and display

7.4.4. Post-shipment inspection

Post shipment inspections are performed to check whether the received goods hold all necessary documentation and import papers / certifications (e. g. Certificate of Conformity). Furthermore, the post-shipment inspection shall document any transportation damages to enable claims based on such damages. Again, a prior agreement on acceptance criteria is of major importance.

7.5. Specific requirements per key component

Generally, spelling out solid requirements for key components is one of the most mission critical items. The importance of this topic can hardly be underestimated when it comes to the long term technical and financial success of a PV project. Going into great detail is outside the scope of this document and therefore, the following subchapters only give an outline. One of the biggest challenges comes from time pressure during the construction phase combined with manufacturing or delivery problems that may occur during the project execution. An additional challenge is related to reviewing quality, as the variety of testing and inspection services offered in the market is quite wide when it comes to reliability, accuracy, validity, viability etc.

There is no principal reason behind underperforming PV assets. System faults occur the most frequently, but individual components can also have defects. It is absolutely crucial for EPC service providers to ensure the quality and reliability of all the components that they use.

7.5.1. Modules

Modules are the engine of the final system and represent a significant proportion of a project’s CAPEX and labour corrective maintenance measures need to be carried out. In the planning phase one should verify that modules are, in theory at least, capable of operating in the given working environment for the anticipated lifetime and with the assumed durability. It is often wrongly assumed that this will be the case if the module type has passed the IEC 61215 / IEC 61730 type/safety approval test. These standards have been one of the most successful contributions to reducing problems in the array field but are only a design qualification standard. They are limited to evaluating known failure mechanisms and assume a moderate climate. Examples of failure modes being missed include backsheet issues or PID and Light and elevated Temperature Induced Degradation (LeTID) related issues. The main impact has been to reduce early failures in the first few years in operation. It does not give any information on the durability of a module, nor does it verify the quality of the product actually being installed, just the general suitability of the product family for the intended application.

Ideally one should verify whether the modules will operate at conditions represented by the tests they have undergone or account for an increased quality risk if conditions in the field are expected to be out of the test standard’s scope. An example of modules potentially operating outside tested specification could be building integrated mounting or systems in arid climate zones, as such systems may run much hotter than they have been tested for. IEC TS 63126 Guidelines for qualifying PV modules, components, and materials for operation at high temperatures gives guidance on testing modules and components for high temperatures. As some standards also allow variants of test conditions based on manufacturer’s definition, reviewing the testing protocol alongside the certificate is recommended.

Integrating testing requirements for PV modules in the procurement conditions allows for claims against underperformance as well as identifying design deficiencies. PV modules from one system supplied by various production sites or batches may require separate assessment.

There are three groups of quality tests described:

1.       Performance characterisation testing

2.       Qualification testing

3.       Module Reliability Tests (Stress Tests, Accelerated Aging Tests)

Performance characterisation testing mainly addresses the electric performance of the PV modules and the condition of the cell interconnection circuit (cell cracks or interrupts). Regarding the power warranty, the performance of the entire delivery can be deduced from a random sample according to ISO 2859-1. As budget and timing is usually critical, mostly General Inspection Level based on the total number of modules per production batch is applied. As an alternative, a combination of a smaller sample size (e.g., 50 per batch) and the manufacturer’s flash list will allow a robust product verification if the measurements have been carried out with a sufficiently low uncertainty and the service provider has an appropriate quality system. It is advisable to combine power measurement with electroluminescence imaging for crack detection. The performance at low irradiance is something needed for the energy yield calculation, but samples size can be small (e.g., S 1). In the absence of third party verified PAN files it is advisable to base PAN files on independent measurements as simulations based solely on data sheet information may lead to high uncertainties in energy yield simulation.

Product qualification tests are typically destructive or longer-term tests and sample sizes are kept smaller. It is important to perform tests on modules that represent the material combinations (bill of materials) of the module type. The tests shall check the functioning manufacturing processes, the production control and are helpful in determining general workmanship. Some suitable qualification tests are defined in the standard IEC 61215-2, which is the basis for type approval and design qualification of PV modules. The sampling method is typically Special Inspection Level S 1 to S 3 acc. to ISO 2859-1 with consideration of all bills of materials and potentially different production lines to be represented. Induced degradation tests (such as PID and LeTID) are screening tests and are suggested if sufficient proof of resistance to such degradation is not provided. Here sampling rate could be reduced to two modules per bill of materials to minimise testing cost.

Product reliability tests shall evaluate the long-term behaviour with a focus on module performance but also on electrical safety. Several test sequences for investigating a module’s resistance to environmental conditions, such as high UV level, strong temperature changes, high temperatures combined with high relative humidity and mechanical stress both from wind forces and snow loads are described in IEC TS 63209 Photovoltaic modules - Extended-stress testing - Part 1: Modules. Depending on the application and the project region the stress level may vary. The suggested sample size is two modules per test and bill of materials. In particular polymeric material degradation has caused major reliability concerns in the recent years. Here the technical specification, issued in 2021, provides a combination of damp heat testing, UV testing and thermal stress in its sequence three that is designed to screen for long-term backsheet failures.

7.5.2. Inverters

The inverter is one of the most complex components in a PV power plant and includes multi-functional power electronics for optimising the power output. This element is the interface with the grid and reads and communicates operational data to the monitoring system. A fault with the inverter leads to an immediate decrease in power output, which grows in proportion to the size of the inverter. Owners should not simply rely on data sheets but invest in quality review services, conducted by experienced technical advisors. In a quality assurance process, the key steps of design, manufacturing, installation, and commissioning are independently evaluated, to prevent potential issues that could decrease performance across the inverter’s lifecycle.

The key risk mitigation steps are a factory audit, the review of a manufacturer’s factory-out inspection and the commissioning, which are presented in sections 7.3. Supply review. and 7.4. Delivery.

Aside from the general comments above, key areas for potential issues with inverters include:

• Adaptation to voltage and power design

• Isolation issues

• Blocked air vents, filters etc.

• Derating characteristic of inverters, high temperature shut off

• Rating or spacing not suitable for location (e. g. high altitude)

• Grid code compliance

• Unavailable required national certification

• Inverter metrology

• Interference with radio signals etc. (electromagnetic compliance and adaptability)

• Optimisers

• Local transportation including unloading opportunities

• Local service

Inverters need to be chosen depending on system topology. There is no formal assessment available currently, but a risk assessment when choosing a system topology considering performance, maintainability, impact of failures, likelihood of failure and reparability. As an example, a central inverter may have a higher efficiency, be cheaper to install, but in case of a failure takes down the system and will take weeks to repair, while spare string inverters could be stocked, and any failure could be corrected in a short time. The evaluation of risks will depend on design objectives, but it should be documented for later verification and any future process improvements.

When planning a system, it is critical to match the operating characteristics of the inverter (efficiency, load-related derating, voltage window) to the real operating conditions.

Sufficient diligence needs to be exercised when it comes to:

• Specific requirements for inverters, e.g., compliance with (EU) 2016/631 for Europe

• Performance characterisation testing (INV File generation for energy yield simulations)

• Product qualification testing

• Product reliability testing according to appropriate standards

7.5.3. Mounting structure (fixed tilt)

Racking systems hold valuable modules in place and ensure stability of the installation of the PV system. Mounting components consist of various metal parts with different coatings or materials, such as aluminium, alloy, stainless steel, or galvanised steel. Corrosion can occur due to the constant and long-term exposure of these materials to each other, to soil conditions and to environmental stresses, such as rain and moisture and other atmospheric pollutants like chlorides in marine environments or sulphur dioxide and nitrous oxides in industrial locations. As corrosion intensifies over time, serious structural failures in racking and mounting components can result in instability in the PV system and cause it to malfunction. It lays at hand that quality of mounting systems plays a tremendous role in each step from manufacturing to installation, maintenance, and recycling.

As lifespans of solar PV systems can reach up to 30 years, racking manufacturers must target a similar life span for the racking materials. The following norms and guidelines are of great significance and should be adhered to during the project development and during the construction stage:

• The manufacturing process of mounting systems should be in accordance with Eurocodes 1991 1-1 - 1-6 Actions on Structures. The norm includes guidance on the actions to be performed on structures designed for use in buildings and other civil engineering works

• In addition, to prevent corrosion of the mounting structure, manufacturers should comply with the standards “Specifications and test methods on hot dip galvanised coatings on fabricated iron and steel articles” (EN ISO 1461) and “Continuously hot-dip coated steel flat products for cold forming - Technical delivery conditions” (EN 10346). The two quality standards underline the importance of corrosion free purlins, aluminium mounting brackets and bolts and focus on the chemical composition and mechanical characteristics of the components for racking systems in general. Information on coating thickness (e. g., zinc coated steel, anodised aluminium, etc.) can be determined by measurements in testing labs or on site

• A third standard, the “Execution of steel structures and aluminium structures - Part 1: Requirements for conformity assessment of structural components” (DIN EN 1090-1), assures the quality of steel components, aluminium components, and kits in the manufacturing process

• The material quality should be verified on documentation basis (alloy, etc.). Spot checks of the anti-corrosion coating thickness can be performed in factory or onsite. Further the dimensions and tolerances of the delivered parts shall be verified against the available documentation

7.5.4. Mounting structure (trackers)

Tracker systems offer a significant additional complexity to a PV power plant system as it entails moving parts being added to an otherwise static system. When considering tracking, be it single axis or dual axis tracking, in addition to the previous section, the following points should be considered:

Tracker system selection

• Structural calculation according to applicable standards in the country of the project and international codes like ASCE or Eurocodes. This calculation should consider the specific conditions known or foreseen for the soil conditions. It is highly recommended to check whether the tracker system has undergone wind tunnel testing, and in addition, CFD (computer fluid dynamics) modelling to simulate wind situations. This is particularly important for resonant frequency conditions that can occur at wind angles of attack that can hardly be simulated in a wind tunnel. Note that catastrophic failure at resonant frequencies does not necessarily require high wind speeds

• Certification of the PV tracker against relevant standards like IEC 62817, UL 3703 or UL 2703. Specific confirmation that the components used in the trackers to be supplied are listed in those certificates

• Accelerated lifetime tests beyond those associated with the certifications mentioned above

• Justification in the form of studies, wind tunnel measurements or tracker measurements showing that all the aero-elastic stabilities are properly added to the structural calculation mentioned above. Particularly, the following instabilities should be considered as a minimum: flutter/galloping, torsional divergence, buffeting, vortex-induced vibrations, and aero-elastic deflection. Justification of the values used for the damping ration and natural frequency should be provided.

Tracker system reception and installation. Once on site, the delivered equipment should be verified by collecting a sample of each element of the structure which is then measured and verified against the specifications. Certificates for the steel and galvanisation are provided directly from the manufacturer’s sub-suppliers with site measurements of dimensions and thickness.

It is recommended that the installation process should be overseen by a representative of the manufacturer and the following recommendations should be a general checklist for this stage, being part of the project commissioning stage.

1. Torque verification according to manufacturer specifications
2. Tolerances in installation are within the levels accepted by the manufacturer
3. Piles driving are tested (pull-out) showing minimum recommendation by the manufacturer
4. Tracker Control Units (TCUs) and Network Control Units (NCUs) are installed and connected with configuration approved by the manufacturer and Owner’s engineer
5. Meteorological stations are commissioned according to manufacturer recommendations and testing to see whether the stowing strategy is working

Special care should be taken if material is galvanised. To maintain the corrosion protection, the galvanisation must not be damaged by scratching or any machining.

7.5.5. Cabling (including connectors)

Proper cabling and connections must be ensured. The list of partially serious problems is virtually endless but here are a few examples:

• Cabling specification

         -  Cable cross-sections are undersized

         - Cross sections of safety fuses are undersized

         - Cables sheathing is made of inferior material not capable of weathering (e.g., low UV light resistance, low permeability)

         - Cable wire material is inferior (e.g., not compliant with strand construction class 5 or 6 as per IEC 62930:2017 or not compliant with stranding class B or higher as per UL: ZKLA or PV-wire requirements)

• String / combiner boxes
• Non-matching or “compliant” or “compatible” connectors

          - Connections have a too small contact surface or are not suitable for the specific current (and voltage) application

          - Materials used between different manufacturers can be slightly different causing contact corrosion

          - Metal contact size not fitting with the cable conductor cross section

          - Seal gasket not fitting with the PV cable outer diameter

• Fuses (e. g. power rating/wire diameter, housing, temperature derating)
• Earthing, potential bonding

Various standards refer to proper cabling and connection practices, such as IEC 62930:2017 resp. EN 50618 (Electric cables for PV systems), IEC 62790 (junction boxes for PV modules), IEC 62852 for DC connectors and IEC 62738 (Design guidelines). There are also other international and national standards and codes related to cabling and connectors. In addition to the pertinent standards, the IECRE offers a conformity assessment system referring to most relevant standards, like IECRE OD-401 and OD-403.

Qualification requirements may depend on the application. For example, when cables are planned to be laid underground, they must be qualified and tested for this application. Having systems close to the coast or on floating systems, will bring additional requirements like resistance to salt laden atmospheres.

When defining system components, it is also important to check compatibility of components and their interfaces. For example, a connector on a module might mate to a connector on a string cable, but its connection with the “mating” connector of another make may not be approved. Warranties may exclude such cross connection, so caution should be taken.

7.5.6. Transformers

The power transformer testing (Factory Acceptance Test) should be performed once the assembly is completed at the manufacturing facility. The power transformer procurement process should include a design review and quality control of the manufacturing process. Factory Acceptance Tests are done at the factory to make sure that applicable standards are met, to assure high quality products, considering IEC 60076-1, 2, 3, 10, 18.

The following table summarises the tests to be performed for the transformers to be provided.

TABLE 2 - TESTS TO BE PERFORMED FOR TRANSFORMERS