Rooftop Solar Onsite Testing: Harnessing the Potential of Solar

With over 10 GW of installed capacity, India is on the verge of becoming one of the fastest growing nations in terms of PV installation. It is worth mentioning the exponential growth of the market during the last few months. The almost exponential increase of global solar PV installations in the recent two decades have enabled both rooftop systems and utility-scale PV farms to become an increasingly important mainstream source of electricity. Solar photovoltaic (PV) systems are being installed in ever increasing numbers throughout the world and are expected to safely and reliably produce electricity over several decades of operation. However, many systems are not satisfactorily evaluated prior to being put into service and many have little, if any, scheduled maintenance or testing over their lifetime. Unfortunately, this leads to unsafe and underperforming systems with reduced value to their owners. Any electrical system can be tested to verify performance and to evaluate the condition of wiring systems and equipment. This is particularly important for PV installations which can be subjected to extreme environmental conditions and deteriorating effects of the elements over many years.

Many of installed PV systems are showing throughout their lifetime significant problems. Also, due to the increasingly extreme weather phenomena, there is growing need for action with regard to a fast and efficient recognition and repair of failures for existing solar power plants.

A primary objective for any power plant is to ensure the plant continuously and reliably operates, thereby generating the maximum economic and energy performance returns.  plants are no exception. The onsite testing of large PV plants requires an integrated management system that is implemented throughout the entire lifecycle. Actual system performance of a PV system can differ from its expected behavior. This is the main reason why the performance of PV systems should be monitored, analyzed and, if needed, improved on. Some of the current testing procedures relating to the electrical behavior of PV systems are appropriated for detecting electrical performance losses, but they are not well-suited to reveal hidden defects in the modules of PV plants and BIPV, which can lead to future losses. It is important to conduct tests and procedures that are used to evaluate the performance of PV systems, and especially on a novel procedure for quick on-site measurements and defect recognition caused by overheating in PV modules located in operating PV installations. During the last few years, grid-connected photovoltaic (PV) systems totaling a nominal power of more than 275 GW have been installed all over the world. In order to make possible to sustain this high installation rate in the future, it is needed to ensure a proper operation of the PV systems that guarantees their good performance. Therefore, testing and monitoring procedures are critical to check whether the PV systems’ actual performance is in line with expected behavior. Otherwise, they should be improved on, by repairing or replacing the faulty devices.

TEST METHODS FOR PHOTOVOLTAIC SYSTEMS

Visual Inspection

The modules are integrated in the service workflow per scanner. All identifiable faults are detected and recorded by camera during the optical check and cleaning.

These include loose junction boxes, faulty cables, deformed frames, cell inclusions and breakages, delamination, visible spots and scratches on the rear.

Insulation Test According to IEC61215

The modules are placed in a water bath and 1,000 volts direct current are applied in this test. Only modules that uphold a resistance according to their surface for 2 minutes pass the insulation test. The test criteria comply with the current IEC standards.

Infra-Red Tests under Load

The modules are powered backwards during the infra-red test to identify possible weak spots in the cell area, connectors and above all the junction box by means of thermal imaging and infra-red camera.

STC Performance Measurements (Flash Test)

The current performance is normally measured for all modules that pass through the PV service centre. During so-called “fl ashing” (acc. to IEC 60904), the module is fl ashed with a calibrated fl asher and the electrical output is measured (“fl ashed”) and the power determined under standard test conditions (STC).

Electroluminescence

Damage to the cells can be visualized by means of an electroluminescence test. The module is photographed with three high-resolution NIR-CCD cameras at such a high resolution that even the smallest of cracks are detected. The test documentation comprises images of the complete module and of every damaged cell.

UV-Fluorescence

This technique allows detecting changes in the EVA film caused by UV radiation in the sunlight. With the UV fl uorescence it is possible to identify cell cracks and draw conclusions regarding the age of the cracks. This helps to identify the cause of detected damages.

Parameter Detection

The electrical parameters in accordance with IEC 60904 are measured with a repeating tolerance of +/- 0.8%. The following values can be recorded in the PV service centre or on site at the system:

• Open-circuit voltage Uoc

• Short-circuit current Isc

• Voltage at the best possible operating point Umpp

• Current at the operating point with maxi- mum output Impp

• Maximum achievable output Pmpp

• Filling factor

PHOTOVOLTAIC BALANCE OF SYSTEM COMPONENT TESTING AND CERTIFICATION

The balance of system (BOS) covers all the electrical and mechanical components of a photovoltaic (PV) system other than the PV modules. BOS component quality is essential for the safety and performance reliability of PV systems throughout their lifetime.

PV module power will degrade if the BOS components are not integrated within the entire installation. In addition, the BOS components must withstand harsh weather conditions. Manufacturers of BOS components need to be aware of product- related design and safety requirements to ensure that their products comply with national and international standards and codes.

Product Development

The service provider offers support with BOS component testing during research and development (R&D). Tests are conducted according to ISO/IEC 17025, which comprises verification of scope and accreditations, testing structure and laboratory layout, ope- rations and maintenance requirements.

Product Testing and Certification

Rigorous testing and certification are done in accordance with national and international standards.

• PV inverters in accordance with IEC 62109 and countryspecificgrid connection requirements.

• PV electrical components, which include junction box, cables and connectors.

• PV mounting systems in accordance with PPP 59029.

• PV batteries and energy storage systems (ESS) according to IEC 62619 and IEC 62620, as well as specifi c country safety requirements and standards.

• PV materials, which include backsheet (PPP 58066), EVA encapsulant (PPP 58065) and adhesive and potting compound (PPP 58064).

• PV trackers and storage systems according to a combination of IEC standards.

• Performance and safety-related environmental testing

Performance and Safety-Related Environmental Testing

The onsite testing assessors provides fast- motion simulation of various environmental conditions (heat, cold, dry, wet, etc.) and testing for corrosion resistance and long-term durability performance (endurance testing with outdoor field conditions, temperature change examination and moisture-heat examination).

Product Quality Monitoring and Inspections

It is recommended that assessors conduct initial and follow-up surveillance for manufacturing facilities. There should be annual routine inspections comprising the predefined routine testing, special inspections and on-site assessments, preshipment inspection (PSI) and during-production audits (DuPro), as well as bankability audits.

CONSTRUCTION MONITORING

Investors, owners and developers of photovoltaic (PV) power plant projects need to minimize risk in order to maximize their returns. When PV projects enter the implementation phase, stakeholders have to monitor construction to ensure all contractual obligations are met, from foundation construction to project completion. Any deviation in the quality of finished work could affect the projected energy yield. All aspects of construction quality – as well as the overall operability of installed parts and components – will have an impact on the expected yield through the whole lifetime of the plant.

Monitoring Quality

Assessors should monitor the overall qua-lity of work during construction, if required.

Time Schedule Monitoring

It is important for the assessors to com- pares the progress of solar project with the approved time schedule to ensure that it meets approved key milestones in the construction process.

Meeting Contractual Obligations

Assessor should take responsibility to verify that the work meets all conditions stated in the EPC contract and that the construction runs in accordance with the approved project schedule.

Technical Requirements

A team of experts focused on evaluating electrical and civil engineering aspects should monitor for compliance of PV technology with international and best practice requirements and standards (e.g. IEC 61215/61730/61646).

Civil Engineering Assessment

The civil engineering assessors should confirm compliance of the mounting structure with the approved design (e.g. applied material, surface finish, inclination, foundings, thickness of the structure). The evaluation should cover fixed, single- axis as well as two-axis structures. Also, the assessment includes compliance of components and finished work with stated parameters in the project documentation and with relevant technical standards.

Electrical Engineering Assessment

The assessors should confirm compliance of the electrical part of the plant with approved project documentation. They should ensure that all electrical components are installed according to local / international standards and best practices.

PV Technology Assessment

The assessors should ensure that PV technology comply with approved project documentation. All components should be properly installed according to local/international standards and best practices, and have valid product certificates.

Periodical Reporting

A report with the overall evaluation for each visit performed during the construction phase should be handed over to the clients. Typical testing procedures to evaluate the performance and quality of PV systems can be divided in two groups: electrical procedures and other procedures. The electrical procedures aim to determine the behavior of PV arrays and PV inverters, as well as their compliance to the established international standards. Some of the usual electrical tests that should be carried out in the fi eld on PV installations are:

Measurement of the PV Array I/V Curve

This characteristic is very useful to verify if the actual array power is coherent with the nominal power installed (sum of the power of all the modules in the array). It allows detecting if a set of modules has a power below the manufacturer’s datasheet (or flash-list) and/or if a string is disconnected (due to burnt out fuses, disconnected cables, defective PV modules resulting into open-circuit conditions for string they belong to, etc.). This test can also notify on aging and degradation with time and/or anomalies such as shading and potential induced degradation (PID), etc. There are several commercial devices that are designed to measure IV curves.

Measurement of the PV Inverter Efficiency

This test is carried out simultaneously at the input and at the output of the PV inverter, and it is useful to verify if the actual inverter’s behavior complies with the manufacturer’s specifications. It can be performed with a wattmeter.

Measurement of the PV System’s Performance

This test allows knowing in detail the inst- antaneous behavior of the PV system and its performance under abnormal conditions, such as shading, inverter saturation, strings disconnection, etc. It can also be achieved with a wattmeter.

Measurement of the PV Array Insulation Resistance

It is very important to ensure the electrical safety of the system, because ground faults represent safety hazards to personnel. In fact, inverters typically are equipped with an insulation alarm that warns when this defect appears.

Measurement of PID

At a later stage, it is common to carry out on-site tests to determine if modules are affected by PID. This is usually done during night hours with the help of a power source and a CCD camera, but new procedures that can be achieved with only a power source have been proposed.

The other procedures (other than electrical) mainly consist of visual inspections that aim to detect defects on civil works, structures, PV modules, connection boxes, inverters, etc. Some of these defects are the hotspots in PV modules, which are invisible for the naked eye. Nevertheless, they can be detected with infrared (IR) imaging. In the last years, IR cameras have become in a very valuable device for detecting hot-spots.

It was noted that larger PV plants and Building Integrated PV (BIPV) are making more difficult and more time consuming to complete on-field IR test on PV modules to assess the good performance of the installation. In this contribution, new and advanced procedures have been developed to quickly detect hot-spots in large PV plants and BIPV of difficult access through IR imaging. This procedure makes use of hardware composed of a PV drone, a microcontroller, and an IR camera. The main result of this testing procedure is a shorter time for visual and thermographic inspection of a PV installation than the standard procedure.

A quality control at the end of the production process in the solar panel manufacturer’s production site isn’t enough because at times unsuitable packaging and incorrect hand- ling during logistic can lead to damage to solar PV systems. The damage done is often invisible to the contractors during the installation process and can have a negative effect on the output. To help ensure the long term safe operation of these systems, quality PV installation and service contractors execute a thorough commissioning process followed by a regular periodic testing and maintenance programme. As India marches towards its ambitious goal of 100 GW by 2022 and spends bill-ions setting up new solar capacity and the associated infrastructure, both the government and private sector need to make more efforts to ensure highest possible returns from these investments. In the con- text of onsite testing of solar PV plants, conducting detailed studies, tests and inspections to improve understanding of solar plants and operational issues – these onsite testing of solar PV installations can help to promote safety and optimize performance, and provide essential information to those who may need to effectively troubleshoot, diagnose and resolve any problems arising with the system. In this way, all PV systems require testing for performance and safety verification. The level of testing required will depend on local regulators, customer requirements and the quality commitments of the installation and maintenance contractors.