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EPC contractors have a key role to play in ensuring the long-term health of PV power plants Credit: Sterling & Wilson

EPC contractors have a key role to play in ensuring the long-term health of PV power plants Credit: Sterling & Wilson

Solar PV is a reliable and stable technology, with innovation in modules only increasing its life expectancy. While PV modules generally receive most of the attention in the solar sector, there are a variety of technologies and services that are essential to the success of any PV installation; many of these come under the umbrella of EPC – engineering, procurement, and construction – encompassing end-to-end solar services, from system design and procuring components to installing the project.

After the success of SolarPower Europe’s Operations and Maintenance (O&M) Best Practice Guidelines – now in its fourth version – and its first edition of guidelines related to Asset Management (published in December 2019), the task force behind the two documents is now developing its first-ever EPC Best Practice Guidelines, which aims to help the industry standardise and optimise the EPC segment. The document, which will be published later this year, is not only targeted towards EPC providers, but all relevant stakeholders, including investors, financiers, monitoring solution providers, asset managers and even O&M contractors. An important element of the EPC guidelines will be to benefit from the long-term experience that the European solar industry has in the operational phase and create a feedback loop and dialogue with all providers.

This article aims to introduce some of the core elements of EPC: inverters, trackers, junction boxes, and monitoring technology. These are the technologies that ensure the long-term success and efficiency of solar installations, and, if appropriate attention is given to them, can end up saving developers significant resources over the course of the solar PV system’s lifecycle.

Inverters: the heart of PV plants

Inverters and their associated technologies are central components in all solar PV systems. Inverters ensure downstream that the power generated by the PV array can be fed into the grid, used by connected AC consumers or temporarily stored in conjunction with storage systems. Upstream, they perform important safety functions, such as earth fault detection, arc detection and anti-islanding. Due to the continuously increasing share of PV in the energy mix, inverters must perform more and more tasks, also related to grid stabilisation; as grids become smarter, inverters must also take over more grid-related services. In order to perform these services at all times, an increasing number of PV power plants will be combined with energy storage systems. The inverter can thus be described as the heart of any PV power plant – its failure therefore leads to serious problems with the larger system components.

Topology

The topology of a PV power plant usually follows three different concepts: (1) large parts of the plant can operate via a central inverter; (2) the inverter can be used at string level, combining single or multiple strings; or (3) it can be operated on a module level, via module-level power electronics (MLPE). With regard to ease of maintenance and availability of the plant, it should be noted that central inverters are easy to maintain, and in the best-case scenario can be repaired on site, therefore offering a high overall lifetime of 20 years or more. However, in the event of a problem, large parts of the power plant are separated from the feed-in. MLPEs as well as string inverters cannot usually be repaired on site and should not be touched until environmental influences have been eliminated. In the event of their failure, only smaller system parts or even only one PV module is affected. Such inverters usually have a lifetime that is shorter than the plant’s operation time, so they need to be replaced during the life of the system.

In addition, the specific number of failures for less accessible components increases with the number of electronic components used in the system. Market analyses in relation to the frequency of the use of different topologies in industrial and utility plants show an even distribution of string inverter and central inverter designs, and a growing number of MLPE-based plants (although on a much lower level). Availability also plays a major role in the selection of the appropriate design or provider. In the event of a defect, short-term availability of replacements is crucial to keep yield losses to a minimum.

Planning and commissioning

The importance of planning when it comes to PV installations cannot be emphasised enough. In addition to the quality and reliability of the components used, it is at this stage that the quality of the system’s subsequent performance is determined. Besides standard-compliant planning, the environmental conditions and working windows recommended by the manufacturer must be observed. Non-observance of these requirements usually leads to increased failure rates during operation. It is therefore recommended to have each system of relevant size inspected by an independent party before and after commissioning, and to have any deviations corrected. The documents of the IEC 62446 series, for example, provide guidance on the appropriate procedures.

Downtimes of PV systems are often caused by inverters [1] [InvRel]. However, many of the interruptions underlying these evaluations are ultimately due to problems with other system components. Here, ground-fault problems and, if a corresponding detection is available, actual or incorrectly detected arc-faults play a role. In addition to the plant design, the quality of the components used is of decisive importance. However, a plant designer or installer has only limited possibilities to comprehensively assess quality without being able to rely on field data and other empirical values. The conformity of the inverters to qualifying standards is mandatory but does not allow a detailed statement about their durability in the field. This can only be determined by a long lifetime test, in connection with simulations based on inverter lifetime models.

This is an extract of an article first published in Volume 24 of PV Tech Power. The full article can be read here, or in the full digital copy of PV Tech Power 24, which can be downloaded via the PV Tech Store here

References

[1] Nagarajan, Adarsh, Ramanathan Thiagarajan, Ingrid Repins, and Peter Hacke. 2019. Photovoltaic Inverter Reliability Assessment. Golden, CO: National Renewable Energy Laboratory. NREL/TP-5D00-74462. https://www.nrel.gov/docs/fy20osti/74462.pdf.

Authors

Máté Heisz, head of international cooperation, SolarPower Europe

Máté Heisz is the Head of International Cooperation at SolarPower Europe, and Coordinator of the association's Lifecycle Quality Workstream and Emerging Markets Workstream. Prior to joining SolarPower Europe in 2017, Máté spent four years in Tunisia working as a Renewable Energy Advisor at the Tunisian Ministry of Energy on behalf of the German Development Cooperation (GIZ). Máté holds a Master’s degree in International Relations from the Free University of Berlin, and a Master’s degree in Economics from the Corvinus University of Budapest.


Ralph Gottschalg, director Fraunhofer CSP, Department Reliability and Technology for Grid Parity

Prof. Dr. Gottschalg has been Director at Fraunhofer Centre for Silicon Photovoltaics, Department Reliability and Technology for Grid Parity since 2018. He is also Professor of Photovoltaic Energy Systems at Anhalt University of Applied Sciences in Köthen, Germany. Before this he was Professor of Applied Photovoltaics at the Centre for Renewable Energy Systems Technology (CREST) at Loughborough University, UK.

 

Martin Lütgens, senior project manager solar international, ABO Wind AG

Martin Lütgens is Senior Project Manager Solar International at ABO Wind AG in Germany. Before this he was Head of Purchasing and Project Management at CTF Solar in Germany. He has also held positions at Günther Spelsberg Photovoltaic and BP Solar.

 

Nicolas Bogdanski, global head of energy storage systems, TÜV Rheinland AG

Dr. Bogdanski studied Electrical Engineering at Wuppertal University, Germany. He has been working as a scientist in the field of semiconductor science and polymer mechanics for several years. At TÜV Rheinland he is part of the solar energy R&D group where he is responsible for PV reliability projects.

 

César Hidalgo, Principal Engineer Solar, DNV GL

César is a Principal Engineer at DNV GL, and has led an international solar team from 2007 until 2015. His previous roles within DNV GL have included Senior Independent Engineer in Spain and Latin America, representative for DNV GL’s WindFarmer software in Spain, Latin America and the Balkans, and responsibility for training courses in Spanish, among other marketing positions. César worked for Haskoning International Consultants in renewable energy consulting in the period 1995-2000, and was also the director of a cogeneration plant based on gas and fuel-oil engines of 15MW during the commissioning and first months of operation.

Tags: o&m, operations and maintenance, solarpower europe, dnv gl, tüv rheinland ag, abo wind ag, fraunhofer csp, epc, maintenance

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