Connectors: Key Components In PV Bankability
The concept of bankability plays an important role in assessing the viability of new solar power projects. Selwyn Corns, managing director of Stäubli Electrical Connectors UK, discusses the effect PV connectors have on bankability and the importance of their reliability over a plant’s expected 25+ year lifetime.
The term bankability was coined during the credit crunch in the aftermath of the financial crisis in 2008 and is a measure of the willingness of financial institutions to finance a project. Banks often took a gatekeeper role in the project approval process and in their bankability assessments they developed a systematic approach to identifying the legal, technical and economic risks of projects.
For photovoltaic plants, bankability has become an established risk management tool covering the whole lifetime of the project and can be a deal breaker for investors in large projects. In recent years this emphasis on a PV project's overall efficiency has been heightened by the withdrawal of government subsidies for solar power in many countries.
The MC4-EVO2, whose voltage rating of up to 1500V dc means that solar power operators can benefit from longer cable strings when linking solar fields.
The key to long term efficiency
The guiding principle for bankability is to minimise risk while maximising return. This can only be achieved by ensuring efficiency over the long term through the use of high quality components.
During the project planning stage of a PV system, the main focus is understandably on the initial costs, predominantly on the two highest cost items: the solar panels and the power inverters. The components for cabling (connectors, junction boxes and cables) typically make up less than 1% of the total initial costs.
This is a reason not to consider cutting costs by using cheaper connectors, since any initial savings would only amount to a small proportion of the total capital expenditure. Any savings would be outweighed by the resulting higher failure rates over the project's 25 year or more lifespan that would lead to higher operating costs and a negative impact on return on investment.
Failure Mode and Effects Analysis ratings of PV module failures
The assessment of technical risks covers the whole project life cycle and involves all parties and associated contracts along with the corresponding components and associated suppliers. Technical risks can arise from PV modules, inverters, connectors and other mechanical or electrical components, as well as from system engineering, energy prediction, installation and operation.
Technical risks very much depend on the individual framework conditions of the underlying PV system, i.e. system design and size, module technology and inverter configuration, site characteristics (whether ground or roof-top mounted), geographic and climatic conditions as well as international standards and local regulations. These risks were a major consideration in the EU-funded Solar Bankability project, which ran from 2015 to 2017, and was set up to establish a common practice for professional risk assessment based on existing studies and statistical data of failures in PV plants.
One of the reports it produced presents a cost-based Failure Mode and Effects Analysis (FMEA) for the estimation of economic losses due to planning failures, system downtime and replacement of components. High among the risk factors affecting downtime were those relating to connectors, with the use of mixed connectors from different manufacturers having by far the highest risk. Other causes of downtime were connectors not being correctly connected, and unprotected connectors.
Connectors and cable problems were also prominent among the top 20 technical failures in terms of economic impact, taking into account downtime and cost of repairs. Wrong or absent cable connections caused the largest losses, followed by broken or burnt connections.
One reason connectors have such a large impact on bankability is the large number of them required. For example, in a 5 MW ground-mounted plant, there could be 20 blocks of solar panel modules, each containing 18 strings of 45 modules. This amounts to about 32,000 connectors on the modules plus another 3000 for the field assembly, making 35,000 in all. The failure of one connector can cause the outage of a whole string of 45 modules.
Another factor is if operators try to cut corners by using cheaper connectors or mixing connectors from different manufacturers. It is important for connectors to have a very low electrical resistance as this maximises the current flow without generating significant amounts of heat. For mismatched connectors or those not made to the highest specifications, there can be a vicious circle effect, with a higher contact resistance leading to component heating, which leads to a higher contact resistance and so on until the connector eventually fails or even causes a fire.
Mixing connectors from different manufacturers can cause problems due to differences in production processes and quality standards, loose fitting and insufficiently high contact forces, and chemical incompatibility of component materials. Cross-mating is specifically disallowed in the relevant standards as studies have shown that there is no compatibility between different brand connectors.
Long term reliability
Stäubli's PV connectors maximise bankability through their low contact resistance and long term reliability, which have helped to make the MC4 series the market leader. They use the company's proprietary MULTILAM contact technology, which has spring contact elements to produce multiple current-carrying contact points. Each spring forms an independent current bridge, so that the many parallel springs substantially reduce the overall contact resistance. This helps them to provide long term reliability, with a lifetime of 25 years and more.
Stäubli's expertise built up from over 20 years' experience of manufacturing PV connectors and accessories makes the company a bankable partner for the long term, helping to lower the risk of any PV project.