Matt Denninger, Advanced Energy Industries, Inc., Fort Collins, CO USA
September 20, 2011 | 2 Comments
Ongoing operations and maintenance (O&M) can have a huge impact on how well a PV site performs, yet system designers don't often give adequate consideration to O&M when system design work is initially performed -- particularly the O&M requirements of balance of system (BoS) components. (BoS comprises all component of a PV system other than the PV panels. This includes wiring, switches, support racks, and inverters. In the case of free-standing systems, land is sometimes included as part of the BoS .)
The primary reason designers do not include O&M of BoS components in the planning phase is simply due to the lack of benchmarking data; there just isn't a mechanism to provide comprehensive, industry-wide information in a "lessons-learned" format. Having this kind of information would enable designers to see specific issues, potential impacts on power production, probable root causes and efficient ways to remedy them.
|Figure 1. Pareto chart of the top issues and non-conformances found through the servicing of PV sites across North America in 2009 and 2010.|
This article discusses the types of data that can be collected in analyzing BoS maintenance and reliability issues, with examples outlining how future performance problems can be avoided by early planning from an O&M perspective. The conclusion is that maximizing performance and decreasing the levelized cost of energy production (LCOE) of a PV site calls for putting as much vision, strategy, and understanding into the O&M plans as the initial PV site plans when a system is in development.
There are a number of methods that can be used to collect O&M data. In fact, many statistical process control tools can be applied to O&M practices. For this article, only the most common field issues will be discussed as well as the probable root causes. Data was collected from a contracted install base in 2009 and 2010 of North American PV sites of various sizes and with various mounting configurations. Almost two-thirds of the install base is roof-mounted, and over one-half of the base is in the range of 250kW to 1MW, with several sites larger than 1MW.
The Pareto chart in Fig. 1 shows the frequency of each documented occurrence at PV sites during the two-year period of the study. In this article, we examine the top five areas that can lead to power production losses as demonstrated by actual case data.
Unsealed Enclosures and Conduit. Left alone, unsealed enclosures and conduits can cause rust, corrosion, or ground faults that lead to future power production losses. The example below illustrates one example of how enclosure and conduit problems can negatively impact power output.
Heavy rain can cause erosion around cement pedestals and electrical junction boxes, damaging the sealing around them. Under normal conditions this would not create a significant issue. However, if the conduit in the box was improperly sealed during its installation, it would likely expose underground strings to dirt and water (Fig. 2). Having dirt and water in the conduit may lead to the ground faults, triggering the inverter to shut down. Highlighting this potential risk can help material selection and installation procedures mitigate risks caused by heavy erosion.
In the event of a ground fault, the site shown in Fig. 2 would lose power production capacity from a 500kW inverter and could take two weeks to recover – due to troubleshooting, trenching and rewiring.
Hot and Loose Connections. Improper connections can create arcs and cause fires, introducing safety hazards to field service personnel and other non-related parties who may respond to the problem. (For the purpose of this article, loss of power due to thermal losses is not factored.) For example, electrical boxes can accept wires with long and short termination. One common occurrence documented during the study was when wires with a short termination wire were placed in terminal receptors for long-terminated wires. Figure 3 shows black arc marks on the copper wires as a result of the arcing that occurred between that wire and the terminal block.
|Figure 2. Dirt and water in the electrical box will trigger the inverter to shut down.|
Some sites may have loose strings in the combiner boxes at a higher frequency than others. A problem will arise when a combiner box has several connections that fail to meet the IAW torque specifications. If the specifications are not met, than a DC arc can occur in the combiner box that would terminate one combiner box worth of power production. This is a simple matter to rectify; however, it may not be clear whether the error was a recent occurrence or had occurred long before. A complete check of all combiner boxes on an ongoing basis helps to mitigate the risk of loose connections. This process can be established during the planning phase of the project by an O&M partner.
|Figure 3. Occurrence of arcing between the wire and the termination block.|
Tracking Equipment Issues. If electrical connections are loose in the tracker encoder boxes, there may be false or lost encoder counts. As a result, the tracker loses its position and causes intermittent loss of tracker synchronicity with the rest of the system. While the entire production would not be lost to a tracker malfunction, some percentage of the total power production would be lost because the modules are not perpendicular to the sun’s rays.
|Figure 4. Failed connectors lead to loss of power production.|
Under-performing and Non-performing Strings. String issues pose an immediate and direct impact on power production. Examples include blown fuses, failed power connections, connectors not tightened to torque specifications, poorly performing strings and grounded strings. For example, one site in the study had a string that was not producing any power due to a blown fuse. This string was brought back to production after replacing the terminal block and inserting a new fuse. Planning a site inspection within a few months of operation can address these types of issues before the accumulated power production losses create a serious impact on the site’s total production.
Non-performing strings can also be caused by failed connectors. The connectors in Fig. 4 became disconnected despite a functional locking mechanism. This is likely attributed to improperly joined connectors that eventually became disconnected by the wind or other disturbances.
Breached Equipment. Electrical equipment, when improperly sealed or closed, is exposed to Mother Nature. Insects, spiders, scorpions, reptiles and rodents find their way into electrical boxes and other equipment on the site. Rodents have been known to chew through electrical wires and communications cables running through their nests and PV sites are not immune. While it may be difficult to prevent all intrusions, identifying likely entry points during the design planning stage can provide the best opportunity to have a design review or mitigation plan for construction.
On one site from the study, mouse droppings and shredded paper were found in a cabinet, potentially damaging communications for the site. The mouse’s entry point and nest were in a partially buried conduit at the base of the concrete slab. While power production and reimbursement may not be impacted by lost communications, many clients feel communications are as important as power because of the loss of performance visibility.
The examples presented here demonstrate the need for O&M at the beginning of PV design and planning, especially as it relates to BoS components. Experienced and knowledgeable O&M providers should be consulted early in the process to help mitigate the potential for the above events occurring on a future PV site. In addition to maintaining the entire site, they will collect data, track and measure system performance, and address any non-conformances or irregularities. A world-class O&M partner will also have a broad perspective and provide industry benchmarking and recommendations based on a portfolio of sites under the O&M contract to engineers and designers, ensuring maximum power production.
If there is more effort toward collecting the type of data presented in this article, design teams will be able to focus on issues that are important to them from a strategic or materials selection point of view, while benchmarking against the rest of the industry. The vision is to provide the market with unbiased, independent and useful information to enable the complete sharing of O&M issues that help all parties to improve designs, drive down costs, increase power production, and support the growth of the PV market.
1. Definition from Wikipedia, 2010 entry.
Matt Denninger received his BS in marketing and a BA in Japanese language and literature from Indiana U. He is the Director of Services Marketing at Advanced Energy Industries, Inc., 1625 Sharp Point Drive, Fort Collins, CO 80525 USA; ph.: 970-221-5583; email email@example.com
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