Electrical issues in photovoltaic systems

The solar energy market is growing exponentially these years, and it causes an escalating disparity between the number of photovoltaic (PV) power plants and the available resources for operations and maintenance (O&M).

The current landscape of O&M resources, notably PV technicians, is displaying signs of stagnation. Consequently, a competitive dynamic has emerged between PV installation companies and O&M firms in their pursuit of highly skilled personnel. While this scenario presents opportunities for PV technicians seeking employment, its long-term implications for the industry may be less favourable.

Recent investigations point at a high level of technical diversity in the electro-technical challenges observed in the field. PV technicians face more and more complicated tasks, when they go to work to keep up with a skyrocketing amount of new and aging PV installations. In this article we discuss the challenges and look at what we can expect for the future of solar PV O&M, in terms of field work.

The cost of solar energy ownership is changing

The expected demand for skilled labour combined with exponential growth in capacity leads to two main challenges:

1. The quality of work in installations will over time decrease, leading to more faulty installations and thereby reduced production and higher risk of accidents and fires. This happens because the resources will have to be spread thinner and thinner.
2. We can expect higher prices on maintenance work, as the demand increases dramatically over a short time span.

The total effect of this dynamic is a significant change in the structure of the total cost of ownership (TCO). Since prices for components are decreasing rapidly, O&M thus takes a relatively higher share of the TCO for PV systems. If we look at a 100kWp PV system, the O&M share will go up from 23% to 33% towards 2030.

Operations and maintenance field work

According to Wood Mackenzie, solar installations nearing inverter end of life reached 21 GW by the end of 2019, representing 3.4% of the global market — increasing to more than 14% of the total cumulative capacity over the following five years. Today a portion of solar PV projects have a monitoring system implemented, however few are synchronised in real-time with diagnostics tools in place, and even fewer have basic periodical performance assessments carried out.

In a market with so little automation in place, there is an increasing demand for in-field inspections that can lead to actionable insights to improve the state of a PV power plant. The process of identifying and solving electro-technical issues in solar PV power plants can however be quite demanding, often leading to a significant drain on team resources and truck rolls in vain.

Using standard electrical and optical tools for troubleshooting complex and invisible issues can lead to ambiguous test results and interventions that do not solve the problems. This means that experts with specialised PV knowledge must be involved every time, to ensure correct reporting and data analysis.

Conventional test methods, such as curve tracing, are essential for documenting the light to electricity conversion in new PV systems. A useful measurement however requires that results are captured close to Standard Test Conditions (STC). In short, comparative performance-based field studies are limited to the sunny season.

More importantly, performance-based methods were never developed to troubleshoot and locate discrete point-like issues in PV module strings. This is important because the main share of electro-technical issues found in the field seems to have a root cause that is not necessarily related to the absorber efficiency and its degradation over time.

Novel studies point at the importance of understanding the interplay between all components in the solar panel circuit, and to avoid entirely focusing on the semiconductors. The latter is the traditional approach, transferred from academia to the field — but it is time to revise the strategies.

Faults in DC cables and connectors

In 2016 there was clear evidence that failure modes in PV power plants were not only linked to material degradation in the light absorbers. The report “Technical Risks in PV Projects” by TÜV indeed concluded that unmated PV module connectors was a leading cause of PV module failure.

This is an excellent example of the above-mentioned electro-technical testing and analysis challenges. Imagine that a string of PV modules presents itself with a reduced open circuit voltage. In a case with unmated connectors, the lower voltage is not caused by cell efficiency degradation, but comes from an increase in series resistance in one or more connector sets.

When the connectors are unmated, there can be a mechanical tolerance that leads to separation of the current-carrying tips inside the connector set. The PV system voltages set up an electrical arc to transport the electrical current, creating a plasma which is several thousand degrees. Such extreme temperatures lead to chemical transformations in the metal parts of the connector, resulting in a “welded” connector made of a different material and left in a state of high resistance.

Photovoltaic module bypass diodes

A PV module normally features bypass diodes in a junction box on the back of the module. The diodes serve as an effective point for disabling DC power in the event of an arc fault or other unsafe operating conditions.

But when the bypass diodes blow and disconnect after an arc fault or a lightning strike, you end up with an unprotected PV module. If a single cell is shaded while the bypass diode is open circuit, the module can no longer shift the voltage polarity, and the string current must pass through a shaded non-conducting cell. The local dissipation of electrical power becomes a hot spot that has the capacity to melt the lamination and cause an ingress of water — escalating to a severe isolation resistance issue.

Recent sources have pointed out that DC distribution and wire management is a prominent root cause of issues in PV power plants. By counting fault events using workflow software, a database of knowledge was created — the statistical analysis confirms that 59% of all issues are related to field-made connectors and wire management in DC distribution.

The great thing about workflow software reporting is that the findings tend to be free of confirmation bias and guesswork. The data is simply a description of the issues found by technicians during their daily work.

Newer investigations show evidence of continuing safety issues related to Rapid Shutdown Devices (RSD) and mismatched connectors. Module Level Power Electronics (MLPE) may solve one task, but in some cases the MLPE introduce new technical risks as well. MLPE components carry high currents, sit in outdoor environments, and endure challenging conditions — thermal cycling, moisture exposure and mechanical movements.

The growing prevalence of extra module electronics has contributed to a statistically significant increase in failure points within rooftop solar systems across the United States and elsewhere. In conclusion: unmated connectors are the root cause of most problems, and even fire prevention electronics can overheat due to faulty connectors.

Electrical isolation and ground faults

Another failure mode in PV systems not directly linked to cell efficiency degradation is electrical isolation faults.

In the beginning of the PV industry era, many projects were sold as “maintenance free” — but over the years we have learned that with currents of 10-100A DC and millions of components at play, electrical isolation plays a critical role with regards to safety and financial ROI in PV.

A solar PV ground fault is the condition of current flowing through the grounding conductor — an unintentional electrical connection between a current-carrying conductor in the PV system and the equipment grounding conductor. When a ground fault is present, the electric current that was supposed to flow to the inverter is flowing directly to ground. Above a certain current threshold, the inverter shuts down for safety reasons.

When a technician goes to the field to locate ground faults, standard testing can lead to prolonged service interventions. Traditionally one would impose a high voltage pulse and test the strength of the electrical isolation resistance (Riso). While this method is a rational safety requirement, it can unfortunately also ionize the equipment — and knowing the isolation resistance as a number will not reveal where the fault is sitting in the system.

The problem occurs when the Riso value is higher, but still low enough to create a permanent or intermittent power shutdown. When testing isolation resistance, the value you get is often much higher than the highest resistance value that will register on a standard multi-meter. In this way only clear-cut faults can be found. The intermittent faults stay present in the systems and demand repeated service interventions, until they manifest strongly enough to be found.

The exponential mindset — a necessity for the future of solar PV

For many years the field approach to testing PV module strings has been focused on the light to electricity conversion. In the field however, we have learned that the external circuit is of equal importance when it comes to the long-term operations and maintenance of the power plant.

The amount of experienced PV engineers per GW of installed solar capacity is dropping by the hour, and it will likely lead to disruptions in automation solutions. Already we see entirely new concepts for on-site inspections in the form of autonomous robots and alternatives to conventional test equipment.

Such innovations enable the use of artificial intelligence — a somewhat proven way to deal with a workforce that is simply not growing fast enough to keep up. Solar energy has an enormous and proven potential, but it is time to adopt an exponential mindset and try new technologies while constantly striving to make things simpler.