Designing for arc flash mitigation in solar photovoltaic systems

Rising solar capacity comes with rising risk. The challenge is mitigating that risk.

01/05/2018


Learning objectives:

  • Understand the needs and requirements for designing solar photovoltaic (PV) systems.
  • Review the codes, standards, and guidelines that dictate the design of PV systems.
  • Design electrical and power systems for arc flash mitigation in a PV system. 

Photovoltaic (PV) solar arrays introduce new challenges to arc flash analysis and mitigation within the energy industry, particularly within dc power distribution systems. As more large-scale arrays come online, it has become increasingly important for the industry to develop uniform design calculation standards, set proper maintenance procedures, and develop arc flash mitigation strategies. 

The solar industry is in a state of rapid growth. The National Solar Jobs Census states that one out of every 50 new jobs added in the U.S. in 2016 was created by the solar industry, representing 2% of all new jobs.

Figure 1: This graph shows how the number of workers employed by the solar industry has changed each year from 2010 to 2016. The blue bars show cumulative job numbers while the orange line represents the growth rate from year to year as a percentage, and it shows significant increases from 2011 to 2012 and from 2015 to 2016. Courtesy: SEIA/GTM Research U.S. Solar Market InsightWhen an industry grows this quickly and begins to employ a large workforce with relatively few years of design, construction, or operating experience, clear guidelines are required to promote efficient facility performance and protect the safety of personnel (see Figure 1).

Over the course of the past 5 years, much has evolved in the large-scale PV sector. According to the Solar Energy Industries Association (SEIA), total solar capacity in the U.S. was 2 GW in 2012. By the end of 2016, total solar capacity had increased to nearly 15 GW—and more than 70% of this capacity came from utility-scale plants. Presently, it is estimated that 2,500 solar plants have come online or are currently under development, representing 41 GW of power. This trend is expected to continue as the cost of development falls (see Figure 2).

Article 690, Solar Photovoltaic (PV) Systems, has been in NFPA 70-2017: National Electrical Code (NEC) since 1984. Recognizing the growth of large-scale solar, the 2017 edition of the NEC dedicated an entire new article: Article 691, Large-Scale Photovoltaic (PV) Electric Power Production Facility, sets standards for safer installation practices within the industry.

Similar to other energy production facilities, solar plants must maximize energy production while maintaining a safe work environment. These facilities often have legal or contractual requirements to produce a specific amount of energy over the course of a defined period of time or to maintain a minimum capacity level during all operational hours. The inability to meet these requirements frequently has significant financial and operational impacts for all parties involved.

Figure 2: This graph shows the growth of solar installations in the U.S. over the past decade. It differentiates installations by type: residential (blue), nonresidential (orange), and utility (gray). There is a continual growth trend from 2011 to 2015 for both residential and utility installations and a strong spike in utility installations from 2015 to 2016. Courtesy: SEIA/GTM Research U.S. Solar Market InsightOne of the greatest risks to any person maintaining an electrical production facility is arc flash. Arc flash risk is eliminated by de-energizing equipment when it is being worked on. However, it has become more common to maintain solar plants while they are still energized. This is not only due to operational requirements, but also because the dc side of the solar array will always be energized whenever it is exposed to solar radiation. This is during nearly all daytime hours, when maintenance is most often performed.

Understanding arc flash hazards

An arc flash hazard is defined by NFPA 70E-2018: Standard for Electrical Safety in the Workplace as “a dangerous condition associated with the possible release of energy caused by an electric arc.” This release of energy is caused by a line-to-line or line-to-ground fault in which energy is transformed into destructive elements, such as intense heat, light, and pressure. The energy released during these events can cause extreme sound and vaporization of metal. In short, it is an explosion. An arc flash can only occur in the presence of an energized source.

In a 2015 study, the Fire Protection Research Foundation found that 30,100 individuals were injured over the course of a 10-year period from electrical shock or arc flash across the U.S. The same study indicated that a substantial number of the injuries were the result of “work inappropriately performed on energized equipment.” Further research indicated that time pressures, supervisor demands, and a lack of clear organizational communications led these workers to take shortcuts that caused these events to occur. If proper protocols had been followed, these incidents could have been prevented entirely.

NFPA 70E’s Article 110.3(F) requires the implementation of an electrical safety program, which includes identification and risk assessment procedures, for systems with energized conductors operating at or above 50 V or where an electrical hazard exists. Furthermore, NFPA 70E Article 130.3(B)(1) requires an arc flash hazard analysis be performed. This analysis produces hazard warning labels for each piece of equipment likely to require examination, adjustment, servicing, or maintenance while energized, indicating the available incident energy and the corresponding working distance. Additionally, each label must indicate the required personnel protective equipment (PPE) that must be worn by service personnel. IEEE 1584-2002: Guide for Performing Arc Flash Hazard Calculations provides the industry’s standard guide for performing these calculations.

Unfortunately, there is minimal guidance in IEEE 1584 to quantify the arc flash hazard on the dc power distribution system of a PV array. Most electrical sources are at a constant voltage whereas the dc side of a solar array is a constant-current source and must be modeled accordingly. Most calculation strategies revolve around the conservative approach indicated in 2015 NFPA 70E Annex D.5. The annex includes two different calculation methods: the maximum power method and the detailed arcing current method.

Both methods have been questioned and investigated by industry experts, and the annex itself indicates that the maximum power method is conservatively high. In 2013, David Smith from Colorado State University wrote a detailed report regarding arc flash hazards in PV arrays in which he concluded that, while the detailed arcing current method produces a more realistic representation of dc arc flash hazards, “no consensus standard exists for calculating arc energies in dc systems.”

NFPA 70E does not exclude the use of alternative calculation methods. In the absence of clear and convincing scientific guidance or a recognized design standard, many designers have become reliant on outside studies and recommendations from industry leaders. Multiple publications on dc calculations have provided guidance in recent years. However, before determining any arc flash mitigation techniques, an arc flash hazard analysis must still be performed. The results of this analysis will guide designers and help them tailor the correct strategies to the needs of a specific project.

Competing demands

Maintaining and operating large-scale solar arrays requires a delicate balance between system availability, reliability, and integrity. These plants often have a power purchase agreement (PPA) or other contractual agreement in place that requires that energy be produced per specific terms, and if it is not, a financial penalty may be levied as compensation. While fulfilling these terms is an important factor, safety remains the highest priority.

Many utility solar plants are designed specifically to offset daily peak energy demands and/or reduce the costs to run auxiliary generation facilities during these peak times. The activation of these auxiliary generation plants can be a significant financial burden to most utilities due to the cost of the fuel required to cycle the systems on and off.

To effectively operate a PV system under these circumstances, the plant must first be correctly commissioned during construction and regularly maintained after it is turned over to its owner-operator. During the commissioning process, the array must be energized to some degree to test the system components and ensure the equipment is operating correctly and to specification. Typically, the dc side of the array is sequentially energized as the commissioning progresses, culminating in a full test of the array. Additionally, in many cases, the system is at least partially energized during maintenance to avoid downtime.


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