Power Engineering Software for Large-Scale Solar Applications: Page 2 of 5
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Short-circuit study. Understanding the magnitude and duration of short-circuit currents within an electrical system not only is a basic life and safety issue, but also is essential to the power engineering studies described below. NEC section 110.9 requires that equipment intended to clear a short circuit carry an appropriate interrupting rating. Simply speaking, the nominal maximum current rating of an overcurrent protection device—which manufacturers and testing laboratories determine based on performance under standard test conditions—must be at least equal to the available fault current at the line terminals of the equipment.
In practice, calculating maximum available short-circuit current available in an electrical system is not as intuitive as you might expect, in part because OCPDs with the same nominal interrupting rating can have very different time-current characteristics. Figure 1, for example, shows that a 100 A–rated Bussmann Series LPJ Class J fuse will open a 200 A fault in roughly 300 seconds (5 minutes) and a 1,000 A fault in approximately 0.2 seconds. Another 100 A–rated fuse may have different time-current characteristics even though its nominal interrupt rating is the same.
Though the IEEE standards pertaining to ac circuit breakers date back to the 1990s, working groups have recently revised the standards relating to both high-voltage and low-voltage devices. In 2016, IEEE published a revised version of IEEE C37.010, which covers the application of OCPDs rated above 1,000 Vac. In 2015, the international standards association published the most recent edition of IEEE C.37.13, which relates to the application of low-voltage ac OCPDs. These IEEE standards describe two short-circuit calculation methods, both of which are based on Thévenin-equivalent circuit models for each bus node within a single-line diagram.
Large-scale PV power plants integrate dozens or even hundreds of inverters, each of which the IEEE standards define as a generator. While it is impractical to manually determine all of the short-circuit calculations at each electrical node in a utility-scale PV plant, power engineering software can perform these calculations quickly and accurately.
Minimum AIC ratings. While this analysis goes hand-in-hand with a short-circuit study, it emphasizes not the magnitude and duration of a fault but rather the ability of the OCPD to clear a fault without extensive damage to the equipment or electrical system. Manufacturers and testing laboratories certify and mark listed OCPDs with an available interrupting capacity, expressed in the units AIC or kAIC. For example, low-voltage ac panelboards and circuit breakers typically carry withstand ratings from 14 kAIC to as high as 65 kAIC, whereas fuses for some applications carry ratings as high as 200 kAIC.
Not surprisingly, equipment costs within the same voltage class increase for products with a higher withstand rating. All else being equal, the installing contractor will purchase the lowest-cost product that meets engineering design specifications, which means it is very important that electrical engineers specify minimum AIC ratings for OCPDs, panelboards and other electrical equipment. Fielding improperly rated equipment could lead to catastrophic failures in the event that instantaneous fault currents exceed equipment withstand ratings.
When reviewing interrupting capacity ratings, note that NEC Section 240.86 allows for both fully rated and series-rated protection systems. In a fully rated system, each OCPD carries an AIC rating greater than or equal to the available fault current. Evaluating compliance with this parameter is a very straightforward process, and the fully rated design resists the introduction of errors over time. In a series-rated system, the available short-circuit fault current may exceed individual component withstand ratings for tested combinations of equipment or combinations of equipment selected under engineering supervision. While this option may allow for lower up-front installation costs, a licensed professional engineer needs to evaluate, document and stamp every subsequent change to the electrical system.
Protection coordination. Each branch of a multibranch electrical system has its own overcurrent protection, whether a fuse, breaker, relay or other similar device. Should a fault occur somewhere in the system, the OCPD installed between the fault-current source(s) and closest to the fault should be the one that opens and clears the fault. The main purpose of a protection coordination study, therefore, is to review the settings and ratings of each OCPD to ensure the proper fault-clearance time sequence. Short-circuit study values are obviously a primary input for a protection coordination study.
The IEEE Buff Book (Standard 242-2001) is the primary sourcebook for protection and coordination principals. Depending on the project-specific equipment, a multitude of other IEEE standards may also apply to the protection coordination study calculations. Standards in the IEEE C37 series provide information pertinent to different types of breakers, relays, air break switches and other equipment. IEEE C57.12.59-2015 provides guidance on fault-current duration calculations for dry-type transformers, whereas IEEE C57.109-1993 provides similar guidance for liquid-immersed transformers.