Power Engineering Software for Large-Scale Solar Applications: Page 3 of 5
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Arc-flash hazards. NEC 110.16 provides a representative list of electrical equipment—switchboards, switchgear, panelboards, industrial control panels, meter socket enclosures and motor control centers—that requires field- or factory-applied markings to warn qualified persons of potential arc-flash hazards. In general, electrical equipment on or in commercial, industrial or utility facilities that requires servicing, adjustment or maintenance should have an arc-flash hazard warning label so that workers know what level of PPE to wear when working on the equipment. These arc-flash hazard analysis and warning requirements extend to PV system dc collection equipment, including combiner boxes, recombiners and inverters.
IEEE 1584-2002 is the international standard that governs arc-flash hazard calculations. A companion standard, IEEE 1584.1-2013, details the scope and deliverables for an arc-flash hazard study. The standards provide guidance regarding the input data and calculation methods for determining the arc-flash hazard distance and incident energy in a variety of scenarios, including dc systems, ac systems under 1,000 V, and ac systems between 1,000 V and 20 kV.
Whereas general arc-flash hazard warning labels meet minimum code requirements for equipment rated less than 1,200 A, Section 110.16(B) requires more-detailed labels for equipment rated 1,200 A or greater. To assist the servicing electrician with PPE selection, these equipment labels should detail the nominal system voltage, available fault current, clearing times for OCPDs and date of label application. Note that an exception to NEC 110.16(B) allows the use of alternative labels that conform to NFPA 70E, Standard for Electrical Safety in the Workplace.
Harmonic resonance. The shattering of a wine glass when a singer hits a particular note is a well-known example of harmonic resonance, a phenomenon described by physics that affects many different types of systems. A system tends to oscillate or vibrate at a larger amplitude when exposed to certain frequencies. The fact that materials and systems have a resonant frequency is generally a nonissue, as evidenced by the fact that a wine glass does not routinely shatter at the slightest vocal provocation. However, harmonic resonance can have unexpected and destructive consequences. These can occur when a source applies input energy to a system at a specific resonant frequency, having a substantially stronger physical effect on the system than expected.
The fundamental frequency of the North American electrical power system is 60 Hz. Each multiple of this frequency is an ordinal harmonic, meaning that 120 Hz is the second harmonic, 180 Hz is the third harmonic and so on. Generators and power supplies have a tendency to produce harmonics when they operate. We refer to the sum total of these harmonics as total harmonic distortion (THD). To comply with UL 1741, the inverter listing and test standard, inverters must also meet the requirements in the IEEE 1547 and IEEE 519 standards, which cover distributed resource interconnection and harmonic control requirements, respectively. According to IEEE 519, the THD a given inverter in isolation produces is limited to 3% current waveform distortion over the first 60 harmonics. Meanwhile, IEEE 1547 states that the total demand distortion (TDD) at the point of interconnection cannot exceed 3% of the current waveform distortion. These requirements are related as TDD is a measure of THD that also accounts for the peak demand load current.
The ac collection system for a large-scale PV power plant has a natural resonant frequency. Generally speaking, the resonant point for most electrical systems is located somewhere between the fifth and seventh harmonic. While resonance is usually not a problem in PV power plants, issues do arise in some projects because the resonant condition is excited where the harmonic characteristics of the inverters and any harmonic contributions from the utility match up exactly with the resonant frequency of the collection system. Though this issue may occur in just a handful of projects, it can incur substantial mitigation costs due to the time required to design, implement and test the efficacy of mitigation measures.
Performing a harmonic resonance study is an insurance policy against possible commissioning costs in the future. It is the only way to determine whether a project will experience resonance issues. If so, engineers can design mitigation measures in advance, which will avoid difficulties and delays during the compressed timelines associated with many commissioning and performance-testing activities. After the engineering team has completed the previously discussed engineering studies, it only needs the inverter manufacturer’s IEEE 519 test results and the interconnecting utility’s harmonic content information to perform a harmonic resonance study. With proper planning, you can obtain this information from both parties at the same time that you gather the fault-current contribution information needed for the short-circuit study.