Ungrounded PV Power Systems in the NEC: Page 3 of 12
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Isolated inverters. Most utility-provided ac circuits have a grounded current-carrying conductor, as do traditional dc PV source and output circuits. Therefore, a mechanism is needed to prevent the injection of normal operating current from one system into the other through the equipment-grounding paths, which are bonded to the grounded conductors.
The most common method of separating the grounded ac system from the grounded dc system in PV applications is through the use of frequency-based or transformer-based inverter topologies. These inverter designs include either high- or low-frequency transformers that electrically isolate the ac and dc systems. (For an overview of inverter operation and topologies, see “How Inverters Work,” April/May 2009, SolarPro magazine.)
In UL 1741, the standard for inverters and other equipment intended for use with distributed energy resources, an isolation transformer is defined as “a transformer having its primary winding electrically isolated from its secondary winding and constructed so that there is no electrical connection—under normal and overload conditions—between the secondary and primary windings, between the primary windings and the core, or between separate adjacent secondary windings, where such connection results in a risk of fire or electric shock.”
Isolation transformers provide what is known as galvanic isolation between electrical systems, meaning that there is no physical or electrical connection between the input and output power. Because transformers use magnetic induction to transfer power from one circuit to another, there is no need for a metallic conduction path between the two systems. A dielectric barrier—such as an air gap, electrical insulation or both— is used to provide adequate isolation between the primary and secondary sides of the transformer, based on the highest expected voltages in the two systems. The presence of isolation transformers in the inverter prevents any stray or fault currents from traveling between the grounded ac system and the grounded PV system.
Historically, all residential and many commercial utilityinteractive inverters include an isolation transformer within the inverter enclosure. To simplify transportation and installation, inverters used for large commercial applications may have an external isolation-transformer cabinet. Inverters that include an isolation-transformer as part of the listed product are specifically designed and tested for use with grounded PV systems and are often referred to as isolated inverters or transformer-isolated inverters.
Non-isolated inverters. When we switch to an ungrounded PV system, we eliminate the need for electrical isolation. Since neither the positive nor the negative PV system circuit conductor is intentionally connected to ground, there is no parallel current-carrying path between the dc and the ac electrical systems. This means that inverters used with ungrounded PV systems do not require an isolation transformer. In grid-direct applications, utility-interactive transformer-isolated inverters cannot be used with ungrounded PV arrays, because the GFDI is internally bonded to one of the PV input-circuit conductors. Since these inverters ground the array by default, they are incompatible with an ungrounded PV array. A different inverter topology is needed.
In 2005, when Section 690.35 was added to the Code, UL 1741 did not address safety concerns related to inverters intended for use with ungrounded PV systems. Therefore, a draft approach was developed based on IEC 62109, the equivalent International Electrotechnical Commission standard. It was not until April 2010 that UL formally published a Certification Requirement Decision (CRD) containing content specifically related to inverters designed for use with ungrounded PV systems. The expectation is that the definitions and safety requirements in this CRD will be incorporated into UL 1741 after the Standards Technical Panel has reviewed and commented on them.
Inverters listed to UL 1741 for use with ungrounded PV systems do not include an isolation transformer. While as a result these are commonly referred to as transformerless inverters, it is worth noting that this particular term does not appear in either the UL standard for utilityinteractive inverters or the NEC. Instead, UL 1741 defines an “inverter that does not provide galvanic isolation between its input and output circuits” as a non-isolated inverter. The NEC includes neither term, referring simply to ungrounded PV systems and inverters “listed for the purpose.” The problem with this inconsistent nomenclature is that it can be confusing.