Ungrounded PV Power Systems in the NEC: Page 4 of 12

Bill Brooks, principal at Brooks Engineering, is an active participant in the UL 1741 Standards Technical Panel as well as in the code-making panel that has purview over NEC Article 690. According to Brooks, “The NEC is going to have to adopt a more accurate concept of isolated and non-isolated.” He continues: “The most common ungrounded PV systems installed in the US are non-isolated systems. While 690.35 is very clear on the requirements for ungrounded PV systems, contractors and AHJs do not necessarily understand that non-isolated means ungrounded and that Section 690.35 of the NEC applies. To make matters worse, not all literature provided by the manufacturers of non-isolated inverters makes it clear that 690.35 must be followed.”

Eliminating the Isolation Transformer

Inverter topology determines whether an application calls for a grounded or an ungrounded PV system. In the US, where grounded systems are common, most inverters incorporate an isolation transformer. Figure 1 shows a representative single-phase grid-tied inverter used in the US.

While the isolation transformer represents a significant part of the overall volume and cost of the inverter, it also performs the following three important functions.

Output filtering. The transformer, being a reactive component, helps filter the inverter’s pulse-width modulation signal. The transformer is generally not the only reactive component in the filter design. An inductor (identified in Figure 1) and a capacitor (the large sky-blue component in the top right corner near the control board of the inverter) provide additional filtering so that a pure sine wave is generated at the inverter’s ac output.

Voltage step-up. The maximum voltage that an inverter can output is about 10% less than the maximum voltage that can be produced on the dc side of the system. On the one hand, the typical maximum dc voltage in the US is 600 Vdc, and the actual operating voltage can be as low as 330 Vdc. On the other, the output voltage of the inverter must match the grid’s maximum voltage. For a 240 Vac installation, the peak grid voltage can be as high as 373 Vac. For a 480 V 3-phase ac system, the maximum peak grid voltage can be as much as 747 Vac. An isolation transformer makes it possible to step up the dc input voltage to match the grid voltage.

Decoupling ac from dc. With grounded PV systems, the dc system ground needs to be isolated from the ac system ground so that they are not coupled through the source circuits.

Unfortunately, an isolation transformer also decreases the inverter efficiency by 1%–2% and lowers the overall system efficiency as a result. If the isolation transformer is eliminated, then inverter and system efficiency can be improved. However, the functions performed by the isolation transformer need to be addressed before it can be eliminated.

Benefits of Non-Isolated Inverters

As we will show, ungrounded PV systems have additional BOS requirements compared to conventional grounded PV systems. Ungrounded PV systems also require special inverters specifically designed and listed for use with ungrounded arrays. So why would anyone choose to go this route?

It turns out that many of the potential benefits of deploying ungrounded PV systems are specifically associated with the use of non-isolated inverters. The advantages most commonly attributed to non-isolated inverters include higher efficiency, improved economics and increased ground-fault sensitivity.

Higher efficiency. Advanced Energy has sold its bipolar transformerless inverters into commercial and utility-scale PV applications since August 2007. According to Tucker Ruberti, the company’s director of segment marketing, “The most obvious benefit of a transformerless architecture is higher inverter efficiency.” As an example, the weighted CEC efficiency of Advanced Energy’s transformerless Solaron 250 kW inverter is 97.5%, which is 1% higher than that of the company’s transformer-isolated PVP250kW inverter.

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