Inverter Manufacturer Recommends Action on SW Units

Xantrex Technology Inc has released special instructions for its dealers to address questions surrounding its Trace SW inverter.

ARLINGTON, Washington, US, 2002-01-02 [SolarAccess.com] Last month, Underwriters Laboratories Inc said models SW4024, SW4048 and SW5548 do not comply with certain requirements of the UL 1741 standard, and may pose a hazard under specific circumstances. The standards organization said some grid-tie installations with the ‘sell’ mode enabled may support an unintentional island, and may continuously generate and feed power into the transmission line at the tested power levels of 25, 50 and 100 percent. In addition, total harmonic distortion may exceed the limit of 5 percent when the inverter is exporting power, and the problem is most apparent when SW inverters are connected to low impedance utility lines, the bulletin explains. “Unintentional islanding occurs when a utility interactive inverter energizes a portion of the utility grid system when power from the grid is interrupted,” explains UL. “Unintentional islanding may create a risk of electric shock for utility line personnel by causing a utility line to remain energized when it is assumed to be disconnected from all energy sources.” Islanding can also damage utility equipment, and interfere with operation or restoration of utility service. The report noted that SW inverters manufactured before November 2000 or installed in off grid or in backup applications comply fully with all UL requirements and retain a full listing. The alert did not require grid interactive units to be removed from service, and left each utility to make its own decision. The SW inverter includes four levels of protective features to avoid islanding, and exceeds UL requirements that were in place last year, says a Xantrex bulletin. Testing since then has revealed that the model “may exhibit islanding in some rare test conditions which impose an intentionally severe balanced electrical condition,” but that condition has never been observed in any grid tie system, and the severe test intentionally bypasses the first three levels of protection in the inverter. “Based on field experience and the judgment of third party industry experts, it is Xantrex’s position that no material safety hazard exists for any SW series inverter currently in service,” it explains in a note circulated to dealers on December 17. The west coast manufacturer recommends action that is “reasonable, protects the interests of all stakeholders, and will help advance the PV industry.” Testing since the SW model was approved was based on a different test procedure than IEEE 929 and, since that standard and UL 1741 are intended to be harmonized, UL considers the SW to not comply with UL 1741. “The risk of islanding is so small and the risk of injury even more remote” and Xantrex recommends that utilities accept or ‘grandfather’ the pre-November anti-island protection levels for existing SW installations, explains the bulletin. “Based on over six years of SW field experience and the judgement of third party industry experts, it is Xantrex’s position that no material safety hazard exists for homeowners or utility personnel.” If utilities require additional information, Xantrex recommends that system owners disable the ‘sell’ feature to prevent the export of power. Testing will be complete soon on an accessory that can be installed on grid-tied installations. This Grid Tie Interface will ensure that SW inverters meets all applicable CSA, IEEE and UL regulatory standards for grid-interactive use. “The product is not dangerous,” notes Kevin Hagan. “It does not present any dangerous risk to homeowners.” There is no estimate of the number of inverters affected by the UL decision, but Hagan says it would be in “the hundreds.” The new grid interface, which should be approved by January, will cost less than US$500, although he added that details are not yet available. The interface involves a new control board and will not be easily compatible with existing SW models, and Hagan says the existing inventory of affected units will be sold for off-grid applications. Due to the uncertainty for grid connection, the California Energy commission has temporarily suspended approval for the SW series, which affects rebates for some homeowners in that state. Hagan says Xantrex has posted an online registration form to identify affected homeowners.
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