With federal Nuclear Regulatory Commission investigators picking through the aftermath of last Monday’s emergency shutdown of Arizona’s Palo Verde Nuclear Power Plant, the first of its three units (Unit #1) was ramping up to full operations Friday, according to a spokesperson for Phoenix-based Arizona Public Service (APS), the majority owner and operator of the plant.

In addition, a number of neighboring utilities and natural gas-fired power plant operators were reviewing the incident that knocked out other plants and threatened Las Vegas, NV, with blackouts, according to reports from Sierra Pacific Resources’ Nevada Power Co.

Nevada Power’s regional transmission director Mark Shanks said the utility “got very close; (but) from a supply perspective we didn’t see any impact at all from having these units down.” Although there were outages in Arizona and New Mexico, Shanks told the Las Vegas Review-Journal that the local system held up well and there was no added uncertainty about reliability going into the official start of summer.

The APS spokesperson said the utility plans to bring all three 1,300-MW units back sequentially, taking two to three days for each unit, so by the end of the month Palo Verde should be fully restored.

A failed insulator on a 230-kV transmission line in the northwest Phoenix area caused the shutdown of the nearly 4,000-MW nuclear plant west of Arizona’s largest city. In the aftermath, APS earlier in the week said that the incident had limited operating impact. The incident was watched closely by stakeholders through the Southwest, including the California power transmission grid operator (CAISO).

A CAISO spokesperson said the incident could have turned into a so-called “cascading event” with widespread implications throughout the West. It did cause a severe frequency drop that was immediately noticed by CAISO and some of the largest power-consuming entities on its three-control-area grid. Within an 11-14-minute interval on Monday, the situation was stabilized, the CAISO spokesperson said.

Up to 190 MW of interruptible customers’ load — mostly in Pacific Gas and Electric Co.’s territory — automatically shut down with the frequency drop, and the grid operator began offering 500-800 MW of emergency juice to Arizona to help balance the overall grids in the two states for about a 2.5-hour period, said the CAISO spokesperson, who added that the grid operator cancelled some planned maintenance on a northern California transmission line to keep maximum capacity available from the north.

Noting that it would monitor the financial impact, Standard & Poor’s Ratings Services said Tuesday it did not expect any immediate rating consequences for APS because the outage was “expected to be short, manageable,” with import capability from California and other available resources sufficient to make up for the loss of about 3,900 MW.

APS credit quality could still be hurt if the federal Nuclear Regulatory Commission determined to delay bringing the units back, S&P said in a short bulletin issued Tuesday. “The utility is currently dispatching its other owned baseload resources, as well as purchasing wholesale power to meet loads that peaked yesterday at 5,280 MW,” S&P said.

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