Unprecedented market-purchase curtailments of up to 1.2 GW contributed to NV Energy's call to customers to conserve energy in August, the company said in an Oct. 21 filing to the Nevada Public Utilities Commission.

The comments were filed in compliance with a resource-adequacy investigation opened by commission Chair Hayley Williamson on Aug. 26 (see CEM No. 1605). The first set of comments was filed on Oct. 5, in which NV Energy said a "severely restrictive" market led to the company's inability to secure additional electricity resources (see CEM No. 1611).

NV Energy told regulators that while it prefers to purchase energy as much as 24 months in advance with terms that include a high commitment of deliverability on the part of the seller, it had made an atypical number of "nonfirm," day-ahead and real-time market purchases during the week of Aug. 17.

Also during that week, the market experienced purchase curtailments for 11 hours per day on average for a total of 76 hours. During nine of those hours—seven of them on Aug. 18— curtailments exceeded 500 MW. The largest curtailment, 1,243 MW on Aug. 18, occurred at the 6 p.m. hour.

"NV Energy has never experienced a curtailment of that size," utility attorneys Michael Greene and Justina Caviglia said in the filing.

The company said it made its best effort to resupply through real-time market purchases, but the restricted market meant it was unable to procure additional resources. 

The issue was compounded by its transmission-only customers, whose total supply exceeded load needs. NV Energy said it has developed an internal tool to identify imbalances in supply and load with its transmission-only customers. 

"If firm purchases had been delivered and Transmission customers avoided leaning on NV Energy's resources, the shortfall would have largely been avoided," the utility said.

The company told regulators it experienced no forced outages from its generating units, which it said were achieving 99 percent of potential output during the week of Aug. 17. The utility's 43 active power-purchase agreements also had no supply constraints. 

"In general, the generating units operated by NV Energy performed as expected and, in some cases, exceeded expectations providing anticipated supply for use by all customers," Greene and Caviglia said. They reported that the unit ramp rate, the number of megawatts per minute by which a generating unit can increase or decrease output based on need, had no impact on the company's ability to respond to energy supply limits.

The data do show one hour, from 3 p.m. to 4 p.m. on Aug. 18, in which NV Energy was 95 MW deficient from its required operating reserve of 532 MW and used Northwest Power Pool imports to supplement.

The highest discrepancy between projected and actual loads occurred on Aug. 19 at 6 p.m. Day-ahead forecasts at that time differed from actual load by 16 percent, and real-time estimates differed by 13 percent. The discrepancies were mainly due to weather, the company said.

The PUCN has scheduled a workshop Dec. 16, following the submission of NV Energy's final comments on the Western Electricity Coordinating Council regional transmission system and the state of the wholesale electric market for the week of Aug. 17, which are due Nov. 24.

Aria covers California and the Southwest from Albuquerque. Her work has appeared in a variety of popular and academic publications.