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Declaring Emergencies in California: The Realities of ISO Operation

An independent system operator's view on the energy crisis and the realities of maintaining reliability.
Fortnightly Magazine - October 15 2001

this net dependable capacity is adjusted to reflect generating unit outages. This report will explain each factor and how the CAISO calculates the net dependable capacity and generator outages. The section begins with the installed generation within the CAISO control area and walks through how the CAISO determines what resources it can rely upon to serve load for a given operating period.

Though the capacity of existing resources in CAISO control area capacity is approximately 46,612 MW, not all that capacity is available at all times. Capacity limitations must be factored in for a true representation of available capacity; however, several types of these limitations are very difficult to forecast.

General estimates of an average expected unavailability of generating resources for 2001 considering the limitation of Qualifying facilities (QFs)-is the most difficult limitation to discretely estimate. Also, one must take into consideration qualifying facility limitations due to weather conditions including ambient temperature, wind speed and humidity. Also consider the native load 4 of qualifying facility operations which is served by those units (netted load behind the metering point), outages which are not required to be submitted to the CAISO, the affects of power purchase agreements 5 with the utilities, the fact that the simultaneous output of a field of geothermal and portfolio of cascading hydro units is typically not equal to the maximum output of each unit, and depleting steam field pressure that affects the power output of geothermal units. All of these factors thereby reduce the overall maximum "dependable" generating capability of qualifying facilities. In evaluating emergency situations, the CAISO relies on qualifying facility data submitted by the Utility Distribution Companies 6 (UDCs) to the CAISO to determine the level of qualifying facility output.

For our example dates, data was obtained for the peak time period identified under the operating date. The level of qualifying facility output ranged from 3,311 MW to 5,855 MW out of a maximum nameplate capacity estimated at 10,313 MW. The total dependable capacity for these dates is generally lower than the average estimated dependable capacity (42,113 MW) shown in, but is within 6 percent of the estimated average.

In addition to limitations from qualifying facilities, many other factors contribute to the reduction of MW that can be considered dependable and available to the CAISO. The actual generating capacity available to meet load on any given day is dynamic and is interrelated to planned transmission limitations (congestion), unit de-rates, hydro conditions and forced outages. Furthermore, numerous generating plants within the CAISO control area are near or beyond their expected life. As plants get older, availability rates of those plants decline. The following elaborates further on the various factors that contribute to lower than maximum (actual nameplate value) capacity levels available to the ISO to meet system load at all times.

Dealing with the Dreaded Transmission Constraint on Path 15, and other issues

Due to the locational dispersion of generation resources in California, the CAISO usually transmits power from generation capacity in Southern California to Northern California to meet Northern California load under high demand situations. Path 15