Declaring Emergencies in California: The Realities of ISO Operation
is used to transmit power to the north or south on a daily basis; however congestion usually occurs when Northern California's load is high and generation and imports from the north are not sufficient to cover the demand. Under these circumstances, there may be available and dependable generation capacity in the south that cannot assist in averting firm load curtailments in the north. For example, on Jan. 17 and 18, 2001, excessive Path 15 flows were a significant factor for firm load to be shed on those days. From April 1, 1998 to Jan. 15, 2001, there were 228 instances where Path 15 was constrained in the south-to-north direction. Path 15 7 limitations could limit additional power to be transmitted into Northern California (where there continues to be more resource deficient).
Moreover, the generation from the hydroelectric resources in California and in the Northwest varies on a daily basis due to the following factors: snow pack conditions, water availability, run of the river issues, contractual requirements, and unit outages. In addition to capacity limitations, hydroelectric resources are energy limited. Only a finite amount of water storage (energy) is available during any given day. Furthermore, during any peak summer hour, the simultaneous peak capacity of a portfolio of hydro resources on a cascading watershed is simply not equal to the sum of the peak output of each resource of the watershed. These resources are and will continue to be limited due to low water levels and related environmental restrictions. Hydroelectric limitations are another component that is hard to accurately quantify.
For the examples, limitations were estimated by first subtracting the hydro load measured for the peak period of each date from the total maximum hydro (11,801 MW) as presented in the CAISO 2001 Summer Assessment. Then, since the known hydro load considers hydro outages, any planned and unplanned outages for hydro facilities were subtracted out so not to double count the limitations. The CAISO was faced with situations where actual hydroelectric facility resources operated as low as 43 percent of normal available capacity. This translates into reductions ranging up to 6,800 MW that would normally be available to meet system demand. Hydroelectric limitations of this magnitude increase reliance (24 hours per day) on thermal resources, which further contributes to the increased number of unplanned outages of the thermal generation fleet in the California system.
Like the other internal generation components, these hydro limitations must be subtracted from the maximum installed internal generation.
Power Plant Outages: How to explain the Increase in Frequency
Outages are one of two types: planned and unplanned (forced). Historically, the CAISO experienced approximately 2,500 MW in unplanned outages, with fewer outages in the summer due to deliberate efforts to prepare generation for the peak summer period. The number of forced outages has significantly increased since the summer of 2000 due to (a) mechanical failures caused by high usage rates on aging generation units in California, and (b) environmental constraints 8 including emissions limits. In general, these aging plants that have been run harder than ever before have suffered from