2009 Scenario Reliability Assessment - NERC

Scenario Reliability Self-AssessmentsLow-load/low-variable generation poses the risk that variable generation would increase in theabsence of other generation that can ramp down. This situation happens during off-peak periodsand is worsened when it (frequently) coincides with periods of high hydro generation, whichlimits the flexibility of the hydro generators. Although there is not a significant resourceadequacy risk, there are significant impacts to system operations and to what generation must beon-line. The increase in variable generation would impact what generation must be madeavailable during this time, such as making combustion turbines run during off-peak to providethe necessary ramping capacity. In addition, the curtailment of the variable generation duringthese times would likely increase.The low-load/high-variable generation poses the risk that variable generation would drop offduring off-peak when much of the flexible generation is off-line. This is the least risk toreliability and operations since there are often other generation resources available to ramp up.However, with an increase in the level of variable generation, situations may arise where thelevel of variable and must-run generation exceeds load. As with the previous case, this situationoften coincides with periods of high hydro generation.WECC expects that a number of additional standards, guidelines, and rules would emerge fromstudies of the increase in variable generation and its impact on operations and reliability.Specifically, these would directly address the need to better forecast and control the fleet ofvariable generators in the Western Interconnection. In addition, changes to business practices andtechnologies that would enable higher levels of variable generation are anticipated. Theseinclude sub-hourly scheduling of energy and transmission, increased dynamic schedulingcapability, and the expansion of ancillary service markets.Currently, some entities in the Western Interconnection are making changes to their LargeGenerator Interconnection Agreements to address the controllability of large wind generatingstations. These include provisions that range from forecasting requirements to enabling the BA todirectly control output of the generator. 108A large number of variable generation (mostly wind) integration studies have been performed byentities in the Western Interconnection. Although the focus and intent of the studies are notidentical, the primary objective is the same quantify the impact of variable generation onoperations and costs.To facilitate a greater understanding and address of the effects of variable generation, WECC’sJoint Guidance Committee created the Variable Generation Subcommittee (VGS) in October2008. The subcommittee is made up of a broad set of stakeholders in the WesternInterconnection. The purpose of the VGS is to provide a holistic perspective of the issues andopportunities related to the presence of variable generation in the Western Interconnection. Italso serves to add value for WECC members by facilitating the development and implementationof solutions and by assuring reliability in the Western Interconnection. These challenges arebeing met through the compilation of information and member issues, coordination of issueanalysis, and dissemination of information. As a recently formed subcommittee, the VGS is108 http://www.bpa.gov/corporate/WindPower/WIT.cfm,http://www.caiso.com/docs/2003/01/29/2003012914230517586.htmlPage 1562009 Scenario Reliability Assessment