2009 Scenario Reliability Assessment - NERC

Scenario Reliability Self-Assessmentsmore contingency reserves be available in the day-ahead market. The variability and rampingcharacteristics of wind turbine output would also require additional spinning and contingencyreserve margins, both positive and negative, the precise amount of which changes day to daydepending on the quality of Regional wind prevalence. These factors would dictate that dayaheadAncillary Service Market (ASM) must be closely coordinated with wind forecasts andreal-time monitoring of wind output.(d) Increased Risk of Baseload Unit RetirementsBaseload units reduce market price volatility and provide reliable operational reserve margins,precisely the characteristics needed in an environment striving to accommodate large-scaledwind power integration. Many of these facilities are located where their dynamic reactivecapabilities are also crucial to voltage regulation and local system reliability. However, whenwind penetration reaches a level where many of these older and less efficient baseload facilitiesbecome sporadically dispatched day-in and day-out, there is the danger of their owners retiringthese facilities.(e) Lower System InertiaMore wind generation is being added which tends to lower the system inertia. During light loadperiods baseload units maybe displaced with wind generation, which contributes no inertia. Thelack of system inertia has a negative impact on system stability, especially in the MRO footprintwhere transmission distances are typically long.(f) Ambient Temperature Operating LimitsThe standard thermal limitation on a wind generator is -20 degrees Celsius (-4 degreesFahrenheit). Many wind generation owners in the MRO Region purchase the “cold weatherpackage” to increase the thermal limit to -30 degrees Celsius (-22 degrees Fahrenheit). Thearea in MRO with abundant wind is concentrated in a corridor where extreme cold weather maylikely occur simultaneously, affecting all facilities in the area. If such extreme weather was notaccurately forecasted day-ahead, automatic shutdown of a large amount of wind output in a shorttime span could occur.(g) Impacts on Protective RelayingWind facilities contribute minimal fault current in the event of a system disturbance. The lack ofadequate fault current may in turn jeopardize the applicability of many local system protectionschemes. This issue has not been dealt with in the industry enough for protection engineering todevelop guidelines for relaying scheme changes. To date, experiences in the MRO footprinthave not raised significant concern, as real-life experiences in the Buffalo Ridge area withground faults since 2003 have indicated zero sequence current sources and short-circuit currentcontributions from the transmission system have been adequate in clearing faults within normaltime. However, some perplexing misoperations of some digital relays had been observed, theroot cause of which have not been totally understood. It is imperative, therefore, that the issue ofshort-circuit current be rigorously studied and understood.(h) Accurate Day-Ahead and Hourly Wind ForecastingAn aspect of wind generation is the limited ability to predict with reasonable confidence what theoutput level would be at some time in the future. Conventional plants also cannot be counted on2009 Scenario Reliability Assessment Page 49