2009 Scenario Reliability Assessment - NERC

Scenario Reliability Assessment Summarygeneration pose significant operational challenges due to inherent variability and uncertainty. 14The following operational issues were considered the most important:1. Minimum Generation Limits During Light Load ConditionsLarge additions of variable generation may present a challenge in managing the generationfleet output for day-ahead unit commitment. For example, system demand may fall belowthe aggregate minimum output of existing conventional steam plants. Increased cross-Regional communication and coordination would be needed to alleviate these potential overgenerationscenarios. Because renewable portfolio standards require meeting minimumenergy levels, operators must balance these requirements without affecting bulk powersystem reliability.2. Increased Ramp Requirements and Out-of-Phase RampingRamps are the increase or decrease of generation output. Wind generation ramps can have aninverse correlation (out-of-phase ramping) to daily load profiles resulting in the need foradditional reserves. Operators may need to closely monitor the system and introduceoperational resources, such as Demand Response or energy storage, that support thevariability and ancillary services needed to reliably support integration. Additionally,enhanced operational measures, in particular re-dispatch of conventional generation anddynamic curtailment/dispatch of wind resources, can mitigate ramping impacts.Photovoltaic (PV) generation ramps have different characteristics than wind generationramps. Since there are no moving parts, there is no inertia, resulting in significant rampswhen fuel (sunlight) becomes obscured by clouds. PV generation can experience variationsin output of +/- 50 percent in a 30 to 90 second time frame and +/- 70 percent in a five to tenminute timeframe. 15 These ramps in PV generation output can occur many times in a singleday due to varying weather conditions. Managing the energy contributions of solar resourceswith conventional generation represents a significant challenge for reliable integration. Theability to rely on faster-acting resources may reduce these impacts and enable large-scaleintegration.3. Accurate Day-Ahead and Hourly Wind ForecastingPredicting the output level of wind generation for a future time period (e.g., day ahead, hourahead, etc.) is vital to maintain bulk power system reliability. Because wind generation isdriven by the same physical phenomena as weather, the uncertainty of wind generation at afuture hour (even the next hour) may be significant. As wind generation penetration levelsincrease, the forecast accuracy becomes essential to operate a reliable system. Additionally,accurate wind forecasts and timely updates are necessary in order to incorporate windgeneration into the Day Ahead markets.14 http://www.nerc.com/files/IVGTF_Report_041609.pdf15 NERC Special Report: Accommodating High Levels of Variable Generation (Page 27).Page 102009 Scenario Reliability Assessment