ASSESSING THE IMPACT OF FLEXIBLE RAMP CAPABILITY PRODUCTS IN THE MIDCONTINENT ISO

Abstract

In electric power systems, balancing authorities adjust the output of dispatchable coal and natural gas generators in response to changes in net load (electricity demand minus variable generation such as wind). As penetration of renewable energy increases, so do the variability and uncertainty surrounding net load, making balancing more difficult. The flexibility of the system to ramp power output up and down (i.e. ramping capability) may be insufficient to accommodate large changes in net load, potentially leading to scarcity events and threatening system security. The Midcontinent Independent System Operator (MISO) has proposed ancillary service products called up-ramp capability (URC) and down-ramp capability (DRC) intended to increase system flexibility. The purpose of this study is to explore the economic, environmental, and reliability impacts of MISO’s proposed ramp capability products. Two versions of the unit commitment and economic dispatch processes used by MISO to optimally schedule generators were modeled: (1) a baseline model representing current MISO practices, and (2) a ramp capability model that includes the proposed products. These models were applied to a small power system representative of MISO’s mix of generators under low and high wind penetration levels. In this model the DRC product had no impact, indicating that the representative power system was more flexible in the downward direction than MISO’s actual system, perhaps due to model simplifications or inaccurate assumptions. The URC product, however, did benefit the system. Results show a reduction in the frequency of energy and operating reserve shortages when compared to the baseline model, particularly with high wind penetration, thereby indicating improved reliability. While there was a small price increase in non-shortage intervals due to procurement of URC, this was outweighed by the avoidance of high penalty prices incurred in shortage intervals; the overall average market clearing price was significantly reduced. The URC product also caused a small amount of fuel switching from coal to the more flexible natural gas, slightly reducing the system’s CO2 emissions. However, the more pronounced environmental benefit was the URC product’s ability to help the system absorb increased wind penetration while avoiding most of the corresponding increase in reliability problems.