IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 23, NO. 1, FEBRUARY 2008 127 GENCO’s Risk-Based Maintenance Outage Scheduling Lei Wu, Member, IEEE, Mohammad Shahidehpour, Fellow, IEEE, and Tao Li, Member, IEEE Abstract—This paper presents a stochastic model for the op- timal risk-based generation maintenance outage scheduling based on hourly price-based unit commitment in a generation company (GENCO). Such maintenance outage schedules will be submitted by GENCOs to the ISO for approval before implementation. The objective of a GENCO is to consider financial risks when scheduling its midterm maintenance outages. The GENCO also coordinates its proposed outage scheduling with short-term unit commitment for maximizing payoffs. The proposed model is a stochastic mixed integer linear program in which random hourly prices of energy, ancillary services, and fuel are modeled as sce- narios in the Monte Carlo method. Financial risks associated with price uncertainty are considered by applying expected downside risks which are incorporated explicitly as constraints. This paper shows that GENCOs could decrease financial risks by adjusting expected payoffs. Illustrative examples show the calculation of GENCO’s midterm generation maintenance schedule, risk level, hourly unit commitment, and hourly dispatch for bidding into energy and ancillary services markets. Index Terms—Monte Carlo simulation, optimal generation maintenance outage scheduling, price-based unit commitment, risk-based GENCOs, stochastic mixed integer programming. NOMENCLATURE Variables: Fuel consumption of a unit. A group of units that share an emission allowance constraint. A group of units that share a fuel allocation constraint. Thermal unit. Unit status indicator (1/0). Indicator for providing non-spinning reserve when a unit is off. Non-spinning reserve offered by a unit when on. Non-spinning reserve offered by a unit when off. Generation of a unit. Manuscript received February 27, 2007; revised May 30, 2007. This work was supported in part by the National Science Foundation under grant # ECCS- 0725666. Paper no. TPWRS-00150-2007. The authors are with the Electrical and Computer Engineering Department, Illinois Institute of Technology, Chicago, IL 60616 USA (e-mail: lwu10@iit. edu; ms@iit.edu; litao@iit.edu). Digital Object Identifier 10.1109/TPWRS.2007.913295 Generation for segment in linearized fuel cost curve. Total payoff for a scenario. Spinning reserve of a unit. Scenario. Hour. Total generation offered to (positive value) or purchased from (negative value) a market by a unit. Total non-spinning reserve offered by a unit. Indicates whether a unit is started at segment of startup cost curve (1/0). Indicates whether a unit is started at segment of startup emission curve (1/0). Unit maintenance indicator (1/0) with 0 means on maintenance. Shutdown indicator. Constants: Slope of segment in linearized emission curve. Slope of segment in linearized heat curve. Heat quantity at the minimum generating capacity. Maintenance duration of a unit. Maximum sustained ramp rate (MW/min) for a unit. Number of units of a GENCO. Maximum number of units that can be simultaneously on maintenance. Number of times under study. Number of segments for the piece-wise linearized emission curve. Number of segments for the piece-wise linearized heat curve. Number of segments for the startup fuel curve. 0885-8950/$25.00 © 2007 IEEE