Cost of Reliability Analysis Based on Stochastic Unit Commitment

TLDR

Traditional reliability analyses used deterministic unit commitment with predefined reserve constraints or economic dispatch, whereas this work solves an hourly stochastic unit commitment that incorporates spatial constraints, random outages, and load forecast uncertainty. The study develops a stochastic optimization model for long‑term security‑constrained unit commitment to calculate power system reliability cost while balancing operating cost and reserve adequacy. The model employs scenario trees generated by Monte Carlo simulation to represent random generator and line outages and load forecast errors, and includes loss‑of‑load expectation as a constraint to determine reserve cost. Simulations demonstrate that the model, applicable to both vertically integrated utilities and ISOs, effectively reduces reliability cost by integrating fuel and emission scheduling and constraints into the unit commitment problem.

Abstract

This paper presents a model for calculating the cost of power system reliability based on the stochastic optimization of long-term security-constrained unit commitment. Random outages of generating units and transmission lines as well as load forecasting inaccuracy are modeled as scenario trees in the Monte Carlo simulation. Unlike previous reliability analyses methods in the literature which considered the solution of an economic dispatch problem, this model solves an hourly unit commitment problem, which incorporates spatial constraints of generating units and transmission lines, random component outages, and load forecast uncertainty into the reliability problem. The classical methods considered predefined reserve constraints in the deterministic solution of unit commitment. However, this study considers possible uncertainties when calculating the optimal reserve in the unit commitment solution as a tradeoff between minimizing operating costs and satisfying power system reliability requirements. Loss-of-load-expectation (LOLE) is included as a constraint in the stochastic unit commitment for calculating the cost of supplying the reserve. The proposed model can be used by a vertically integrated utility or an ISO. In the first case, the utility considers the impact of long-term fuel and emission scheduling on power system reliability studies. In the second case, fuel and emission constraints of individual generating companies are submitted as energy constraints when solving the ISO's reliability problem. Numerical simulations indicate the effectiveness of the proposed approach for minimizing the cost of reliability in stochastic power systems.