Power system scheduling methodologies and their applications

Date of Completion

January 1996


Engineering, Electronics and Electrical




Scheduling of power system and determining inter-utility power transactions are important problems faced by electric utilities. Scheduling is the determination of the commitment and dispatch of generating units, while transactions involve the purchase or sale of power between utilities. These two problems are actually coupled through the system demand and reserve requirement, and should be considered as an integrated problem. The problem is to determine the commitment and generation levels of all generating units, and the levels and durations of transactions over a specified period of time with the objective to supply economic power. Each unit or transaction may also have a variety of individual constraints such as minimum up/down times and ramp rate constraints for a thermal unit. This problem involved both discrete variables (up/down of units, take/not-take of transactions), and continuous variables (generation level of up units, purchase levels of purchase transactions), and belongs to the class of NP-hard mixed-integer programming problems which are extremely difficult to solve. However, potential cost savings are quite significant.^ There are two major parts of the research. The first one is to develop a method for scheduling hydrothermal power systems based on the Lagrangian relaxation technique. By using Lagrange multipliers to relax system-wide demand and reserve requirements, the problem is decomposed and converted into a two-level optimization problem. Given the sets of Lagrange multipliers, a hydro unit subproblem is solved by a merit order allocation method, and a thermal unit subproblem is solved by using dynamic programming without discretizing generation levels. A subgradient algorithm is used to update the Lagrange multipliers. Numerical results based on Northeast Utilities system data show that this algorithm is efficient, and that near-optimal solutions are obtained.^ The second one investigates uncertainties in the integrated problem of power system scheduling and inter-utility transactions, since the effects of these uncertainties can propagate through the time horizon, significantly affecting the economics of schedules and transactions. With deregulation in the utility industry and increasing competition in the electricity market, these uncertainties need to be properly managed. In this research, system demand, reserve requirements and prices of future purchase transactions are considered as uncertain, and the integrated scheduling and transaction problem is formulated as a fuzzy mixed integer programming problem for a power system consisting of thermal units and purchase transactions. Based on the symmetric approach of fuzzy optimization and the Lagrangian relaxation technique, a fuzzy optimization-based algorithm is developed. Test results using fuzzy simulation show that the method produces robust scheduling and transaction decisions to hedge against uncertainties. ^