EFFECTS OF RAMP-RATE LIMITS ON UNIT COMMITMENT AND ECONOMIC-DISPATCH

This paper proposes an algorithm to consider the ramp characteristics in starting up and shutting down the generating units as well as increasing and decreasing power generation. In power system generation scheduling, a number of studies have focused upon the economical aspects of the problem under the assumptions that the changes in the generating capacity follow a step function characteristic and unit generation can be adjusted instantaneously. Even though these hypotheses greatly simplify the problem, they do not reflect the actual operating processes of generating units. The use of ramp-rate constraints to simulate the unit state and generation changes is an effective and acceptable approach in the view of theoretical developments of industrial processes. Since implementing ramp-rate constraints is a dynamic process, dynamic programming (DP) is a proper tool to treat this problem. In order to overcome the computational expense which is the main drawback of DP, this study initially employs artificial intelligence techniques to produce a unit commitment schedule which satisfies all system and unit operation constraints except unit ramp-rate limits. Then, a dynamic procedure is used to consider the ramp properties as units are started up and shut down. According to this adjustment maximum generating capabilities of units will change with the unit operation status instead of following a step function. Finally, a dynamic dispatch procedure is adopted to obtain a suitable power allocation which incorporates the unit generating capability information given by unit commitment and unit ramping constraints, as well as the economical considerations. Two examples are presented to demonstrate the efficiency of the method.