Mathematical Programming for Power Systems Operation: From Theory to Applications in Python by Alejandro Garcés

Author:Alejandro Garcés [Garcés, Alejandro]
Language: eng
Format: epub
ISBN: 9781119747284
Published: 2021-11-15T15:01:05+00:00

8

Unit commitment

Learning outcomes

By the end of this chapter, the student will be able to:

Identify the difference between the economic dispatch and the unit commitment

Solve basic problems of deterministic unit commitment

Include transmission constraints into the model by a linear power flow formulation.

8.1 Problem definition

The economic dispatch presented in Chapter 7 is a continuous problem decoupled in time; the latter means that current decisions do not affect future operation. However, this picture is incomplete since a power system presents a dynamic behavior due to loads’ daily changes. Therefore, a dynamical model is required.

The starting-up of a thermal power plant is not instantaneous since the boiler requires suitable pressure and temperature conditions to generate power, as shown in Figure 8.1. Shutting-down is not instantaneous either. Besides, several physical and economic limitations such as the minimum operative time and the maximum off-line time must be considered in the model. All these constraints introduce binary variables into the optimization problem. Thus, the unit starts generating power when the temperature is between Tmin and Tnom; below Tmin the unit requires fuel to maintain the temperature, but the output power is zero. This implies costs that must be included in an optimization model named unit commitment. This problem is discrete since the time in which the unit is connected (committed) or disconnected from the grid (de-committed) is part of the decision; hence the problem dynamic. Additional constraints are also included in the model, related to the ramps of start-up and start-down in each thermal unit. In the following sections, we present the basic model with the most common constraints. As in all the previous chapters, we offer only toy-models in order to understand the problem.

Figure 8.1 Simplified model for the starting-up of a thermal power plant.

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