Research Article  Open Access
A. M. Elaiw, X. Xia, A. M. Shehata, "Minimization of Fuel Costs and Gaseous Emissions of Electric Power Generation by Model Predictive Control", Mathematical Problems in Engineering, vol. 2013, Article ID 906958, 15 pages, 2013. https://doi.org/10.1155/2013/906958
Minimization of Fuel Costs and Gaseous Emissions of Electric Power Generation by Model Predictive Control
Abstract
The purpose of this paper is to present a model predictive control (MPC) approach for the periodic implementation of the optimal solutions of two optimal dynamic dispatch problems with emission and transmission line losses. The first problem is the dynamic economic emission dispatch (DEED) which is a multiobjective optimization problem which minimizes both fuel cost and pollutants emission simultaneously under a set of constraints. The second one is the profitbased dynamic economic emission dispatch (PBDEED) which is also a multiobjective optimization problem which maximizes the profit and minimizes the emission simultaneously under a set of constraints. Both the demand and energy price are assumed to be periodic and the total transmission loss is assumed to be a quadratic function of the generator power outputs. We assume that there are certain disturbances or uncertainties in the execution of the optimal controller and in the forecasted demand. The convergence and robustness of the MPC algorithm are demonstrated through the application of MPC to the DEED and PBDEED problems with fiveunit and sixunit test systems, respectively.
1. Introduction
Reducing fuel cost in electricpowergenerating plants has received considerable attention in recent years. Dynamic economic dispatch (DED) is a realtime power system problem which is used to schedule the committed electricpowergenerating units’ outputs so as to meet the load demand over a dispatch period at minimum operating cost while satisfying ramp rate constraints and other constraints (see, e.g., [1–11]). Since the ramp rate constraints couple the time intervals, the DED problem is a difficult optimization problem. Since the DED problem was introduced, several optimization techniques and procedures have been used for solving the DED problem with complex objective functions or constraints (see the review paper [1]). After deregulation of the electric power system, the objective of the dynamic dispatch problem has been changed from cost minimization to profit maximization and the problem is known as profitbased dynamic economic dispatch (PBDED) problem [11].
The emission of gaseous pollutants including SO_{2}, , CO, and CO_{2} from fossilfuelfired thermal plants affects the human health directly or indirectly. Therefore, electric utilities and generation companies (GENCOs) are requested to reduce emission from their plants. As a result of public awareness of environmental protection, diverse compliance strategies have emerged. These strategies include installation of pollutant cleaning, switching to lowemission fuels, replacement of the aged fuel burners with cleaner ones, and emission dispatching [12]. The last strategy is usually preferred to the existing systems because it is easy to implement and requires less additional cost.
The emission can be taken into the DED problem by minimizing both fuel cost and emission simultaneously under load demand constraint, ramp rate constraint, and other constraints, resulting in a multiobjective optimization problem. This problem is referred to as dynamic economic emission dispatch (DEED) [13–15]. The emission can also be considered into the PBDED problem by formulating the profitbased dynamic economic emission dispatch (PBDEED) problem which is also a multiobjective optimization problem which maximizes the profit and minimizes the emission simultaneously under ramp rate constraint and other constraints.
Recently, many optimization techniques have been used to solve the DEED problem with complex objective functions or constraints (see, e.g., [13–18]). In [16], by assuming that the decision maker has goals for each of the two objective functions, the multiobjective optimization problem is transformed into a singleobjective optimization by the goalattainment method and is solved by particle swarm optimization (PSO) method. In [17], it was assumed that the decision maker has a fuzzy goal for each of the objective functions. The optimal noninferior generation schedule is determined by the evaluationprogramming (EP) based fuzzy satisfying method. In [13], the multiobjective optimization problem is solved by nondominated sorting genetic algorithmII. In [18], the multiobjective optimization problem is converted into a singleobjective optimization and is solved by an improved patternsearch (PS) based algorithm. Pandit et al. [14] proposed an improved bacterial foraging algorithm for solving the DEED problem. Group search optimizer with multiple producers has been used to solve the DEED problem in [15].
In [13–18], the main attention focuses on finding the optimal dispatch over a fixed time horizon and the periodic implementation of such an optimal dispatch solution has been though regarded as straightforward and thus left to the practitioners. This periodicity assumption comes from the fact that the demand is periodic due to cyclic consumption behavior and seasonal changes [2]. From a control theoretical point of view, both the DEED and PBDEED formulations provide only openloop optimal solutions to the generation dispatch problem; that is, the optimal solutions are predetermined before actual execution, and there is no measurement on the system states which is fed back to the optimization model. Therefore, a closedloop control by the MPC method is introduced in this paper so that the measurement of states can be fed back to the optimization model, and the optimal solution is updated according to the feedback information at each time step. The MPC method has been successfully applied in power systems in [2, 19–22]. MPC has many advantages including its facility of handling constraints, being able to use simple models, and its closedloop stability and inherent robustness. Moreover, MPC solves the optimal control problem online for the current state of the plant which is a mathematical programming problem and is much simpler than determining the feedback solution by dynamic programming [2].
MPC has been proposed for the periodic implementation of the optimal solutions of the DEED and PBDEED problems in [20]. However, the transmission line losses were not included in the DEED and PBDEED formulations. The system loss is a very essential factor to be considered in the power system analysis. Particularly, while the DEED and PBDEED are concerned, the system loss will impose a great impact on optimal pattern of the real power generations at different power plants. In the present paper we propose an MPC approach for the periodic implementation of the optimal solutions of the DEED and PBDEED problems taking into account the transmission losses. The transmission losses will be expressed as a quadratic function of the generator’s power outputs.
The remainder of this paper is organized as follows. In Section 2, we introduce the DEED and PBDEED problems under regulated and deregulated power systems. In Section 3, we outline an MPC approach for the DEED problem and summarize the main results obtained in [2]. The simulation results for the application of MPC to the DEED and PBDEED problems are given in Section 4. The last section is the conclusion. Throughout the paper, the following notations and definitions will be used. For a sampling period , the dynamic dispatch problem is considered over dispatch intervals, , where the optimization is considered, for and is the dispatch period. For simplicity, we make the convention throughout the paper that denotes the time interval . For any , define , and . Define and . The total fuel cost and emission from all units and over the dispatch period are denoted by and , respectively. The demand and the energy price are assumed to be periodic with period .
2. Optimal Dynamic Dispatch with Emission
In this section we formulate both DEED and PBDEED problems. Under regulated power systems utilities have the objective to minimize the fuel cost and are obliged to serve all customers and meet all demands. After deregulation and restructuring of the electric power systems, GENCO has the aim to maximize its own profit. In this case, GENCO is not obliged to meet all demands but may sell its energy at less than the system’s forecasted demand equilibrium if this will maximize its profit.
2.1. Dynamic Economic Emission Dispatch
It is well known that the fuel cost and the amount of emission conflict with each other. Minimization of fuel cost maximizes the amount of emission and vice versa. Therefore it is necessary to find out an operating point that strikes a balance between fuel cost and emission. This can be done by formulating the DEED problem which is a multiobjective optimization problem with two conflicting objectives, the fuel cost and emission.
The following objectives and constraints are taken into account in the formulation of the DEED problem.
Cost. The fuel cost curves are assumed to be a quadratic function of the generator’s active power output. Therefore, the total fuel cost over the dispatch period is given by [15]
Emission. The amount of emission of SO_{2}, CO_{2}, and can be expressed as a quadratic function of the generator’s active power output. The total pollutants emission over the dispatch period is given by [23]
Constraints. (i) Power balance constraint where is the transmission line loss at time . The Bcoefficient method is one of the most commonly used method by the power utility industry to calculate the network losses. In this method the network losses are expressed as a quadratic function of the generator’s power outputs that can be approximated in the following [13]: (ii) Generation limits (iii) Generating unit ramp rate limits
The Optimization Problem. Aggregating the objectives and constraints, the DEED problem can be mathematically formulated as a nonlinear constrained multiobjective optimization problem which can be converted into a singleobjective optimization using the weighting method as follows where the feasible domain is defined to be the set of satisfying constraints (3)–(7); is a weighting factor, . It will be noted that, when , the problem (8)(9) determines the optimal amount of the generated power by minimizing the cost regardless of emission and the DEED problem leads to the DED problem [2]. If then, the DEED problem determines the optimal amount of the generated power by minimizing the emission regardless of cost and the DEED problem leads to the pure dynamic emission dispatch (PDED) [13].
The constraints (3)–(6) are usually used in the conventional DEED problem [13, 16, 17]. Since the demand and constraints are periodic, one may obtain the solution of the conventional DEED problem (i.e., problem (8) under constraints (3)–(6)) over, for example, 24 hours ( and ), and then, this solution is implemented not only for the first day but also for all the other week days. Sometimes such an optimal solution is not able to be practically implemented, or, in other words, the solution is not practically feasible. The ramp rate constraint may be violated when the generators are moved from the th hour of a day to the first hour of the next day. This problem can be resolved by including the ramp limit on the difference between and . This can be achieved by adding the constraint (7) to the conventional DEED problem (see [2, 20]).
We note that the above DEED problem can be solved over the dispatch period for any and it can be formulated as We define the following set of parameters: Since the demand is periodic and do not change over time, then , and satisfies and then is shift invariant (see [21]). The shiftinvariant property of is needed for the application of the MPC approach to the DEED problem.
2.2. ProfitBased Dynamic Economic Emission Dispatch
Now we introduce the PBDEED formulation with the aim to produce electricity with minimum operating cost and sell it with maximum profits and environmental protection by limiting the emission of greenhouse gases into the atmosphere.
The total revenue of the GENCO over the dispatch period is given by [11] where is the forecasted energy price at time .
The total profit over the dispatch period is given by [11, 20] Let us define a function which measures the profit attained by the conversion of the energy available in fossil fuels into electric energy.
The objective of the PBDEED is to simultaneously minimize the emission and maximize the profit satisfying a set of constraints. The PBDEED can be mathematically stated as follows: where the feasible domain is defined to be the set of satisfying the following constraints: It will be noted that, when , the problem (15) determines the optimal amount of the generated power by maximizing the profit regardless of emission and the PBDEED problem leads to the PBDED problem [11]. If , then the PBDEED problem determines the optimal amount of the generated power by minimizing the emission regardless of profit [24]. Of course this case is not useful for GENCOs.
Constraint (16) means that under the deregulated environment GENCO is not obliged to meet all demands and energy lost due to transmission lines but may sell its energy at less than the system’s forecasted demand plus power loss. Similar to Section 2.1, one can show that the set is shift invariant.
In the next section we propose an MPC approach for the periodic implementation of the optimal solutions of the DEED problem. MPC obtains a feedback control which is constructed by solving a finite horizon optimal control problem at each sampling instant using the current state of the plant as the initial state for the optimization and applying only “the first part” of the optimal control [25].
3. MPC Approach to DEED
In this section, we introduce an MPC approach proposed in [2, 20] for the optimal dynamic dispatch problem with emission and transmission line losses. We first show the application of MPC to the DEED problem and then show that this MPC version can also be applied to the PBDEED problem. We introduce the control variables as [8, 9] where is the ramping action of unit at time . This equation actually defines coordinate transformation between the variables and the variables . It is obvious that the inverse coordinate transformation is given by The optimal solution of the DEED problem is implemented repeatedly at instants which equal to multiples of . To introduce the MPC approach, let us consider the DEED problem starting at an arbitrary instant and over a dispatch interval . Then the optimization variables are changed into . By the transformation defined in (21), the optimization variables are transformed into .
In the MPC method, a finitehorizon optimal control problem is repeatedly solved and the input is applied to the system based on the obtained optimal openloop control. In our case, the horizon is chosen to be . Instead of solving the DEED problem with number of variables , the MPC algorithm solves the following problem which has only number of variables .
Problem . Given , and , let and solves the following minimization problem: where the feasible domain is defined to be the set of satisfying The notation denotes that the optimization problem is solved over the interval with variables and for known inputs , .
In order to make the MPCDEED problem solvable, the following hypothesis is needed as in [2, 5, 8].
Feasibility Hypothesis. After the change of variables in (22) over any dispatch interval with , the set is not empty. This hypothesis ensures the solvability of the problem . Denote the optimal solution of MPCDEED for a given initial generation by . In the MPC method, the optimal solution is applied only in the first sampling period ; that is, is applied to the state . Since depends on the current state , in this way a feedback control can be obtained. We define the MPC feedback controller by .The closedloop solution given by is actually executed over the time period . The idea of the MPC can be formulated into the following MPC algorithm.
MPC Algorithm Initialization. Input the initial status and let .(1) Compute the openloop optimal solution to the problem .(2) The (closedloop) MPC controller is applied to the plant in the sampling interval to obtain the closedloop MPC solution over the period .(3) Let and go to step (1).
Theorem 1 (see [2]). Suppose the Feasibility Hypothesis holds, is the globally optimal solution of the DEED problem, and the initial power output at time satisfies , then the MPC algorithm converges to .
This theorem tells that the solutions of the MPC algorithm converge to the optimal solution of the DEED problem. Now we consider the inherent robustness properties of the MPC algorithm. The uncertainties in energy demand, price, and reserve demand for the PBDED problem are discussed by fuzzy optimization in [11]. However, no theoretical result is given. First, we suppose that the disturbances happen in the execution of the controller. That is, the disturbance happens only in step (2) of the MPC algorithm so that when the control is applied to the plant in the sampling interval , the system actually executes over the period , where is the disturbance. We assume that the disturbances satisfy the following bound:
Theorem 2 (see [2]). Suppose the Feasibility Hypothesis holds, is the globally optimal solution of the DEED problem, the norm of the gradient of the function of DEED problem has the upper bound on , is a small enough positive constant, is a positive constant which is less than , (26) is executed in step (2) of the MPC algorithm instead of (25), and the constant disturbance satisfies (27) where is small enough so that , then there exists an integer such that for any , the optimal MPC solution of the th loop in the MPC algorithm belongs to the domain .
Theorem 2 shows that the MPC algorithm is robust against certain disturbances in the execution of the optimal controller. It may happen that there is disturbance or uncertainty in the forecasted demand; that is, the demand is disturbed so that the actual demand is . It has been shown in [2] that the demand disturbances or uncertainties can also be written in the form of (26) and the result of Theorem 2 is applicable when is small enough.
Theorems 1 and 2 are based on the assumption that the objective function of the DEED problem is strictly convex and differentiable over the set which is bounded. Since both the fuel cost and emission functions are assumed to be quadratic, then all the objective functions of the DEED and PBDEED problems are strictly convex and differentiable over their feasible constraint sets. Also since the demand and energy price and all constraints are assumed to be periodic, then all feasible constraint sets, and , are shift invariant. Therefore, Theorems 1 and 2 are valid for both the DEED and PBDEED problems.
4. Simulation Results
In this section we present two test systems consisting of five units and six units. The first system is used to show that the MPC algorithm converges to the optimal solutions of the DEED problem. The inherent robustness properties of the model predictive control (IRPMPC) algorithm are also shown. In the second test system, we show the converge and robustness properties of the proposed MPC algorithm to the PBDEED problem. The load demand and energy price are assumed to be periodic over a dispatch period of one day and the sampling period is chosen to be one hour. The optimization problem is solved by sequential quadratic programming. All computations were carried out by the MATLAB program. In particular, the optimal control sequence is computed by the fmincon code of the MATLAB Optimization Toolbox.
4.1. FiveUnit Test System
This example presents an application of MPC to the DEED problem consisting of five units. The technical data of the units are taken from [16]. First, we present the optimal solutions of the DEED problems for different values of the weighting factor and then show that the solutions of the MPC converge to the optimal solutions of the DEED problem. The optimal solutions of the DEED problem with different values of are given in Tables 1 and 2. Table 1 shows the hourly power schedule and transmission loss obtained from the DEED problem with and . Table 2 shows the hourly power schedule and transmission loss obtained from the DEED problem with and . The optimal cost and emission obtained from the DEED problem with different values of are given in Table 3. It is seen from Table 3 that the cost is $40,121 under DEED (), but it increases to $40,851 under DEED () and emission obtained from DEED () is 20,363 lb but decreases to 16,546 lb under DEED (). Figure 1 shows the effect of the transmission loss on the outputs of the power units. We can see that neglecting transmission loss would often introduce inaccuracy into the resulting dispatch.
