Evaluation of Emerging Metaheuristic Strategies on Opimal Transmission Pricing
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Evaluation of Emerging Metaheuristic Strategies on Opimal Transmission Pricing José L. Rueda, Senior Member, IEEE István Erlich, Senior Member, IEEE Institute of Electrical Power Systems Institute of Electrical Power Systems University Duisburg-Essen University Duisburg-Essen Duisburg, Germany Duisburg, Germany [email protected] [email protected] Abstract--This paper provides a comparative assessment of the In practice, there are several factors that can influence the capabilities of three metaheuristic algorithms for solving the adoption of a particular scheme of transmission pricing. Thus, problem of optimal transmission pricing, whose formulation is existing literature on definition of pricing mechanisms is vast. based on principle of equivalent bilateral exchanges. Among the Particularly, the principle of Equivalent Bilateral Exchange selected algorithms are Covariance Matrix Adaptation (EBE), which was originally proposed in [5], has proven to be Evolution Strategy (CMA-ES), Linearized Biogeography-based useful for pool system by providing suitable price signals Optimization (LBBO), and a novel swarm variant of the Mean- reflecting variability in the usage rates and charges across Variance Mapping Optimization (MVMO-SM). The IEEE 30 transmission network. In [6], an optimization problem was bus system is used to perform numerical comparisons on devised based on EBE in order enable exploration of multiple convergence speed, achieved optimum solutions, and computing solutions in deciding equivalent bilateral exchanges. In the effort. 2011 competition on testing evolutionary algorithms for real- Index Terms--Equivalent bilateral exchanges, evolutionary world optimization problems (CEC11), this problem was mechanism, metaheuristics, transmission pricing. solved by different metaheuristic algorithms, most of them constituting extended or hybridized variants of genetic I. INTRODUCTION algorithm and differential evolution [7]. Optimal allocation of transmission costs is among key Results of CEC11 showed the potentials of some problems on power system operation, especially in the new metaheuristic strategies as generic optimization engines for context, where higher uncertainties and complex agent solving a variety of problems by using the same parameter interactions will be reflected in more complicated problem setting. Moreover, considerable performance differences were formulation having a high dimensional, non-linear, non- evidenced by statistical assessment of results obtained by convex, discontinuous, and multimodal landscape. These repetitive optimization with the contestant algorithms [7], features motivate the exploration of new solution strategies, thus, highlighting the need of further algorithmic for which heuristic optimization framework is an attractive developments. Along this spirit, the work presented in this alternative due to conceptual simplicity, easy adaptability, and paper concerns a performance comparison between three open architecture. Reported applications of heuristic emerging metaheuristic algorithms, namely, Covariance optimization algorithms to transmission pricing include Ant Matrix Adaptation Evolution Strategy (CMA-ES), Linearized Colony Optimization [1], and Tabu Search [2]. Biogeography-based Optimization (LBBO), and a newly developed swarm variant of the Mean-Variance Mapping Due to their stochastic nature, heuristic optimization Optimization (MVMO-SM), which are used for solving the algorithms do not strictly guarantee global optimality, EBE based formulation of optimal transmission pricing (EBE- especially for high-dimensional and complex problems, but OTP). Numerical comparisons, obtained by application on a usually provide near-to-optimal or good enough solutions in benchmark power system, include convergence speed, reasonable time [3]. Thus, their effectiveness and robustness achieved optimum solutions and computing effort. can be questioned. With the aim of achieving fast and enhanced global search capability, high level schemes, known Following this introduction, the rest of the paper is as metaheuristic algorithms, have been conceived to organized as follows: Section II introduces the formulation of subordinate and exploit strategically different heuristic EBE-OTP. The theoretical background of the compared procedures, which could include especial schemes for adaptive algorithms is briefly outlined in Section III. Numerical results parameter change, local search, reinitialization, adaptive are provided in Section IV. Finally, conclusions are population sizing and population information exchange [4]. summarized in Section 5. 978-1-4799-6415-4/14/$31.00 ©2014 IEEE II. PRICING SCHEME where Xmn is the reactance of line connecting buses m and n, The transmission pricing mechanism defined in [6] is whereas Xmi , Xni , Xmj , and Xnj are entries into the adopted in this paper. Broadly speaking, it assigns some of the reactance matrix X. total charges due transmission system usage to bilateral customers, whereas the rest is distributed through pool The set of optimization variables is defined by a vector customers. The first assignment is due to the fact that bilateral containing all GDij . Each GD accounts for an exchange transactions are usually known a-priori, so the usage rate between a generator and a load in the system, so the size of the associated to them can be determined, for instance, by vector is Ng⋅Nb. Therefore, the scale of the optimization evaluating power transfer distribution factors (PTDs) [8]. By problem increases with system size. Besides, the problem contrast, optimization is needed to properly decide equivalent possesses a multimodal search space, so a powerful bilateral exchanges such that the charges on pool customers optimization solver is needed to find the most feasible are close to those had bilateral transaction been absent. solution. The lower bound for each GDij is set to zero, A. Optimization problem statement whereas the upper bound is determined by using (6). It is assumed that an input file, containing bus and line max{GD} =−− min{ P BT , P BT } (6) specifications, fixed cost to be recovered and bilateral ij gi ij dj ij transactions data, is previously defined. Then, EBE is applied Once the best values of GDs are found, the usage rates for on the given information and the transmission charges are pool generations and pool demands can be easily calculated by evaluated at each bus, such that the transmission usage rates using simple expressions, which are functions of GDs and can i j be found in [6]. They are not reproduced here due to space for a generation at bus i (TU g ) and a demand at bus j (TU d ) constraints. are obtained. Next, equivalent bilateral transactions between pool generations and pool demands are obtained by III. METAHEURISTIC ALGORITHMS minimizing usage rate deviations due to bilateral transactions. This Section provides a brief review on background of the Mathematically, this optimization problem can be formulated employed algorithms. N and N are used to denote number as: p var of particles and number of optimization variables, Minimize respectively. The superscript g denotes iteration (generation) 22number. ⎛⎞⎛⎞FGD() FGD () OF=−+−⎜⎟⎜⎟ij TUij ij TU (1) A. CM A-ES ∑∑⎜⎟⎜⎟PP−−**gd PP ij⎝⎠⎝⎠gi gi dj dj The framework of this type of evolutionary strategy bases considering that on the assumption that the elements (optimization variables) xi of the optimization vector x follow a multivariate normal FCk FGD()= GD γk (2) distribution with mean vector m ∈ℜn , and covariance matrix ijkk∑∑ ij ij ⋅γ + ⋅γ nn× ∑∑GDij ij ∑∑ BT ij ij jk C∈ℜ . Additionally, a step size σ is used to perturb C [9]. ij ij The underlying iterative procedure is summarized as follows: and subject to • Step 1: Set N , the initial covariance matrix C=I, draw =−* ∀∈ p ∑GDij P gi P gi i Ng (3) the elements of m from a random uniform distribution i within the problem space, set initial σ to 30% of the =−* ∀∈ ∑GDij P dj P dj j Nd (4) constrained region in search space and the evolution j paths pc and pσ to zero. Set the number of best parents where Ng and Nb are the number of generator and load buses, μ to Np/2. Additional strategy parameters to be k ω μ μ respectively. FC stands for fixed cost of a line k that needs initialized are i (i=1… ), eff, cσ, dσ, cc, c1, and cμ. to be recovered, BT denotes bilateral transaction between Default values for these parameters can be found in ij [9]. generator at bus i and demand at bus j, Pgi is total generation • Step 2: Sample a new population of N particles at bus i, P* is the sum of generations due to all bilateral p gi according to: * gggg+1 transactions at bus i, Pdj is the total demand at bus j, Pdj is the =+σ = xmk N (0, C ) k 1K Np (7) sum of demands due to all bilateral transactions at bus j, and • GD is the equivalent bilateral transaction that need to be Step 3: Evaluate all particles x, rank them in ij ascending order according to their fitness, and update γk evaluated. ij denotes the sensitivity of a line k connecting m by (8), which computes the weighted intermediate buses m and n for a transaction between buses i and j, and is recombination of the best μ parents. given by (5). μμ mxgg++11=ω ω=ωω ω XXXX−− − ∑∑ii, where i 1, 12> >K >μ >0 (8) γ=k mi ni mj nj ii==11 ij (5) Xmn Each ωi is decreased linearly throughout iterations. • Step 4: Update pc by using (9). gg+1 gg+1 gg + mm− xx=+μ−() xx (15) ppgg1 =−()12ccc +cc() − μ (9) ij,, ij i k,, j ij cccσg cceff • Step 4: The new habitats are evaluated and the worst are μeff is inversely proportional to the sum of squared ωi. replaced with the elites of previous generation. Step 5: Update C according to • Step 5: Perform local, boundary, global grid, or Latin μ + TTHypercube search if there is no improvement of fitness in CCppyyg11111=1--()cc g + c g+() g+ + c ω g+ () g+ (10) 1 μ 1c c μ ∑ ii i two consecutive generations according to the predefined i=1 thresholds in Step 1. gggg++11=+ σ yxmii()/ (11) • Step 6: Re-initialization is performed, that is, Np new • Step 6: Update σ according to (12) and (13).