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2.Energy Saving Technologies in transmission, distribution of electrical energy Energy Cost and Power Loss Minimization in Distribution Networks with Distributed Generation

Jordan Radosavljević, Nebojša Arsić

Faculty of Technical Sciences, University of Priština in Kosovska Mitrovica, Kneza Miloša 7, 38220 Kosovska Mitrovica, Serbia.

Introduction

Electricity distribution companies tend to integrate different types of distributed generation (DG) units in the distribution network [1]. Integration of DG units with renewable energy such as photovoltaic (pv) systems, small wind turbines (wts) and small hydro power plants is influenced by geographical and meteorological conditions. The DG units with non renewable energy, including fuel cells (fcs), micro turbines (mts) and diesel generators (dgs), can be connected to any point in the distribution network but fuel cost must be taken into account.

Depending on the DG units technologies, the primary energy of DG units may be injected to the distribution network through either a synchronous or asynchronous electric machine which is directly connected to the grid, a combination of an electric machine and a power electronic interface, or only through power electronic interface. If electric machine is directly connected to the grid, its operation determines the model of DG unit (PQ or PV node) for power flow studies. In other cases, characteristics of the interface control circuit determine the DG unit model. As a general rule, in case the control circuit of the of the converter is designed to control active power and voltage independently, the DG unit model shall be as a PV node and being designed to control active and reactive power independently, the DG unit model shall be as a PQ node [2].

Generally, DG units, if they are properly planned and controlled, may offer the improved voltage profile and power losses reduction of the distribution network, better economics and a reduced dependence on the local utility [3]. Benefits can be achieved only by efficiently coordination of the DG units operation, voltage regulation (voltage regulators, ULTC) and reactive power compensation (VAR compensators) in the distribution network. In fact, it is an optimal power flow (OPF) problem. Main objective of the OPF problem for distribution network is to minimize the fuel cost of DG units, voltage profile improvement and minimization of power losses, though optimal settings of the control variables, while at the same time satisfying various distribution system operating constraints.

In [4], an optimal distribution power flow strategy based on decomposition of the overall system problem into two components: economic dispatch for energy at the system level and loss minimization at the distribution level has been proposed. A combined quadratic programming and sectioning algorithm is used to find solution.

Generalized formulation to determine the optimal operating strategy and cost optimization scheme for a micro-grid consisting of a wt, a mt, a dg, a pv array, a fc, and a battery storage has been presented in [5]. In this strategy, the mesh adaptive direct search algorithm is used to minimize cost function of the system while constraining it to meet the customer demand and safety of the system. Other objectives, such as power losses minimization and voltage profile improvement, as well as power flow constraints, bus voltage constraints are not considered.

The OPF is formulated as a nonlinear multi-objective optimization problem with equality and inequality constraints. The following objective functions are considered: (i) fuel cost minimization for DG units (ii) simultaneous minimization of the fuel cost and power losses (iii) simultaneous minimization of the fuel cost and voltage deviation and (iv) simultaneous minimization of the fuel cost, power losses and voltage deviation.

The chapter is organized as follows. After introduction, the mathematical formulation of the OPF problem for distribution networks is exposed in Section 2. Optimization models of DG units are formulated in Section 3. OPF solution using multi-objective genetic algorithm is presented in Section 4. In Section 5, gravitational search algorithm for solution of the OPF problem is proposed.

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