DETERMINATION OF THÉVENIN IMPEDANCES TO ALLOCATE TRANSMISSION POWER LOSSES A. V. Adebayo*, C. T. Gaunt * , M. Malengret * , K. Awodele* *Dept. of Electrical and Electronics Engineering, University of Cape Town, South Africa Abstract: Restructuring and unbundling the electricity industry brings about changes in the regulation in the sector and a need to allocate costs to the participants. Power losses become important, including how to charge participants for their contribution to grid losses. Methods include the Pro Rata approach, marginal procedure allocation, and proportional sharing. Most other methods are either mathematically complex or time consuming and none can allocate losses with fairness and transparency. In this paper a Thévenin equivalent network measured from its point of common coupling is used in determining the I 2 R losses associated with a load or generation. The Thévenin parameters are derived using the change in phasor voltage and phasor current resulting from a change of load or generation at the PCC. This method is transparent, fair and quick. Key words: Thévenin parameters, Fairness and Transparency, Restructuring. 1. INTRODUCTION The Electric power industry (EPI) worldwide is going through transformation with the aim of introducing competition, increasing efficiency and quality of service, and with customer price reduction through competition. Generally, transformation has unbundled vertically integrated utilities into different companies for generation, transmission and distribution, while in some countries the system operation company is separate from the transmission company. Competition exists in the generation and distribution sectors, while transmission is an intermediary to transmit electricity and remains monopolistic in most countries. Markets in power systems take the form of a power pool and bilateral contracts. In the power pool, the generating utilities (or independent power producers) and customers bid for selling and buying power at the pool. The power pool conducts time-differentiated markets such as day before market, hour before market and real time market operation. The consumers’ meters measure actual consumption and the generators’ meters measure actual production including the network losses. Of course, the problem of “who pays for losses?” arises, and those payments are significant. Bilateral contracts are usually long-term agreements determined through negotiations between a buyer and a seller, with the seller usually arranging the transportation of the contracted power over the grid network [1]. On a bilateral exchange, the price is based on market forces other than under potential system security violations [2]. In order to improve efficiency, the knowledge of transmission losses is important information needed by the bilateral market in every proposed bilateral transaction. This information helps both buyer and seller to incorporate the level and cost of transmission losses into their negotiations. A key issue in the restructured environment is that the cost of transmission losses is satisfactorily allocated among all partners in spite of the non-linear nature of power flow. Regardless of market structure, a transparent and fair method of allocating delivery losses to all participants is needed in terms of the economic theory that goods and services should be charged on a basis of marginal cost. An appropriate model and algorithm for determining accurately the delivery losses must allow for many influencing factors including:  The location of generation and load connections.  Types of connected loads and network configuration.  Voltage levels and voltage instability.  Dynamic factors associated with the operational system of large ac networks (e.g. power factor, harmonics and the control of active and reactive, or non-active, power).  The length of the lines - this is an almost linear relationship (e.g. doubling the line length would double the line loss).  The design of lines, size, material and type of cables; with the types of transformers and their loadings. In a deregulated environment, allocating losses is a difficult task since it is part of the generated energy. Moreover, following economic theory, the short-term marginal cost (SRMC) pricing of transmission service has failed to recover the total incurred network cost due to the unpredictability of the method used which is not economically marginal. Although, there is no perfect system, but the independent system operator (ISO) should adopt a safer path of specifying the energy capacity and energy bid price of all the generators in order to select the