I. Introduction
Restructuring involves a transformation in the structure and organization of power industry. Under restructuring, vertically integrated utilities, in which producers generate, transmit, and distribute electricity, have been legally or functionally unbundled. Transmission and distribution are still considered natural monopolies that require regulation whereas competition will occur in both the generation market and retail electricity services market. The main argument used to support competition is that a free market promotes fairness and open access to the transmission network. Fairness can only be achieved by adopting a fair and transparent usage allocation methodology acceptable for all parties. In view of market operation it becomes more important to know the role of individual generators and loads to the networks and power transfer between individual generators to loads. This is necessary for the competitive market to operate economically, efficiently and for the guarantee of open access to all system users. Several schemes have been developed to solve the allocation problem in the last few years. Methods based on Y-bus or Z-bus matrix of the system have recently received great attentions, since these methods can integrate the network characteristics and circuit theories into line usage and loss allocation. The method reported in [1] is based on Kirchhoff's Current Law (KCL), equivalent linear circuit transformation and superposition principle. In general it assumed that the current at each network injection point may reach all lines and loads. Based on these assumptions, a recursive procedure is used to construct an equivalent circuit for each bus and followed by application of superposition theorem to the bus's equivalent circuit, starting from a bus whose injected currents are known. The procedure is applied to all the nodes in the system. Another circuit concept method was proposed by Chang in [2]. It was based on the system Ybus matrix and Z-bus modification. Starting from the load flow solution, branch current are determined as a function of generators' injected current by using information from the bus impedance matrix. Similarly, contribution to bus voltages is computed as a function of each generator current injection by decomposing the network into different networks. Using the computed voltages and currents, the power flowing on the transmission lines are unbundled. It uses approximate formulation to calculate the unbundled loss components. This algorithm utilizes the network decomposition concept proposed in [3] which determines the use of transmission network by individual bilateral contracts. Reference [4] proposed a systematic method, which is very similar to [2], to allocate the power flow and loss for deregulated transmission systems.