Contingency screening method for voltage stability analysis in the assessment of available transfer capability in deregulated power systems

The electric utility industry is going through the phase of restructuring towards evolving into a competitive market. In order to facilitate that transformation, the access to the transmission network is provided on a non discriminatory basis, which is limited only by the capability of the network to transfer power. Available Transfer Capability (ATC) has become the standard tool to compute the transfer capability remaining in the network over and above already committed use. The calculation of ATC involves network limitations due to thermal and voltage stability limits. Network limitations are to be calculated not only for the current operating condition but also for a worse case that could occur due to a set of contingency cases, which require huge amount of computation. This puts constraint on ATC to be an effective tool in real time operating environment. The aim of this dissertation is to study the computational efforts involved in the computation of ATC and propose a method to minimize the computation involved in the assessment of ATC due to the voltage stability limit. The proposed method performs ranking of the contingencies using a severity index based on the incremental reactive power losses between the base case and the contingency case. Based on the ranking, a short list of contingencies is to be selected for full simulation. Since the impact of the contingencies is not known until the power flow equations are solved, the convergence of the contingency cases is not known a priori. For the purpose of identifying the base case load level for contingency screening, couple of measures of convergence of contingency cases at each load level on the base case curve are proposed, one for large correction errors and the other for divergence behavior due to load increase. The proposed method is validated with simulations on practical large power systems.