EFFECTS OF LOAD MODELING IN SECURITY CONSTRAINED OPF STUDIES

Load modeling in OPF studies minimizing fuel costs is known to give lower fuel costs by lowering the power demand. When quite a few loads are modeled, the minimum fuel costs may be much lower than the corresponding standard OPF fuel cost with a significant decrease in power demand. A similar effect can be expected when load models are incorporated in security constrained OPF studies as well. The decrease in the power demand when load models are incorporated in OPF studies may not be desirable under normal operating conditions. This problem can be avoided in a security constrained OPF by incorporating load models during contingencies only. This not only gives results that are more comparable with standard OPF results, but may also give lower fuel costs without lowering the power demand of the intact system. A sample system is chosen to demonstrate this effect. The modeled loads are assumed to be fed by fixed tap distribution transformers and are modeled using an exponential type of load model. It is concluded that a decrease in fuel cost can be obtained in some instances when load models are incorporated in security constrained OPF studies during contingencies only. In situations where a decrease in fuel cost is obtained in this manner, the magnitude of decrease depends on the total percentage of load fed by fixed tap transformers and the sensitivity of these loads to modeling. The tap settings of these fixed tap transformers have an influence on the results as well. An increase in fuel cost can also occur in some isolated cases. However in either case, given accurate load models, optimal power flow solutions that are more accurate than the conventional OPF solutions can be obtained. An alternate approach for normal OPF as well as security constrained OPF is also suggested.