Effects of nontransposed lines and unbalanced loads on state estimation

This paper investigates the errors introduced in the positive sequence state estimation due to the usual assumptions of having fully balanced bus loads/generations and continuously transposed transmission lines. A three-phase state estimator is first developed in order to verify the actual error free solution in the phase coordinates. Then, several tests are conducted using different assumptions regarding the availability of single and multi-phase measurements. It is demonstrated that incomplete metering of three-phase system quantities may lead to significant errors in the positive sequence state estimates for certain cases. Such cases may also lead to incorrect bad data detection and elimination, further deteriorating the quality of the state estimate. IEEE 30 bus test system is used to illustrate these cases.