A Proposed Method for Processing Unbalanced Conditions and DC Offset Currents in Transient Stability Analysis

An approach has been proposed in this paper for processing unbalanced operating conditions in transient stability calculations. The suggested method combined positive sequence network usually used for traditionally balanced transient stability program with zero and negative sequence network which are available for traditional unsymmetrical fault routines. The zero and negative bus matrixes are usually available in sparse nature within any commercial software to handle large scale power system analysis. Therefore, they can be easily used in unbalanced transient stability routines, as described in this paper. Besides, a method is presented to evaluate DC offset currents in system branches during the phasor-based transient stability run. The DC offset arise from the stator and network transients during switching (including faults). Conventional phasor-based transient stability programs ignore the occurrences of such a DC offset in their simulation process. Approximating of DC and negative sequence current braking effects in AC machines and modeling negative, ground and overcurrent instantaneous protective equipments behavior in transient stability simulation are the main benefits of the proposed method. Computer test results in comparison with results carried out by detailed three-phase time-domain simulations using power system blockset (PSB) of MATLAB, show clearly the effectiveness of the method.