A Method of Modeling Tap-Changing Transformers for Power-Flow and Short-Circuit Analysis Studies

Abstract

Tap changing transformers are commonly used to reduce the imbalance in phase voltages or maintain the voltage magnitude in the system at a range, often 0.95 to 1.05 p.u. If an electric fault occurs on a power distribution network with such a tap-changing transformer, the tap adjusted on either the primary or secondary side changes the magnitude and angle of the short-circuit current. Therefore, power-flow analysis algorithms must be able to model tap-changing transformers. For example, several such algorithms (e.g., the Newton-Raphson, Gauss-Seidel, and fast decoupled methods that use the bus admittance matrix) require an inversion of a Jacobian matrix. If the matrix size is sufficiently large (e.g., up to thousands of dimensions), such methods may fail to calculate the inversion of a large matrix within a limited time despite the matrix reduction techniques. Moreover, if the admittance matrix of a tap-changing transformer is singular, its inverse matrix could not be found by the inversion of the matrix. The bus impedance matrix is commonly necessary for short-circuit studies. Thus, the singularity problem has been solved by a backward and forward sweep method. But, the method may not work for heavily-meshed distribution networks. Thus, this study presents a novel method that models tap-changing transformers, not causing the singularity. Then, the proposed method is verified in various case studies.