A load flow method using line-to-line voltages for ungrounded distribution power systems

Some load flow methods in distribution power systems, such as backward-forward sweep method, Zbus method, and loop impedance method, advocate phase-to-ground voltage component. But in ungrounded distribution networks, the ground reference is lost and the phase-to-ground voltage is undefined, so the solution can´t be unique. Also in these ungrounded systems, admittance submatrices of transformers become singularity. The traditional backward-forward sweep method can´t handle these problems. This paper advocates line-to-line voltage component, avoiding the singularity caused by admittance submatrices of transformers in ungrounded distribution power systems. Models of loads, transmission lines, transformers and induction motors are modified using this component. We note that these models can be easily incorporated into an implicit backward-forward sweep method, and it can easily deal with load flow calculation in ungrounded distribution systems. The load flow method based on line-to-line voltage component models can also decrease the calculation, and it coincides with the real circumstance in ungrounded distribution power systems. Numerical examples demonstrate the validity and effectiveness of the proposed method.