How to model a Single-Phase Mid-tap PQ-Type Generator to run Unbalance Short-Circuit study when Swing-Bus (SB) is not defined in system model.

Question: If user is modeling a single-phase mid-tap generator as an emergency power source in my system and the software is warning the user that there is no Swing-Bus connected even though there is a Utility connected on the “Normal” power source side, how to get unbalance system studies to run?




Answer: The reason the Unbalance Single Phase short-circuit study is not allowing to model this configuration is ATS switch, which is isolating the Utility network, therefore no swing bus is really defined for the study. Since we do not allow Single-Phase generators to be a “Swing-Bus” then the alternative and the only other way to model it would be to insert a “dummy” utility in parallel to the Emergency Generator and enter large impedance values so that the utility contribution is close to 0.

See below example where we have added a “dummy” utility to allow software to run Unbalance short-circuit studies and how the contribution is close to zero for utility, but non-zero for the single-phase mid-tap generator: