(A) Impedance for D-Yg and Yg-Yg transformer types:
- Shell: The zero sequence impedance is same as that of positive sequence impedance.
- Core: The zero sequence impedance is 0.85 times the positive sequence impedance.
Note: We are assuming the calculate button is used (on the Impedance tab)
(B) Transformer zero sequence model and impedance for Y-Yg:
- Shell: The model is an open circuit.
- Core: The zero sequence connection model is same as that of D-Yg (the impedance is grounded on one side and connected at the secondary side). The impedance is 5 times the positive sequence impedance.
Figure 1 is an EasyPower model with five different configurations at an identical MVA rating and positive sequence impedance. It has two D-Yg and two Y-Yg transformers—one each in core and shell configurations. Additionally, it has a Y-Y transformer to show one of the open circuit transformers.
Figure 1: EasyPower Model Showing Different Two Winding Transformer Types
When a single Line-to-Ground (SLG) fault is applied at all the buses, the following observations can be made:
- Fault current at BUS-2 is higher than that of BUS-2_A even though both have the same configuration type, D-Yg. This is because the core transformer has a lower impedance than the shell type as mentioned in (A).
- There is a significant amount of fault current at BUS-3 while there is zero current at BUS-3_A although both the buses are connected to the same configuration type, Y-Yg. As mentioned in (B), the shell type is open circuit and core type is modeled similar to D-Yg, but the fault current is significantly lower than BUS-2 because of the high impedance.
Figure 2: Single Line to Ground Fault Results (the Displayed Currents are Phase Values)