WYE-DELTA-WYE TX BLOWN HS FUSE PROTECTION

[rogerj1]

Folks,
We typically protect a 3PH Delta-Wye power transformer from blown high side fuse operations with negative sequence overvoltage detection in the low side tiebreaker. In this case, a dropped phase on the high side of a Delta/Wye produces one phase at nominal and the other two at half nominal values. (I'm not even sure how this is shown in the vector math, but its a method we've employed according to a SEL application guide... I'd like to see the solution to this if anyone can explain!) Anyhow, how would this protection apply to a 115-2.4-12.5 kV Wye-Delta-Wye transformer? (the delta is tertiary) Would the same values result as in a Delta-Wye connection, or would it act like the primary with one phase at zero and the others at nominal....??? Thanks for the help!

 

[waross]

If "A" Phase fuse blows, the "A" Phase and the "C" Phase transformers go in series across "B"-"C". The voltages of the two transformers connected to the blown fuse will sum to normal voltage. The actual voltages will be in inverse proportion to loading on the two phases. That the voltages are equal indicates that the load is fairly well balanced. All three voltages will be in phase although one will be opposite polarity.
If there is a fairly large motor running single phased on the system, it will try to replace the missing phase. In this case there will be a phase displacement. The voltages and phase displacements in a particular system will depend to a great extent on the size of the motor relative to the size of the system.
respectfully