Wye Grounded Broken Delta Transformer Regenerating Voltage On Open Phase

[DM61850]

I have an auxillary wye 1:1 120V grounded broken delta transformer that is feeding a Basler 59N relay. When I hooked up a test set to the secondary of bus PT's (15 kV:120V) I was seeing voltage any of the phases when I pulled it from the test set. I thought it was a wiring issue at first but I figured out that the broken delta regenerates the voltage on the high side of the auxiliary PTs/low side of the bus PTs. Is this something that is just inherent if you have this type of a transformer. I don't have a second set of PT's to use to separate the broken delta. I am concerned that my transfer scheme might misoperate during a blown pt fuse or loss of a phase.


I haven't found much on this but I found this NERC lessons learned where a utility had a misoperation due to a wye-broken delta pt backfeeding a phase that had a blown fuse.

https://www.nerc.com/pa/rrm/ea/Less...ssion_Relaying_Removing_Unused_Components.pdf

How much concern should I have of a wye-broken delta transformer backfeeding voltage to my relaying? My transfer scheme depends solely on line and bus voltages.

 

[waross]

removed and the broken delta side of the auxiliary PTs was incorrectly shorted.

With the improper jumper in place this is no longer a broken delta.
It is a grounded wye:delta.
If this connection is used for distribution transformers It will back-feed and supply the loads on a missing phase in the event of single phasing of the distribution circuit.
If you find yourself in an area where this connection is in use, you may notice many transformer banks with one fused cutout open running happily on open delta.
They have been overloaded while back feeding a missing phase and lost one fuse.
The connection then becomes an open delta with a third transformer connected across the open side, but disconnected on the primary side.
Moral:
Don't close the delta.
Note to our European friends.
What we call a broken delta, you call an open delta.
What we call an open delta is a two transformer connection.

This thread discusses the problems associated with wye:delta distribution transformers in the real world.
Some of the same effects may be seen in your metering setup.

Circulating currents in banks of transformers with star-delta connection
thread238-444324


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DM61850]

I don't have a true broken delta because I have 750 watt ballast resistors on the broken delta side for voltage stability and preventing ferroresonance.

 

[waross]

Those resistors will limit any short circuit currents.
That arrangement may be used as a grounding transformer, but the resistors are more commonly installed between the primary wye point and ground.
On an open circuit the resistors will act as a jumper.
Remember, open circuit = no current = zero voltage drop across the resistor = jumper = full voltage across the unloaded secondary windings.


Bill
--------------------
"Why not the best?"
Jimmy Carter