3PH Transformer Open Phase Detection

[hibb1]

I need some advice. Looking at a hospital complex with a mix of 3 phase DY, YY, and YD transformers (not sure of the exact numbers of different types of transformers yet).
Hospital has installed wye type voltage magnitude sensors on utility interconnect end to detect single phase loss.
During test of single phase loss (utility pulled open a fuse), backfeed from the hospital complex kept these voltage sensors picked up, so they didn't detect a single phase loss.
My question, does the voltage angle change on the backfeed from the hospital system after a phase loss? Also, could we detect that with a negative sequence voltage element?
Any comments would be appreciated. I'm starting to hear of several cases where the voltage magnitude sensors on UG cable boots have not picked up for a loss of phase.

 

[jghrist]

You can detect an open phase condition from the low side of a DY transformer. See Detecting High-Side Fuse Operations Using an SEL-251 Relay Ed Schweitzer; John J. Kumm, AG95-26.

https://www.selinc.com/literature/literature.aspx?fid=1813

Registration might be required.

 

[hibb1]

I'm familiar with that paper, but it's not practical at this site (physically or politically) to sense on the low side of a DY transformer. We need to sense only on the high side.

 

[davidbeach]

Your best tool for detecting that condition is negative sequence current.

 

[hibb1]

I've thought about using negative sequence current, but there is a question about sensitivity when the hospital is at minimum load. We also would need to struggle with the concept of picking a CT ratio that is low enough to be sensitive, yet high enough to allow for system load growth. If I pick a CT ratio that is high enough to match the incoming breaker ampacity, then it looks like it isn't sensitive enough to pick up for a loss of phase under minimum load growth.

 

[waross]

Are your YD wye points connected to the system neutral?

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

 

[jghrist]

Is the backfeed from connected generation or is it from the transformer connections? The Ø-n voltage on the open phase of a DY transformer would be 1/2 of the normal Ø-Ø voltage if there was no active load connected.

 

[davidbeach]

If one of those YY transformers is really a YDY with the tertiary buried, you can get close to full voltage on teh phantom phase. It is cheaper to build a 3-legged YDY than to build a 4/5 legged YY, and smaller too. They really bite when trying to do phase loss detection based simply on voltage magnitude.

 

[hibb1]

Thanks for all the comments. I don't know enough details on the hospital's system yet. I do know that the hospital receives power at 34.5kV and steps it down for about 3/4 of their load through a 3000kVA YgYg padmount transformer. I have been told that this transformer has a five legged core to meet energy efficiency (not sure if that is affecting things here).
The other 1/4 of the load is stepped down through a DYg transformer to 13.2kV.
The main backup generation is only used in an islanding mode. There are other small gensets, but they are strictly for backup mode.