Gnd wye Gnd wye PMT with one primary phase off - Good voltage on s

[daviandgo]

One of our customers is served from a 2500 kVA PMT, 5-legged, 35 kV Gnd Wye - 480V Gnd Wye. The 35 kV feed is from a fused Overhead lateral line to a fused terminal pole. The cable from the TP to the transformer is about 100' 1/0 AL EPR cable. The customer has a transfer switch that senses loss of utility voltage and transfers to his generators.

Recently B-phase on the lateral pole blew. The customer load transferred to his generator. However, once load was removed from the transformer the transfer switch sensed good utility voltage and retransferred. Once load was added to the transformer the transfer switch transferred back to the generator. It did this until the customer opened his main breaker.

We duplicated this by opening B-phase on the TP with the customer's main breaker open. Voltages were:
Before Opening Fuse
LG = 284, 284, 283
LL = 491, 491, 490

After Opening Fuse
LG =284. 284, 267
LL = 491, 480, 496

Could someone explain this occurrence and maybe offer a solution to this problem.

Thanks

 

[alehman]

This is a common problem with wye-wye transformers, particularly 5-leg core types. These are becomming more common as utilities look for ways to save money. The fact that you mention the specific transformer configuration makes me think you may be somewhat aware of the issue. This type of transformer can induce voltage on the secondary leg corresponding to the open primary. It is incapable of supporting significant load, so once the ATS retransfers the voltage collapses causing the ATS to go back to emergency.

Basler has a very good article on the subject.

http://basler.com/php/download.php?file=practical.pdf&id=204&type=res

I guess you could add some type of load between the transformer and ATS to pull the voltage down or change the transformer to a different type. If you could find a way to sense the voltage at the primary, that may work also.

 

[daviandgo]

Alehman, thanks for your response.
The most practical solution at this location is to add a "small" load to the transformer. Any guidance in estimating the "small" load?

Thanks.

 

[alehman]

I haven't tried this, so I really couldn't say. I need to investigate this as well. I'll try to reach someone at Basler and will reply here if they provide any useful information.

 

[tommom]

I was thinking that this "phantom" voltage could also be alleviated by adding a delta-wye transformer to the same high-voltage circuit, in order to give the third harmonic voltages a path to cancel out.

Am I wrong about this?

 

[mc5w]

One of the reasons for using a 5 legged core on wye wye transformers is that when a primary fuse blows on a short circuit that short circuit cannot draw significant energy from the other 2 phases. This is a serious drawback with 3 legged cores.

The line fuse holder can also be very slightly conductive with the fuse blown hence the need to ground out disconnected wires.