blown fuse detection

[alehman]

Getting back to the subject of blown fuse protection (thread238-175697), one of my clients has experienced a few cases where an automatic transfer switch (custom built breaker transfer-pairs) begin cycling after a transformer primary fuse opens. These cases all involve pad-mount medium voltage to 480V utility-owned transformers supplying the normal source input of the breaker pair ATS.

An article on the Basler web site discusses the phenomenon where wye-wye 5-leg shell-form transformers regenerate voltage on open phases, particularly when there is no load applied. I suspect many pad-mount transformers are of this construction. This would explain the confused ATS's.

The ATS's typically utilize a 47/27 relay, connected through open-delta PT's, for sensing on the utility source.

Possible solutions I see are to add some load upstream of the ATS, so the transformer always has some load, or replace the transformer with delta-wye (may not be an option). The Basler article mentions a "small" load, but how small is small?

Has anyone dealt with this situation or have any other suggestions to deal with this regenerated no-load voltage?

 

[ePOWEReng]

Hi Alehman. Could you send the link to the balser article?

 

[alehman]

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

I think you have to register or something to download it.

 

[magoo2]

When you get an open phase on the primary of a wye-wye transformer based on a 5-legged core design, the voltage that gets coupled to that winding depends on its location, i.e., whether it's the inner leg or one of the outer legs. If it is the center winding, you'll get enough flux in the core from the 2 good phases to give you rated voltage. If its an outer leg, you'll get about half voltage.

In either case, it's a high impedance phenomena. A small amount of load should collapse the voltage, but the magic question is how much. I'd try about 2% load on the winding, in other words put about 2% of rated current on the winding and see if the voltage collapses.

What kVA size transformer is involved?

 

[alehman]

Thanks. Transformers range from 500 to 2000 kVA.

 

[daviandgo]

We had a similar situation. The ATS manufacturer installed a Negative Sequence Voltage Detection Relay. The relay was programmed to prevent retransfer to utility if the negative to positive sequence voltage percentage was greater than 2%. This solved the problem.

 

[magoo2]

That's interesting, daviandgo. I had picked the 2% figure along the lines of some situations I had been involved with on avoiding ferroresonance. That was usually enough load to minimize the overvoltage under those conditions. That too is a high impedance phenomena and is really the same analysis if you just get an open circuit condition.

It's always good to see a recommendation from another source. It is comforting when the two agree with each other.

 

[slavag]

Hi.
it's very intresting topic.
I think more from 2% it's not simple value.
According to theory (I haven't any excpirience with 47 protection): several reason of NPS on busbur are:
Conductor broken, asimmetrical load and motor load(voltages unbalance as result of current unbalance of motor).
As I remebare (please correct me), it's more or less linear
proportianal ( about k*Un%---5k*In%). That means
in normal situation with motor load U NPS is not more 2%.
Daviandgo, I assume your setting about 3-4%.
Problem, I think is time delay, it's depend.
I assume, NPS not used as trigger of ATS, it's only prevent transfer.
Alehman, thanks for start of this topic.
Hm, I think , I'll put this function in one of projects
and check what is result. Hey, I know were, next month we'll
install relay for decoupling of plant on 36kV side.
Regards.
Slava

 

[alehman]

The Basler article predicts the following for 5-leg shell-form wye-wye xfmr:

B-phase open:
Van: 1.0<0
Vbn: approx. 0.5<-120
Vcn: 1.0<120

V2/V1 = 0.2

C-phase open:
Van: 1.0<0
Vbn: 1.0<-120
Vcn: 0.58<90

V2/V1 = 0.23

 

[daviandgo]

Here is some more information.
Pad-Mounted Transformer 2500 kVA, 34.5kV YG/480V YG
System Phase Rotation: CBA
Measured Values:
With all fuses in (Load on transformer)
Negative-to-positive Ratio: 0.2%
With C-phase fuse removed (No load on transformer)
Negative-to-positive Ratio: 4.3%
With B-phase fuse removed (No load on transformer)
Negative-to-positive Ratio: 2.4%

This was the basis of the 2% setting to prevent transfer back to the utility source in the event of a blown fuse.

 

[alehman]

Interesting. 4.3% V2 would correspond to a single phase imbalance of about 5%.

 

[slavag]

Thanks a lot Daviandgo.
Are you check NPS voltage on 34.5kV side ?
Are you check NPS voltage with some load ?
Q?
Why according to Alehman's post ( sorry to Basler) it's 23% and 20%?
According to theory/practic,"first" setting of NPS (47) is 3%.
Regards.
Slava

 

[alehman]

Salvag,
You say "According to theory/practic,"first" setting of NPS (47) is 3%.".

Just curious if you've seen that in a standard somewhere?

 

[slavag]

Alehman.
No, of course no.
I wrote,that I haven't any expirience with this function.
Before some time, we installed protective relay of some
big compnay ( for feeder protection). Those relays are include NPS and PPS ( U2> and U1< )voltage protection.
We never used this protection and I asked mnf.:
What is a typical appl for this protection.
U1< for the decoupling.
U2>, for open conductor, asimmetrical loads.
in manual recommendaed typical setting is 3%.
I'm only translate mnf. recommendation, sorry.
BTW, my second problem, we haven't Y/Y trnasformers in our area, only D/Y.
Why, it's intresting for me, before half year we used
NPS voltage for alarms in VT secondary wiring and it's helped us.
Regards.
Slava

 

[slavag]

Hi.
Forgot something.
According to Basler result.
U=0.5Un or 0.58Un.
Sorry, for what I need 47 function?
27 is good function for this data.
Regards.
Slava

 

[alehman]

Thanks for the clarification slavag. I agree the Basler results would be easy to sense with a 27 relay. We see a mix of D-Y and Y-Y, but Y-Y seems to be more common.

 

[alehman]

daviandgo, magoo - Thanks for the info. Do you know if there have been many undesirable activations? Do you use a time delay function with the 47?

 

[daviandgo]

I am not aware on any

 

[daviandgo]

I am not aware of any undesirable activation. No time delay is used. Recall that this is to block transfer back to the utility. No change was made to the transfer from the utility in the event of a loss of voltage on one or more phases.