PT Fuse Failure 1

[sngpl]

We have a 3.6 MW Hydro Electric Power Generator. The unit was shut down for a couple of days and when we attempted to start it, it won't develop voltage. We found that one of the generator PT High Tension side fuses was sort of blown. I said sort of because it’s damaged in the center. I think if a fuse blows... it blows .... But this one is only damaged in the center and not blown completely.

My gut feeling is that the fuse was defective and got damaged. We should megger the generator with PT etc connected and then to further sectionalize if the results aren't good. If the results are good then simply replace the fuse and run the unit.

Honorable Forum: Please correct me if I am wrong!!

What could be the problem and its causes?
What tests to perform?

Thanks.

 

[advidana]

I have experience this type of fuse failure. I think the fuse will weaken over time with minor power surges just below blowing the fuse out.

 

[rbulsara]

What was the function of this PT? Is this feeding some controls that are essential for generator to develop power? or its just a measuring PT?

And when you say it was not developing voltage, did it show no voltage on all the phases?

Where is this PT connected, betwee gen leads and the breaker?

If no work was done on the generator during the shut down, I would be inclined to replace the fuse and give it a try. If anythig you need to meggar the PT itself.

Worst that can happen is it will not develop a voltage! So meggaring generator may not be of much value but meggaring test will not hurt, but you need to disconnect any connection to ground for meggaring.

 

[jbartos]

Suggestion: Higher harmonics may cause the fuse filament to be partially thermally eroded. If there happen to be ferroresonance, then the PTs secondary connection should have a suitably sized resistor.

 

[DanDel]

sngpl, was the fuse actually opened or just damaged at the center? If the damaged fuse measures the same(very low) resistance as the others of the same type, then it probably should be replaced due to the damage you see, but it is not causing your problem.

 

[wagsdm]

How old is this fuse? I work in an old plant built in the 30's. Over time we have replaced most of the fuses because they fail due to old age. Ironicallly I have found the older equipment to be much more reliable and well built that the newer stuff we have installed over the years.

 

[edison123]

sngpl,

Is it a possible during the generator shut down, the PT fuses removed (to megger the stator) and then someone damaged one while refixing them ?

As rbulsara asked, did no voltage appear at the generator terminals at all ? Does your AVR need all 3 phase input voltage from the PT's to excite the machine ?

Anyway, you can megger the stator, replace the fuse and restart the machine to see what happens ?

 

[dpc]

PT fuses are extremely low amperage, generally 1 amp. They are well known for failing or opening due to vibration, aging, or whatever.

 

[busbar]

One note about MV-primary fuses on a open-delta PT set. GE mentions in their literature that a single fuse that serves two 1ø PTs may be more likely to operate than the other two phases that serve only one PT. IIRC, it has to do with inrush from two PT primaries versus one.

Try

http://www.geindustrial.com/products/applications/GET-97D.pdf

or others at

http://www.geindustrial.com/cwc/library?famid=42 &view=doctype&SMSESSION=NO

or page 10 of

http://www.geindustrial.com/products/buyersguide/GEP-9186-6.pdf

Then again, some PT-primary fuses seem to blow if you so much as look at ‘em crosseyed.

 

[sngpl]

rbulsara (Electrical)
The PTs are in open delta for voltage regulation.
The generator was not developing voltage.

jbartos (Electrical)
Can you please explain the phenomenon of ferroresonanne and its effect on the fuse in a bit more detail. Sounds really interesting.

DanDel (Electrical)
The fuse is damaged and open, that’s why the generator wasn’t building up voltage.

wagsdm (Electrical
The fuse was installed in May this year.

edison123 (Electrical)
The generator was not meggered nor were the fuses removed.
No voltage appeared at the generator terminals … yes the AVR senses voltage through two PTs connected in open delta.

busbar (Electrical)

Thanks for very useful links.


I'll keep you posted about any further findings.
Any further suggestions are most welcome...... I am grateful for all the time and help.

 

[X2K3]

Sngpl,

PT connected in open delta should have one[middle] phase grounded. Was the ground connection intact?

 

[DanDel]

sngpl, I'm confused because you said "this one is only damaged in the center and not blown completely". But it was open(blown), right?
Burning in the center of a fuse that has opened may be an indication of fault current above the fuses rating, or an overload which heated and burnt the fuse tube as it opened, or cumulative damage over time.
Hopefully everthing tested out OK and the unit is back in service?

 

[nasmith]

Info-Recently one of our plants had a Bussman LMN infantile PT fuse failure. It was a manufacture defect problem. Appeared to be a cold solder joint.

-Nehtisha

 

[sngpl]

DanDel
What I meant by not blown completely was that the 95% of the casing was still in tact.... normally its just shattered completely!! It was open of course.

 

[sngpl]

X2K3 (Electrical)


"PT connected in open delta should have one[middle] phase grounded. Was the ground connection intact?"

I would rather disagree... Pts connected in Y have their Y-point grounded but not PTs in open delta.
Please feel free to correct me, if I am wrong?

 

[alehman]

"normally its just shattered completely!!"
I think you're using the wrong kind of fuse. Either the fault clearing rating or voltage rating may be inadequate. Either is very dangerous. You should correct this situation immediately!!

 

[X2K3]

Sngpl,

What I meant by open delta configuration - PTs with two windid only on HV and LV. HV side both wdgs connected in series - voltages are applied at one phase at each end of wdg (A&C) at middle pt third phase (B). Secondary too is connected in similar way but middle point is grounded.

By Open delta configuration I did not mean three PT windings connected in "Broken Delta" configuration. This is used for measuring residual voltage / zero sequence components to detect the ground faults.

 

[busbar]

X2K3, I believe this may be the open-delta configuration your are initially describing:

http://www.beckwithelectric.com/lc/protection/vtcongnd/images/fig8.jpg

 

[hmchi]

Most PT fuses are current-limiting type because they are close to a stiff source and need high interrupting capacity which current limiting fuses excel. However, CL fuses have damageability issues such as ferro-resonanc and thermal cycling, etc. [CL fuses must replace all 3 phases even if only one fuse operated after a fault --- due to the damageability issue].

Many large HV substations use S&C fusistor [

http://www.sandc.com/pdfs/300/341_30.pdf,

and

http://www.sandc.com/pdfs/300/341_31.pdf

] to get away from the damageabilty issue. The resistor will also limit the fault current level making it easy for use in high IC applications such as generating stations. The same resistor also limits the continuous current rating thus making it viable only for PT fusing ...

 

[asymptote]

Sngpl.
There were problems with this type of failure on withdrawable 11kV VT's, where HV HRC fuses passed through the steel tank wall of oil filled voltage transformers within the ceramic VT spout. It's a high stress area and causes the fuse element to fail in service or on resoration of supplies.
asymptote.