Fuse Protection of Power Transformers 4

[Mbrooke]

What are the cons of protecting 5-60MVA power transformers with primary fuses? Why is the practice frowned upon?

 

[itsmoked]

The potential for single phasing.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

Poorer protection when compared with breaker + relays.

 

[Mbrooke]

What are the negative effects of single phasing? How does poorer protection present a disadvantage?

 

[dpc]

I don't get it. Are you just trolling?

 

[davidbeach]

If you're going to wash your hands of the transformer as soon as it's powered up, sure put in fuses. But if you, or a future you, is going to replace that transformer at end of life and future-future you is going to replace that one, you'll find that a full protection package is far less expensive that doing it half-arsed with fuses. Never seen a decent event file from a fuse. Never seen a delta high-side fuse that clears a wye low-side SLG fault in anything close to a reasonable amount of time.

I've had a transformer with differential trip within cycles of a high-side turn-turn fault, way before any fuse would have done anything. I've had a fuse protected transformer spew oil all over the place (fortunately no ignition) because the fault was deep enough to not blow fuses faster than the line trips; a low enough magnitude fault that the line tripping took long enough that the reclosing reset between trips. Fuses are great for VTs, but not for real transformers.

 

[Mbrooke]

Is clearing any better delta-delta? Good points btw.

 

[RRaghunath]

Availability of fuses for such large ratings (transformer MVA 5-60) is also an issue.
I have seen an American O&G company installing fuse only protection for 4MVA, 33kV transformer. But, it was a temporary installation and they didn't want to spend much money (willing to forego the transformer in case of fault). It was in desert, feeding a 1200kW pump motor directly.

 

[stevenal]

Single phasing is bad for three phase motors. If the transformer is delta-wye, the delta collapses leaving two phases at reduced magnitude, so single phasing can even affect single phase loads that remain energized.

Fuses can fail dramatically. If exposed to moisture, the boric acid inside the tube can swell and prevent the element from separating when melted. The fault current then continues until cleared elsewhere.

For arc flash purposes, the section between the transformer secondary and the next protective device is problematic. The transformer impedance ensures long clearing times. The effect can be mitigated with relay settings.

One advantage: Fuses are self powered and complete. No DC, CTs, or low voltage wiring; all possible failure points.

 

[Mbrooke]

Is there any indication of water ingress?

 

[stevenal]

Fuses left hanging open and inverted in the holders during wet weather is one indication.
Another is failure to pass an air flow test. S&C sells an air flow testing device for their fuses.

 

[Mbrooke]

Did not know that. Can a fuse become filled with water left in the "closed" position and not give indication? I'd hate to install a SMD-2B, get wet internally and then fail to blow correctly 30 years latter.

 

[davidbeach]

Yes, they can become water damaged while closed and in use. Happened to our SMU cap bank fuses until we started replacing fuses older than some number of years. Fuse on a cap bank is more likely to gather moisture than one on a transformer would be due to the daily thermal cycle from off to fully loaded to off.

 

[Mbrooke]

How many years? I did not know S&C fuses had that limitation.

 

[stevenal]

To determine that SMD Fuse Units are in proper operating
condition, the condition of the fuse unit bore should be
checked periodically—whenever the protected device
is taken out of service for routine maintenance. SMD-2B
Fuse Units can be easily checked in the field using the S&C
Airflow Test Instrument.

Link

 

[Mbrooke]

Thanks

Can't believe they don't make a better fuse.

What about Southern States trafo fuses? Are they similar?

 

[stevenal]

Using a combination of boric acid-filled auxiliary fuse tube...

Link


Sounds similar to me.

 

[Mbrooke]

Bummer

Any idea of the failure rates for trafo fuses? Are they more reliable than properly tested and maintained relaying?

 

[PSEng01]

Time for fuses to blow is much slower than proper breaker. The scale of damage will be worse.
I am not familiar with fuses of these sizes but do they have to be replaced after each operation like distribution fuses?
If yes then you probably want to make sure you have spares handy, and of the correct fuse size!

 

[Mbrooke]

Yes, the fuse unit must be replaced after its blown. S&C even has instructions which include a pulley for the larger fuses. The plus side its rare for them to blow on the primary side. Ditto for VT protection.