Transformer Fuse Protection

[Fuselink]

Link below shows transformer protection with two different rating fuses. 40A and 50A

Transformer is 1500KVA 27.6KV to 600V. It seems that 40A fuse provides better protection however existing fuses are 50A. Secondary breaker is old H1-25 Federal pioneer breaker which I can not find any info on the relay. All I know is that this breaker has a dashpot thermal magnetic relay. Three questions.
Can I still use the 50A fuses to protect the transformer?
What is the point that fuse curve can cross the transformer curve and still provides protection?
What’s the way around it, if I don't have the relay curve info.

Thanks

 

[DanDel]

Yes, the 40A fuse provides better protection while still clearing the transformer inrush point, but since the FLA of the transformer is 31.4A, the 50A fuse is quite acceptable, and usually preferred for temporary overloads.

I notice that you are using the Frequent Fault curve for the transformer proection; this may not be necessary unless it feeds a load that typically sees many faults, and the Infrequent Fault curve will allow added clearance for your fuse curve.

Per IEEE 242, the primary fuse curve "may cross the through-fault protection curve at lower current levels since low-current overload proection is a function of the secondary-side protective device".

 

[Fuselink]

Dandel,

Is there an excat number which can be calculated for this lower limit current which fuse curve can cross the fuse curve?

How about estimating the relay curve? Have you had experience with dashpot relays?

 

[DanDel]

As long as the primary device covers the high current region for short circuit protection (which it does) it should be OK.

The FLA of a 1500kVA 3-phase transformer at 600V would be 1445A. What are the settings of the F-P H1-25? I don't have access to the curves.

In any case, old dashpot devices are notoriously inconsistent in their response to overloads. The trip bands for these devices (even when new) are very wide. This makes them hard to coordinate and difficult to rely on for protection. My recommendation would be to replace or retrofit that C/B to a newer trip unit.

 

[Fuselink]

it's not possible to retrofit to a newer trip unit.
in anycase, I have told them that there is no ground fault protection on the secondary too.
Dashpot settings are 1280A and don't know the instantaneous settings.
I was told that the primary fuse on the transformer should be 1.25 to 1.33 times the primary current. By choosing 50A I have 1.6 times the primary current. I realize that it covers the high current region. But I just want to make sure that in the future Ican choose up to this range for xfmr protection.

Thanks

 

[Borti]

FuseLink,

What is the transformer winding arrangement? If it is delta-y with the neutral grounded you have to look at the ground fault on the secondary. The transformer curve has to be shifted by the factor of 0.58. Then you can look at the fuse curve.

When selecting the fuse on the transformer primary, there are many factors determining the fuse rating; the transformer application, inrush current, through fault protection curve, fuse TCC, transformer secondary protective device, upstream protective device and available fault current are some i can think of right now. I found that the fuse rating between 140% and 160% transformer full load current usually works the best.

 

[Fuselink]

Transformer winding is wye-wye 27.6Kv to .6Kv.
Software will shift the curve automatically by that factor.
I never heard of through fault protection curve!
Borti,
can you please explain?
Thanks

 

[stevenal]

Fuselink,

The transformer curve in your link is the through fault protection curve or damage curve depending on how you look at it. Frequent faults should be below the dog leg, infrequent faults may be above it in the middle region. Through faults are distinguished from internal faults. Internal faults should be cleared instantly if possible.

 

[dpc]

The transformer damage curve has nothing to do with faults within the transformer. It only relates to through-faults. For a fault on the secondary side, the transformer must withstand the mechanical and thermal stresses that the high current places on the windings.

If the fault is in the transformer winding, it is already "damaged" so the damage curve is no relevant.

 

[DanDel]

Whichever fault curve is used, the protective device should be below it to provide the protection.

The high-current, low-time end of the curve is what changes between the Frequent and Infrequent fault curve. That part of the Frequent fault curve is drawn lower than that of the Infrequent fault curve, because if the transformer is subject to frequent through-faults, better fault protection is needed to prevent transformer damage.

For a transformer subject to frequent through-faults, the Frequent Fault curve should be chosen to help prevent cumulative damage from multiple through-faults.

 

[Borti]

Hi Fuselink,

No need to explain the through fault protection curve. Stevenal, dpc and DanDel have done it already.

Ideally you should plot the breaker curve and then make a decision on the fuse rating but since you have an old oil dashpot trip unit, I believe that the lowest possible fuse rating is the best choice (40E).