TCC Curves - Transformer Protection

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lielec11

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Charlotte, NC
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Electrical Engineer (PE)
I am having a hard time trying to protect a transformer properly using SKM. In the image below, the primary side protection (fuse "PD-0001") should be to the right of the inrush point at the bottom and then to the left of the primary side damage curve at the top. However, no matter what fuse I choose in the SKM library, none of them achieves both of these principals. In fact, I cannot even get close. I am sizing my fuse based on 125%. Am I missing something here? Test project parameters are below for your reference, any help or input is greatly appreciated.

1000kVA Dry Type Transformer
480V Delta - 208Y/120V
PD0001: Gould Shawmut A4BT 600V, 600A fuse
PD0002: Cutler Hammer DSLII-840 4000A circuit breaker
tcc-test.jpg
 
With a 480V primary 1000KVA transformer, the full load current on the primary is going to be about 1200 amps, so the primary fuse should be more like 1500 amps.

The fuse curve shown is shown for the transformer secondary current (notice how a 600 amp fuse crossed the top at around 300 amps? Think of the ratio of 480 volts to 208 volts.)

it looks like the transformer damage curve is also shown for the secondary current, but I believe the inrush point is shown for the primary current.

So I think your fuse curve is for the secondary current, while the inrush is for the primary current. They are apples to oranges. You need to set them so they are apples to apples.
 
As a side note, I'm curious what other people think - Does everyone else expect the primary fuse to prevent the transformer from running into its damage curve?

Or do you assume that job belongs to the circuit breaker on the secondary side?

Or both? Or neither?
 
With a properly sized secondary breaker, you are not limited to 125% for the primary protection.

IMHO, the primary protection is more for short circuits than it is for running overload.
 
jim dungar said:
With a properly sized secondary breaker, you are not limited to 125% for the primary protection.

IMHO, the primary protection is more for short circuits than it is for running overload.
Click to expand...
And, as has been noted elsewhere, the primary breaker can at best protect the secondary winding of the transformer against short circuit or overload. It cannot be used to protect the wires attached to that secondary.
 
GoldDigger said:
And, as has been noted elsewhere, the primary breaker can at best protect the secondary winding of the transformer against short circuit ...
Click to expand...
:thumbsup:

And, protection cannot prevent a short circuit, it can only reduce the amount of damage. Typically the primary protection is to protect the rest of the system from a failed transformer.
 
steve66 said:
With a 480V primary 1000KVA transformer, the full load current on the primary is going to be about 1200 amps, so the primary fuse should be more like 1500 amps.

The fuse curve shown is shown for the transformer secondary current (notice how a 600 amp fuse crossed the top at around 300 amps? Think of the ratio of 480 volts to 208 volts.)

it looks like the transformer damage curve is also shown for the secondary current, but I believe the inrush point is shown for the primary current.

So I think your fuse curve is for the secondary current, while the inrush is for the primary current. They are apples to oranges. You need to set them so they are apples to apples.
Click to expand...

It was a typo, it was supposed to say 1600A... I'll take a look at the reference voltage for all curves and inrush points and let you know what I find. Thanks..
 
jim dungar said:
With a properly sized secondary breaker, you are not limited to 125% for the primary protection.

IMHO, the primary protection is more for short circuits than it is for running overload.
Click to expand...

I've been hearing through others that it is okay to have the primary fuse ride inside the transformer damage curve lines in the long time region towards the top of the curve. In your opinion, as long as I'm to the right of the inrush point in the short time region do you think this will work?
 
IMHO, as reinforced by a client whose transformer failed, the primary protection should clear the transformer before a secondary through fault reaches the damage curve. On a project not designed/built by us, the 18 kV -4.16 kV transformer was destroyed by a phase-phase fault in the bus to the switchgear. Through fault current destroyed the 15 MVA transformer before relaying cleared the unit. Primary protection should clear the transformer by being under and to the left of the lower portion of the damage curve. The upper, overload portion of the damage curve is covered by the secondary protection.

Your issue may be trying to get a fuse to do both overload and short circuit protection. Use the secondary breaker for overload.

Note that your chart shows two damage curves based on frequent or rare through faults. If it is a distribution transformer feeding a few miles of overhead lines subject ot lightining and trees, it will have frequent through faults that it needs to withstand over its life. If it is a commercial or industrial transformer it will probably only see 5-10 faults at most in its lifetime and the infrequent fault damage curve can be used.

Look for a fuse with an almost vertical characteristic to protect the transformer and not blow on inrush. Use the circuit breaker to protect from overload.
 
If you are into the damage curve when fusing at 100%, I would say that the fuse type is wrong or you are misinterpreting the damage curve(s).

Tapatalk!
 
rcwilson said:
Note that your chart shows two damage curves based on frequent or rare through faults. If it is a distribution transformer feeding a few miles of overhead lines subject ot lightining and trees, it will have frequent through faults that it needs to withstand over its life. If it is a commercial or industrial transformer it will probably only see 5-10 faults at most in its lifetime and the infrequent fault damage curve can be used.
Click to expand...

Just so we're clear, the frequent through faults are represented by the mechanical damage curves, which are the lower "legs" of the damage... and the infrequent faults are the two standard curves that go from 2s - 1000s. Am I correct? I am confused by the two main damage curves, all the white papers I come across refer tot hem as primary and secondary thermal limit curves.
 
Have a look at

http://www.skm.com/applicationguides18.html

It says

LV Transformer Fused Switch Feeder Unit

Class J (<600A) and Class L (>600A) fuses are typically used in fused switches serving LV transformers. The fuse characteristics are plotted on a phase TCC along with the transformer and feeder damage curves.

The purpose of the fuse is to allow for full use of the transformer, and to protect the transformer and cable from faults. To accomplish this, the fuse curve should be to the right of the transformer inrush point and to the left of the cable damage curve. Typically the fuse will cross the transformer damage curve. The secondary main device provides overcurrent protection for the circuit. LV Transformer Fused Switch Feeder Unit
 
rcwilson said:
Through fault current destroyed the 15 MVA transformer before relaying cleared the unit. Primary protection should clear the transformer by being under and to the left of the lower portion of the damage curve. The upper, overload portion of the damage curve is covered by the secondary protection.
Click to expand...

I believe any transformer larger than 7.5MVA should have at least two protective device curves for proper protection. This could be a fuse and a differential relay, it could be an overcurrent relay with multiple steps in its curve, or even multiple fuses. But complete protection can almost never be done with a single fuse.
 
dkidd said:
Have a look at

http://www.skm.com/applicationguides18.html

It says

LV Transformer Fused Switch Feeder Unit

Class J (<600A) and Class L (>600A) fuses are typically used in fused switches serving LV transformers. The fuse characteristics are plotted on a phase TCC along with the transformer and feeder damage curves.

The purpose of the fuse is to allow for full use of the transformer, and to protect the transformer and cable from faults. To accomplish this, the fuse curve should be to the right of the transformer inrush point and to the left of the cable damage curve. Typically the fuse will cross the transformer damage curve. The secondary main device provides overcurrent protection for the circuit. LV Transformer Fused Switch Feeder Unit
Click to expand...

This was just what I need... I swear I was on SKM's website and must've looked at every tutorial except this one :roll: Thank you!
 
lielec11 said:
This was just what I need... I swear I was on SKM's website and must've looked at every tutorial except this one :roll: Thank you!
Click to expand...

Based on what I've seen in the field doing coordination studies, the MV fuses typically do cross the middle of the transformer damage curve AND the inrush is to the right. Obviously, this isn't what you want, but these transformers have been in service for 20+ years without problems. I've been told to indicate that the transformers are working without any reported tripping problems upon energization and move on. The crazy thing is that these are almost always packaged units (the transformer and integral fuses are straight from the manufacturer). I've also been told that 12x FLA at 0.1 s is an assumed inrush point and is not typically the real world value of a transformer.

I have been looking for more clarification on where exactly the cross-over point of the damage curves exists between a fuse being acceptable and not acceptable... no luck so far.

Keri :D:D
 
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