Potential Transformer Primary Fuse Basis for Sizing 2

[brainsalad]

Greetings:

Would be grateful for your thoughts regarding the origin of any bases used for the inrush capability of primary fuses for potential transformers.

I have a GE JVM-3 PT (2400-kV, 750-VA) with an EJ-1 fuse. Also an ancient ITE PT-6 PT (600-VA, 480-V) with Gould A6Y7E-96 fuses......

Can anyone recall the origin or basis of the requirement to have primary fuses tolerate at least 12x (or 25x) rated primary current?

Thanks for your help.

 

[waross]

The fuses are there to protect the source and the wiring to the PTs. PTs by nature of their duty are not liable to overloads. Any internal fault in a PT which results in a blown fuse is not going to be prevented by the fuse clearing. However, if a PT shorts internally you don't want the PT primary conductors vaporizing, hence the use of primary fuses for PTs.
A transformer may take up to 25 X FLC during the energization inrush. Many fuses rated at 12 X FLC will carry through an energization inrush due to the thermal lag, but some designers may prefer the added safety of 25 X FLC rated fuses.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

Thank you Waross. Is there a trend to avoid primary fuses in MV or HV PTs? To develop a current of 12 or 25 FLC fault current from an interturn fault in PT will may take quite a bit of time and by then PT may have blasted.Recently I saw in 25 kV Pts( electromagnetic) used in Railway Electrificatication with out fuses.But earlier projects they did use fuses.

Any change in fuse setting if Pts are of capacitor voltage type instead of electromagnetic?

 

[waross]

First, I am not in a part of the industry where I can comment in an informed manner as to present practice.
Possibly some others will post in here.
As to interturn faults, the purpose of the fuse is not to protect the transformer, it is to protect the wiring feeding the transformer.
When an inter turn fault does develop into a short, the fuses will disconnect and prevent the primary conductors from vaporizing and destroying the cubicle.
Interturn faults tend to be short lived. The heat generated causes the fault to deteriorate quite rapidly.
Even a short on the transformer secondary should not blow the primary fuses on a PT.
Consider a PT fed with #10 AWG conductors. The thousands of Amps developed by a fault will exceed the withstand capacity of the #10 AWG conductors and most likely vaporize the conductors.
The purpose of the fuses is to avoid this possible damage.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

In a typical installation with 750 VA 15 kv PT's, the primary fuses are 0.5E (half an amp. Since the transformer ratio is 120:1, this translates to 60 amps of secondary current. Again, speaking of a typical installation, the secondary fuses are 5 amps. I never expect the primary fuses to blow on a secondary fault. NEVER!

Further, if the primary fuses DO blow, my maintenace staff is told "DO NOT replace the fuses and re-energize the PT until tests have been done but a competent and knowledgeable testing technician."

Internal faults in a PT are often of a nature that does not show up on the typical insulation resistance (DC) tests, no on low voltage ratio tests, a subsequent energization of a failed PT may result in catastrophic (read 'explosive') failure of the PT and much switchgear damage.


old field guy

 

[waross]

Thanks for filling the gaps, OFG

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

I have experienced some problems with these 0.5E and 1E fuses failing (open) for no apparent electrical reason. I think these fuse elements are very fragile and sometimes open up due to rough handling, vibration, or thermal cycling. But we do specify fuses for 15 kV class and below.

Utilities generally do not fuse high-voltage PTs, at least in my limited experience.

 

[oldfieldguy]

dpc-

I have seen a few installations where the 0.5E fuses blew fairly often on energization of the gear. The last one that comes to mind sits across the fence behind my office, and they used to lose power several times a week. About one in twenty of those events resulted in a blown PT fuse on power restoration. We diligently tested PT's and finally decided to go up to 1E fuse. Problem solved.

This was on an indoor, metal-clad lineup. The practice in the utility I used to work with was mixed. At transmission voltages, no fuses. Of course, do YOU want to find or pay for a 230-kV, one-amp fuse?

At distribution voltages, fuses were common.



old field guy

 

[prc]

I once came across a case of 25 Kv single phase dry type PT blowing out( may be due to ferro-resonnance)Secondary winding was in tact and failure was in primary winding. To prevent blowing out and cause collateral damage, some of the suggestions were to use a Primary fuse or use a thin wire to connnect to source( meaning it will melt before the PT burst). Do you think these alternatives will be effective?

 

[waross]

The thin wire may light up an arc, phase to phase or phase to ground.
It depends.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

True waross.But will it prevent bursting of PT?

 

[oldfieldguy]

Use a proper fuse. if there is an event that melts the "thin wire", when it burns out, it's going to leave in its place a long thin bit of plasma, ready to conduct where ever it lands. This is prescription for disaster. It might prevent the PT from bursting, but that fact may be hard to determine with all the of the other debris from the follow-on fault in the area.

The construction of the fuse is such that the burning of the current-carrying element is quenched by non-conductive materials.

old field guy

 

[prc]

Tahank you waross & oldfieldguy!

 

[stevenal]

"if there is an event that melts the "thin wire", when it burns out, it's going to leave in its place a long thin bit of plasma, ready to conduct where ever it lands."

Exactly how staged fault testing and arc flash testing is performed.

 

[busbar]

No one has cited this one yet…

From IEEE C57.13 7.7 Short-circuit capability
Voltage transformers shall be capable of withstanding for 1 s the mechanical and thermal stresses resulting from a short circuit on the secondary terminals with full voltage maintained on the primary terminals. “Capable of withstanding” shall be interpreted to mean that, if subjected to this duty, the voltage transformer shall show no damage and shall be capable of meeting the other applicable requirements of this standard.

 

[scottf]

busbar...I think the assumption is secondary fusing is a given, so primary fuses shouldn't have to handle primary current resulting from a secondary short-circuit.

 

[waross]

I think that we are talking about the case of an internal short in the transformer. Once a transformer shorts turn to turn internally, the listed impedance values are meaningless.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scottf]

As is stated above, primary fusing is not going to stop a VT from failing due to a shorted turn. The current draw prior to failure is generally no high enough to trip a 0.5E or 1.0E fuse that is typically used.

As stated, the purpose of fusing the primary of a VT is to keep the system protection from operating in the event of a VT failure, i.e. clear the VT from the line.

 

[prc]

Scottf, I felt some contradiction.In first sentence you are saying that interturn fault will not result in fuse operation.Next sentence it is said that fuse will operate in the event of a PT failure.

I thought, may be interturn fault will not create fuse operation immediately.But it will lead to higher currents and then fuse will save from a violent explosion of PT

 

[scottf]

prc-

What I'm saying is that a primary fuse will generally not operate until AFTER the VT blows-up/cracks/fails violently. The primary current flow prior to a physical failure is generally not high enough to blow a primary fuse.

In some cases with ferro-resonance, a primary fuse will operate prior to a physical failure, but that depends on the actual failure mode as a result of the ferro-resonance.