Transformer Protection regarding Frequent Fault Damage Curve

[ish1284]

I was wondering in what applications a transformer frequent fault damage curve can be ignored when setting primary and secondary protective devices. I have heard that there are certain uses where a transformer is susceptible to damage from frequent faults and other applications where not. I was wondering what the rule of thumb was if there is any. Thanks!

 

[JoeStillman]

I can imagine ignoring it with a primary breaker or fuse as a way of coordinating with a secondary main, but only as long as the secondary main coordinates with the damage curve. Is there a reason why you can't keep the secondary main below the damage curve?

"Frequent Faults" sounds like a scary term. Is it defined anywhere for the sake of generating the damage curve?

 

[gray.one]

The frequent fault portion of the curve applies when the secondary of the transformer is overhead lines. With overhead lines you get things like tree branches that cause a fault and burn clear.


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[Sahib]

I was wondering what the rule of thumb was if there is any. Thanks!

IMO, a street transformer with network protector is allowed to feed external fault without the network protector operating but if it develops any internal fault causing power flow to reverse, the network protecter operates to remove the transformer from the power network.

 

[rian0201]

imo, i don't care if the transformer experiences frequent faults or not. it is a basic protection principle that protection should be below to damage curves.
at certain moment, it may not experience frequent faults, but we cannot be sure it will remain like at all times.

There might be times, that a fault that will occur is very low and you may treat it as overload because of its value, but still it is a fault. And it could occur on whatever system. so i think it is best to keep it below the damage curve, and it cant achieved it, determine what could have been differently.


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[mivey]

imo, i don't care if the transformer experiences frequent faults or not. it is a basic protection principle that protection should be below to damage curves.
at certain moment, it may not experience frequent faults, but we cannot be sure it will remain like at all times.

There might be times, that a fault that will occur is very low and you may treat it as overload because of its value, but still it is a fault. And it could occur on whatever system. so i think it is best to keep it below the damage curve, and it cant achieved it, determine what could have been differently.


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Some basic protection schemes actually cross the damage curve. By always protecting to the left of the damage curve, some schemes will not allow you to take advantage of the transformer overload capability. There are trade-offs with protection schemes and one must always look beyond any single rule.

 

[mivey]

I was wondering in what applications a transformer frequent fault damage curve can be ignored when setting primary and secondary protective devices. I have heard that there are certain uses where a transformer is susceptible to damage from frequent faults and other applications where not. I was wondering what the rule of thumb was if there is any. Thanks!

In some cases the mechanical damage portion of the curve is dismissed. IEEE standards cover the different fault cases. Four or five cases but I don't recall off the top of my head or recall the standard number. I would look but am not where I can right now.

 

[rian0201]

Some basic protection schemes actually cross the damage curve. By always protecting to the left of the damage curve, some schemes will not allow you to take advantage of the transformer overload capability. There are trade-offs with protection schemes and one must always look beyond any single rule.

yes i agree, there are trade offs, but like what i have said, what could have done differently? if i can set the devices that there will be no trade offs, much better.


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[Sahib]

I believe that damage curve of a transformer is of little use when the transformer is part of utility network. May be because of that, many external faults are allowed to burn free to clear in utility network.

 

[mivey]

I believe that damage curve of a transformer is of little use when the transformer is part of utility network. May be because of that, many external faults are allowed to burn free to clear in utility network.

Utilities use the transformer damage curve. Many faults can clear without damaging the transformer but that does not mean the utility ignores the damage curve.

 

[Sahib]

Utilities use the transformer damage curve. Many faults can clear without damaging the transformer but that does not mean the utility ignores the damage curve.

Would you please elaborate, because it's stated that an electricity metering arrangement may not be placed inside a building as there is no o/c protection to upstream service conductors?

 

[mivey]

Would you please elaborate, because it's stated that an electricity metering arrangement may not be placed inside a building as there is no o/c protection to upstream service conductors?

What does that have to do with transformer protection as was being discussed?

 

[Sahib]

What does that have to do with transformer protection as was being discussed?

The utility transformer secondary O/C protection, if provided, is also for transformer itself. So would you answer my question?

 

[mivey]

The utility transformer secondary O/C protection, if provided, is also for transformer itself. So would you answer my question?

Primary side protection provides fault protection for the transformer with the damage curve being factored in: thus the damage curve is used when the transformer is part of the utility network.

 

[philly]

Primary side protection provides fault protection for the transformer with the damage curve being factored in: thus the damage curve is used when the transformer is part of the utility network.

Keep in mind the transformer damage curve only applies to thru-faults that occur on the secondary of the transformer, and not within the transformer itself. I see quite often people mis-interpreting the damage curve as applying to internal transformer faults.

Also for Delta-Wye transformers the damage curve is shifted to account for a secondary ground fault appearing as on overload on the primary. I believe the primary device should be below the sifted damage curve however the secondary device only need be below the original non-shifted damage curve which if further to the right than the sifted one.

 

[mivey]

Keep in mind the transformer damage curve only applies to thru-faults that occur on the secondary of the transformer, and not within the transformer itself. I see quite often people mis-interpreting the damage curve as applying to internal transformer faults.

By definition, an internal fault means the transformer is damaged. We can differential substation transformers to watch for internal faults (along with other means). Sudden pressure indicators on distribution transformers can indicate internal faults. We do not want to re-energize after an internal fault and make matters worse.

Also for Delta-Wye transformers the damage curve is shifted to account for a secondary ground fault appearing as on overload on the primary. I believe the primary device should be below the sifted damage curve however the secondary device only need be below the original non-shifted damage curve which if further to the right than the sifted one.

Primary fuses often cross the damage curve for the reasons given before.

 

[Sahib]

I was wondering in what applications a transformer frequent fault damage curve can be ignored when setting primary and secondary protective devices. I have heard that there are certain uses where a transformer is susceptible to damage from frequent faults and other applications where not. I was wondering what the rule of thumb was if there is any. Thanks!

In applications where number of through faults will not exceed a specific number, as defined in relevant ANCI/IEEE standard, throughout transformer rated life, then frequent fault damage curve which also takes into account mechanical effect of through fault on transformer may not be used; just ordinary transformer damage curve may be used.