Dry Type Transformer Short Circuits 1

[kevd]

I am trying to understand the short circuit mechanism of dry type transformers.

How many short circuits on the secondary should a well manufactured dry type transformer withstand in its lifetime?

What mitigating strategies do transformer manufacturers take to withstand the forces induced?

If type testing to IEC60076-5 is undertaken, how many short circuits are tested?

Thanks
Kev. D.

 

[dpc]

There is not a simple answer to your question. It depends on the magnitude of the short circuit currents and the duration.

ANSI has transformer damage curves that attempt to address this question. For larger transformer, they have separate curves for "frequent" and "infrequent" through-faults.

But every through-fault is stressing the transformer. Some of this is cumulative.

In some respects, tolerance of through-faults is a measure of the quality of the transformer construction. I'm not familiar with IEC test requirements, but I would imagine that testing is based on a small number of through-faults and not every transformer is tested - only a few that are representative of the manufacturer's product line.

 

[peebee]

ANSI/IEEE Std. 141 (Red Book) has a nice discussion of this as well as references, includes graphs of the damage curves mentioned by dpc, and has references to other ANSI standards. All information below is summarized from the Red Book:

At 400 to 600% load, the damage curve is approximated by I**2t, a straight line (log-log) with -2 slope extending to and terminating at the trasformer withstand point.

The break between "frequent" and "infrequent" is more than 5 faults in lifetime (for tx's thru 5000 kVA 3-phase) or more than 10 in lifetime (over 5000 kVA 3-ph). The curve for frequent faults is at a lower fault level than for infrequent faults.

 

[electricuwe]

See thread238-16730

Please consider the following:

All transformer standards I know require that a transformer is able to withstand a short circuit without mechanical damage, but usually the customer has to rely on the ability of the manufacturer to guarantee this by design and sound workmanship.

Performing a short cicuit test is very expensive and is only done if it has been specified by the customer e.g.

- as a type test

- for special transformers for special applications (since it is difficult to get a replacement in this case)

 

[busbar]

It may be apparent that basic premises are that thermal damage varies with I²t, and magnetic effects vary with I_peak. With that, thermal withstand of insulating materials and internal bracing quality and quantity should be seen as critical points affecting long-term durability.

 

[jbartos]

Suggestion: The better, more sensitive short circuit current protection will allow the transformer to withstand more short circuits. I.e. if there is a current limiting fuse (low peak), then the transformer will be disconnected before it reaches its full short circuit current value, e.g. Isc=1.0/Zsc in pu.