Power Transformer Through Faults 3

[Zenmatters]

I am interested in two aspects of Power Transformer Through Faults:

First, Have you had a transformer failure that was attributed to the aggregate accumulation of transformer through faults over the course of time? If so I would be interested in the details.

Second, I am interested in knowing of effective formal programmatic plans that identify and evaluate the aggregate effects of transformer through faults as a fault preventation and mitigation measure.

Thanks for your time.

 

[rbulsara]

Research assignment??

 

[Zenmatters]

Just due diligence. I have used some industry search engines and am trying to make the case that this phenomena happens more frequently than many people realize and want to lay out a proposed program for contending with this phenomena.

 

[dpc]

The average number of through faults will vary significantly depending on the transformer application. Distribution substation transformers are normally subjected to a high number of through faults during the transformer life, while many industrial/commercial transformers will see a limited number.

It is difficult to determine if a transformer failed due to excessive through-faults since the damage done is cumulative.

In a few cases, a single through-fault will destroy a transformer due to breaker failure. But this is rare. Normally it is the combined effect of numerous through-faults over many years (along with transformer overloading).

There no question that through-faults damage transformers - that's why ANSI establishes transformer damage curves.

 

[apowerengr]

I've seen transfomrer winding failures on through-faults. Since the protection was coordinated with the transformer I'd say the through fault caused the failure (telescoping winding failure - turn-to-turn). That's why I've started requesting keyed windings or CTC for winding conductors. I feel the greater surface area on the CTC provides better mechanical performance in fault conditions. Any of you transformer guru's - please correct me if I'm wrong.

 

[prc]

The short circuit current withstand strength of transformer depends on the magnitude of it and also the number of faults during a time period.It is well accepted that in the life of a tranfsormer it is very rare that the unit will see the worst assymetric SC current on it as it depends on the system fault level(rarely it is infinitive and always system impedance will be limiting the current. Maximum assymetry (up to 2.6 times ) will occur only in case fault happens when sinusoidal voltage is near to zero)

Number of faults do have an effect on the SC withstand strength of transformer.Certain types of transformers have to meet several faults in a day eg : Rail Traction track side transformers supplying power to overhead lines,transformers supplying power to steel arc furnaces.Such transformers are specially made and prototype unit is short circuit tested at high Power laborataries.

Extra Short circuit withstand strength is built in to transformers by special design/manufacturing and processing measures.CTC(continuously transposed cable)will not add any SC withstand strength.In fact SC strength is less due to smaller dimensions of the several parallel enamelled conductors constituting the CTC.But special CTC (with hardened conductors and with additional semi cured coating over enamelled conductors-which during drying process will cure and bond all conductors together giving higher buckling strength)will give extra mechanical strength for inner windings to with stand buckling forces experienced during fault current flow.

 

[jghrist]

See IEEE guide for liquid-immersed transformer through-fault-current duration, IEEE Std C57.109-1993.