25. How many short circuits can a transformer withstand?

A frequent question raised by transformer users is how many external line short circuits can a transformer withstand without loss of life or before a failure in service. I believe it is impossible to give a solid number, due to the numerous service variables involved for the generated forces in windings. The short circuit withstand strength of windings also diminishes based on the extent of insulation aging and cumulative effects of earlier short circuits. Radial and axial forces developed in windings during the short circuit depend on the magnitude of over current flow through the transformer. The peak value of short circuit current varies based on (a) Fault level of feeding source (b) Point on voltage wave when the short occurred (c) Distance of fault from transformer secondary terminal (secondary line impedance) (d) X/R of the system (e) Duration of current flow (fault clearing time by breaker.) Please see part 1 of this series, published in previous weeks. When a transformer is short circuit tested at a power lab, all the above four factors ( a-c) are taken for the worst scenario and three current shots with maximum asymmetrical peak (with fully DC offset) are applied on windings of each phase for 0.25 to 0.5 seconds. But in reality, most of the transformers in service, will never see such over currents in their whole life. Only few units will ever be subjected to even 75% of such peak current (50 % of force) in their service life.

Transformers will be able to with stand short circuits many times during their life, provided the current values are small. Eg. Rail track side supply transformers and furnace transformers will be subjected to short circuits several times in a day. But a severe fault similar to the one done in a power lab, may be withstood by transformers only during the initial stages of life but difficult to stand in later stages due to lowered withstand strength of windings from the ageing of insulation, loosening of windings etc. My understanding today is quality transformers can stand short circuits many times, if the first peak current is less than 50 % of the theoretical maximum short circuit peak current.

Quoting from the ABB book “Short Circuit Duty of Power Transformers” by G. Bertagnolli ed1.0-1996 Clause 10.5 Test Vs Field Experience “Experience shows that, if a transformer has passed the test (short circuit test as per IEC 60076-5) without damage, it seems reasonable to assume that it will certainly be able to stand up to a hostile environment, where many more random short-circuit events are expected as compared with the limited number of test shots (three as per IEC), though they may be much less severe.” This statement reminds users, that they should not be under the false notion that since the transformer was short-circuit tested, care need not be taken to prevent or minimize feeder faults on the down-stream of transformers. Frequent feeder faults do have a cumulative effect in mechanical weakening of clamping pressure of windings, insulation supports and spacers, aggravating the probability of a pre-mature failure of the transformer.

Another interesting point is transformers may work for several years without issues even after a damaging deformity in winding from a severe secondary line short circuit (eg buckling of inner winding or axial tilting of turns) provided, there were no insulation damages in windings to cause any electrical shorts during the short circuit incident.

A caution for transformer short circuit strength in service- old wet transformers shall not be dried fully at site. When moisture is removed fully, press board spacers will shrink, loosening the winding axially and reducing axial clamping forces in winding. This will reduce short circuit withstand strength of windings drastically.

There are two IEEE standards (guides) recommending the permissible symmetrical RMS  short circuit current durations versus current magnitudes for transformers. These guides give recommendations for the permissible exposure time of transformers to short circuit currents so that suitable settings can be used (relay co-ordination) in over current protective devices used with transformers.

C57.12.59-2015 Guide for dry-type transformer through-fault-current duration

C57.12.109-2018 Guide for liquid-immersed transformers through-fault-current duration.

Quoting from C57.12.109 Clause 1.3, quote “The magnitude and duration of fault currents are of utmost importance in establishing a coordinated protection practice for transformers, as both the mechanical and thermal effects of fault currents should be considered. For fault-current magnitudes near the design capability of the transformer, mechanical effects are more significant than thermal effects. At low fault-current magnitudes approaching the overload range, mechanical effects assume less importance, unless the frequency of fault occurrence is high. The point of transition between mechanical concern and thermal concern cannot be precisely defined, but mechanical effects tend to have a more prominent role in larger kilovolt-ampere ratings, because the mechanical stresses are higher” unquote. It means power transformer failures from short circuit currents (when they are high and near to theoretical maximum limit) are due to the dynamic mechanical forces during the first couple of cycles and not due to thermal effects (from long duration). Advantage of fast clearing breakers is that it will limit energy flow in to the arc flash inside tank (in the event of winding failure from mechanical forces due to first peak of current), preventing tank rupture and a transformer fire.

These standards recommend to consider only transformer impedance for estimating fault currents for ratings up to three phase 5 MVA. But sum of transformer impedance and system impedance is considered in current calculation for larger ratings. A typical short circuit current versus permissible time periods for large transformers (more than  30 MVA three phase ) as given in C57.109  is given below.

In case of dry type transformers, heating thermal time constants are rather low compared to liquid filled transformers and hence for a specific rating and impedance, permissible durations are less.

(IEEE C57.109)

Tip of the week:

https://www.usbr.gov/ - Resources &Research > Manuals &Guide Lines & FIST Documents – There are several documents at this web site, in connection with selection and maintenance of power equipment, useful for Power Engineers.

https://www.usbr.gov/tsc/techreferences/mands/mands-pdfs/Trnsfrmr.pdf - See the Manual on Transformer Basics, Maintenance and Diagnostics (256 pages, Year 2008)

https://www.usbr.gov/power/data/fist_pub.html - FIST 3-30-Transformer Maintenance Guide lines, Year 2000

https://www.usbr.gov/power/data/fist/fist3_31/fist3-31.pdf - FDIST 3-31-Best Document on Transformer Diagnostics second only to IEEE C57.152 Standard (71 Pages, Year 2003)

https://www.usbr.gov/power/data/fist/fist3_32/Redacted%20FIST%203-32%20NOV%202016.pdf -FIST 3-32-Transformer Fire Protection (43 pages, Year 2016)