Transformer Yoke and Frame 1

[camo1978]

Many of the power transformers I have worked on have there core and Yoke shorted to the transformer case when in service. This is done through external links which are removable. Can any one explain to me why these links are put there - why not solidly connect the core and yoke to the chasis. What would occur if these links were left off?

 

[oldfieldguy]

The link results in a single-point ground. If there were more than one point of grounding of the core to the tank or other structure, then the result would be the equivalent of a shorted single conductive (more or less) turn within the magnetic path of the transformer. This would result in a high current flowing through the resultant path, and usually shows up as heat and unusual excitation current.

By using a removable link, the user can remove this link and test the core's insulation to ground to determine if a second, unwanted ground of the core exists.

If the links are left off, a charge may build in the core in relation to ground. When it reaches sufficient voltage, it will "punch through" the core to ground insulation. At the very least, this results in some unexpected gases in the oil during your periodic analysis. At the worst, it can deteriorate the oil, and carbonize enough material to result in a full-blown fault of the transformer.

old field guy

 

[dewone]

The yoke connection can also be a high resistance grounding point, a neutral or four wire connection point, or simply grounded direct: low resistance ground. Depends on how your using the transformer.

 

[camo1978]

Thanks guys that makes sense

 

[prc]

1)In case the core and core frame is not earthed,floating potential can develop which will cause arcing in service.This can be severe especially during transients like impulse /lighning surges.There are cases of LV winding failure(LV being innermost) at factory during impulse test when this earthing was forgotten.

2)Removable earthing links are provided so that insulation between core to earth ,core frame to earth and core to core frame can be checked separately at factory, site (to see that no damage during transport) and during service.

3)In small transformers multiple earthing of core frame may not cause any problem.But in large transformers,there will be appreciable leakage flux from winding(zero at no-load and maximum at full load) which will be impinging the core frame and inducing small voltages.This can cause appreciable circulating currents in core/core frame causing heating of metal and sometime even melting of metal.But, I think, this is not due to "shorted-turn effect" as turn effect will come only when a conducting turn or path crosses through the core window.