What core magnetic balance test of transformer?
How to do this test in a three phase Star delta (star-LV,
Delta-HV) transformer?
How to measure and where to measure?
thanks and regards
Austin
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Answer Posted / engr. ziaul ashraf shafi
Answer #5 is fine.. but i want to add something more..
normally this test is made in star side of the transformer.
the result should be varified like following:
1) if 2u-2n is fed by 230 volt.
- the nearest phase (2v-2n) will experience the 85% to 90%
of fed voltage at 2u-2n. (i mean 2v-2n = 195.5 to 207
volt). And the distant phase (2w-2n) will experience the
10% to 15% of fed voltage at 2u-2n. (i mean 2w-2n = 23 to
34.5 volt).
2) if 2v-2n is fed by 230 volt.
- then the fed pahase is middle phase, and both other phase
are magnetically in equal distance.. so the measured
voltages in 2u-2n or 2w-2n = 50 % (or 40 to 60 % is also
accaptable). so the 2u-2n or 2w-2n = 92v to 138v is
accaptable.
3) if 2w-2n is fed by 230 volt.
- the nearest phase (2v-2n) will experience the 85% to 90%
of fed voltage at 2w-2n. (i mean 2v-2n = 195.5 to 207
volt). And the distant phase (2u-2n) will experience the
10% to 15% of fed voltage at 2u-2n. (i mean 2w-2n = 23 to
34.5 volt).
*If the results are found beyond these above mentioned
limits then other tests (like open ckt test with rated
voltage) should be performed to be sure about core and
winding.
*You can measure and note down the current of each injected
phase. it will give u a signaficant idea about winding. if
the winding has an inter-tern short then the faulty phase
will draw comapretively large magnetizing current like 3A
or 5A, wheather other phases will draw 60mA to 100mA.
*Remember, the magnetic reluctance path of a core is not
lenier, so the result veries itself. But big differences
are remarkable.
*Sometime due to non-lenier characteristic of magnatic
circuit and the presense of harmonics in the fluxes of un-
excited phases, the algebric sum of induced volage in two
phases may not be equal to the applied voltage.
*if there is a turn to turn fault in any phase winding, the
faulty section will draw comparetively large magnatizing
current which will oppose the magnetic fluxes and will
result a very low induced voltage (tends to zero) in the
faulty phase.
*this test should be performed after demagnaizing the
transformer properly. slightly magnatized transformer (like
after insulation test) can give you an unexpacted and
distorted result of magnetic balance test.
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