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Transformer Riddle No.46 - Magnetic Balance test
During magnetic balance test in auto transformers, It is experienced that during U & W phases magnetic balance test the addition of voltage measured in other limbs is 5 to 25% more than the voltage applied. Why?
Please share your experience...

#1
Thu, July 29th, 2010 - 14:26
For others information I try to explain the transformer magnetic balance voltage test then to reply your question.
Magnetic Balance Voltage should be within ± 5 per cent.
It is recommended a tolerance of (+) 15 per cent on the component losses, subject to a maximum of (+) 10 per cent on total losses. However, most of the power utilities do not go by such recommendations. It is, therefore, advisable to follow the customer’s specification with regard to tolerance on losses and no-load current.
Single phase voltage of pre-determined value is fed to one of the two primary terminals and voltage across each of the secondaries with respect to neutral are measured and recorded.

(a) Single phase AC voltage of pre-determined value, preferably 400 to 500 V is applied between
the primary terminals (1 U–1 V) and voltage across all the secondaries with respect to neutral
are recorded.
(b) Single phase AC voltage of same magnitude as above is again applied between the primary
terminals (1 V–1 W) and voltage across all the secondaries with respect to neutral are recorded.
(c) The test is repeated with single phase AC voltage of same magnitude applied between primary
terminals (1 W–1 U) and voltage across all the secondaries with respect to neutral are recorded.

A typical case of measurement done on 5 MVA, 33.11 kV transformer is given in Table

Conclusion
(i) Sequence (a)    (2 U – 2 N) = (2 V – 2 N) + (2 W – 2 N)
84 V = 56 V + 28 V
and (2 V – 2 N) > (2 W – 2 N)
56 V > 28 V
(ii) Sequence (b)   (2 V – 2 N)= (2 U – 2 N) + (2 W – 2 N)
84 V = 42 V + 42 V
and (2 U – 2 N) = (2 W – 2 N)
42 V = 42 V
(iii) Sequence (c)  (2 W – 2 N) = (2 U – 2 N) + (2 V – 2 N)
84 V = 32 V + 52 V
and (2 U – 2 N) < (2 V – 2 N)
32 V < 52 V
However, the relations (i), (ii) and (iii) are subject to a tolerance of ± 5 per cent.

But about your question, when you apply the voltage in two primary terminal, because of electrical and magnetically links you may face to additional vector voltage of an electrical element (U/2) and percentage of consequent induced voltage in other limb (k.U) between third terminal and earth.

V= U/2+k.U
where k is related to transformer magnetic circuits.

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