Testing on 3 limb VT

[DiscoP]

Hello All
A colleague of mine was recently testing a 415/110 V 3 limb VT. When injecting phase-phase on the 415 V side using a variac the secondary measurements were hopelessly out, but if a full three phase injection was performed with a test set then the secondary measurements were as you would expect.
The VT was totally isolated from the rest of the network.
3 limb VTs won't produce zero sequence voltage but there shouldnt be any zero sequence for a phase to phase injection.
Is there anyone here who might be able to offer an explanation. It is probably something obvious but none of us here can think of it !

 

[RRaghunath]

DiscoP,

This is due to the diffrence in magnetic path reluctance values. To explain further, when you energise 415V/sqrt3 winding on the middle limb, the voltage induced in the 110V/sqrt3 winding on the middle limb will show expected voltage whereas the other two would show half the middle limb winding voltage (for a star / star VT). In case of extreme limbs, the induced voltage will be 2/3 in the adjacent limb and 1/3 in the far limb winding,approximately.
This phenomenon is familiar for the engineers involved in transformer testing and is called Magnetic balance test. Raghunath