Transformer Ratio Test

The performance of a transformer largely depends upon perfection of specific turns or voltage ratio of transformer. So transformer ration test is an essential type test of transformer. The voltage should be applied only in the high voltage winding in order to avoid unsafe voltage.

Ratio test of transformer and check of phase displacement

Actually the no load voltage ratio of transformer is equal to the turn ratio. So ratio test of transformer

Procedure of transformer ration test
1) First, the tap changer of transformer is kept in the lowest position and LV terminals are kept open. 2) Then apply 3-phase 415 V supply on HV terminals. Measure the voltages applied on each phase (Phase-Phase) on HV and induced voltages at LV terminals simultaneously. 3) After measuring the voltages at HV and LV terminals, the tap changer of transformer should be raised by one position and repeat test. 4) Repeat the same for each of the tap position separately.

The above transformer ratio test can also be performed by portable Transformer Turns Ratio (TTR) Meter. They have an in built power supply, with the voltages commonly used being very low, such as 8-10 V and 50 Hz. The HV and LV windings of one phase of a transformer are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector.

Let's have a discussion on Transformer Turns Ratio (TTR) Meter method of turn ratio test of transformer
A phase voltage is applied to the one of the windings by means of a bridge circuit and the ratio of induced voltage is measured at the bridge. The accuracy of the measuring instrument is < 0.1 %