- Transformer Turn Ratio Definition: The transformer turn ratio is defined as the ratio of the number of turns in the HV winding to the number of turns in the LV winding.
- Voltage Ratio Test of Transformer: This test checks if the voltage ratio matches the expected turn ratio by applying voltage to the HV winding and measuring the induced voltage in the LV winding.
- Testing Procedure: The procedure involves setting the tap changer, applying voltage, measuring voltages, and repeating for different tap positions.
- Use of TTR Meter: A portable TTR meter can perform the test using a bridge circuit to measure the induced voltage ratio accurately.
- Detecting Faults: Deviations in the measured ratio can reveal faults like shorted or open turns and high resistance connections in the transformer.
Transformer Ratio Test
The performance of a transformer largely depends on the accuracy of its turn or voltage ratio. Therefore, the transformer ratio test is essential. To ensure safety, voltage should only be applied to the high voltage (HV) winding.
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 Ratio Test
- First, the tap changer of transformer is kept in the lowest position and LV terminals are kept open.
- 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.
- After measuring the voltages at HV and LV terminals, the tap changer of transformer should be raised by one position and repeat test.
- 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 %. 
The theoretical turn ratio is adjusted on the TTR meter by changing the settings on the adjustable transformer until the percentage error indicator shows a balance.
The reading on this indicator implies the deviaton of measured turn ratio from expected turn ratio in percentage.
Out-of-tolerance, ratio test of transformer can be due to shorted turns, especially if there is an associated high excitation current. Open turns in HV winding will indicate very low exciting current and no output voltage since open turns in HV winding causes no excitation current in the winding means no flux hence no induced voltage.
An open turn in the LV winding causes low, fluctuating LV voltage but normal excitation current in the HV winding. This results in normal exciting current but very low and unstable output voltage. The turn ratio test can also detect high resistance connections in the leads or high contact resistance in tap changers, indicated by higher excitation current and difficulty in balancing the bridge.
