- 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
A transformer is checked by comparing its turn or voltage ratio with the design. The transformer ratio test measures that ratio. Apply the test voltage only to the HV winding so the induced LV stays lower.
Ratio Test of Transformer and Check of Phase Displacement
At no load the voltage ratio of a transformer equals the turns ratio, neglecting a small winding-resistance drop. IEC 60076-1 lists voltage ratio and phase-displacement check as a routine test.
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 same transformer ratio test can be done with a portable transformer turns ratio (TTR) meter. Those meters have a built-in supply, often 8 to 10 V at 50 Hz, far below a 415 V site supply. One HV and one LV winding of a phase are connected to the instrument, and the bridge elements are adjusted until the detector reads a null.
A TTR meter measures the turn ratio test of transformer with a bridge.
A phase voltage is applied to one winding through that bridge and the induced-voltage ratio is read on the other side. Typical accuracy of the measuring instrument is < 0.1 %. That figure is an instrument specification, not an IEC 60076 pass/fail band. 
The expected turns ratio is set on the TTR meter with the adjustable transformer until the percentage-error indicator balances.
That indicator reading is the percentage deviation of the measured turns ratio from the expected turns ratio.
An out-of-tolerance ratio test of transformer can come from shorted turns, especially when excitation current is also high. Open turns in the HV winding give a very low exciting current and no output voltage, because with no excitation current there is no flux and no induced voltage.
An open turn in the LV winding leaves HV excitation current about normal but makes the LV voltage low and unstable. The turns-ratio test can also show high-resistance connections in the leads or high contact resistance in tap changers, seen as higher excitation current and a bridge that will not balance.
