The TTR (Transformer Turns Ratio) test answers a central question: does the transformer's actual turns ratio match its design? A small deviation in this number can conceal a turn-to-turn short, a connection error, or a tap changer problem.
The principle of the test
A TTR instrument applies a low, safe alternating voltage to one winding and measures the voltage induced in the other, calculating the actual ratio with high precision and comparing it to the nominal ratio computed from the nameplate. In three-phase transformers, each phase is tested individually, taking the connection group into account.
When is it performed?
- Upon receipt and before initial commissioning.
- After any fault, major maintenance, or suspicion regarding the windings.
- As part of periodic preventive tests and to monitor tap changer positions.
Interpreting the results
| Result | Possible significance |
|---|---|
| Within tolerance (around 0.5% in common specifications) | The windings and ratio are sound at this position |
| Uniform deviation across all three phases | A tap changer position that doesn't match the assumed one, or an error in the reference ratio |
| Deviation in a single phase | A turn-to-turn short, winding damage, or a connection problem in that phase |
| An extremely abnormal or unstable reading | An open winding or incorrect test connections |
Record the tap changer position before every measurement and compare it to the ratio for that specific position. Comparing a TTR reading to the ratio of a different position leads to a wrong judgment about a sound transformer — or worse, clearing a faulty transformer.
Sample answer: A large deviation in a single phase, with the others normal, points to a localized problem in that phase: a turn-to-turn short (reducing the effective turns), winding damage, or an error/fault in its connections. I would first check the test connections themselves and the tap changer position, then support the diagnosis with a winding resistance test and a comparison across phases, and DGA may be needed if the transformer is an in-service oil-filled unit.
Performing a TTR test without recording the tap changer position, or comparing it against the ratio of the wrong position. The ratio changes with every tap, and this is the most common cause of false "failed" results in the field.
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