Turns ratio testing is one of the fastest, most definitive checks available on a de-energized transformer. It directly verifies the electrical relationship between windings and catches shorted turns, LTC connection errors, and winding damage that other tests may not isolate as clearly. A ratio that deviates from nameplate by more than a fraction of a percent is always worth investigating.
The turns ratio of a transformer is the ratio of the number of turns in the high-voltage winding to the number of turns in the low-voltage winding. For an ideal transformer, this ratio equals the voltage ratio: a transformer with a 10:1 turns ratio steps 115 kV down to 11.5 kV. The nameplate voltage ratio and the actual turns ratio must match within tolerance for the transformer to perform as designed.
On a transformer with an LTC or de-energized tap changer, each tap position changes the effective number of turns in the winding, changing the ratio. TTR testing on every tap position verifies that every tap is connected correctly and that the LTC mechanism is moving to the right tap position when commanded.
A TTR test set applies a low AC voltage — typically a few volts to a few hundred volts — to the high-voltage winding of one phase and measures the induced voltage on the low-voltage winding. The ratio of applied to induced voltage is the measured turns ratio. The test set compares this to the calculated nameplate ratio and reports the deviation as a percentage.
All three phases are tested, and on a transformer with an LTC, every accessible tap position is tested on each phase. A complete TTR test on a full-range LTC transformer may generate dozens of data points, but each measurement only takes seconds. The full test on a three-phase transformer typically takes 30 to 60 minutes depending on the number of taps.
The test is run with the transformer de-energized and isolated but does not require draining oil or opening the tank. It is commonly run alongside the power factor test as part of a complete maintenance or acceptance test sequence.
IEEE C57.12.90 specifies that the measured turns ratio should be within 0.5% of the calculated nameplate ratio for most transformers. Some manufacturers specify tighter tolerances on specific designs. A deviation within 0.5% on all three phases and all tap positions confirms the winding configuration matches nameplate and the LTC connections are correct.
The three phases should also be consistent with each other. On a balanced three-phase transformer, the ratios on all three phases should be essentially identical at the same tap position. A reading that is within 0.5% of nameplate but significantly different from the other two phases — even if it nominally passes — is worth a second look. Phase-to-phase imbalance in the ratio can indicate turn shorts in one phase that cancel out to produce a ratio that still falls within tolerance.
Shorted turns reduce the effective number of turns in the affected winding. A shorted portion of the high-voltage winding reduces the measured ratio; a shorted portion of the low-voltage winding increases it. Even a small number of shorted turns — a few out of hundreds — produces a measurable deviation. This is one of the most important uses of TTR testing: shorted turns often produce no obvious effect during normal operation until the fault progresses to a more serious failure, but they show up clearly in the ratio.
Wrong tap position is the most common cause of an out-of-tolerance reading and the easiest to resolve. If the LTC is on the wrong tap, the measured ratio corresponds to the actual tap, not the intended one. Comparing the measured ratio to the ratio expected for each available tap position identifies which tap the LTC is actually on. This can be a wiring error in the LTC control, a mechanical jam that left the LTC one position off, or simply a documentation error in the test setup.
Open-circuit turns — a break somewhere in the winding — produce a very large deviation or a test set error rather than a slight deviation. An open in the high-voltage winding can cause the test set to read an essentially infinite ratio or fail to obtain a stable reading. This is a clear failure indicator.
LTC contact or connection problems show up as inconsistent ratios across tap positions or as a ratio that jumps unexpectedly when the LTC is stepped through its range. If the ratio is correct at tap 1, wrong at tap 2, and correct again at tap 3, the LTC connection for tap 2 is suspect — either a loose connection or a contact that is not seating properly at that position.
TTR is standard practice during acceptance testing of a new transformer before it is energized. It confirms that the unit was built to nameplate, that it was not damaged in shipping, and that connections are correct before the transformer is put into service. A transformer that fails TTR at acceptance does not get energized — it goes back to the manufacturer or is subjected to further diagnostic investigation.
After a through-fault event — when the transformer has seen a nearby fault and the resulting fault current — TTR is one of the first tests run to assess whether winding deformation has occurred. Severe through-fault currents create electromagnetic forces that can mechanically deform the winding, pulling turns together or distorting coil geometry. Deformed windings can develop turn-to-turn shorts that appear as ratio deviations. A TTR test that matches the pre-fault baseline gives confidence that no winding damage occurred. A deviation that was not present before the fault demands investigation before the transformer is returned to service.
We perform TTR, power factor, winding resistance, excitation current, and SFRA testing on power transformers across Florida and the Southeast. Send us your scope and we’ll respond within one business day.