Transformer Turns-Ratio Test

What it is

The turns-ratio test compares the number of winding turns on the high-voltage side of a transformer to the number on the low-voltage side, expressed as a ratio, against the ratio printed on the nameplate or calculated from the nameplate voltages for the tap position under test. A TTR test set applies a known AC voltage to one winding and measures the voltage induced in the paired winding, then reports the ratio (and, on most modern sets, the excitation current and phase deviation) directly. NETA lists it as an electrical test item for both dry-type and liquid-filled power and distribution transformers, tested at every de-energized tap position, one phase at a time.

Why it is performed

A transformer only delivers the secondary voltage a downstream system expects if its internal winding connections and turns count are exactly what the nameplate claims. A ratio that is off calls out a shorted or open turn, a miswired tap connection, or a manufacturing defect - any of which can produce an unexpected secondary voltage, unbalanced loading, or an internal fault once the unit is energized. Confirming the ratio before energization, and again after any event that could disturb the windings (a through-fault, tap-changer service, shipping damage), is a direct check against putting a damaged unit into service undetected.

When it is performed

Acceptance: run at every tap position before a new or reconditioned transformer is first energized, as part of the standard electrical test sequence alongside insulation resistance and winding resistance. Maintenance: repeated on a periodic interval or after a triggering event, typically at the transformer's normal in-service tap rather than every tap, and compared against the acceptance-test baseline or the calculated ratio rather than re-derived from scratch.

How it is typically performed

With the transformer de-energized, isolated, and grounded per the shop's safety procedures, any bus, cable, or surge-arrester connections at the bushings under test are disconnected so the transformer stands electrically isolated for the measurement, and the de-energized tap-changer (DETC) is set to the position under test. The test leads connect to the corresponding high-side and low-side bushings for one phase, and the TTR set applies its test voltage and reads the ratio, repeating the measurement at each de-energized tap position and then at each phase before the leads move to the next winding pair. Calculated (nameplate) ratios for each tap are worked out ahead of time so the measured value can be compared as each reading comes in, rather than after the fact.

What gets recorded

For each tap position tested: the calculated (nameplate) ratio and the measured ratio for phases A, B, and C, plus the overall percent deviation of the measured value from the calculated or adjacent-phase ratio. The as-found and as-left de-energized tap-changer position, ambient temperature, and test-instrument identification (make, model, serial, calibration due date) are captured alongside the readings so the result can be reproduced and audited later.

How results are evaluated

NETA ATS-2025 7.2.2.D.3 (and its 7.2.1.1 dry-type counterpart) states the tolerance in explanatory terms: a turns-ratio result should not deviate by more than one-half of one percent from either the adjacent phases' ratio or the calculated ratio for that tap. A result outside that band is flagged for investigation before the unit is energized rather than accepted on the strength of the raw reading alone. The maintenance twin in NETA MTS-2023 carries the same numeric tolerance for units already in service. See the purchased standard for the complete text and any tap- or connection-specific exceptions.