Transformer turns-ratio test (TTR) — why this reveals a different fault than oil analysis or the Buchholz relay

The transformer oil analysis guide (BDV/DGA) and the Buchholz relay guide cover ways to detect a developing problem in a transformer that is already in service. This article covers a tool that is applied precisely before the transformer is ever placed on voltage and load (during factory acceptance, after transport, after maintenance, or during periodic offline inspection): the turns-ratio test, commonly referred to by the abbreviation TTR (Turns Ratio Test).

What the test measures

A TTR test set applies a low, safe AC test voltage (typically a few volts to a few tens of volts, far below nominal operating voltage) to the winding with the highest number of turns, and measures the induced voltage on the corresponding winding on the other side. The ratio between the two voltages — the turns ratio — should match the ratio of the number of turns between the two windings, and therefore the ratio stated on the nameplate for the relevant tap position.

The test is performed:

Per phase (so, for a three-phase transformer, three separate measurements, or combined via a three-phase test set).
Per tap position of the voltage-regulating transformer (see also the OLTC/regulating transformer guide), because each tap position should give a slightly different turns ratio, and the OLTC mechanism itself can be checked for correct operation across the full tap range via this test.

What a deviation reveals

A measured turns ratio that deviates from the nameplate value (with a common acceptance limit of roughly 0.5% deviation, though this can differ by application standard or test set manufacturer) points to one of the following problems:

Shorted turns within one winding — a common internal failure mode that lowers the effective turns ratio of that phase, and which could go unnoticed without this test until the transformer overheats or burns out under load.
An incorrectly connected or faulty tap changer — the measured ratio at a specific tap position then does not match the expected value for that position, pointing to a contact problem or an incorrectly positioned OLTC.
An open connection or a poor joint within a winding — instead of a small deviation, this often gives a completely unusable or infinite reading.

Note: the exact acceptance limit for the turns ratio, and the precise test voltage and procedure, follow from the applicable product standard (for example IEC 60076-1) and the transformer manufacturer's recommendations — this article covers the principle of the test, not a universal figure.

Why TTR detects something different from oil analysis or the Buchholz relay

Oil analysis (BDV, DGA) and the Buchholz relay both detect a problem via a by-product of a developing fault — a reduced breakdown voltage of the oil, dissolved gases, or released gas bubbles. These methods only work while the transformer is actually on voltage and/or load and the problem has already been active for some time. TTR, by contrast, measures an electrical property of the winding itself, entirely independent of the oil or operating conditions, and can therefore reveal an already-existing, dormant winding problem before the transformer has ever been put on power — for example immediately after manufacture, after transport (where mechanical shocks can damage turns), or after a repair.

Practical relevance

TTR is a standard part of the factory acceptance test (FAT) of a new power transformer, of the test performed after transport to the installation site (to rule out transport damage), and of periodic offline maintenance where the transformer is already de-energised anyway. Combined with a winding resistance measurement and a polarisation index/DAR insulation test, the TTR test gives a fairly complete offline picture of the winding's electrical health, complementary to the oil- and gas-based methods that are specifically intended for continuous monitoring during service.

Common mistakes

Treating TTR as a replacement for oil analysis or the Buchholz relay — the test detects a different type of problem (winding geometry/continuity) and at a different moment (before service, not continuously during service).
Testing only one tap position instead of the full tap range of the tap changer — a contact problem on one specific position can thereby be missed.
Comparing test results against a generic expected value instead of the actual nameplate ratio for the specific tap position of that specific transformer.
Not retaining a reference measurement from the first (factory or commissioning) TTR test — without a reliable baseline, a later deviation is harder to interpret unambiguously as an actual change.