Transformer Turns Ratio (TTR) testing (also known as ratio testing or turns ratio testing) is a core and fundamental test in transformer manufacturing, installation, and maintenance. Its main purposes include the following aspects:

1.Verify Design Intent and Manufacturing Correctness:

This is the most fundamental purpose. The turns ratio is one of the most critical design parameters of a transformer, directly determining the proportional relationship between input and output voltage (neglecting losses and excitation current).

The test ensures that the windings were wound with the exact number of turns specified by the design, confirming the transformer can provide the intended voltage transformation function.

2.Detect Winding Defects:

Interturn Short Circuits: This is one of the most important fault detection purposes. Even a short circuit involving only a few turns will significantly alter the effective number of turns in that winding, causing the turns ratio for that phase to deviate from the rated value or become inconsistent with other phases. TTR testing is one of the most sensitive and direct methods for detecting interturn shorts.

Open Circuit Faults: The test can detect problems like broken connections, poor tap changer contact, or broken leads causing a partial or complete open circuit in a winding (an open circuit usually prevents obtaining a valid ratio or yields abnormal results).

Connection Errors: Ensures correct start/finish connections of windings, proper tap changer position connections, and correct phase winding connections (e.g., star/delta configuration) in three-phase transformers.

3.Evaluate Tap Changer Performance:

Testing is performed across all tap changer positions. The purposes are:

To verify that the actual ratio at each tap position matches its design value.

To check if the tap changer's mechanical operation positions correctly and if contacts are sound (poor contact can cause unstable or incorrect ratio readings for that tap position).

To ensure the step change and overall range of ratio variation after tap changer operation meet requirements.

4.Confirm Three-Phase Balance:

For three-phase transformers, the turns ratio of each phase (HV to LV) is tested. The purpose is to ensure that the three-phase ratios are consistent or very close (within allowable tolerance).

Significant differences in ratio between phases may indicate:

An interturn short circuit in one phase winding.

An incorrect connection in one phase winding.

Magnetic circuit imbalance (less common, but TTR combined with other tests can aid diagnosis).

5.Provide Baseline Data for Future Diagnostics:

TTR test results obtained during factory acceptance, before commissioning, or after major overhauls are recorded as critical baseline data for the transformer.

Results from subsequent periodic preventative maintenance tests or fault diagnostics are compared against this baseline data. A significant change (exceeding allowable error limits or deviating from historical trends) is a strong indicator of potential winding problems (especially interturn shorts), signaling the need for further inspection or maintenance.

6.Comply with Standards and Specifications:

Transformer TTR testing is a mandatory routine test specified by international (e.g., IEC), national (e.g., GB), and industry standards (e.g., IEEE) for factory acceptance, commissioning (after installation), and preventative maintenance. Performing the test ensures compliance with these regulations, confirming the transformer's basic performance and quality meet requirements.