What is transformer turns ratio test definition?

Definition

The Transformer Turns Ratio (TTR) test is a fundamental diagnostic and acceptance test performed on transformers to verify that the ratio of primary winding turns to secondary winding turns is correct and matches the nameplate specifications.

In simpler terms, it confirms that the transformer will step up or step down the voltage by the expected amount. A correct turns ratio is critical for the proper operation, performance, and parallel operation of transformers within a power system.

Purpose of the Test

The TTR test is performed to:

Verify Nameplate Rating: Ensure the manufactured or repaired transformer has the correct ratio as designed and stated on its nameplate.

Assess Winding Integrity: Detect shorted turns within a winding (which would change the ratio) and open circuits.

Check Tap Changer Operation: Verify the correctness of every tap position on a transformer's tap changer. Incorrect operation of a tap changer can lead to improper voltage regulation.

Ensure Parallel Operation: For transformers to be connected in parallel, their turns ratios (and other parameters) must be nearly identical to prevent circulating currents.

Commissioning and Maintenance: It is a standard test during initial installation, after maintenance, or during troubleshooting.

 

The Underlying Theory

The test is based on the fundamental principle of transformer operation:

The voltage transformation ratio is equal to the turns ratio.

This is expressed by the formula:
Vₚ / Vₛ = Nₚ / Nₛ = a
Where:

Vₚ = Primary Voltage

Vₛ = Secondary Voltage

Nₚ = Number of turns on the primary winding

Nₛ = Number of turns on the secondary winding

a = Turns Ratio

For example, a transformer with a ratio of 10:1 should have exactly 10 times more voltage on the primary winding than on the secondary winding when energized.

 

How the Test is Performed

The test involves applying a known low-voltage AC signal to one winding and measuring the induced voltage on the other winding(s). It is performed de-energized (with the transformer disconnected from the power system).

There are two primary methods:

1. Voltage Method (The Classic Method):

A low-voltage (typically 120V or 240V AC) supply is connected to the high-voltage (HV) winding.

The voltages on the HV and low-voltage (LV) windings are measured simultaneously using two voltmeters.

The measured ratio (V_HV / V_LV) is calculated and compared to the nameplate ratio.

Limitation: The accuracy of this method depends on the accuracy of the two voltmeters.

2. Using a Specialized TTR Test Set (Modern Method):

This is the most common and accurate method used in the field. A dedicated TTR test set (e.g., from Megger, OMICRON, or HVVI) is used.

The instrument contains a variable voltage source and a precision bridge circuit that compares the transformer's ratio to a known internal reference ratio.

The operator connects the leads to the transformer windings, selects the expected ratio, and the instrument automatically applies voltage and gives a direct reading of the percent error (%) or the actual measured ratio.

A null meter (galvanometer) on the device is adjusted until it reads zero. The reading on the dial at this null point indicates the ratio.

Process for Tap Changers:

The test is repeated for every tap position.

As the tap changes, the ratio should change in precise, predictable steps. Any deviation on a specific tap indicates a problem with that particular tap connection.

 

Interpreting the Results

A Good Result: The measured ratio matches the nameplate ratio within a very small tolerance (often <±0.5%). For a TTR test set, the percent error is very close to 0%.

A Bad Result:

Shorted Turns: A short circuit between turns effectively reduces the number of turns in that winding. This causes the measured ratio to deviate from the nameplate value.

On an HV winding: The ratio will be lower than expected.

On an LV winding: The ratio will be higher than expected.

Open Windings: An open circuit will result in no voltage induced on the other winding.

Faulty Tap Changer: A specific tap position showing an incorrect ratio indicates dirty, worn, or misaligned contacts on that tap.

Summary

Aspect	Description
Test	Transformer Turns Ratio (TTR) Test
Purpose	To verify the ratio of primary to secondary turns matches the design and nameplate rating.
Principle	V_primary / V_secondary = N_primary / N_secondary
Method	Apply a low AC voltage to one winding, measure the induced voltage on the other, and calculate the ratio.
Equipment	Dual voltmeters or a dedicated TTR Test Set.
Importance	Critical for ensuring correct voltage output, winding integrity, and safe parallel operation.