What is the purpose of the TTR test?

The Transformer Turns Ratio (TTR) test is a fundamental, non‑destructive diagnostic test whose core purpose is to verify that the transformer's primary‑to‑secondary winding turns ratio matches its design/nameplate specification, ensuring correct voltage transformation and detecting internal winding faults.

Primary Purposes

Verify Design & Voltage Transformation

Confirm the transformer steps voltage up/down as intended (voltage ratio ≈ turns ratio).

Validate compliance with nameplate ratings and industry standards (e.g., IEEE C57.12, IEC).

Detect Internal Winding Faults

Shorted turns/inter‑turn faults: Reduces effective turns → ratio deviation.

Open windings: Causes extreme ratio error or no reading.

Incorrect winding connections: Reversed phases, wrong vector group, or miswiring.

Insulation degradation: Early sign of winding damage.

Validate Tap Changer Operation

For transformers with on‑load tap changers (OLTC) or off‑load taps: confirm each tap position yields the correct ratio.

Identify tap switch contact issues or mechanical misalignment.

Quality Control & Commissioning

Factory acceptance: Ensure manufacturing/assembly correctness.

Site commissioning: Verify no damage occurred during transport/installation.

Post‑repair: Confirm repairs restored proper winding configuration.

Preventive Maintenance & System Protection

Early fault detection to avoid catastrophic failure.

Prevent over/under‑voltage damage to connected loads.

Avoid misoperation of protective relays and inaccurate metering.

Ensure stable parallel operation (prevents harmful circulating currents).