The purpose of a turn ratio test (TTR test) is to verify the actual turns ratio of a transformer's windings and ensure it matches the designed, nameplate, or required ratio. This fundamental test is critical for transformer manufacturing, acceptance, commissioning, and maintenance for several key reasons:
Verify Correct Construction & Identify Faults:
Manufacturing Defects: Confirms the windings were wound with the correct number of turns during production.
Winding Damage: Detects issues like shorted turns (a major fault where adjacent turns short together, effectively reducing the number of turns in that winding) or open circuits.
Transport/Installation Damage: Checks if the transformer was damaged after leaving the factory.
Ensure Proper Connection of Taps:
Verifies that the tap changer (de-energized or on-load) is correctly connected to the intended tap positions, ensuring the expected voltage ratio is achieved at each tap.
Confirm Polarity and Phase Relation (for multi-winding transformers):
While primarily a ratio test, the method (applying voltage and measuring phase) inherently helps confirm if the windings are connected with the correct polarity (additive/subtractive) and phase displacement (especially important for 3-phase transformers and potential transformers (PTs)).
Calculate Performance Parameters:
The actual turns ratio is essential for accurately predicting transformer performance under load, including voltage regulation, efficiency, and fault current levels.
Compliance with Standards:
It's a mandatory test specified in international standards (like IEEE C57.12.90, IEC 60076) for factory acceptance, commissioning, and often during routine maintenance.
In essence:
A correct turns ratio means: The transformer will transform voltage as designed (V_primary / V_secondary ≈ N_primary / N_secondary). This is fundamental to its core function.
An incorrect turns ratio indicates: A problem exists (shorted turns, wrong connections, physical damage) that will prevent the transformer from operating correctly. It could lead to:
Incorrect output voltage.
Excessive heating (especially from shorted turns).
Circulating currents (in parallel operation).
Reduced efficiency.
Potential damage to connected equipment.
Protective relay maloperation.
How it's done (Simplified):
A low AC voltage (safe for testing, often from a specialized TTR test set) is applied to one winding (usually the HV winding). The induced voltages on all other windings (and at all tap positions) are measured simultaneously. The ratio of the applied voltage to each measured voltage is calculated and compared to the expected ratio.
In summary: The turn ratio test is a vital, non-destructive diagnostic tool to confirm the fundamental integrity and correctness of a transformer's windings and their interconnections, ensuring it will perform its voltage transformation function reliably and safely. It's often one of the first tests performed when a transformer is received or suspected of having a problem.