Excitation Current: The Vital Companion Measurement in TTR Testing
Beyond the Ratio: Understanding the Magnetizing Circuit
While the turns ratio is the headline measurement in a TTR test, the simultaneous measurement of excitation current (also called magnetizing current) provides an equally vital diagnostic window into the transformer's condition. This current is the quantity required to establish the magnetic flux in the core when a voltage is applied to a winding with the others open-circuited. It is highly sensitive to the integrity of both the magnetic core and the geometric arrangement of the windings. A modern TTR test is therefore incomplete without recording excitation current; it transforms the test from a simple verification of turns count into a powerful tool for detecting core faults, winding deformation, and subtle problems that a ratio measurement alone would miss.
Excitation current analysis is based on three principles: its absolute magnitude, its balance across phases in three-phase units, and its harmonic composition. Deviations in any of these areas point to specific types of internal degradation.
What Does Excitation Current Reveal?
The excitation current flows through the shunt branch of the transformer's equivalent circuit, which models the core. Its value and waveform are influenced by:
Core Material and Condition: The quality of the core steel lamination insulation and the tightness of the core clamping structure.
Core Saturation: The point at which increasing voltage no longer produces a proportional increase in flux. Test voltage must be chosen to avoid saturation for a valid diagnostic.
Winding Geometry: Mechanical displacement of windings changes the magnetic leakage paths, which can alter the current required to establish the mutual flux.
Shorted Turns: A short circuit within a winding acts as an additional load, dramatically increasing the apparent excitation current drawn.
Thus, a normal excitation current signifies a healthy magnetic circuit and winding geometry, while an abnormal current signals a problem in one of these domains.
Interpreting Excitation Current Measurements
Interpretation requires comparison—either to the other phases of the same unit or to a historical baseline for that specific transformer.
| Excitation Current Observation | Comparison Reference | Likely Diagnosis |
|---|---|---|
| High and/or Distorted Waveform on all phases. | Manufacturer's data or historical baseline. | Core Fault: Shorted core laminations, poor core ground, or general deterioration of core insulation. The core losses are increased. |
| Unbalanced (one phase significantly different). | Other phases on the same transformer. | Localized Core Fault (e.g., short in one leg), Winding Deformation on one phase, or an Problem in an Untested Winding (e.g., shorted turns in a tertiary affecting one phase's magnetic circuit). |
| Very High on one phase with a corresponding ratio error. | Other phases and nameplate ratio. | Shorted Turns in the winding of that phase. This is a classic, definitive signature. |
| Normal magnitude but elevated harmonics (particularly 3rd and 5th). | Baseline harmonic signature. | Early stages of core insulation degradation or minor winding movement affecting magnetic symmetry. |
| Low or Zero. | Expected normal range. | Open circuit in the test connection or in the winding itself. The magnetic circuit cannot be established. |
Practical Testing Considerations for Accurate Current Measurement
To ensure excitation current data is reliable, follow these guidelines:
Use a Consistent Test Voltage: Always use the same test voltage (e.g., 120V) for trending comparisons. The magnitude of excitation current is non-linear and varies with voltage.
Allow Stabilization: After applying voltage, wait a moment for the reading to stabilize, especially on large transformers where the core may have residual magnetism.
Demagnetize if Necessary: If a transformer has been DC tested (winding resistance), it may retain residual flux, distorting the excitation current. Use the demag function on your meter or a variable AC source to remove it before TTR testing.
Test with Correct Winding Configuration: For three-phase transformers, test each phase individually with the other windings open-circuited but grounded for safety. Automated three-phase testers handle this internally.
Record Ambient Temperature: While less sensitive than DC resistance, excitation current can have a slight temperature dependence. Note it for precise long-term trending.
Case Example: The Silent Core Fault
Consider a transformer with a perfect 1.000 turns ratio on all phases. A TTR test focusing only on ratio would pronounce it healthy. However, the excitation current measurements show: Phase A: 15 mA, Phase B: 14 mA, Phase C: 45 mA. This severe unbalance (Phase C is 3x higher) with correct ratios is a strong indicator of a core fault isolated to the C-phase leg, such as a short between laminations. This fault would increase core losses, cause localized heating, and generate gases over time, yet it left the turns ratio completely unaffected. Without excitation current measurement, this developing problem would remain undetected until it triggered a gas alarm or caused a major failure.
In summary, the excitation current measurement elevates the TTR test from a functional check to a true diagnostic procedure. It provides an independent assessment of the magnetic circuit, offering early warning of core and winding problems that are invisible to the ratio test. For any critical transformer assessment, recording and analyzing excitation current is not optional—it is an essential component of a thorough and predictive maintenance strategy.
