3.2 Transformer Turns Ratio (TTR) and Winding Resistance Testing

Transformer Turns Ratio (TTR) and Winding Resistance Testing

Diagnostic Focus: Transformer Turns Ratio (TTR) and Winding DC Resistance testing are primary baseline electrical tests performed during commissioning and routine maintenance. TTR verifies the magnetic turns geometry and tap connections, while winding resistance verifies the physical integrity of current-carrying conductors, internal joints, and tap contacts.

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Transformer Turns Ratio (TTR) Theory and Principles

Under ideal no-load conditions, the ratio of voltages induced in transformer windings is directly proportional to the physical ratio of turns in those windings:

$\text{Turns Ratio} = N_{primary} / N_{secondary} = V_{primary\ phase} / V_{secondary\ phase}$

Ratio Calculations for Three-Phase Winding Connections

Because nameplate voltages are stated as line-to-line (V_LL) values, calculating the theoretical phase turns ratio requires accounting for Delta (Δ) and Wye (Y) internal connections:

      DELTA (Δ) WINDING                      WYE (Y) WINDING
   V_phase = V_line-to-line               V_phase = V_line-to-line / √3
          A                                      A
         / \                                     |
        /   \                                    |
       /     \                                   • Neutral
      B-------C                                 / \
                                               /   \
                                              B     C

Worked Calculation: Delta-Wye Turns Ratio

A 13,800 V Delta to 480/277 V Wye distribution transformer is tested on its nominal tap:

$\text{Theoretical Ratio} = V_{p,HV} / V_{p,LV} = 13,800\text{ V} / (480\text{ V} / \sqrt{3}) = 13,800\text{ V} / 277.13\text{ V} = 49.796$

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TTR Test Procedure and Tap Sweep Protocol

TTR testing per IEEE C57.12.90 is executed by applying an AC excitation voltage (typically 8 V to 100 V AC, single-phase or three-phase) to the high-voltage winding while measuring the induced voltage and phase angle on the corresponding low-voltage winding.

+-------------------------------------------------------------------------+
|                        TTR TEST EXECUTION RULES                         |
| 1. ALWAYS excite the HIGH-VOLTAGE winding to prevent dangerous step-up  |
|    voltages on the HV terminals.                                        |
| 2. TEST ALL TAPS: Every DETC position (1-5) and all 33 LTC positions    |
|    (16 Raise through 16 Lower).                                         |
| 3. RECORD ratio, excitation current, and phase angle deviation.         |
+-------------------------------------------------------------------------+

Acceptance Criteria (NETA ATS/MTS Section 7.2.2)

According to NETA ATS Section 7.2.2.B.2 and NETA MTS Section 7.2.2.B.2, measured turns ratio results must not deviate from the calculated nameplate ratio by more than ±0.5%:

$\%\text{ Deviation} = |(\text{Measured Ratio} - \text{Calculated Ratio}) / \text{Calculated Ratio}| \times 100\% \le 0.50\%$

Failure Modes Identified by TTR

• Turn-to-Turn Short Circuit: Causes a substantial change in measured ratio accompanied by abnormally high excitation current on the faulted phase winding.
• Open Winding or Broken Tap Lead: Results in an open-circuit condition with zero induced secondary voltage (infinite ratio).
• Misaligned Tap Mechanism: Generates ratios that do not match the designated tap step sequence or repeat previous tap ratios.
• Reversed Winding Polarity / Internal Wiring Error: Results in a 180° phase angle error or incorrect phase displacement.

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Winding DC Resistance Testing

Winding resistance testing measures the DC resistance of transformer coils to verify the health of current-carrying paths. It is extremely sensitive to mechanical defects, including loose bolted internal busbars, fractured conductor strands, poor crimps, and burnt, pitted, or misaligned LTC/DETC contacts.

4-Wire Kelvin Measurement Technique

To eliminate the resistance of test leads and contact probes, winding resistance is measured using a 4-wire (Kelvin) configuration:

         [ CONSTANT CURRENT SOURCE (I) ]
           +                         -
           | Current Lead (+)        | Current Lead (-)
        +--▼-------------------------▼--+
        |   C1                     C2   |
        |        TRANSFORMER COIL       |
        |   P1                     P2   |
        +--▲-------------------------▲--+
           | Potential Lead (+)      | Potential Lead (-)
           +                         -
         [ HIGH-IMPEDANCE VOLTMETER (V) ]

                  R_coil = V / I

Core Saturation Physics and Stabilization Time

Power transformer windings exhibit enormous inductance (L) coupled with very low resistance (R), creating a large magnetic time constant:

$\tau = L / R$

When DC test current is initially injected, the current rises exponentially (I(t) = I₀(1 - e^-t/τ)). As the core iron becomes magnetically saturated, inductance drops dramatically and the test current stabilizes. Technicians must not record the resistance value until the digital reading has completely stabilized (which may require 30 seconds to several minutes on large power transformers).

Inductive Kickback Hazard: A transformer core stores substantial magnetic energy (E = 0.5 × L × I²). Sudden disconnection of DC leads while current is flowing produces a destructive inductive kickback voltage (V = -L × di/dt) that can exceed several thousand volts. Technicians must use test instruments equipped with automated discharge circuits and never disconnect test leads until the discharge sequence is complete and discharge indicator lamps confirm zero residual energy.

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Temperature Correction of Winding Resistance

Copper and aluminum winding resistance varies directly with conductor temperature. All field measurements must be converted to a standard reference temperature (75°C for oil-filled units rated for 55°C or 65°C rise; 85°C or 100°C for dry-type units):

$R_{corrected} = R_{measured} \times [(T_k + T_{reference}) / (T_k + T_{measured})]$

Where the inferred zero-resistance temperature constant (T_k) is:

• T_k = 234.5°C for Copper (Cu) conductors.
• T_k = 225.0°C (or 230.0°C) for Aluminum (Al) conductors.

Step-by-Step Worked Example: Temperature Correction

A copper winding on an oil-filled transformer is measured at a top oil temperature of 22.0°C with a measured resistance of 0.04520 Ω. Correct the reading to the 75.0°C standard baseline:

1. Identify constants: $T_k = 234.5^\circ\text{C},\quad T_{ref} = 75.0^\circ\text{C},\quad T_{meas} = 22.0^\circ\text{C},\quad R_{meas} = 0.04520\ \Omega$
2. Calculate temperature correction factor (F_T): $F_T = (234.5 + 75.0) / (234.5 + 22.0) = 309.5 / 256.5 = 1.20663$
3. Calculate corrected resistance (R₇₅°C): $R_{75^\circ\text{C}} = 0.04520\ \Omega \times 1.20663 = 0.05454\ \Omega\ (54.54\text{ m}\Omega)$

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Acceptance Criteria for Winding Resistance

According to NETA ATS / MTS Section 7.2.2.B.1 and IEEE C57.152:

1. Phase-to-Phase Comparison: Temperature-corrected resistance values of adjacent winding phases (e.g., A-B vs. B-C vs. C-A on Delta, or A-N vs. B-N vs. C-N on Wye) must agree within 2.0% of each other.
2. Factory Baseline Comparison (NETA ATS): Field measurements must not deviate by more than 1.0% from factory test report values corrected to the same temperature.
3. LTC Tap Sweep Profiling: Resistance measurements across all 33 LTC tap steps must transition smoothly and monotonically without abrupt spikes or dropouts. A resistance spike on a specific tap indicates contact erosion, poor spring tension, or carbon buildup on that transition finger.