7.2 Winding Resistance & Transformer Polarity

Winding Resistance Test Method

The winding resistance test measures the DC resistance of each transformer winding. It is performed with a DC micro-ohmmeter (Ductor) using 4-wire Kelvin connections so that test-lead resistance is excluded from the reading. A standard DMM on a 200-ohm range is not acceptable — its two-lead connection includes lead and contact resistance that swamps the small winding resistance values being measured.

Test current must be sufficient to magnetize the core (typically 1 to 10 A DC for power transformers). After connection, the technician must wait for the inductive transient to settle — the winding is a large inductor, and the current takes several seconds to stabilize. Reading too early gives a falsely high resistance. Many modern instruments detect settling automatically.

Measurement Patterns

On a single-phase transformer, measure HV winding end-to-end and LV winding end-to-end.

On a three-phase wye winding, measure each phase to the neutral (H1-N, H2-N, H3-N).

On a three-phase delta winding, measure each phase pair (H1-H2, H2-H3, H3-H1). Each reading includes two windings in series with the third in parallel, so a single open or degraded winding distorts the pattern in a predictable way: the two readings that include the bad winding both shift, while the reading of the two good windings stays normal.

For three-winding transformers (a primary, a secondary, and a tertiary), repeat the suite on all three windings.

Temperature Correction

Winding resistance varies with temperature. To compare readings against factory data, prior field readings, or phase-to-phase, correct the measured resistance to a reference temperature — 75 degrees C for oil-filled transformers and often 85 degrees C or the insulation class rating for dry-type. For copper windings the correction is:

R75 = Rm x (234.5 + 75) / (234.5 + Tm)

where Rm is the measured resistance and Tm is the measured winding temperature in degrees C. For aluminum windings, replace 234.5 with 225. The winding temperature for an oil-filled unit is usually taken as the top-oil temperature; for a de-energized unit that has sat, ambient is acceptable.

What Winding Resistance Reveals

NETA MTS guidance generally treats phase-to-phase winding resistance differences greater than about 5% (or the manufacturer's stated value) as a flag for investigation, because the three phases of a healthy winding are within a few percent of each other after temperature correction.

Transformer Polarity Test

The polarity test determines whether a transformer's primary and secondary windings are subtractive or additive and confirms the H1/X1 polarity marks on the bushings.

• Subtractive polarity: when H1 and X1 are adjacent and the voltage measured between H1 and X1 is less than the primary voltage alone, the windings oppose. Most distribution transformers above about 200 kVA and virtually all power transformers are subtractive.
• Additive polarity: when H1 and X1 are adjacent and the voltage between them is greater than the primary voltage alone, the windings aid. Common on small distribution transformers below about 200 kVA and on some instrument transformers.

Two methods are used in the field:

1. DC kick method — a small DC battery is momentary connected across the primary; a galvanometer or DC voltmeter on the secondary deflects. The direction of the first deflection identifies polarity. This is the classic Level 2 method.
2. AC comparison method — a known AC voltage is applied to the primary, and the voltage between a primary bushing and the adjacent secondary bushing is compared to the two winding voltages. If the reading is V_primary - V_secondary, polarity is subtractive; if it is V_primary + V_secondary, polarity is additive.

Why Polarity Matters

Polarity and vector group (e.g., Dyn1, YNd11) determine how a transformer connects into a system. Two transformers can only be paralleled if they have the same vector group, the same polarity orientation, and the same turns ratio. Getting polarity wrong on a parallel pair causes circulating current between the units that can trip them or damage them on energization.

Polarity is equally critical for differential (87) protection. The 87 scheme compares CT secondary currents on the primary and secondary sides of the transformer; if the CT polarity does not account for the transformer's polarity and vector group, the relay sees a through-current as an internal fault and trips on load or on through-faults. Polarity verification is part of commissioning for every new 87 installation.

Short-Circuit Impedance Estimate

The winding resistance test gives the DC resistance of each phase. The transformer's short-circuit (leakage) impedance is larger than the DC resistance because it includes leakage reactance. A rough first-order estimate is:

Z_sc approx R_winding + jX_leakage

But X_leakage cannot be derived from a DC resistance test — it requires an actual short-circuit test (apply reduced voltage to the primary with the secondary shorted and measure voltage and current at rated current). NETA Level 2 candidates should know that winding resistance is only the resistive part of the impedance and that the leakage reactance test is a separate, full-voltage-equivalent test. The winding resistance value is most useful for I-squared-R loss checks, OLTC contact health, and trending — not for impedance calculations.