Potential Transformer Testing: Ratio, Polarity, Burden, and Excitation

A potential transformer (PT), also called a voltage transformer (VT), steps high system voltage down to a standard secondary voltage, typically 120V or 69.3V, that protective relays, revenue meters, and control circuits can use safely. The nameplate turns ratio establishes the proportional relationship between primary and secondary, and the accuracy class specifies how close that ratio must be to theoretical across a range of burden and voltage conditions. A PT with an incorrect ratio, wrong polarity, or degraded insulation produces incorrect inputs to every relay and meter it feeds, errors that propagate silently into protection and metering until a fault or an audit exposes them.

PT testing at acceptance verifies the ratio and polarity before any relay is calibrated or any revenue meter is commissioned. Insulation testing verifies that the winding insulation has not been damaged in shipping or installation. Burden testing confirms that the connected load does not exceed the PT's rated burden, which would push it out of its accuracy class even with a correct ratio. For a parallel with CT testing (which follows the same logic), see our article on current transformer testing.

Voltage ratio testing

The ratio test applies a known AC voltage to the secondary winding and measures the resulting voltage on the primary, then calculates the actual turns ratio from the two measurements. Applying voltage to the secondary rather than the primary is standard field practice because it keeps the test voltage within a safe, accessible range, you don't need to apply 15 kV to verify a 15 kV:120 V PT. The ratio is calculated as V_primary / V_secondary, and the result is compared to the nameplate ratio. IEEE C57.13 allows a ratio error of ±0.3% for accuracy classes 0.3 and better, and ±0.6% for class 0.6.

The ratiometer or TTR (transformer turns ratio) test set is the standard instrument for this measurement. It applies a precise AC voltage and measures both windings simultaneously, computing the ratio to three or four decimal places with better accuracy than the percentage uncertainty of a calibrated voltmeter pair. For multi-ratio PTs with multiple secondary taps (such as a 14400:120/69.3 V unit with H1-H2 and H1-H3 connections), each ratio is tested independently.

A ratio error outside tolerance almost always indicates one of three things: the wrong ratio tap was selected (on a multi-tap unit), a partial winding short reducing the effective turns count, or a connection error. Ratio errors that are consistent across multiple test voltage levels point to a winding condition issue; errors that vary with test voltage suggest a nonlinear core saturation problem, which can indicate core damage or that the applied test voltage is too high relative to the PT's rated voltage.

Polarity testing

PT polarity determines the phase relationship between the primary and secondary terminals. A PT with subtractive polarity (the standard for most North American PTs) produces a secondary voltage that is in phase with the primary voltage when the primary H1 terminal and the secondary X1 terminal are the marked terminals. Reversing the polarity connection, connecting the relay to X2 instead of X1, inverts the voltage signal, which causes directional relays to operate in the wrong direction and differential relays to compute incorrect differential quantities.

The polarity test is simple: a small DC voltage (a flashlight battery will work) is applied momentarily to the primary winding H1-H2 terminals and a DC millivoltmeter is connected to the secondary X1-X2 terminals. A positive deflection on the meter when voltage is applied indicates subtractive (correct) polarity; a negative deflection indicates additive polarity or a reversed winding connection. The test takes seconds and unambiguously confirms polarity before any relay wiring is connected.

Insulation resistance testing

Insulation resistance testing on a PT follows the same approach used for power transformers, a DC megohmmeter applied between each winding and ground, and between primary and secondary windings. NETA ATS acceptance criteria require the measured insulation resistance to be compared to manufacturer specifications; in the absence of manufacturer data, the minimum acceptable value per NETA MTS is 100 MΩ for the primary winding at 1000V or 2500V DC test voltage. Secondary winding insulation is tested at 500V DC.

Elevated test voltage for PT insulation measurement must be handled carefully. The capacitance of a high-voltage PT primary winding stores significant energy at 2500V DC, the discharge after the test must be performed deliberately and the winding treated as energized during discharge. For oil-filled PTs, the insulation resistance result should also be compared against the power factor test result; a PT with marginal IR that also shows elevated power factor has degraded insulation on both DC and AC measures and should be investigated further before it is placed in service.

Excitation test

The excitation test applies increasing AC voltage to the secondary winding and measures the resulting current. The current-versus-voltage relationship is plotted as the excitation curve. In the linear region, the PT core is unsaturated and current rises proportionally with voltage. At the knee point, the voltage at which the core begins to saturate, the current rises sharply for small increments in voltage. The position of the knee point confirms that the PT has the correct core design for its application.

For protective PTs, the knee point voltage is important because the PT must be able to reproduce voltage accurately during fault conditions, when system voltage may be depressed or distorted. A PT with a low knee point saturates easily under off-nominal voltage conditions and produces a secondary voltage that no longer accurately represents the primary, which corrupts the inputs to distance relays and voltage-supervised overcurrent elements precisely when they need accurate data most. The excitation curve from the factory test report, if available, should be compared to the field excitation test result as a winding condition check.

Burden measurement and accuracy class verification

PT accuracy class specifies the maximum ratio and phase angle error at a defined burden level. A Class 0.3 PT maintains ratio error within ±0.3% and phase angle error within ±15 minutes of arc across the burden range from 25% to 100% of rated burden. If the connected burden (the sum of relay inputs, meter inputs, and control circuits on the secondary) exceeds the rated burden, the PT's accuracy is not guaranteed, the increased secondary current demand causes a larger voltage drop in the winding impedance, introducing ratio error above the rated specification.

Burden measurement requires measuring the impedance of the secondary circuit connected to the PT. This is done either by measuring each connected device's input impedance individually and summing (the preferred approach for new installations where devices can be characterized before connection), or by applying a known test voltage to the secondary circuit with the PT disconnected and measuring the resulting current. The total VA burden at rated secondary voltage is compared to the PT's rated burden. If the measured burden exceeds the rated burden, either the connected devices must be reduced or a higher-rated PT must be installed.

Revenue metering circuits have particularly stringent accuracy requirements, ANSI C12.20 accuracy class for revenue meters requires PT accuracy class 0.3 or better under the actual connected burden. A utility that has installed a Class 0.3 PT on a circuit where the actual burden exceeds rated will have revenue metering errors that may not surface until an audit or complaint triggers a meter test.

Electromagnetic versus capacitive voltage transformers

At transmission voltages (typically 115 kV and above), conventional electromagnetic PTs become physically impractical and expensive. Capacitive voltage transformers (CVTs) use a capacitor voltage divider to step the primary voltage down to an intermediate level, then an electromagnetic step-down stage to produce the 120V secondary output. CVTs introduce additional phase angle error and transient response characteristics that differ from electromagnetic PTs, and the testing approach accounts for these differences, the burden and ratio tests are similar, but CVT transient response verification and ferroresonance testing are additional considerations that don't apply to electromagnetic units.

Most distribution and subtransmission substations use conventional electromagnetic PTs, and the tests described above apply fully. For transmission CVTs, the manufacturer's test procedure supplements the standard NETA requirements.