Instrument Transformer Ratio and Phase Angle Verification

What it is

Ratio and phase angle verification is the field measurement of how faithfully a current or voltage transformer reproduces its primary quantity at the secondary. Ratio is the scale factor between primary and secondary, and the nameplate states a marked ratio, with several taps on multi-ratio units. In practice the true ratio departs slightly from the marked ratio, and the secondary quantity is also displaced in time from the primary by a small angle. The ratio error and the phase angle error together define the accuracy class of the unit. The test is applied to current transformers and voltage transformers in switchgear, bushing current transformers in other equipment, and the instrument transformers that supply the relays of network protectors. It is performed with the transformer disconnected from the secondary load, or with the load accounted for, so that the measured error describes the transformer under conditions the standards recognize. For many acceptance purposes the ratio is verified at the field level, while the angle is confirmed where metering accuracy or protection performance depends on it.

Why it is done

Errors in ratio or angle translate directly into errors in what meters and relays see. A metering transformer whose ratio is off by a small fraction causes consistent billing errors across years of operation. An angle error distorts power and energy measurement, especially at low power factor, and disturbs directional and distance relays that depend on the phase relation between voltage and current. A transformer installed with a different ratio than the design assumed, or a multi-ratio unit left on the wrong tap, will make relay settings wrong across the whole scheme. Verification also catches shorted turns in a winding, wrong internal connections and damage from transport. Finding any of these at commissioning, before the circuit is relied upon, avoids misoperation of protection and the cost of correcting revenue data after the fact.

The principle

An ideal transformer would deliver a secondary quantity exactly proportional to the primary and exactly in phase with it. A real instrument transformer needs a small exciting current to magnetize its core, and this current causes the secondary current to differ slightly in magnitude and to lead or lag the primary. The difference in magnitude is the ratio error, usually expressed as a fraction of the true value, and the displacement in time is the phase angle error. Voltage transformers also have error from the voltage drop across their winding impedance when load flows. Both errors depend on the burden, on the level of the primary quantity, and on the condition of the core. Because errors are small, they cannot be seen by ordinary measurement and must be found by a comparison method. One approach compares the transformer against a reference of known superior accuracy, another injects a known primary quantity and measures the secondary with an accurate instrument, and another measures the ratio of voltages applied and induced. All of them reduce the question to the same comparison between what went in and what came out.

How it is performed

The circuit is de-energized, isolated and grounded under the owner's safety procedures, and the transformer is disconnected from relays and meters unless the procedure calls for the load to remain. For current transformers, a known current is injected through the primary, either by passing a lead through the window or by connecting across a bar or bushing conductor, and the secondary current is measured with an accurate instrument, or a purpose-built analyzer measures ratio and angle directly. Where a high-current source is not practical, the ratio can be verified by applying a voltage to the secondary and measuring the voltage induced on the primary, a method acceptable for field confirmation of the marked ratio. For voltage transformers, a known voltage is applied to the primary at a safe level, or the transformer is energized from a test source, and the secondary voltage is measured. Each phase and each tap in use is measured. The secondary circuit of a current transformer is never opened with primary current flowing. Instrument calibration, lead arrangement and the burden connected during the test are noted, since they all affect the result.

What is recorded

The record shows the transformer and phase tested, the tap or ratio in service, the method used and the quantity applied. It holds the calculated ratio, the ratio error for each phase and tap, and the phase angle where it was measured, together with the burden connected during the measurement. The instrument and its calibration status, the date and the ambient conditions are noted, along with the as-found state of the unit, for example whether it was new, repaired, or removed from service. Keeping the method with the result matters, because ratio obtained by a voltage method on the secondary is not strictly the same quantity as one obtained by passing current through the primary.

How results are judged

Results are compared with the marked ratio on the nameplate and with the accuracy class assigned to the unit. The measured ratio error and angle error should be consistent with that class at the burden that was applied, and the three phases of the same installation should agree closely with each other, since a phase that departs from its neighbors points to a defect. Comparison with earlier results on the same unit reveals drift. For revenue metering the evaluation is stricter and may involve correction factors based on measured errors. The classes, the way errors are defined and the conditions of validity are found in IEEE C57.13-2016 and IEEE C57.13.1-2017 and, for high-accuracy units, IEEE C57.13.6-2005. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 call for ratio verification on instrument transformers and network protectors; consult your licensed copies for the values and the criteria that govern.

Variations

At acceptance, ratio is verified on every current and voltage transformer, and angle is added where the class or the application demands it, such as revenue metering and directional protection. In maintenance, the test is repeated after repairs, after a through-fault, or when meter or relay behavior is suspect. Current transformers are measured by injecting current or by the voltage method, and bushing units inside other equipment are measured from the terminals available. Multi-ratio units are checked on each tap that will be used. Voltage transformers, including those fed from capacitor dividers, depend on the burden more strongly, so the load connected during the test should resemble the real one. The transformers in network protectors are small, often enclosed units that supply relay inputs, and their verification follows the same logic with the limits and class definitions found in the standards named above. High-accuracy and metering-class units are judged on tighter classes than relaying-class units.