Solutions for Transformer Ratio Test – EuroSMC

Transformers are subject to several stresses and changes in their operational life, and so utilities and service companies schedule different transformer testing programs to assess their condition throughout the life of the transformer.

The transformer condition depends much on the winding performance to provide the expected voltage ratio, which is directly related to the turns ratio (the ratio of wire turns in the primary winding to the number of wire turns in the secondary winding); therefore, the transformer ratio test is an essential test of the transformer, as one of the most effective for its status evaluation; the changes in turn ratio, and the corresponding deviations beyond the maximum error tolerance from the nameplate ratio, can be an indication of winding damage or deterioration; the transformer ratio test helps to identify problems such as shorted-turns or open-turns, incorrect connections, tap changer and internal magnetic core problems, etc.

Transformer Ratio Test in Power transformers

International standards require that the measured voltage ratio between any two windings be within ±0.5% of the ratio indicated in the nameplate; to verify this requirement, the transformer ratio test measures the actual voltage ratio, which is compared to the nameplate. The transformer ratio test in power transformers is made through low ac test voltage injection in de-energized transformers and the corresponding measurement of the voltage induced in other winding; the transformer ratio test is made for each phase and for each tap position if the transformer has taps for changing its voltage ratio.

Transformer Ratio Test in Instrument Transformers

More info

In current transformers, for metering and/or protection, the transformer ratio test is mainly based in current injection, since the CT ratios are expressed as a ratio of the rated primary current to the rated secondary current at specified burden.

If it is not possible the current ratio test, as in CTs situated directly in the high voltage bushings of power transformers, the ratio can be tested through voltage injection, but it should not be a substitute for the current method. The current transformer ratio test at rated current offers more assurance that it will perform as expected when placed in service.

The Raptor System provides several test templates and functions for transformer testing of CT/VT and PT; the great portability of the Raptor also enables placing the test set closer to the transformer under test and reducing the length of test leads and the power required; regarding the transformer ratio test, the Raptor includes the following templates:

  • CT ratio: performs the current transformer ratio test through current injection in the primary and measurement of the secondary current; with same template it determines the polarity and angle error, and the connected burden in VA at the test current, in impedance and the power factor of the burden; if the nominal ratio is entered, the transformer ratio test also displays the ratio error; since it is advisable to test the ratio error and phase angle at different percentages of nominal current, including full load current and higher, the Raptor provides the high currents and power required for the application to meet the different transformer ratio test jobs.
  • CT ratio, voltage method: this template is an alternative when, for whatever reason, is not possible to directly inject primary current into the CT; the transformer ratio test is performed applying voltage on its secondary side while measuring the primary voltage; this method obviously offers no information about the influence of the magnetic core on the total accuracy at its precision load.
  • CT ratio for Rogowski and low power CTs: the transformer ratio test is made through primary current injection and measurement of the secondary voltage; it measures CT ratio, polarity and phase angle of this specific type of current transformers or sensors, in which the turn ratio is defined as a proportion of the primary current to the low level secondary voltage.
  • VT ratio: this transformer ratio test for voltage measurement transformers, is made applying voltage to the primary and measuring the secondary voltage, checking the VT turn ratio, polarity and phase angle.
  • PT ratio: The Raptor also includes the power transformer ratio test in single phase, measuring the voltage ratio between the primary winding and the relative secondary winding of a PT; if the PT is three‐phase, a measurement must be carried out in each phase; If the neutral is not accessible, the injection is between two phases and measure at the low side according to the calculation and required connection diagram depending on the PT connection group.

Any of the primary and secondary injection test sets of the EuroSMC product range can be also used as current or voltage source, provided that is reached the injection range required, in combination with a suitable external meter for the transformer ratio test, but it is preferable an integrated injection and measurement system as the Raptor, which also includes all data and results in the same report, among other advantages.

For more information, please check the related EuroSMC products or contact us.