Solutions for Transformer Ratio Test – SMC

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

power transformer testing

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.

Specifically designed transformer ratio test sets are used generally for making this kind of tests; the transformer ratio test set made the measurement phase by phase, regardless of the configuration, but usually employs internal switching arrangements to facilitate testing of 3-phase transformers, keeping in mind the connection group.  The transformer ratio test set measures the voltage ratio, and for single-phase and most 3-phase cases, it is the same as the transformer turn ratio (with the ratio error difference), but in some power transformer configurations it is different from the voltage ratio and it is calculated using some conversion formulas and factors which depend on the PT connection group.

The ETP-1 of SMC meet all the required features as a 3-phase transformer ratio test set and excitation current meter; the ETP-1 module determines the turn ratio on the three phases of power and distribution transformers and autotransformers of any type by applying a test voltage on the high-voltage winding and measuring the resulting voltage on the low-voltage winding.

This transformer ratio test set also provides the evaluation of other parameters, such as the excitation current (which gives an estimation of the no load losses), phase angle between the test voltage and the excitation current (which enables vector group checking) and phase angle between the test voltage and the voltage measured (this phase deviation enables to identify damages in the magnetic cores).

The ETP-1 provides a fully guided and automatic software controlled transformer ratio test, with operator instructions during the execution, with graphic representation of the results for each tap position, and avoids the need to perform calculations, reducing human errors.

The database storage of results and immediate reporting of this transformer ratio test set enables individual tracking of each transformer, which can be compared with previous results with optional trending and diagnosis software. The ETP-1 is part of ETP system for predictive power transformer testing, so that it can cross results with the other units to provide a complete diagnostic.

Transformer Ratio Test in Instrument Transformers

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 SMC 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 and the ETP-1, which also includes all data and results in the same report, among other advantages.

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