Solutions for Winding Resistance Test – SMC

Resistance testing of electrical equipment covers a wide range of techniques depending on the nature of the element tested and the target of the test. The winding resistance test of transformers, tap changers or motors involves a particular challenge in resistance testing, due to the highly magnetic nature of the windings.  Related testers must cope with the strong inductive load to apply the necessary DC current to magnetize the winding and stabilize the current, besides taking into account the connection group and the temperature.

The winding resistance test is used both in factory tests and for field operating verification. In the factory, the winding resistance test assure proper manufacturing according to design, serves also to calculate conductor losses and as variable magnitude during thermal test related to winding temperature. On site, the winding resistance test is performed to evaluate possible winding problems or damages, such as short turns, open windings, internal connection problems, hot points and the condition of the tap changer. Winding resistance tests are also typical on new transformers after delivery to site to detect any damage during the transport, and as initial transformer reference for comparison with future measurements. The winding resistance tests are also part of periodic maintenance programs for finding problems that may reduce the system performance and can lead to unexpected power failures.

Required characteristics for winding resistance test sets

What makes a winding resistance test set different than a traditional low resistance ohmmeter, is that they are able to overcome the effects of large inductances which are associated with transformer windings. Also, those components within a winding circuit, such as bushing connections and load-tap changer switches, are included in this testing. The winding resistance test is performed with specific transformer ohm-meters which, unlike the typical micro-ohmmeters for contact resistance measurements, have higher compliance voltage to deliver the energy needed to saturate the windings and stabilize quicker the reading, to provide an accurate and repeatable measurement.

The winding resistance test is done with four wires method, by injecting DC current through the winding and measuring the voltage drop. A 3-phase connection is preferable in the winding resistance test, even if the testing is performed phase by phase, because it reduces testing time as the user has to go on the top of the transformer less times.

The winding resistance test depends much on temperature, so that the resistance results are referenced to a normalized temperature (temperature-corrected). Before returning the transformer to service, it is also important that the core be demagnetized, to avoid personal or equipment damages, and therefore the winding resistance test set must provide this capacity once the measuring is finished.

The problems in the transformer can involve different sources which might require crossing results of different parameters in the same transformer, for a complete diagnosis, so that the winding resistance test is also used to complement information from other transformer diagnostic tests, for example to confirm suspect problems from the TTR test results and viceversa.

SMC solutions for winding resistance test

The ETP-3 of SMC meet the features required, performing a 3-phase winding resistance test with a guided-software, automatic magnetization and de-magnetization and graphical display of the test evolution, for assessing the transformer and tap changer possible problems.

The ETP-3 performs the following measurements in all type of power, distribution and instrument transformers:

  • Automatic winding resistance value in single or three phases.
  • Automatic correction of the resistance per phase to a reference temperature.
  • Automatic calculation of the compound winding resistance, for different configurations, Delta, Star, etc,

The ETP-3 performs the winding resistance test of each phase, sequentially, without any change in the connection to the transformer, for each tap changer position. It uses a four wires measuring method to avoid any error from the connection cables, with adjustable (auto-manual) output current and quick testing way, fully guided and automatic; this computer controlled winding resistance test set is supplied with its associated software, which indicates how to perform the test, connections, tap changing, phases,…with graphic display of the results and database management for trending analysis. The results are automatically saved and the user can issue an instantaneous complete report.

Winding resistance test etp trends softwareThe ETP-3 unit is also part of ETP System, the integrated SMC predictive power transformer maintenance system, crossing results from different units to provide a complete diagnostic. The ETP System is composed by four independent measurement modules controlled from common software in a PC. Any of the modules can also be used separately. Trending and diagnostics are performed by expert software which confirms the existence of problems by evolution and correlation of the measurements done by each module.

The ETP-3 is supplied with the necessary 3-phase test cables, for the primary and secondary connection. These are 8-meter lengths with an optional 12-meter extension lead, enabling to reach all transformer sizes.

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

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