AVO New Zealand

Abstract:

Measuring a transformer's winding (DC) resistance from one external terminal to another can reveal a great deal of information about the transformer. In addition to the obvious faulted winding (i.e., an open winding), more subtle problems can be detected. The DC current, in addition to flowing through the winding, must also flow through the off-load ratio adjusting switch (De-Energized Tap Changer or DETC) and the on-load ratio adjusting switch (Load Tap Changer or LTC), as well as numerous welded and mechanical connections. Hence, the integrity of all these components can be verified.

Authors note: Specific aspects of safety are addressed herein; however comprehensive procedures are not detailed. It is assumed the operator has sufficient knowledge of electrical theory and safe working practices to use the test instrument in a safe and responsible manner.

Full Paper (400KB PDF)

1 Introduction

Winding resistance measurements in transformers are of fundamental importance for the following purposes:

• Calculations of the I2R component of conductor losses.
• Calculation of winding temperature at the end of a temperature test cycle.
• As a diagnostic tool for assessing possible damage in the field.

Transformers are subject to vibration. Problems or faults occur due to poor design, assembly, handing, poor environments, overloading or poor maintenance. Measuring the resistance of the windings assures that the connections are correct and the resistance measurements indicate that there are no severe mismatches or opens. Many transformers have taps built into them. These taps allow ratio to be increased or decreased by fractions of a percent. Any of the ratio changes involve a mechanical movement of a contact from one position to another. These tap changes should also be checked during a winding resistance test.

Regardless of the configuration, either star or delta, the measurements are normally made phase to phase and comparisons are made to determine if the readings are comparable. If all readings are within one percent of each other, then they are acceptable. Keep in mind that the purpose of the test is to check for gross differences between the windings and for opens in the connections. The tests are not made to duplicate the readings of the manufactured device which was tested in the factory under controlled conditions and perhaps at other temperatures.

This application note is focusing on using winding resistance measurements for diagnostic purposes.