Transformer Stories: From Testing and Analysis to Maintenance Case Studies

Transformer DC Resistance Measurements

Winding resistance measurements in transformers are of fundamental importance for the purposes of calculations of the I²R component of conductor losses; calculation of winding temperature at the end of a temperature test cycle; and 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, handling, 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.

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.

Figure 1 – Common three-phase transformer connections

2. Transformer Dc Resistance Measurements

2.1 At Installation

Risk of damage is significant whenever a transformer is moved. This is inherent to the typical transformer design and modes of transportation employed. Damage can also occur during unloading and assembly. The damage will often involve a current carrying component such as the LTC, RA switch or a connector.

Damage to such components may result in a change to the DC resistance measured through them. Hence, it is recommended that the DC resistance be measured on all on-load and off-load taps prior to energizing.

If the transformer is new, the resistance test also serves as a verification of the manufacturer’s work. Installation measurements should be filed for future reference.

2.2 At Routine (Scheduled) Transformer Maintenance

Routine maintenance is performed to verify operating integrity and to assure reliability. Tests are performed to detect incipient problems. What kind of problems will the resistance test detect?

Figure 2 – Alternative 3-phase transformer connections

2.2.1 Ratio Adjusting Switch (Ratio Adjusting Off-load Tap Changer)

Contact pressure is usually obtained through the use of springs. In time, metal fatigue will result in lower contact pressure. Oxygen and fault gases (if they exist) will attack the contact surfaces. Additionally, mechanical damage resulting in poor contact pressure is not uncommon. (e.g. A misaligned switch handle linkage may result in switch damage when operated).

Such problems will affect the DC resistance measured through the RA switch and may be detected.

Further Study – Mastering stability test of power transformer: Differential and Restricted Earth Fault (REF) protection

Mastering stability test of power transformer: Differential and Restricted Earth Fault (REF) protection