A Guide To Transformer DC Resistance Measurements, Part 1

By Bruce Hembroff, CET
Measuring a transformer's 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 or shorted turn), 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 (RA switch), 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.

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 manufacturers work. Installation measurements should be filed for future reference.

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?

Ratio Adjusting Switch: 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.

Load Tap Changer: The LTC contains the majority of the contacts and connections in the transformer. It is one of few non-static devices in the transformer and is required to transfer load current several thousand times a year. Hence, it demands special consideration during routine maintenance.

In addition to detecting problems associated with high resistance contacts and connectors, the Multi-Amp Transformer Ohmmeter will also detect open circuits as short as 1ms (drop-out test). LTCs transfer load current. They are not designed to interrupt load current. An open circuit would likely result in catastrophic failure. On installation and after maintenance it is certainly prudent to verify operating integrity by checking for open circuits. LTC maintenance often involves considerable disassembly and the test will provide confidence in the reassembly.

It is recommended DC resistance measurements be made on all on-load and off- load taps to detect problems and verify operating integrity of the RA switch and LTC.

Unscheduled Maintenance Troubleshooting
Unscheduled maintenance generally occurs following a system event. The objectives of unscheduled maintenance are:

Many transformer faults or problems will cause a change in the DC resistance measured from the bushings (shorted turns, open turns, poor joints or contacts). Hence, the information derived from the resistance test is very useful in analyzing faults or problems complimenting information derived from other diagnostic tests such as DGA, capacitance bridge, ratio, and excitation current measurements. The winding resistance test is particularly useful in isolating the location of a fault or problem and assessing the severity of the damage.

Internal Transformer Inspections
Internal inspections are expensive due primarily to the cost of oil processing. When such opportunities do present themselves the inspection should be planned and thorough. Prior to dumping the oil, all possible diagnostic tests including the resistance test should be performed.

Safety Considerations: While performing winding resistance tests hazardous voltages could appear on the terminals of the transformer under test and/or the test equipment if appropriate safety precautions are not observed. There are two sources to consider:

  1. A-C induction from surrounding energized conductors, and
  2. The D-C test current.
A-C Induction
When a transformer is located in an A-C switch yard in close proximity to energized conductors it is quite probable an electrostatic charge would be induced onto a floating winding. This hazard can be eliminated by simply tying all windings to ground. However, to perform a winding resistance test only one terminal of any winding can be tied to ground. Grounding a second terminal will short that winding making it impossible to measure the resistance of the winding. Two grounds on the winding under test would probably result in measuring the resistance of the ground loop. Two grounds on a winding which is not under test will create a closed loop inductor. Because all windings of a transformer are magnetically coupled the DC test current will continually circulate within the closed loop inductor (the shorted winding). The instrument display would probably not stabilize and accurate measurements would not be possible.

It does not matter which terminal is grounded as long there is only one terminal of each winding tied to ground. When test leads are moved to subsequent phases or windings on the transformer it is not necessary to move the ground connections. Ensure the winding is grounded prior to connecting the current and potential test leads, and when disconnecting leads remove the ground last.

D-C Test Current
Should the test circuit become open while DC current is flowing hazardous voltages (possibly resulting in flash over) will occur. Care must be taken to ensure the test circuit does not accidentally become open:

  1. Ensure the test leads are securely attached to the winding's terminals.
  2. Do not operate any instrument control which would open the measured circuit while DC current is flowing. Discharge the winding first.
  3. Do not disconnect any test leads while DC current is flowing. Ensure the winding is discharged first.
To terminate the test, discharge the winding following the instructions recommended by the instrument manufacturer. Then, connect a jumper across the transformer terminals shorting the winding. When testing larger transformers it may take 30 seconds or more to discharge the winding. If a longer time (30 seconds plus) is required to charge a winding when the current is initiated a corresponding longer time will be required to discharge the winding.

Summary of Safety Precautions

  1. Ensure all transformer windings and the test instrument chassis are grounded prior to connecting the test leads.
  2. Take appropriate precautions to ensure the test circuit is not opened while DC (test) current is flowing.
Failure to take appropriate precautions can result in hazardous potentials which could be harmful to both personnel and test equipment. It should be noted that transformer windings are essentially large inductors. The higher the voltage and the larger the (MVA) capacity, the higher the induction and hence the potential hazard.


Selecting The Proper Current Range
Ensure that the current output selected does not exceed 15% of the rated winding current. This could cause erroneous readings due to heating of the winding (e.g. A transformer rated 150 kVA, 1 ph: the rated current of the 66 kV winding is 2.25 amps; therefore the test current should not exceed 340 mA. Select the 50 mA current output.)

Choose the highest current output possible for the expected resistance value without exceeding 15% of rated winding current.

If the instrument will not energize the transformer at the highest current output, check connections and try again. Then, if necessary, reduce the current to the next lower value and repeat.

CAUTION: Do not operate the current selector switch without first discharging the winding (move the current control switch to discharge). Failure to discharge the winding could damage the instrument.

Measurements
Wait until the display has stabilized prior to recording resistance values. Generally, readings should stabilize in 10-15 seconds. However, the time required for readings to stabilize will vary based on the rating of the transformer, the winding configuration and the current output selected. On large transformers with high inductance windings it could take one to two minutes for readings to stabilize. If the readings don't stabilize within two minutes, check leads, connections and instrument. It may be necessary to reduce the test current.

Use Form I-89D, Transformer Resistance Test, to record measurements:

As a general rule, the first measurement made is repeated at the end of the test. Consistent first and last readings give credibility to all measurements. Whenever an unexpected measurement is obtained the test method and procedure is questioned. If the measurement can be repeated the doubt is removed. In situations where time is of concern the repeat measurement can be omitted if all measurements are consistent.

Always check the winding schematic on the nameplate, and trace the current path(s) through the windings. The nameplate vector representation may be misleading. Also, check the location of grounds on the windings and ensure the grounds do not shunt the DC test current.

When a winding has both an RA switch (ratio adjusting off-load tap switch) and an LTC (load tapchanger) take measurements as follows:

RA Switch Measurements
The recommended procedure for testing RA switches is as follows:
  1. Prior to moving the RA switch measure the resistance on the as found tap. Note: This measurement is particularly useful when investigating problems.
  2. Exercise the switch by operating it a half dozen times through full range. This will remove surface oxidization. See "Interpretation of Measurements - Confusion Factors".
  3. Measure and record the resistance on all off-load taps.
  4. Set the RA switch to the as left tap and take one final measurement to ensure good contact. Do not move the RA switch after this final measurement has been made.
LTC Measurements
As found measurements are performed for diagnostic purposes in both routine and non-routine situations. As left measurements are performed to verify operating integrity following work on the LTC. The resistance test on a transformer with an LTC is time consuming; hence the value of the as found test in each particular situation should be evaluated. Consider maintenance history and design. Certainly, if the proposed work involves an internal inspection (main tank) or a problem is suspected the as found test should be performed.

Prior to taking as left measurements exercise the LTC. Operating the LTC through its full range of taps two to six times should remove the surface oxidation.

When testing windings with LTCs, leave the transformer ohmmeter on while changing from tap to tap. Should the LTC open circuit for even 1 ms, the Transformer Ohmmeter will automatically shut off. Such transformers should not be returned to service as catastrophic failure would be possible.

Part 2 of this article will appear in the April issue. Bruce Hembroff is with Manitoba Hydro.