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 tapchanger or LTC), as well as numerous welded and mechanical connections. Hence, the integrity of all these components can be verified.
Author's 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.
CONNECTIONS
General
Prior to connecting the instrument leads to the transformer all transformer windings must be grounded. See Safety Considerations.
Make connections in the following order:
1. Ensure winding terminals are not shorted together and tie to ground (the transformer tank) one terminal only of each transformer winding (i.e. both the winding to be tested as well as those not being tested). Note: It does not matter which terminal is grounded (a line terminal or neutral) as long as only one terminal on each winding is grounded. There is no need to move the ground as the test progresses to measuring subsequent phases or windings.
2. Ensure the instrument's power switch is in the off position and connect it to a 115 volt supply. Note: The instruments chassis is grounded through the supply cable to the station service.
(On occasion it has not been possible to stabilize the display when the instrument's chassis ground was not connected to the same ground point as the winding (i.e., the transformer tank) This problem is most likely to occur when the station service ground is not bonded to the transformer tank and is easily remedied by connecting a jumper between the instrument chassis and the transformer tank.)
3. Connect the current and potential leads to the instrument.
4. Connect the current and potential leads to the transformer winding.The potential leads must be connected between the current leads. Do not clip the potential leads to the current leads. Observe polarity. If the potential leads are connected opposite to the current leads erroneous measurements will result.
Upon completion of the test ensure the winding is discharged before disconnecting any test leads. Remove the ground from the transformer winding last. Caution: Do not open the test circuit in any way (i.e. disconnecting test leads, or operating the current selector switch) while DC current is flowing. Hazardous voltages (probably resulting in flash-over ) will occur.
Wye Windings
Refer to Figure 1. Measure two windings simultaneously. Take resistance measurements with the indicatedconnections.
Connecting the test equipment as per Figure 1 is the preferred method because it allows the operator to measure two phases simultaneously. Compared to measuring each phase individually there is a significant time saving particularly when measuring a winding with an LTC.
Alternately, if the instrument will not energize both windings simultaneously, measure one winding at a time. Refer to Figure 2 and take measurements with theindicated connections.
If time is of concern the last test set up, which is a repeat of the first, may be omitted if all measurements are consistent when comparing one phase to the next or to previous tests.
Delta Windings
Refer to Figure 3. Take resistance measurement with the indicated connections. Again, if time is of concern the last test set up, which is a repeat of the first, may be omitted if all measurements are consistent when comparing one phase to the next or to previous tests.
Interpretation of Measurements
Measurements are evaluated by:
1. Comparing to original factory measurements
2. Comparing to previous field measurements
3. Comparing one phase to another
The latter will usually suffice. The industry standard (factory) permits a maximum difference of 1/2 per cent from the average of the three phase windings. Field readings may vary slightly more than this due to the many variables.
Variation from one phase to another or inconsistent measurements can be indicative of many different problems:
- Shorted turns
- Open turns
- Defective ratio adjusting (RA) switch or LTC
- Poor connections (brazed or mechanical)
The winding resistance test is very useful in identifying and isolating the location of suspected problems.
Confusion Factors
Apparent problems (i.e., inconsistent measurements or variations between phases) can also be the result of a number of factors which are not indicative of problems at all. Failure to recognize these factors when evaluating test data can result in confusion and possibly unwarranted concern.
Temperature change: The DC resistance of a conductor (hence winding) will vary as its temperature changes. This is generally not a significant consideration when comparing one phase to another of a power transformer. Loading of power transformers is generally balanced, hence temperatures should be very similar . However, when comparing to factory measurements or previous field measurements small but consistent changes should be expected. In addition to loading, temperature variations (likewise resistance variations) can be due to:
- Cooling or warming of the transformer during test. It is not uncommon for one to two hours to pass between taking a first and last measurement when testing a large power transformer with an LTC. A transformer which has been on load can have a significant temperature change in the first few hours off- load.
- When measuring the DC resistance of smaller transformers care should be exercised to ensure that the test current does not cause heating in the winding. The test current should not exceed 15 per cent of the windings rating.
Contact oxidization: The dissolved gases in transformer oil will attack the contact surfaces of the RA switch and LTC.The problem is more prevalent in older transformers and heavily loaded transformers. Higher resistance measurements will be noticed on taps which are not used. (Typically a load tapchanger installed on a subtransmission system will only operate on 25-50 per cent of its taps.) This apparent problem can be rectified by merely exercising the switch. The design of most LTC and RA switch contacts incorporate a wiping action which will remove the surface oxidization. Hence, operating the switch through its full range 2 to 6 times will remove the surface oxidization.
A potential transformer installed in one phase could become part of the measured circuit and affect the measured DC resistance of that phase.
A two winding CT installed in one phase would have a similar effect. Usually donut bushing type CTS are used in power transformers. However, on rare occasions an in-line two winding CT may be encountered.
A measuring error: There are many possibilities:
- A wrong connection or poor connection
- A defective instrument or one requiring calibration
- An operating error
- A recording error
Ambiguous or poorly defined test data: There is often more than one way to measure the resistance of a transformer winding (e.g., line terminal to line terminal or line to neutral). Typically, field measurements are taken from external bushing terminals. Shop or factory measurements are not limited to the bushing terminals. Additionally internal winding connections can be opened (e.g. opening the corner of a delta) making measurements possible which are not practical in the field .Details of test set ups and connections area often omitted in test reports which can lead to confusion when comparing test data.
How Bad is Bad?
When a higher than expected measurement is encountered what does it mean? Is failure imminent? Can the transformer be returned to service? Is corrective action needed? To answer these questions more information along with some analytical thinking is usually required.
1. Firstly, have the confusion factors been eliminated?
2. Secondly, what are the circumstances which initiated the resistance test? Was it routine maintenance or did a system event (e.g. lightning or through fault) result in a forced outage?
3. Is other information available? Maintenance history? Loading? DGA? Capacitance bridge? Excitation current? If not do the circumstances warrant performing additional tests?
4. Consider the transformer schematic. What components are in the circuit being measured? Has the location of the higher resistance been isolated? See "Isolating Problems".
5. How much heat is being generated by the higher resistance? This can be calculated (I2R) using the rated full load current. Is this sufficient heat to generate fault gases and possibly result in catastrophic failure? This will depend on the rate at which heat is being generated and dissipated. Consider the mass of the connector or contact involved, the size of the conductor, and its location with respect to the flow of the cooling medium and the general efficiency of the transformer design.
Isolating Problems
The resistance test is particularly useful in isolating the location of suspected problems. In addition to isolating a problem to a particular phase or winding, more subtle conclusions can be drawn. Consider the transformer schematic (nameplate). What components are in the test circuit? Is there an RA switch, LTC, diverter isolating switch, link board connectors, etc.? By merely examining the test data, problems can often be isolated to specific components. Consider:
RA Switch: In which position does the higher resistance measurement occur? Are repeat measurements (after moving the RA switch) identical to the first measurement or do they change.
LTC: The current carrying components of the typical LTC are the step switches, reversing switch and diverter switches. Carefully examine the test data looking for the following observations:
Component: Step Switch Observation: A higher resistance measurement occurring on a particular tap position both boost and buck (e.g., both +1 and-1, +2 and -2, etc.)
The above observation would indicate a problem with a particular step switch. Each step switch is in the circuit twice. Once in the boost direction and once in the buck direction.
Component: Reversing Switch Observation: All boost or buck measurements on a phase are quantatively and consistently higher, than measurements in the opposite direction or other phases.
The reversing switch has two positions, buck and boost, and operates only when the LTC travels through neutral to positions +1 and -1. Hence a poor contact would affect all boost or buck measurements. If the LTC is operated between +1 and -1 the resistance measured through a poor reversing switch contact would likely change.
Component: Diverter Switch Observation: All odd step or all even step measurements in both the buck and boost direction are high.
There are two diverter switches. One is in the current circuit for all odd steps and the other for all even steps.
The foregoing discussion is only typical. LTC designs vary. To draw conclusion based on resistance measurements, the specific LTC schematic must be examined to identify the components which are being measured on each step. This information is usually available on the transformer nameplate.
Contacts vs Connectors or Joints: Is the higher resistance measurement consistent and stable when the RA switch or LTC is operated? Generally inconsistent measurements are indicative of contact problems while a consistent and stable high measurement would point to a joint or connector.
Limitations
The transformer resistance test has several limitations which should be recognized when performing the test and interpreting test data:
The information obtained from winding resistance measurements on delta connected windings is somewhat limited. Measuring from the corners of a closed delta the circuit is two windings in series, in parallel with the third winding (see Figure 4).
The individual winding resistances can be calculated; however this is a long tedious computation and is generally of little value. Comparison of one 'phase' to another will usually suffice for most purposes. Additionally, since there are two parallel paths an open circuit (drop out) test does not mean too much. However, the test is still recommended. Problems involving LTCs and RA switches will yield measurements which are not uniform, and often unstable and inconsistent. Hence the resistance test will detect most problems.
The resistance of the transformer's winding can limit the effectiveness of the test in detecting problems. The lower the resistance of a winding the more sensitive the test is with respect to detecting problems. Windings with high DC resistance will mask problems.
The detection of shorted turns is not possible in all situations. Often shorted turns at rated AC voltage cannot be detected with the DC test. If the fault is a carbon path through the turn to turn insulation it is a dead short at operating potentials. However, at test potential, 30 V DC, the carbon path may be a high resistance parallel path and have no influence on the measured resistance. Certainly if the conductors are welded together the fault should be detectable.
It is not possible on some transformer designs to check the LTC using the resistance test (e.g., series winding). The circuit between external terminals simply excludes the LTC.
On such units the resistance test is of no value in verifying the operating integrity of the LTC. If the LTC selector switch is in the main tank (i.e., same tank as windings) and cannot be physically inspected it is recommended that samples for DGA be taken as part of routine LTC maintenance.
Bruce Hembroff is with Manitoba Hydro. Part 1 of this article appeared in the March issue of Electricity Today.