Power transformers are one of the most expensive and critical assets of the power sector. In this blog, we will discuss  principles of winding resistance measurement, a common transformer test prescribed in various standards such as IEEE C57.12.90-2006 Clause 5, IEC 60076-1 Clause 10.2, IS 2026-1 Clause 10.2;  OLTC condition analysis; and  share our thoughts on the specifications of the ideal kit for transformer testing.
Why Measure Winding Resistance?
The table below discusses the need for measuring winding resistance in the factory and on-site:
|Factory test benefits||On-site test benefits|
|Ensure proper manufacturing||Evaluate possible winding damage, such as open turn and inter-turn short of winding|
|Calculation of copper loss (I2R loss)||Identify contact problems|
|As a base for assessing possible damage on-site||Check proper functioning of the tap changer|
|To identify loose connection and broken strands of conductor||Calculate the winding temperature|
|Calculate the winding temperature|
Principles of Winding Resistance Measurement
In this technique, the idea is to inject DC current through the winding, and measure corresponding voltage drop across the windings of the same phase after stabilization of current. While applying DC voltage to transformer winding, the current increases slowly. This rate of increase depends on winding inductance and applied voltage. The larger the winding (inductance), the greater the difficulty in achieving current stabilization. Once required current is established, the voltage should remain constant to maintain constant current.
To remove the effect of inductance (L), the change in current needs to be zero. To achieve this, transformer core saturation must be ensured and current must be stable.
To calculate winding resistance, we use Ohm’s law:
R = Vdc ⁄ Idc
Where, Vdc = voltage across transformer winding,
Idc = DC current through transformer winding,
R = resistance of the transformer winding
Open Circuit Voltage
- Open circuit voltage rating of the test-kit is critical to understand the time required by the test-kit to generate enough magnetic flux to saturate the transformer core before conducting the WRM test
- The time needed also depends upon the number of windings being tested simultaneously and prior level of core magnetization.
Selecting Test Current Range
- According to IEC 60076/ IEEE C57, it is recommended that test currents are 0.1-10% of rated current
- Care should be taken to never exceed 10% of rated current. This can cause erroneous readings due to heating of the winding
- Keeping these factors in mind, typically, test current range is 1-3% of rated current.
Details of the Test
The following steps illustrate how to conduct this test:
- For star connected winding, the resistance is to be measured between the line and neutral terminal. On the other hand, for delta connected windings, measurement of winding resistance should be done between pairs of line terminals.
- In delta connection the resistance of individual winding cannot be measured separately. Thus, the resistance per winding shall be calculated with the following formula:
Resistance per winding = 1.5 x Measured value
- The resistance is measured at ambient temperature and then converted to resistance at 75˚C for purposes of comparison with specified design values, previous results and diagnostics, as per the formula below:
R75 = Rt(235+75 / 235 +Rt)
Where, Rt = Winding Resistance at temp t
t = Winding Temp
- Very small value of resistance (in mΩ range) can be measured by 4-wire Kelvin bridge method and Kelvin clips are made to facilitate this kind of connection across a subject resistance which avoids errors caused by wire resistance.
There are two distinctive types of DC Winding Resistance measurement: a) static and b) dynamic. Static test is the standard test used to measure the actual resistance value of a transformer winding and associated series components. Dynamic Test is typically applied to load-tap changing (LTC) transformers. The dynamic winding resistance measurement tracks the changing resistive behavior as the LTC operates.
The steps below describe the process of OLTC testing:
- Dynamic winding resistance measurement is conducted to track the changing resistive behaviour as the LTC operates
- Measure the fast-switching process of the diverter switch
- Test current is applied through the on-load tap changer to detect increased contact resistances and current interruptions. Typically, arcing contacts, switching times of the diverter switch, switching interruptions due to broken commutating resistors or broken leads can be detected.
- Analysis by Current Curve: The moment a tap position is changed from one tap to another, the device detects a sudden, very short drop of the test current. By analyzing the recordings, it is possible to draw a number of conclusions related to the condition of the OLTC. The winding during tap change is sampled at high speed, and measure:
- During tap change, the current is sampled and plotted against time
- % drop in current during tap change and
- Time required to change the tap position from one tap to another tap i.e. transition time
Interpretation of Measurements
On completion of the tests, measurements can be compared as shown below:
- Comparing to original factory measurements
- While comparing absolute resistance values in the field with factory values may be difficult due to the problem of exactly estimating the winding temperature, values within 5% are normally acceptable.
- The industry standard permits a maximum difference of 0.5% from the average of the three phase windings.
- Comparing one phase to another
- Variation from one phase to another or inconsistent measurements can be indicative of many different problems
- Comparing to previous field measurements
- Field readings may vary slightly more than this due to the many variables. If all readings are within one percent of each other, they are acceptable.
- The transformer core may remain magnetized as a result of DC measurements of winding resistance. This can lead to issues when re-energizing the transformer or when performing frequency response analysis.
Demagnetization feature in winding resistance kits allows the users to demagnetize the core of the transformer at the completion of the test.
Ideal Kit Specification
In our opinion, the ideal kit for transformer winding resistance measurement should adhere to the following:
- It should be capable of working in electrostatic noisy conditions in charged EHV substations up to 765kV & should offer repeatability, consistency & immunity to interference in live switchyard conditions up to 765kV levels
- It should be capable of sustaining Back EMF offered by large power transformers or other Inductive Loads. Necessary protection with indication should be available in the kit.
- It should have a minimum 3/6 channels for Winding Resistance measurement and 1 channel for external temperature measurement. One time connection of the Test Lead to all HV (3 Nos) or/ and LV (3 Nos) windings should be possible. All the winding resistances should be calculated automatically by the instrument in a single connection.
- Minimum selectable current ranges should be from 10mA -25A or 10mA -50A DC with various selectable currents (25A/50A DC current makes it possible to measure low resistances with high accuracy)
- Instrument should use Kelvin’s 4 wire method for resistance measurement to compensate for lead resistance.
- Kit should have Resistance measurement Range up to 2000Ω with best resolution 0.1µΩ
- Kit should have minimum 50V open circuit voltage for quick stabilization of current
- The kit should have the facility of automatic temperature coefficient correction, for Copper and Aluminum etc
- The instrument should have touchscreen display with interactive menu and result display, keyboard for setting and operation functions & messages like status of current rising in the test object, value of stabilized current, warning message during discharging of test object etc.
- The instrument should have a facility of automatically discharging the specimen when test is completed or when current cable is accidentally disconnected or when instrument’s power supply is lost.
- It should have Measurements of winding resistance of all taps of phases in one go.
- The kit should have the automatic demagnetization feature. The operator should be able to conduct the demagnetization as an independent function as well as at the end of every test.
- The kit should be able to conduct the measurement of resistances of all taps with automated tap change function.
- It should check any discontinuity in OLTC while changing tap from one position to another
- To analyze OLTC Healthiness the kit should be able to conduct Resistance Vs Time test i.e. the variation in the winding resistance sampled and plotted against time (sec) in graphical way also display % drop in current during tap change & Time required to change the tap position from one tap to other tap (i.e. transition time).
- Instrument should have Internal Memory to store at least 1000 test results for future ref
- Facility to operate it externally through a Notebook Computer through software. Kit should have Ethernet and / or a USB port for connection with the PC.
- The kit should work on single phase input power supply.
SCOPE manufactured TRM instrument is able to conduct the above-mentioned tests in an effective manner, while adhering to all the specifications of the ideal kit for transformer winding resistance measurement.