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Winding Resistance Testing. ndb Technologies WRT-10. Winding resistance measurements in transformers / generators & large motors are of fundamental importance for the following purposes:

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Winding Resistance Testing

ndb Technologies



Winding resistance measurements in transformers / generators & large motors are of fundamental importance for the following purposes:

  • Manufacturers will use the test to determine “copper losses” which are calculated using the formula I2*R
  • At the time of commissioning, a winding resistance test can be used to detect loose connections, typically at the connections to bushings, and abnormalities with the tap-changer as a result of poor workmanship or shipping damage.

• Calculation of winding temperature at the end of a temperature test cycle.


The voltage across an inductor is proportional to the time rate of change of the current through it.

  • The DC current source must be extremely stable. Refer to formula for DC voltage across a transformer below: v = I * R + (L di/dt) where:
  • vdc = voltage across transformer winding
  • I = DC current through transformer winding
  • R = resistance of the transformer winding
  • L = inductance of the transformer winding
  • di/dt = changing value of current (ripple)

Selecting the Proper Test Current Range

  • Transformer manufacturers typically recommend that the current output selected should not exceed 10% of the rated winding current. This could cause erroneous readings due to heating of the winding. Test current ranges are typically from 0.01% to maximum of 10 % of rated winding current.
stabilised resistance measurements
Stabilised resistance measurements
  • On large transformers with high inductance windings, it could take a few minutes for readings to stabilize. The time required for readings to stabilize will vary based on the rating of the transformer, the winding configuration, the instruments output current selected.
  • Readings on a Star/Wye-configured transformer should typically stabilize in 10 to 30 seconds
  • For large transformers with delta configuration, magnetization and getting stable readings can take significantly longer time, sometimes as long as 30-60 minutes.
stabilised resistance measurements b
Stabilised resistance measurements - B

By connecting the primary and secondary windings in series (using a Jumper cable), the speed of saturation and stability of the readings is increased because there are more amp-turns contributing to the flux in the core. The resistance measurement of both windings is made at the same time on channels A and B of the WRT-10.

Delta Windings

If possible, always inject test current to HV & LV simultaneously. Correct measurements values can be obtained approximately 7 times faster when injecting in both HV & LV in comparison to measuring LV only

wrt 10 reading stability indicator
WRT-10 Reading Stability indicator

Indicates if the voltage reading (and displayed resistance reading) has stabilized.

wrt 10 reading stability indicator1
WRT-10 Reading Stability indicator

- Gray when the WRT-10 is idle, when it is dumping current, or when measuring has begun but is not yet stabilized.

- Yellow when approaching stability (1% or less variation in the value for ten consecutive readings).

- Green once the nominal current value is reached and the resistance measurement is almost the same each time (within a 0.25% tolerance for ten readings in a row).


Winding resistance measurements

  • How much heat is being generated by the higher resistance of a transformer winding? This can be calculated (I²R) using the rated full load current.

Is this heat sufficient 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



  • Windings resistance measurements can reveal a great deal of information.
  • Faulted winding (open winding or shorted turn)
  • Integrity of numerous welded and mechanical connections
  • Poor Joints
  • RA switch, LTC (Diverter switch, Tap Selector switch) damage
transformer preventive maintenance winding resistance measurements vibrations a
Transformer preventive maintenance - winding resistance measurements (vibrations - a)

Slight changes of length exhibited by a ferromagnetic object when magnetized, result in the familiar “hum” heard around large power transformers, it’s the sound of the iron core expanding and contracting at 120 Hz (twice the system frequency, which is 60 Hz in the Canada & the USA), one cycle of core contraction and expansion for every peak of the magnetic flux waveform. The “hum” also includes the noises created by mechanical forces (Magneto-Motive Forces - mmf) between primary and secondary windings.


Transformer preventive maintenance

- windingresistancemeasurements (vibrations - b)

  • When the secondary winding is “loaded” (current supplied to a load), the winding generates an mmf (magneto-motive force), which becomes counteracted by a “reflected” mmf in the primary winding to prevent core flux levels from changing. These opposing mmf's generated between primary and secondary windings as a result of secondary (load) current produce a repulsive, physical force between the windings which will tend to make them vibrate.These vibrations can affect the integrity of different joints and connections in power transformers
tap changers maintenance
Tap Changers maintenance
  • A transformer tap is a connection point along a transformer winding that allows a certain number of turns to be selected. By this means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the output. The tap selection is made via a tap changermechanism.
  • Tap changers can be On-load (LTC) or Off-load (RA switch)
  • In power distribution networks, transformers commonly include an off-load tap changer on the primary winding to accommodate system variations within a narrow band around the nominal rating. The tap changer will often be set just once, at the time of installation, although it may be changed later during a scheduled outage in order to accommodate a long-term change in the system voltage profile.


About 96% of all power transformers today above 10MVA incorporate on load tap changers as a means of voltage regulation

For many power transformer applications, a supply interruption during a tap change is unacceptable, and the transformer is often fitted with a more expensive and complex on-load tap-changing mechanism. On-load tap changers may be generally classified as either mechanical, electronically assisted, or fully electronic. typical LTC are step switches, reversing switch & diverter switches.

These mechanisims have many different joints and connections that can be tested with winding resistance measurement.

reversing switches
Reversing switches
  • In most transformers the tap-windings are connected to the main winding through a reversing switch. The reversing switch allows the tap-windings to be either additive or subtractive (boost and buck) to the main winding. This effectively reduces the number of required taps by half. In other words, a tap changer that has for example; 16 raise taps and 16 lower taps, would have 33 different ratios, but only 16 tap selector switches (plus the neutral position).
reversing switch b
Reversing Switch-B
  • The diagram below illustrates the complete tap changer circuit. Note the switches “R” (raise) and “L” (lower) in the dotted circle.
  • N1 is the main winding. N2 is the tap winding. The diagram on the right is just a simplified version of that on the left.
ltc open circuit tap change
LTC – Open circuittap change
  • LTC’s transfer load currents and are designed for make-before-break, they are NOT designed to interrupt load currents. An open circuit would likely result in catastrophic failure. On installation and after maintenance it is certainly advised to verify operating integrity by checking for open circuits.
  • If an open circuit is detected while changing taps the WRT-10 will automatically stop and go into its discharge cycle indicated by the discharge indicator.
v graph analysis tool for tap changers
V-graph analysis tool for Tap-changers

ndb Report Manager Software

  • This graphical representation of the Lower to Riser tap winding resistance is an easy and fast tool for Tap-changer analysis. We are looking for an ideal smooth & symetrical V shape graph.
v graph 3 phase overlaping possible reversing switch problem on phase b
V-Graph 3 phase overlaping ( possible Reversing switch problem on Phase B )

ndb Report Manager Software

v graph before and after analysis
V-Graph, Before and After analysis

ndb Report Manager Software

example of v graph information
Example of V-Graph information
  • High Resistance in Diverter Switch
  • In the example below, it can be seen that all of the “even numbered” taps (2, 4, 6, 8…) are higher than the norm. (Norm being the imaginary V-shaped line drawn through the data points). Note that all the odd taps are connected through side “B” of the diverter switch and all the even taps are connected through side “A” of the diverter switch. The pattern of results below would indicate that the contact resistance of the “A” side of the diverter switch is high.

Please note that when graphed, there will normally be some deviation from the norm between the odd and even values. This does not necessarily indicate a problem with the diverter switch. This may be due to physical installation conditions. For example the internal cables connected to one side of the diverter switch may be longer than those connected to the opposite side. In fact if the diverter switch is damaged, the results will be inconsistent. In other words, some of the odd (or even) taps will have elevated resistance and other will appear normal. Or, if a given tap is tested more than once, after operating the tap changer, the result for the first test will be different from the second test and so on. 

  • Sound judgment and the ability to compare readings to factory values, historic values, similar transformers or other phases will help in drawing the correct conclusion.

High Resistance in Reversing Switch

  • In the example below, when the reversing switch is in the “raise” position all the resistance values are high than when the reversing switch is in the “lower” position. In this case, a high resistance on the “raise” side of the reversing switch is suspected.
  • Refer back to figure 2. The reversing switch only operates as the tap changer changes from operating in the additive connection to a subtractive connection. For example if the tap changer has been in the subtractive position (taps -1 to -16) and is then required to operate in the additive mode, the reversing switch will reverse the connections of the tap winding as the mechanism moves through neutral and makes its connection to the first raise tap (+1).

High Resistance in Tap Selector Switch

  • In the example below, it can be seen that tap +5 and -5 both have higher than normal resistance.
  • In this example both these taps use the same tap selection, but with the reversing switch in different positions, it is likely that the cause of this inconsistency is a high resistance in position five of the tap selector switch.
  • Note that the entire circuit to tap 5 is suspect, not just the tap switch itself. This would include any connections between the taps switch and the winding. 
  • The reversing switch can not be to blame because in every case the resistance of the odd tap is similar to its even counter part.
induction noise
Induction / Noise

The WRT-10 has Sigma-delta A/D converters and notch filters on input channels to eliminate effects of substation noise.

safety induction in substation
Safety - Induction in Substation

transformer located in substations in close proximity to energized conductors are subject electrostatic charges induced onto floating windings. This hazard can be eliminated by grounding all windings. However, to perform a winding resistance test only one terminal of any winding can be tied to ground. It’s important that the windings are grounded prior to connecting the current and potential test leads, and when disconnecting leads the ground is the last to be removed.

wrt 10 color screen
WRT-10 Color Screen
  • Large, Bright, Color LCD screen, 120 mm x 90 mm (4.75" x 3.5"), easy viewing on a sunny day.
what is the transformer temperature rise test or heat run test
What is the Transformer Temperature Rise Test or “Heat Run Test”?
  • This test determines the average winding temperature rise of the transformer under rated load (generally performed by Transformer manufacturers)
  • Thermometers are installed to measure ambient temperatures, oil temperatures, etc... Readings on these are taken before the test is started to obtain base temperatures on which to determine the rise at rated load, These are also read periodically during the test, and continued until temperatures do not vary more than a few degrees over a period of hours.
  • If the temperature rises is above the transformers winding heat Class tolerance, overheating of the insulation takes place and that leads to an accelerated ageing of the insulation and, in excessive cases, could damage the transformer.
what does the winding resistance measurements have to do with the heat run test
What does the Winding Resistance Measurements have to do with the « Heat Run Test »?
  • The average winding temperature is determined by the equation:

T = R/R0 (Tk + T0 ) – Tk (note: IEEE: C57.12.90)

  • Where:T is the temperature (°C) corresponding to hot resistance R,T0 is the temperature (°C) at which cold resistance R0 was measuredR0 is the cold resistance (Ohm)R is the hot resistance (Ohm)Tk is 234,5 °C for copper (resp. 225,0 for aluminium)
The WRT-10 is equiped with a timed interval resistance measurement logging feature for heat run testing

IEEE C57.12.90 requirements for Hot Resistance measurements.

  • The time from instant of shutdown shall be recorded for each resistance measurements.
  • At least one resistance measurement shall be taken on all terminal pairs within 4 min after shutdown. Resistance-time measurements shall be made on all windings.
  • The resistance-time data shall be corrected to the instant of shutdown using a resistance-time cooling curve determined by plotting data on suitable coordinate paper, or by using a curve fitting program.
  • The resistance-time data obtained on one phase of a winding shall be used to determine the correction to shutdown for the other phases of the same winding, provided the first measurement on each of the other phases has been taken within 4 min after shutdown.
wrt 10 advantages
WRT-10 Advantages
  • High accuracy 0.25%
  • 0.01A, 0.1A, 1A & 10A (30V) Current regulated output source for fast stabilization and saturation
  • V-Graph analysis for fast tap-changer diagnostics ( PC software )
  • Reading stabilization indicator, ( we look for no more than 0.25% deviation for 10 measurements ) to indicate stabilization is completed.
  • Load Tap Changer channel, ( we can control change the taps of a transformer from the WRT-10 )
  • Auto-Tap Changer mode ( automatically performs measurement , data log at tap change sequence )
  • Duty Cycle:  the maximum tested 4.5 hours continuous at full 10A output.
wrt 10 advantages1
WRT-10 Advantages…
  • RS-232C & USB communications
  • Passive Circuit discharge indicator and .. LED for applications where while the instrument is charging a transformer a power outage occurs , the user can see that the instrument is still discharging the transformer – SAFETY !
  • In the event that the current circuit is opened, the WRT-10 has a fast Crowbar discharge circuit (voltage channel)
  • 50 feet / 16 meters of testing cables + 1 jumper cable for dual channel performance measurements ( included)
  • Special Sigma / Delta input measurement filter, looks out substation noise and induction influence of measurements
  • Large Memory, 100Files of 120 Measurements each
  • Break before Make feature for On-Load Tap-Changer analysis,  the instrument automatically discharges the transformer when inductive kick back is detected showing that there is an open circuit between 2 taps of the LTC!!
wrt 10 advantages2
WRT-10 Advantages …
  • Heat Run testing mode, where the resistance data can be logged for cool down curve information.  The instrument can record the time and interval of the measurements automatically ( 5 seconds to 100 seconds intervals )
  • Transformer information can be entered directly on the instrument
  • FULL keyboard, for easy data entry.
  • Beep sound level adjustment ( off, low- med – high )
  • FULL Color display, easy to see even in bright daylight !
  • Housing includes a storage compartment for cables, or accessories
  • Housing has wheel and a retractable handle
  • Lightweight instrument, 25 LB or 11.4 KG
  • Resistance , temperature correction feature ( Aluminum , Cooper and custom )
  • On board Printer
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