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Sweep Frequency Response Analysis Test | SFRA Test
Electrical Power Transformer | Definition and Types of Transformer
What is transformer? Definition & Working Principle of Transformer
Ideal Transformer
Theory of Transformer on load and no load operation
EMF Equation of Transformer | Turns Voltage Transformation Ratio of Transformer
Resistance & Leakage Reactance or Impedance of Transformer
Equivalent Circuit of Transformer referred to Primary and Secondary
Voltage Regulation of Transformer
Hysteresis Eddy Current Iron or Core Losses and Copper Loss in Transformer
Open and Short Circuit Test on Transformer
What is Auto Transformer ?
Tertiary Winding of Transformer | Three Winding Transformer
Parallel operation of Transformers
Transformer Cooling System and Methods
Core of Transformer and Design of Transformer Core
Transformer Insulating Oil and Types of Transformer Oil
Transformer Accessories | Breather and Conservator Tank | Radiator
DGA or Dissolved Gas Analysis of Transformer Oil | Furfural or Furfuraldehyde Analysis
Over Fluxing in Transformer
Single Three Phase Transformer vs bank of three Single Phase Transformers
Current Transformer CT class Ratio Error Phase Angle Error in Current Transformer
Voltage Transformer or Potential Transformer Theory
Accuracy Limit Factor and Instrument Security Factor of Current Transformer
Knee Point Voltage of Current Transformer PS Class
What is Earthing Transformer or Grounding Transformer
Transformer Winding Resistance Measurement
Voltage and Turn Ratio Test of Transformer
Vector Group Test of Power Transformer
Insulation Dielectric Test of Transformer
Transformer Oil and Winding Temperature Rise Test
Impulse Test of Transformer
Magnetizing Inrush Current in Power Transformer
Air Core Transformer
High Voltage Transformer
Silica Gel Breather of Transformer
Conservator Tank of Transformer
Maintenance of Transformer
Radiator of Transformer | Function of Radiator
Magnetic Oil Gauge or MOG | Magnetic Oil Level Indicator of Transformer
Oil Winding and Remote Temperature Indicator of Transformer
On Load and No Load Tap Changer of Transformer | OLTC and NLTC
Distribution Transformer | All Day Efficiency of Distribution Transformer
Installation of Power Transformer
Commissioning of Power Transformer

Transformer Winding Resistance Measurement

Under Electrical Transformer Transformer winding resistance measurement is carried out as a type test, routine test and also as a field test.

In the factory, it helps in determining the following :

  1. Calculation of the I2R losses in transformer.
  2. Calculation of winding temperature at the end of temperature rise test of transformer.
  3. As a benchmark for assessing possible damages in the field.
It is done at site in order to check for abnormalities due to loose connections, broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings.

Procedure of Transformer Winding Resistance Measurement

For star connected winding, the resistance shall be measured between the line and neutral terminal.

For star connected auto-transformers the resistance of the HV side is measured between HV terminal and IV terminal, then between IV terminal and the neutral.

For delta connected windings, measurement of winding resistance shall be done between pairs of line terminals. As in delta connection the resistance of individual winding can not be measured separately, the resistance per winding shall be calculated as per 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 all practical purposes of comparison with specified design values, previous results and diagnostics.

Winding Resistance at standard temperature of 75° C

Rt = Winding resistance at temperature t. t = Winding temperature.

Generally transformer windings are immersed in insulation liquid and covered with paper insulation, hence it is impossible to measure the actual winding temperature in a de-energizing transformer at time of transformer winding resistance measurement. An approximation is developed to calculate temperature of winding at that condition, as follows

Temperature of winding = Average temperature of insulating oil.
(Average temperature of insulating oil should be taken 3 to 8 hours after de-energizing transformer and when the difference between top & bottom oil temperatures becomes less than 5° C.)

The resistance can be measured by simple voltmeter ammeter method, Kelvin Bridge meter or automatic winding resistance measurement kit. (ohm meter, preferably 25 Amps kit)

Caution for voltmeter ammeter method: Current shall not exceed 15% of the rated current of the winding. Large values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance. NB: - Measurement of winding resistance of transformer shall be carried out at each tap.

Current Voltage Method of Measurement of Winding Resistance

The transformer winding resistances can be measured by current voltage method. In this method of measurement of winding resistance, the test current is injected to the winding and corresponding voltage drop across the winding is measured.

By applying simple Ohm's law i.e. Rx = V ⁄ I, one can easily determine the value of resistance.

Procedure of Current Voltage Method of Measurement of Winding Resistance

  1. Before measurement the transformer should be kept in OFF condition without excitation at least for 3 to 4 hours. During this time the winding temperature will become equal to its oil temperature.
  2. Measurement is done with D.C.
  3. To minimize observation errors, polarity of the core magnetization shall be kept constant during all resistance readings.
  4. Voltmeter leads shall be independent of the current leads to protect it from high voltages which may occur during switching on and off the current circuit.
  5. The readings shall be taken after the current and voltage have reached steady state values. In some cases this may take several minutes depending upon the winding impedance.
  6. The test current shall not exceed 15% of the rated current of the winding. Large values may cause inaccuracy by heating the winding and thereby changing its resistance.
  7. For expressing resistance, the corresponding temperature of the winding at the time of measurement must be mentioned along with resistance value. As we said earlier that after remaining in switch off condition for 3 to 4 hours, the winding temperature would become equal to oil temperature. The oil temperature at the time of testing is taken as the average of top oil and bottom oil temperatures of transformer.
current voltage method of star
  1. For star connected three phase winding, the resistance per phase would be half of measured resistance between two line terminals of the transformer.
  2. For delta connected three phase winding, the resistance per phase would be 0.67 times of measured resistance between two line terminals of the transformer.
  3. This current voltage method of measurement of winding resistance of transformer should be repeated for each pair of line terminals of winding at every tap position.
current voltage method


Bridge Method of Measurement of Winding Resistance

kelvin bridge

The main principle of bridge method is based on comparing an unknown resistance with a known resistance. When currents flowing through the arms of bridge circuit become balanced, the reading of galvanometer shows zero deflection that means at balanced condition no current will flow through the galvanometer. Very small value of resistance ( in milli-ohms range) can be accurately measured by Kelvin bridge method whereas for higher value Wheatstone bridge method of resistance measurement is applied. In bridge method of measurement of winding resistance, the errors is minimized. kelvin bridge to measure low resistance

The resistance measured by Kelvin bridge,

All other steps to be taken during transformer winding resistance measurement in these methods are similar to that of current voltage method of measurement of winding resistance of transformer, except the measuring technique of resistance.

whetstone bridge method of resistance mesurement The resistance measured by Wheatstone bridge,

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