Online Electrical Engineering
Design of Inductor in Switched Mode Power Supply Systems
Electrical Power Transformer | Definition and Types of Transformer
What is transformer? Definition & Working Principle of 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
Hysteresis Eddy Current Iron or Core Losses and Copper Loss in Transformer
Open and Short Circuit Test on Transformer
Voltage Regulation of Transformer
Over Fluxing in Transformer
Parallel operation of Transformers
Transformer Accessories | Breather and Conservator Tank | Radiator
Transformer Insulating Oil and Types of Transformer Oil
Silica Gel Breather of Transformer
Conservator Tank 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
Transformer Cooling System and Methods
Maintenance of Transformer
What is Auto Transformer ?
Buchholz Relay in transformer | Buchholz Relay operation and principle
Transformer Testing | Type Test and Routine Test of Transformer
Voltage and Turn Ratio Test of Transformer
Transformer Winding Resistance Measurement
Insulation Dielectric Test of Transformer
Transformer Oil and Winding Temperature Rise Test
Impulse Test of Transformer
Vector Group Test of Power Transformer
Sweep Frequency Response Analysis Test | SFRA Test
DGA or Dissolved Gas Analysis of Transformer Oil | Furfural or Furfuraldehyde Analysis
Distribution Transformer | All Day Efficiency of Distribution Transformer
Installation of Power Transformer
Commissioning of Power Transformer
Magnetizing Inrush Current in Power Transformer
Tertiary Winding of Transformer | Three Winding Transformer
Single Three Phase Transformer vs bank of three Single Phase Transformers
What is Earthing Transformer or Grounding Transformer
Air Core Transformer
High Voltage Transformer
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
Differential Protection of Transformer | Differential Relays
Restricted Earth Fault Protection of Transformer | REF Protection
Core of Transformer and Design of Transformer Core
Design of High Frequency Pulse Transformer
Transformer Winding Resistance Measurement
Under Electrical Transformer
In the factory, it helps in determining the following :
- Calculation of the I2R losses in transformer.
- Calculation of winding temperature at the end of temperature rise test of transformer.
- As a benchmark for assessing possible damages in the field.
Procedure of Transformer Winding Resistance MeasurementFor 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:
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
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 ResistanceThe 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.
Procedure of Current Voltage Method of Measurement of Winding Resistance
- 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.
- Measurement is done with D.C.
- To minimize observation errors, polarity of the core magnetization shall be kept constant during all resistance readings.
- 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.
- 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.
- 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.
- 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.
- For star connected three phase winding, the resistance per phase would be half of measured resistance between two line terminals of the transformer.
- 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.
- 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.
Bridge Method of Measurement of Winding Resistance
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.
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.
The resistance measured by Wheatstone bridge,