Tan Delta Test for Transformer

Tan Delta Test for Transformer In a pure capacitor current leads the voltage by 90 degrees. When a resistor is introduced in between the current leads an angle less than 90 degrees. Similarly in a Dielectric material when a cavity or deterioration starts, the life of the material starts deteriorating, as there is a resistance getting added and hence leakage current increases. In tan Delta we find the difference in the angle and periodically note down the pace at which deterioration takes place. Tan Delta is also known as the 'loss angle' or 'dissipation factor'. It is used to test the level of degradation in insulation materials of electrical machines and power cables. Tan Delta, also called Loss Angle or Dissipation Factor testing, is a diagnostic method of testing electrical equipment to determine the integrity of the insulation. This is done to try to predict the remaining life expectancy of the equipment. The winding insulation of an electrical machine, or a cable free from defects, will act as a perfect capacitor, that is, it will only store energy and not dissipate it. In an ideal capacitor, the voltage and current are out of phase by 90 degrees and the current flowing through the insulation is purely capacitive Ic. However, dielectric losses within the insulation due to deterioration or other factors, will result in the reduction of the resistance in the insulation. This will in turn increase the resistive current Ir. The phase angle between the current and voltage is now less than 90 degrees, and the extent to which this is less than 90 degrees gives the level of degradation in the insulation. So this 'loss angle' is required to be measured. If the angle is Delta, then the tangent of Delta can be calculated by dividing is 'opposite over adjacent', which is Ir/Ic. There are various methods of how this is tested and industrial standards, such as the IEC provide acceptable values of Tan-delta for different types of insulation. Tan Delta, also called Loss Angle or Dissipation Factor testing, is a diagnostic method of testing electrical equipment to determine the integrity of the insulation. This is done to try to predict the remaining life expectancy of the equipment. If the insulation free from defects, it approaches the properties of a perfect capacitor. It is very similar to a parallel plate capacitor with the conductor and the neutral being the two plates separated by the insulation material. In a perfect capacitor, the voltage and current are phase shifted 90 degrees and the current through the insulation is capacitive. If there are impurities in the insulation,for example, moisture, the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation. It is no longer a perfect capacitor. The current and voltage will no longer be shifted 90 degrees. It will be something less than 90 degrees. The extent to which the phase shift is less than 90 degrees is indicative of the level of insulation contamination, hence quality/reliability. This "Loss Angle" is measured and analyzed. Below is a representation of an insulation. The tangent of the angle δ is measured. This will indicate the level of resistance in the insulation. By measuring IR/IC (opposite over adjacent –

the tangent), we can determine the quality of the insulation. In a perfect insulation, the angle would be nearly zero. An increasing angle indicates an increase in the resistive current through the insulation, meaning contamination. The greater the angle, the worse is the insulation.

Insulation power factor is the angle 90-δ. If the insulation is excellent, the tan delta or dissipation factor is equal to the power factor. Essentially, both tan delta and power factor are just the same. Method of Testing The cable or winding whose insulation is to be tested is first disconnected and isolated. The test voltage is applied from the Very Low Frequency power source and the Tan delta controller takes the measurements. The test voltage is increased in steps upto the rated voltage of the cable. The readings are plotted in a graph against the applied voltage and the trend is studied. A healthy insulation would produce a straight line. The test should be continued only if the graph is a straight line. A rising trend would indicate weak insulation which may fail if the test voltage is increased beyond the rated voltage of the cable.

Interpretation of the test data There are not standard formulae or benchmarks to ascertain the success of a tan delta test. The health of the insulation which is measured is obtained by observing the nature of the trend which is plotted. A steady, straight trend would indicate a healthy insulation, while a rising trend would indicate an insulation that has been contaminated with water and other impurities.