Tan Delta Test

What Is Tan(delta) or Tan Delta?

Tan Delta also called as Loss Angle or Dissipation Factor testing is a diagnostic method to determine the quality of the cable insulation. This is done to predict the remaining life expectancy of the cable, so that you can replace the cable in time. It is also useful for determining what other tests has to be perform.

Working principle?

If the insulation of a cable is free from defects, like water trees, electrical trees, moisture and air pockets etc., the cable shows the properties of a perfect capacitor. It is very similar to a parallel plate capacitor with the conductor and the neutral being 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,the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation hence 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.

Example of a cable:The tangent of the angle displacement is measured. This will indicate the level of resistance in the insulation. By measuring IR/IC we can determine the quality of the cable insulation. Incase of a perfect cable, 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 the cable.

How are The Test Results Interpreted?

This tests has no standards and benchmark.The very first test on a cable yields valuable information about the insulation. Also, most tan delta testing is performed on a comparative basis.If a cable’s insulation is perfect, the loss factor (tan delta) will change little as the applied voltage is increased. The capacitance and loss will be similar with 1 Kv or 10Kv applied to the cable. If the cable has water tree contamination, thus changing the capacitive/resistive nature of the insulation, then the tan delta numbers will be higher at higher voltages. Rather than a flat curve for the loss number versus voltage, the curve will be non linear.From below graph, we can see that the aged cable has extensive water tree damage. The Loss Angle increases with increasing voltage, indicating a high resistive current element to the insulation. These results can be compared to other cables tested to determine which cables are in need of immediate replacement and which can wait a bit longer.An average value for the tan delta can be calculated and possibly used as a future benchmark.

Limitations of using Tan Delta Testing?

Since we are measuring the loss angle of an insulating material and making an analysis about the test results possibly based on historical data, it is not advisable to test a cable length that contains more than one type of cable.Different cables have different loss characteristics. It is not a good practice to test a cable length of XLPE insulation spliced to an EPR or PILC cable. The only way in which this is meaningful is when many tests are done on the same cable length over time and the results are carefully trended.