VLF Tan/Delta Cable Testing

Tan Delta is an effective tool in determining the insulation condition of power cables using a VLF-TD System.

Tan Delta measures the resistive component of a cable system; a perfect cable is all capacitance and has no resistance. The resistive component is caused by voids in the insulation, electrical trees, water trees etc. The resistive element causes the voltage not to lead the current by 90° (something less) This is difference in angle (less than 1° ) is the loss angle. The tangent of this loss angle is the Tan Delta / or the tangent of the loss angle.

Some of the many Questions we receive on Tan Delta Testing are:

    1. Isn’t that form of test destructive?

    2. Do I have to disconnect the cable from the switchgear, Transformer, I.E.?

    3. What test voltages do I use?

    4. What are good & what is bad Tan Delta Readings?

    5. What does 2.2*10 ¯ ³mean?

    6. What does 2Vo or 2Uo mean?

    7. Can water trees be detected?

# 1. Isn’t that form of test destructive?

VLF Testing can be if you are using a pass / fail method, but Tan Delta is one of six diagnostics methods, with the newer test equipment on the market today it is easy to say that VLF- TD is not a destructive test as most of the equipment will come with a waveform or meter and an emergency shut off much like the old Dc Hy-pot test set.

# 2 Do I have to disconnect the cable from the switchgear, Transformer, I.E.?

Yes you do have to disconnect the cable, you are measuring the insulation of the cable not the insulation of the switchgear & or the transformer.

#2A But the customer does not have time to disconnect the cables what do I do?

If the cable are connect to a open point I.E. Switchgear / breaker test it, if the cables are connected to a winding, I Say do not test it connected to any type of winding.

# 3 What test voltages do I use? Using the tables 4 and 5 from IEEE 400.2 is a good start, but if you follow the table from IEEE 400-2001 table 2 (See below) two times the rated voltage gives you a differential to make your assessment.  This is always up to the customer but, at least use the rated voltage to ground of the cable & cable accessories.

For Example 15 kV is = up to 8,300 volts to ground.

#4 What are good and what are bad Tan Delta Readings ?  If we look at table 2 from IEEE 400 - 2001 anyhing higher than a 2.2 x 10-3 is highly degraded.

The base line Tan Delta for EPR insulation will vary by the composition of the insulation by manufacturer.  As all EPR insulation is not created equal,  Tan Delta readings will generally be higher for XLPE insulation,  but good readings on a Tan Delta gradient should remain flat,  indicating good cable insulation.  If the readings are increasing it is indicating various levels of insulation agining.   In addition you also want to look at the phase comparisons.

# 5 What dies 2.2 x 10-3 mean ?  If you have a tangent 1 degree = 0.01746 = 7.46 x 10-3,  so that would mean that your tangent > 2.2 x 10-3 and the cable may need to be replaced.

# 6 What does 2Vo or 2Uo mean ?   IEEE uses this designation to represent 2 times the rated voltage to ground.