TAN DELTA and PD with VLF for MV Cables

VLF stands for Very Low frequency. VLF supply is recommended for testing cables due to their capacitance.

When High voltage 50/60Hz AC is applied to the cables for HV test or Tan delta / PD test, the test equipment needs to be heavy in order to supply the capacitive charging currents of cable.

As you may already know DC is not recommended for HV testing of cables due to the possible insulation deterioration with XLPE.

This is how the evolution of VLF happened. It doesn't have the disadvantages associated with 50/60Hz AC supply or those of DC.

IEC as well as ANSI standard for cables can give more details.

This subject has been covered in this forum many times, Mildred. All you have to do is to type those words into the Search this Forum box over there. ->

You need to do it and you don't know how. How did that come about, again?

Good electrical insulation acts like a capacitor. As insulation ages, leakage increases so insulation acts like a capacitor in parallel with a resistance.

Resistance is the same regardless of frequency, but the impedance of a capacitor increases as the frequency is lowered. The tan delta test measures the ratio of the impedance of the capacitance compared with the resistance of the leakage, so low frequency (VLF) is preferable to get a good reading.

The current through a capacitor leads the voltage by 90 degrees. The current through a parallel resistance and capacitance leads the voltage by less than 90 degrees. The tan delta test measures the tangent of this difference in angle, hence the name.

This site provides a good explanation.

https://hvtechnologies.com/blog/vlf-tan-delta-testing

https://cr4.globalspec.com/search/sitesearch?do=show&srchobjs=t%2Cbe%2Cc&order=asc&query=tan+delta&sort=textmatchrank&ts=7