Very Low Frequency-Application in Power equipment

In Wiki, very low frequency being explained:

"Very low frequency or VLF refers to radio frequencies (RF) in the range of 3 kHz to 30 kHz. "
In the area of electric engineering, VLF commonly refers to frequencies below or equal to 0.1Hz.
Very low frequency(VLF) method is now expanding its application in electric power engineering area, two recently announced products are:
a. transformer moisture analyzing system, where VLF plays very important role.
b. cable insulation and dielectric loss test, where VLF reduces the weight and power compared with traditional methods.