According to IEEE 400.2 VLF tests should last between 15 and 60 minutes with a recommended minimum duration of 30 minutes. This length of time hasn’t just been plucked out of the air; there is some theory behind it.

According to IEEE 400, when testing at 3 times the line to ground system voltage the tree growth rate of XLPE at a 0.1 Hz sinusoidal test voltage is 10.9 – 12.6 mm/h.

A 15 kV 133% cable has an insulation thickness of 5.9 mm. Therefore in a 30 minute test nearly all defects will grow to failure.

What is a Tree?

At stressed points in insulation where there are protrusions, voids or contaminants a phenomenon called treeing may occur. Treeing is the preferred name due to the branch like structure of these trees. There are two types of tree effect:

Water tree: Produced by electro-oxidation fuelled by electrical stress and water ingress within insulation. Water trees do not emit partial discharges so PD testing cannot be used to test for the presence of water trees. During service conditions the growth of water trees is extremely slow taking years to completely penetrate the insulation. Like protrusions, voids and contaminants, water trees act as stress enhancements. They can increase the local electrical field and also create local mechanical stresses. If these electrical and/or mechanical stresses are high enough an electrical tree can initiate.

Electrical tree: Micro-channels of carbonisation or non-insulation found within insulation that is irreversibly damaged. Electrical treeing will lead to a completed failure path and failure of insulation relatively quickly. Below is an electrical tree that is growing from a water tree.