At some point in our lives we’ve probably all heard the old saying which states: “There’s no such thing as a dumb question…” which is often followed by “…except for the one you don’t ask!” or something similar.
Random questions are part of the daily routine for many of us. From time to time a pattern starts to emerge related to a specific topic which often means “everyone has the same question” and maybe we need to address it more broadly. One such topic which I’ve been hearing a lot lately is related to “VLF Testing”. To that end here’s my version of FAQ’s related to VLF testing.
So, what exactly is VLF testing?
VLF stands for Very Low Frequency testing; which is an AC method of testing high capacitance loads.
Isn’t that the same as a Hi-Pot test?
A VLF test set is essentially an AC Hi-Pot operating at a very low frequency.
What can you test with VLF?
VLF testing is well suited for testing high capacitance loads such as cable, insulators, arrestors, and rotating machinery, such as motors and generators.
What’s the advantage of VLF compared to a Hi-Pot test?
One of the advantages is, due to the use of low frequency signals, the size of the power source needed for testing is minimized. This means the test set is smaller, lighter, requires less power to be injected into the cable system and is more portable when compared to a traditional Hi-Pot test set. In addition, due to the lower power demands, the test set can efficiently be used to test systems with long cable lengths.
Can VLF testing cause my cable to fail under test?
While VLF testing will not lead to premature failure or damage healthy insulation, if there is a defect in the insulation system, such as a water tree in the insulation or a bad splice, the damaged cable or component will fail under test; when repairs can be made while the cable is isolated as opposed to an unscheduled failure while in service. If the cable fails under test it was going to fail shortly anyway and you’ve just prevented an unplanned outage.
It sounds like VLF testing is still just a stress test.
While it is a stress test, VLF testing doesn’t create voids or ‘trees” in the insulation and isn’t prone to flashovers.
Why not just stick with the DC Hi-Pot?
Traditionally DC voltage sources have been used for field testing of cables, specifically dielectric strength and the resistance of electrical insulation. However, DC sources have been shown to be ineffective at detecting insulation weaknesses and defects for several of the most commonly occurring forms of insulation damage. In addition, VLF is less likely to leave residual charges that contribute to premature cable insulation degradation.
Lastly, DC testing can yield ambiguous results and are generally considered ineffective for testing MV and HV power cables. In contrast VLF testing utilize AC voltage signals produced at frequencies in the range of 0.01 Hz to 1 Hz.
When would you use VLF testing?
VLF testing can be used for installation, acceptance, and maintenance testing of insulated power system cables. The tests typically are based on withstand and diagnostic testing standards, per IEEE 400.2.
There are three types of tests which are covered by the standard:
• Installation Test: Completed after the cable is installed but prior to installing any accessories (splices, terminations, etc.). These tests are intended to detect any cable insulation defects related to manufacturing, transport, and installation.
• Acceptance Test: These tests are conducted after the installation of the cabling system, including all cable and accessories, but prior to final energization of the cable. The purpose is to detect installation damage in the cable and cable accessories.
• Maintenance Test: Performed periodically throughout the operating life of the cable system to assess the condition and serviceability of the cable system. For lifecycle and maintenance planning, scheduling replacement, repairs and upgrades.
So, remember, there’s no such thing as a dumb question…keep asking and keep learning!!