Is it ok to perform an off-line PD test with a VLF Test set? If NETA/ANSI standards say it is OK? 1

[0neManRiot]

Is there an acceptable work around? if the manufacturer also claims that their PD TEST set cannot be used with VLF (0.1Hz) test sets and is designed for use with the power frequency (i.e. 50/60Hz) either on-line (in normal service condition) or off-line (via energisation from a transformer).

I understand that PD testing with VLF is quite difficult due to the requirement for a HV filter (to eliminate the noise) and specific software. For acceptance/commissioning testing often a VLF withstand (overvoltage) test is conducted off-line and then an on-line PD test is conducted directly after initial energisation and then on a 6-monthly or annual basis to trend the levels over time.

Is there a sufficient solution to OFF-LINE PD testing?

Thanks,
0neManRiot

 

[Zogzog]

The key here is what will the owner of the equipment do for future testing? You want to establish some baseline data for the type of testing they will do, if the plan is on line PD tests annually then do VLF followed by on line PD. If planned outages will be the norm then do Tan Delta testing.

NETA dosent say much either way, they just list the acceptable methods. PD, VLF, and TD are fairly new additions to the standard.

 

[0neManRiot]

Thank you Zog... I am fairly new to NETA testing standards(eight months applied engineering)
When you speak of Tan Delta testing, Is this also referred to as Time Domain Reflectometer testing?

I am still trying to get a handle on the relative High Voltage test equipment and procedures.

 

[ScottyUK]

Tan Delta measures the losses in a capacitive insulation, for example a bushing core.

TDR injects a pulse with short duration and fast rise and fall times into a conductor, for example a cable or a winding, and then analyses the reflections from discontinuities in the conductor.

 

[pwrtran]

Another term for Tan Delta is "dissipation factor"

 

[ScottyUK]

I should probably clarify that 'discontinuities' in my previous post means abrupt changes in characteristic impedance which causes part of the pulse energy to be reflected. The mechanism at work is explained in transmission line theory derived from Maxwell's equations. I'm not even going to try to explain that on a text-only forum. Plenty of better mathematicians than me have already written books on the subject!

 

[VLFit]

VLF or power frequency can be used for TD and PD testing cables, as well as AC Withstand testing. Of course 50/60 Hz testing is the best but usually the capacitance of the cable prohibits this approach, hence VLF. There are vendors around the world that produce TD and PD detection equipment that works reliably with VLF @ 0.1 Hz or 0.05 Hz. Much of the world has adopted VLF TD testing and some have done VLF PD, although it is more expensive and interpretive, but useful when done right. Any search engine will take you to the vendors. You can also check out

http://www.neetrac.gatech.edu.