Medium Voltage Switchgear Megger Testing 4

[mellomain]

When conducting a megger test of 4160V switchgear bus (cubicle to cubicle), how much resistance would a CT add to my result?

Man is troubled by what might be called the Dog Wish, a strange and involved compulsion to be as happy and carefree as a dog --- James Thurber

 

[wbd]

Would not the CT's be window type and encircle each phase bus, so they should not be in the meggar circuit. Unless you mean PTs, which should be disconnected when meggaring.

 

[mellomain]

Thanks. My technicians said it was a CT. By removing the CT the meg reading dropped by 70-80 micro-ohms. Maybe they were supposed to take it out of the circuit to start with and just did not know. Most of the runs in this switchgear do not have CTs where we are measuring. This one measurement did have CTs. Our procedures are generally detailed enough to tell them to remove the CT prior to taking the measurement or the acceptance criteria accounts for it. Neither was the case here. So I was just wondering how much would a CT typically add so we can fix the procedure.

Man is troubled by what might be called the Dog Wish, a strange and involved compulsion to be as happy and carefree as a dog --- James Thurber

 

[DTR2011]

I think some terminology is being misused. A Megger test yields results in Meg (or Greater) Ohms. I Ductor (DLRO, contact resistance, etc) yields results in
micro (or milli) ohms. If a Megger test was being performed with VT's (L-N connected primary), a very low value would be measured, through the VT winding itself. On a Megger, it may read very close to Zero.

As mentioned earlier, if the CT's are window type, they are not in physical connection with the bus bar. If the CT's are bar type, they would be included in the measurement. I'm fairly sure that a 70-80 micro ohm difference would not be picked up by a 1kV or 5kV Megger test set.

To directly answer the first question - A CT (bar type), would typically be expected to Lower the Insulation Resistance value slightly, depending on the condition of the CT an Switchgear.

 

[waross]

70-80 micro-ohms

That sounds more like a ductor test than a megger test.
A CT may make a small difference to the impedance with an AC ductor test.
The ductor may have been made by the Megger company and have a Megger logo.
Also, checking cubicle to cubical sounds more like a ductor test than a megger test.
Megger test are generally conducted from phase to phase or phase to ground rather fro cubicle to cubicle.
In very very old text books you may find a reference to a "wireless reactor".
This was a stack of iron laminations with a center hole. One of the secondary cables of a distribution transformer was passed through the hole in the iron laminations.
This would add some series impedance to facilitate the parallel operation of transformers with differing inpedances.
The core of the CT may act as a wireless reactor and add a small amount of impedance to the circuit being tested by a ductor.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Further to
DTR2011's comment; "I'm fairly sure that a 70-80 micro ohm difference would not be picked up by a 1kV or 5kV Megger test set."
Most of the original meggers with a crank driven generator would not see a difference of 1000 Ohms.
The scales typically started at about 10,000 Ohms or 100,000 Ohms.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mellomain]

My huge apologies. We were not doing insulation resistance testing (megger testing). We were doing DRLO testing using a megger branded instrument. I should have been more careful with my words. So reading back over everyone's responses who correctly assumed that I meant ductor testing, it seems that 70-80 micro-ohms may be expected with the CT in the circuit. Is this accurate?

Man is troubled by what might be called the Dog Wish, a strange and involved compulsion to be as happy and carefree as a dog --- James Thurber

 

[waross]

Yes. The core of the CT will add a little inductive reactance and increase the impedance slightly.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

That depends on whether the test current is DC or AC (or ripply DC). The T&R ductor uses an unsmoothed DC which will - and does - interact with inductive components, but the Megger DLRO's push out a fairly pure DC which should result in a straightforward resistance measurement.

 

[edison123]

I assume this is a bar primary CT. Don't the CT's have to be demagnetized after passing DC current (during DC resistance measurement)?

Muthu

http://www.edison.co.in