IR of transformer is very low 3

[Jed057]

Hi anyone,
TR 2000 kVA (6.6 /0.4 kV)feeder is tripped.

Restric earth fault relay (64R) is activated
(Setting is 0.06In)
after that we checked and found that the problem form wrong polarity CT of 64R and already changed.
But the problem is that
we measured insulation primary side(6.6 kV)& Earth of this TR is around 60 Mohm (It is very low),Pri and Sec side is 400 mOhm .So ,Please suggest that we can energize this transformer and what the actual problem coming from ??

 

[Zogzog]

ANSI/NETA spec is 5000 Meg minumum, so you have a problem. What was the winding temp at the time of the test? Are your posted results tempature corrected, that makes a huge difference.

Also, was the transformer isolated at the time of the test? All connections removed?

 

[j3n5]

May I suggest measuring the polarization index?
Is it possible to disconnect the core earthing on this transformer (to see if the conductivity is phase-to-core, phase-to-tank or both)?

 

[edison123]

I would say 60 megohms is good enough for 6.6 KV. Charge the trafo and see what happens.

Muthu

http://www.edison.co.in

 

[oldfieldguy]

What was the IR the last time you tested it? Zog makes a good point about temperature, and j3n5 has an good point about isolating the core ground, but a simple megohmmeter test is only one of several tests you might want to do. A power factor test will corroborate your megohmmeter findings.

Is it liquid-filled? A dissolved gas analysis should be done immediately. It will show if there were any heat or energy discharge issues in the insulating fluid. If the transformer is nitrogen padded, then a TCG test of the pad gases will also indicate failures. In either case, there are absolute levels, but it is always helpful to compare readings with historical data for the equipment.

The single CT for a ground fault relay is generally not polarity sensitive. for device 64. It is non-directional. a directional element would be a 67G, and would have a second circuit input to establish polarity, either potential or current.

old field guy

 

[rhatcher]

I agree with edison123 that 60Mohms is normally considered acceptable for your voltage. But, I would say that if you were running normally before the relay trip and now there is a trip and a question of why, the question needs to be answered. What is the change from before to now? What is the source of the trip problem?

oldfieldguy has some good suggestions of where to go from here to find out why.

 

[Zogzog]

Ok, the guys saying 60M is good, you have a basis for that? Any spec you can reference?

 

[rhatcher]

The basis for this statement is the 'rule of thumb' that for rotating equipment that the minimum acceptable insulation resistance is 1M per 1000V + 1. So, the minimum insulation resistance (IR) by this 'rule' for your gear would be 7.6M.

So, if you are having a problem, we are sugesting that IR may not be the source. I would not turn it on until you understand what device caused the trip and why. Remember that this may mean there is a failure in the protective relays or this may mean a failure unrelated to the IR of the motor. You should know which one it is before proceeding.

 

[oldfieldguy]

Zog-

That's why I said check it against past tests. I've seen some strangely low IR numbers, but still within the "thousand ohms per volt" rule of thumb, with decent power factor results.

You and I both know that transformers may fault and still test "pretty good" for IR and power factor. The TCG/dissolved gas analysis generally tells the tale, though.

I'd think that a checkout of the 64R circuit and relay would be on my agenda along with a more complete battery of transformer tests.

old field guy

 

[edison123]

Zog - IIRM, we had this discussion before about trafo IR. I've rarely seen 5000 megs on a 6.6 KV trafo but have seen in-service 11 KV trafos with as low as 10 megs running ok for years.

I'm with rhatcher about the thumb-rule and with ofg on ruling out any other issues before energizing this trafo.

BTW, does that ANSI/NETA spec insist on a min 5000 meg or is it only recommended ?

Muthu

http://www.edison.co.in

 

[j3n5]

How about the comparison 60 Mohm primary v.s. 400 Mohm secondary?
The primary has more turns, but the BIL should also be higher, right?
I would question the use of acceptance values for rotating machines used for transformers (especially if liquid filled). The insulation systems are very different, based on different materials.
I also agree that a single IR reading is way too limited info to give reliable advice in this matter.

 

[edison123]

j3n5 - IR value is not an indication of the insulation dielectric strength.

Muthu

http://www.edison.co.in