IR of transformer is very low
IR of transformer is very low
(OP)
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 ??
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 ??






RE: IR of transformer is very low
Also, was the transformer isolated at the time of the test? All connections removed?
RE: IR of transformer is very low
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)?
RE: IR of transformer is very low
Muthu
www.edison.co.in
RE: IR of transformer is very low
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
RE: IR of transformer is very low
oldfieldguy has some good suggestions of where to go from here to find out why.
RE: IR of transformer is very low
RE: IR of transformer is very low
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.
RE: IR of transformer is very low
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
RE: IR of transformer is very low
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
www.edison.co.in
RE: IR of transformer is very low
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.
RE: IR of transformer is very low
Muthu
www.edison.co.in