Earth fault with Meduim voltage motor
Earth fault with Meduim voltage motor
(OP)
dear all
The mentioned motor 2600kw 6.6kv was tripped by earth fault (50N/51N),after disassembling the motor found the insulation resistance 4MΩ ,no sign of burn or moisture inside winding ,all winding all the physically checks shows no sign of damage .we tried to clean winding by air but we get the same results PI=0.65.
What is your advices for diagnostic and solving?????
The mentioned motor 2600kw 6.6kv was tripped by earth fault (50N/51N),after disassembling the motor found the insulation resistance 4MΩ ,no sign of burn or moisture inside winding ,all winding all the physically checks shows no sign of damage .we tried to clean winding by air but we get the same results PI=0.65.
What is your advices for diagnostic and solving?????





RE: Earth fault with Meduim voltage motor
RE: Earth fault with Meduim voltage motor
RE: Earth fault with Meduim voltage motor
for sure the motor cable disconnected
RE: Earth fault with Meduim voltage motor
RE: Earth fault with Meduim voltage motor
"..the more, the merrier" Genghis Khan
RE: Earth fault with Meduim voltage motor
im sorry it was wrong reading that i have received,the correct are
motor >>>>>> 2.6MW & 6.6KV . setting I0=(2.5A,1.5sec) & (30A,300sec),resistive neutral with 150 ohm
fault>>>>>>> 4M ohm,PI=0.65,fault current 42A.
motor cable power supply has good insulation resistance.
RE: Earth fault with Meduim voltage motor
Later inspection showed signs of water getting into the motor. Also a fault on the very end of the knuckle at bottom end of the motor. Apparently water was dripping from the knuckle causing fault.
That’s not a likely scenario (for one thing, water should not be getting into motor..ideally...), but just mentioning for info
How is the ground current sensed? (derived from three phase currents sensed by the relay? or residual connection of ct? or window ct?
Just a curiosity, seems like the setting is too high.
6600/sqrt3/150 = 25A
could have sustained ground fault without trip?
or did I misunderstand?
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(2B)+(2B)' ?
RE: Earth fault with Meduim voltage motor
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(2B)+(2B)' ?
RE: Earth fault with Meduim voltage motor
thank you for you time,the setting are 2 stages uint1=2.5A ,1.5sec
unit2=30A ,300sce
RE: Earth fault with Meduim voltage motor
Both 2.5A and 30A refer to the same quantity? (I assume primary-side I0)?
Then the 30A ,300sec would be irrelevant because it can never trip without the (2.5A, 1.5sec) tripping first.
In contrast an inverse characteristic would be
(2.5A, 300 sec) (30A, 1.5sec). Neither one is redundant. Either one can trip without the other one tripping. Long low current trips the first, shorter duration higher current trips the 2nd.
Maybe I am misunderstanding something..
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(2B)+(2B)' ?
RE: Earth fault with Meduim voltage motor
but now i have low insulation motor and i do not have any sing of winding defection or moisture,what are steps that must to be do
RE: Earth fault with Meduim voltage motor
RE: Earth fault with Meduim voltage motor
i did not mentioned that maximum ground fault is 160,by the way the grounding resistance is star point at primary of 20kv/6.6kv transformer ,is that make any difference?
RE: Earth fault with Meduim voltage motor
RE: Earth fault with Meduim voltage motor
RE: Earth fault with Meduim voltage motor
You can use a meggar for spot checks of a 6.6kV winding but you should expect to see a reading of several hundred Megohms, not a low value like 4 Megohms.
Also, the low IR readings indicates that the motor winding is wet or that the winding insulation is very brittle, cracked, and degraded.
In conclusion, I do not think that you have a relay problem, you probably have a grounded motor. Look for signs of the fault at the end of the slot where the winding exits the core iron or in the middle of the slot. The fault will appear to be a small black spot that may or may not have copper splatter around it. Sometimes it is difficult to see the fault location on a dirty winding so do not be fooled simply because you cannot see the fault. The insulation resistance test does not lie.
RE: Earth fault with Meduim voltage motor
Anyway, you need to assume you have a problem in the motor until proven otherwise. I'll leave it to the others to continue going around this circle....
RE: Earth fault with Meduim voltage motor
RE: Earth fault with Meduim voltage motor
RE: Earth fault with Meduim voltage motor
This type of earth fault is typical where a coil has blown out at the end or middle of
a stator slot.
The earth fault blows itself open & clear so subsequent tests do not reveal any poor
megger readings.
Look for hole in winding isulation, copper spatter & carbon footprint.