Earth fault with Meduim voltage motor

[emadshaaban]

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?????

 

[dpc]

What type of neutral grounding are you using and what was the magnitude of the ground fault current that caused the relay to trip?

 

[ScottyUK]

Is the cable ok? A cable or cable box fault would cause the same relays to operate.

 

[emadshaaban]

the neutral grounded through 150ohm resistor,fault current is around 160A.
for sure the motor cable disconnected

 

[dpc]

Scotty is asking if the fault could be in the cable and not the motor. With current limited to 150 A, there may not be any obvious signs of a fault. What is the relay pickup setting?

 

[powerjunx]

dpc is right..for sure try check the settings and checking system pertaining to motor and protection.



"..the more, the merrier" Genghis Khan

 

[emadshaaban]

there sir
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.

 

[electricpete]

I had a 13.2kv vertical outdoor motor that tripped on ground fault only intermittenly and only during rainstorm, and meggered fine.

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?

setting I0=(2.5A,1.5sec) & (30A,300sec)

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?


=====================================
(2B)+(2B)' ?

 

[electricpete]

setting I0=(2.5A,1.5sec) & (30A,300sec)

Also, are those numbers swapped? If it were inverse characteristic should be high current for short time or lower current for longer time.. . or am I misunderstanding what these numbers mean?

=====================================
(2B)+(2B)' ?

 

[emadshaaban]

dear sir
thank you for you time,the setting are 2 stages uint1=2.5A ,1.5sec
unit2=30A ,300sce

 

[electricpete]

I still don't get it.

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..


=====================================
(2B)+(2B)' ?

 

[emadshaaban]

yes MR electricpete you analytic is correct and is logical.
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

 

[dpc]

Something is off. If you have a 150 ohm grounding resistor on a 6.6 kV system, the maximum ground fault current is going to be about 25 A, not 160 A.

 

[emadshaaban]

dear dpc
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?

 

[crthompson]

If you have the wye primary(20,000 V) neutral grounded through a 160 Ohm resistor and you have a phase to ground fault, a maximum of 72 A will flow through the neutral grounding resistor. (20,000 V/sqrt3/160 ohms) = 72.17 A. You should never be able to have a ground fault of 150 A on the primary. You mentioned that your motor is on the secondary of this transformer and your neutral grounding resistor is on the primary. Where is your protective device located? Are you monitoring the ground cable of the motor? The three phases of the motor(zero sequence)? Where did you read 150A of ground fault current?

 

[crthompson]

Sorry, (20,000 V/sqrt3/150 Ohms) = 76.98 A

 

[rhatcher]

What test voltage are you using to check the insulation resistance? A 500V or 1000V meggar is insufficient. In fact, if you are reading 4 Megohms with a 500V or 1000V meggar then it is very likely that you would find the motor to be grounded if the test were performed with a DC high pot at the proper voltage. The correct voltage for the testing insulation resistance of a 6.6kV motor is about 14kV DC. Keep in mind that the DC peak value of the 6.6kV line voltage is 9.3kV.

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.

 

[dpc]

Uh, actually you did:

the neutral grounded through 150ohm resistor,fault current is around 160A.
for sure the motor cable disconnected

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....

 

[burnt2x]

A PI below 1.0 indicates bad insulation, IMO. Please isolate motor from the supply line and see if the motor (only) PI is "good".

 

[emadshaaban]

Thank you.