×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Earth fault with Meduim voltage motor

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?????

RE: Earth fault with Meduim voltage motor

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

RE: Earth fault with Meduim voltage motor

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

RE: Earth fault with Meduim voltage motor

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

RE: Earth fault with Meduim voltage motor

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?

RE: Earth fault with Meduim voltage motor

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


"..the more, the merrier" Genghis Khan

RE: Earth fault with Meduim voltage motor

(OP)
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.

RE: Earth fault with Meduim voltage motor

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?

Quote:

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)' ?

RE: Earth fault with Meduim voltage motor

Quote:

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)' ?

RE: Earth fault with Meduim voltage motor

(OP)
dear sir
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

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)' ?

RE: Earth fault with Meduim voltage motor

(OP)
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

RE: Earth fault with Meduim voltage motor

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.

RE: Earth fault with Meduim voltage motor

(OP)
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?

RE: Earth fault with Meduim voltage motor

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?

RE: Earth fault with Meduim voltage motor

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

RE: Earth fault with Meduim voltage motor

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.

RE: Earth fault with Meduim voltage motor

Uh, actually you did:

Quote:

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....

RE: Earth fault with Meduim voltage motor

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".

RE: Earth fault with Meduim voltage motor

(OP)
Thank you.   

RE: Earth fault with Meduim voltage motor

Agree with rhatcher.
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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources