Motor insulation and conductor failure - which came first
Motor insulation and conductor failure - which came first
(OP)
OVERVIEW: Large motor tripped on ground fault while running. No other protection tripped (not time overcurrent alarm, time overcurrent trip, HDO OC trip, instantaneous overcurrent trip, negative sequence trip). Motor meggered sat (>1000 megaohms, PI > 2) but showed an open circuit on B phase. Disassembly inspection showed that the B phase line side T-lead (T2) was damaged a few feet inside the motor. There was approx 1.5" of copper missing from the T-lead. There was an arc strike on a grounded surface approx 4" from the fault. The location of the fault was not a joint within the T-lead.
MACHINE DESCRIPTION: 13.2kv, 3500hp, 324 rpm, vertical squirrel cage motor in outdoor environment, WP2 construction.
HISTORY:
Motor rewound in 2005 with epoxy VPI windings. T-leads are 2AWG silicon-rubber insulated with an insulation temperature rating of 150C by the T-lead manufacturer. Motor winding temperature as measured by the hottest of 6 RTD's in the slot section is less than 200F while near full load following rewind - no change in this pattern at any time since rewind.
Motor was idle (not running) during the period 1/22/09 to 3/10/09. Review of temperature trends indicates winding temperature 10-15C above ambient - suggests space heaters were working. Other checks of space heaters before and after the event were also sat.
Motor was started 3/10/09 and ran until the event.
Rain occurred 3/14-3/15 and ended on 3/15.
TRIP:
Motor tripped on ground fault relay (51G) the afternoon of 3/16. It was sunny at the time and no plant evolutions in progress. No other flags were tripped and no disks noted in abnormal position. The motor had been running continuously since 3/10/09.
IMMEDIATE CHECKS: Immediate checks show B phase open-circuited, A and C phases have balanced resistances. Megger is > 1000 megaohms.
PROTECTIVE RELAYING:
(** Protection curves are shown in slide 1 of the attachment.)
51G relay - ground overcurrent - fed from 50/5 donut CT. It is GE type 12IAC77A801A with TD setting 2 and pickup settin 1A.
The following additional relays fed from 200:5 phase CT's:
A: 50/51 IAC66 phase overcurrent relays (one per phase). These are GE IAC66M. These have three functions:
1 - time overcurrent trip (pickup is 5A secondary, TD=2)
2 - HDO = trip at 30A secondary/1200A primary if that level persists for 0.1 seconds or more.
3 - Instantaneous = trip at 50A secondary / 2000A primary (no intentional delay but curve is provided by GE).
B: 50/51BX time overcurrent alarm function sensed on B phase. This is also a GE IAC66M
time overcurrent alarm (pickup is 4.3A secondary, TD=1.5)
46 - current balance relay - GE IJC51E. TD=4, TT=1.15sec, PU = 1.1A, Slope = 5%
Again the only flag of the above that flagged was the 51G ground relay.
SHOP INSPECTION:
Electrical testing at the shop confirmed same results as at the plant (B phase open, megger test sat).
The motor was disassembled and the location of the open circuit was found. It is at the B-phase line-side T-lead (T2) in the endwinding area of the motor (top end of motor which is connection end). The fault location is approx the 1:00 position if 12:00 is the motor term box. The conductor was melted open at this point and the insulation is compromised at this point, and there is evidence of current flow to ground at the closest non-insulated ground location on the side of the stator approx 4" away. There are no other anomalies evident during visual inspection to suggest the presence of any other fault location.
Slide 2 shows overview of the fault location. Slide 3 is zoom-in on fault location. Note that even though there is powder on the surface of the T5 lead (which is believed to be vaporized silicon insulation from the T2 lead), when cleaned up later T5 lead is found to be unaffected.
Slides 4 and 5 show the adjacent location on the stator which shows evidence of arc damage. (approx 4" away).
Slide 6 is closer view of the fault after cleaned up a little. You can see some copper is missing.
Slide 8 shows the pieces involved in the conclusion that 1.5" of copper is missing.
Slide 6 shows an overview of the T-lead. Note that downstream of the fault there is a transition point where the original lead insulation is stripped off of the lead, and further downstream (not even shown here) there is a brazed/crimped connection to the winding. However there was no connection or transition at the location of the fault. There was a mechanical tie within a few inches of the fault, but no evidence of any unusual mechanical stresses in this area.
Slides 9 and 10 show powder from the top bracket directly above the fault. This is believed to be vaporized silicon insulation.
Slides 11 and 12 show excessive bugs within the motor. After stator was later steam cleaned, there was absolutely no PD evident visually anywhere.
Slide 13 shows trails which indicate water droplets at some point in time had flowed radially outward along the blade (cannot be to gravity - must be due to centrifugal force indicating motor was running at the time).
The bugs and water evidence are clearly undesirable conditions and we plan to re-examine our filter changeout practices and other aspects of our enclosure very closely to address them. I am not particularly interested in opinions on whether they need to be addressed since we plan to address them. I am interested in the main question listed below.
REWORK: The failed connection was reworked. Motor passed dc step votlage test to 30kvdc (one phase at a time - very linear). Motor passed surge test to 22.6kv. Motor reassembled, dc step voltage test repeated, installed into the plant, running fine at this time.
THE MAIN QUESTION: What do you think caused the failure?
Did ground insulation fail first leading to copper damage? If so, did the bugs and or water contribute? Other possible contributors? (this scenario seems unlikely since the ground relay is sensitive and should trip before significant damage occurs, especially given that no other relays tripped).
Did the conductor fail first leading to insulation damage? What would cause the conductor to fail? (this scenario seems unusual since this was not location of connection... wouldn't expect conductor to fail in the middle... also would expect a resistive conductor failure to show up during starting rather than steady state running).
MACHINE DESCRIPTION: 13.2kv, 3500hp, 324 rpm, vertical squirrel cage motor in outdoor environment, WP2 construction.
HISTORY:
Motor rewound in 2005 with epoxy VPI windings. T-leads are 2AWG silicon-rubber insulated with an insulation temperature rating of 150C by the T-lead manufacturer. Motor winding temperature as measured by the hottest of 6 RTD's in the slot section is less than 200F while near full load following rewind - no change in this pattern at any time since rewind.
Motor was idle (not running) during the period 1/22/09 to 3/10/09. Review of temperature trends indicates winding temperature 10-15C above ambient - suggests space heaters were working. Other checks of space heaters before and after the event were also sat.
Motor was started 3/10/09 and ran until the event.
Rain occurred 3/14-3/15 and ended on 3/15.
TRIP:
Motor tripped on ground fault relay (51G) the afternoon of 3/16. It was sunny at the time and no plant evolutions in progress. No other flags were tripped and no disks noted in abnormal position. The motor had been running continuously since 3/10/09.
IMMEDIATE CHECKS: Immediate checks show B phase open-circuited, A and C phases have balanced resistances. Megger is > 1000 megaohms.
PROTECTIVE RELAYING:
(** Protection curves are shown in slide 1 of the attachment.)
51G relay - ground overcurrent - fed from 50/5 donut CT. It is GE type 12IAC77A801A with TD setting 2 and pickup settin 1A.
The following additional relays fed from 200:5 phase CT's:
A: 50/51 IAC66 phase overcurrent relays (one per phase). These are GE IAC66M. These have three functions:
1 - time overcurrent trip (pickup is 5A secondary, TD=2)
2 - HDO = trip at 30A secondary/1200A primary if that level persists for 0.1 seconds or more.
3 - Instantaneous = trip at 50A secondary / 2000A primary (no intentional delay but curve is provided by GE).
B: 50/51BX time overcurrent alarm function sensed on B phase. This is also a GE IAC66M
time overcurrent alarm (pickup is 4.3A secondary, TD=1.5)
46 - current balance relay - GE IJC51E. TD=4, TT=1.15sec, PU = 1.1A, Slope = 5%
Again the only flag of the above that flagged was the 51G ground relay.
SHOP INSPECTION:
Electrical testing at the shop confirmed same results as at the plant (B phase open, megger test sat).
The motor was disassembled and the location of the open circuit was found. It is at the B-phase line-side T-lead (T2) in the endwinding area of the motor (top end of motor which is connection end). The fault location is approx the 1:00 position if 12:00 is the motor term box. The conductor was melted open at this point and the insulation is compromised at this point, and there is evidence of current flow to ground at the closest non-insulated ground location on the side of the stator approx 4" away. There are no other anomalies evident during visual inspection to suggest the presence of any other fault location.
Slide 2 shows overview of the fault location. Slide 3 is zoom-in on fault location. Note that even though there is powder on the surface of the T5 lead (which is believed to be vaporized silicon insulation from the T2 lead), when cleaned up later T5 lead is found to be unaffected.
Slides 4 and 5 show the adjacent location on the stator which shows evidence of arc damage. (approx 4" away).
Slide 6 is closer view of the fault after cleaned up a little. You can see some copper is missing.
Slide 8 shows the pieces involved in the conclusion that 1.5" of copper is missing.
Slide 6 shows an overview of the T-lead. Note that downstream of the fault there is a transition point where the original lead insulation is stripped off of the lead, and further downstream (not even shown here) there is a brazed/crimped connection to the winding. However there was no connection or transition at the location of the fault. There was a mechanical tie within a few inches of the fault, but no evidence of any unusual mechanical stresses in this area.
Slides 9 and 10 show powder from the top bracket directly above the fault. This is believed to be vaporized silicon insulation.
Slides 11 and 12 show excessive bugs within the motor. After stator was later steam cleaned, there was absolutely no PD evident visually anywhere.
Slide 13 shows trails which indicate water droplets at some point in time had flowed radially outward along the blade (cannot be to gravity - must be due to centrifugal force indicating motor was running at the time).
The bugs and water evidence are clearly undesirable conditions and we plan to re-examine our filter changeout practices and other aspects of our enclosure very closely to address them. I am not particularly interested in opinions on whether they need to be addressed since we plan to address them. I am interested in the main question listed below.
REWORK: The failed connection was reworked. Motor passed dc step votlage test to 30kvdc (one phase at a time - very linear). Motor passed surge test to 22.6kv. Motor reassembled, dc step voltage test repeated, installed into the plant, running fine at this time.
THE MAIN QUESTION: What do you think caused the failure?
Did ground insulation fail first leading to copper damage? If so, did the bugs and or water contribute? Other possible contributors? (this scenario seems unlikely since the ground relay is sensitive and should trip before significant damage occurs, especially given that no other relays tripped).
Did the conductor fail first leading to insulation damage? What would cause the conductor to fail? (this scenario seems unusual since this was not location of connection... wouldn't expect conductor to fail in the middle... also would expect a resistive conductor failure to show up during starting rather than steady state running).
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RE: Motor insulation and conductor failure - which came first
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Motor insulation and conductor failure - which came first
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Motor insulation and conductor failure - which came first
RE: Motor insulation and conductor failure - which came first
If a few strands break (but not all), do we expect arcing? What would be the mechanism?
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RE: Motor insulation and conductor failure - which came first
That's a good question. I believe the mechanism might be based on the fact that when you postulate a broken or damaged strand, it is typically not a clean break with a large gap. It could start with small discontinuities that would intermittently contact (due to vibration of the machine or movement of the conductor during starting, etc), arcing during the contact-and-break cycle, and progressively worsen.
RE: Motor insulation and conductor failure - which came first
------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Motor insulation and conductor failure - which came first
We will check the ground relay. Also as KB3HHC reminded me, we have ground resistor that should limit ground current somewhere in the neighborhood below 13.2/sqrt(3) / 20 ~ 400A.
I do agree 100% that traditional thinking among folks that who are very experienced/knowledgeable in motor failure analysis (moreso than myself) is you don't expect to lose any significant copper unless you have either a phase-to-phase fault or a turn fault (both of which we can completely rule out).
I don't fully understand the physics or logic that would "prove" that traditional thinking. I did do an I^2*T calculation using the stuff I learned in thread237-238191: Time to delta T and plotted the results along with my relay time current curves on slide 1 of previous attachment. I plotted time to reach 150C (insulation rating) and time to reach 1000C (copper melting temperature). And of course those times are far far above all the protective curves. But that really proves nothing because the high temperatures associated with arcing come from some source other than I^2*R losses in the metal. And taking a look at the other end of the arc at the steel, we have a nice little chunk of steel melted away. And steel has a melting point about 3000F compared to 2000F for copper.
The steel was subject to the same limitations on magnitude and duration of ground current, yet it reached 3000F and melted or vaporized a big chunk.
=====================================
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RE: Motor insulation and conductor failure - which came first
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Motor insulation and conductor failure - which came first
But when I go back ook at the photo that Slide 5/13 which I was calling the "arc strike point" on the steel, the appearance of the surface of that hole is reddish. Either rust or paint... either way not what I would expect after an arc. Also I did see a few copper bee-bee's spread around but nothing that I thought was a steel bee-bee. Hmmm.
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RE: Motor insulation and conductor failure - which came first
------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Motor insulation and conductor failure - which came first
Any further thoughts by anyone? I am still scratching my head on this one.
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RE: Motor insulation and conductor failure - which came first
I'll throw one out. What about a manufacturing flaw in the lead wire conductor and/or insulation that was exacerbated by a high transient voltage.
Thanks
RE: Motor insulation and conductor failure - which came first
Of course the weld is a mechanical stress point, and in the case of HV cable also a voltage stress point.
When I inquired about butt welds in a wire from the manufacturer, I was told that in certain kinds of cable this was normal practice! Oddly enough one type of cable where is is not permitted is building wire. The reason being the stresses imposed on the wire while pulling it through conduit.
RE: Motor insulation and conductor failure - which came first
It would never even occur to me that a cable OEM would or could sell a cable with a hidden splice embedded. That is just plain bizarre. But I guess bizarre things sometimes do happen.
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