electricpete
Electrical
- May 4, 2001
- 16,774
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).
=====================================
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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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