electricpete
Electrical
- May 4, 2001
- 16,774
Motor Nameplate data – 3500hp, 13.2kv 324rpm, 60hz, vertical squirrel cage induction motor. FLA = 153A. LRC = 989 at full current.
Motor Protection
CT ratio is 40:1
Instantaneous trip, High dropout trip and Time Overcurrent functions provided by IAC66M3A relay as follows:
Instantaneous trip 50A CT secondary (2000A primary)
High dropout trip – 30A CT secondary (1200A primary). Only trips if current remains above this level after 0.1 seconds.
Time overcurrent trip – 5A CT secondary (200A primary), Time Dial setting 2
Time overcurrent alarm (B-phase only) – 5A CT secondary (200A primary), Time Dial setting 1.5
Note that this relay is described at Negative sequence current trip (46 device) – GE 12KJC51E2A – set at slope=125%, TD=4
Ground overcurrent alarm – IAC77A relay fed from window type CT enclosing all three phases.
Time vs current curves for motor protection and starting are shown in slide 1 of the following powerpoint
History – motor was rewound in 2000. T-leads were 2AWG. 1/0 lug was crimped on using hydraulic crimping tool. As far as I know this within the lug manufacturer’s recommendations. All testing including surge testing and resistance testing was sat and balanced (we have the results).
Motor operated with no problem from 2000 until Fall 2005 (started and stopped approximately 20 times)
Upon starting motor in Oct 2005 following a system maintenance period (no motor work done, space heaters were energized), the motor tripped 2.5 seconds after start.
The following relays were tripped: 51G, A-phase HDO. No other alarms or flags were received.
The term box was inspected. Phases are ordered A, B, C from left to right. The rectangular plate is where the T-leads exit the motor in the following order (L to R): 5,1,2,6,4,3. Inspection of the term box showed evidence of slight surface tracking at where an insulating tie was used to position the unshielded cables in the vicinity of T5 (B-phase neutral), T1 (A-phase line), and the B-phase coupling capacitor jumper (slides 2,3). Further inspection of these points, including removal of the insulating tie, showed only a black residue, but no apparent insulation damage. Visual inspection did not show any apparent insulation damage at other locations including the C-phase. There is a single pock-mark on the back of the panel which would either be an arc strike mark or a copper bead.
The Raychem tape insulation was removed which revealed that T-3 (C-phase line lead) lug barrel was melted away. (SLIDES 4,5,6). The other T-lead lugs were inspected. On T2 lug there were 2 or 3 strands that appeared to be broken. Otherwise everything looked good.
The T1,T2,T3 lugs were replaced (not T4,T5,T6), residue removed, motor reterminated and re-taped. Passed PI, megger (5,000 megaohms plus), bridge test, and dc step voltage test to 24kvdc (I know that’s a little low) with no nonlinearities.
Upon starting the motor, further anomalies were noted which were eventually traced to the pump. Current went from LRC to 140A (normal), then back up to 190, 140, 190, 140 over the course of several minutes. Unusual noises were heard. Inspection of the pump showed the impeller had rubbed against the bowl due to loosening of anti-rotation pins outisde the bowl.
So now I am trying to make sense of all of this:
1 – Why did the A phase high dropout relay trip upon open-circuit of the C-phase lug? It seems to me that what initially looked like tracking at the A/B phase locations was just residue from a fault on C-phase.... based on inspection results and the fact that hi-pot later passed (although we only tested phase-to-ground, not phase-to-phase). I think that the failure of C-phase was energetic and pushed a pin-hole in the raychem taped which was missed during visual inspection (how could the Raychem tape stay intact when copper inside was melted) and through black soot in the direction of A/B phases and a single copper bead behind A/B phases. I think that due to failure of the C-phase during start (before motor up to speed), A and B phases were drawing more than locked rotor current resulting in the A-phase HDO. And the ground trip was either a result of arcing associated with the C phase failure or else due to unbalanced motor terminal voltages during the fault which would create unbalanced current to ground through the ground-connected motor surge caps. Reasonable?
2 – Any relationship between the pump event and motor event? Most people here believe the lug was already bad and the pump problem created longer duration starting which pushed it over the edge. That may be reasonable, but I’m a little skeptical since we didn’t get a time-overcurrent trip which is what might be expected in that scenario. Looking to explore that further. The pump engineer is also exploring some scenario’s where the trip upon start may have created an unusual hydraulic transient which pushed the pump over the edge, but that’s outside my scope of interest.
3 – How can we detect or prevent this? We do not do periodic tests of winding resistance because we don’t believe it is a very sensitive test, particularly when performed from the switchgear through several hundred yards of cable. To perform from the motor would require determinating. Note also that winding resistance test at rewind shop was sat. After this event I fear the only possible
4 – I have the failed lug as well as a sister crimped connection that we removed on the motor. Are there any inspections that should be done? I am a little skeptical whether there is anything useful that will come out of sending the sister lug out for analysis ($).
5 – Anything else we should look at to understand and prevent this occurence?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
Motor Protection
CT ratio is 40:1
Instantaneous trip, High dropout trip and Time Overcurrent functions provided by IAC66M3A relay as follows:
Instantaneous trip 50A CT secondary (2000A primary)
High dropout trip – 30A CT secondary (1200A primary). Only trips if current remains above this level after 0.1 seconds.
Time overcurrent trip – 5A CT secondary (200A primary), Time Dial setting 2
Time overcurrent alarm (B-phase only) – 5A CT secondary (200A primary), Time Dial setting 1.5
Note that this relay is described at Negative sequence current trip (46 device) – GE 12KJC51E2A – set at slope=125%, TD=4
Ground overcurrent alarm – IAC77A relay fed from window type CT enclosing all three phases.
Time vs current curves for motor protection and starting are shown in slide 1 of the following powerpoint
History – motor was rewound in 2000. T-leads were 2AWG. 1/0 lug was crimped on using hydraulic crimping tool. As far as I know this within the lug manufacturer’s recommendations. All testing including surge testing and resistance testing was sat and balanced (we have the results).
Motor operated with no problem from 2000 until Fall 2005 (started and stopped approximately 20 times)
Upon starting motor in Oct 2005 following a system maintenance period (no motor work done, space heaters were energized), the motor tripped 2.5 seconds after start.
The following relays were tripped: 51G, A-phase HDO. No other alarms or flags were received.
The term box was inspected. Phases are ordered A, B, C from left to right. The rectangular plate is where the T-leads exit the motor in the following order (L to R): 5,1,2,6,4,3. Inspection of the term box showed evidence of slight surface tracking at where an insulating tie was used to position the unshielded cables in the vicinity of T5 (B-phase neutral), T1 (A-phase line), and the B-phase coupling capacitor jumper (slides 2,3). Further inspection of these points, including removal of the insulating tie, showed only a black residue, but no apparent insulation damage. Visual inspection did not show any apparent insulation damage at other locations including the C-phase. There is a single pock-mark on the back of the panel which would either be an arc strike mark or a copper bead.
The Raychem tape insulation was removed which revealed that T-3 (C-phase line lead) lug barrel was melted away. (SLIDES 4,5,6). The other T-lead lugs were inspected. On T2 lug there were 2 or 3 strands that appeared to be broken. Otherwise everything looked good.
The T1,T2,T3 lugs were replaced (not T4,T5,T6), residue removed, motor reterminated and re-taped. Passed PI, megger (5,000 megaohms plus), bridge test, and dc step voltage test to 24kvdc (I know that’s a little low) with no nonlinearities.
Upon starting the motor, further anomalies were noted which were eventually traced to the pump. Current went from LRC to 140A (normal), then back up to 190, 140, 190, 140 over the course of several minutes. Unusual noises were heard. Inspection of the pump showed the impeller had rubbed against the bowl due to loosening of anti-rotation pins outisde the bowl.
So now I am trying to make sense of all of this:
1 – Why did the A phase high dropout relay trip upon open-circuit of the C-phase lug? It seems to me that what initially looked like tracking at the A/B phase locations was just residue from a fault on C-phase.... based on inspection results and the fact that hi-pot later passed (although we only tested phase-to-ground, not phase-to-phase). I think that the failure of C-phase was energetic and pushed a pin-hole in the raychem taped which was missed during visual inspection (how could the Raychem tape stay intact when copper inside was melted) and through black soot in the direction of A/B phases and a single copper bead behind A/B phases. I think that due to failure of the C-phase during start (before motor up to speed), A and B phases were drawing more than locked rotor current resulting in the A-phase HDO. And the ground trip was either a result of arcing associated with the C phase failure or else due to unbalanced motor terminal voltages during the fault which would create unbalanced current to ground through the ground-connected motor surge caps. Reasonable?
2 – Any relationship between the pump event and motor event? Most people here believe the lug was already bad and the pump problem created longer duration starting which pushed it over the edge. That may be reasonable, but I’m a little skeptical since we didn’t get a time-overcurrent trip which is what might be expected in that scenario. Looking to explore that further. The pump engineer is also exploring some scenario’s where the trip upon start may have created an unusual hydraulic transient which pushed the pump over the edge, but that’s outside my scope of interest.
3 – How can we detect or prevent this? We do not do periodic tests of winding resistance because we don’t believe it is a very sensitive test, particularly when performed from the switchgear through several hundred yards of cable. To perform from the motor would require determinating. Note also that winding resistance test at rewind shop was sat. After this event I fear the only possible
4 – I have the failed lug as well as a sister crimped connection that we removed on the motor. Are there any inspections that should be done? I am a little skeptical whether there is anything useful that will come out of sending the sister lug out for analysis ($).
5 – Anything else we should look at to understand and prevent this occurence?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
