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Simultaneous 4kV Motor Failures
2

Simultaneous 4kV Motor Failures

Simultaneous 4kV Motor Failures

(OP)
We experienced the simultaneous failures of two 4kV fan motors on a high resistance grounded system.  They were on the same switchgear bus, a 6000 HP fan (located outside) and a 600 HP fan (inside) – both less than 300 feet from the bus.  The 6000 HP fan has surge protection at its terminals – the only other surge protection is on high side of main transformer.  Our control system shows the loss of the 6000 HP fan one second before the 600 HP fan, however, the points have a 0.5 second scan time and it is too close to call if that is real or not.  The 6000 HP fan went on instantaneous at 8920 A (740 FLA) and one end coil was visibly blown out, and the 600 HP fan on time overcurrent at 1160 A (76 FLA).  No abnormal weather or operating conditions.  A similar situation occurred previously wherer we got a 2-fer in failures. Any ideas?

RE: Simultaneous 4kV Motor Failures

Did the 600HP motor fail or just trip out on overcurrent? If it did fail, what type of damage, if any, has been determined to have caused the failure, insulation breakdown or overheated windings?

RE: Simultaneous 4kV Motor Failures

The failures happened during start up or the system was running?
If it happened switching do, you have a vacuum breaker or switch and three-phase cable feed from the switchgear to motors?

RE: Simultaneous 4kV Motor Failures

There are a large number of possibilities.  Easy to speculate, tough to prove

#1 - An abnormal voltage (surge or steady state) was initiating event and caused both motors to fail.

#2 - Failure of first motor (likely the 6000hp motor) caused voltage transient which caused 2nd motor to fail.

#3 - Ungrounded system may have contributed to item #1 or #2.  There have been lots of threads on intermittent arcing faults on ungrounded system.

#4 - Failure of 6000hp was the initating event and the 600hp tripped due to the high current in 600hp motor as a result of the 6000hp motor fault not being cleared rapidly?  An induction motor acts like a generator during a fault.

Maybe some monitoring of system transient and steady state voltages to ground would shed some light if there are occasional destructive conditions in the power system.

RE: Simultaneous 4kV Motor Failures


Are the short-circuit numbers {8920/1160A} ø-ø or ø-g values?  They seem large for ground faults on a typical high-resistance grounded system.  In this configuration, when a phase-to-ground fault occurs, voltage on the unfaulted phases raises to phase-to-phase values, but normally this is allowed for in the system insulation levels.  If the resistive ground-fault current does not equal or exceed the system capacitive charging current, some oscillatory/transient overvoltage may occur.  Have equipment and cables been added after the grounding resistor was sized for then-present conditions?
  

RE: Simultaneous 4kV Motor Failures

Whoops. Delete my #3. I was thinking ungrounded, not high-resistance grounded.

RE: Simultaneous 4kV Motor Failures

Suggestion: Assuming that the high-resistance system grounding did not fail, then the shorts were phase to phase. The reasons can be:
1. Large surge beyond surge protector ratings
2. Harmonic distortion, i.e. high voltage harmonic content, especially, if there are ac motor variable speed drives
3. Voltage spikes in the power supply, (Are there any static switcher?).
4. Winding insulation failure due to insulation defects causing short(s).

RE: Simultaneous 4kV Motor Failures

I would tend to agree with Busbar. We had a similar instance on a 5 kv MCC where 2 motors failed within about 45 sec of each other. At that time all of our motors on the MCC ( about 12) had surge caps. We found that we exceeded to maximum charging current rule for HRG systems if we had 2 or more caps connected. In your case, it sounds as if you have one surge cap and the capacitance of the cables to consider.

We were able to show that our motor cable length was adequate enough to protect our motors (basically the cable is also a capacitor, albeit smaller than a surge cap). So we disconnected the surge caps.

RE: Simultaneous 4kV Motor Failures

I need some time to work thru thisw discussion of high-r grounding by busbar and Gord.

One item was mentioned a max expected fault current delievered from the source.  But doesn't that exclude the capacitive current mentioned by Gord which in theory can be much higher limited only by fault resisstance as capacitance discharges thru the fault.

We have high R systems with many long high-C cabbles and many motor surge caps attached.  I'll have to look at that scenario closer to see if it applies to us.  

RE: Simultaneous 4kV Motor Failures

I did locate the calculation used to size our neutral grounding resistors.  It does compare total estimated system cacitive reactance to ground to grounding resistance as described above.

I was a little surprised to see that they considered cable capacitance to ground (based on cable type and lenght) and motor capacitance to ground (based on horsepower and speed), but not surge capacitor. I'm guessing perhaps the surge caps are small contributor to the total?

RE: Simultaneous 4kV Motor Failures

(OP)
The motors were running under normal full load conditions when the event happened with non other switching.  No major system changes have been made since the system was designed or the surge protection added.  There are no harmnonic generators on the system such as VFDs.  Both motors did fail, and the rapir shop just said that the 6000 HP motor was burned in at least three places, but I don't have the full report yet on it or the 600 HP motor.  The maximum charging current rule is intriguing and I've also got to do some research there.

RE: Simultaneous 4kV Motor Failures


Unlike most power-factor capacitors in medium-voltage applications, the surge-protective capacitor wyepoint is intended to be connected to machine frames and station-ground bus.  [Example at www.geindustrial.com/products/manuals/GEH-2730B.pdf]  This could significantly increase charging current as effectively in parallel with the dielectric of shielded cables, transformers and rotating equipment—lowering zero-sequence capacitive reactance, with the prospect of increased phase-to-ground oscillatory/transient overvoltage.
  

RE: Simultaneous 4kV Motor Failures

Pete...in our installation, the surge capacitor current was a significant portion of our resistor let-thru current. We used 0.5 uF caps which result in a charging current of 1.35 A or about 50% of our resistor let-thru.

We had operated for about 20 years in the above configuration. Old-timers did indicate that we used to lose motors in groups ( within days or a few weks of each other). My previous posting was the first time that motors failed within sec's of each other.

So just because the charging current exceeds the resistor let-thru does not mean you imediately have a problem. The problem occurs when you have an arcing ground fault. I modelled our installation using PSpice and found that when charging current equalled resistor let-thru current, the resulting transient voltage rise was about 200% - this matched to textbooks I had read on HRG systems. As the charging current increased, transient voltage increased.

I think, in simplistic terms, a ground fault results in the ground capacitances getting charged up when the fault is applied. If the fault is removed (ie arcing), these capacitances try to discharge thru the neutral resistor. If the resistor is small, the ground capacitances can almost totally discharge befor the next application of the fault; if the resistor is to big, the energy from successive applications of the fault adds onto the capacitor voltage remaining.

RE: Simultaneous 4kV Motor Failures

I agree with everything that has been said. I see in IEEE documents where the importance of keeping neutral resistance < Xc is stated to prevent damaing transients is stated.

I analysed the system consisting of balanced 3-phase resistance grounded source powering three identical capacitances connected to ground, with a time-varying fault resistance connected across phase as shown in:

http://www.geocities.com/pschimpf/HiRGroundedSys.htm

Equation 11 gives a solution:

d(En(t))/dt = -1/3*En(t)/Rn/C - 1/3*(En(t)+Ea(t))/C/Rf(t)

where En(t) is neutral voltage referenced to ground, Ea(t) is A phase voltage referenced to ground, C is capacitance to ground, Rf(t) is time-varying fault resistance, Rn is neutral grounding resistance.

At times when Rf(t) is infinity, the first term gives a decaying exponential response which always decreases |En| over time.

When Rf becomes low, the 2nd term can act to increase |En(t)| over time, but ONLY when En and Ea are opposite sign AND |Ea| > |En|.

Therefore we can see the peak value that En can attain is the peak value of Ea.

If we have no capacitance and a solid ground short, then En(t)= - Ea(t).
in that case Vb(t) = En(t)+Eb(t) = -Ea(t)+Eb(t)  will have a peak value of sqrt(3) times the nominal line-to-ground voltage, since there is 120 degree angle between Ea and Eb.

Now if we add the capacitance and remove the short when Ea hits a peak, the peak value of En(t) will decay slowly.  At 60-degrees later it will be 180 degrees apart from Eb or Ec and will create a voltage approaching 2x nominal line-to-ground voltage. (exactly 2x if no decay occurs during that 60 degree time span).

Now the question…. how do we expect a machine to respond to line-to-ground voltage increasing by a factor of 2 above nominal?

IEEE432-92 and others specify that ac hi-pot tests for machines with service-aged insualtion be performed at a level of 125% to 150% of rated machine line-to-line voltage for one minute.    Taking the lower limit of 125%, that corresponds to a 1.25*sqrt(3) ~ 215% of nominal line-to-ground voltage.    If a machine were to fail at less than 200% of nominal line-to-ground voltage for duration of less than one minute, it seems to me that the insulation was already weak.  What do you guys think?

It also makes me wonder whether voltage can increase higher than the factor of 2 predicted above if we change the model....

I think that if we modeled other motors connected to the system ot would have no effect… they continue to see balanced voltage applied to their terminals (even though line-to-ground voltage is changing).  

But I do think that we can get higher voltages and surges if we add a series inductance into the supply circuit representing transformer and cable impedances.  That can give some oscillatory behavior in reponse to a step-change in resistance.  If I get a chance I will try to model that.

Although we have spent a lot of time in discussion of this particular aspect (partly due to my comments) I would recommend to the original poster to keep an open mind to a wide range of possibilities for his problem.

RE: Simultaneous 4kV Motor Failures

I see the high resistance grounding as problematic on two counts.

One that it cannot and should not be applied where there are capacitors (including surge suppressors) in the system or / and where the equipment is spread out in the plant involving lot of cabling (capaciatance of cables). The thumb rule is that the total capacitive currents in the system shall not exceed 10A.

The second pertains to detection of earth faults or more precisely identifying the faulty feeder to isolate. It is generally not possible considering the low fault current magnitude and the earth fault in the system is identified by measurement of residual voltage and annunciated. It also happens in many plants that annunciations are not given much attention (rightly so though, some times) and as a result, the feeder continues to be in service with an earth fault in the motor or the connecting cable.

Under above conditions, the voltage of the healthy phases is raised, putting more stress on the insulation. No surprise, that a fault in another phase occurs sooner than later and both the feeders (fault in one phase in one feeder and another fault in the second feeder being one of the possible cases) trip which means the client is saddled with two simultaneous failures, like the one in the post.

I believe that high resistance grounding or isolated systems demand high level of education and awareness and education among plant operation and maintenance personnel to reap the benefits of the system (such as continity in production even with a ground fault). Further, proper design, good quality of equipment and installtion too is a prerequisite.

Raghunath

RE: Simultaneous 4kV Motor Failures

This has been an interesting discussion.  Just a couple of points:

In the U.S., HRG is not generally used for medium-voltage distribution system, mainly because it is not possible to selectively trip on ground faults.  I believe I have a paper by Louie Powell or other GE engineer regarding renewed interest in HRG on medium voltage systems to limit damage to motor stator iron on ground faults.  

My experience with HRG is primarily on large generators with unit transformers.  Selectivity is not an issue in this case.  Surge capacitors are universally used in combination with surge arresters at the generator terminals. The capacitance is factored into the resistor sizing calculations.

I don't see an inherent problem with use of surge capacitors on an HRG system, provided the capacitance is taken into account.  Use of PF correction caps is probably not a great idea.  Another drawback to HRG systems.

I'll see if I can dig up that paper on Monday.  

RE: Simultaneous 4kV Motor Failures


faulty, you raise some good points about high-resistance grounding limitations.  The facility owner/staff must understand that the first ground fault allows process continuity, but that, if not acted upon, the likelihood of two faults increases, and will disrupt more equipment and cause progressively greater expense and disruption.

If immediate efforts are not dedicated to eliminate the first fault, then the method loses its operational advantage.  

In the case of expecting a ground alarm not to be acted upon, then yes, definitely it is not the best choice, and a first ground fault should operate an overcurrent device, as in the case of low-resistnce or solid grounding.  That capability and limitation is ~50 years old, but is not a cure-all.

Also, high-resistance grounding should be applied to the load serving a manageable section of the electrical system, and if too large (high zero-sequence capacitive reactance) then it is also a poor choice.

If the client operation does not understand the tradeoffs, then it is probably counterproductive to implement this grounding method.  

dpc, I may be wrong, but my understanding is that surge capacitors are intended to be phase-to-ground connected, whereas PF-correction capacitors typically are not.  
  

RE: Simultaneous 4kV Motor Failures

Maybe I'm mistaken but I think the comments about education, responding to ground alarms etc apply to ungrounded, not high-R grounded.

We have 13.2kv system with 20ohm neutral grounding resistor. I think it falls in the category of high-R grounded.

I know we don't have any ground alarms on our high-resistance grounded systems, only ground fault trips.  

Individual motor loads are protected with residually connected ct's or differential-connected ct's.  The transformer supplying the bus is protected by ground current sensed in neutral (higher lelel). I think this provides plenty of sensitivity for selective tripping.

RE: Simultaneous 4kV Motor Failures

I'm not sure about that resistance value. Maybe 400 ohms and 20 amps. I'll look it up on Monday.

RE: Simultaneous 4kV Motor Failures

Pete...the theoretical voltage rise due to an arcing fault can approach that of an ungrounded system (ie 6X). I think you will see this when you add the effect of inductance and continue the analysis for several cycles.

In your analysis you say you remove the short when the voltage was at a peak. Shouldn't you apply the short when the voltage exceeds a level - insulation breakdown depending on a vols/mil being exceeded?

When I did the PSpice analysis of my situation, I got a voltage peak of about 4-5X rated. The frequency of the oscillation was also quite high so the motor was shown to be overstressed when compared to the surge voltage withstand envelope (this is the curve relating motor insulation capabity vs microseconds). This is a much snaller scale than the hi-pot test and reflects the fact that transients take a finite time to propogate through the winding.

RE: Simultaneous 4kV Motor Failures


electricpete, the quoted resistor values seem logical: 20 amperes and 400Ω correspond to 8kV line-to-neutral.  It also figures at 160kW, so that’s more likely a short-time {id est, 10-second} rating and not intended for continuous dissipation.  This would suggest inverse-time tripping and not an annunciation-only scheme.  A fraction of the resistor current would be desirable for ground-fault relaying...a 5-10A primary pickup for zero-sequence CT scheme is reasonable—or possibly for a residual-CT configuration.  This corresponds reasonable relay sensitivity weighed against minimal thermal stress to 5-mil copper-tape-shielded MV cable.  
  

RE: Simultaneous 4kV Motor Failures

busbar - I'm pretty sure that's what we have (400 ohms 20A). Not sure if it falls exactly in the definition of high-R grounded. We also have a 4kv system with neutral grounding resistor and similar relaying (no alarms). I will double check both resistance values.

Hi Gord. I will try the system with inductance and I expect higher voltages as we have said.

The reason I turn the fault off at the peak of Ea  is because that is how I would achieve the highest peak phase-to-ground voltage (Vb=En+Eb) approaching twice peak nominal line-to-neutral voltage (in my simulation without inductance).

Compare  two HiR cases in my simulation (ignore the LowR cases):

(Note for the following discussion I have assumed the magnitude of nominal line-to-neutral voltages Ea, Eb, Ec vary between –1 and 1.)

Case 1 – graph labeled 'HiR_3_cycle_short' - represents continued application of the fault for 3 cycles.  En(t) remains very nearly equal –Ea(t).  Vb(t)=Eb(t)+En(t) = Eb(t) –Ea(t) which has maximum value of the line-to-line voltage which we know is sqrt(3)~1.7

Case 2 – graph labeled 'HiR_Intermittent_Half_Cycle_Short' - represents removal of the short when Ea reaches it's positive peak and En reaches it's negative peak (at t=8.33 msec +k*16.7msec).    At approx 60 degrees later (2.8msec later) at time = 11 msec Eb will reach it's negative peak (-1) and Vb will be Vb= Eb +En = -1 + ~-1 ~ -2 where En has decayed only slightly from it's value of –1.  (looks like –0.84 in my graph giving –1.84…. would be higher magnitude for slower RC time constant).  

Case 2 (where the fault is removed at the peak) gives a higher maximum line-to-ground voltage (for the case with no inductance)

RE: Simultaneous 4kV Motor Failures

Suggestion: Visit
http://www.ecmweb.com/ar/electric_choosing_grounding_options/
for high resistance grounding.
Above approximately 4.16kV, especially on 13.8kV the high-resistance grounding cannot be applied because of high charging currents. These are obtained by calculations. The medium resistance grounding is used. Sometimes, when the charging current is too high on 4.16kV, the medium resistance grounding is used. This may be the case, if there happen to be surge arrestors/capacitors protecting rotating machinery. In some cases, the surge protection is implemented upstream of the system grounding location, e.g. on the transformer primary side.

RE: Simultaneous 4kV Motor Failures

I took a closer look at our system.

I did see that motor surge caps used on 13.2kv motors are fractions of a millifarad… approx 1000 times more than motor capacitance which are on the order of microfarads.

13.8 kv system – 20ohm resistor, 400A ground current, 10 sec rating. Since ground current >10A I believe this is considered a LOW-resistance grounded system =>   We have surge caps on our 13.2kv motors. We have no calculation to demonstrate Rn<Xc0.  Perhaps this is not required for a  LOW-resistance grounded system ?

4.16 kv system – 240ohm resistor, 10A ground current, 60 minute rating. Hi-resistance grounded system => We have Calcuation to demonstrate Rn<Xc0.  We have no surge caps on our 4 kv motors, which is why I previously thought we left surge caps out of our calc.

Ou 13.2kv motors have 51G ground trip.  Our non-critical 4kv motors have 51G ground trip. Our critical safety 4kv motors have only 50G ground alarm.   I’m not sure the reason we use time overcurrent 51G vs 50G for our motor ground trip functions. Also I was surprised to see the long time rating on our 4kv neutral resistor... I haven't looked closely at protection applied to the bus/transformer.

RE: Simultaneous 4kV Motor Failures

(OP)
Original poster here.  For the high resistance grounding, the transformer feeding the bus has a single 10 kVA, 1-phase, 4160/120V transformer in its neutral with a 0.866 ohm, 8 kW resistor on its secondary with a 64 relay.  The bus the motors are fed from has a similar setup but with three single phase trasnformers connected grounded wye - open delta with a 4.3 ohm, 10 KW resistor for selectivity on ground faults with the transformer. Still researching surge protective device parameters.

RE: Simultaneous 4kV Motor Failures

Two different grounding devices on the same bus?

RE: Simultaneous 4kV Motor Failures

joepower, I have some questions about your last explanation.
What you describe for high resistance grounding appears to be a neutral grounding transformer with a resistor in the secondary circuit. Typically in this setup, there is a 59(overvoltage) relay across the resistor to sense a GF in the primary. A 64 relay senses current. Is the relay in series or parallel with the resistor?
For the three single-phase transformers connected grounded-wye, open-delta; open-delta is for a two transformer system. Possibly, is the resistor in series with the three secondary windings of the delta connection? Where and what type of GF protection is present?

RE: Simultaneous 4kV Motor Failures

Looking at the bus transformrs, I suspect they are for relaying (grounded wye primary, broken-delta secondary).  If primary voltages to ground are balanced then the sum of the three voltages induced in secondary are balanced.

I think the ground fault current produced by that set of but pt's will be very low because the secondary relay is likely very high impedance (only magnetizing curent flows in primary if secondary is open-circuited).

RE: Simultaneous 4kV Motor Failures

Referring again to your bus pt's:
IEEE 242-1986 (Buff Book) section 4.12 - Overvoltage relay (59) - Section (2) - Ground Fault Detection - "One method measures the zero sequence across the corner of a broken delta secondary of three voltage transformers that are connected grounded wye..... a resistor may be required accross the relay to prevent damage due to ferroresonance."

Can you confirm you have a broken delta, not an open delta?

Is there a 59 relay associated with the bus PT's?

Is the 4.3 ohm resistor in parallel with that 59 relay?

RE: Simultaneous 4kV Motor Failures

I agree for systems with large capacitance that would rule out the possibility of limiting the earth fault current magnitudes to a low value such as 10A, it is still possible to go in for high resistance grounding system. The difference is that the current magnitudes could be 20A or so as the design demands and the motors in the system have to be tripped on detection of earth fault, as there is risk of core burnout.

The above method fails to offer the benefit of continued process and thus is like low / medium resistance grounding system. In fact, it is less preferred considering the difficulty in detecting reliably such low fault currents compared to the other systems.

Raghunath

RE: Simultaneous 4kV Motor Failures


joepower, a single-phase grounding transformer is typically applied where there is a physical neutral connection, such as a serving wye-secondary bank, or a dedicated outboard zigzag or grounded-wye/broken-delta grounding bank applied to a delta system that has no physically-accessible neutral bushing.  
  
I believe the differentiation of high-resistance versus low- or “medium”-resistance grounding lies in having a continuous-rated resistor and ground annunciation, and generally excluding tripping functions—such that manual intervention is needed to reset the annunciation.  

The applicable overvoltage relay should be able to withstand 1.73 p-u continuausly, but be able to pickup at under 0.1 p-u, and typically restrained for third voltage harmonic.
  

RE: Simultaneous 4kV Motor Failures

(OP)
It is broken delta, and the resistor is in parallel, but teh relay is called out as a 64.  However, it appears from further research that only the transformer neutral ground was in the circuit as the bus tie to teh section of gear with the broken delta configuration was open and the buses were independently fed.

RE: Simultaneous 4kV Motor Failures


A device 64 in protective relaying systems is intended to be a generator-field ground relay per ANSI C37.2.   For the application the OP describes, there has been either a misapplication or typo.  A usual/slang term for ground  detection in ungrounded/high-resistance-grounded systems is “59G”, for a [ground]-overvoltage relay.  

Now, if bus transfer occurs between a non-high-resistance-grounded and a high-resistance-grounded system, unreliable {or certainly varying} operation may occur.    
  

RE: Simultaneous 4kV Motor Failures

We have the same situation on our generator isophase bus.  When powered from the generator, the system is high-resistance grounded. When backfed through the generator stepup (delta-winding on low-side), it is ungrounded.  We have ground protection sensing generator neutral current, and an additional broken-delta scheme to provided continued ground proteciton on the isophase bus for use when backfeeding with generator off-line.  

We also call ours 64B, by the way. And we have 64R and 64S on the generator neutral and 64F on the field. I don't know what's up with the numbering.... call it what you like.

RE: Simultaneous 4kV Motor Failures

But I see now that Joe is saying that those bus pt's and relays were not connected during the fault.  Sorry for the tangent.

RE: Simultaneous 4kV Motor Failures

Suggestion: Reference:
J. Lewis Blackburn, Protective Relaying Principles and Applications, 2nd Ed., Marcel Dekker, Inc., 1998,
Section 7.4 Ground-Detection Methods for Ungrounded Systems
Section 7.4.1 Three-Voltage Transformer
For ballast resistors that are used to reduce the shift of the neutral from either unbalanced excitation path of the voltage transformers or from ferroresonance between the inductive reactance of the voltage transformers and relays and the capacitive system. Typical resistance values across the secondary windings are derived from experience and are shown in Table 7.1.

RE: Simultaneous 4kV Motor Failures

It does seem like Gord and busbar have identified a likely problem.

I have read where 0.5 micro-Farads per phase is typical value for the 3-pole surge caps in 4kv, as Gord says.

And going through the calc, you get
V_LG = 4KV/SQRT(3) ~ 2300
xc = 1/2/Pi/c = 5300
I per phase = 2400/Xc ~ 0.5
Ic total = 3*I ~ 1.5A.

Compare to your 0.866 ohm resistor. On the primary it looks like 0.866*(4160/120)^2 = 1040 ohms.
Resistive curent = 2300/1040 ~ 2.3A.

You have used up more than half of your allowed capacitive current already, exactly as Gord said.  So that does sound like a very fruitful area to pursue.  

Some other wandering thoughts:

Simultaneous failure of two motors must be connected through the power system.  Sustained voltage excursion, surge (from external source of from first motor failure) or one motor feeding the other as a generator and fault not cleared quick enough.  If you have 0.5 sec voltage magnitude recordings on the computer might help shed some small light (waveforms from fault recorder or digital relay are a lot nicer of course).

Even though you say you have ruled out power system problems, are there capacitors nearby which may have been switching?  Also utility may be able to provide you info on known activites/trips at the time of your event.

A fault in the first motor creates a steep-front wave which travels and can cause other damage.  (it is well known that if you have a flashover during a hi-pot test you stand a good chance of damaging motor or cable at other locations from the traveling wave).  One question that crosses my mind... if the large motor failed first internally, are its surge caps effective at preventing the internally-generated surge from  LEAVING the motor to the power system?    Since the standards make a big point of requiring a surge cap to be connected very close to the motor, I would say maybe not, because those caps on the large motor are not close to the 2nd motor that failed.

Surge caps generally have to be close to the protected equipment and have good soild ground connection to the motor frame ground (don't rely on cap frame ground).  

If a turn or ground failure appears to be on the first coil (especially first turn, connection end), that increases the likelihood of cause being surge-related.

Turn failures typically result in a lot of copper melting because they don't trip until they go to phase or ground.

Can they narrow down turn insualtion failure or ground instulation failure and how far in from the terminal?  If multiple failures look at how they relate to each other in physical space and electrical connection.

If you see heavy evidence of movement of end-turns in one phase of the smaller motor, I would say that may would support a theory that the 2nd motor failed due to the fault current that it supplied to the fault in the first motor.

Some types of motors are very susceptible to turn insulation problems, possibly even in the presence of surge protection. Those are form wound motors which do not have dedicated turn insulation (they use the strand insulation to serve as the turn insulation).  

Our 4kv high-R grounded system does meet the criteria for capacitive current < resistive (at least on paper). We have also had simultaneous failure of two 4kv motors on that system. Cause unknown.  (I'll have to look at the report to refresh my memory… it was before my time).

RE: Simultaneous 4kV Motor Failures

Some more discussion.

My scenario of 2nd motor acting as generator feeding fault in first motor (not cleared promptly) would be expected to lead to instantaneous trip of 2nd motor, if not both motors. That does not appear to be the case for yours.

It will be interesting to know how far these two motors are from the bus.  Cable can act somewhat like a surge capacitor.   If there is enough capacitance in those cables, maybe we rule out most of the scenario’s associated with surge?

Also, if you are giving thought to removing surge capacitors from your motor, there are two things that may help support that decision:
#1 – the capacitance of the cable between motor and the bus.
#2 – the use of dedicated turn insulation on the replacement motor.
Also the type of breakers used may affect your decision (vaccum contactors suspected to create worse transient than electromechanical breakers).

One other thing to note is that complete surge protection requires not only capacitors (to reduce dv/dt) but also arresters (to reduce the peak).  Capacitors need to be close to the load but arresters can be at the source of the switching or anywhere betweeen there and the motor.  I think most installations provide arresters on the transformer hi-side and possibly low side to protect from surges coming into the facility.

I think that the type of voltage oscillation resulting from excessive capacitance connected to your system results in temporary overvoltages threatening the ground insulation, but not what we would call surges that threaten the turn insulation.  The rise time of surges affecting turn insulation would be below approx 2-5 microsec.  LC oscillation will have a resonant frequency probably much lower.  At least that’s what I’m thinking now. So examine your 2nd smaller motor carefully to see if you can discern turn insulation damage (signifcant melting of copper).

RE: Simultaneous 4kV Motor Failures

For 0.5 sec scan time on both motor points, and difference in time recorded at 1.0 sec, I would expect the actual time between signals reaching the computer was between 0.5sec and 1.5 sec.  0.5 sec is a long time considering typical relay and breaker response times and does suggest strongly that the large motor went first.

To round out your calc of capacitances you want to add in your cables and motors.

Cable capacitance can be found from physics of cylinder within cylinder.
Substituting in unit conversions gives the following formula from Okonite:
C= 7 * 35*S1C / LOG(D/d)
Where
C = Cap in picoFarads per foot
S1C = dielectric constant from 2.8 to 3.5 (use higher value for conservatism).
D = Outer Diameter
D = inner diameter (usually conductor diameter).

Our 4kv cables range from 0.15 to 0.122 microfarads per 1000 foot per phase.

Motor capacitance to ground can be determined during Doble test.  Westinghouse provided us some typical values ranging from 0.037 microfarads for 400hp 2-pole to 0.149 microfarads for 1500hp 6-pole motor (4kv motors).

RE: Simultaneous 4kV Motor Failures

Correction
D=outer diameter, d = inner diameter, both in inches.

RE: Simultaneous 4kV Motor Failures

More values of SIC
PVC: 3.5-8.0
EP - 2.8-3.5 (that is what I reported above).
Polyethelene - 2.3
XLPE - 2.3-6.0

RE: Simultaneous 4kV Motor Failures

(OP)
Awesome input.  I should be at the motor repair shop tomorrow to get the full report on the nature of the failures.  I'll post some of the digital pictures to my website as soon as I get them.

RE: Simultaneous 4kV Motor Failures

Can you tell us a little better what we're looking at?   

Can we see bare copper?  Evidence of melting => turn-to-turn fault?

Was it found this way or pulled apart for inspection?

Is this the connection end. I think I can see jumpers attached on the outer edge just out of view, but not sure. Does it look like perhaps the fault started at the point where the jumper enters the knuckle?  That "scarf joint" in taping is a natural weak point.

Or does it look like it started closer to the slot where there might have been ground fault. I'm guessing not.

One more question, surely not related to the failure:  are those rub marks that we see on the stator bore?  Surely not since the pattern is so regular.  What is it?

RE: Simultaneous 4kV Motor Failures

Conductors blown apart make me think of currents flowing in opposite directions. One scenario is turn to turn fault. Current induced in the shorted turn is opposite direction from other turn currents and can be very high magnitude.

RE: Simultaneous 4kV Motor Failures

Gentlemen (oops, and ladies if present):

When you discuss continuity of production or "production safe" you are, of course, using the same convoluted logic used in ungrounded LV systems.  Additionally, even though you are generalizing, hopefully you are limiting such systems to "unclassified area" installations!

RE: Simultaneous 4kV Motor Failures

Suggestion to the first link showing the motor winding damage. It appears to be a turn-to-turn short as stated in the above electricpete posting; however, the root cause analysis may or may not be very conclusive for one cause because of the surge protection, age of motor winding, power supply quality, etc. factors.

RE: Simultaneous 4kV Motor Failures

(OP)
The multiple electrical failures in the 6000 HP motor were generally "in the slot", or phase to ground rather than phase to phase.  The rub marks seen in the photo are interpreted by the rewind shop to be wear marks from an abrasive dust environment, although I still need some convinving.  Both motors did have heavy crud buildup which probably didn't help their temperature any.  And for those fans of "almost in time" maintenance, the 6000 HP was scheduled to be sent out for refurbishment 4 days after the failure.  The probability is high that I have a set of 30-year old motors that have seen some hard times and are at the end of their insulation life and some relatively minor item could trigger the failure.

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