Fault Analysis Help 3

[rockman7892]

I am investigating a fault on a 4.16kV 2300hp motor and was looking to get opinions on the attached event report showing the fault.

The fault happened on the motor and blew all (3) power fuses on the motor contactor, tripped an upstream breaker protected by a Multilin 750 relay, and tripped the utility circuit switcher. There are obviously some coordination issues that need to be addressed, but I am also trying to make sense of the events as they occured.

In looking at the attached event report from the main breaker I have the following observations:

It appears that the system was operating with steady state current up until point (1) on the event waveform at which time there was a current transient with the current on all three phases increasing. I am assuming that this current transient is a result of the fault at the motor. This current transient only lasts for about 1/2 cycle before the current returns to steady state. I am trying to figure out why the current returns to steady state value so quickly, and the only thing I can think of is that after the 1/2 cycle falut transient the fuses at the motor blew returing the current seen at this breaker relay to steady state current at point (2).

At point (3) it appears that the current goes to zero and looks as if this is the point the breaker opened. The event report shows the relay tripping the breaker on an Neutral Time Overcurrent trip (51N). The part that is confusing is why there are about 3 cycles of steady state current after the current transient before the breaker appears to trip (between points (2) and (3)) The only thing I can come up with is that during the transient the relay pick's up and issues a command to the breaker to trip and these 3 cycles of steady state current is seen due to a 3 cycle delay for the breaker to clear. In other words although the downstream fuses at the motor cleared the fault and returned system current to steady state, the realy had already issued a breaker trip command and the breaker clearing time had about a 3 cycle delay before it actually opened. Does this make sense?

The breaker trip output is "Output 3" seen at the bottom point (4) and can be seen issuing the trip command and then dropping out after the breaker appears to open. This trip output appears to be on for about 3 cycles while the breaker clears. I believe 3 cycles may be a typical clearing time for a vaccum type breaker?

The voltage waveform at point (5) shows a small transient at the time of the current transient or fault. There does not appear to be much of a distortion or drop however and I wanted to see if this made sense? Could this be due to the fact that the fuses cleared the fault current so quickly?

The strange part is what happens to the voltage waveforms after the breaker appers to open. When the breaker opens at point (3)/(4) the current drops off however the voltage appears to taper off slowly. The voltage is coming from a set of PT's on the switchgear bus located on the secondary of the breaker that is opened. Why would the voltage appear to taper off slowly for several more cycles as opposed to going to zero right after breaker opening. The voltage tapers off to where at point (6) it is about 3kV and at the last cycle shown of the waveform the voltage is slightly under 1kV. What could cause the voltage to drop so slowly?

I appreciate any suggestions or corrections on this event as I am trying to make sense of what happened. Thanks for the help.

 

[alehman]

I think that pretty much makes sense if the breaker was serving other loads. It looks like the current may have dropped a bit (by the amount of the motor load?) after point 2.

After the breaker tripped, it's likely there were other motors on the system that continued spinning and producing voltage due to their residual flux.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[AMBMI]

I mostly agree with Alehman; the other asynchronous motors should be quite big (big time constants, big inertia) to produce a so low decaying voltage for so many cycles.
Other stuff: the relay is tripping in about a cycle. I wonder why the 51N element has been set with no delay. It seems that the breaker is operated by the 51N starting signal. It could be the reason of the mis coordination.

 

[rockman7892]

alehman

Yes this breaker is serving other loads in the plant.

Excellent point about motors on the system generating voltage with their residual voltage after breaker tripped. There were several 500hp+ motors running on the system at time of fault.

My relay did not show me the value that the 51N element saw. The 51N pickup was set at 150A. The relay however does give me the individual phase currents and angles at the time of the 51N pickup. Can I add these phase current vectorally to arrive at the value of ground current that the 51N element saw in its residual calculation?

Something else that has me curious is the fact that the main relay picked up on a 51N element (ground fault) although looking at the current waveform it shows that the fault at some point turned into a three phase fault. The relay did not pick up at all on a overcurrent condition for this 3 phase fault. Is it possible that the relay picked up on the 51N element first an iniated the trip to the breaker before it regognized a phase fault condition? In other words if the relay has already picked up and issued a trip command from one of the elements will it ignore the pickup from any other elements that may occur after?

Thanks for the help.

 

[jghrist]

looking at the current waveform it shows that the fault at some point turned into a three phase fault.

It looks like a 3Ø fault from the beginning at Point 1. The neutral current probably was a result of the motor fuses blowing at slightly different times on each phase. It certainly appears that you need more time delay in the 51N element. The current after Point 2 does not seem high enough to be a 3Ø fault. Seems more like load current from the other motors.

 

[alehman]

I think the 750 will log the elements individually. Perhaps the event was cleared before the phase overcurrent had time to activate, or maybe the 51N is set for lower pickup. Someone would need to review the settings in detail. Can you tell from the logs if it picked up and dropped out, without tripping?

As AMBMI suggested, it seems like 51N is much too fast.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[rockman7892]

I also agree that the 51N is too fast. I am going to have setting reviewed and add a delay.

I am still waiting on utility relay event report but from what they have told me they picked up on a ground current as well. I'll let you know what I find.

Am I right as far as adding the phase currents and angles vectorally to see what magnitude the relay picked up on?

I dont know if I totally understand a 3 wire 51N residual calculation. I always thought this would only show current if there was current flowing outside of the three phase conductors ie a neutral or ground current. Are you saying that this can pickup current unbalanced phase current in the three phases only? Can you briefly explain how this calculation works?

 

[ASimmons23]

The three phase CT's are connected in a wye configuration with the residual current element (51N) on the neutral leg of this connection. It's not a calculation but more of a measurement and will pick up any phase current imbalance. I agree with JGHRIST that the imbalance most likely came from the fuses blowing at different times. When re-coordinating your fuses and upstream devices, play with the 51N curve in order to give the neutral current enough time to dissipate on its own before relays kick in. You'll want to check with the facility and see what the maximum tolerable ground current magnitude and duration is before adjusting settings.

 

[rockman7892]

So even if the system is operating at steady state an there is a current unbalance between the three phases (unbalanced L-L loading) the 51N element will still see this current imbalance. I guess this is different from a zero sequence CT which stricly looks at ground current and does not pickup and phase curren unbalance?

Does the fact that a 51N element can pickup on steady state unbalanced conditions factor into the pickup. I imagine it must to avoid nusience tripping. 5% minimum pikup seems like a number that comes to mind.

That makes sense about the fuses blowing. I guess with the fuses blowing at different times this caused the current magnitudes on each of the three phases to be different and thus trigger the 51N element.

 

[davidbeach]

The residual, whether calculated or summed, is only 3I0, nothing more, nothing less. Exactly the same thing that a core balance CT (CBCT - aka zero sequence CT) around the same conductors would see. The only advantage of a CBCT is that it can have a much small ratio than CTs that see phase current and can therefore be more sensitive, but it still provides exactly the same 3I0 that a residual connection/calculation would.

 

[gokulkrish2]

David beach,

Can you please elaborate more on this CBCT. Are they similar to GFCT? Actually we had some trips on a Main breaker at a sub which feeds a few pumps and a feeder.

Trips actually started recently when more 1 phase transformers were added to the feeder.

The main breaker for the sub has a 351S which has inputs from both a GFCT(50:5) and 3 phase CT's (1200/5). The settings of 50N1 was 20A and 40cycles time delay. It tripped on IG being 24A.

I am reluctant to increase the setting to more than 50 as the system ground fault is already limited to 100A. If more resistance is added to the ground fault return path through arc resistance then there is a chance that ground faults are not sensed...

I want to learn more abt the GFCT's. Does it have the capability to measure phases?? How is it adding the 3 phases vectorily??

I am not trying to deviate from the original thread... but sense that the issue i brought was related to GFCT too. If you guys need i can start a new thread...

gokul

 

[dpc]

I think CBCT and GFCT are two four-letter acronyms for the same thing. I generally refer to these as flux summation or zero-sequence CTs.

The CBCT that David was talking about is a toroidal CT that encloses all three phases (and neutral if necessary). It sees the flux summation of the three phases. If current are equal, there is no net flux and the CT has zero current. If there is current imbalance, whether due to a fault, or unbalanced loading, it will see the resulting net flux and the secondary current will be proportional to the imbalance.



David Castor

http://www.cvoes.com

 

[AMBMI]

A core CT (toroidal CT enclosing all phases) or 3 CTs using Holmgreen connection (the CT current vectors are summed together) are not sensitive to unbalanced loading or to ph-ph faults not involving the ground. Indeed both configuration are measuring the 3I0 (zero sequence) current which is present only for faults involving the ground. This from a theoretical point of view. In the real world the response of the "Holmgreen" CTs during a transitory or their magnetizing characteristics can be different so we get some % of unexpected current also without a ground fault. It's the reason why a 50N/51N element using an Holmgreen connection was usually set with 20% In threshold. More recent rules are asking for smaller thresholds (10%) but perhaps a bit bigger time delay should be used in that cases
Much better the response of the core CT. So less problems and a smaller threshold but an additional stuff (the core CT) is needed.

Regarding the Gokulkrish2 trip I would say that 24 A of ground current with 100A as max grnd short circuit current are a lot. The element has also a great 40 cycles delay so any transitory should be gone. I would expect that the system experienced a real fault. Critical point: the 50/5 core CT is quite small, here we are more used to see 100/1 core CTs, more iron better performances..

 

[rbulsara]

51N function in Multilin SR 750 relay is a calculated value, vectorial sum of the three phase currents, by the relay. So yes, it will pick up the load unbalance. You need to set it higher than the normal unbalance plus some time delay.

For a measured ground fault current using a separate residual CT in the neutral bonding jumper, there is a separate input connection and is designated as 51G in SR 750.

All mfrs are not the same. See thread238-238200.

Rafiq Bulsara

http://www.srengineersct.com

 

[AMBMI]

I apologize rbulsara but even if there is a load unbalance the vectorial sum of the phase currents is zero if no grnd fault is present. So under these ideal conditions the 51N element won't pickup. I think that in this case the phase CTs reacted differently during the fault transitory so the relay calculated internally some zero sequence current and due to the 51N missing delay we got a 51N trip....

 

[rbulsara]

Yes, only if there are no L-N loads and proper application/design. The relay does not know that. 51N is a neutral overcurrent function and not a ground overcurrent, but can be used as such. It is not necessarily summing up neutral and phase currents, just the 3 phase currents. There is a fine line between Neutral overcurrent and Ground overcurrent.

Plus I was particularly responding to:

I dont know if I totally understand a 3 wire 51N residual calculation. I always thought this would only show current if there was current flowing outside of the three phase conductors ie a neutral or ground current. Are you saying that this can pickup current unbalanced phase current in the three phases only? Can you briefly explain how this calculation works?

and in reference to SR 750.

Without a neutral connection to loads (3-wire circuit), the sum of three phase currents will always be zero in normal conditions.

Rafiq Bulsara

http://www.srengineersct.com

 

[rockman7892]

Thanks for all the help guys. I think though now I have become confused.

For a residual CT configuration used for this 51N function the three phase CT's sum the current in the three phases. Since these three CT's are connected in a wye configuraton then for any unbalance there would be current flowing in the neutral of the CT connection. This current flowing in the neutral of the CT connection is where the relay element would pickup this unbalanced current. This would be similar to adding a neutral to a 3 phase wye connected load, where any unbalanced current on the phases would flow in the neutral. This neutral current would flow for unbalanced conditions aside from fault conditions? Is this not correct?

If a newer solid state relay (such as 750) does not have an element in the neutral CT connection but rather uses a vectoral summation of the 3 phase currents wouldn't you expect the same results? Esentailly the vectoral sum of phase currents indicates the current flowing in the neutral. This would hold true if there is indeed a neutral present in the priamry circuit but what about this case where there is no neutral present on this 3 wire circuit? Shouldn't this vectoral sum still provide a neutral current value even for unbalanced conditions that are not a ground fault?

Now with a zero sequence CT (core CT) I always thought that this was strictly only picking up on ground currents. That even under unbalanced conditions the net flux in the core was zero due to the fact that the unbalanced current was still being balanced on the return legs and therefore the net flow through the core is zero. I am starting to wonder now if this is true? Is this not true and the zero sequence CT will sum the flux of the current and output a neutral current value if there is any unbalance.

I am still anxiously awaiting utility relay report.

 

[rbulsara]

Again, no neutral connection in primary, no neural current in Wye Connected CT secondaries. The sum of the phase currents is already zero.

Rafiq Bulsara

http://www.srengineersct.com

 

[rockman7892]

rbulsara

I can understand what you are saying in concept but cant seem to visualize it.

Let me present an example:

We have a 3 wire 3 phase load with unbalanced currents. We have 3 CTs (one on each phase) connected in a wye configuration measuring current. As you said because there is no neutral in the primary circuit then there will be no neutral current flowing in the CT secondaries and therefore not neutral current flowing in the CT connection neutral.

Now lets say we go ahead and connect a neutral to the primary load so that the unbalanced phase current flows in this neutral. Now since there is neutral current flowing in the primary circuit we will see neutral flowing in the secondary CT circuit from what you are saying. However the phase angles of the current through the CT did not change and therfore the phase angles of the current in the CT secondaries did not change. So then how by having neutral current in the primary does this effect the netural in the CT secondaries?

The realy gave the pickup currents of:

Ia = 5973A @38deg
Ib = 6798A @169deg
Ic = 6000A @ 270deg

The relay did not give a 51N current value but adding these vectorally I come up with 2,244A as the pickup value. Clearly these currents here are shown to be unbalanced however we cannot tell if there was actually a ground fault presnet from these phasors.

 

[rbulsara]

There could have been a ground fault involved in this instant. I am not trying to solve this "mystery" over internet. You would need someone experienced on site to look in to this and spend some time and effort.

Rafiq Bulsara

http://www.srengineersct.com