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.

 

[rockman7892]

rbulsara

Thank you very much for your help I appreciate your input you have offered thus far. Of course I do not expect you to solve this mystery over the internet however I do appreciate your thoughts thus far on a general idea of what might have been going on. I am bringing a knowlegable consultant on-site to review this event and others with me however I wanted to hear others thought on this site to get additional input/ideas.

I finally got the relay event files from the utility relays and have posted these files. The 501-2 relay is looking at the utility transformer high side, low side, and tank ground CT's. The 387E relay is looking at the utility transformer high side, low side, and line side of the NRG CT between the NGR and the neutral bushing.

It appears that the utility circuit switcher opened when the 501-2 relay picked up on ground current. This relay input is looking at a CT which is around a 4/0 bare length of copper connected to the substation ground grid and bonded to the transformer tank. As you can see from the attached plot this CT saw about 250A of current through this CT. I am trying to think why this CT would see this current and the only thing I can come up with is the possibility that the path through the ground grid, through the transformer tank and back to the ground grid to the NGR presents a low impedance path that allows this ground current to flow through this path. I am not really familiar with the function of a tank ground CT. What typically is the purpose of a tank ground CT? Should it see current for a fault downstream on the system?

This tank ground CT has a very low pickup value (20A) and thus falls below the pickup value of my main breaker and downstream feeder breakers. I'm thinking the reason this tank ground CT has such a low pickup is due to the fact that it is not supposed to see much fault current during the fault? If this tank ground will see substantial fualt current during a fault then this pickup probably needs to be coordinated with the downstream ground pickups. I am going to discuss with utility but wanted to hear others thoughts.

It appears that the CT on the NGR saw about 50A in the neutral during this event. The other thing that strikes me as strange is the fact that the NGR saw 50A while the tank ground saw 250A. Assuming that all the ground current must return to neutral bushing through the NGR I do not see how the additional 200A going through the tank ground is getting back to the neutral. In this case I am thinking there is a shunt path around the NGR that is allowing current to flow back to the neutral.

The 387 relay is also looking at the load side of the NGR between the NGR and the ground grid. While the CT on the line side of the NGR saw 50A this CT on the load side of the NGR only appears to see about 0.5A. These currents on the line and load side of the NGR should match and I find it strange that they dont. Another question for the utility.

I'd appreciate greatly any additional input based off this new information and thank you greatly for your help.

 

[alehman]

We really need to know the design of the substation to respond to these questions. I don't know the purpose of the tank bond relay. It seems strange to me.



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

 

[rockman7892]

Attached is a single line of the substation as well as a 3 line diagram of the substation design on the third page.

I am confused as well about the tank ground. I am going to question the Utility about its purpose.

Thanks for your input.

 

[rcw retired EE]

The tank ground CT is probably a "64" case ground protection scheme.

In my experience in switchyards, older transformers were mounted on railroad wheels and tracks for easy removal for repair and maintenance. The wheel bearings were insulated so the ground CT was the only connection to ground for the tank. The winding neutral and arrestors were grounded separately. Any current flowing in the ground CT had to be from an internal transformer winding failure. The relay pickup could be set quite low, because there should never be any current except for internal faults.

(A ground fault in a 208V radiator cooling fan will also trip off the transformer. We discovered this on a retrofit.)

If an arrestor or neutral got bonded to the tank, or a conduit support or other ground path connected the tank to ground, a circulating current might flow. Maybe someone didn't like seeing only one ground on a transformer and added another, creating a ground loop that carries current during an external fault.

The 64 protection is also used on rectifier units, high voltage boilers, and some utility's live tank circuit breakers.

 

[rockman7892]

rcwilson

Thank you for your insight.

Upon inspection of the substation I found that there were two tank sides of the transformer that had a bonding connection to the ground grid which is right in line with what you mentioned about a ground loop carrying current during an external fault. I guess its possible that the path from the ground grid to one side of the tank transformer case around the transformer case and back to the ground grid through the connection on the other side may have a low enough impedance that is a lower impedance path than other paths between the ground grid and NGR that current flow through this path during an external fault. Even if this did occur however I would expect to see all of this current travel through the NGR to the neutral of the transformer. Even if we removed one of the (2) tank ground connections I still suspect that there could be circulating currents through the remaining tank ground connection and through the transformer concrete foundation which I'm sure presnets a pretty low impedance path to ground as well.

I agree with your comment on being able to set this relay pickup very low if the tank was isolated from ground (insulated from foundation) and only had one connection due to the fact as you mentioned the only current that would be flowing through this CT would be from an internal winding fault. Because of the fact that this existing connection seems to create a low impedance path for current to flow during an external fault the pickup will need to be coordinated with the downstream pickup if this relay does indeed see this circulating current.

One other thing that I noticed during inspection was the fact that there was a single phase single bushing station service transformer that was connected to the neutral on the line side of the NGR and on the transformer the primary and secondary neutral was bonded together. The secondary of the transformer then went on to feed a station 120/240V feeder panel which appeard to have this secondary neutral bonded to ground inside the panel with the ground of the panel then tied into the substation ground grid. Seeing this I guess it is possible that current flowing on the ground grid from an external fault can travel up the ground connection to the feeder panel through the bonded neutral in the panel back to the severice transformer secondary, through the bonded primary/secondary neutral connection on the service transformer back to the neutral of the utility transformer completely bypassing the NGR. The neutral wire on the secondary of the 120/240 transformer is only #10 or so for large fault current we would expect to see some damage if current was indeed taking this path. But if were only talking about 200A or so as shown in the event report then I guess its possible. Something to investigate further with the Utility.

Thanks for the help.