3 Phase Trip 2

[rogerj1]

Has anyone ever recorded a 3 Phase Trip with all phases having same magnitude and same phase angle? We've never seen this before.... just looking for answers or possible causes. Thanks, very nice forum here!
-rogerj1

 

[DanDel]

How was this recorded? The exact same magnitude and same phase angle on all three phases?

 

[rbulsara]

Why the question? Homework?

 

[rogerj1]

SEL Relay and I'll see if I can get the event report file on it. And no, I'm a Relay EE at a G&T Co-op. Thanks for your interest

 

[rbulsara]

I have personally have not recorded, but a bolted three phase fault would do it, I guess.

 

[stevenal]

If you look into relay event records far enough back you will likely see some strange events like the one described. Most likely caused during commissioning by a tech who connected each relay input in series to his single phase test set.

 

[oldfieldguy]

rbulsara and stevenal both have good scenarios.

I've been responsible for several records like stevenal describes.

It's an interesting exercise to run the oscillography data from a fault recorder through a vector generation program and watch the vectors swing around as a fault progresses.

old field guy

 

[Bung]

What about a very high zero sequence infeed to a ground fault from a remote ground point (tranfarmer star point, etc) with no active source behind it?


Bung
Life is non-linear...

 

[jghrist]

I just got done downloading a fault record that has all three phases identical in magnitude and phase. The fault was a single phase to ground fault on a substation 12.5 kV bus. The fault record was from an SEL-587 relay protecting a 480V-12.5kV generator step-up transformer. The 12.5kV side is connected grd-wye, the 480 volt side is connected delta. The 480 volt breaker was open.

The 12.5kV line current was pure zero-sequence. Primary current was 1072A and resulted in a 51P trip in 0.42 sec.

The substation bus was fed by a 100kV-12.5kV power transformer. A racoon was found below the bus and a bus insulator showed burn marks. The racoon did not survive the incident.

 

[rogerj1]

jghrist,
This is pretty much what happened in our case. We had a similar generation source open up on the delta side of the step-up TX. A ground fault on the secondary produced zero seq. current in the wye connection, thus all three phases same in magnitude and phase. We are putting together a trouble report study on this event and maybe I can share more details once that is complete. Thanks to everyone for their interest and input.

 

[stevenal]

jghrist, rogerj1

The current on the faulted phase during a an SLG fault is equal parts +, -, and 0 sequence. Please explain how you are seeing a pure zero sequence phase quantity.

 

[jghrist]

With the fault on the 12.5 kV bus, there are parallel paths in the sequence networks. One is through the power transformer to the transmission system source. The other path is through the step-up generator transformer to the generator.

In the zero-sequence network, there is an open circuit on the delta side of both paths; on the transmission system side of the power transformer impedance (the power transformer is delta-grd wye) and on the generator side of the generator transformer impedance.

There is a short to neutral on the wye side. This short to neutral on the generator transformer provides a path in the zero-sequence network. There is no path on the generator side of the positive- and negative-sequence networks because the generator side breaker is open.

The only path on the generator side is in the zero-sequence network, so only zero-sequence current flows through the generator transformer to the fault point. The total fault current includes all sequences, but not the portion flowing from the generator transformer.

Looking at it in a more physical manner, consider that with the generator side breaker open, there is no line current on the delta side of the generator transformer. The current in each delta winding must be the same because the the windings are in series with no other connections. The current "circulates" in the delta. The current in the high side wye windings are directly proportional to the current in the low side windings, so they are also all equal.

The line current from the generator transformer in the unfaulted phases goes through the 12.5 kV bus, through the power transformer windings to neutral and through the racoon to the faulted phase.

 

[stevenal]

Thanks.