Current N RMS Trigger calculation

[alsafaj]

Hello,

For an IDT 132 kV, a primary value of a CT is given at 3200 A -- A trigger is set for a residual or neutral current at 8% for the HV side. I would like to understand the purpose and how to read this out from the below graph.



Thank you,

 

[Sahib]

Hello,

For an IDT 132 kV, a primary value of a CT is given at 3200 A -- A trigger is set for a residual or neutral current at 8% for the HV side. I would like to understand the purpose and how to read this out from the below graph.

View attachment 18927

Thank you,

The purpose is to sense and trip the breaker for ground fault. The graph is not clear to me............

 

[alsafaj]

The purpose is to sense and trip the breaker for ground fault. The graph is not clear to me............

Thank you. That I understand .. but I am trying to see the 8% when it trips mathmatically -- Am I not supposed to sum phase currents and notice the difference prior and after trigger conditions. See below:







Can you please explain how to arrive at the trigger condition with the math.

Thank you

 

[Sahib]

Suppose trigger value of 8% is set at 1500A of neutral current i.e at 120A. In your case the actual neutral current has a maximum value of 117.5A only. So the breaker would not trip. You may have to reduce the trigger value for that.

 

[Smart $]

Suppose trigger value of 8% is set at 1500A of neutral current i.e at 120A. In your case the actual neutral current has a maximum value of 117.5A only. So the breaker would not trip. You may have to reduce the trigger value for that.

From the graph, there appears to be several positive peaks which exceed 200A (8% out of balance) but being only positive and no negative, a DC offset is being introduced into the system somewhere. Another question I have is whether that 8% is from system 0 volts or a calculated expected neutral current. The latter will require vector math and compensation for nonlinear loads.

 

[alsafaj]

From the graph, there appears to be several positive peaks which exceed 200A (8% out of balance) but being only positive and no negative, a DC offset is being introduced into the system somewhere. Another question I have is whether that 8% is from system 0 volts or a calculated expected neutral current. The latter will require vector math and compensation for nonlinear loads.

This is a calculated expected In (trigger setting in primary 120 A)

Looking at the graph, I was able to see the maximum current is about 224.1 A prior to trigger activation .. Once triggered I saw the value dropped at 204.7 which's > 8% but drop. Isn't this supposed to be overcurrent trigger.

I am confused. What am I missing here?

 

[Sahib]

This is a calculated expected In (trigger setting in primary 120 A)

Looking at the graph, I was able to see the maximum current is about 224.1 A prior to trigger activation .. Once triggered I saw the value dropped at 204.7 which's > 8% but drop. Isn't this supposed to be overcurrent trigger.

I am confused. What am I missing here?

The trigger value 120A is RMS value and not peak value and 117.5A is also RMS value. Otherwise the breaker would have tripped at peak values of 224.1A or 204.7A. Did it?

 

[alsafaj]

The trigger value 120A is RMS value and not peak value and 117.5A is also RMS value. Otherwise the breaker would have tripped at peak values of 224.1A or 204.7A. Did it?

That is right, but it is still not working the way it should. Please check the below. The trigger starts at 0 seconds and I have included two cursors, red cursor before the trigger and greenish one after the trigger (during fault violation) -- Please note the In (black label) -- both RMS values didn't exceed 120 A -- The values are almost the same. What does it mean?

 

[Sahib]

alsafaj: Your problem is the breaker should not trip under the given conditions but it trips-fault violation. Correct?

 

[Ingenieur]

it has to see |In| > 120 (rms) for 1 cycle (20 mSec)
that doesn't appear to be happening
the neg swing magnitude appears to be <120 so resets the counter

 

[alsafaj]

alsafaj: Your problem is the breaker should not trip under the given conditions but it trips-fault violation. Correct?

That is true -- I am not able to simulate this with a a secondary injection due to unavailability, so I am just relying on the existing fault record.