# Knowledge Base

## Applying Multiple Faults Simultaneously in Transient Stability

A common question that PowerWorld support staff receives is as follows.

## Answer: Yes, absolutely!

We agree that normally this would not happen in real life situations, however there are studies that power engineers do that may require this. Our customers in North American who are running NERC CIP-14 studies often do this for example and apply faults to multiple buses inside the same substation. There is nothing special you need to do in software tool to do this.  All you do in PowerWorld Simulator do this is apply a fault at multiple locations with timing that overlaps and this will happen.

If you need more convincing that this is possible in software, it is useful to step back and think about what software is really doing numerically when you say “apply a fault”.  To numerically simulate applying a fault in transient stability, software tools like PowerWorld Simulator simply place a very small impedance between the bus and ground.  Putting in a very small impedance to ground is the same as placing a very large admittance at the bus in the numerical simulation.  Even if you choose to place a “solid fault” in the software, that does not mean that a 0.0 impedance (or infinite admittance) is used.  Software tools instead just put in an extremely large admittance value.  What this means is that “applying a fault” in a software tool is exactly the same as putting in an extremely large constant impedance Mvar load at the bus and closing it in to represent a fault.  There is nothing wrong with putting in multiple very large constant impedance Mvar loads and closing them in, so there is nothing wrong with simulating multiple faults.  Obviously you need to be careful with doing this in too many locations at once because at some point you’ll just get a voltage collapse, but presumably when applying multiple faults like this you are trying to understand if a voltage collapse will occur.

In the extreme, you may choose to apply a fault at every bus in a single substation simultaneously.  That is allowable. Sometimes we have seen customers instead go in an apply multiple line faults at end of every AC line that connects to a single substation.  That is fine too, but it may be more cumbersome than necessary and may be easier to just apply a fault at every voltage level within the substation instead.

January 8, 2021