# How to estimate the maximum asymmetric short circuit current?

Hi folks,

I want to determine the maximum asymmetric short circuit current due to a bus fault at a particular bus in my saved case. How do I determine this using the PSSE API?

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Running a short circuit analysis and getting the results back can be difficult with the PSSE API. The documentation doesn't have any examples, and the API is a bit confusing. Here is how to do it:

ASCC_CURRENTS

ascc_currents in the pssarrays module will run the short circuit analysis and return the results in a custom made Python type that PSSE refers to as rlst.

# this script assumes you have already initalised PSSE and loaded a saved case.
import pssarrays

# create a subsystem of buses you will apply the fault to.
DOWNTOWN_SID = 1
psspy.bsys(sid=DOWNTOWN_SID, numbus=2, buses=[2001, 2002])

results = pssarrays.ascc_currents(sid=DOWNTOWN_SID,
all=0,
fltlg=1, # report on line to ground faults.
flat=0) # use prefault voltage from working case


This code will run a line to ground fault on two buses 2001 and 2002 and store the result in the scfilename file. We don't use flat start conditions, so the pre-fault voltage is the same as the one in the solved case.

Lets unpack the results variable to see what is inside:

# get the phase A current for this line to ground fault
phase_a = [i.ia for i in results.fltlg]
# and match the phase currents up against the faulted buses
fault_currents = zip(results.fltbus, phase_a)
print fault_currents


That final print statement should show something like this:

[(2001, (30.4 + 12.j) ),
(2002, (30.1 + 11j) ]


You asked about how to find the maximum. There are a number of things that contribute to a maximum fault level:

1. Prefault voltage (higher voltages have higher fault currents)
2. All generation connected (each generator contributes to fault current)
3. Buses at terminal stations are tied (will increase the fault current if the fault is downstream of the bus tie)

There are a few other things to watch for that will increase your fault currents unrealistically. If you model your SVC units as a synchronous condensor (like we do) then you need to replace it with an equivalent capacitor bank. SVC units behave like a capacitor during short circuits, not like a spinning machine.

#### Warning

I don't have a copy of PSSE here with me (and I wont for a few months). You should check that the code to get phase_a works. Let me know if it doesn't so I can update the answer with working code!

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@JervisW, my understanding is that the ASCC routine gives symmetrical fault currents. I am trying to ascertain the max asymmetric fault current for the purpose of CT sizing. Do you think the bkdy API is appropriate?

( 2012-06-18 10:55:07 -0500 )edit
1

@amaity this procedure will certainly give asymmetric currents. A line to ground is one example of an asymmetric fault. While a three phase fault is symmetric. The example ASCC routine I've shown calculates asymmetric line to ground currents.

( 2012-06-19 02:13:32 -0500 )edit

@JervisW perhaps I did not explain my requirement clearly. I am trying to find the most severe fault condition at a bus and capture the current in the sub-transient region. I hope to use this to size all CTs connected to the bus.

( 2012-06-19 11:08:21 -0500 )edit

We use the subtransient reactance value when populating the sequence data in our models.

My procedure only calculates the line-ground asymmetric currents, but you can extend it to: line-line ,line-line-ground and three phase and take whichever is greater.

( 2012-06-20 02:10:53 -0500 )edit

Jervis, have you attempted to generate any graphical results from an ASCC routine? The PSSE api doc shows command ASCC has a parameter scfile which creates scfile. The PSSE api doc then shows command ASCC_SCFILE can read this file into memory to enable displaying ASCC results onto slider diagrams.

( 2017-05-25 15:52:51 -0500 )edit