why neglect load current in fault studies?
why neglect load current in fault studies?
(OP)
Why load current is usually neglected in fault studies? Fault current is usually 5-6 times the load current. How we can neglect load current which is as high as 20%?
regards
Sunil
regards
Sunil






RE: why neglect load current in fault studies?
The fault current is the maximum current the transformer is capable of.
The only impedance considered for for fault current is the transformer impedance.
Fault current is based on bolted fault conditions, or zero impedance. Putting a load impedance in parallel with a zero impedance bolted fault still results in zero impedance.
Under bolted fault conditions, the load voltage is zero and with zero voltage, the load current is zero.
Bill
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"Why not the best?"
Jimmy Carter
RE: why neglect load current in fault studies?
http://GeneralPAC.com - A platform to ASK, LEARN, SHARE, & GROW in power protection, automation, and controls.
The Wye Wye Transformer Connection video: http://gpac.link/1GdGRSQ
RE: why neglect load current in fault studies?
The motors act as induction generators for a brief time. The actual time depends on the inertia of the motors and, for overhauling loads, may be continuous.
In any event, the motor contribution is greatest at the moment of the fault and contributes to the fault current and to the maximum magnetic forces on the conductors carrying the combined fault current.
Bill
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"Why not the best?"
Jimmy Carter
RE: why neglect load current in fault studies?
Number two, you can't simply add load current to fault current because load current is at an angle near zero, and fault current is closer to 80 to 90 degrees. When you do the vector math, you will find the added current is not very much.
Also, the load does decrease during a fault because of the drop in positive sequence voltage.
So if you want to include the load current in a fault, you need to first determine what the faulted condition load current is.
In the case of large motor load, the inductive feedback might exceed the normal load (rare on transmission, which is why I don't consider it).
Also, you need to consiter the contribution from grounded wye capacitor banks in your fault calculations.
RE: why neglect load current in fault studies?
Thanks
RE: why neglect load current in fault studies?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: why neglect load current in fault studies?
Load in the faulted phase may go to zero, but the unfaulted phases still wants to do its own thing. The direction on load current vs. fault current can either desensitize or increase the sensitivity of the protection, particularly when dealing with distance protection.
It was a huge wake up call for me when I transitioned from the radial world to the networked world. Somethings I thought I knew turned out to not apply at all any more.
RE: why neglect load current in fault studies?
I took the question to be in relation to fault studies where the load current could be ignored. I am going to do some browsing to try to get a better feeling for the implications of load current on fault protection in the networked world.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: why neglect load current in fault studies?
RE: why neglect load current in fault studies?
I see my error.
The maximum fault current is limited by the transformer impedance and the load impedance is effectively shorted out.
However in the event of a far out fault, where the conductor impedance combines with the transformer impedance to further limit the fault current, there may be upstream issues.
My comments are valid for the maximum fault current, but there may be issues with fault currents that are less than the maximum.
The current on the feeder under consideration will not be more than the transformer can supply, but the less than maximum fault current may combine with loads on healhty feeders and result in coordination issues.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: why neglect load current in fault studies?
RE: why neglect load current in fault studies?
Our protection folks typically use Aspen to run their fault studies. I found you can add load to those models if you like but most protection folks do their studies without load.
Like Cranky says, I'm used to fault currents being much higher than the load current, but if you situation has a 5 to 1 ratio of fault to load current, then superposition will work.
RE: why neglect load current in fault studies?
RE: why neglect load current in fault studies?