I don't know what the meaning of "realistic" is since in order to design or verify the installations one has to use a good statistical appreciation for at least 10-20 years in the future. The actual value, depending on number and power of power stations on the Grid and the distance from the measured point to the center of the power system, is useless in my opinion. The Utilities are analyzing the shortcircuit current periodically for the entire interconnected system but always for expected-at least- 10 years.
As I encountered, on 110 kV [50 Hz] -close to the center- 30 kA was expected.
220 kV may be up to 50 kA. 160 kV may be 63 kA- top and 400 kV from 50 to 63 kA also.
I think these data may be considered for 60 Hz system too.
a client is connected to a transmission network and the SCMVA is 2000 MVA. A other client is on the SAME line, but at the end of the line, and the SCMVA is 500 MVA. This is for a 120 kV network
It's going to mainly be a function of the transformer size that is serving the transmission line, and how far down the line the fault is.
Take the transformer MVA, divide it by the transformer impedance and you'll have an upper limit for a three-phase fault. 200 MVA transformer, 10% impedance - 2000 MVA. The transformer impedance is primarily a function of the primary voltage and BIL. ANSI has standard impedances based on the BIL.
"The perfect is the enemy of the good." -- Voltaire
Thanks all for your response!
However, I am more interesting on the transmission grid level, i.e. 230kV and 115kV ring buses other than a tapping transmission customer, any ideas?
I am working on a protection study for one of our transformer stations. The station (DESN) is tapped on the 230kV transmission lines. The normal operation is to run two transformers in parallel with the bus tie closed. If there is a fault on one 230kV line, as a backup, the directional distance element will prevent the back feed from the healthy 230 line to the fault through the paralleled transformers. So, my first step is to do a fault flow calculation. However, I don't have the fault level at the supply authority's ring bus. We had a similar study done by a consultant 7 years ago and the consultant didn't have that info either, what they did is to assume 6600MVA (I have no idea where they took this estimation from)at the 230kV bus - 17kA 3Ph fault at the nominal voltage 220kV, which I thought is underestimated.
Infinite bus at the other end of the line then. There are no typical values, ours range all over the place. If you work from a value that is too high your costs will be higher than necessary. If you work from a value that is too low you aren't going to have the right equipment.
A rural location will be different than in an urban core; near generation will be different than far from generation; the Atlantic seaboard will be different than the wide open spaces of the inter-mountain west. You need to be working from a system model of the specific system you are planning to interconnect with.
If made up values are really all you need, go ahead and make them up, but there are no usable "typical" values.
Here is another sample value for a 115kV station supply (within about 50km of the 230kV point mentioned earlier and both within about 50km of a large generating station
Much appreciated guys! You are all absolutely correct, I am after some reasonable values just for some the reference. The actual value should be obtained from the transmitter.
Substation circuit breakers are rated for voltage, continuous current, and fault interrupting capability. In transmission substations (69 kV and up), typical fault current ratings include 25, 31.5, 40, 50 and 63 kA, and some manufacturers now can attain 80 kA.
This should give you some indication of what the market supports. As the engineer, you have to know what conditions you are going to encounter to decide how much of a breaker to buy. This is where the fault study comes in.
In my rural California utility, 40 kA is plenty for every part of my system. In fact, I've got old OCB's from the 70's at 14.5 kA and 19 kA that are still adequate *for my system*. But it would be foolish for my counterpart at the utility next door to try to apply what works for me to his system (not to mention the big urban utilities nearby).
