Fault level Vs Switchgear Capacity

[FernandoCFE]

I have the next situation:

In a substation (115 kV system) I found a fault level of 43 kA (symmetrical value) in the busbar, and the switchgears in this substation are 40 kA (symmetrical).

The 43 kA are the result of a vectorial sum of the contributions of the derived circuits conected to this substation, but in a fault after a switchgear, the fault current that will flow through this switchgear will be less than the total fault current in the busbar and less than 40 kA.

Can I assume that this switchgear will withstand the fault?
Any reference about this issue, will be appreciate!

FernandoM

 

[alehman]

The bus must be braced for 43kA. If the switchgear is such that it cannot see more than 40kA under any conditions (including a fault within the switchgear equipment), it would be OK. More information is needed on the exact configuration to evaluate.

 

[dpc]

When you are this close to the switchgear interrupting rating, you must follow the short circuit calculation procedure specified by the standards to which the switchgear is manufactured. For ANSI switchgear, this is covered in ANSI C37, for IEC gear, I have no idea.

You have to consider the x/r ratio that the switchgear interrupting rating is based on.

 

[jbartos]

Suggestion: The switchgear has to be rated 43kA or higher to be able to interrupt 43kA. It has to be braced for close and latch capability value that is 1.6K rated short circuit current (rms), where K is rated voltage range factor. K is usually equal to 1 for 121kV (115kV falls under 121kV rated max voltage under ANSI C37.06).
The next size of the switchgear will be 63kA, which is available for rated continuous current 2000A (rms). It is not advised to assume any higher than rated values to be withstand since the switchgear is not tested for such values, and it may explode for untested values. Some designers and engineers are leaving certain design margin to 40kA or 63kA.

 

[cuky2000]

Fernando,

Appear unclear to me how the 43 kA fault was found. Perhaps someone could be able to help if you could please clarify the following:

1- Describe briefly the one line diagram with sources and components (transf, gen, TL, etc).
2- Available max. fault current and X/R ratio at the substation incoming/outgoing TL.
3- Confirm 40kA sym. is the lowest interrupting rating of the substation components (breaker, sw, etc.)
and bus withstands SC capacity based on clearance, insulator mech. strength, bus conf, etc.

 

[Area]

In any case, you need a complete short circuit calculation of your network. The how to do is described in the IEC 60909-1. Do not only consider the symmetrical velue of the short circuit (Ik" in kA rms) but also the initial peak value (ip in kA peak) of the short circuit, where Ik" is referring to the thermal stress that occurs during a short circuit while ip is referring to the mechanical stress. ip is derived from Ik", by multiplication of a factor which depends on R/X of your network.

The rated values of your switchgear must be:
Icw > Ik"
Ipk, Icm > ip for the busbar, cabinets and all devices such as short circuit breakers.

In case of a short circuit in one of the connected sub stations, the short circuit will be less than the max. value that you have given. But in case of a short circuit in your main switchgear, you will have the full short circuit. If it is above the design value of the switchgear you might have a problem. In this case, you cannot assume that the switchgear will withstand the short circuit.

 

[jbartos]

Suggestion: As it can be seen from the above postings, ANSI/IEEE Stds and IEC Stds set limits on the circuit breaker rated values, interrupting capacity and Close & Latch or Withstand capability. These limits must not be exceeded to avoid serious damages that potentially may happen.