3-Phase Faults Vs Line to ground

[Wfg42438]

Hello Everyone,

In section 7.2 of the IEEE 399 book there is some discussion about 3Ph faults vs LG fault for protective device duty analysis. (see attached image)


Typically its most common to focus on the 3Ph faults however for the special condition where LG faults results are larger than 3Ph the following questions come to mind:

In the IEEE 399 we are told MV and HV CBs have 15% higher interrupting ratings for LG faults.

Can someone elaborate on following:

1. By this statement does that mean the breaker is somehow designed to have a 15% higher int. rating for LG faults exclusively or am i misinterpreting the text?


2. Based on item 1 does that mean data sheets provide an additional LG int rating and if so, is there a sample anyone can share?


3. The section touches on MV and HV CBs what about all other devices?
For example under the same conditon where LG is worst case whats to be done for Bus bars, Fuses, LVCBs, etc?
Has anyone seen any guidance for such cases or is there some practical reason IEEE 399 does not mention other equipment for such special cases?

 

[dpc]

You are probably worrying too much about SLG faults when it comes to equipment duty. Three-phase faults will always be the worst-case in terms of the total fault energy. The 15% allowance is just inherent to the performance of the equipment and is not a function of design. I've never seen a SLG rating on a protective device except in very exceptional situations. See ANSI C37.010 if you have access to it.

If the calculated SLG fault current exceeds the published three-phase rating, I'd be concerned and would assume that the device is underrated. In most areas of a power systems, the SLG current will be less than the three-phase.

 

[Wfg42438]

You are probably worrying too much about SLG faults when it comes to equipment duty. Three-phase faults will always be the worst-case in terms of the total fault energy. The 15% allowance is just inherent to the performance of the equipment and is not a function of design. I've never seen a SLG rating on a protective device except in very exceptional situations. See ANSI C37.010 if you have access to it.

If the calculated SLG fault current exceeds the published three-phase rating, I'd be concerned and would assume that the device is underrated. In most areas of a power systems, the SLG current will be less than the three-phase.

Thanks for the feedback I get where you are coming from thats its most common 3Ph currents are worst case.

Do you mind elaborating on the 15% allowance for the equipment rating?

Can you maybe break it down in terms of an example, lets say a CB has an SC current rating of 50kA how does the 15% allowance come into play?

Also it seems this allowance is mentioned for breakers does that mean other equipment wont have the additional allowance?

Also in practice if for these special cases it is identified that the LG current is bigger than the 3Ph current is it typically within the 15% margin which is why your saying its not a concern?

Additionally, if it is above that margin would the solution simply be to introduce resistor grounding on the XFMR or generator?

 

[dpc]

Each type of equipment has its own standard, and standards are different in US and other countries. There's no single rule you can apply. For breakers covered under C37.010, for SLG faults you can multiply the interrupting rating by 1.15.

If the breaker is underrated, the simplest approach is to replace it with one that has a higher SC rating. If I were doing a study and determined that the SLG fault current exceeded the tested 3 ph rating, I'd recommend replacing it. After 50 years, I can think of maybe one case where that situation actually came up.

 

[mparenteau]

I would just add to dpc's commentary: close-coupled switchgear, i.e. perhaps a power style transformer that is a few feet away from a switchgear lineup, will likely have a SLG fault current (3*Io) greater than the 3PH fault current. As dpc addresses above, this is cause for concern.

Mike

 

[7anoter4]

According to IEC 60909-0 Short-circuit currents in three-phase
a.c. systems -Part 0 Calculation of currents Fig.10, the maximum
ratio between three-phase symmetrical short-current and line-to-Earth fault current is 1.3 indeed if Z2/Z1=0.7-1 or Z0/Z2=1-1.4.
In connection with breaker rating, according to IEC 60694, there are two levels of rated short circuit current:
Rated short-time withstand current (Ik) and
Rated peak withstand current (Ip)
The definitions are:
For rated:
The r.m.s. value of the current which the switchgear and controlgear can carry in the closed position during a specified short time under prescribed conditions of use and behaviour.
For peak:
The peak current associated with the first major loop of the rated short-time withstand current which switchgear and controlgear can carry in the closed position under prescribed conditions of use and behaviour.
The ratio Ip/Ik=2.5 for 50 Hz 2.6 for 60 Hz

 

[7anoter4]

Sorry, I missed the conclusion.
The peak value depends on rated -rms-current.
Ip=k*sqrt(2)*I"k and k = 1 ,02 + 0,98e^(-3 (R/X))
There are not two rating values for a circuit breaker but different AFC
[Available Fault Current] for the same point of a circuit. The engineer has to consider all the possibilities and take the maximum AFC.