Switchgear Fault Level 4

[Deansharafi]

What is the remedy for a switchgear when its fault capacity is exceeded? Let's consider a case where generators are added to a busbar without considering the fault capacity of the board.
Is there anything that can be done to rectify the situation?
Thanks for your thoughts.

 

[ctpt]

One option is perhaps introduction of a fault current limiter. See link below :

http://www.gwelec.com/product_current/clip.cfm

 

[sykimk]

Using fault current limiter to reduce short circuit contrbution currents is a good solution. Alternatively, a current limiting reactor (CLR) is a commonly used solution. However, you should carefully consider to utilize CLR because it could cause serious voltage drops on system due to high values of reactance.

Therefore, you must conduct load flow study as well as short circuit current study to verify whether there will be serious voltage dips on system.

 

[FernandoCFE]

Visit this link:

http://www.tyndall.ac.uk/publications/working_papers/wp25.pdf

 

[cuky2000]

Beware that current limiter are not cheap. Sometime the option of new switchgear could be more cost effective option. Consider in your analysis the cost of replacement, spare part and downtime in the event of trip.

I understand that only the manufacturer is allow to replace the internal components.

 

[Rod62]

Also be aware Fault current limiters may not be approriate if installed on a system with capacitors (pfc or filters) or where large direct-on-line motor starting currents can flow through the Is Limiter

Presume you can not mitigate the high fault level by, say, opening a bus section switch. Additional generation may also increase supply security and permit this. Finally it may be worth contacting the switchgear manufacturer to see if they can change some components to enhance the fault rating. I've experience of doing this a few times on older types of switchgear

Rod62

 

[hmchi]

Yet another option is to consider current limiting fuses. The concept is that the let through current of a current limiting fuse will be much smaller for the downstream protective devices to handle. It is cheap and efficient [no steady state losses as in current limiting reactors, and no voltage drop/regulation issue either].

However, it does take a specialist to perform fault current study and coordination and it is a single phase device, although most motor starters should have already have single phase protection already.

 

[Deansharafi]

Hi All,
How close can your fault capacity get to the fault rating of a switchboard. Let's imagin that a switchboard is rated for 30KA for 3 seconds, what fault capacity can you have on that switchboard to be safe?

 

[powercam]

hi have the same problem.we put in larger capacity cables but the busbars and cts are under-rated. so now the only solution is to re-design and increase the capacity of the b/b including all the ct's. so unfortunately theres nothing else that can be done .if you really want the circuit to carry more load then upgrade.

 

[reactive]

In answer to your question regarding the 30kA for 3s.

This is simply the rated short time current of the of the board (busbars etc.). The amount of current that the board can handle depends on the duration of the fault i.e. k=I^2t = 2700kA^2s. If the max. fault duration was 2s then rated short time current(2s) = sqrt(2700/2) = 36,7kA for 2s.

What is normally more important than the above is the 'make' and 'break' ratings of the circuit breakers. The 'break' rating would typically be the same as the rated short time current (30kArms) and the make on an IEC breaker would typically be 2,5 x 'break' = 75kApeak.

Careful consideration needs to be given to the asymmetry during the fault and any contribution from large motors. It is no good using steady state fault currents if the protection will trip instantaneously.

In other words be VERY careful.

The most cost effective solution would probably be fault limiting reactors. As mentioned in another post volt drop needs to be considered, although this should not be a problem unless large motors are being started DOL (still, it needs to be considered). A calculation of the cost of the reactor losses should also be carried out before a decision is made.

Current limiters are very expensive and tend to compromise protection grading.

As an outside chance, are there no bus sections that can be run open ?

 

[CJFlatters]

In some countries, such as the UK, fault current limiters such as the ABB unit are not used since they are not regarded as 'fail safe'.

How about using an interposing transformer for the generator ?

_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager

 

[Deansharafi]

Reactive,
Thanks for the valuable post.
Fault level is regarded the available current after normally 8 cycles. This is the value that is compared against the short circuit capacity of a switchgear. I know that the protection may break the fault current sooner than 3 seconds, however if the available current is close to the short circuit capacity of the switchboard then we must consider rectifying the problem.
Lets say we have come close to the limit for this switchgear. Adding more generation capacity to this can not be considered unless rectification measures such as fault limiters are used.