Is Limiters & Intterupting Current

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eanton22

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So I guess I have a few questions.

I specified that our MV SWGR be 63KAIC. The vendor does not offer 63 KAIC but offers 50 KAIC. The vendor than comes back with a method to attain 25 KAIC by the use of these Is limiters. They are to be connected at each end of the SWGR to the main circuit breakers.

1. Why would I use these Is limiters when I want 63 KAIC rather than a lower KAIC?

2. It states it is only connected to the main circuit breakers what about the feeder breakers? If the limiters are just connected to the main breakers, the feeder breakers would still be 50 kAIC?

3. Does anybody have experience with these Is limiters? Do I want to have a smaller KAIC? is it more economical?

I know these are questions to the vendor, but I suppose i run by you guys so I know what questions to ask. Plus its Friday night and everybody is gone;)
 
Mv @ 63k!

Mv @ 63k!

Did you say that you specd medium voltage (12kV?) switchgear at 63k. That will need a 150MVA or larger transformer to be able to supply that much fault current. Are you sure you need 63kA MV switchgear?
 
eanton22 said:
So I guess I have a few questions.

I specified that our MV SWGR be 63KAIC. The vendor does not offer 63 KAIC but offers 50 KAIC. The vendor than comes back with a method to attain 25 KAIC by the use of these Is limiters. They are to be connected at each end of the SWGR to the main circuit breakers.

1. Why would I use these Is limiters when I want 63 KAIC rather than a lower KAIC?

2. It states it is only connected to the main circuit breakers what about the feeder breakers? If the limiters are just connected to the main breakers, the feeder breakers would still be 50 kAIC?

3. Does anybody have experience with these Is limiters? Do I want to have a smaller KAIC? is it more economical?

I know these are questions to the vendor, but I suppose i run by you guys so I know what questions to ask. Plus its Friday night and everybody is gone;)
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Did you check to see what the available fault current was? Is it in the range of 63 ka?

1. You do not "want" 63 ka. The available fault current is what the utility can
generate at the low side terminals of the transformer. The fault current ampacity, be it 63 ka, 50 ka, 25 ka or 10 ka, does nothing for you. It is what it is. You design the equipment to with stand the available fault current. you
don't need 63 ka equipment if the fault current is 15 ka. Your Main breaker panel and breakers must be rated for the available fault current.
 
The specification calls out for a 4.16 kV, 63kA MV SWGR. I have not seen the transformers but yes, the transformer will be large (greater than 1000 MVA). The vendor submitted a bid with a switchgear that is 50kA. Is this acceptable? Then they say that I could attain 25kA with Is limiters. I need 63kA not 25 kA.
 
eanton22 said:
The specification calls out for a 4.16 kV, 63kA MV SWGR. I have not seen the transformers but yes, the transformer will be large (greater than 1000 MVA). The vendor submitted a bid with a switchgear that is 50kA. Is this acceptable? Then they say that I could attain 25kA with Is limiters. I need 63kA not 25 kA.
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You need what ever the fault current on the secondary side of the transformer dictates. Calculate the fault current first.
 
It looks as though you are the Consulting Engineer in this scenario. You therefore have the right (IMHO) to insist on their meeting your specification as written, or consider a well documented value engineering proposal. So far it sounds as if they are just running an idea up the flag pole without providing you with adequate proof that their concept is valid. I don't see how you can accept it on that basis. I'm not saying the idea doesn't have merit, it just looks as though they are not doing a good job of explaining the concept t you to your satisfaction.

Most CE firms I have worked with will have provisions in the contract that states, if a vendor wants to substitute something that does not explicitly meet the specifications, they must bear the added cost of your evaluation and still suffer the risk of rejection. So my advice is, insist on that with the vendor and if he is serious about getting the job, he will put his money where his mouth is. If he refuses, he was just scamming to make some extra profit and put the risk burden all on you.
 
The numbers and terminology the OP is using tells me this is an IEC design outside the US. In that regard, 63KAIC is very common for MV switchgear. I know ABB makes it. Current limiters are essentially inductance. They have a similar effect as a transformer. However the drawback can cause poor voltage regulation when starting a large motor, but there are many applications for them, usually at the substation level.

63KA gear is expensive. If you can live with the limiters you may want to consider them. A good trade off may be to use them to get you down to the 50kA level. This will open it up to many more suppliers.

An alternative is to also go with a higher than normal impedance on the transformer. There is a cost though, and still voltage regulation can be an issue. But taps on the trafo may help overcome this.

You will need to look at the economics of each set-up and compare it to worst case operational requirements/restrictions in order to chose. Sounds like you may need some help from your Sr. Engineer.
 
Is Limiter

Is Limiter

Per ABB literature, the Is Limiter is a current limiting fuse in parallel with a fast-acting explosive-operated contact. It is a one-shot device; use it, replace it. Using such a device as backup protection may be valuable but to rely on it as part of the initial design seems a poor choice.

Other have said the same thing: start by determing the actual available fault current. Perhaps 63kA is more than what is needed and the specifications can be eased. For a load this large, voltage-regulating transformers will probably be used, so a higher-impedance transformer may be appropriate. Series line reactors may be appropriate. Can you split the load between two (or more) transformers and reduce the AFC?
 
After your calculcations if you still need 63kAIC, I know that Siemens builds 63kAIC at 5kV level and it is ANSI as well too. Look at Siemens GM-SG brochure for ratings
 
ABB does not offer a 63 kA MV SWGR only 50 kA. I have discussed this with my senior engineer and he states this is not acceptable. I am trying to get a deeper understanding of what is going on. When I determine my fault current (based on the calculations) do I want to size my interrupting current to be greater, equal to or less than my fault current?
 
You want the interrupting current capability to be greater than or equal to the available (present now or planned for) fault current.

If the available fault current is greater than the interrupting current capability of the gear, than a fault could cause more current to flow than the gear is capable of safely interrupting. In this case, the gear could fail, and either not interrupt the current to the fault, or interrupt the current but only after significant damage.

-Jon
 
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The bracing and interrupting capacity of the switchgear MUST be more than the available fault current, otherwise it may fail to interrupt current during a fault. Duh!
 
sorry for what seems to be a stupid question, DUH??? That's what I thought, it just that if we specify a 63KA why is the vendor giving me 5OkA or lower. DUH?????!!
 
The terms here are breaking current and peak current. The peak is approx. 2.5 x the breaking current rating.

IEC 60909 fault calcs are different than US method, they take into consideration some decay time, and that's why they call breaking current instead of withstand. You can't use US ratings and expect the same reuslts.

Simens makes 63kA gear, so does Hitachi. Are you sure you need 63kA breaking capability, or do you need at least 63kA peak? Because 40kA for 1s rating has a peak rating of 100kA. Make sure your talikng apples and apples.
 
Yes, 63kA breaking capability and 164 kA peak (close and latch duty).

I am consulting engineer just looking at MV SWGR bids, i did not do the calcs for the short circuit current. I am just looking at the spec. I am now questioning if we really need a 63 kA MV SWGR. I asked if there was a short circuit calc performed. The answer was no, she got it from the standards. Standards, what standards???????????
 
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