Reducing Fault Level Issues
Reducing Fault Level Issues
(OP)
Hello,
We are looking at reducing the fault level on a new system in order to save costs on cables and MCC boards. The fault level is only an issue on the MV side. We do not want to put in reactors as it will increase the losses. We can't increase the transformers impedance upstream as they are the utilities, the utility cannot provide a guarantee to not parallel their transformers so we are designing on the basis of maximum fault level. So we are looking at putting in fault limiting fuses on the incoming feeders. We want to reduce the fault level from 25kA to around 16kA.
Can anyone tell me what are the issues/disadvantages or things we need to investigate before limiting the fault current? Protection grading shouldn't be a problem as our plant has not been built yet and we will grade at a later stage. I am thinking of voltage stability etc?
We are looking at reducing the fault level on a new system in order to save costs on cables and MCC boards. The fault level is only an issue on the MV side. We do not want to put in reactors as it will increase the losses. We can't increase the transformers impedance upstream as they are the utilities, the utility cannot provide a guarantee to not parallel their transformers so we are designing on the basis of maximum fault level. So we are looking at putting in fault limiting fuses on the incoming feeders. We want to reduce the fault level from 25kA to around 16kA.
Can anyone tell me what are the issues/disadvantages or things we need to investigate before limiting the fault current? Protection grading shouldn't be a problem as our plant has not been built yet and we will grade at a later stage. I am thinking of voltage stability etc?






RE: Reducing Fault Level Issues
How much losses do you expect from reactors? If you keep the PF near 1 then they should not be that high.
RE: Reducing Fault Level Issues
I don't think 25 kA of fault current is going to have much impact on the cost of the cables.
In the US at least, certain circuit breakers are tested in combination with upstream fuses and this "series rating" can increase the rating of the circuit breaker dramatically. But again, this eliminates any reasonable chance of coordination.
In today's systems, 25 kA is not a lot of of fault current. It is fairly easy to buy MCCBs with 42 kA of interrupting, even in smaller sizes.
RE: Reducing Fault Level Issues
It uses a buss bar fitted with explosive charges to short out the reactor and a current limiting fuse. A CT senses the fault current and triggers the shaped charges to sever the bus bar. That transfers the load to the current limiting fuse which blows and places the reactor in the circuit.
No reactor losses.
But the cost is probably more than just going with 65 KAIC gear.
RE: Reducing Fault Level Issues
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RE: Reducing Fault Level Issues
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RE: Reducing Fault Level Issues
But fuses still don't reduce fault current even on the MV side.
RE: Reducing Fault Level Issues
I am aware that the fuses dont reduce the fault only limit it. The idea behind the fuses was to put them on the incoming side of the MV SWB in order to use smaller cable sizes on the outgoing side of the MV switchboard. We have about 15km worth of cables.
Can someone tell me if there is a fault on one of these cables and assuming it pulls enough fault current to blow one of these fuses. Will all four have to be replaced, will they have started to fuse and hence cannot be relied upon. I am assuming that they wont all blow at once but maybe they are damaged? We have calculated that if 1 fuse blows we should be able to still run the whole plant.
Also can anyone point me in the direction of some good literature on reactors i have spent ages on the web trolling for how they work to reduce fault current and how they are installed on the network. (Our PF is assumed to be around 0.95)
From the sounds of it the fuses are the cheapest way to reduce (limit) the fault enough to use smaller cables. Are there any issues with limiting the fault that I need to consider?
RE: Reducing Fault Level Issues
A possible problem with reactors would be voltage regulation. The reactors will increase voltage drop.
What four are you talking about? Fuses? Why four fuses? If one phase blows, you don't want to run the plant. The possibility of doing so is a problem with using fuses for protection.
What's the difference between reducing fault current and limiting fault current? If the available fault current is above the current-limiting range of the fuse, it reduces the current.
Is there such a thing as series rated protection at MV?
RE: Reducing Fault Level Issues
Did I miss something, how do you run a plant with a phase missing?
There was a scheme I saw once with a current limiting fuse and a reactor. The fuse blows and requires all the current to flow through the reactor. It kept fault currents down without the problems of the reactor.