Current-limiting fuse to allow transformer replacement
Current-limiting fuse to allow transformer replacement
(OP)
Hi all,
I've recently done a short-circuit analysis on EasyPower to evaluate the impact of replacing a 1.5 MVA xformer by a 2 MVA. What came out is that the old FPE 50H breakers cant withstand the new Icc of over 50kA (about 55kA) so I am considering two options:
1. Replace every breaker by an appropriate model ($$$);
2. Add a current-limiting fuse at the secondary of the xformer.
I'd like to know if option 2 is viable and if the fuse will blow at every single fault. What I'd like is for the fuse to blow ONLY when the fault current is about to get higher than 50kA and let the breakers open if it's not the case.
So what do you think?
Mike
I've recently done a short-circuit analysis on EasyPower to evaluate the impact of replacing a 1.5 MVA xformer by a 2 MVA. What came out is that the old FPE 50H breakers cant withstand the new Icc of over 50kA (about 55kA) so I am considering two options:
1. Replace every breaker by an appropriate model ($$$);
2. Add a current-limiting fuse at the secondary of the xformer.
I'd like to know if option 2 is viable and if the fuse will blow at every single fault. What I'd like is for the fuse to blow ONLY when the fault current is about to get higher than 50kA and let the breakers open if it's not the case.
So what do you think?
Mike






RE: Current-limiting fuse to allow transformer replacement
Can you rise the impedance of the 2 MVA transformer to lower the SC current?
RE: Current-limiting fuse to allow transformer replacement
RE: Current-limiting fuse to allow transformer replacement
I'd just replace the entire secondary lineup rather than trying to replace every breaker.
Another option would be a current-limiting reactor but I suspect you'd need a lot of reactance, and they have their own problems.
RE: Current-limiting fuse to allow transformer replacement
What is the impedance of the transformer you have?
Why can't you order one with a higher %Z?
RE: Current-limiting fuse to allow transformer replacement
What voltage is this at? 55kA sounds high for a 2MVA units (assuming 480V) unless you have other sources connected. Have you taken the source impedance behind the transformer into account?
RE: Current-limiting fuse to allow transformer replacement
The designer calculated that 100 feet of cable would add enough impedance to drop the available fault current to a level that the breakers could safely interrupt. He then spec'ed that every breaker must be fed with no less than 100 FT of cable. This was the poor man's series reactor. If you are close to the rating of the breakers, you may make up the difference with cable impedance.
I have also seen series reactors used.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Current-limiting fuse to allow transformer replacement
The transformer was installed before I was hired, they've just never calculated the impact on the equipments. There are many similar cases (20 transformers + 100+ branch breakers) covered in the analysis so I won't list every impedance but it should be around 5.5%.
The Icc sometimes reaches 60kA so adding cable length is out of the question for most cases.
dpc I don't understand what you mean by replacing the secondary lineup. Does it mean rising the % of the tranformer?
Thanx again for the help.
Mike
RE: Current-limiting fuse to allow transformer replacement
Keith Cress
kcress - http://www.flaminsystems.com
RE: Current-limiting fuse to allow transformer replacement
RE: Current-limiting fuse to allow transformer replacement
RE: Current-limiting fuse to allow transformer replacement
Bill
--------------------
"Why not the best?"
Jimmy Carter