[Milsurplus] Fuses to protect transformers

[Gene Smar]

Wayne:

     I've nevefr heard of this approach being used to limit SC current in 
electronic equipment before but you can consider what the electric utilities 
do to limit current through large distribution power transformers.

     I work for our local electric utility company and we are working with a 
large commercial customer that wants to connect a sizeable generator to our 
network to power their much of their equipment load (using all of their 
generator output) and use our lines to supply the remainder.  Our load-flow 
studies showed that our circuit breakers would become overdutied if a fault 
were to occur on our lines while their generator was operating.  So we are 
going to install reactors (no, not that kind, but choke coils) in the 
secondaries of our substation's transformers feeding this customer's 
circuits.  The rapid rise in current during a fault will produce a 
high-impedance point at the chokes in the secondaries, thus limiting the SC 
current and protecting the transformers.

     You might try placing some amount of inductance (RF choke?) in the 
transformer secondaries.  I can't help you with an inductance value, but 
it's worht considering and researching.


73 de
Gene Smar  AD3F


----- Original Message ----- 
From: "Richard Brunner" <brunneraa1p at comcast.net>
To: <milsurplus at mailman.qth.net>; <hwhall at compuserve.com>
Sent: Saturday, March 15, 2014 2:59 AM
Subject: Re: [Milsurplus] Fuses to protect transformers


> That has to be a typo, it's probably 45 volts dc.  Slow-blow is no 
> surprise. Fuses are inherently not fast devices, they will carry rated 
> current forever, will operate "eventually" at 135% of rating, in about two 
> minutes at 200% of rating, and fairly fast at 10 or more times rating. At 
> ten times it may take 5 cycles to clear, and with a good short circuit 
> it's fairly fast.  To interrupt, the fuse element must melt and/or 
> vaporize, which takes time. Circuit breakers are usually faster.  There 
> are now current-limiting fuses which will interrupt before the current 
> wave reaches its peak, but you have to hit it hard to current-limit, at 
> least 20,000 amps.
>
> Richard, AA1P
>
>
>>ac interruption is easier than dc because there is the current zeroThat 
>>makes a great deal of sense but leads me to ask if you, or someone, knows 
>>why the Littelfuse series 808 is rated for 450VDC but only 250VAC? Seems 
>>backwards, but that's what's on the datasheet. So far, I haven't found a 
>>450VAC or higher pigtail fuse in the neighborhood of 1A that isn't 
>>slowblow.
>
>  Wayne
>  WB4OGM
>
>
>  -----Original Message-----
>  From: Richard Brunner <brunneraa1p at comcast.net>
>  To: milsurplus <milsurplus at mailman.qth.net>; hwhall 
> <hwhall at compuserve.com>
>  Sent: Thu, Mar 13, 2014 3:06 pm
>  Subject: Re: [Milsurplus] Fuses to protect transformers
>
>
> Firstly, those are ac rated fuses and will not work anywhere as well on 
> dc.
> ac interruption is easier than dc because there is the current zero.  The
> voltage rating on a fuse is the highest voltage it will reliably 
> interrupt.
> At higher voltage and dc the fuse will melt and arc, dropping the voltage,
> which may be enough to save the equipment and transformer, but a better
> solution is using hv rated dc fuses, if you can find them.
>
> Second thought: Since it will be feeding rectifiers, the current will be 
> dc
> pulses with a current pause, so there is some opportunity for current
> interruption.  However, since it is way over the ac fuse voltage rating it
> is hit-or-miss whether it will interrupt, and probably not.
>
> Richard, AA1P
>
>> For this 40s vintage Link Trainer electronics I'm working on, I'm 
>> thinking
>> that it'd be a good idea to fuse not only the primary but the secondary 
>> HV
>> winding, since these transformers are not too easy to find anymore. I'm
>> thinking of pigtail fuses inserted in the wiring under the chassis, to
>> maintain appearances. But I'm not sure about what fuse selection would be
>> adequate protection.
>>
>> Would a 1A fast blow fuse be OK for the secondary that probably delivers
>> around 100ma DC? Or to put it another way, how much over the normal
>> current output should I select a fast blow fuse? The filters are dual
>> section choke input with 15 & 25 mfd caps. I haven't reached the point of
>> firing up the whole thing, so I have to guess for now at the current.
>>
>> The common fuses I have available are marked 250V and the HV is 450V. Can
>> they be relied on to open and not arc?
>>
>> Any advice? Thanks!!
>>
>> Wayne
>> WB4OGM
>
>
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