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In-house fusible link - design for medium voltage?

In-house fusible link - design for medium voltage?

In-house fusible link - design for medium voltage?

(OP)
Hi Gents,

I want to design a custom fuse link, and am trying to find resources to help understand the principles and design rules that govern it.  

The biggest question I have is:  what design parameters determine the fuse voltage rating?  Intuitively it seems like the distance bridged by the think (between large conductors) needs to be sufficient to stand off the rated voltage, but does it get more complex than that?

If any of you consult or know of a good resource/consultant that can talk us through fuse design, that would be great.  Thanks for the help!


Cheers,

Rob

RE: In-house fusible link - design for medium voltage?

Medium voltage?

I wouldn't even try that for low voltage. Once, when prospective short circuit currents were moderate, you could use wire fuse links. I would not do that today. And certainly not for medium voltage. What voltage are we discussing? And why on earth do you want to blow your own?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: In-house fusible link - design for medium voltage?

(OP)
Whoops, meant to add that detail... let's say ~400V

The primary reasons for wanting to go this way are cost and reliability - integrating a purchased fuse link into our system means having several critical electrical connections to attach it, which escalate cost and reliability concerns dramatically.

This fuse is an emergency last-ditch safety effort - if I can just stamp a thin section into our busbar and replace the whole thing if the link blows, that would be much better.

RE: In-house fusible link - design for medium voltage?

400V is well under 1000V and therefore is low voltage.  Otherwise, I'm sure that Gunnar is dead-on with his assessment.

RE: In-house fusible link - design for medium voltage?

Busbar. Sounds like hefty currents.

A weak bridge that opens under high current will start an arc. And arcs at 400 V are nasty things.

I repeat: It is a very bad idea. Do not do it!

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: In-house fusible link - design for medium voltage?

TheSwener,

If this is to be connected to a power system of any size then there's no way that you should be thinking about an open fuselink. HRC fuselinks use a high strength ceramic tube containing silica grains around a (typically) silver alloy element. The fusible element ruptures and the resulting arc is quenched as the silica fuses (melts) around it. These things are tested over and over again in a high power test lab delivering typically 80kA or more until their capability to break the fault current is proven. An open link just burns back until the arc hopefully goes out. Which is ok, except when the arc doesn't go out - then you have a problem.

If you want an idea of what an electrical arc running amok in a LV switchboard does, have a look at http://www.youtube.com/watch?v=YwI48O0MUXY, or just search Google or YouTube for hundreds of examples. Personally I think this stuff is scary enough that penny-pinching over the cost of an LV fuse is too ludicrous to entertain. But maybe that's just me.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: In-house fusible link - design for medium voltage?

(OP)
Hi All,

For starters, we are currently using an open fuse link designed specifically for this application by a major fuse manufacturer.  Unless they are risking their entire reputation for our sake, it must be doable with high repeatability.

I simply want to integrate that same fuse shape directly into our custom "busbar" (90A peak rating, really just nickel sheet).  This necessitates a change in the fuse material and thickness.  Even the cheap-ish open link fuses are a significant percentage of total system cost, so yes it's worth trying to "penny-pinch."

I'm looking for design guidance before we start making custom shapes and attempt to blow them in the lab (yes, with a reliable breaker also in the test circuit).  Thanks!

RE: In-house fusible link - design for medium voltage?

What is your prospective short-circuit current?

When you say bus-bar, I - for one - think high currents. Is that not what you have?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: In-house fusible link - design for medium voltage?

TheSwener:

Fuse manufacturing is not a DIY item.

You came here for an advice and you got it from some very knowledgeable and among the wisest members on this forum. Heed their advice.  If you knew anything about the hazard involved in what you are asking, you would not have asked the question.

Better yet, you would do well to stay away from any type of electrical work for the sake of your own safety and that of others.

Plus "low voltage" is a misleading the term for those outside electrical power field. 400V system faults can carry a lot of energy and be very damaging and lethal.

Rafiq Bulsara
http://www.srengineersct.com

RE: In-house fusible link - design for medium voltage?

(OP)
Sorry Gunnar, probably need to clean up my terminology when dealing with electrical folk - our group in automotive has different associations to the lingo :)

Voltage:  ~400V (medium to us, low to you)
Current:  ~90A (peak normal operating conditions, 60A continuous)
Fuse:     ~250A, 3s blow

We are currently getting a custom designed open fuse link that meets these requirements, but integrating it into our circuit as a separate piece creates new weak points.  Unless they are welded the reliability of these connections are unacceptable, and se far we haven't found a good enough welding method.

RE: In-house fusible link - design for medium voltage?

OK. If you can't get more than 90 A, it may be safe. Is it DC or AC? How come that a standard connection fails? Do you have high vibration? Corrosive environment? There are remedies for that.

Anyhow, I would never take the risk of putting my own device there. Lawyers will spot it immediately and you will be a very bad guy if something happens.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: In-house fusible link - design for medium voltage?

But, the maximum fault current question has not yet been answered.  Has to be well more than 90A, or there is no point to the existing 250A fuse.

RE: In-house fusible link - design for medium voltage?

Yes, determining the safe fault current for a fuse is one of many important factors. Load current itself does not determine if the setup would be safe.

Start here to see just the list of considerations, never mind the how to manufacture and test those.

http://www.littelfuse.com/data/zh/Product_Selection_Guides/Fuseology.pdf

If an open fuse does not open properly, you can end up with ionized air surrounding it and quickly (read ms) in a fire ball.
 

Rafiq Bulsara
http://www.srengineersct.com

RE: In-house fusible link - design for medium voltage?

When I look at some of the applications where we use bolted connections in the power industry I'm really surprised that you can't get a reliable connection without welding. I know automotive is a fairly tough environment, but compared to, say, a ladle arc furnace, or a turbo-generator or an open yard substation it begins to look rather tame. Perhaps efforts could be better focussed on trying to get a reliable connection to a commercial fuse? What failure mode are you seeing - vibration damage?

Gunnar has a good point about lawyers. If you do decide to roll your own, get it proof-tested by one of the high power labs. If you tell us where you are we could probably point you toward one.

Breaking a 250A overload isn't the problem you need to solve - it's breaking a dead short across the terminals which you need to worry about: on a 400V battery it could be a good few thousand amps, and the arc just won't want to go out once it strikes. I've worked around industrial batteries for a fair few years now and I am very cautious around big banks of cells. I really suggest you look again at using a DC-qualifed HRC fuse. The cost of a bolt-in fuse just can't be that significant compared to the cost of a traction battery, can it?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: In-house fusible link - design for medium voltage?

(OP)
Hi Gunnar, Hi David,

DC battery powered system, ~300A is the max fault current.  We are basically just trying to break the circuit in the event of a dead short.

And it's automotive environment, so "yes" to vibration and corrosion.  We have multiple fuses in parallel (to protect batteries in parallel), meaning that under certain circumstances we will not be able to detect a single poor connection *and* that connection will have high current pushed through it... so welded connections (or continuous metal) are highly preferred for peace of mind.



Rafiq,

Again, a major fuse manufacturer has provided us with an open fusible link that meets all of these requirements.  Would you like to tell them that they would "do well to stay away from any type of electrical work for the sake of their own safety and that of others?"  And if not, are you implying that they have mystic knowledge that a helpless professional like myself could not possibly hope to understand?  If not, then I am trying to learn and would appreciate your help.

I have designed, qualified and implemented an open link of this type before for a major OEM, but at lower current and in a different design.  So for this version I am hoping to understand the basic principles a little better in the hopes of shortening our development time.

If there are fuseology experts out there I am looking for your help to create a robust, integrated, and above all else safe fuse design.


Cheers,

Rob

RE: In-house fusible link - design for medium voltage?

(OP)
Hi Scotty,

We have one HRC fuse across the pack terminals, but in the event of a sub-pack event or even shorting failure of one cell within a parallel group we need localized backup fuses.  The Tesla roadster for example has 6821 cells, individually fused... 6821 HRCs would be bigger than the rest of the pack (and more expensive).

RE: In-house fusible link - design for medium voltage?

I'm surprised that the short circuit current is 'only' 300A from a fairly high voltage battery capable of supplying 60A of load current. For my own education, what cell type are they?

I think I understand your connection problem too - you're running mutliple strings of cells in parallel, each string being individually fused?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: In-house fusible link - design for medium voltage?

(OP)
Hi Scotty,

I'd prefer not to give too many hints :) but 300A is a theoretical maximum based on the cell's voltage and internal resistance.  At lower state of charge the cell voltage is lower and the peak fault current goes down.

And you are close on the interconnection strategy - cells are connected in parallel first, then in series.  This dramatically reduces the number of unique voltages that we have to keep track of.

For most circumstances the fuse only needs to stand off a single cell voltage, but there are some circumstances in which all the fuses of one parallel group could break, and then they would suddenly see the entire pack voltage.  Maybe an arc would keep enough current running to blow the HRC, but I'm assuming it wouldn't be wise to rely on that...

RE: In-house fusible link - design for medium voltage?

Could you draw out a simplified sketch of your battery showing the fuse locations? I'm starting to wonder if the fuse we're discussing is a low voltage type such as those designed for electric forklift trucks, which are rated for about 60V.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: In-house fusible link - design for medium voltage?

(OP)
Hi Scotty,

Under most circumstances you are right, these fuses will just stand off the voltage of the parallel group they belong to (<5V).  HOWEVER, if a massive short breaks all of the fuses in a parallel group AND the contactor + HRC fail to open the circuit, those fuses will stand off full pack voltage.  That extremely obscure case is the one I am concerned with.

RE: In-house fusible link - design for medium voltage?

Swener, you might want to start with a commercial fuse that will meet your load current, max fault current and voltage requirements. Then, if mounting cost, complexity, weight, or volume are an issue, you should go to the manufacturer and inquire about the possibility of integrating their product into your battery pack.

Most fuses are designed to be components in field assembled, field maintained equipment. As such, they are intended to be installed in fuse holders (or bolted to cable lugs). That increases weight, volume and cost. If fuse links from a tested design are integrated into your battery assembly together with their tested expulsion tubes, surrounding quenching media, etc. (also tested), you'll be better off than starting a design from scratch.

Odds are (and I'm just guessing here) that once a fuse has blown in a battery assembly, that whole assembly will be replaced. There will be no need to replace the fuse in the field.  

RE: In-house fusible link - design for medium voltage?

Are you looking for 'the perfect storm'? You need two failures of independent protective elements AND a battery fault before you even get to a dangerous condition. How likely is that to occur? Desinging for that number of contingencies isn't normally seen until you are working on very high-value or high-risk plants such as power stations, nuclear facilities, major transmission substations, and so on.

Somewhere it will be 'possible' for a combination of faults to occur which you can't protect against. You need to look at likelihood, cost of mitigation, and consequences.
   

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: In-house fusible link - design for medium voltage?

(OP)
Hi Scotty,

Glad the conversation has transitioned from "you are a lunatic!" to "you are overdesigning!"  smile

There are still two scenarios that concern me:

#1

- Vehicle crash in which a short circuit is created somewhere inside the battery, upstream of the HRC fuse and contactor.  With the HRC + contactor out of the circuit, the cell fuses will have to blow and stand off whatever voltage is in the circuit (less than <400).

- In the midst of writing this I realized that we should move the HRC to the middle of the pack, that way the worst possible short without an HRC in the circuit is less than half of pack voltage, so perhaps this is acceptable.


#2

- For higher power (HEV) applications it may not be possible to ensure that the HRC blows 1st in a dead short.  In this scenario the cell fuses could blow first and face full pack voltage.

- If they were to arc, can we be confident that the arc will be a "low resistance" connection?  My concern is that the flow of current will not be high enough to guarantee that we blow the HRC if the cell fuses arc, therefore they would need to stand off full pack voltage in this scenario.


What do you think?

RE: In-house fusible link - design for medium voltage?

Is the pack earthed to the vehicle body, or floating? That would influence whether fusing it midway would be a good idea or not. If the pack is earthed then you need to fuse at the point of greatest potential relative to the body.

It should be possible to find an HRC fuse value which coordinates with the individual cell fuses, or at least it is a reasonable target to aim for. Have you tried plotting the respective fuse curves on a log-log graph of time and current? That's what the normal method is for plotting a protection grading study.

Arcs tend to be higher resistance than a bolted fault, so I understand your concern in #2. I'm not sufficently familiar with automotive crash requirements to comment on what would be a reasonable expectation in a major crash - hopefully someone who is familiar will comment.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: In-house fusible link - design for medium voltage?

(OP)
Hi Scotty,

We will plot the fuses and design for the HRC to blow first.  And the battery floats relative to the chassis, so all fuse locations should be equally valid.

Cheers,

Rob

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