In-house fusible link - design for medium voltage? 1

[TheSwener]

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

 

[PHovnanian]

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.

 

[ScottyUK]

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.


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

 

[TheSwener]

Hi Scotty,

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

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?

 

[ScottyUK]

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.


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

 

[TheSwener]

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