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 Post subject: Interesting question of utility fuse ratings
PostPosted: Sat Oct 09, 2021 9:33 pm 

Joined: Tue Apr 20, 2021 4:07 pm
Posts: 2

First post here on the forum. I saw some discussion previously regarding utility fuses and interrupting ratings. What I have found that is interesting is that I have seen a few utilities that are applying fused cutouts above their asymmetrical interrupting ratings. What I found out, interestingly enough, is that many of these fused cutouts have been tested on a one-time basis to have higher interrupting ratings when both the fuse tube and the link is replaced instead of just replacing the link. This is referred to as a single-shot rating.

There are a few "balancing" things that come to mind when I think about this. The first thing is the tradeoff between a high impedance transformer (I'm speaking of larger sized units) and voltage regulation. You can go with a higher impedance transformer, to reduce the short circuit down to fit into the typical ratings of cutouts but you lose the ability to regulate voltage. This is especially true at lower power factors. To get the regulation better you may need to have more capacitors on the system, which adds cost. The transformer also will have more losses, which adds cost. See a pattern here?

Now the second part of the equation is what types of fuses you are using. Noting the above, you could say, ok well I'll spec a transformer that is a 50.4/67.2/84MVA unit with 26.4kV on the low side and use power fuses good to 16kA asymmetrical. So with that and a fairly stiff utility you are at about 10% impedance on your 50.4MVA base. If you wanted to go with a cutout, typically most cutouts are limited to around 12kA asymmetrical max. So if you figure the above now you go up to about 14% impedance. Cutouts are significantly less costly than power fuses, again tying this whole topic back to the design/cost benefit.

What is typically done, however, is the utilities will just use the 12kA asymmetrical cutout and extend the ratings a bit. It has been tested that the cutout can withstand a single shot rating (see above) of 16kA asymmetrical. However, the disadvantage of this method, is if you did get a fault at that level, you have to replace both the tube and the link. Given the fuse cutout is normally seen as a multi operation device (i.e., you can replace the link after one operation and put a new one back in the tube), what thoughts do you have of this method? The obvious cost savings to using fuse cutouts is pretty substantial benefit, but my concern with this method is contractor/linemen safety. Also keeping in mind the whole cost/benefit factor of keeping the impedance "reasonable" on the transformer for better regulation. I suppose it really depends on how many fuses you plan to deploy and where on the system as well. Fuses applied close to the station will run into these issues with ratings where fuses after a few thousand feet of line will naturally fall within their marked ratings.


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 Post subject: Re: Interesting question of utility fuse ratings
PostPosted: Mon Oct 18, 2021 1:25 pm 
Sparks Level

Joined: Sat Nov 21, 2009 10:23 pm
Posts: 116
Location: Ohio
An opinion:

1. I have never seen a cutout fail from a short circuit, I have seen creepage issues that lead to failure on several cutouts.

2. If this is an outdoor utility type installation, 99% of the time a failure can be tolerated. If this is an expulsion fuse, you have some inherent physical danger to begin with. When you look at the $20,000 for 5/15kV switch and fuse, that usually forces the decision to a cut-out,therefore, the plant uses a cut-out, versus a current-limiting fuse. You will also have a more difficult time protecting the transformer, the current-limiting fuse curve falls further (protects) to the left on the TCC, therefore, offering better protection.

3. If this is an expulsion fuse and the installation is customer owned, that is a misapplication (opinion) from the start. The rapid current/voltage collapse during fault clearing on a expulsion fuse can cause a large damaging transient within the plant, which can lead to equipment failure. I investigate one such occurrence, in this specific case over (50) VFD's failed.

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