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 Post subject: fused and non-fused DISCONNECTS on sec. side of a transfomer
PostPosted: Sat Oct 17, 2015 1:29 pm 
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Joined: Tue Oct 02, 2007 7:23 pm
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Location: Oak Ridge, Tennessee
What mitigation strategies do you recommend when there is a transformer feeding a fused or non-fused disconnect on the secondary and you want to lower the PPE Hazard Category (or incident energy)? I can think of many alternatives, but what is the simple solution...

Situation is simply a tap off a utility line to a large transformer, so upstream fuse cannot be changed (or transformer size).

Thanks!


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 Post subject: Re: fused and non-fused DISCONNECTS on sec. side of a transf
PostPosted: Sun Oct 18, 2015 6:44 am 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
Generally speaking, the upstream device has minimal impact anyway. Generally above 1500 kva at 480 V it will exceed 40 cal no matter what you do. Some possible solutions are:
1. Add an upstream breaker/switch ground/pole mounted. Runs around $25k installed. If its really bad, install. bushing CT's and another trip relay. This gives the effect of a secondary side breaker with almost "zero" distance.
2. Install new smaller transformers. May want to consider fuses in the transformer body so there are no tasks which generate an exposure.
3. Some folks have used cable limiters. These are small fuse packages that can bolt directly on the transformer lugs so the fit inside a live front termination compartment. Ignore the sizing method and size directly using TCCs. This moderately bumps arc flash down.
4. Install a second set of fuses (no disconnect) on transformer cables such as class J or L. Can't be serviced without disconnecting the main but that's not the point. Knocks down incident energy at the disconnect only. Disconnect fuses or breaker is rated less than "arcing fault fuses".
5. Implement procedures to operate on primary side only if practical. Test on load side of disconnect, downstream of enclosure.
7. Replace disconnect with arc rated equipment (breaker or fuse). Can't service it without opening upstream side but fused disconnects usually only call for servicing once every 10 years especially if its vacuum. So with a 30-40 year life it only requires service 3-4 times over its operating life. A lot of European stuff has the breaker/fuse on the "high" side with a non-load break switch or some similar arrangement and a load break switch with a grounding position on the load side. Look for solidly insulated switchgear or SF6 insulated switchgear with vacuum interrupters. Also a lot of underground utility breakers in the US are built the same way for up to 15 or 35 kV. See for instance S&C, Eaton,or Thomas and Bette. These run $15-$40k depending on bells and whistles with of course S&C is the priciest. According to the T&B guy I talked to the failure mode is not "boom" but it just sort of mushrooms out if the vacuum interrupter fails with the rubber jacket with snares the potential shrapnel based on lab tests they did.
8. S&C has had the smart fuse "fault fiter" for years. Recently a competitor has come out with the same device except it has an external trigger but I can't think of the name. This is a standard medium voltage fuse with a very high interrupting rating. On the side is a CT and electronics that charges a capacitor. On command from a typical relay you would use for a breaker, it discharges the capacitor across the fuse which is low voltage but high enough current to melt the fuse link. This gives you fuse speeds (1/4 cycle) with breaker programmability. Again, you either insert them on the primary side or between the intended disconnect and the transformer.

It should be obvious that even with arc flash considerations the only protection for the feeder between the transformer and the first over current protection device is the primary side protection which typically to avoid transformer inrush is too large to protect against anything except short circuits. Overload protection comes from the downstream devices for the most part. The same applies to arcing faults. You either accept this as a necessary evil or reduce the length of unprotected equipment either by adding secondary side sensors and tripping on the primary side, or adding an extra layer of protection that it serviced from the primary side which is either external or internal to the transformer, or dropping the transformer size down to an acceptable level and spreading your distribution out more. The old practice of using one giant transformer (I've seen 10 MVA 480 V transformers) was a bad idea even before arc flash. If nothing else as voltage increases, so does the threshold. So at 4160 V the same "1500 kva barrier" becomes around 20 MVA. So it should be an obvious indication that you should be using a secondary distribution voltage at that point. Perhaps the next edition of IEEE 1584.1 or the Red book will point this out.

Using these techniques, you can set a realistic goal of 8-12 cal incident energy for all service points. Watch out for the popular (with vendors) arc resistant gear trap. Properly serviced/designed/installed equipment doesn't need arc flash PPE to operate it (as per 70E). This gear only affords protection for this single class of tasks, so it really doesn't have a purpose except to increase prices 10-50%. When you service the equipment (open doors), the arc resistant status goes away. I have seen some recently from the most expensive switchgear vendor in the US (Powell) and similar offerings from Eaton, S&C, ABB, and Siemens in the SF6 insulated gear where racking tasks are still arc resistant to that makes the most critical (and failure prone) area arc resistant, or with the SF6 stuff it is sealed for life so you can't service it anyway (its like a panel board). And with advertised (but not necessarily achieved) 40+ year life's, it makes the shutdown issue moot. Note that ABB's "40 year" magnetic actuator breaker is bull-s*. Their manuals give much shorter service intervals and the problem is all in the linkage from the actuator to the vacuum interrupter. ABB requires greasing every 3 years and factory overhaul every 12 years. The design doesn't have sleeve bearings made of PTFE/brass with a single shaft unlike the competition so the linkage (which is a very mechanically weak design to reduce inertia and get faster operation) is the issue. Otherwise their "maintenance free" claim would be true. Competitor designs directly drive the vacuum interrupter with the magnetic actuator and avoid the weakness. The magnetic actuators themselves are all designed by another company and as far as I can tell that company designs them for all manufacturers.


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 Post subject: Re: fused and non-fused DISCONNECTS on sec. side of a transf
PostPosted: Mon Oct 19, 2015 7:09 am 
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Joined: Tue Jan 19, 2010 2:35 pm
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Can you give some detail on the size transformer you're dealing with?
We had a similar issue. In this case, a water system's well field was fed by the owners 12.47 KV line. Each of the 25 wells was fed by a step down transformer. They varied in sizes but were mainly in the 500 kVA range. In one case, the pump house with the gear was very small, so space was one issue. Instead of installing a large 600 amp fused disconnect, they installed 600 amp fuses in a much smaller junction box. Following the install, if it became necessary to replace replace blown fuses, they could shut off the 12.47 line. However, shutting off the 12.47 line would have been necessary even with a 600 amp disconnect. That was because the very high hazard would still be present at the line side of the fuses in a disconnect or the junction box.

The positive effect was the fuses lowered the arc flash hazard enough that they could leave power on, suit up and perform any testing or measuring.


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 Post subject: Re: fused and non-fused DISCONNECTS on sec. side of a transf
PostPosted: Mon Oct 19, 2015 7:27 pm 
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In the situation described, you could install fused cutouts for about the same price. These are typically mounted off the ground, at least 10+ feet up. Since it's open air, the incident energy falls off with the square of distance, much faster than enclosed gear. The cutouts are easily operated with a hot stick and on the medium voltage side incident energy is lower in the first place. In the previous post, I have recently built a fleet of 10 portable 480 V substations with 500 kva transformers on board. Masts are 20 feet tall with lightning arresters, cutouts, and the insulators on the cutouts support the cables that are clipped to the overhead line. A simple telescoping pole is all you need to operate the cutouts and/or swap out fuses. Just because the utility is simply supplying connections to the customer equipment doesn't mean you don't need your own cutouts. It makes them happier too because it takes a lot of coordination for one of their operations personnel to come out to de-energize your service and then re-energize later.


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