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 Post subject: Bus Tie Breaker
PostPosted: Tue Apr 29, 2014 12:17 pm 
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Perhaps I missed a posting on this but here is something I have been thinking about. Consider the following scenario:

Two lineups of 15kV metalclad switchgear with a bus tie breaker. Each bus is fed by a separate power transformer thru a breaker in the metalclad switchgear. Each transformer has differential and overcurrent protection. One transformers protection scheme has been upgraded to a microprocessor relay while the other transformer has electromechanical relays. Due to this difference in relaying and settings, Bus 1 has an IE of 7 cal/cm^2 while Bus 2 has an IE of 1.5 cal/cm^2.

Now when racking out the bus tie breaker, there are several possibilities for arc flash:
1. Fault begins and terminates at one of the 2 buses. So AFH could be either 7 or 1.5 cal/cm^2
2. Fault begins at Bus 1 and incorporates Bus 2 also.
3. Fault begins at Bus 2 and incorporates Bus 1 also.

For #2 & 3 above, would one use the higher IE (7) for PPE or combine the 2 IEs for 8.5 cal/cm^2 which would move one into a higher class of PPE?

Looking for some interesting discussion on this.

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PostPosted: Tue Apr 29, 2014 12:28 pm 
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Without doing any math, I would assume 7 cal for this. However, anywhere I have worked, policy has been to wear a 40 cal suit any time you were racking switchgear, regardless of the individual numbers.

I actually witnessed (i.e. was in the room) an arc flash when a tie breaker closed in while both ends of a double ended sub (which is essentially your scenario) were closed in on different sources. The tie breaker was supposed to be interlocked so that couldn't happen, but during switchgear maintenance the wrong breaker was put back in this position, one that didn't have the right contacts installed for that interlocking. The workers doing the work were wearing blast suits, but I (improperly) was there without even full category two equipment on.


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PostPosted: Tue Apr 29, 2014 1:51 pm 
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To get the two IE values, you must have calculated them with the tie breaker open. There is no straight forward method to combine them to get an IE for scenarios 2 or 3. Suggest you rerun the study with the two sources paralleled (tie breaker closed).


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PostPosted: Fri May 02, 2014 5:32 am 
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Incorrect on both counts. Two more items to consider:

1. You have additional loads that are changing the X/R ratio and/or adding transient reactance to the system. This increases the fault current at least over the short term so since short circuit current increases, so does incident energy. How much? I have an extensive distribution system here where the main sub is double ended and most of the intermediate distribution subs are also double ended. With the bus ties open, we're looking at usually around 12.5 kA. With the bus ties closed, it gets up to around 20 kA. The additional loads could in fact improve things in that trip times decrease substantially, or it could actually make it worse. All the modelling I've done says that for us, it gets worse, because most of the loads are inductive in nature and we already have fairly fast tripping.
2. If your switching scheme is to always operate single ended...ie, you must open a main before closing the tie, then you don't have to consider recirculating loads. Otherwise as with paralleling transformers, you can end up with recirculating loads contributing additional fault energy.


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PostPosted: Fri May 02, 2014 5:40 am 
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As I mentioned in my post above, I witnessed a situation with a double ended sub, where the procedure was break main before engaging the tie, where the electricians did it backward and made the tie before breaking the main. Given that, I would definitely say you should consider the scenario of both mains being made and the tie being made when considering the worst case for the substation. After all, we're more likely to see an arc flash in a situation where a mistake is made.


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PostPosted: Mon May 12, 2014 8:57 am 
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You also have to consider that the two sources may not be in sync, or in phase if they are in sync.


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PostPosted: Mon May 12, 2014 9:33 am 
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That's why I had an arc flash in my case, because the two sources were not in sync (one was a utility and the other was a generator) and they didn't play nice together when they touched.


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PostPosted: Mon May 12, 2014 2:14 pm 
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these are fed from the same utility source so are in phase. I did run the scenario with the bus tie breaker closed so the sources are in parallel to simulate a arc engulfing both bused and used the integrated feature of EasyPower.

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PostPosted: Wed May 14, 2014 5:46 am 
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I have 3 sources (utility x 2, on site cogen). We have 2 levels of distribution for the most part, a main sub that is main-tie-main, and then all of the subs and switch houses off of it are also for the most part main-tie-main fed from 2 breakers on the main sub. Just to complicate things even further, someone made the decision in the past to save money by putting two feeders on each of the main breakers so that although in theory we get the same result, we have half as many breakers and we lose half of two subs for every fault that triggers a breaker at the main sub.

And those sync relays are pretty much required. Although this is an example of extreme phase mismatch and circulating currents, I recently got called out to a sub where they had just replaced the transformer that had failed. The sync relay was holding them out. It took all of 5 minutes to look at the name plates on the transformers...delta-wye, and delta-delta. Good thing they didn't try to bypass the sync relay! The original installation was delta-wye but kind of strange. It has peg grounded transformers and a zig-zag transformer with a separate resistor. Kind of a ridiculously overcomplicated setup since a zig-zag is only required when converting from delta-delta. This confused the whole grounding issue even more, especially with peg grounding.


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