Switch over breaker vs fused switch with Main secondary

[JBUDA54]

OK here's the thing. I normally research my topic thoroughly prior to coming into the eng-tips forums, but I am on a tight deadline to have answers on this topic very soon. So I am apologizing up front for now making attempts at due diligence.

Now that I got that out of the way...

My question is I have always, always, always seen a Fused isolation switch on the primary of the Substation XFMR with a main secondary breaker (My industrial sector that I have have exposure to is Pulp and Paper some chemical and some semiconductor). I am being asked to investigate the switch over breaker option where the primary switch has a breaker that has relaying to protect the primary and secondary of the transformer, and the secondary is forgoing the main breaker. I did have briefly seen the advertisement by Eaton concerning their switch over breaker product line. I thought that a fuse reacts faster then a breaker so I am afraid that the switch over breaker design would be not as effective as what my experience is with the proposed scheme. My guess is that they are putting the breaker in maintenance mode when maintenance personnel have to deal with the arc flash occurrences. Does breakers with fast acting instantaneous settings protect for arc flash more effectively than a fuse?

General question is this becoming a new design that industry is moving towards? Thanks in advance for your comments!

 

[davidbeach]

There are some high-side faults for which a current limiting fuse can be faster than relay plus breaker. If the fuse isn't current limiting it may never be faster than the relay/breaker combination. But for any secondary fault, the relay and breaker can be much faster than the high-side fuse. There are many transformer faults that relays can detect and breakers interrupt long before the fuse would ever respond; those faults have to become much worse before the fuse blows. I'll always take differential relays and breakers over fuses any time I'm given the choice.

 

[GroovyGuy]

I generally agree with David, but as always there are many factors to be considered.
[li] What size of transformer are we talking about here? 500kVA or 5MVA?[/li]
[li]What is secondary voltage? If MV, how is neutral treated? If lo-resistance grounded (ie 25A for 30s) then a CB is a must?[/li]
[li]Coordination, with the Utility, is usually easier with a CB.[/li]
87 protection is considered a luxury on smaller transformers. Having said that; I recently installed several 1500KVA transformers at a Site c/w;
[li]Primary 3-cycle CBs with GE T60 relays (with 87 protection)[/li]
[li] Outdoor oil-filled transformers with secondary CTs[/li]
[li] Cable-bus from transformer secondary to LV MCC[/li]
[li] Dedicated LV MCC with mains-only (ie no incoming CB)[/li]
My original thinking here was to use the cost savings (of not having a secondary CB) to upgrade the transformer protection to include 87 and 50/51 on the secondary. This in turn would reduce the AF levels on the LV side of the transformer, including the incoming section of the MCC. While this did help to reduce the LV AF levels, they were not as low as I had originally envisioned. At the end of the day, the Client ended up with 87 protection, and reduced AF protection, for less cost.

From my experience a primary CB, should provide lower secondary AF levels, when compared to a fused LISw. Even more so if the primary CB is equipped with transformer LV 50/51 protection.

Don't forget to spec a 3-cycle CB, and not a 5-cycle CB, if you plan to go down this (reduced AF) path.