Ratings on double-ended switchgear 1

[Chuk1t]

What is the proper way to size transformers for double-ended switchgear?? For example I have a switchgear with a left and right section with a normally open tie breaker . Each bus rated for 4000A. The load on each side is approximately 3400A at 480 volts 3-ph. If one transformer fails is it customery to size the transformers so that either one can pick up the entire load i.e. 6800A ??

 

[Hertz]

It mostly depends on the application. In most cases the non-essential load can be shed from the substation, and the remaining transformer is required to supply the essential load on both buses. Also it is customary to use the fan-cooled rating of the transformer to supply the total essential load. Based on the type and design of the transformer, one can get up to 1.8 times the nameplate rating of the transformer. It all comes down to $'s.

 

[jbartos]

Suggestion: Double ended substations are more suitable for non-essential loads rather than emergency load since the tiebreaker may experience a catastrophic failure thus eliminating all emergency supply.
The transformer on the double-ended substation normally is specified for carrying all load of the double-ended substation. If specification allows some load to be discarded on each side, than the transformer size may be smaller. However, the savings are not that significant considering the overall project and two service electrical utility. Unless justified, it may be viewed as the "penny pinching policy."

 

[hmchi]

The question was how to size the transformer. In addition to specifying a bigger transformer, there are various industry standard 'tricks' that many have specified before ..

I believe that ANSI standards requires a substation type transformer be able to sustain 125% of its nameplate rating for one hour. Then there are the 55/65 degree rise [another 15%] that one can specify for liquid transformers to run cooler as well as prepare for overloading. [The ventilated dry transformers based on Class H insulation can have ??% additional loading [I seem to remember it was 25% but not sure] if one specifies 80C rise instead of 150C rise].

Then there are fan cooling, and ANSI has specified the extra loading allowed when they are specified. [Sorry, I only remember 33% for Class H vent dry transformers].

I agree that it is more appropriate to look at the loads first, essential vs. non-essential, etc. Then the thing to do seems to be fans. And if you would like to baby your equipment for longer life, the 55/65 or 80/150 rise thing can be considered after that.

 

[CHDean]

Considering the loads you have, double ending as 100% back-up is not a good idea. At 3400A per side you can't even get a standard breaker/fusible switch to use for the mains or tie as each could be loaded to 6800 in a disaster. The best you're going to do is a 5000A DE substation and I wouldn't even call that standard. I would strongly suggest the you consider the 4000A DE you spoke of and an additional SE sub that has provision to add the opposite end in the future. This will allow you to get the loading down to a more reasonable level for back-up of critical loads and allows some options for the future unknown/unseen event.

 

[dpc]

In a past life I was involved in the design of numerous double-ended substations for high-reliability installations. Our basic approach was to size the transformer such that it could carry the expected peak demand load of the entire substation using its highest forced-cooled rating. Not the connected load, but the expected peak load.

As others have said, the type of facility and needed reliability will determine your level of conservatism.

I disagree with JBartos to some extent. Double-ended substations are the norm in industrial facilities such as power plants, wastewater treatment plants and other facilities where extended downtime due to the loss of a transformer is not acceptable. The common-mode failures associated with tie breakers can be mitigated by physical separation of the two buses and by use a second tie breaker or disconnecting means. This allows one end of the substation to be quickly restored in the unlikely event of a tie breaker failure. The use of double-ended substations must be combined with use of backup equipment and in some cases, multiple feeds to critical loads.