Transformers on loop

[m sleem]

We have double ended 13.8kv switchgear feeding no of loops, each loop serving more than transformer. My question, would we size the breaker in switchgear as 6x of the total transformers or just we can apply 6x of the largest one plus 1x of the rest?

Where, the transformer's impedance is less than 6%.

 

[mayanees]

In this type of arrangement, the 13.8 kV breakers provide power to a group of transformers, but the transformers all have individual primary fusing in accordance with 450.3(A). If your arrangement is typical of a double-ended campus-type distribution system, the A & B sides are brought into each transformer disconnect switch, such that if either side is lost, you can select the alternate feeder. So each side could power all of the transformers in a given group.
The issue then becomes selection of an instantaneous settings that will tolerate the inrush of the group, which in theory is 8-12X. I've tried to find data on that cumulative effect, to no avail. We will be doing measurements in the next few weeks to get at a good number, where we'll energize a group of 8-10, 13.8 kV transformers ranging in size from 75-500 kVA, for a total of about 1500 kVA. I'll use that instantaneous measurement to establish a limit as to how much I can energize at once in a given group of transformers.
Power systems are always constrained by the Utility overcurrent protective device (ocpd), which ultimately determines how much cumulative KVA can be switched on at once. Ideally you'd like to bring the system up with as little sectionalizing as possible, but you must stay within the limits of the Utility ocpd.
The other consideration is conductor overcurrent protection, where 240.100 allows you to exceed the ampacity of the conductors, with conductor service calculations essentially being the overload protection.
John M

 

[m sleem]

Good, i'm waiting for your measurments. But you know, where we are in design phase, we don't know the utility ocpd. Do we have approximately approach in such case?

 

[mayanees]

I would set it up as if there were no Utility constraints, and then modify accordingly.
450.3(A) for transformer fusing. Size cables to accommodate the transformer loads. Adjust the 15 kV breakers for 10X cumulative instantaneous pickup, with LTPU to coordinate with 15 kV mains. When you get a Utility ocpd, coordinate the mians wtih the Utility, then the tie, and then the largest distribution transformer fuse. Give priority to keeping the A & B sides energized when faced with any uncoordinated distribution transfromers.
John M