We have a 34.5 kV Utility Source feeding a customer-owned 34.5kV: 480Y/277V, 500 KVA padmount, which in turn directly feeds a 480V: 208Y/120V, 225 kVA Dry-type Transformer. (We could not find a 34.5kV: 208Y/120V padmount in time). The 208Y/120V secondary will supply an office building/warehouse for a Data Center. There are no loads between the two transformers, which are located around 8 ft apart. Two questions:
1) If the Utility OH line tap is protected with fuses, and the padmount has primary fuses, where is OCPD required between the transformers?
2) If a 480V Breaker is used ahead of the Dry-type, is this where the Main Bonding Jumper should exist? I'm confused as to what is considered "Service Disconnect" in this situation.
Service disconnect would be at 34.5 kV since transformer is customer owned. It separates the utility and customer side of things. It does not need to be a safety disconnect and you won’t find many available at 34.5 kV. For some customers we would just use cutouts. But if you want a real “disconnect” use a GOAB (gang operated air break switch). But these are long lead time for 34.5 kV. I have one sitting in our shop right now in the crate. They are huge and require a line crew to install. You will need something like this if you go VCB regardless…
The 480 and 208 buses can be separately derived sources so treat each one separately. So just tie X0 to a ground rod for the 34.5 kV transformer and call it a day. On the 208 side you can connect the ground rod at the transformer but it’s much more convenient to do it at the MDP.
The secondary protection on the low side of the 34.5 kV transformer is there to protect the transformer against failures in the wiring between the two transformers. You don’t have to have it but NEC then severely limits the primary protection. It CAN be done but what you typically need is a microprocessor protective relay controlling a vacuum breaker. The relay would be programmed so that it starts on very high settings to avoid the magnetic inrush. Once that passes in a couple cycles it engages the normal settings to meet Code. Since you are on this path though you could add a second relay and bushing CTs on the secondary side. The second relay serves as both secondary protection of the 34.5 kV transformer and primary protection of the 480 V transformer. A third relay could provide secondary protection as well. This leaves you with just one switching device. I’m a big SEL fan. The 751 relay will do everything you want/need. 651s also work very well but they look a little screwy because they’re intended as reclosers but they have 6 CT inputs instead of only 3. Having done it both ways pay the extra and use a second 751.
Make sure you include batteries for the relays. Or you may want to consider a small 1 kVA 34.5 kV:120 control power transformer. It’s simple and cheap and readily available. BAY-O-NET fuses or cutouts provide convenient primary protection if the relays are still booting up or out of service for some reason,
You are not going to want a switch between the two transformers anyway. The AIC is going to push you into some very expensive hardware with long lead times and the arc flash potential is obscenely high. This solution addresses both in a simple way.
The only downside to vacuum breakers is you MUST have a manual switch inline somewhere. Needed for servicing and testing and because VCBs aren’t a good choice for LOTO. You’ve only got about a 3/8” gap and it cannot be inspected. Cutouts are cheap and work well. The previously mentioned GOAB is convenient if using hot sticks makes you nervous.
