Reverse Connected 480V Delta to 208Y Transformer
Reverse Connected 480V Delta to 208Y Transformer
So I just want to check my thoughts here
Below is a picture of a step up transformer being fed via 120/208 800 amp
To 277/480. As you can see the neutrals are not even hooked up nor is the transformer bonded. Even though there are no neutral loads.To my knowledge this is a dangerous set up as the fault current can not be carried back to trip the breaker. am I right here or over thinking this. We will be replacing all this anyway but I just want to make sure I'm thinking correctly here
Thanks
So regardless my two neutrals from the feed. should be landed on XO and bonded to the tranny case and a rod. The secondaries stay isolated from XO
For what its worth, here's what I see from the provided photos. A transformer nameplate would have been good, to see the type and size for sure. At 800A 208V feeder, that would equate to a 300KVA rated transformer, at 832A on the 208 side and 360A on the 480V side.
It looks to be a standard 480V delta primary, and a 208V Wye secondary, used in reverse as others have suggested. Cant be much else, because it doesn't have terminals for anything else. The XO terminal is definitely on the 208Y side, due to the size of the common bus strap and welded connections from each Y connected coil.
So as it sits, its an ungrounded 480V delta output. It doesn't appear that it has ground fault monitoring, nor is it corner grounded. What's worse is that your 208V Wye neutrals from the supply side are spliced to the load neutrals going to the 480V load side, completely bypassing the transformer coils and frame connections.
This can be a very bad thing, for a couple reasons. First one is, that there is no low impedance fault current path from the 480V load side, back to the 480V delta output coil. Second is that the fault current, if and when it happens, will be directed back to the 208V Wye supply source, likely the service transformer, because the neutrals and grounds are spliced back through.
What this does is make a high impedance connection, for fault current, to get back to the 480V delta primary winding. The only way that current can get back to that 480V delta coil is through the magnetic induction of the transformer core, and stray capacitive coupling of the windings and conductors. An arcing fault, on the 480V side, under these conditions can produce some high ringing voltages in the circuits, as well as not trip the overcurrent devices.
The 208V feeder, did not require neural conductors to supply the transformer windings, and should not be connected to it either. Large circulating currents will result, if you do so.
The 480V delta side can only be corner grounded, since that's all the terminals you have to connect to, the corners of the delta.
The transformer case and core appear to be bonded to the grounding conductor from the 208V side. A lug to the case in the right rear corner, and a case to core strap in the right front corner. Then the bonding jumpers to the conduits. Most of the bonding jumpers appear to be undersized, as compared to the ungrounded conductors.
The other thing that appears missing is a grounding electrode conductor and connection to the 480V delta coil. The delta coil is a separately derived system and should be bonded back to the building steel, water service entrance or main ground buss in the service where the electrode conductors terminate, unless there is a ground detection system installed.
For all practical purposes the existing transformer should be corner grounded or replaced with a 208 delta primary /480V Wye secondary and wired correctly. The way it is existing, could be a serious hazard during a fault conditions, especially an arcing fault. Personally I wouldn't touch it if I couldn't get the OK to fix it properly, one way or the other.
MTW
