Load Imbalance

[davidbeach]

In a project I am working on, I have a 208Y/120V unit substation supplying a load that is essentially all 120V, 20A, receptacle circuits - many panels, each with 30 or so 20/1 breakers. The design loading is reasonably well balanced from phase to phase, the bulk of the load being 180VA receptacles.

Does anybody have any empirical data as to how much imbalance I might see in actual operation? The worst possible case would be, with the building fully loaded, somebody went through and turned off and the phase 'B' and phase 'C' breakers, giving 100% load on 'A' and nothing on the other two. But that wouldn't be very likely. On the other hand, there could be, by happenstance mostly, a noticeable unbalance. How unbalanced could one reasonably expect to see?

Thanks.

 

[BJC]

There are problems with wye-wye transformer connections. I'm not sure I remember all the details and if it applies to all types of transformers.
Will an unbalanced load try ot shift the secondary neutral point to somewhere other than the center of the secondary voltage triangle? IF I remember correctly the problem was current flowing in the link between the two grounds.

A 1000 kVA transormer is going to have a super fault current. I think industry experience will verify ( I don't have any statistics at hand to bear this out however.) that faults in the gear are man caused. That is someone leaves a tool in the gear or something like that. In this case I would opt for fully insulated bus. Some of that red plastic on the bus may save you a lot of blue smoke some time in the future.

 

[jbartos]

Comment on the previous posting: Depending on the neutral grounding on primary and secondary sides, there will be third harmonic component in the current and voltage causing various negative effects.
Reference:
Electrical Transmission and Distribution Reference Book by Central Station Engineers of the Westinghouse Electric Corporation, East Pittsburgh, Pennsylvania, 1964.
Table 15 Influence of transformer connections on third-harmonic voltages and currents on page 129