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Phase Voltages for an Open Phase Condition on a Ungrounded System

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umrpwr

Electrical
Dec 21, 2002
71
Presently I performing post event fault analysis of the following situation and would appreciate input in trying clarify the concepts of phase voltages with one open phase.

The situation is as follows:

Customer has a facility that is served by incoming utility services both from transformers that are connected in delta on the primary and wye-ungrounded on the secondary. The utility transformer neutrals (XO) are ungrounded and not brought in from the secondary to the customer main switchboard. Thus the customer system operates an ungrounded system. Downstream of the cutomer main switchboard is a 480V UPS system which then feeds a 500kVA step down transformer which is 480V primary-208/120V secondary and is connected delta-primary and wye-grounded secondary. This transformer then feeds the branch distribution panels within the facility.

During maintenace there was a short resulting in a line-to-line fault on the 208V system. The upstream protective device operated on the 208V secondary side of the transformer as would be expected. In addition as might also be expected for a secondary line-to-line fault one of the fuses on the H.S. side of the step-down operated. During this sequence there was an open phase condition on the primary side of the transformer which is delta or could also be considered wye-ungrounded from the utility.

Besides all of the other issues the question is what would be the expected voltages along with the sequence networks for calculation of the two other phases with one phase open for the ungrounded system? I would expect that the two un-opened or healthy phase voltages would become elevated since the system is ungrounded and there is nothing to hold the voltages down even though there isn't current flow similar to a line-to-ground fault on an ungrounded system even though they are capacitively coupled.

Assistance in clarifying this is appreciated.

Thanks,

UMRPWR
 
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The answer rquires information on the UPS as the faulted and unfaulted phases are supplied from the output of th e UPS not the ungrounded wye of the utility transformer. Can you explain its output connecton?
 
The open phase is upstream of the 500kVA transformer and in the static bypass switch of UPS. One of static switch fuses opened. Thus the question regarding the overvoltages for an open phase since the UPS would probably see some type of overvoltage on the other two phases. What would be the magnitude of the overvoltages the UPS would see?
 
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