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PWM UPS fed by a constant voltage transformer (Ferro)

PWM UPS fed by a constant voltage transformer (Ferro)

PWM UPS fed by a constant voltage transformer (Ferro)

(OP)
We have a PWM UPS that replaced an old Ferro UPS. The site decided to re-use the bypass tarnsformer from the original UPS. This transformer is a ferroresonant constant voltage transformer.
When commissioning the new UPS it was found that the UPS seems to work as designed. The problem only arises when a static transfer to the bypass supply is initiated. As soon as the command is given the inverter grunts loudly as if it is feeding a short circuit. The UPS does not transfer to bypass and remains on Inverter static switch. If the inverter OFF command is given to the UPS then a transfer to static bypass happens without any problems.
Therefore my question is: does anyone know of a reason that a PWM inverter cannot be fed by a Ferro bypass supply? It seems that the only problem is when both static switches are on for the very short overlap period. This seems to cause a high amount of circulating current.
Thanks as always for any help.

P.S. I will not be at this site anytime soon to perform any further investigations at this time.

UPS Service Manager http://www.nolanpower.com

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