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Ups Nuisant Transfer To Bypass


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I am presently working on a problem with one of the UPS systems in the plant. There are two Single UPS systems (480-208/120) feeding two UPS buses (A and B) separately. The two separate bus circuits feeds a couple of 208VAC-24Vdc rectifiers which in turn individually feed as redundant power supplies to downstream PLCs, SIS, etc.


Of the 24Vdc rectifiers, the 24Vdc, 400A rectifier is the largest. The UPS systems always shifts to bypass everytime the UPS output MCCB is closed or this Rectifier is energized. The UPS alarm shows "Output Overload". We suspect that it was the downstream in-rush which is causing the UPS to nuisantly transfer. An idea by one of the contractor was to put a reactor on the input of the rectifier. A 5% impedance reactor was connected in series but still the UPS keep to transfering to bypass. Everytime the staticon charger is energized, the UPS transfers to bypass and indicates an "output overload" and after the staticon charger is started, it shifts back to the inverter. The concern also is that if there is a plant wide shutdown, then the UPS may not transfer to the batteries everytime the rectifier is energized since it sees a fault in its output.


It is observed that the UPS output transformer secondary is wye and the downstream rectifier isolation transformer primary is delta...is it possible that the problem is also caused by phase mismatch between the UPS and downstream charger?


is there a possibility that the UPS settings may be too sensitive?


The contractor's consultant also suspected that the downstream charger is in resonance with the UPS meaning, the UPS sees a zero impedance and erroneously see as a fault and was suggesting phase shifting isolation transformer between the UPS output and the downstream charger input to correct the phase shift mismatch. I do not see anything wrong of connecting a delta connected load from a wye source.


I am gathering all the data so I can post them here soon. But any help or guidance will be very much appreciated. Thank you.

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Hello pgb1925


Welcome to the forum.


I is possible that the waveform being drawn by the rectifier is very peaky and that the peak currents are overloading the UPS.

Recitfiers supplying batteries and capacitors often have a very narrow conduction angle. There may be an average of 400 amps, but a peak of say four times that.

Putting a DC choke after the rectifier in one DC output leg may help to reduce the peak current and alleviate the problem.


I would try to get a picture of the output current waveform and the input current waveform and then present those to the UPS manufacturer for comment.


Best regards,


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Hello Marke. I will be suggesting the UPS be tested further at site to determine what is causing it to sense an "output overload".


1. Energize the the 24Vdc rectifier with the converter isolated and only the input transformer is energized. If UPS do not transfer,

proceed to item 2 below.


2. Energize the 24Vdc rectifier with both input transformer and inverter connected but the 24Vdc loads and batteries isolated. If UPS

do not transfer, proceed to item 3 below.


3. Energize the 24Vdc rectifier with both input transformer, inverter and batteries connected but the 24Vdc loads isolated. If UPS

do not transfer, proceed to item 4 below.


4. Energize the 24Vdc rectifier with both input transformer, inverter, batteries and 24Vdc loads connected.


I am proposing these operational tests to determine which current in-rush is making the UPS transfer to bypass and then from there propose some mitigation. Is a series reactor on the input transformer of the rectifier work as a mitigation?


Is phase shift mismatch an issue here?


May I request for more technical info on this DC choke (coil?).


Thanks for all the support.

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