UPS DC Bar

[karalahana]

Hi, I wonder how is the DC bar of an online ups can be kept constant? I mean if your utility voltage is fluctuating in a wide range, so it means your DC voltage rectified on DC busbar will behave also so isnt it? I guess to be able to chop the voltage on this bar it is needed that this voltage must be constant ,at least I guess so..

 

[Skogsgurra]

Simple UPS have diode rectifier and the DC link voltage changes when grid voltage changes. But that is not a problem for the IGBTs - they switch happily as long as there isn't a severe overvoltage.

The output voltage of a UPS is usually kept constant, no overvoltage if grid voltage goes up. But if input voltage goes below output voltage, the latter is also reduced.

There are more sophisticated input stages for UPS. Like sine forming and AFE rectifiers. Everything can be had. But cheapo UPS don't have those features.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[karalahana]

so you say if voltage raises the IGBT part will take care of it, but if it decreases so it is naturally a situation to bypass! So is there any solution for keeping the voltage constant via perhaps inductors or capacitors maybe?

 

[Skogsgurra]

The more expensive ones have various ways of doing that. Input transformer is simple and reliable. An active PFC circuit usually takes care of grid voltage swings. It also makes the current drawn from grid perfectly sine shaped.

Others work more intimately with UPS and will surely add/comment later today.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sibeen]

Older UPS systems (those that don't use an active PWM rectifier) used thyristors in the rectifier in either a 6 or 12 pulse configuration. The thyristor is of course controllable and it was this feature that is used to control the output of the rectifier voltage.

Without getting too mathematical, the output of the rectifier was approximately:

Vdc = 1.35 * Vac * cos alpha

where Vac is the line to line incoming voltage

cos alpha is the firing angle of the thyristor.

The resultant output voltage would also be filtered which would remove much of the ripple voltage, using either a DC choke or a DC link capacitor, depending upon the configuration of the inverter.

 

[raithrovers1]

The DC bus of a UPS system also has a battery connected. This acts as a buffer during any fluctuations in the input mains voltage that the rectifier can not handle.

UPS engineer

http://www.calibrepower.co.uk