Articles on UPS's and Generators

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your input

your input

Thanks for all of the input. Have downloaded several articles from the sources you suggested.

Ron - you asked more specifically what I was looking for. I'm writing a paper on why generators need to be oversized to handle down stream UPS's. I've found that while it's common knowledge that this is so and that the gen sizing programs most of us use keep us out of trouble, an understanding of what drives the concern is less common. In general, I'll be discussing how current harmonics interact with the high impedances of generators to create high levels of voltage harmonics and the troubles this can cause.

Frankly, I'm still toying around with whether or not this scope is too narrow or not and am of an open mind as to what exactly the paper transpires to be. Hence the interest in a broad range of subject matter provided they have to do with the interaction of these two devices.

Thanks,

Mike
 
mshields said:
Thanks for all of the input. Have downloaded several articles from the sources you suggested.

Ron - you asked more specifically what I was looking for. I'm writing a paper on why generators need to be oversized to handle down stream UPS's. I've found that while it's common knowledge that this is so and that the gen sizing programs most of us use keep us out of trouble, an understanding of what drives the concern is less common. In general, I'll be discussing how current harmonics interact with the high impedances of generators to create high levels of voltage harmonics and the troubles this can cause.

Frankly, I'm still toying around with whether or not this scope is too narrow or not and am of an open mind as to what exactly the paper transpires to be. Hence the interest in a broad range of subject matter provided they have to do with the interaction of these two devices.

Thanks,

Mike
Click to expand...

Harmonics are just one of the issues. The characteristics of the front end of UPS's also of concern. Like switching power supplies they can represent up to 400% "inrush" when they turn on and that can collapse the generator voltage, which in turn would turn off the front end, that would allow the generator to recover and so on the cycle would run.
The other issue is regulation and what is the bypass tolerance of V and Hz of the UPS and the stability of the generator when you slamm full bypass load on in one step.

Intriquing subject.
 
Aha

Aha

Hmmm - very interesting. So there are two scenario's that can result in the UPS toggling between gen power and battery power:

1. High current harmonics interacting with high reactance of genset results in high voltage distortion which the front end of the UPS rejects.

2. Inrush you describe above resulting in a voltage dip which the front end of the UPS rejects.

True?

And let me ask you this, how do the inrush characteristics of an SCR front end compare with that of an IGBT front end?

Mike
 
mshields said:
Hmmm - very interesting. So there are two scenario's that can result in the UPS toggling between gen power and battery power:

1. High current harmonics interacting with high reactance of genset results in high voltage distortion which the front end of the UPS rejects.

2. Inrush you describe above resulting in a voltage dip which the front end of the UPS rejects.

True?

And let me ask you this, how do the inrush characteristics of an SCR front end compare with that of an IGBT front end?

Mike
Click to expand...

I beieve it to be true and also have poersonally experienced it.

It does not matter if it is SCR or IGBT, it depends on how the rest of the circuit is designed, the caps and chokes and the firing circuit. The individual design has to be scrutinized and matched with the generators charactersitics. There was a time when Excide UPS's (later Powerware and now Eaton) would only able guarantee their unit to work with one specific generator manuafcturer.

The other issue you need to be concerned about UPS's is the line harmonic content. With the emergence of mass applications of ASD's the harmonic content can mess up what the UPS' "sees" as acceptable power source. Your bypass has always less tolerance than your main input.

Other stuff to watch out for: large rectifiers such as chlorine cells, electroplating, electrical furnaces and capacitor banks.
 
Toggling of UPS

Toggling of UPS

I'm starting to suspect that the two phenomena are more of a simultaneous "one two" punch that occur during the subtransient phase of the inrush current surge. Sounds like the inrush combined with the high current harmonic content interacts with the subtransient reactance of the generator. This interaction causes high levels of voltage distortion which the UPS may reject. The inrush itself causes a voltage dip which too may be a problem. The latter however is not strictly limited to non-linear loads of course.

Further thoughts?

Mike
 
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