UPS Input Power Factor

[peebee]

Until recently, I suffered under the illusion that input power factor for a double-conversion UPS system would be near unity -- based on all power flowing through a DC bus, where power factor has no meaning.

However, it has recently come to my attention that UPS power factor is actually much lower. During some recent tests, I was quite surprised to measure a 90% input power factor at the UPS while the system was at 100% kVA load (100% kW + 100% reactive kVAR load applied). I was even more surprised to dig up the vendor catalog cut indicating a 90% input power factor -- and another competing vendor indicated 95% input power factor for their equipment.

Can someone explain to me why the power factor is not closer to unity? Perhaps input filters (but I thought they were usually capacitive)?

Guesses are OK, please just identify them as such.

 

[ScottyUK]

Depends what you call power factor (or what your instrument calls PF):

Displacement power factor is the cosine of the phase angle between the fundamental frequency components of both voltage and current. Normally voltage is sinusoidal (or pretty close) so the calculation is simplified when doing it manually. A decent power analyser will extract the fundamental of both voltage and current and use these values in its calculation.

Distortion PF is the ratio of RMS fundamental frequency current divided by the total RMS current for all frequencies.

True PF = distortion PF x displacement PF

also

True PF = kW / kVA

Displacement PF for a plain rectifier is pretty good - normally better than 0.9. Distortion PF is not particularly good due to the high harmonic levels. Making the rectifier a half-controlled or fully-controlled type further complicates the analysis as the both displacement and distortion PF vary with delay angle.


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If we learn from our mistakes I'm getting a great education!

 

[peebee]

Hmmmm.... This vendor indicates 7% THD, which would imply 93% distortion PF. Total PF per documentation and as measured is 90%.

The calculated displacement power factor would therefore 90%/93% = 97%.

I guess that sounds about right, thanks.

 

[Senselessticker]

Knowing you said "guesses" are okay... My "guess" is that the reason is $$$$$. It cost more to develop an UPS that doesn't want to pull VARS and reflect horrible power qaulity conditions back upstream (poor pf, high THD, etc..). Double conversion UPS means you go from input AC to DC and back to AC on the output. You have to "use" power within the UPS to clean up its owm mess (due to PWM or other switching mode characteristics), if you don't want to reflect all the distortion and reactive currents back upstream.

 

[sibeen]

This question is a bit hard to answer without knowing the size and input topology of the UPS.
Generally, UPS systems up to about 200 kVA now use an active rectifier (boost) as the input stage and the input power factor of these units will be better than 0.95. These units are transformerless in the output and therefore the DC link voltage must be higher then the peak of the output voltage waveform x 2. For example, in a system with an output voltage of 240 volts phase – neutral the DC link voltage must be higher than sqtr(2) x 240 x 2. Generally the DC link voltage would be in the order of 700 volts. In these systems the battery is not connected directly to the rectifier voltage and a separate battery charger is used.

For older UPS systems, or systems larger than 200 kVA, the input configuration is normally a 6 or 12 pulse controlled rectifier (thyristors). In these units the batteries are normally connected to the DC bus and therefore the DC voltage must be controlled to avoid damaging the batteries. The DC voltage is controlled by delaying the firing of the thyristors in the bridge. A ‘rule of thunb’ equation is that the DC voltage = 1.35 Vac (line-line)*cos (theta), where theta is the delay angle of the thyristor firing. As a UPS must operate with a reasonable large variation of input voltages, the rectifier is normally designed so that at nominal input voltage the firing angle (theta) of the thyristors will be approximately 30 degrees.

The firing angle of the thyristors is directly reflected in the incoming displacement power factor of the unit. With a firing angle of 30 degees (cos 30 = 0.866) the displacement power factor is then 0.866. This will vary with the input voltage as the UPS will keep the DC link voltage stable under different voltages. If the input voltage increases the firing angle will need to increase to keep the DC voltage at nominal and therefore the input power factor will get worse. Conversely, if the input voltage decreases the input P.F will improve.

Of course you then need to take into account the other factors that will influence, for the worse, the input P.F. This includes the harmonic content of the input current, the ‘cleanliness’ of the source voltage as well as any magnetising current that is ‘used’ by the input transformers or inductors.
With larger systems, without any P.F correction at their input, I’d normally expect the input P.F to be in the order of 0.75 – 0.85, depending upon site conditions.

 

[ScottyUK]

Now we have another undefined term!

Input power factor = true power factor?


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If we learn from our mistakes I'm getting a great education!

 

[peebee]

Input power factor = power factor drawn by the input (rectifier) of the UPS.

"Input" is used to differentiate from the rated output power factor of the UPS, which might be anything from 60 to 90%, and describes the amount of kW that can be sourced by a UPS (which are typically rated in kVA). A 60% PF UPS might be a good deal if you're powering very low PF load (such as HID lighting fixtures) whereas a 90% PF UPS might be a better deal when you're powering higher PF loads (such as newer computer equipment).

 

[peebee]

The units in question were 800kVA/720kW.

 

[ScottyUK]

Input power factor = power factor drawn by the input (rectifier) of the UPS.

Back to where we started! Distortion PF, Displacement PF, or True PF. These are the only PF measurements which I can recall seeing a generally agreed defintion of. See my earlier post for the definitions. Which of these is 'input PF'?


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If we learn from our mistakes I'm getting a great education!

 

[sibeen]

Scotty, I always mean true P.F (W/VA) when I am quoing a P.F. The term input P.F is just to indicate where the P.F is taken.