power factor calculation

[RobWard]

Hi.
I've just started monitoring our power using a Northern Designs Multicube 6.
The display shows (amongst other measurements)the combined power factor, and the individual phase power factors.
I've hooked it up to a pc using RS485 comms, but at the pc you can only read the individual power factors.
So the question is: how do I calculate the combined power factor from the individual ones.
(I'm missing something obvious but I just can't get it right. Must be a Monday...)

Any help very much appreciated.


Rob

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[waross]

It may be more acurrate to add the KW and KVA from each phase respectively and recalculate the combined power factor.

 

[cuky2000]

“Combined power factor” is not an actual value. It is just a statistic indicator determine by calculation considering the individual power factors per phase. This could be either determined by averaging the phase power factors (sum of PF /3 or Sum of kW/Sum kVA).

If the average power factor is close to each individual power factor, we could say that this value is representative of the system power factor. On the other hand, if there is unusual large deviation among phase power factor, them the system should be analyzed further and a single number may not be representative of the system power factor.

 

[RobWard]

Thank you.
(Sum PF)/3 isn't anything like the result the meter's giving.
I'll try (Sum kW)/(sum kVa), which I should have tried earlier.
Though I'm new to this, there does indeed seem a large variation between phases.
We're monitoring because we're building a new factory and we need to know exactly what we need to pay for with respect to incomers, availability etc.
The meter assigns capacative values a negative value, so that's what's probably thrown me.

I'm using Specview and it's not the easiest package to start playing around with calculations in, but this should be easy enough.

Thanks again.

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[waross]

Hi cuky2000
Point accepted.
I think I agree with you wholeheartedly. I chose the description as being less 'Verbose' than;
" On the other hand, if there is unusual large deviation among phase power factor, them the system should be analyzed further and a single number may not be representative of the system power factor."
But I agree with you. Your statment is exactly what I was trying to say.
(But that is semantics which may be more appropriate in the "Engineering Language/Grammer Skills Forum.")

RobWard,
Remembering that the point of the forum is to better help your understanding;
You may be able to get a better feeling for this by doing some sample calculations.
I would set up an Exel spread sheet to calculate power factor with both methods. Set up the formula and then plug in different values for KVA, and/or power factor.
The system Power Factor is represented by Total KW over Total KVA. A few quick trials on the spreadsheet will demonstrate that the average power factor is not always equal to the system power factor.

 

[RobWard]

Thank you Waross and cuky2000.

It's always nice to learn new things but sadly this learning curve is approaching vertical!
(This is all too new to me and I don't have anyone here to ask(small family company)).

I'm not designing the new system (I'm only qualified as a basic electrician, and have spent my career here trying to keep this old place from falling apart), but I'm the one who has to deal with the contractors who are, and I'm just trying to do it to the best of my ability. Or slightly beyond in this case...

Now I'm wondering why on earth one phase is reading as capacitive, and the others not.

I just don't have the experience to know whether getting readings such as R=0.97, Y=0.27, and B= (-)0.72 looks "right" or not, and how the "combined" (for want of a better term) PF is about 0.6 (for example).

Our load is mainly resistive, with some small three phase fans on our industrial ovens and extract system.

I suppose the question I should have asked, right at the very beginning, was "How do the utilities measure the power factor that they then use in calculating your bill?", as that's the figure I want to know and monitor.
Then I can look into what I need to do to get the figures nearer to unity.

Anyway, thanks for taking the time to help me out there.


Rob



"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[RobWard]

Had a bit of a think.
Phoned the manufacturer and asked WHY 3 phase PF wasn't being displayed over 485 comms.
He didn't know but said that there was indeed a Modbus address for it.
Spoke to Specview, who will look into why it's not being displayed and hopefully fix it for me.
Job sorted.

Now, if only I'd made that phone call at 9 am this morning...


Anyway, thanks again.


Rob

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[waross]

If you care to phone the power company also, and ask two questions,
1> How do they measure the power factor.
2> How do they calculate the penalty.
If your not paying a penalty, no problem. If you are paying penalties get back to us and we will all do our best to help.
One thing, as you describe your load, it should be close to 100% power factor. A couple of things;
1> Check if there ever were capacitors added for power factor correction. An old forgotten capacitor bank that was oversized in the first place, with one or more fuses blown could account for an apparent leading power factor on one phase.
2> I am not familiar with your test equipment, but one of your current transformers may be reversed.
If the instrument has clamp-on type ct's one may have to be removed and replaced so that the current flows through it in the other direction.
yours