Power Factor 4

[aprilee]

A trick question regarding lagging and leading power factor:

Is this determination based on the direction of var flow (in or out), or is it based on watt and var flow in the same direction?

If watts and vars are flowing out of a substation, we have a lagging PF.
If watts are flowing out, and vars are coming in, we have a leading PF.
What about watts in / vars in?
and watts in / vars out?

Can a PF be lagging at the feed end of a line and leading at the load end of a line?

 

[Skogsgurra]

The directions of W and var are totally independent. And, yes; If the PF is lagging in the var consuming end, it is leading in the var producing end.

The convention is that inductive loads (current lags voltage) has negative P.F. while capacitive loads (current leads voltage) has leading P.F.

Gunnar Englund

http://www.gke.org

 

[dpc]

If watts and vars (convention that positive vars are inductive reactive power) are flowing in the same direction, power factor is lagging. It doesn't matter which direction the power is flowing. This is consistent with the concept of the current lagging the voltage at a lagging power factor.

If watts and vars are flowing in opposing directions, power factor is leading.

So for an induction motor - power is flowing into the motor, reactive power is flowing into the motor - power factor is lagging.

Synchronous generator that is producing vars - watts flowing out, vars flowing out - power factor is lagging.

Capacitor bank - power flows in (losses), vars flow out - leading power factor.

There is an IEEE metering standard that is generally depicted in any power meter instruction book that covers this. If you go to the GE website and download the instruction book on their PQM, I believe you will find a diagram there.

Also, you might search this forum - there have been some extended discussions on this in the past.

 

[tinfoil]

We were taught the mnemonic 'ELI the ICE man' (works in English, anyways):

ELI: Current(I) is after ('lags') voltage (E), in an inductive (L) load.

ICE: Current is before ('leads') voltage in a capacitive (C) load

 

[aprilee]

Thanks a lot for the info. They helped a lot!

 

[electricpete]

"Can a PF be lagging at the feed end of a line and leading at the load end of a line?"

Yes if it is a short heavily-loaded line and the load is capacitive.

The model for a short transmission line is a simple series inductance.

Let's say the load is dissipating 10MW and producing 1MVAR (leading since MW and MVAR flow in opposite directions).

Let's say the transmission line at this loading level dissipates 3MVAR in the series inductive element.

We have to injuect 10MW and 3-1 = 2MVAR at the supply end. Since P and Q are in the same direction this is lagging.

Note this scenario becomes less likely with longer tranmission lines where the equivalent circuit includes a a shut-capacitor at the end.

The more likely scenario with a long transmission line is that we may have an inductive load at the load end and leading power factor at the supply end (due to line capacitance generating more MVAR than is used by the load)

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