leading/lagging terminoglogy 4

[electricpete]

For a load it is easy to figure out what lagging and leading are (inductive and capacitive). Also the terminology of vars flowing in a given direction is unambiguous. But the terminology of leading/lagging as applied to generators seems a little confusing to me. I excerpt here from the post on "Voltage Drop on Large Motor Startup"

Maak - (2) A induction motor driven above its rated speed delivers leading power factor current back into the line -- it must derive its exciting current from the line. This is a very simple form of generation of power and for certain power systems is an almost ideal method of increasing line capacity."

aolalde - (2) It should read "delivers lagging power factor current back into the line"

dpc - Actually, it is correct as written. A generator that is producing kW and absorbing kVars is leading. A generator producing kW and producing kVars is lagging. This is true for synchronous and induction machines.

aolalde - dpc: As far as I know, LEADING CURRENT is obtained from static capacitors or overexcited synchronous machines connected in parallel with the load. LAGGING CURRENT is obtained from inductances.
The power will follow the current since for a given voltage (V), P = V*I.

electricpete - fwiw - I agree with the power factor terminology as described by aolalde.

Scotty UK - Lordy, I'm going to disagree with Aolalde and ePete in one post! I'm in agreement with dpc and Maak re. generator VAr terminology: In the UK a generator exporting VAr's (over-excited) is deemed lagging, while a generator importing VAr's (under-excited) is deemed to be leading. For a load, a consumer of VAr's is deemed to be lagging (inductive) and a producer of VArs is deemed to be leading (capacitive).

What Scotty and dpc are saying is a generator has the same terminology (lagging or leading) as the load it supplies. That sort of makes sense when we consider that we restrict the power factor angle to be less than 90 degrees. If we viewed the generator and load as having opposite properties (one leading and one lagging), remembering they share the same voltage and the 180-degree opposite current, we would conclude one has power factor angle below 90 and one has power factor angle above 90 (not permitted).

Explain it to me again.

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[electricpete]

... But there is one thing I don't like about this approach. Let's say I have a machine with vars flowing in. If it's a generator I call it leading and if it's a motor I call it lagging.

I think the clearest terminology is vars in and vars out.

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[cbarn24050]

I think the confusion comes about because you think that leading/lagging is the same as +direction/-direction which it's not.

 

[buzzp]

We have serveral generating facilities and no one here ever refers to leading or lagging from the generator end. We use over or under excited or absorbing or sourcing VARs.

Seems the IEEE (for us US people) would have this defined for generators. Anyone have access to the IEEE definitions or generator standards? (Ours are inaccessible right now due to a move).

 

[dpc]

I have in front of me a generator capability curve from General Electric for a 24.5 MVA generator. The underexcited region (absorbing vars) is clearly marked "Leading". This is the same as any generator capability curve I have ever seen from any country. This is also consistent with definitions in all of the generator relaying instruction books that I have ever seen.

I checked Blackburn's "Protective Relaying" book and he gives the same definition. See Figure 8.16.

I believe there is an IEEE definition of power flow for metering purposes, but I don't have a reference number. It defines four quadrants with all possible combinations of power and var flow.

Basically: If power and vars are flowing in the same direction, power factor is lagging. If power and vars are flowing in opposite directions, power factor is leading. It makes no difference if you are talking about a synchronous generator, an induction generator, a motor, or any other type of load.

I don't really think there is any discrepancy on this in the technical literature.

Of course, we could start debating the definition of power factor if we really want to have an interesting discussion...

 

[rbulsara]

I agree with dpc. Good clarification. Gave a star.

 

[electricpete]

"Basically: If power and vars are flowing in the same direction, power factor is lagging. If power and vars are flowing in opposite directions, power factor is leading. It makes no difference if you are talking about a synchronous generator, an induction generator, a motor, or any other type of load. "

OK, that makes some sense. I think I understand the standard terminology you're using better now.

But when you throw DIRECTION into the mix ("A induction motor driven above its rated speed delivers leading power factor current back into the line it must derive its exciting current from the line.") you have something really really weird. The current flowing into the motor lags the voltage in a very straightforward manner (angle less than 90). The current flowing out of the motor "leads" voltage by an angle between 90 and 180 - not what I would normally call leading. I would say the only angle between voltage and current that we can find less than 90 is between current flowing into the motor and voltage... and it is lagging.

I'm going to stick with vars in and vars out myself. But I think I understand the terminology better now.
Thx.

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[electricpete]

BTW

"But when you throw DIRECTION into the mix..."

I didn't mean "you" as in dpc, I meant "you" as in anyone.

dpc - you get a star from me too.

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[electricpete]

I agree what dpc presneted is standard. Just to argue the logic a little more:

In mathematical terms, leading and lagging apply to relationship between current and voltage, not between real and reactive power.

Look back at my above example without clouding it by interpetting the original statement about leading power factor into the line. Let's say it's a sync generator operating in vars in . The current flowing into the generator motor lags the voltage in a very straightforward manner (angle less than 90). The current flowing out of the motor "leads" voltage by an angle between 90 and 180 - not what I would normally call leading. I would say the only angle between voltage and current that we can find less than 90 is between current flowing into the motor and voltage... and it is lagging (current lags voltage).

You all (dpc and company) would call it leading. What is the current that leads voltage and is the angle less than or more than 90? (my opinion is the only leading current we can find has an angle more than 90).

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[electricpete]

delete reference to "motor" in my above example... it is a generator.

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[jghrist]

I think things are clear if you consider what the terms "leading" and "lagging" mean. They indicate whether the current leads or lags the voltage. In vector terms, a lagging current vector angle will be less than the voltage vector angle, using the normal counterclockwise rotation convention. In the time domain, lagging current will reach a peak after the voltage reaches a peak.

Clearly, then, if the generator, whether induction or synchronous, "delivers lagging power factor current back into the line" the current going into the line lags the voltage. The load is inductive and the generator is providing the inductive vars to the load. The real power is also flowing into the line (otherwise we would be talking about a motor not a generator).

Conversely, if the generator delivers leading power factor current into the line, the current going into the line leads the voltage. The load is capacitive and the generator is absorbing vars that are provided by the capacitive load.

 

[electricpete]

Conversely, if the generator delivers leading power factor current into the line, the current going into the line leads the voltage. The load is capacitive and the generator is absorbing vars that are provided by the capacitive load."

This is the example I used. I see your logic. My logic for same example produces opposite concluision. My logic was that the the current going into the generator must lag voltage by between 0 and 90. For current leaving generator and we change by 180 degrees - either between 180 and 270 lagging or between 90 and 180 leading... not a simple 0-90 leading.

Which answer is right I think lies in how we model the generator and power system. I will think about it a little more when I get a chance tonight.

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[aolalde]

I think DPC statement makes sense.
If I have a synchronous generator feeding a pure resistive load, the PF is unit (not leading not lagging).
If I increase the excitation, the PF remains unit but the voltage will increase.
If I connect an inductive load to the generator the current lags the voltage and the generator voltage drops unless I increase the excitation.
Then to make VARS circulate out of the generator I need a load that makes the current lag the voltage and now the generator is providing VARS with lagging PF.
I confess I was thinking wrongly all my life.

 

[dpc]

Power system engineers always think of vars as "lagging" vars. That is why capacitors "produce" vars. They produce lagging vars and absorb leading vars. They operate at a leading power factor because they consume power (losses) while producing vars.

If you think of vars as always inductance-based (lagging), the terminology might make a little more sense. Or not..

 

[electricpete]

I came to the conclusion jghrist was right and I was confused.

My error was to assume vars into a generator equated to current lagging between 0 and 90. Actually this is true for a motor but not a generator (due to reversal of the real component). Drawing it on a paper, a situation where vars flowing into the generator, current lags voltage between 90 and 180.

Sorry if I have muddied a simple issue. Thanks for helping me through it.

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[electricpete]

Just to clarify my previous statement:

IF we define the reference direction for current is INTO the machine, THEN a motor consuming real power and consuming vars has current lag voltage by between 0 and 90 whereas a generator producing real power and consuming vars has current lag voltage by between 90 and 180.

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[jghrist]

A lot of confusion can be eliminated by assuming that the current flows in the same direction as the real power. If real power flows out of the machine, call it a generator and define positive current direction as into the line. If real power flows into the machine, call it a motor and define positive current direction as into the machine.

 

[electricpete]

Yes, it is pretty simple in retrospect as you point out.

I had never given much thought before to power factor of a generator (always vars in or vars out) but it makes sense now. They key fact already mentioned that I didn't really absorb earlier is that leading/lagging polarity depends not just on direction of reactive flow but also on direction of real power flow.

At first glimpse, it didn't seem like it should be that way. After all, If I model "vars in" as an internal inductor in parallel with my ideal machine terminals, I know that the current within that inductor (the reactive component of my machine current) will always lag the terminal voltage by 90 degrees. But the angle of interest is not the angle from voltage to reactive current but from voltage to total current. Flipping the real component 180 degrees changes the vector diagram.

A lot of rambling on my part but a straightforward explanation by dpc and jgrist.
Thx.

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