Poor power factor - induction generator 1

[ebarba]

Hi all,

We're getting a much lower power factor than anticipated on a 100 kW asynchronous generator intended for 100% power export to the grid. It is connected via a dedicated line that serves only this generator and it's auxiliary systems (about 5 kW worth of 3 phase motors).

This is a follow up of thread237-376121, please see that thread for details. Please note that we added a power reactive power control to the configuration described in the previous thread. This power factor controller was designed so it would bring the power factor to 0.95 from the 0.85 figure this machine has at 100 kW

A power analyzer on-board the genset recorded the following, while the reactive power control was on-line
Real power: 54.7 kW
Apparent power: 80.3 kVA
Reactive power: 62.2 kVAR
PF: 0.63

The generator is rated at 0.73 @ 50kW with no reactive power control. Why is the PF so poor?

Thanks and regards,
Ernesto.

 

[DF56]

An induction (asynchronous) generator will absorb reactive power from the system it is connected to, unless capacitors or other reactive power sources are installed.

DF.

 

[crshears]

Hello ebarba / Ernesto,

Where are the measurements being taken?

At the risk of being obvious or considered obnoxious...

As others have already stated, the induction machine itself will ALWAYS absorb inductive VARs from the system when the unit is connected to the grid, no matter whether there be delivery or absorption of real power.

The power factor controller is [or should be?] configured to deliver almost all of the inductive VARs being drawn by the machine, and is therefore connected between the terminals of the machine and the POCC to the grid. As a result only a small amount of inductive VARs need be drawn from the grid.

Measurements of power factor should therefore be taken on the grid side of both the induction generator and the power factor controller.

If the option exists to place the PFC on manual control, adjustments of the same should be undertaken in small increments and the changes in readings confirmed to be of a magnitude and direction as electrical theory tells us they must inescapably be.

If the measurements don't accord with the first principles of electrical theory, something is perforce amok with the metering set-up....but then again, maybe that's just me...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ebarba]

Between the generator and the street meter there are a manual breaker and a contactor in series. The manual breaker is just after the generator terminals, the contactor in the switchgear, on the meter side.

The PFC is connected in parallel Between the meter and the contactor
The analyzer measures current and voltage between the breaker and the contactor (is this wrong?).

The size of the PFC is enough to bring the PF of the generator from 0.85 to 0.95, about 35 kVAR and it's totally static (this makes sense, as this generator won't be operating at part load... When it runs, it does it at the rated power constantly). What we can do is disconnect it and make the genset run without any PF correction.

Hope the above is clear and detailed enough.

 

[crshears]

If I'm following your description, your equipment sequence is as follows:

GENERATOR < > BREAKER < > ANALYZER < > CONTACTOR < > PFC INJECTION POINT < > METERING UNIT < > STREET

If the above sequence is correct, it should be clear that the analyzer is measuring the amount of inductive VARS drawn by the generator, NOT by the site...and it is the site power factor that matters.

Therefore in my view your configuration should be:

GENERATOR < > BREAKER < > CONTACTOR < > PFC INJECTION POINT < > ANALYZER < > METERING UNIT < > STREET

The utility will bill/credit based on the measurements taken and values recorded by the METERING UNIT, so your ANALYZER should be set up to see the vectorial sum of the sites' real and reactive power flows [which, after all, is what the utility sees], and the PFC adjusted to supply the lion's [lioness's ?] share of the reactive demands of the site, meaning generator and station loads - after all, the utility will be using their trusty four-quadrant interval meter to bill you for whatever reactive you don't supply for yourself.

Bottom line: in config 2, adjust the PFC output until the analyzer shows a site power factor just under unity, lagging.

Hope this helps.

 

[ebarba]

crshears, the current config is your first one. Being the PFC connected in parallel, isn't it possible to assume that the whole line between the generator and the meter has the same characteristics? I mean, isn't the effect of the capacitors going to be "felt" the same in all the line?

We can't adjust the PFC output, we can only change it or turn it off by opening the connection to the line. What we'll do next time is run the generator without the PFC, to see if we approach the figures from the generator manufacturer.

What puzzles me is that we're having lower PF than the one stated by the manufacturer WITH the PFC on...

 

[ebarba]

I think that my initial question could be condensed to: can a Power Factor Controller worsen the PF of a motor/generator?

The PFC in question is 100% capacitive (i.e. a bank of capacitors) and static (no automatic variations, you either connect or disconnect it).

If the answer is "no", then we are measuring wrongly, the PFC is damaged or there are other issues in the line
If the answer is "yes", then we should get the plate PF just by disconnecting the PFC.

 

[davidbeach]

You're at 54% load, not 100% load. That means you have fewer kW in the equation but a nearly constant amount of kVAr. The nameplate pf value is for a single point, nameplate real power. Consider what happens if you have the machine at exactly synchronous speed (assuming that's possible) and you have no Watts in or out; you'll still have somewhere around 60 kVAr of reactive current.

Measuring between the generator terminals and the PFC all you'll see is what the generator does, there's no way to tell whether the PFC is there or not. Move your measurement point to between the PFC and the meter; there you should see a significant difference between PFC on and PFC off.

 

[ebarba]

The curve provided by the manufacturer shows we should have a PF=0.72 @ 54 kW and no PFC. Why is the PF so low? (see attached curves).

Just found something: we requested a PFC rated 35 kVAR but in reality it was delivered one rated rated 37.5 kVAR. According to the manufacturer, the generator needs 34.9 kVAR at 50 kW. Is all this situation due to the PF being overcorrected?

 

[davidbeach]

Are you precisely at nameplate voltage? Should be slightly less than .7, is about .63. Is that really a problem?

The generator doesn't know, nor care, where the needed VArs are coming from. Measuring between the generator and the PFC you just have the generator, the PFC might as well not exist. Move your measurement point to the meter side of the PFC connection and forget about what's happening between the motor and the PFC. Do you have a suitable power factor at the meter? If so, you're done, project completed.

 

[crshears]

Hello ebarba,

You wrote:

"crshears, the current config is your first one. Being the PFC connected in parallel, isn't it possible to assume that the whole line between the generator and the meter has the same characteristics? I mean, isn't the effect of the capacitors going to be "felt" the same in all the line?"

To which I answer: um, no, it won't; the PFC should be connected shunt, that is, across the generator windings. Only a capacitor connected in series with the generator windings would force the current and voltage throughout the circuit to have the same angular displacement throughout.

Re-wording/anagramming what davidbeach wrote:

Try to visualize it like the letter T, with the generator at the left end of the cross-bar, the utility supply at the right end of the crossbar, and the capacitor at the bottom of the pendant line.

The power factor when measured at the generator terminals will be very poor in the inductive direction [current lagging the voltage] when the unit is close to zero real power output [think kW/kX]; and since the amount of reactive power drawn is more or less constant, as the unit is loaded up, meaning producing more real power, kW/kX will rise, meaning the power factor of the generator will "improve," despite the fact that the generator is absorbing virtually the same amount of reactive power as before. [Ironically, at zero real load the power factor will be poorest, whereas the power factor will "improve" either side of that, meaning whether the unit is fully loaded - maximum energy input - or motoring - no energy input at all, meaning drawing real power from the grid.]

Bottom line: the power factor of the generator will change with the loading of the unit.

The behaviour of the capacitor is fundamentally different; it is connected across the line voltage source, and its current will lead its voltage by almost 90 degrees, regardless of what happens to the generator. Hence the capacitor has, and always will have, an almost purely leading power factor.

The amount of reactive drawn from the system to which the generator is connected will thus be the vectorial sum of the reactive demand of the generator minus the reactive supply of the capacitor. If the cap is too small, there will always be a net draw of reactive power from the system, whereas if the capacitor produces more reactive than the generator can absorb, the balance will flow out to the system. The goal should therefore be to meet the "Goldilocks" factor, erring on the smaller side.

As I have mentioned in previous posts, the fact that real power can be controlled by means of throttle valves / fuel rack / wicket gates etc. and reactive power can be controlled by means of excitation adjustments / thyristor firing rates etc. is the reason why my utility has almost never bothered installing power factor meters anywhere, much preferring to directly measure the real and reactive power flows through individual pieces of equipment by means of wattmeters and VARmeters rather than by means of the more derivative power factor meter.

Hope this helps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ebarba]

OK, now I got why measuring in the generator side is not useful. My mistake was not reasoning fully that the two plates of each capacitor bank had no other possibility but to be connected ACROSS the windings.

On the other hand, I reasoned that a 10% power variation during testing was possible and the real and reactive power readings are in different pages of the display. If a 10% or so variation occurred when going from a page to another, all the mystery about a higher VAR reading than expected can be explained.

The original cause of concern (from the original thread) was that the utility was charging a lot for the VAR's and the plant seemed to be drawing lots and lots of them. We'll measure at the meter side and see what's going on.

We have to wait for some problems in the prime mover engine to be solved and then we'll make a test accordingly.

Thanks for all your help, will let you know.