What may happen to an alternator operated other than 0.8 P.F

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T.M.Haja Sahib

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In most of the alternator nameplates,the power factor is given as 0.8.I think its exciter is likely to be overloaded if the alternator is operated below 0.8 P.F and if it is operated above 0.8 and up to unity P.F,there may not be any problem to its exciter.Your inputs please.
 
Generator alternators are typically designed such that the steady state operating region is fully capable between 0.8 and 1.0 power factors. A leading power factor is to be avoided, as this can easily cause pole slip instability and generator overspeed. A low (lagging) power factor should also be avoided, as the rotor damage curve falls below the rated power output as you go below 0.8 power factor.

This is all highly dependent on the alternator synchronous reactance, where a lower value will result in higher tolerance to any of the above conditions.
 
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Look at a generator capability diagram. The generator is rated in kVA, usually expresed as rated kW at a power factor. A 10 kW 0.8 pf generator is rated for 12.5 kVA, lagging power factor.

Between 0.8 and 1.0 power factor the KVA rating is constant. (Think constant armature current). The engine or turbine may not be able to put out more kW, but the generator could handle 12.5 kW at 1.0 power factor.

Below 0.8 power factor on the capability curve, rotor and exciter heating limit the output, so total KVA capability drops below the 12.5 KVA.

What if we ran the generator at a constant KVAR output? 0.8 pf 10 kW, 12.5 kVA is 7.5 kVAR. The exciter and field are designed to supply 7.5 kVAR. Now drop the kW output to 50 Watts, 0.05 kW at the same 7.5 kVAR. KVA is = sqrt(0.05^2 + 7.5^2) = 7.5001 kVA. Power factor is = 0.05/7.5 = 0.007.

Nothing is overloaded even though our power factor is well below the rating point's 0.8.

Conclusion, operating at power factor less than the machine rating point is not a problem as long as the operating point does not exceed the generator's capability curve.
 
The alternator name plate details refer to full load conditions.So if the power factor of a fully loaded alternator is reduced below 0.8, keeping the KVA at its rated capacity,the exciter of the alternator is likely to be overloaded.
If the generator capability diagram is not readily available,the safe loading of the generator may be carried out this way. Say the alternator capacity is 30KVA at 0.8P.F. Its kW rating is 24kW and KVAR rating is 18 KVAR. This KVAR rating should not be exceeded.(courtesy by rcwilson) So,for this alternator,a load of 27KVA at 0.7 PF is permissible?No,because then kW=19 and KVAR=19. Is a load of 20 KVA at 0.7 PF permissible? Yes. Because kW=14 and KVAR=14.
Similarly for alternators of other sizes.
 
Further to post#4:

However the remark by a member of another forum complicates the matter:

''Most generators that I am aware of have an operating curve of their limits and they can be operated at zero power factor to 1.00 power factor. For instance at zero power factor our 1,037,000 KVA 0.8 P.F. generator is limited to about 450,000 KVAR leading(0.00 PF or a synchronous condenser) and about 700,000 KVAR lagging (0.00 PF). Most large power generators operate at 0.95 to 0.99 pf. The nameplate is stated at 0.8 pf but we mostly try to keep it about 0.97 PF. As long as you stay within the operating curve supplied by the manufacturer you should be okay. The 0.8 pf is just a nice average due to the fact that most of the electrical power is used by electric motors.''
 
rcwilson said:
Look at a generator capability diagram. The generator is rated in kVA, usually expresed as rated kW at a power factor. A 10 kW 0.8 pf generator is rated for 12.5 kVA, lagging power factor.
Click to expand...
Although it isn't mainstream business for us we make static exciter panels alternators. Some of these were for rotary frequency converters. The drive motor rating was given in kW and the alternator in kVA.
In fact, the only "generator" I've seen was one driven by a water turbine and we put in an asynchronous motor operating in the supersynchronous region. It's grid connected and never runs in stand alone mode. When we first put it in, we used a wound rotor machine with resistors in the rotor to give a soft transfer. Now it is a cage motor driven at about 1515 rpm. (50Hz here).
 
My comment above about keeping KVAR's constant was only to demonstrate that it is permissible to operate a generator at low power factor.

If the exciter/generator/field can produce 17 kVAr at 0.8 power factor they can certainly do the same at low power factors. Actually, the generator reactive capability goes up as the kW loading goes down. I just was holding it constant to prove the point that low power factor operation is possible and that the 0.8 pf on the nameplate is not a power factor limit, just a rating point.

Most generator capability curves graph kW on the horizontal axis and kVAr on the vertical axis. The fictional 12.5 kVA ,0.8 pf generator's curve will be a circle arc of constant radius starting at 12.5 kW, 0 kVAR and going up to 10 kW, 7.5 kVAR, the full load 0.8 power factor point. The circle radius gets shortened from that point over to the vertical axis (0 kW). It is not a straight horizontal line = to constant kVAR's as my example may have implied. The curve goes up, indicating the generator can produce more KVAR's at lighter loads. The limit is the rotor field current heating limit or exciter heating.

(I don't have time to upload a curve. Do a search and look at some of the fine examples on the internet. If the curve is drawn by someone in the IEC world, lagging power factor KVARS out of generator, is drawn as negative below the axis, instead of on top as we do in ANSI land. Don't get confused.)
 
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