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Diagnosing a power factor problem.

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I hope this does not fall under the heading of "commercial" as I am a Maintenance Supervisor on an offshore drilling rig, if so, I do appologise.

I have recently joined this rig and I find that we have a serious Power factor problem with our main generators.

We are running four 2000 KW, 600 volt, 60 Hz, 3 phase generators and usually three out of the four will be running in paralell on a common 600 volt buss.

Apart from a hotel load of about 800 KW, most of the demand on these generators is from a group of high current SCR rectifier units that power large variable speed DC motors.

The output of each of the SCR units is 0- 750 volts DC, at 0-1500 amps.

I find that with no SCR units assigned, I have a normal buss voltage of 600 volts and a power factor of 0.8 However, as soon as any of the SCR units start loading up, the power factor drops to 0.5 and the buss volts drops to 580 volts.

Could you explain to me how the drop in the power factor causes the voltage to be unsustainable by the voltage regulators and what is the likely cause of this chronically low PF.


Yours sincerely,

Terence Smart

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Hi Terence,


I suspect the poor power factor you observe is only a symptom, not a cause of the Voltage drop you have noted.


From your description, you seem to have in excess of 50% of your total load being non-linear in nature, ie, rectifiers - DC drives. Such loads likely draw non sinusoidal current, as harmonics. The result being 'distortion power factor' - the 'other type of power factor', see previous posts on this forum - and as the generators are exposed to a high percentage total harmonic current distortion, the AVRs cannot regulate as you may expect.


A high percentage of harmonic current distortion can cause bad effects by names such as 'negative phase sequences'. Such effects can result in excessive generator prime mover loading. A quick check for this effect is to compare prime mover mechanical power to generator output true electrical power. A discrepancy between the two, not accounted for by basic system inefficiency, could be cause for concern. By the way, a simple way of determining average prime mover power is to measure fuel flow over time, again allowing for inefficiencies.


It seems to me that 20V drop on a 600V buss (3%) should not be too problematic though? Of more concern may be the total Voltage harmonic distortion, ie, power quality, which may effect the entire installation.


I don't know of any simple solution to the above (assuming I'm correct, that is!).


Hope this helps.





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Hello Terence


I think that Bigmax has given you a very good answer. The problem is harmonic related with the poor powerfactor being due to the high level of harmonics.

You will probably find that the KVA loading is high due to the poor power factor and the output voltage is sagging because of the high loading and the harmonic content. Some AVRs are better suited to operating under high harmonics than others.

Basically, there are two major forms of AVR, three phase average reading and single phase peak reading. The three phase average reading is much less affected by the presence of high harmonic currents.


Best regards,

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