Diagnosing a power factor problem.

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