Generators and Power Factor

Hello Sapper

The PF of the load varies, and more often the connected load is inductive (lagging current) rather than capacitive (leading current).

Because most uses of an alternator are going to have transformers, motors and other inductive loads connected, the alternator is rated at the PF 0.8 figure, as that is a "normal" status of most alternators.

A very rare load for any alternator would be one which is purely resistive, and of course there is still the alternator winding inductance itself to be taken into the calculation.

Most alternators are designed to cope well at the 80% figure above without any bother, but as overload is approached, most alternator control systems lose control of the alternator more and more easily.

Thus the 80% in kW of the 100% kVA rating is quoted as a safe working standard for alternators.

In cases of emergency, the 80% rating may be exceeded, but for a very short time only, as if damage occurs during such a period, the cost is quite extensive and expensive = a rewind job.

If a user regularly is needing a larger output than the 80% rating, it is more prudent to purchase a larger alternator unit.

I have also seen quite large crankshafts broken in the diesel motors of gensets, because of sudden overloads, and this means the genset may be out of action for many months, waiting replacement crankshaft + bearings and more.

Power transformers are similarly rated, with the normal effective kW being some 80% of the rated kVA rating on the nameplate.

Kind Regards....

There are many people here that I am sure will answer this better than I but I dealt with gens and gen controls for quite a while so I'll try and help.

As I understand it, Generators generate Voltage and supply current in phase.

Only in theory on paper. Yes, if one hooks a purely resistive load to a generator you get a unity pf of 1. But reality is no. When you hook up a big resistor bank the cables you hook it up with will have an amount of capacitance and will also have an amount of inductive properties even if you are careful and have no coils rolled over at all. I've seen .995 or so on both sides of unity with 1MW resistive load banks. You look twice then realize your error and run to find out why the load bank fans aren't going.

Generators are rated at PF=0.8 lagging meaning that they can handle loads down to that Power Factor before stalling or going offline.

No...never heard that one before. An overloaded generator is just that ... overloaded. The engine is going to its knees and is about to get tripped off by protective relaying like under freq., overcurrent, undervoltage. If a 480v gen rated 1500kW/1875kVA is humming along at say...1500amps @ 0.8pf then a dip to 0.7 or 0.6 is not going to knock the unit off as there's plenty of kVA headroom. Now...bear in mind in multiple unit paralleling to the grid you'll usually find var/power factor controllers and associated protective devices for this application but this is designed by the engineer for system operation and protection, not because the generator can't handle it.

There are other issues of wasted energy, eddy currents and rotor heating from power factor being to far out there.

I also understand that they can handle slightly leading Power Factors (PF=0.95-1.0) and after that the Generator controller will shutdown due to overspeed.

Nope, never ran into that one either. Overspeed is a function of throttle control and usually occurs when a significant load is slammed on and the generator responds by going balls out and then ...just as copious amounts of beautiful black smoke is pouring forth...the load trips off. If the gen governor gain and damping aren't set to handle this the governor will not respond fast enough to back off fuel and then over rev.

ELI the ICE man... old industry saying. Purely resistive is unity or 1.0. eli the ice man is easy to remember which side of unity you are on if inductive or capacitive. I've seen plenty of big data centers that were running .65pf leading and added synchronous motors to bring the pf back up towards unity.

Which in the real world means that a 1000kVA Generator can Generate 800kW or better lagging and 950kW or better leading.

No...it's the same leading or lagging it's still (IE)(pf)

Yes in theory at .99 you get 990kW.. but it's the reality that kills. It can be really expensive to add enough cap banks to pull back towards unity. An engineer has to try to meet a happy medium between different expenses.

I've seen some wild whacked pf. I once did a job out at a new gold mine in Nevada where they were to operate two huge P&H shovels. They put in 3 paralleled Caterpillar 3516 gens at 2000kW each for 6MW. We built the switchgear and tied the gens to the grid which was a long skinny line up a little desert valley. One of the wildest things I ever seen. They put the gens in to hold up the line while the shovels ran and then to absorb the regenerative power when the shovels stopped. Those 3 gens literally would slam from .2 one direction to point 2 the other. Shovel would go...wham .2pf and i gotta hold all 3 together and on the grid while shoving so hard to push the pf back up... then slammo...shovel stops and we were using the gens to literally absorb....motor the power off the line. The gens would literally slam back and forth on the frames. They parked a welder in the compound because the peckerhead frames broke so often and they stuck a box of 100 diodes at each gen because they would frag out diodes several times a day. The mine ran like that for 6 months while the utility was running a 235kV dedicated line to them. So..point being... I know the abuse these modern gens will take.

Hope this does help some... also you can do some chasing with good ole Google. Lots of very good info regarding inductive and capacitive loads. Go to Basler and perhaps Woodward and read some of their generator control white papers.