Utility Powerhouse Generators 1

[peetey]

I was wondering, are the large generators that utilities use to generate power, synchronous type? I was wondering how they produce a pure sine wave and don't translate a power factor reflective of a wound motor turned into a generator, say a power factor of 30 degrees leading .8660 PF. When I do load checks on large motors and they are no longer using power but are still rotating they generate power and it seems to be the angle of the load (30 degrees) plus 180 degrees due to Current transformers. This equals 210 degrees of delivered power with a power factor of .8660 so how does a generator at a utility produce a power factor of 1.0? Peetey

 

[alehman]

Utility generators are almost all synchronous type. Synchronous generators and motors have externally powered fields which can be controlled to adjust the reactive power flow. As the field is increased, more reactive power is available. Once increased past the level required to make up the machine's own reative power needs, surplus reactive power is available to export to the system. At that point the machine is operating at leading power factor.

Your measurements are most likely on induction motors. Induction motors by definition have self-induced fields which always require an external source of reactive power. That means they operate with lagging power factor.

http://www.itee.uq.edu.au/~sansom/lecture7_syn_mach_big.pdf

You may wish to find a text book on electric machines to get a better explanation.

 

[mc5w]

You also get into the problem that reactive power for a larger induction motor varies from no load to full load.

On one machine that I maintained for the better part of 5 years the no load current of a 60 Horsepower motor was 30% of full load current. This would imply that no load reactive power is 50% of full load reactive power.

One explanation is that half the reative power contributes to production of countervoltage and the other half helps push power across the air gap in order to produce torque. There are more complicated models and explanations but that is the simplest.

Mike Cole

 

[busbar]

 
On a related note, there are four-quadrant definitions and “polarities” assigned to power flow in AC systems, based on combinations of real and reactive power. {Harmonic quantities add another ‘dimension’ to the diagram.}

 

[peetey]

Hey I really appreciate your comments. I have one more question about this topic. What is the voltage produced at the generator before it is stepped up to transmissions voltage? I had a lineman ask me this question and I didn't have an answer. Hey Busbar, that diagram looks awfully familiar, although very relevant to the conventions of powerflow. Peetey

 

[byrdj]

There is a post below on "VAR tripping" that states his CT generators are 11KV. The data sheet for a ATB2-620MW i'm looking at is 22KV.

 

[busbar]

 
Peetey, SWAGwise, 60Hz stator voltage may be 13-25kV in synchronous generators. Note that this voltage, although in a similar range, usually does not correspond with local distribution voltage.