Synching Feeling 5

[marada93]

What is it that hold a generator in synch with the rest of the grid. My way of thinking, the magnetic field produced by the generator must be of opposite polarity as the magnetic field of the grid...so they attract each other.

Is that close...? Or should I go ahead and take that fuel handler job they just posted...

 

[Jensgisla]

marada93: youre understanding is quite accurate. In fact the grid's alternating voltage uses the stator winding to produce a rotating magnetic field in the airgap of the machine. (you could say this was the "magnetic field of the grid"). The rotor has it's own field that is "attached" to the rotating stator field. If you use some kind of a prime mover to put mechanical torque on the rotor, the rotor field will "pull" the stator filed, and energy is coupled from the prime mover to the stator winding and the grid, youre machine works as a generator. If you use load to put mechanical torque on the rotor, the energy is coupled from the stator to the load, youre machine works as a motor.

cheers

 

[buzzp]

The thinking is the grid is so powerful that virtually no generator has the capability to operate out of synch with the grid - it either gets pulled in sync, damaging the generator and switchgear, or follows the grid. If your a large producer then you may have the ability to mess with this 'infinite bus'. In the west, the infinite bus is considered to be Grand Coulee.

 

[marada93]

Thankya to both Jensgisla & buzzp!

AS a CRO, I know how to make the turbine generator operate...I'm just wanting a better understanding of why it works that way.

I reckon it follows then, that the grids alternating voltage would tend to oppose allowing the generator speed to either slow OR speed up (to a point). If you increase the torque of the prime mover without a corresponding strenghtening of the generators magnetic field, it is possible then to overpower the the grid. Is that correct?

Or...if you reduce the prime mover, you would need to decrease excitation as well.

I sorta get this. I was always told the prime mover directly controls MW load, while excitation controls voltage...but I never could see how this happened independently. It seems to me that while this may be true, one without the other doesn't go too far.

Let me try to follow what I think I think...

If you increase the prime mover in order to increase generator load, then the corresponding excitation needs to be increased to maintain proper voltage. The strengthened magnetic field then has more lines of flux cutting thru the stator winding...Is this why load actually increases?

Thanks in advance for helping me out. If my questions seem a bit novice, I apologize in advance.

 

[davidbeach]

The two are more independent of each other than that. As you increase the power to the shaft, the generator moves forward relative to the grid, increasing the power angle delta. Power transmitted from the generator to the system is the product of the absolute values of the generator internal voltage and the system voltage divided by the reactive impedance between the two voltages, all multiplied by the sin of delta. So, with constant voltages, more power means a higher delta. All power comes from the shaft, the field can not produce power.

With the shaft supplying a constant power, changing the internal voltage produced by the generator will change the power angle, but will not change the amount of power.

The amount of reactive power is the product of the magnitudes of the two voltages divided by the reactive impedance, multiplied by the cos of delta, all that minus the square of the magnitude of the system voltage divided by the reactive impedance. Increasing the internally generated voltage (increasing the field current) increase the reactive power delivered by increasing the product of the magnitudes of the voltages. In response to that increase the power angle reduces so that the real power output remains constant (controlled by the power in at the shaft). The reduction in power angle increases the cos value, so reactive power output goes up with both the increase in generated voltage magnitude and with the decreased power angel.

If someone else knows how to put the equations in here so they show up well formatted, that would be great.

 

[alehman]

Basically the load power is proportional to the shaft torque. You adjust the excitation to change the reactive power output. A generator can be operated to maintain constant power factor (ratio of real power to KVA). In this mode, the excitation must be increased as the shaft power is increased to keep the power factor constant. The generator can also be operated to maintain constant reactive power output. In this mode, basically the excitation is only adjusted as needed to compensate for variations in the utility voltage to maintain constant reactive power.

This article explains operation of a network connected geneartor fairly well.

http://basler.com/php/download.php?file=parallel_op.pdf&id=38&type=res

 

[marada93]

Thanks for the additional information. The web site article was quite helpful, though it'll take me awhile to digest some of it. Good examples of the physics involved, and will no doubt lead me to post more questions in the future. I'm very much impressed with the expertise demonstrated in response to my post!