Situation:
Generator produced an output as 500MW where the load demand are same as 500MW (sych into grid).
If the load suddenly increase up to 800MW or decreased down to 300MW (value is just for the example-assume that the grid is small/weak). What will happen to the generator?

Question:
1) Can you please help me to brief this phenomena which is related with the frequency? - with formula, better. I can't see the relationship between Watt, Hertz and Governor when the situation above happened. Those 3 elements struggling me.
additional where; F=(P*N)/120 and P=(3^.5) VI cos phi.

2) And what is happening to the excitation, when Watt increased/decreased? Is the excitation should act to support with VAr action, because the power factor must maintain at 0.9. If yes, How it works?

Thanks with Regard

 

Huzairi,

Self-challenge is always good.

To answer the question in your 'Situation' it would be necessary to know how much power the generator is capable of producing.

Let me give you an example. You are pedaling a bicycle on a long flat road and you are carrying some packages. You have been told to maintain a constant speed on your journey and are applying the proper amount of torque to the pedals/crankshaft to maintain the desired speed while carrying the packages.

Suddenly, the package weight increases by 60% (as per your Situation). Would you be able to maintain the same speed with the additional weight? Perhaps; perhaps not. It depends on your strength and stamina. You might be able to propel the additional weight at the desired speed for a short distance. It depends on your ability ("rating"). Also, it would depend on the strength of the pedals/crankshaft and of the chain being used to transmit the torque to the rear wheel.

Generator prime movers usually have "limiters" to prevent overloading the generator much past its nameplate rating to protect against damaging the prime mover, the coupling between the prime mover, and/or the generator.

Now, let's say the package weight suddenly decreased from the original package weight to approximately 60% of the original weight. If you did nothing to decrease the amount of torque you were applying to the pedals/crankshaft the speed would increase, and you would need to decrease the amount of torque in order to maintain the desired speed with the reduced package weight.

It's EXACTLY the same for an electrical generator--really for the prime mover driving the electrical generator. The electrical generator is just a device for converting torque into amperes. And at the other ends of the wires connected to the generator (the "grid") are electric motors and lights and computers and computer monitors that convert the amperes back into torque or some other useful work (lights, and 1's and 0's in the case of computers--what I like to refer to as "virtual torque").

That's why we produce electricity in the first place: to be able to easily transmit torque (in the form of amperes via wires) to far-flung locations where torque can be easily produced to other locations where torque is required or desired.

A generator's ability to produce electric power is really a function of two things: The amount of torque that the prime mover driving the generator can provide. And, two, the ability of the generator to be cooled (since amperes flowing in conductors also produces heat, the generators need to have some method of cooling in order to be able to produce power without suffering damage caused by heat--burnt insulation, etc.).

The more heat which can be removed from a generator over a sustained period the more torque the generator can generally convert to amperes over a sustained period (presuming the coupling between the generator and the prime mover can handle the torque load).

As far as answering your two questions, the first formula only relates speed and frequency--which are very important. Just like in the bicycle example above, if you were carrying zero packages it would still require some effort (torque) on your part to maintain a constant bicycle speed while traveling on a flat road. Generators operate at a specific speed (constant frequency) and there is a certain amount of torque required just to achieve and maintain that speed (frequency) even if there is no electric load.

The second formula is just a method for calculating three-phase power using generator terminal voltage, generator stator current (real current, not reactive current) and knowing the ratio of reactive current to real current.

The formula you are "missing" is the one that converts electric power (Watts) to real power (usually horsepower, or whatever units you use in your part of the world). For horsepower ("real work"), 1 HP = 746 Watts. Again, generators are devices for converting torque (from the prime mover driving the generator) to amperes.

Electric motors and lights and computers and computer monitors are devices for converting amperes back into some "useful" work at some remote location from where the torque is available or produced.

The second question is a little more difficult to answer. What happens in a generator when power is being produced can be researched using your preferred Internet search engine. (Look for topics like "counter emf" and "back emf" and "load angle" and "armature reaction" and such.) But, basically as amperes flowing in the generator stator increase (as the power being produced by the prime mover and generator is increased) this has the net effect of decreasing the generator terminal voltage, so the excitation must also be slightly increased in order to maintain the generator terminal voltage at the desired value in order to hold the power factor or Var setpoint constant. (The exact mechanism by which generator terminal voltage decreases as generator stator current increases is very complicated and involves a lot of maths and vectors and formulae, which can all be found on the World Wide Web.

What I have described is the net effect of what happens when generator stator current changes--as the result of changing the torque being applied to the generator rotor by the prime mover. Again, the exact "mechanics" is not very easy to describe without being able to draw pictures and without providing a lot of maths which is easily available elsewhere on the Web--but the net effect is still the same. As the torque being applied to the generator rotor by the prime mover changes, if it is desired to maintain the same power factor- or VAr setpoint then the excitation must also change slightly in order.

It's important to note that generator terminal voltage is usually a relatively constant value. In fact, most syncyhronous generators (and their excitation systems) are rated for no more than plus or minus 5% of rated terminal voltage. So, by looking at your second formula if we presume the 'V' value does not change by more than approximately 5%, and if the value of 'phi' remains constant, the only way to change 'P' is to change 'I'--which is the generator stator current.

And generator stator current is directly proportional to the torque being applied to the generator rotor by the prime mover. Increase the torque and you will increase the generator stator current (amperes). Decrease the torque and you will decrease the generator stator current (amperes).

Perhaps other respondents will provide more information. These topics have been covered MANY times in other threads on control.com--some more informative than others, I'll admit. There is a 'Search' field at the far right of the Menu bar of every control.com webpage. (I suggest using the Search 'Help' before beginning; search terms and context are not exactly like most Internet search engines.)

Hope this helps!

Do some research, and if you have more questions we'll try to help!