Hi Folks,

I m a summer student working at a hydro generation company in North America.

I am missing a linchpin in my understanding of how a generator puts power on a grid. I have to confess, I am not sure what exactly what I am asking for.

Because in my most basic understanding: Voltage needs to be higher at your end to pump current (thus power) to the other end.

while it is very easy to understand the open circuit voltage of a generator that is off the grid: The windings on the stator are cutting the M lines of an rotating and excited rotor, a motive/tendency/voltage is created in the stator windings propelling charges to flow. But, since the endings are open, no charges are flowing in the stator, So the rotor is not experiencing any counter force from the stator at all.

But I have huge troubles of understanding once the current of the grid starts flowing in the stator, and the voltage of the generator (after Xs and Rs and the transformer) matches that of the grid.

Once you have the same current flowing in the stator from the grid and that same current flowing out of the stator back to the grid; And a matching voltage (to me it means no current should flow between the same potential at all), are we generating power or are we moving like a motor? or when the rotor is not even excited or rotating, the whole stator is just an big stationary resistor and inductor.

My understanding so far is that: 3 phase grid current in the stator winding will be manifested (as you add 3 phases of sinusoidal up as a function of angle and time) as a rotating magnetic field. the rotor as a magnet needs to try to rotate faster than that stator's field to experience a "drag force", and that drag force was neutralized by the water rotating the turbine. This understanding is only good "energy conservation" wise. It does not explain the voltage/current/power confusion I mentioned above.

Since we are here,
two more questions:

[1]
the load of the grid is really the amount of current flowing on it right?

[2]
is it possible for the grid current to be so low and your wicket gate to be opened so wide to have the rotor spinning faster than "60hz" (as in 1.5 hz for a 90 pole machine)

 

Hi Ugene,

And welcome to the power generation industry! Even if it's only for a summer. I presume you are working with large synchronous generators, and not asynchronous (induction) generators, and my discussion below is for synchronous generators--not asynchronous (induction) generators.

I've read and re-read your post several times and it seems you are reading too much into the whole process, and confusing the various EMFs and such. If you want help with understanding armature reaction and EMF and counter-EMF, please contact one of the frequent contributors to control.com, a Mr. Phil Corso, at ([email protected]). You will need to provide your name and affiliation (school; company you are interning at) and in turn he will be happy to educate you about all the "backroom" stuff going on with generators and generator design and theory.

But, basically, generators are devices for converting torque into amperes. Motors are devices for converting amperes into torque. That's what electricity does and is for: To transmit torque over long distances and distribute it to many places where it's needed--all using wires, and transformers. In fact some of the largest early generating stations were hydro-electric units, located long distances from the factories and mills where torque was needed. (Remember a lot of early factories and mills were located next to streams where water wheels were used to produce torque for the processes.)So, the prime movers driving the generators (in your case: a hydro turbine) are actually doing the work of some of the motors and lights and computers and computer monitors connected to the grid. Many generators--and their prime movers--connected together (synchronized) are all acting as one large generator powering one large load (all of the motors and lights and computers and computer monitors.)

A synchronous generator, once synchronized--that's a VERY important word and term and concept to understand--spins at a speed that is directly proportional to the grid frequency. The formula that relates speed and frequency is:

F = (P * N) / 120

Where F = Frequency (Hz)
P = Number of poles of generator rotor
N = Speed of generator rotor (RPM)

Magnetism won't let it spin any faster or slower. Once synchronized, the generator rotor's magnetic field is "captured"/"locked to" the magnetic field of the generator stator that appears to rotating (due to it's alternating nature). If more torque is applied to the generator rotor by the prime mover (the hydro turbine in your case) to try to spin it faster the generator converts that torque into amperes. If, however, the amount of torque being provided by the prime mover to the generator rotor is less than that required to keep the rotor spinning at the same frequency as the grid then amperes will flow from the grid--from other generators and their prime movers--to the generator to keep it spinning at synchronous speed. The generator is said to be "motorized" at that point/time. But, when that generator breaker is closed that rotor doesn't spin any faster or slower than dictated by the above formula.

(Caveat: When more torque is applied to the generator rotor by the prime mover there is an instantaneous change in speed which increases the load angle but, again, there is no appreciable long-term (even half a second!) change in speed--it's fixed by the frequency of the grid (since the number of poles of the generator rotor is fixed and can't be varied).

You are very wise to comprehend that the AC (Alternating Current) amperes flowing in the generator stator when it's providing current to the grid are flowing back and forth. It's like a hydraulic pump and a hydraulic motor--the hydraulic fluid is the medium by which the motor driving the hydraulic pump turns the hydraulic motor driving the load--and the fluid is recirculated back to the hydraulic pump suction to be pressurized again to keep the torque transfer going. It's virtually the same in electric systems, it's just that the medium is amperes not hydraulic fluid.

Generator terminal voltage is a function of two things (basically): generator rotor speed, and excitation. (Excitation is the electric power applied to the rotating magnetic coils of the generator rotor to produce the magnetic fields that "pass by" the conductors in the stator. More excitation, stronger rotating magnetic field; less excitation, weaker rotating magnetic field.) As was said before, when a generator is synchronized to the grid it's speed is fixed by grid frequency so the only way to change generator terminal voltage is to change excitation.

When a synchronous generator is synchronized to the grid (it's spinning at a constant speed) and the amount of excitation is exactly equal to what is required to make the generator terminal voltage exactly equal to the grid voltage then zero reactive current will be flowing in the generator stator windings. It will be pure power--watts--with no reactive component. If the excitation is increased from this state in an attempt to try to raise the generator terminal voltage--and the grid voltage, since the generator is synchronized to the grid with other generators!--then lagging reactive current will flow in the generator stator windings along with watts. That reactive current is said to be VArs, and there are leading and lagging VArs.

If the amount of excitation is exactly equal to what is required to make the generator terminal voltage exactly equal to the grid voltage then zero reactive current will be flowing in the generator stator windings. It will be pure power--watts--with no reactive component. If the amount of excitation is decreased from this state in an attempt to try to lower the generator terminal voltage--and the grid voltage, since the generator is synchronized to the grid with other generators!--then leading reactive current will flow in the generator stator windings along with watts.

In general, a small amount of reactive current (VArs) flowing in the generator stator windings isn't a big problem. And, most large hydro units are even built to produce a lot of lagging reactive current (VArs).

So, you see, once the synchronous generator is synchronized to the grid "extra" voltage isn't required to produce power. If the excitation (the variable that changes voltage) is held at the level that keeps the generator terminal voltage exactly equal to the voltage of the grid with which the generator is synchronized as the torque being applied to the generator rotor changes (increase or decreases) then zero reactive current will flow in the generator stator windings--but the generator can still produce lots of watts (real, pure power). Without making the generator voltage higher than grid voltage.

One of my early university instructors used to say, "Voltage doesn't do anything--unless you put in in a Volkswagen and push the Volkswagen down the street." It's current that "does" something--amperes allow the generator to do the work of electric motors and lights and computers and computer monitors. And, all of this work is done at relatively constant voltage. To return to the hydraulic system analogy, hydraulic pressure is akin to volts and hydraulic flow-rate is akin to amperes. If the hydraulic motor can't produce the flow-rate required by the hydraulic motor even though it can increase the pressure of the hydraulic fluid the work to be done by the hydraulic motor won't be what's required by the process.

Don't try to make every aspect of the hydraulic analogy work for understanding electric systems--it doesn't. Just certain aspects work, and they work very well in most cases.

A certain amount of voltage is required to even begin producing amperes--the equation for three-phase electric power (watts) is:

P = Vt * Ia * (3^0.5) * pf

Where P = Watts
Vt = Generator Terminal Voltage
Ia = Generator Stator Current
(3^0.5) = Square root of 3
pf = power factor

Power factor is a measure of how much real power (watts) is being produced for the total power being applied to the machine. It's never greater than 1.0, and as more reactive current flows in the generator stator winding the power factor decreases from 1.0, which means the machine is less efficient at converting energy to real power (watts).

wikipedia.org is a great place for learning about electric power generation fundamentals. A Canadian website, canteach.candu.org, is also a very good site for lots of electric power generation fundamentals.

Hope this helps! There are lots of maths and vectors and egg-head, ivory tower stuff that goes on between the two magnetic fields--but the basic gist of what happens in a generator is described above. Remember: Voltage doesn't do anything.... Well, almost nothing; it's amperes. From the power formula and the knowledge that generator terminal voltage doesn't vary by much during synchronized operation (it's designed to operate at a rated voltage--listed on the nameplate--and operation much higher or much lower is not generally very healthy for the generator) the only variable (assuming the power factor remains constant) in the equation is Ia, armature current. You want to make more real power (watts), you provide more torque. Again, because for all intents and purposes the generator terminal voltage remains stable, and in a small range (for most synchronous generators, that's +-5% of the nameplate voltage rating--which means that the power output of the generator can't be changed by more than approximately +/-5% of rated, which ain't much).

Ask any power plant operator (one who's been working there for a couple of decades), "How do you change the load on a turbine-generator set: by changing the position of the wicket (the amount of water flowing through the hydro turbine), or by changing the voltage (excitation)?" And, any operator worth his/her salt will say, "Why, young man, with the wicket gate, of course." (Don't ask him/her to explain why; they just know that when they are dispatched to change load they do so by changing the amount of water flowing through the hydro turbine by changing the wicket gate position. Amazing, but true--that they don't know why what they do does what it does, just that that's how they do it--because, "That's how we've always done it!")

This should get you started. The aforementioned frequent contributor will chime in now that someone else has responded to your original post. Take your discussion with him off-line (by contacting him with your email and personal info); he's knowledgeable if you want to be able to know how to design a generator or the maths behind everything what goes on in a generator. Which sometimes doesn't help someone who's new to the field and just needs to get some basic fundamentals down, in order to be able to relate to the maths and the egg-head/ivory tower stuff. But, if you want the egg-head/ivory tower stuff, Mr. Corso's your man!)

Best of luck! Enjoy your summer!