Induction Genarator Frequency Control - Wind Turbine 2

[hansforum]

Hi!

I would be grateful if someone can explain how is voltage frequency controlled (kept constant) in the squirrel cage induction generator driven through the gearbox by the turbine connected directly to the grid. I read that generated voltage and frequency are controlled by the grid, but I don't know how. I posted a picture for better understanding. Pic:

http://files.engineering.com/getfil...4a4c-976a-6dcfe98ceb5e&file=induction_gen.JPG

Thanks in advance!

 

[waross]

When the frequency is the same as the grid there is no power flow into or out of the grid. When the frequency is different the difference is the slip frequency. The higher the slip frequency, the more power is transferred. If the turbine frequency is below grid frequency power flow into the induction generator and it acts as a motor trying to speed up. If the frequency is above the grid frequency power flows into the grid. The amount of power depends on the impedance of the induction generator and the slip frequency.
Reactive power will flow from the grid into the induction generator to provide magnetizing current. With any generator, induction or synchronous, where the grid is large in relation to the generator, the grid controls the voltage. A voltage difference between the generator and the grid results in a flow of reactive current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

In a less technical form, think of two machines coupled together. Your generator is a machine, connected to another machine that is the grid. Let's say your machine is 1MW, then the "grid machine" is 1GW. If your machine tries to produce a higher frequency and thus drive the grid machine to a faster speed, it would need to exceed 1GW of power, otherwise it has no effect on the status of the larger machine. But in trying to do so, it will overload. Conversely if your machine produces a lower frequency than the grid machine, the grid machine will drive your machine as a motor load instead of a generator and the wind turbine becomes a useless fan. So the job of your speed control system is just to maintain a minimum speed to avoid becoming a load on the grid, and to maintain a maximum speed to keep it from attempting to drive the grid as a motor and over loading. At either end of that band width the protection systems will take your machine off line, so the rest of it is just minimal speed regulation to maximize production of power.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[hansforum]

Thank you very much for clarification, but I am confused now about the same thing but with the synchronous generator (one that doesn't use ac-dc-ac converter) that is directly connected to the grid. The grid is controlling the rotation frequency by strong rotating magnetic field and doesn't allow any change in rotation speed. As you said above in induction generator if frequency of the rotor goes above the grid frequency power is generated and if it goes under it acts as a motor, but is synchronous generator we don't have this. So under what conditions power is generated and under what conditions power is consumed. Is it because of the torque acting on the rotor, if turbine doesn't produce torque grid is rotating the rotor and theoretically no power is produced or consumed. If turbine starts to produce torque it wants to speed up the generator but the grid doesn't allow, than power is produced. Can someone clarify this!?

 

[IBRCAN]

If turbine starts to produce torque it wants to speed up the generator but the grid doesn't allow, than power is produced.

Exactly. The amount of torque applied to the rotor determines amount of real power the generator feeds to the grid. If no torque is applied to rotor, the generator is motored and consumes power.

 

[jraef]

The difference is only in that for an induction generator to work, the field excitation has to come from the grid connection itself, which then regulates the frequency, whereas the synchronous generator has separate excitation that can be supplied from any other source, such as a DC generator attached to the same shaft. But when the synchronous generator is in "island mode", meaning not connected to a grid, then speed regulation becomes more critical.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[cswilson]

With an asynchronous (induction) generator/motor, the power transfer is a function of the frequency difference between the grid and the rotor mechanics, as others have explained above. In the normal operating range, it is close to a linear function, with the power transfer passing through zero as the frequency difference passes through zero.

With a synchronous generator/motor, the power transfer is a function of the angle difference between the grid and the rotor mechanics. In the normal operating range, it is close to a linear function, with the power transfer passing through zero as the angle difference passes through zero. (It is actually a sinusoidal function of the angle difference, which at small angles can be approximated as linear.)

 

[hansforum]

So because rotor magnetic field advances and goes infront of stator magnetic field by some angle, current in the stator is flowing in opposite direction compared to stator current when working as a motor and power is produced.

What is the purpose of the gearbox between a turbine and a generator if stator magnetic field opposes rotot motion? Isn't generator supposed to control turbine speed with stator magnetic field? Why do we need gearbox then?

 

[waross]

The gear box matches the optimum rpm of the turbine with the optimum rpm of the generator

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Which are vastly different.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[hansforum]

Here are some information about small hydro:
Turbine nominal revolution: 244/min, max. revolution 678/min,
Asynchronous generator nominal revolution 1010/min, max revolution 2798/min (15 min), belt drive, no gearbox!
Grid freq 50Hz
How is that possible that induction generator has that wide rev range?

Back to gearbox question, let'a say we have low speed Archimedes screw turbine 5-25 revolutions per minute (1-4m3/s flow) with 100kw max power and induction generator with nominal revolution 1000/min and 4 speed gearbox. Can someone give some info about gearbox ratios? Is it possible that let's say turbine at 10rpm produce 30kW but dispite gearbox generator at that slip would produce less power so turbine has to accelerate to appropriate slip but there is no enough water for higher turbine speed!

 

[MikeHalloran]

That's not really a 'range'.
The maximum rpm listed for a generator is the speed above which it should be expected to start self-destructing from centrifugal force.


Mike Halloran
Pembroke Pines, FL, USA

 

[LionelHutz]

An induction generator speed of 1010rpm could make sense. 2798 makes no sense though. An induction generator has to be going faster than the synchronous speed to produce power. The speed would be somewhere between synchronous speed and the rated slip speed of the generator depending on the power output. At 50Hz, you can have synchronous speeds such as 3000rpm, 1500rpm, 1000rpm, 750rpm, 600rpm, etc. So, 1010rpm makes sense but 2798rpm doesn't.

I don't really follow your gearbox question. Whatever turbine speed the generator and gearbox set is the speed the turbine has to run at. You need controls to shut-down the unit so the generator doesn't become a motor when there isn't enough water to drive the turbine fast enough to produce power. It can take some work to properly match mechanical speed with generator speed to get the optimal output but it's nothing that isn't already well known and documented.

 

[LionelHutz]

I forgot about your other confusion.

Generator - Power goes into the motor shaft and it comes out of the electrical leads.
Motor - Power goes into the electrical leads and it comes out the motor shaft.

Now, every motor synchronous or induction wants to run at or very near its designed synchronous speed. See examples in the previous post. Connect to the shaft and try to force the machine to a speed below the synchronous speed and it will be a motor. Try to force the machine to a speed above the syncronous speed and it will be a generator. The only difference between induction and synchronous is that the synchronous machine is locked to the synchronous speed and the induction machine speed will vary by say 2% above or below synchronous speed as it switches from full power motoring to full power generating. It might swing from say 980rpm to 1020rpm for example.

 

[waross]

The induction generator may be feeding a rectifier bank so the frequency is more forgiving. None the less I suspect that the maximum speed is a mechanical limit for a machine that has lost its load and is running free.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hansforum]

Does gearbox in small hydro power plants connected to low speed turbines change gears (does this kind of gearboxes exist?) or does it have only one gear ratio. I read that some wind turbines have some kind of CVT gearboxes but I am interested if there is gearbox with three or four gears that works somehow like car auto gearbox.

 

[hansforum]

I read that when ac machine is working as a generator internal voltage E always leads terminal voltage V and this is necessary for active power to flow from generator to grid. Can someone explain why is that? I know that rotor magnetic field leads in front of stator magnetic field. But if we didn't know that how could we explain which way active power flows and why just based on the fact that internal voltage E leads terminal voltage V? I need physical image of what is happening with the power when internal voltage E leads terminal voltage V!

 

[cswilson]

Think of the magnetic fields produced in the rotor and the stator by these voltages and their interaction. When the terminal voltage in the stator leads the internal voltage in rotor, the stator magnetic field leads the rotor magnetic field. You can consider the magnetized rotor to be a compass needle trying to line up with the external (stator) field. If there is a mechanical load on the rotor, this requires a torque on the rotor in the same direction sense as the rotor velocity. The product of the torque and velocity is therefore positive, which indicates power flow into the mechanical system, which is "motoring". The electrical system must supply this power - in the convention of the electrical world, the voltage and current must have the same sign to supply electrical power into the machine.

When the internal voltage in the rotor leads the terminal voltage in the stator, the rotor magnetic field leads the stator magnetic field. In this case, the stator magnetic field is trying to pull back the rotor. That is, it is applying a torque to the rotor in the opposite sense from the velocity. This means that the product of the torque and velocity is negative, which indicates power flow out of the mechanical system, which is "generating". This power must go into the electrical system, where the voltage and current will have the opposite sign.