power flow ac systems

[richerdick32]

Hopefully someone can explain this to me.
In a ac system that can import and export power. what is going on with the waveform of current and voltage as it moves from importing to exporting power. it cannot be a simple reversal of current flow, which would be easy to understand as its AC. im sure it has something to do with the phase angle between voltage and current but am having trouble visualising what is going on

 

[Skogsgurra]

It is simply a polarity reversal of the current. "What?" you say "AC doesn't have any polarity!"

OK, but the current has a phase angle with regard to the voltage. And voltage rules. So you can't do much about that. Now imagine a simple resistive load. Current is in phase with voltage and the load consumes power.

When resistance increases, current decreases and load decreases. But the phase angle is still zero. When resistance is very high (infinite), there is no current.

Now, decrease current below zero (by using a generator that works against the grid voltage) and let that generator push current against the grid voltage. Now, if you look at the phase angle, you will see that it is 180 degrees.

So, there you are. The current has opposite polarity with regard to the voltage. And that is exactly what you have in a DC system. A reversal of current polarity versus voltage.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[richerdick32]

Thankyou, so the equipment i am talking about is a dual fed induction generator I understand that the generator can export energy from both the stator and the rotor. I understand the principles of changing the rotor frequency to compensate for lower rotor speeds to stillproduce 50 hz. I have also heard that these generators can generate at a leading powerfactor, can you help me understand this too

 

[Skogsgurra]

You may need to correct your thinking somewhat. There are no such things as doubly fed induction generators. What you have there is a doubly fed synchronous generator.

You are right saying that you can compensate for over/underspeed by supplying the slip frequency to the rotor and that is a very common design that makes operation at varying speed possible. It has a huge drawback, though. The PWM fed to the rotor leaks from rotor windings to rotor iron and causes problems with bearing currents. It also causes arcing between brushes and slip-rings. The latter is caused by large current peaks (the winding capacitance charge/discharge currents) and, since ordinary current clamps do not show those peaks, the brushes are designed to handle the design current - not the peak currents. That leads to premature brush failure and quite often to eroded slip-rings.

Now the leading power factor: As long as excitation produces a voltage that just can deliver the needed power, the power factor is 1, which is the same as phase angle zero.

If you increase excitation, the generator's voltage increases. The generator will then deliver its peak current (crest of the sine) earlier than before - simply because the voltage is at its peak earlier. Leading current is capacitive power factor and there you are, generating reactive power.

If you reduce excitation, the peak comes later and the generator consumes reactive power. Which isn't very useful.

I made this as simple as I could. I could have added the beer/froth analogy or used complex numbers. I didn't. Others may do so.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Hi Gunnar,

DFIG machines definitely exist - they are basically a WRIM with a power electronic converter driving the rotor winding. I know they can be made to behave in a similar manner to a synchronous machine, but in construction terms they are much closer to a WRIM.

No beer & froth - please!

 

[Skogsgurra]

We are of differing opinions, Scotty.

They may look like induction machines. And they can be made to behave like induction machines. But as long as the rotor is fed with DC or a variable frequency it is not an induction machine. The reason that an induction machine is named an induction machine is that the rotor current is induced in the rotor as a result of the slip. It is a question of working principle and not how it is built.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Ahhh,I see your reasoning now. I was thinking DC field rather than AC field. Constructionally close to a WRIM but I like your way of looking at it. A rotor field which is independent of the stator field, fair summary of your thoughts?

 

[Skogsgurra]

Yes, I think so.
Actually, the machines are (almost) identical construction-wise.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[richerdick32]

"If you increase excitation, the generator's voltage increases. The generator will then deliver its peak current (crest of the sine) earlier than before - simply because the voltage is at its peak earlier.

can you explain why increasing the excitation brings the crest of the sine wave earlier. I dont fully understand this. I can underatand it increasing the magnitude. but not why it hits its peak earlier

 

[Skogsgurra]

Some own thinking and sketching of waveforms may be better than if I try to explain once again. It is the current peak that comes earlier. If you have a problem visualizing the internal EMF coming earlier then do not bother trying that. Concentrate on the current. It is, after all, the leading/lagging current that determines the power factor.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Gunnar and Scotty; Another suggestion.
What speed is the machine running at.
If the speed locks onto the supplied frequency then it is synchronous.
If the speed is slightly below synchronous speed and there is a slip frequency associated with the machine then it is an induction machine.

Re; Importing and exporting power. I agree with Gunnar's explanation but it may not be clear that it applies to reactive power.

Imagine a long conveyor. The conveyor runs uphill for a distance and then runs down hill for a much longer distance, and greater change in elevation.
The conveyor is driven by a synchronous motor.
The conveyor is started empty. The motor draws enough current to drive the empty conveyor.
Now we open the gate and material starts to fall onto the conveyor.
As more mass is added to the conveyor the motor current increases.
When the leading edge of the material load crosses the highest point and starts down hill it balances some of the mass on the uphill side and the motor current starts to decrease. As more and more mass crosses the highest point, the motor current continues to decrease until there is enough mass on the downhill side to balance the other side and to supply the friction and other losses.
At this point the back EMF should equal the applied EMF and the current should be zero. (WE are assuming unity PF)
As the conveyor continues to run more mass is transferred to the downhill side. Now the conveyor is driving the motor. Now the current starts to increase but in the opposite direction. Now we are exporting power rather than importing it.
Assuming synchronous motors at unity power factor avoids the complications that may arise with real current flowing in one direction (Exporting) while reactive current flows sideways in the opposite direction (Importing). Sure this happens in real life but that's an explanation for another day.
A note on shaft positions with load. A synchronous motor locks onto the supply frequency but it will fall behind slightly under load.
Imagine two synchronous motors running side by side on the same supply. They each have a mark on the shaft and we are looking at the marks with a strobe light.
At no load the marks should be at the same position. As one motor is loaded up the mark will drop back a few degrees in relation to the unloaded motor. If the motor is driven overspeed the mark will move a few degrees in the opposite direction.

Bill
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"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

I like that uphill/downhill picture. It adds to the understanding - and still no froth!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.