Grid tie synchronous generator rotating forward and reverse directions

[combatlan]

Hi all,

Is it possible for a sync generator generates continuous power if it rotates forward and reverse (clockwise and counter clockwise) directions? The generator can be connected to power converter and grid tie inverter. The application drive the generator in forward rotation for a fixed amount of time, then reverse for some amount of time and cycle repeated. The time could be could be random. Regardless of the directions, the generator needs to generate power to the grid, is this possible?

Thank you

 

[Skogsgurra]

Yes. Wave energy harvesting?

Gunnar Englund

http://www.gke.org

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

 

[combatlan]

Hi Skog, yes it is. Could you explain?

 

[waross]

The main limitation is cooling. The shaft mounted cooling fans and the air path must be suitable for operation in either direction. Most generator fans are suitable but there are enough generators with uni-directional fans that it needs to be mentioned.
I have seen brush gear that was not suitable for either direction, but most exciter brush gear is suitable for either direction and many generators use brushless excitation. The chances of brush issues are vanishingly small. But, if you have brush gear and the brushes are not perpendicular to the shaft, you may have issues.

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

 

[combatlan]

Thanks Bill. Appreciate cooling may be a challenge, but my concern is whether the generator can generate power in bi-direction. It's for wave power application and I'm considering synchronous brushless AC generator connected to power converter (inverter).

Assuming buoy movement cause the generator's shaft to move clockwise and counterclockwise e.g. 0-30rpm clockwise and then 0-30rpm counterclockwise for every 10 seconds. Would the power converter (inverter) connected to the generator be able to generate power to the grid regardless of the direction?

 

[jraef]

If you are connecting to an inverter, then nothing matters. The inverter just rectifies the AC to DC, then back into AC, so what comes into the inverter is irrelevant (in terms of rotation sequence).

But if you are going to an inverter anyway, why an AC synchro us alternator? Why not use DC and skip the step of rectifying the aC to DC? You can get permanent magnet (brushless) DC genertaors in fairly large sizes now. How big are you thinking?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

To echo Skoggs;
In most instances yes. Induction motors, synchronous motors and large generators are inherently bi-directional. Any limitations are due to occasional construction issues.
You are using a brushless exciter, so if the fan is bi-directional you should be good to go.
You may want to consider rectifiers before the invertor.
Consider also permanent magnet excitation. You may find less losses. The savings in losses may not be an issue and not worth the price for a large scale installation.

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

 

[Skogsgurra]

I said "Yes" because that is what is done in wave energy harvesting. All the time. Details depend a lot on the circumstances. Permanent magnet synchronuous generator and a tie inverter is the standard solution.

Gunnar Englund

http://www.gke.org

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

 

[cswilson]

jraef:

A "DC brushless motor" is a synchronous AC motor, albeit one with a permanent-magnet rotor field. The stator waveforms would be rectified to DC in the same manner as a classic synchronous generator.

But with a permanent magnet motor, you don't really have independent control of the voltage and frequency -- with simple methods for rectification such as a basic diode bridge, the voltage and frequency will be directly proportional, which could be less than ideal.

But you are correct that with a conversion to DC, the direction sense of the generated AC waveforms do not matter.

Curt Wilson
Delta Tau Data Systems

 

[combatlan]

Thanks all for the explanations. It make sense now. My understanding was that the change of direction of rotation would change the phase sequence, hence affecting the power flow to the grid.

 

[waross]

The change of direction will change the phase sequence to the invertor. The invertor should match the frequency sequence to the grid.

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

 

[combatlan]

Bill,

My understanding based on earlier posts is that the AC generator connected to the rectifier can rotates forward or backwards. The change of phase sequence due to change of rotation is 'handled' by the rectifier and converted into DC. This DC amplitude is proportional to the generated AC regardless of the direction. The DC is then fed into inverter converting it to AC which is controlled to match the grid. Correct me if I am wrong.

Thanks

 

[crshears]

Hello combatlan,

The title of your OP is "Grid tie synchronous generator rotating forward and reverse directions..."

Lest there be any misunderstanding:

If a polyphase AC machine's power output is routed through a rectifier and inverter and thence onto the grid, the AC machine in question is not "grid tied," if by grid tied you mean "synchronously grid tied." In this case it for sure exports power onto the grid but the exporting is via an asynchronous tie.

If on the other hand a machine is truly "synchronously grid tied," its phase rotation is that of the system to which it is connected. Our province's main generation company uses AC machines as pump-generating units; these are equipped with Kaplan runners, so to change from pumping mode to generating mode the machine must be taken off line and its rotation reversed by swapping connections to two of the machine's terminals by means of permanently-installed reversing switches.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]