[SOLVED] Difference between Grid Connected and Standalone Wind Energy Conversion systems

[aditya1]

Hi, I would like to know the basic difference in the control strategy in the case of Grid-connected WECS and Stand alone systems. I was able to find a lot of information in terms of the speed-control, converter control etc. for Grid Connected systems but not in the case of stand-alone systems. I am not really clear on the (need for) d-q control of inverter.. Is it needed in the case of stand-alone systems? I would like to know if there are any papers that talk about the difference in terms of control in Stand-alone WECS and Grid-connected WECS.

Aditya

 

[not-a-moderator]

The basic difference between them relates to the output isolation and phase / timing. In grid connected systems your control system has to receive feedback from the grid to stay in phase (timing) with it.
Have a look at: https://www.st.com/internet/com/TEC...AL_LITERATURE/APPLICATION_NOTE/CD00253868.pdf

 

[aditya1]

Thanks. I am going through the PDF and finding it quite helpful. I have a doubt in something mentioned on Pg. 41, could you help me understand it?

"It is worth knowing that if the Vq component is controlled to zero then the Vd component follows the grid-voltage rotation. In this case, the active power injected into the grid may be controlled, transforming the current in the same reference frame and by acting on the amplitude of the Id component. The Iq component must also be controlled in order to ensure zero reactive power injection. On the contrary, if the Vq component is controlled to zero in the PLL, the active power is controlled with the Iq current component and the Id current component is used to control the reactive power to zero or to the desired value."

Something seems quite off in this explanation. If my understanding is right, it should read "On the contrary, if the Vd (not Vq) component is controlled to zero in the PLL, the active power is controlled with the Iq current component and the Id current component is used to control the reactive power to zero or to the desired value"

Could you comment on the need for d-q transformation etc. in the case of Stand-alone loads (for ex. Motors & Resistive banks) In these cases do we need the inverter control specified here, or would there be a simpler way of doing things?

 

[FvM]

The advantage of d-q based control is to have explicite access to real and reactive power transfer to the grid. You can design the grid tied inverter control based on alpha-beta as well.

For the stand-alone operation, you are setting frequency and voltage. Real and reactive power are demanded by the load. The more critical point is to control the generator according to the power demand.

P.S.: I think, you are right about "Vd (not Vq)".