Thevenin equivalent circuit

[FF003]

Hello everyone,

I want to model the inverter side of the three phase grid connected inverter with Thevenin equivalent model. But I found the impedance (both resistive and reactive components) negative. Is it correct?? thanks.

 

[D.A.(Tony)Stewart]

How did you come up with this model?

GTI‘s are generally unity, power factor corrected, but voltage & phase applied must be IN SYNC. The result of generating power is negative when PC is active and voltage applied to the grid is greater.

 

[FF003]

How did you come up with this model?

GTI‘s are generally unity, power factor corrected, but voltage & phase applied must be IN SYNC. The result of generating power is negative when PC is active and voltage applied to the grid is greater.

I have two transformer in the system between inverter and the grid. And I am controlling the voltage and current of the inverter side. Therefore, the voltage and current in the grid side are not in phase (because of having two transformers) (please see the attachment).
For finding the Thevenin voltage and impedance, I injected two different 7th harmonic voltages from the grid side to the system, and based on 7th harmonic changes of current and phase voltage in the PCC, which I consider it at the grid side, I calculated the Thevenin's voltage and impedance.

 

[KlausST]

Hi,

your inverter acts as a "source", or generator.

A resistor consumes energy, and it´s value is positive.
Now if you have something that does the opposite to a resisitor = generating energy, then it´s mathematically expectable to be negative in value.

Klaus

 

[FvM]

Why 7th harmonic? If you don't have harmonic current compensation, it won't pass the RRF (rotating reference frame) based controller. Why do you expect that transformers generate relevant phase shift? I also don't understand the relation of boost converter and DC voltage control, I'd expect MPP controller.

 

[D.A.(Tony)Stewart]

I'm surprised 7th harmonic is used transformer effects in between. I see non-invasive phase methods are being proposed to compute response in each direction.

Are you following this method?

https://www.researchgate.net/publication/360955374_A_New_Stability_Enhancement_Method_Using_KF_Estimation_for_the_PWM-SMC-Based_Grid-Tied_Inverter_under_Weak_Grid_Condition

 

[FF003]

Hi,

your inverter acts as a "source", or generator.

A resistor consumes energy, and it´s value is positive.
Now if you have something that does the opposite to a resisitor = generating energy, then it´s mathematically expectable to be negative in value.

Klaus

I want to model the inverter side (in the PCC) with the thevenin equivalent circuit, and I don't know why I found the thevenin impedance negative.

 

[FF003]

Why 7th harmonic? If you don't have harmonic current compensation, it won't pass the RRF (rotating reference frame) based controller. Why do you expect that transformers generate relevant phase shift? I also don't understand the relation of boost converter and DC voltage control, I'd expect MPP controller.

As I am controling the inverter side voltage and current, these voltage and current after passing through transformer will have phase shift in the PCC. 7th harmonic is an example to find the thevenin model for this harmonic.

 

[FF003]

I'm surprised 7th harmonic is used transformer effects in between. I see non-invasive phase methods are being proposed to compute response in each direction.

Are you following this method?

https://www.researchgate.net/publication/360955374_A_New_Stability_Enhancement_Method_Using_KF_Estimation_for_the_PWM-SMC-Based_Grid-Tied_Inverter_under_Weak_Grid_Condition

7th harmonic is just an example to find the thevenin model for this harmonic. No, I am not following the mentioned method. My system and controller is same as the attached photo in my previous reply.

 

[FvM]

Real impedance of a power source at fundamental grid frequency has to be expected negative. 7th harmonic would be shorted, expect to see filter plus inverter output choke impedance only.

I doubt that "thevenin model" is a useful approach for a system involving pll and rotating reference frame.

--- Updated Feb 22, 2024 ---

A good introduction to grid converter control methods is Remus Teodorescu, Marco Liserre, Pedro Rodríguez, Grid Converters for Photovoltaic and Wind Power Systems

https://onlinelibrary.wiley.com/doi/book/10.1002/9780470667057

 

[D.A.(Tony)Stewart]

This reminds me of an American power engineer program mgr. who asked me for advice from a major power utility down south before CV19 and internet security changes.
They had this solar farm that worked fine with 150 Amp 3ph fuses using Huawei GTI's until a solar farm competitor was installed operating down the road. Then fuses started popping yet monitors did not report any alarming current or thermal changes from the data.

I suggested his GTI impedance was state-of-the-art with very low impedance including harmonics. Then I hypothesized that the new "neighbors" were generating harmonics on the grid with current going in reverse thru the fuses to perhaps to filters or snubbers not monitored. It would have been easy to scope the harmonic emissions with a coil near the fuse panel. Instead he put in 200A fuses which worked. 8>) Not heard from him since.

So negative impedance on fundamental and positive impedance on higher harmonics for output impedance or some kind of resonance(?). I didn't get any details on local grid or test results.