Port impedance of a grid-connected power converter

[sonvalka]

Hello guys, I am working on EMC of Grid connected power converters, I need to find out typical values of port impedance of a grid-connected power converter. Unfortunately I don't understand this term "port impedance of a grid-connected power converter". Can anyone help me to understand the this term and concept. Thank you !

 

[PlanarMetamaterials]

Hi sonvalka,

While I don't work in the power industry, the port impedance in my understanding would be the impedance of the convertor as seen by the grid. I.E. if you were to model the convertor as a single impedance Z, what would the Z value be? Using the infinite source approximation for the Grid, this can be reduced to: what (complex) current does the converter draw? Then Z = V_Grid/I for each phase.

Hope this helps.

 

[sonvalka]

Hey, thank you very much for your reply. However I am still stuck with finding typical values of port impedance of grid connected power converter as function of frequency!

 

[FvM]

I fear it's not so easy to derive a "typical" impedance characteristic of a grid connected power converter. I presume, you're referring to a grid connected inverter or a bidirectional converter with "active front end" (AFE), in other words a PWM bridge connected through an inductor or LC low-pass filter to the grid. Its impedance is determined by the filter as well as the control loop.

Without the control loop effect, the impedance can be modelled in a first order as voltage switching PWM bridge (acting as a short) in series with the grid inductor/filter. Within the control loop bandwidth, the PWM output voltage is steered to follow the grid voltage, raising the port impedance. The impedance at the fundamental grid frequency will be finally determined by the control algorithm and actual converter operation point. Instead of constant R, as suggested in post #2, constant P or constant I are more likely modes.