Hi Ume,

To answer this question, I would assume you have a simple 3-phase VSI inverter, grid-tied through an L-C filter. In this case, it is a popular practice to choose the inductive reactance to be 5-10 % of your base impedance rating (=Vgrid/Igrid) at the grid frequency. For example, if your grid voltage is 230 V(ph) and the inverter AC rated current is say 23 A per phase, then your base impedance is 10 Ohm (=230/23). Now you can calculate the required inductance. Once you have the inductance value, you can calculate the capacitance value depending on the THD requirement at the grid side (usually less than 5 %).

If you do not have L-C and only have 'L', then the value will completely depend on the grid current THD - higher the inductance value, lower the THD. So in this case, your inductance will be much larger.

I would also recommend you to look into L-C-L filter design, which is a little more complicated than L-C filter. But this type of filter interface has some interesting benefits.

Hope this helps. Good luck!

-HSK

First of all thank you Harish for the kind reply.

Base impedance is calculated by grid voltage and grid current, while in calculation we take inverter output current? why so?

on more thing I need to understand is, when we design grid tied inverter we say frequency, phase and voltage should be the same. right?

now the confusion is, how power will flow from inverter to grid if both have same voltage magnitude, same phase and same frequency. it's like  2 same ac voltage sources are connected in series with an inductive load between them, then how and where should the power flow?

Hello Ume,

I think Sandeep has already answered your questions well. There are several methods to control the power transfer to and from the grid.

- If you only want to adjust the active power, you may simply keep the inverter phase angle and fundamental frequency the same as that of the grid and only adjust the magnitude of fundamental voltage component on the AC side of the inverter (by adjusting the modulation index).

- If you want to control the reactive power too, you can vary the modulation index and the phase angle simultaneously, which I think is the most popular method.

- You can use 'droop control' strategy to also vary the inverter fundamental frequency slightly (especially at the transient conditions) to obtain faster response.

One thing I would like to mention in point (2) from Sandeep's reply. If we assume Sine PWM and a simple 3-phase inverter, the inverter fundamental AC voltage equation is given by:

Vac(1,rms) = (ma*Vdc)/(2*sq.root(2)); ma being the modulation index

So if your grid voltage is say 100 V(rms), then you would need at least 282.5 Vdc on the DC bus to make the inverter operate as desired (without saturating the control) since the maximum modulation index with conventional sine PWM is 1.0 (this can be increased as well, using certain other techniques, but let me stick to the simple sine PWM modulation for now). To include the voltage drop and loss effects in the system, we would like to at least have 300 V for proper operation. Theoretically, the inverter would work even if you increase this voltage to infinity, but practically you would not like to go much higher than 300-350 V for this situation. 

But considering your voltage is equal to or higher than 300 V, you will still be able to transfer power both ways (as far as the DC and AC side systems can support that). This can be done by simply adjusting the modulation index and phase angle with respect to the AC (grid) side.

Like Sandeep mentioned, please go through more references to understand the operation in more detail. I would be glad to help you if something is not very clear to you.

Best Regards,

-HSK

I apologize for the late response.Thank you very much for the kind replies Sandeep and Harish.it helped alot. i will go through the references you mentioned and get back to you on this.  if any of you have any related book on Grid Tied inverter please share it with me. That will be really very kind favor.

Thank You

Regards

 All that is O.K. for inductance design. But is their any references for design the core of the inductance that used in PV grid tie inverter? The grid is 50 Hz and the switching frequency of the inverter is 10 kHz and the the transmitted power is 10 kW. If any one can help, I will be very thankful.

Thank you Sandeep, I will see it.