Hi Darshan,

This requirement arises because the power output by PV panels are never at a very high voltage, while the latter happens to be a requirement of the AC side.

The inverter is designed entirely on the basis of AC side requirements - so some kind of DC-to-DC "voltage matching" becomes an inevitable requirement. Of course, this would reduce the current available to the AC side.

-Sanjay

Any Kind of converter either Buck, Boost or Buck-boost is used to match the intrinsic impedance of PV panels with the impedance of load.So that maximum power transfer can occur between them. MPPT is achieved by manipulating the duty ratio of DC-DC converter such that PV panel operates at its maximum power point.You can review the PV characteristics curves for further understanding.

Best,

Asim

Both of previous comments are true. While tracking the Pmax, the impedance of PV panel and should be matched with the load. However, since the PV panel should fed the DC to AC (inverter), you should fed the inverter input with the higher voltage than the nominal output voltage of your inverter . It because the DC to AC converter circuit (not only for single phase out but also for three phase out) was simply derived from buck converter. And, basically the buck converter will decrease the output voltage. That why you need to boost up the PV output voltage before connects it to the inverting stage. 

Here is the pictures to support my comments. And hope this comment can help you :)      

Please see this paper :

https://www.researchgate.net/publication/301912126_Proposal_and_implementation_of_a_novel_perturb_and_observe_algorithm_using_embedded_software

Conference Paper Proposal and implementation of a novel perturb and observe a...

Boost converter has a constant ratio of Vout/Vin with a certain PWM duty cycle. Please let me assume Vout = N* Vin. Then P(PV)= Function (Vin), in terms of the Power-Voltage characteristics of PV panel (The curve increases to a peak, and fast goes down to 0, you must know that). P(load)=Vout^2/R=(N*Vin)^2/R. Assuming the converter don't have power loss, P(PV)=P(Load), that is: Function(Vin) =(N*Vin)^2/R, then Vin can be solved. That means, if you give a constant Duty Cycle, the Vin, Vout, Iin, Iout, Power transferred are all confirmed. Just take any one of them, to be the controlled signal of a closed-loop controller in the Boost converter. The controller will keep justifying the Duty Cycle until it checks the controlled signal identical to a reference. Hope this helps! If I didn't describe clearly, feel free to contact me.

Thank you for the solutions. 

Now it's clear with boost+ mppt. But to reach at mpp, duty cycle needs to be changed. So output voltage will change. But by doing that dc link voltage for the next stage (DC/AC) varies.

So is it permissible variation.? (Sorry to ask this silly question, but i am confused.) 

Not silly at all. It's really a good question. 

The power generated must be consumed equally. When mpp, power generated is confirmed. Load power is confirmed, so V= sqrt (PR) is confirmed as well. However it may not match the next stage. Is this your question?

The key is: EVERY POINT IN A DC MICRO-GRID MUST BE VOLTAGE-CONTROLLED.

The voltage of segment PV to Boost is controlled by the boost to reach mpp. Then the voltage of segment from the boost to next stage must be controlled by the next stage converter. In other words, the next stage (DC/AC) converter should take the rest power away, or add the lack of power, to maintain the voltage there. Even for very large scale DC micro-grid, all the points between converters or batteries must follow this criteria. By the way, battery is a self-voltage controlled device, while it's cannot be changed.

Do I explain clearly? If not, feel free to let me know please.

Hi Darshan,

As Chunpeng rightly said, your query is not silly at all ! There are two specific points that I would like to add, and maybe they can sort out your doubt !

Hope that helped.

-Sanjay

Dear Prof. Roy,

Thanks for adding of specific points. I agree with you. The constant voltage method "reserves" some power output from PV panel to maintain healthy constant voltage. By another method, if the PV panel keeps its maximum power output (maximum power point logic), there must exist "somebody" taking the rest of power away, such as a charging battery, a DC/AC inverter (inverter side control), etc. 

Sincerely,

Chunpeng Li

Actually, the DC-DC converter working principal depend upon the switching of MOSFET or IGBT used in dc-dc converter. So, if provides constant duty cycle it only work as normal boost converter.

In case of MPPT, it is controlled switching of device based upon the implemented MPPT algorithm which tries to switch the device in such a way that to track MPP and definitely it will boost the voltage.

The key point for proper MPPT tracking is the input power and output power for the dc-dc converter is same even though load is changed. But, it is not the case when using dc-dc converter as normal boost converter.

So basically, instead of worrying about voltage, power should be taken care of. I was thinking to adjust the voltage level by varying amplitude modulation index according to the valtage variations at the dc link.. Rectify me if i am wrong..

Proceed step by step. First design the boost converter considering required output voltage for grid integration. To test the proper functioning of boost converter check whether power is constant or not by changing the load. By changing the load voltage and current changes accordingly but output power and input power for boost converter should be same. Just a difference due to some losses in dc-dc converter.

Hello.

Good question Mr. Darshan.

Even I got some of points cleared.

Thanks a lot Prof. Sanjay Roy sir & Mr. Chunpeng Li.

Please refer to these articles:

https://www.researchgate.net/publication/309716325_Designing_And_Simulating_Of_Micro-controller_Based_on_PWM_Solar_Charge_Controller

https://www.researchgate.net/publication/315692275_Maximum_Power_Point_Tracking_Control_of_PV_Electronic_System_using_Neural_Network

Article ‫ Designing And Simulating Of Micro-controller Based on PWM ...

Conference Paper Maximum Power Point Tracking Control of PV Electronic System...

So in MPPT we should always use a boost converter because it's guaranteed we will increase the voltage output? Shouldn't we use buck-boost if we need to decrease the output voltage via MPPT?