Ahmad, the I-V and P-V curves of an unshaded array depend only on the physical properties of the array and the irradiation it experiences. Any PV array will behave sensibly under these condtions and have a single maximum power point. All your converter needs to do is to identify it and lock on to it.

how the existence of DC-DC converter affects I-V and P-V curves

The I-V curve of the array is always the superposition of all parallel strings in I and serial modules per string in V. The DC-DC converter determines the resulting working point on the derived P-V characteristic curve with the mean power value the converter transmits. An intelligent converter will scan for the maximum power point, assuming it can be feed in to a power sink (e.g. grid or battery).  Additionally there may be a ripple of the switching frequency of the converter depending on the used DC-DC technology. The ripple is usually minimized using (high) capacitors on DC input.

Regards, Steffen

The DC-DC didn't affects the I-V or P-V curves. The goal of using DC-DC converter is the MPPT.

Dear Tony

Thank you very much for your answer. So you mean the existence or non-existence of DC-DC converter does not affect PV curve of PV system? Then, how it pushes the operating point to knee point?

Dear Tony

In another words, can you please explain me the detailed effect of D-DC converter in PV System? And can you explain it from circuit analysis point of view?

Sincerely,

Ahmad

To push the operating point to knee point as you said, DC/DC converter chops the input with appropriate duty cycle; the output contains low and high frequency components. High frequencies are eliminated using a low pass filter to obtain the desired operating point.

Regards

If I understood your follow up question, you wanted to know how an inverter can find the MPP.

There are many different ways. This paper gives you a very good idea on most of them

Trishan Esram and Patrick L. Chapman, Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques, IEEE Transactions on Energy Conversion, Vol. 22, No. 2, June 2007, pp. 439-449.

Ahmad, as Mohamed says, the converter doesn't affect the IV curve itself - this is a parameter that defined the behaviour of the array at some temperature, and irradiance. The convertor acts as a DC to DC transformer to match the effective load impedance to the source impedance (the output impedance of the PV array, which isn't linear by the way) and thus you get maximum power transfer.

In order to behave like a DC to DC transformer it has to convert the incoming power to a time varying power, that is, chop it or do something else to it so that you get a time varying electromagnetic field. This can then be transformed into a different voltage current combination. One way of doing this is to vary the duty cycle of the chopping device. Another would be to vary the effective turns ratio of the high frequency transformer. A third way would be to use a phase controlled rectifier.

There are many ways to do this - Charles' references look excellent and I would recommend you follow his suggestions.

Best wishes!

Accrding with the Response of Steffen, the MPP will be searched by the Duty ratio set by the DC-DC that will change the internal impedance of the PV, hence the working point will change.

The I- V curve module is not affected by the converter,. The function of the DC -DC converter is to find the point at which the module will maximum power (MPPT).  Attach a paper that can clarify the concept:

https://www.researchgate.net/publication/245169807_A_reliable_fast_and_low_cost_maximum_power_point_tracker_for_photovoltaic_applications

Article A reliable, fast and low cost maximum power point tracker fo...

You can have a look at these simulation analysis using MATLAB:

https://uk.mathworks.com/matlabcentral/answers/183689-how-does-mppt-maximum-power-point-tracking-simulink-model-work

Regards

The I-V and  P-V curves are operating characteristics of a particular PV system under specific environmental conditions. They have nothing to do with the output of any sort of converter that they are connected to. If you want to look at the output of the entire system, then you ought to see, as system current is increased from zero, a constant voltage output up until the power drawn from the load is equal to the maximum power that can be supplied by the PV array. If you try to draw more power from the system, then what happens depends on the control algorithms in the DC/DC converter. It might  just reduce the output voltage to keep the power drawn constant, or it might shut down completely considering the existence of a fault condition. It might then try to restart sporadically. This also depends on how the load behaves when supplied with a lower than rated voltage. If it is feeding a constant voltage sink like a battery it will - or should - reduce the charge current to keep the PV array at maximum power point.

The DC-DC cxonverter provives the load line to the PV array.  The PV array has a characteristic P-V (const I) and P-I (const V) that depends on illumination, temperature, phase of the moon and alignment of the planets.  For a given illumination, it has an I-V characteristic, of a forward biased junction (many of them in series/paralell combination.  Where the system operates on that I-V characteristic is determined by the "load line" presented by the DC-DC.  I.e., the operating point of the system is at the intersection of the I-V characteristic of the array. and the I-V characteristic of the DC-DC converter.  The I-V char of the DC-DC converter is under your control, and can be included in a control loop that adjusts the I-V characteristic of the DC DC to intersect the PV array I-V at the chosen operating point, often the max power point, but often "close enough" yto be able to operate  with stability over a wide range of illumination, temperature , phase of the moon, etc.

Not all DC-DC converter configurations are appropriate for this task.  Some present "negative resistance" in certain operating regions.  This will be quite difficult to stabilize with linear control techniques, however the feedback loops within the DC/DC can be designed for just this purpose, i.e to present the optimum load to the PV array, and dump that energy into an appropriuate sink, like a battery or the grid..

For more details refer to US patent #5436553 by yours truly "Optical Power Conversion"