Hello everyone. I have reached another concern in my studies of GTIs. If MPPT controls the duty cycle of a boost DC/DC converter and thus varies the output DC voltage of this stage, that means that the DC/AC inverter will have a variable input DC voltage. I have read this in many papers.
So how is possible to connect such a system with the grid since the inverter input Vdc could have a lower voltage than the grid? current would flow INTO the inverter...
The more I read, the more confused I am.
The DC input of the DC/AC is not usually fixed, the inverter is designed to tolerate a specific change in the DC input voltage.
when you design an MPPT control you have some inputs such as input current , voltage , ambient temperature and irradiance for PV or wind speed for wind and the output is the duty cycle as for example for a boost or step-up converter Duty= 1- Vo/Vin so you can choose a fixed value for output DC voltage Vo i.e 400VDC so Duty = 1-400/Vmpp in this way you acheive you delima of MPPT and fixed DC output voltage. from another hand, there are a separate control for the inverter to match the phase and voltage to the grid requirements . Also, there is a link for a practical scenario that could help you.
https://www.researchgate.net/publication/264897184_Enhancing_the_maximum_power_point_tracking_techniques_for_photovoltaic_systems
Article Enhancing the maximum power point tracking techniques for ph...
Thank you everyone! I'm taking notes of all your comments and trying to simulate the system to make it work as I need it.
Hello Joshi. I have finally comprehended it by analysing the "flow of power" rather than the "flow of voltages". Let me explain it a little:
First, the boost converter adjusts its duty cycle to deliver the maximum power to the load (for this stage, let's say the load of the boost is the inverter). If you assume, for example, that your input power in the boost converter is 350W, then, you would expect (ideally) to have 350W at the output (same power, but different voltage).
Since the boost increases the output voltage, to obtain this power, output current should be lower than the input current. This is pretty easy to understand. Now: In the most simple form, the inverter looks like a "adjustable resistor" at the boost output.
Controlling the inverter triggering sequence would "allow" more or less power to be injected into the inverter's load. Right?. So, your current control loop must regulate the amount of current you allow to pass (just like if you had and adjustable resistor!) in order to keep the DC voltage at a certain value (desired Vdc).
If you want a 350Vcc link voltage, and 350Watt, then, you can inject 1A. It means your inverter looks like 350Ohm resistor for the boost. But if your MPPT algorithm delivers only 300W, then you should adjust your inverter to deliver 300W/350V = 0.857A to keep those 350V. This means to control the inverter to look like a 350V/0.857A = 408.4 ohm resistor
That's all. I hope you understand it.
Greetings.
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..
Yes, varying the modulation index would vary the duty cycle of your inverter, injecting more or less current to the grid and thus, regulating the charge/discharge of your Vdc link capacitor keeping it in a desired set-point.
Your modulation index should be varied according to the error between measured and desired Vdc link (as you said) by the use of a Proportional-Integral controller.
Remember that the mentioned above will only give you the amplitude of the modulating signal, but you also need to synchronize this signal (sine like) properly to make the whole system to work as expected.
Yup.. Grid sync is also next probkem,ciam working on... Studying PLL methids for that. But didn't get proper hold on PLL. Do you have Any proper reference.? I need to sync single phase signal..
Hi Darshan,
Look at the papers by Pedro Rodriguez, Frede Blaabjerg and Joseph from Aalborg university on SOGI.
Hello, i have a similar question. I have implemented control upon dc link voltage through power match betwen Pv plant power and grid demand. However, i have found that if this control did not implement, the mppt algorithm does not fixed the maximun power of the pv plant. I had thought that the result of remove the dc link voltage loop would be that dc link voltage would increase or reduce whereas the result was that the mppt algorithm stopped working properly and the dc link tends to a value that is not what i expected. I have used perturb and observe method and incremental conductance with variable step but the result is the same. I do not know what i should do
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