MPPT Questions

marktmarkt Registered Users Posts: 3
Hi Folks,

I just joined the forum as I am working on a project related to solar panels and the Internet of Things. As I am not a trained electrical engineer, I am going to ask some potentially obvious questions about MPP plots.

To start, can someone explain how I or V can be controlled from a panel? While I understand that an MPPT can tell the controller to go more to the left or right of the curve, what is it actually changing? The resisting load?

i.e. Suppose I have a 300W panel in the sun on a slightly cloudy day. Attached to the panel I have a 200W 12V DC motor attached to a buck converter that is fixed to output 12V.

What would be the input voltage from the PV to the Buck converter? How could this be adjusted? Ohms law give me the idea that the resistance load in Ohms can be tuned to change the Voltage or Amps but how does one specifically change the Volts used while holding the amps constant?

Sorry for the obvious questions here but I would really like to get my head around how an MPTT actually actuates the position on the IV curve.

Many thanks,

Mark

Comments

  • solar_davesolar_dave Solar Expert Posts: 2,389 ✭✭✭✭
    This link looks like it explains buck converter design.
    http://powerelectronics.com/dc-dc-converters/buck-converter-design-demystified
  • BB.BB. Super Moderators, Administrators Posts: 32,601 admin
    Mark, welcome to the forum!

    MPPT as implemented can be a pretty complex subject... But starting with Dave's link above about buck converters we can start with some basics.

    Analogies--Always like these . Sometimes they help, sometimes they confuse. You tell me. :blush:

    First one... With AC power you can use a variable transformer to take 120 VAC in and output 0 to 120 VAC out to your loads... Very efficiently. The MPPT is sort of, the DC equivalent of an AC transformer (the DC converter "chops" the DC power and runs it through inductors then rectifies/filters back to DC power (or uses the battery bank as the "filter").

    Another way to think of MPPT (maximum power point tracking) is how a transmission in a car works. The automatic transmission allows the engine to run at optimum RPM while the car is going slow/fast/uphill/down hill. Using sensors and (originally a "hydraulic/mechanical analog) computer, it automatically matches the engine output to the the loads presented by the wheels/environmental conditions.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • marktmarkt Registered Users Posts: 3
    Thanks Bill for the warm welcome and I appreciate the analogy.

    Here is another one for you. The automatic transmission recognizes that it needs to go from 4th gear to the 3rd gear to make it up the hill. That's basically what a discreet MPPT controller does. How in the heck does it actually shift the gear? :)

    The buck converter is simply converting the DC down to a level acceptable to the electric motor (or battery). What gear is the MPPT controller actually shifting?
  • BB.BB. Super Moderators, Administrators Posts: 32,601 admin
    There are many different power converter topologies... Buck is a standard (cheap, reliable, efficient) one of them. It is capable of preserving the energy during the conversion (like an AC Transformer).

    A standard electronic regulator cannot do this... They have a pass transistor that either "restricts" the current flow (sort of like the valve on a faucet). -- Which wastes a lot of heat making the transistor hot... Or the regulator turns on and off 100's of times a second (PWM--Pulse Width Modulation. 50% on, 50% off is 50% power transfer)--Sort of like turning your light switch on/off very quickly to "dim" the lights (no waste heat, but very "electrically noisy").

    PWM controllers use filters/capacitors, or the Lead Acid battery bank to "smooth out" the current flow/average the voltage.

    A buck type switching power supply uses the inductors to "store" the energy as the high voltage/low current is down converted to low voltage/high current on the output to power the load. From a math point (ignoring losses):

    Pin*Vin = Pout*Vout

    100 volts * 5 amps = 500 Watts = 10 volts * 50 amps

    For a MPPT solar power supply--There is a "natural" Vmp (voltage maximum power) for the solar panel (solar panels are current sources, not voltage sources--Another long post). Basically the MPPT controller "measures" the Vmp of the array and varies the "duty cycle of the PWM switches in the Buck Mode converter) to keep the Vinput from the array at ~Vmp. This gives the maximum available power to the battery bank/load (as the load voltage changes, the load current also changes to keep Parray~Vvoltage-load*Current-Load).

    Note that this Maximum Power Point Tracking is only needed/done when the maximum charging current from the solar array is needed. One the battery becomes nearly fully charged, the PWM cycle drops to PWM Mode--Reduced power transfer from the array to the battery bank (to avoid over charging the battery bank/over voltage to the battery/loads).

    With modern electronics, the whole POWER SUPPLY Field has dramatically changed--It is actually quite amazing all of the things that are done today.

    There is a lot of math and circuit topology design issues in this stuff... I am no expert in the field. When you get to AC power and Switching power supplies--A whole bunch of math is needed to full/truly understand the details of what is going on internally. But analogies can give a pretty good appreciation of the over all process.

    If you want to ask more questions, please do--But try to ask very specific questions--You need to understand the building blocks (solar panels, batteries, etc.) and how the all fit together--Then why MPPT is such a neat device makes more sense.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Ethan BrushEthan Brush Solar Expert Posts: 235 ✭✭
    markt wrote: »
    Hi Folks,

    I just joined the forum as I am working on a project related to solar panels and the Internet of Things. As I am not a trained electrical engineer, I am going to ask some potentially obvious questions about MPP plots.

    To start, can someone explain how I or V can be controlled from a panel? While I understand that an MPPT can tell the controller to go more to the left or right of the curve, what is it actually changing? The resisting load?

    i.e. Suppose I have a 300W panel in the sun on a slightly cloudy day. Attached to the panel I have a 200W 12V DC motor attached to a buck converter that is fixed to output 12V.

    What would be the input voltage from the PV to the Buck converter? How could this be adjusted? Ohms law give me the idea that the resistance load in Ohms can be tuned to change the Voltage or Amps but how does one specifically change the Volts used while holding the amps constant?

    Sorry for the obvious questions here but I would really like to get my head around how an MPTT actually actuates the position on the IV curve.

    Many thanks,

    Mark

    Another way to think about it....imagine putting a variable resistor across a PV module. With it set to zero resistance, you have the short circuit current, and very little voltage. Turn the resistor the other way to near infinite resistance, and you have the open circuit voltage and no or very very little current. So of course very little or no power at these extremes. Like a gasoline engine and it's torque and RPM curve, there is a spot in the VI curve where you get the most power. So a mppt controller is able to change its effective input resistance To get to that sweet spot on the power curve, then it's dc-dc converter can "swap" voltage for current (or vice versa) so it matches the load and can thus be fully utilized.
  • marktmarkt Registered Users Posts: 3
    After reading further about some more MPPT designs, it seems that they actuate on the amps load being pulled from the PV to maximize the wattage. Perhaps this is something always assumed by the seasoned electrical engineer, but not by the layman. The penny has fallen!

    Thanks everyone for the patient and thorough explanations.

    Mark

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