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  • How does MPPT work?

    I understand the IV curve and that maximum power is obtained at the knee of the curve due to I*V being greatest there.

    My question is I understand a solar panel as outputing a current and voltage based on what is sees from the sun, how does an inverter change these characteristics to ensure that maximum power is being created?

  • #2
    Well Stompy this is a bit out of scope of this forum, but a MPPT use process control using an algorithm to find the optimum resistance of the solar panel array. It does this in steps over a period of time. The period of time varies between manufactures, but on average about once per second. The advantage is system efficiency, especially in battery systems.
    MSEE, PE

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    • #3
      Originally posted by STOMPY View Post
      I understand the IV curve and that maximum power is obtained at the knee of the curve due to I*V being greatest there.

      My question is I understand a solar panel as outputing a current and voltage based on what is sees from the sun, how does an inverter change these characteristics to ensure that maximum power is being created?
      Sunking went over the theory, in practice, it's a DC-DC converter, that hunts around to find the "best" voltage to run the input at, relative to the output, and it ends up transfering about 97% of the PV harvest, to the lower voltage batteries, when the batteries can accept it. (bulk stage) A DC transformer so to speak.
      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

      solar: http://tinyurl.com/LMR-Solar
      gen: http://tinyurl.com/LMR-Lister

      Comment


      • #4
        Originally posted by Mike90250 View Post
        hunts around to find the "best" voltage to run the input at, relative to the output, and it ends up transfering about 97% of the PV harvest, to the lower voltage batteries, when the batteries can accept it. (bulk stage)
        A MPPT is not limited to batteries is why I did not mention Off-Grid. It works for both off and grid tied. The point is to harvest the MAXIMUM energy possible. Just some do it better than others.
        MSEE, PE

        Comment


        • #5
          So can somebody explain how exactly it adjusts the voltage?

          I know that there are from 2 to 4 the most successful algorithms which are basically used. But they just explain the system`s logic and I still can`t understand how it`s applied practically.
          Maybe it`s a battery of resistors and the system increases/decreases the total resistance by plugging/unplugging them from the string. But in this case the adjustment will be done from I (not V side). Thereby by changing I we can adjust I-V curve of the module.
          But in the most documents it`s mentioned that the principle grounds on the voltage adjustment.
          Maybe its done by connecting the additional power source to the DC converter. And this source sets the strings voltage. Is it possible?

          Thanks for participation in this topic. It is very important question and I`m grateful for everybody who takes part in it.

          Comment


          • #6
            Jim Google Boost Converters. It is too technical to try to cover on a forum.
            MSEE, PE

            Comment


            • #7
              How MPPT Track the Maximium Power

              I am designing an MPPT system. Am following the algorithm as follows.
              "Lets assume am using 75W panel to charge 12V battery, suppose am getting 10v and 4.5a from the panel, then am varying the duty cycle of the DC-DC converter until i get the maximum Power" but I am not sure is this the right way? please help me to sort this issue. Thanks in advance

              Comment


              • #8
                http://www.windsun.com/ChargeControls/MPPT.htm

                Most MPPT controllers take a high PV voltage, and down convert it to lower voltage for batteries, my array is set up for about 80V, to charge a 48v bank.

                there are also sensors to detect the array voltage and current, and the system adjusts the downconverter to get the best power into the battery. Lotsa fancy stuff going on, which is why the cheap knockoffs with a MPPT sticker, are sometimes worse than just wireing the panels right to the battery
                Stick with the big name brands.
                Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                solar: http://tinyurl.com/LMR-Solar
                gen: http://tinyurl.com/LMR-Lister

                Comment


                • #9
                  How MPPT Track the Maximium Power

                  Thanks Mike for your reply, I have a couple of queries please help me.
                  "As you say your array is set for about 80v, IS this voltage-current array is specific for each panel manufacturer or it is generated by the MPPT based on the different conditions?"

                  "Assume am charging 12v battery, and am using 130w panels. Suppose I get 10v and 11a from the panel, that means I get only 110W from the panel only, then How MPPT will change Voltage and Current to maximum power". Thanks in advance.

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                  • #10
                    The charge controller "sweeps" the array with varied loading, and then chooses the best point. That point changes over the course of the day, as sunlight changes, and temperature changes the PV voltage curve. Very tough to do well, Morningstar, Xantrex and Outback, and most >1,000w Grid Tie inverters do this quite well.
                    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                    solar: http://tinyurl.com/LMR-Solar
                    gen: http://tinyurl.com/LMR-Lister

                    Comment


                    • #11
                      Thanks mike.

                      Comment


                      • #12
                        Originally posted by STOMPY View Post
                        I understand the IV curve and that maximum power is obtained at the knee of the curve due to I*V being greatest there.

                        My question is I understand a solar panel as outputing a current and voltage based on what is sees from the sun, how does an inverter change these characteristics to ensure that maximum power is being created?
                        Check out this page called " What is MPPT ?". MPPT controllers and inverters operate by raising the voltage a little and lowering the voltage a little to try and fidn the sweet spot (Maximum Power Point Voltage) of the solar modules. This is called sweeping. Some inverters and controllers sweep all the time and some sweep every few minutes.

                        Comment


                        • #13
                          Originally posted by Jimbo View Post
                          I know that there are from 2 to 4 the most successful algorithms which are basically used. But they just explain the system`s logic and I still can`t understand how it`s applied practically.
                          Let's start from the basic design. A basic MPPT uses a buck converter as a power stage, which is a switchmode topology. (Some use isolated topologies but the control laws are the same.) In a buck converter, the pulse width controls the output voltage, so:

                          Vout = Vin * duty cycle

                          So if you have 60 volts in and a 25% duty cycle you'll get 15 volts out. Losses decrease this, so if you wanted 15 volts to charge a battery at high power, you'd need closer to 30% duty cycle. A voltage control loop uses feedback to maintain the correct pulse width.

                          However, this simple topology will "collapse" the PV system since it exhibits negative impedance when looked at from the PV array side. Most loads exhibit positive resistance, so when voltage goes down, so does current. The feedback inherent in most buck converters tries to draw more current as voltage drops to maintain the same output, and so at some point (when the panels can no longer provide any more current) the voltage quickly collapses to near zero. The feedback loop is too "dumb" to know it can't get any more power by increasing pulse width.

                          That's why you can't just use a regular buck converter.

                          Now imagine that instead of a feedback look you have a knob that sets the pulse width, and a clever operator manning the knob and watching the meters. If he sees the output voltage too low he increases the pulse width. But if the voltage in the panels starts to drop off (especially if it goes below its maximum power point voltage) he would ease off on the pulse width to prevent the panel from collapsing. By twiddling the knob he can keep the system transferring the maximum amount of power.

                          So how do you replace this clever man with an algorithm? There are three basic ways I know of:

                          1) Simple backoff from OCV. Most panels produce their maximum power at about 80% of their OCV. So a simple way to implement this would be to disconnect the array momentarily, measure the voltage, then reconnect it and make sure the voltage never dropped below 80% of that value. You have to do this periodically to track changes due to temperature, clouds etc. The old RV Power Products (now Blue Sky Engineering) MPPT chargers did this.

                          2) Full sweeps. If you ramp your PWM from 0% to 100% you will see a point at which maximum power is produced. Then you limit your minimum voltage to that level. You can't do this if your batteries are close to fully charged because they won't accept all the current you need to run that test - but when you are close to full charge you don't need full power anyway.

                          3) Small sweeps. At startup you make a reasonable guess as to maximum power point (i.e. you measure the input and output voltages, divide and set the PWM value slightly above that) and then go up and down in both directions. If power increases you keep going in that direction; when power starts decreasing again you reverse direction. Since you can do this continually with very little loss in power you don't have to do anything periodically. This system is harder to stabilize than the above two.

                          You can also do a hybrid of 2 and 3 - do a full sweep on startup then small sweeps afterwards. The old MX60 did this.

                          Maybe it`s a battery of resistors and the system increases/decreases the total resistance by plugging/unplugging them from the string.
                          That would be a very crude way to do it. A slightly more sophisticated method would be to use a single programmable resistor (easy to make) that is varied by software. A much better way to do it is to use your load as the resistor, and vary that by varying the pulse width.

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                          • #14
                            mppt working

                            Originally posted by Mike90250 View Post
                            The charge controller "sweeps" the array with varied loading, and then chooses the best point. That point changes over the course of the day, as sunlight changes, and temperature changes the PV voltage curve. Very tough to do well, Morningstar, Xantrex and Outback, and most >1,000w Grid Tie inverters do this quite well.

                            sorry for disturbance.
                            I have a question which is making me uncomfortable... Either i give feedback to controller from dc dc converter output or somewhere else?

                            Comment


                            • #15
                              Originally posted by abdul Baso=it View Post
                              sorry for disturbance.
                              I have a question which is making me uncomfortable... Either i give feedback to controller from dc dc converter output or somewhere else?
                              A MPPT Controller is a small computer, that monitors the loading of the input of it's own DC-DC converter, and varys the output loading to maximize the power harvest.

                              To explain it further, I have to put on my electronic engineers hat, and it's not likely you have one, so our data will not likely connect.
                              Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                              || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                              || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                              solar: http://tinyurl.com/LMR-Solar
                              gen: http://tinyurl.com/LMR-Lister

                              Comment

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