How does MPPT maintain a constant voltage?

Question

I have a slight confusion in the working of the MPPT algorithm in solar inverter. I am confused about how this converter maintains a constant 310V (required for H-bridge) with MPPT. MPPT is a maximum power point tracking algorithm, and it is possible I am getting maximum power at 400 V or 200 V. This would make the resulting AC voltage much higher and lower respectively than required.

Below is an image from a paper that shows how a MPPT DC-DC converter works, but it doesn't talk about how it maintains a constant 310-312 V for a 220 V AC RMS. The load is basically the inverter stage shown below in the second image with three-phase H-bridge configuration:

I do not understand how the MPPT algorithm maintains a constant voltage, which is necessary for home appliances. Please help me understand this as I am not getting a solid answer from anywhere.

Answer 1

MPPT is not a specific "algorithm", it's a concept.

When you pull more current from a solar panel, the panel voltage drops; if you pull too much current, the voltage collapses and you get much less power. By moderating the current that you pull from a solar panel, you avoid the voltage collapsing too dramatically. There is an optimal point, usually around ¾ of the panel's open-circuit voltage, where the achieved power is maximal, although the optimal voltage varies depending on the panel design, sun conditions, temperature, etc.

Maximum Power Point Tracking refers to the idea of a device that continuously moderates how much current it pulls from a solar panel to maximize the total power.

I do not understand how the MPPT algorithm maintains a constant voltage, which is necessary for home appliances. Please help me understand this as I am not getting a solid answer from anywhere.

MPPT is a general concept and doesn't refer to any particular device such as an AC inverter.

For example, one could design an MPPT device whose sole purpose is to dump all solar power into a simple heating coil. That doesn't have a constant output voltage. That's why you're not finding any results for your query.

MPPT is not about trying to maintain a constant output voltage for an appliance; it's about trying to maintain an optimal input voltage coming from the solar panel.

For those devices that do aim to produce a particular output voltage, such as solar battery chargers or grid-tied inverters, they will have a voltage converter with at least two feedback mechanisms: one to prevent the output (appliance) voltage rising above the target, and one to prevent the input (solar panel) voltage falling below the maximum power point. They will move and convert as much power as they can subject to these two limits.

Answer 2

As with any regulator the output voltage can be made stable/fixed/constant with varying input voltages. The diagrams are just so simple that they do not show the presence of feedback, so the output voltage is measured and the pulses are adjusted to keep the output voltage constant.

So it does not really have anything to do with MPPT, these are two concepts working together to make a product.

The MPPT extracts the max power available at some voltage and current. Then that power at some voltage and current is transformed to same power at some other voltage and current, and that just is the 311V DC bus needed for powering AC loads, so there is only the power available that comes from panels minus the power lost in the conversion.

Answer 3

The power converter shown in the MPPT is known as a buck/boost converter; this is a type of DC-DC converter that can produce an output voltage that is either lower (buck) or higher (boost) than the input voltage, depending on the control pulses.

You may be familiar with buck converters; the output voltage of these can vary from 0 to 100% of the input voltage, but can never exceed 100%. Similarly for boost converters, the output voltage can be either 100% or many times the input voltage, but can never be below the input voltage.

Therefore, the buck/boost converter can control its output voltage to be constant at 310Vdc regardless of whether the PV module output voltage is 200Vdc or 400Vdc. The Wikipedia entry does a pretty good introduction to the buck/boost converter:

https://en.wikipedia.org/wiki/Buck%E2%80%93boost_converter

Answer 4

An MPPT can only track the maximum power point if the load can swallow the power, regardless of how much. Usually, you're either charging batteries or feeding the grid. In that case, the load is in control of the voltage, and the MPPT feeds it as much current as it can.

If the load cannot swallow the power, the converter can drop into a mode where it is regulating voltage. In that mode, it's no longer an MPPT.

Answer 5

Your first figure has a flaw.

It's lacking the input capacitors of the DC-DC converter.

Those are massively important.

Let's assume that the L is so big that the current is approximately constant, i.e. very little variation during the switching cycle.

Let's also assume that the duty cycle is 50%, i.e. the switch is open 50% of the time and closed 50% of the time.

Then you have half of the time no current in the solar panel and half of the time twice the average current in the solar panel. That's very far away from the maximum power point. Actually, in this case the controller trying to keep the solar panel at the maximum power point is not keeping it at this point. Without the input capacitors, this is simply a PWM controller.

If you add a huge input capacitor parallel to the solar panel, in this case the twice-the-average current would come half from the solar panel, half from the input capacitor. Then when the switch is opened, the input capacitor voltage has sagged, so the solar panel would still be charging the input capacitor, with the same current the solar panel previously supplied to the inductor. So the solar panel current with huge input capacitors would be constant, and it would be operating at the maximum power point.

A DC-DC converter can work without input capacitors, if (a) it's connected to a device that already has output capacitors, or (b) it's connected to a battery. Neither of these is true in this case. (And besides, input capacitors can help in cases (a) and (b) as well since the input capacitors are closer to the DC-DC converter high frequency part, with less wiring resistance.)

Your figure is a buck-boost converter. To understand how it can reduce or increase the voltage, you need some understanding in DC-DC switched mode power supplies. Essentially when the switch is closed, it will charge the inductor to a certain current. Then when the switch is opened, the only way for the inductor current to go is to charge the capacitor C, and an inductor will maintain whatever current was going through it, and create whatever voltage necessary to maintain this current. It may be the voltage of the inductor will be less or more than what it used to be. Since the derivative of the current switches sign, the voltage will be inverted, and its absolute magnitude can either increase or decrease depending on what voltage C has. The control algorithm just then varies the duty cycle to keep the capacitor C at the optimal voltage.

Most MPPT systems for battery charging are buck converters, so they require the solar panel to have a higher maximum power point voltage than the battery voltage, usually by a margin of few volts, but usually you want far more margin since you want maximal power transfer for cloudy days as well and in those days the maximum power point voltage reduces. The buck-boost converter can allow lower maximum power point voltage as well, because it can ramp up the voltage too. The cost of the buck-boost converter is that it requires a heftier inductor, since all of the energy needs to be stored in the inductor magnetic field. Another cost is that the voltage inverts its sign, so if you want negative side to be ground on both sides, you can't have that. There are some ICs that do synchronous rectification in a buck-boost configuration in a way that allows maintaining the voltage sign by controlling several MOSFETs.