Inverter working principle (Solar Panel)

The inverter is an electronic device that transforms a direct current into an alternating current, see Figure 1 below

Figure 1

In general, the inverters used in photovoltaics can be divided into two large families:

• Inverter for isolated applications

Figure 2

•  Inverters that work in parallel with the national network

Figure 3

The differences between these two categories of equipment are technical and applicative, for this reason, the manufacturers have two separate product lines.

Using the inverter in photovoltaic systems is clear: photovoltaic modules produce energy in the form of direct current, while the devices and user systems (TV, washing machines, refrigerators, routers, etc.) operate in alternating current. The electricity distribution network is also in alternating current (AC), and therefore it is necessary to make the energy produced compatible with that consumed.

Basically, inverters for isolated services have the following characteristics:

•  They draw energy from a constant voltage source (typically an accumulator battery)
• They must maintain the output voltage at a constant value, close to the nominal voltage of the electrical network and, if possible, insensitive to load variations
• They must have a reduced internal self-consumption, possibly using the "standby" mode to avoid discharging the accumulation when the energy source is not available.

Conversely, inverters for grid service must have other characteristics that can be summarized as:

• Ability to stay as close as possible to the point of maximum power on the voltage-current curve of the photovoltaic generator, even in the case of partial shading and rapid variations due to variations in solar radiation on the modules
• High conversion efficiency, even at low power levels
• Ability to synchronize with the national network and to transfer all available power to it in compliance with existing connection rules
• Detection of any insulation losses of the photovoltaic generator
• Disconnection from the network in the event of a fault or anomalies detected
• Protection against the direct current in the event of a fault, implemented using a transformer at the output or suitable protection (separating the direct current section from the alternating current one).

 MPPT (Maximum Power Point Tracker)

The search function of the maximum power point, is made through the MPPT (Maximum Power Point Tracker) function, can only be found in the inverters for the national network, as the inverters for isolated applications typically draw energy from the storage battery and therefore do not they interface directly with the photovoltaic generator.

The MPPT device has the purpose of identifying moment by moment that particular point on the characteristic of the photovoltaic generator for which the power transfer to the load located downstream is maximum, i.e. towards the inverter. As the power of the photovoltaic varies instantaneously with the variation of irradiation and, more slowly, with the variation of the temperature. These variations cause the displacement of the maximum power point, which results in the practical impossibility of predicting its location by controlling the inverter with traditional calculation tools.

Operation 

The low-cost ones "transform" the 12/24 Volts voltage into 220 Volts alternating voltage in order to power low-power electronic devices, and is conceptually composed of two stages; the first raise the voltage from 12/24 Volt to a voltage of about 300 Volts. This voltage, in the simplest and usually cheaper inverters, also called "modified sine wave", is simply modulated by two digital signals, one controls the polarity and one the presence or absence of the output voltage, with the result that the output there is something similar to a square wave rather than a sine wave (see Figure 4 below).

In pure sine wave inverters, which is what I recommend, the second stage is more complex, the 300-volt voltage of the first stage is pulse-modulated (PWM) at a very high frequency and then filtered, the result is that in output, there is a sinusoidal voltage whose quality is often higher than that of the mains. For more details click here to see my project of a pure sine wave inverter (see Figure 6 below)

 Figure 6

The Stand-by of the inverter is used to disconnect the electrical system when there is no absorption for a certain number of minutes and reactivate it when the system requires energy.

Choice of the inverter

1. The sizing is done based on the maximum power required by the user, including the current peaks.
2. The choice is made based on the devices available on the market, which can be square or sine wave. If you choose the sinusoidal, you can decide to order a device with the Stand-By.
3. If pumps are installed or other devices with current peaks at start-up, the rated power must be doubled.

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