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An inverter is an electronic device. The function of the inverter is to change a DC input voltage to a symmetrical AC output voltage, with the magnitude and frequency desired by the user. The inverters use in a great variety of applications, from small power supplies for computers, to industrial applications to control high power. The inverters are also used to convert the direct current generated by photovoltaic solar panels, accumulators or batteries, etc., into alternating current and thus be able to be injected into the electrical network or used in isolated electrical installations.

A simple inverter consists of an oscillator that controls a transistor, which is used to interrupt the incoming current and generate a rectangular wave.

This rectangular wave feeds a transformer that softens its shape, making it look a bit more like a sine wave and producing the necessary output voltage. The voltage output waveform of an ideal inverter should be sinusoidal. A good technique to achieve this is to use the PWM technique getting the main sinusoidal component to be much higher than the higher harmonics.

More modern inverters have started to use more advanced forms of transistors or similar devices, such as thyristors, triacs or IGBTs.

The most efficient inverters use various electronic devices to try to arrive at a wave that reasonably simulates a sine wave at the input of the transformer, instead of depending to smooth the wave.

They can be classified in general into two types:

  • monophase inverters
  • three phase inverters.

Capacitors and inductors can be used to smooth the flow of current to and from the transformer.

In addition, it is possible to produce a so-called "modified sine wave", which is generated from three points: one positive, one negative and one earth. A logic circuit is responsible for activating the transistors so that they alternate properly. Modified sine wave inverters can cause certain loads, such as motors, for example; operate less efficiently.

The more advanced inverters use pulse width modulation with a much higher carrier frequency to get closer to the sine wave or modulations by space vectors improving the harmonic output distortion. You can also predistort the wave to improve the power factor (body & Phi;).

Photovoltaic inverters for the entrance to the network

The photovoltaic inverters for the entrance to the electrical network is a particular type of inverter specifically designed to convert electrical energy in the form of direct current produced by a photovoltaic module into alternating current to be fed directly to the electrical network. The current output of the solar panel is initially in the form of a direct current.

These machines extend the basic function of a generic inverter with extremely sophisticated and advanced functions, through the use of special software and hardware control systems that allow to extract the maximum available power in all the climatic conditions of the solar panels.

This function is called MPPT. The photovoltaic panels, in fact, have a voltage / intensity characteristic curve such that there is an optimal work point, called maximum power point, where it is possible to extract the maximum available power.

This characteristic point varies continuously according to the level of solar radiation that hits the surface of the photovoltaic cells. It is evident that an inverter capable of staying "hooked" at this point will always obtain the maximum power available in any condition. There are several techniques to implement the MPPT function, which differ in terms of dynamic performance (settling time) and precision. Although the accuracy of the MPPT is extremely important, the establishment time is, in some cases, even more. While all inverter manufacturers can achieve high accuracy in the MPPT (typically between 99-99.6% of the maximum available), only a few can combine precision at speed.

In fact, on days with variable cloudiness there are large and sudden changes in solar energy. It is very common to detect variations of 100 watts / m² at 1000-1200 watts / m² in less than 2 seconds. Under these conditions, which are very frequent, an investor with settling times of less than 5 seconds can produce up to 5% -10% more energy than a slow one.

Some photovoltaic inverters are equipped with modular power stages, and some are even equipped with an MPPT for each power stage. In this way, manufacturers allow the engineering system freedom to configure independent master / slave operation or MPPT. The use of independent MPPTs provides an objective advantage in non-uniform panel irradiation conditions. In fact, it is not uncommon for the surface of the solar panels to be exposed to the Sun in a manner that differs throughout the entire field. This is because it is organized in two different levels of the roof, since the modules are not distributed in chains of equal length or due to a partial shading of the same modules. In this case, the use of a single MPPT would cause the inverter to work outside the point of maximum power and, consequently, the production of energy would be damaged.

Another important characteristic of a photovoltaic inverter is the network interface. This function, generally integrated in the machine, must comply with the requirements established by the regulations of the various electricity supply companies.

Inverter applications

In addition to applications in the field of photovoltaic solar energy, the investor's applications are multiple:

  • In the use of solar panels in photovoltaic solar energy, as we have seen, it allows to transform the direct voltage into alternate voltage to be used in the home or to enter the distribution network.
  • In uninterruptible power supplies, the inverter converts the voltage supplied by the battery into alternating current.
  • In the transmission of electrical energy, the inverter converts the energy into direct current transferred in some electric lines to feed the alternating current network.
  • The realization of a switched power supply, for the transformation into direct current, with considerable advantages in terms of efficiency, size and weight
  • In the aerospace sector, they are used to provide aircraft avionics with a highly stable alternating current even if it is supplied by batteries (in the case of an electrical fault)
  • Variation of speed in electric motors.

The simplest type of inverter consists of an oscillator that drives a transistor, which generates a square wave that opens and closes a circuit. The wave is then applied to a transformer that supplies the required voltage at the output, rounding the square wave to a certain point. Often more efficient devices such as the MOSFET, the thyristor or the IGBT are used instead of the common transistor.

The square waveform generated by these devices has the problem of being rich in higher harmonics, while the sinusoidal wave of the electrical network is devoid of it. This implies a lower efficiency of the motorized equipment, greater noise, both acoustic and electrical, and serious problems of electromagnetic compatibility.

The more complex inverters use different approaches to produce a sinusoidal waveform with the highest possible output. An electronic circuit produces a step voltage by means of pulse width modulation (PAM) as close as possible to a sine wave. The signal, called modified sine wave, is leveled by capacitors and inductors placed at the input and output of the transformer to suppress the harmonics.

The best and most expensive inverters base their operation on pulse width modulation (PWM). The system can be fed back to provide a stable output voltage as the input voltage changes. For both types of modulation, the quality of the signal is determined by the number of bits used. It ranges from a minimum of 3 bits to a maximum of 12 bits, capable of describing the sinusoid very well.

In asynchronous electric motors, and even more in synchronous electric motors, the rotation speed is directly related to the frequency of the supply voltage. Wherever it is necessary in the industry to vary the speed of an engine, inverters of alternating current and alternating current (AC-AC) are used. In these systems, the input voltage is first converted to direct current by a rectifier and leveled by capacitors, then applied to the reversing section. In fact, this is, therefore, an " inverter-rectifier" system even if they are only known as "investors" (that is, only "investors"). The purpose of this double operation is only to change the desired frequency within a predetermined interval and the presence of a transformer is not necessary, since it is not necessary to vary the value of the output voltage that remains equal to the input voltage. The output frequency is determined in the simplest cases by an analog signal supplied to the inverter, for example, by a potentiometer, or by a digital signal sent by a PLC.

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Last review: May 22, 2018