A power inverter is an electronic device. The function of the inverter is to change a direct current input voltage to a symmetrical alternating current output voltage, with the magnitude and frequency desired by the user.
Power inverters are also called current converters and current inverters .
In the early days of photovoltaic solar energy, electrification installations used electricity for consumption at the same voltage and in the same way that they received it from photovoltaic solar panels and accumulators, that is, at 12, 24, 48 volts in DC. This made a big difference with users who had an electrical distribution network or generators at 220 volts AC.
Why are power inverters necessary?
The appliance market has adapted to most users and we can find any appliance with 220 volts AC . Therefore, getting reliable, quality and reasonably priced appliances that operate at low voltage and direct current is more difficult.
Therefore, equipment is required to transform direct currents with low voltage values into alternating currents with voltage values of 220 volts. These are inverters (also known as inverters or converters).
The direct current / alternating current converters (inverters, inverters) allow converting the 12, 24, 48 volts of direct current produced by the solar panels and stored in the battery, into alternating current of 125 or 220 V (currently 230 V), like the one normally used in places where the conventional electrical network is.
How does a power inverter work?
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 smooths 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 making the sinusoidal main component much larger than the higher harmonics.
The square waveform generated by these devices has the problem of being rich in higher harmonics, while the sine 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 electromagnetic compatibility problems.
Types of investors
They can be broadly classified into two types:
- single phase inverters
- triphasic inverters.
What are modern converters like?
The most modern investors have begun 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 get to a wave that reasonably simulates a sine wave at the input of the transformer, rather than relying on it to smooth the wave.
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
Capacitors and inductors can be used to smooth the current flow to and from the transformer. These elements are placed at the input and output of the transformer to suppress harmonics.
Furthermore, it is possible to produce a so-called "modified sine wave", which is generated from three points: one positive, one negative and one ground. A logic circuit is responsible for activating the transistors so that they alternate properly.
Modified sine wave inverters can cause certain loads to operate less efficiently. For example, electric motors.
What are the most advanced inverters like?
The most advanced inverters use pulse width modulation with a much higher carrier frequency to get closer to the sine wave or space vector modulations improving the harmonic distortion of the output. The wave can also be pre-distorted to improve the power factor.
The system can be fed back to provide a stable output voltage as the input voltage changes. For both types of modulation, the signal quality is determined by the number of bits used. It spans from a minimum of 3 bits to a maximum of 12 bits, able to describe the sinusoid very well.
In asynchronous electric motors, and even more so in synchronous electric motors, the rotation speed is directly related to the frequency of the supply voltage.
Where necessary in the industry to vary the speed of a motor, alternating current and alternating current (AC-AC) inverters 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 inversion section. In fact, this is therefore an "inverter-rectifier" system even if they are only known as "inverters" (ie only "inverters").
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.
What are the advantages and disadvantages of current converters?
The advantages of having electrical energy in the form of alternating current are diverse:
- It is the type of current that is used worldwide and therefore gives a point of normality.
- It facilitates the purchase of household appliances to access the most efficient ones.
- It allows to maintain stable values of voltage and waveform, despite the variability of the state of charge of the batteries.
- The fact of working with higher voltages (220V is 18 times 12V) allows to work with low electrical currents and, therefore, thinner conductors, habitual electrical protections can be used and losses are minimized.
Not all are advantages, they also have some drawbacks:
- The installation consists of one more element, the converter. Therefore, the reliability of the system decreases.
- The converter has electrical losses to compensate by generating more electricity to the modules (5%).
- In small installations, the converter can represent an important part of the budget; for example, for an installation of about 100 Wp of module power, a 250 W converter can represent 20% of the total cost.
What characteristics must be taken into account in a power inverter?
Main characteristics that define a converter
- Input voltage (Vdc): This value must be equal to that of the accumulator (12, 24, 48 V).
- Output voltage (Vac): this value must be normalized (230 VAC).
- Output / input voltage stability: Variations of up to 10% for square wave converters and 5% for sine wave converters are supported. These are values that the standards allow for the voltage of conventional electrical networks, regardless of the power demanded by consumption. On the other hand, in installations with accumulators, the input voltage may not be greater than 125% nor less than 85% of the nominal input voltage of the converter.
- Wave type : Currently, inverters must present a standard AC type format with a pure sine wave.
- Overload capacity (peak powers) and thermal protection : very useful in installations with motors, since at the moment of starting you can double the power required for nominal operation, although only for a few seconds. Keep in mind that any motor, when starting up, can draw a current of up to five times the nominal current and that, as a general rule, is approximately three times.
- The energy efficiency or performance of the converter is the ratio between the energy supplied by the converter to AC consumption and the energy required by this input converter (from the battery). If the converter designed for a certain power works in a fraction of this power, the performance will drop. A sinusoidal converter must demand a 70% efficiency working at 20% of the nominal power and 85% when it works at a power higher than 40% of the nominal one.
- Automatic start and standby state : it allows the power parts of the same converter to be disconnected in the absence of consumption and reconnected when they detect an energy demand above a previously set threshold.
- Protection against reversal of polarity and short circuits : basic options, given the possibilities of error or malfunction of the consumer circuits that are high during the life of the converter.
- Low harmonic distortion : parameter related to the quality of the generated wave. Harmonics are normally removed by filters, even if this leads to losses. The frequency variation of the output voltage will be less than 3% of the nominal.
- Possibility of being combined in parallel : it will allow a possible growth of the installation and the consumption power.
- Good behavior with temperature variation : operating margin between -5ºC and 40ºC.
- Enough technical documentation . It is required, as a minimum:
- Input and output working voltage.
- Nominal voltage.
- Nominal frequency and distortion factor.
- Output waveform.
- Allowable working temperature range.
- Performance depending on the power demanded.
- Overload that resists.
- Short circuit resistance.
- Power factor.
Photovoltaic inverters for grid input
Photovoltaic inverters for input to the electrical grid 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 into the electrical grid . The current output of the solar panel is initially in the form of direct current.
What is the MPPT?
The MPPT is an added function that this type of inverters has. The MPPT uses special software and hardware control systems that allow the maximum power available in all weather conditions to be extracted from the solar panels.
The photovoltaic panels, in fact, have a voltage / intensity characteristic curve such that there is an optimal working point, called the 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. Clearly, an inverter capable of staying "hooked" at this point will always get 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 settling time is, in some cases, even longer. While all inverter manufacturers can achieve high precision 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 cloud cover large and sudden changes in solar energy occur . It is very common to detect variations from 100 watts / m² to 1000-1200 watts / m² in less than 2 seconds. Under these conditions, which are very frequent, an inverter with settlement times of less than 5 seconds can produce up to 5% -10% more energy than a slow one.
Some PV inverters are equipped with modular power stages, and some are even equipped with an MPPT for each power stage. In this way, manufacturers leave the engineering system free to configure independent master / slave or MPPT operation.
The use of independent MPPTs provides an objective advantage under uneven panel irradiation conditions. In fact, it is not uncommon for the surface of solar panels to be exposed to the Sun in a way that differs across the field. This is because it is organized on two different levels of the ceiling, since the modules are not distributed in chains of equal length or due to 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 energy production would be damaged.
Another important feature of a photovoltaic inverter is the network interface. This function, generally integrated into the machine, must meet the requirements established by the regulations of the various electricity supply companies.
What uses and applications does a power inverter have?
Inverters use a wide variety of applications, from small computer power supplies to industrial applications to control high power. 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.
In addition to the applications in the field of photovoltaic solar energy, the applications of the inverter are multiple:
- In the use of solar panels in photovoltaic solar energy, as we have seen, it allows transforming direct voltage into alternating 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 electrical lines to supply the alternating current network.
- The realization of a switched power supply, for transformation into direct current, with considerable advantages in terms of efficiency, size and weight
- In the aerospace industry, they are used to provide aircraft avionics with highly stable alternating current even if supplied by batteries (in the event of a power failure)
- Variation of speed in electric motors .