A power inverter is an electronic device that can change a direct current input voltage to a symmetrical alternating current output voltage with the magnitude and frequency desired by the user.
In its beginnings, photovoltaic installations used electricity for consumption at the same voltage, and in the same way, they received it from solar panels and batteries. They worked with a direct current of 48, 24, or 12 volts.
This characteristic marked a significant difference with users with an electrical distribution network or generator sets at 220 volts of alternating current. For this reason, they had to introduce a new power tool, the inverter.
What Are the Uses of a Power Inverter?
Inverters are used in various applications, from small power supplies for computers to industrial applications.
Here are some examples where an electrical inverter is used:
In a photovoltaic system, they are used to convert the DC power supplied by the solar panels into alternating currents.
In an uninterruptible power supplies of an electrical installation, the inverter converts the voltage supplied by the battery into alternating current.
In electric power transmission, the inverter converts the power into direct current transferred in some power lines to feed into the alternating current grid.
The realization of a switching power supply, for 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 supplied by batteries (in the event of an electrical failure)
Speed variation in electric motors.
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.
Sine wave inverters have a transformer that softens their shape, making it look a bit more like a sine wave. An excellent technique to achieve this is to use the PWM technique making the main sinusoidal 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 mains sine wave is devoid of it.
In the market, we can find two types of inverters:
Pure sine wave inverter.
Modified sine wave inverters.
Advantages of Power Inverters
It is the type of current used throughout the world and, therefore, gives a point of normality.
It facilitates the purchase of household appliances to be able to access the most efficient ones.
It allows for maintaining stable values of voltage and waveform, despite the variability of the state of charge of the batteries.
Working with higher voltages (220V is 18 times 12V) allows working with a lower current intensity, according to Ohm's law. In this way, thinner electrical conductors can be used, and losses due to the Joule effect are minimized.
Disadvantages of Power Inverters
The installation consists of one more element, the converter. Therefore, the reliability of the system decreases.
The converter has some electrical losses to compensate by generating more electricity for the panels (5%).
In small installations, the converter can represent a significant 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 Is Essential in a Current Inverter?
Main characteristics that define a converter
Input voltage (Vcc): this value must be equal to that of the battery (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.
Wave type: Inverters must present a normalized alternating current type format with a pure sine wave.
Overload capacity ( peak power) and thermal protection: instrumental in installations with motors, since at start-up, the power required for nominal operation can be doubled, although only for a few seconds.
The energy efficiency or performance of the converter is the relationship between the energy that the converter provides to the consumption in alternating current and the power that this input converter needs (from the battery).
Automatic start-up and standby status: it allows the power parts of the same converter to be disconnected in the absence of consumption. Afterward, they reconnect when they detect an energy demand above a previously set threshold.
Protection against polarity reversal and short circuits: basic options, given the possibilities of error or faulty operation 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 typically removed by filters, although this involves losses. As a result, the variation of the output voltage frequency will be less than 3% of the nominal.
The possibility of being combined in parallel will allow a possible growth of the installation and power consumption.
Good behavior with temperature variation: operating margin between -5ºC and 40 degrees Celsius.
Power Inverters for Solar Cells
The inverters of the photovoltaic systems for the entrance to the electrical network are specifically designed for this purpose. Its function is to transform the electrical energy in the form of direct current produced by the solar cells into alternating current to be able to supply it to the electrical network.
Photovoltaic panels have a voltage/current characteristic curve such that there is an optimum working point, called the maximum power point. If we stay at this point, it is possible to extract the maximum power from the solar cells.
This characteristic curve varies continuously according to the level of solar radiation received. Therefore, this type of current inverter is a power optimizer because they are designed to remain at this point and obtain the maximum possible electrical energy production.
In fact, on days with variable cloud cover, large and sudden changes in solar energy occur. For example, detecting variations from 100 watts / m² to 1000-1200 watts / m² in less than 2 seconds is pervasive. Under these widespread conditions, an inverter with less than 5 seconds settlement times can produce up to 5% -10% more energy than a slow one.
What Is the MPPT?
The MPPT is an added function that this type of inverter 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.
Several techniques to implement the MPPT function differ in terms of dynamic performance (settling time) and precision. Although the accuracy of the MPPT is crucial, 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.
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.
Inverter Charger
In the case of batteries, there are inverter chargers on the market. An inverter charger has two functions:
Transforms the direct current coming out of the batteries to alternating current
It has a battery charger to supply power to the PV or car battery from an external electrical source.
Hybrid Inverter
Hybrid inverters can be powered from either the utility grid or a battery providing the same type of electrical output current. The power source can be from a battery in direct current or the electrical network in AC power.
This inverter is also helpful if it works with an electrical generator in a power outage.
String Inverter
This type of solar inverter is used in series-connected solar panels grouped in branches to send all the electrical energy generated to a single power inverter.
Micro Inverter
A solar microinverter is a power tool responsible for converting the direct current from the solar panels into alternating current suitable for domestic use, such as a cigarette lighter.
Microinverters for solar panels have a more advanced technology than conventional inverters and have significant advantages in their use in solar installations. A microinverter is connected to a single solar panel only, although there are also models, duo microinverters, that can be connected to two modules simultaneously.
