An inverter is a device that changes direct current to alternating current. However, that is only half the story, at least in the real world. The real function of the inverter is to change the direct current, which is usually supplied by a solar panel or a battery, to alternating current, which is the same as the power supplied by the mains.

In other words, the inverter should control the voltage, frequency, and quality of the waveform of the power supplied by the mains, as well as match the requirements of devices ranging from simple lighting circuits to complex electronic devices and motor control circuits. Once you understand the real function of the inverter, you will understand the reason for the existence of different kinds of inverters.

Output Waveform and Why It Matters

One of the most important ways to classify an inverter is based on the waveform produced. While this may sound like a theoretical way to classify inverters, rest assured that it is quite important in practice.

The simplest form is the square wave inverter. The waveform produced is one that switches rapidly from positive to negative voltage. While this type is simple and inexpensive to build, it is also one that produces a large amount of harmonic distortion. In practice, this translates to heat, noise, and inefficiency in equipment. Because of this, square wave inverters are no longer used in practice.

A step up from this is the modified sine wave inverter. This has a stepped sine wave output instead of the hard-edged square wave. This is a step up from the square wave inverter in terms of performance. However, it is still not the best option. It will have reduced motor performance and some devices will make noise. It will also have reduced lifespan.

The sine wave inverter is the gold standard in modern inverter designs. It has a smooth sine wave output that is similar to the mains waveform. This is important for compatibility in modern appliances that have electronic control and variable speed drives. In Australia, homes are now being filled with electronic devices and energy-efficient appliances. In this case, sine wave output is not negotiable.

Inverters in Real Systems

Another way of understanding the different types of inverters is to look at them in the context of their application in the field. This is perhaps the best way of looking at the types of inverters, depending on the field of work.

Solar inverters are perhaps the most common types of inverters you are likely to encounter in the field. They are used to convert the DC power generated by solar panels to AC power. In residential installations, string inverters are the most common type of inverter used in solar installations. In this type of inverter, several solar panels are connected to one inverter. This is different in microinverters, in which each solar panel has its own inverter. This makes them suitable for installations in which shading is a problem. In solar farms, central inverters are used.

Battery inverters, on the other hand, have a different function. They draw power from batteries and convert it to AC for use in the home or facility. These inverters are important in backup and off-grid systems, where battery banks play a key role.

Grid connect inverters are meant to function in parallel with the grid. This means they will synchronize to match the voltage and frequency of the grid. One notable feature is that they will automatically switch off in case of a power outage. While this is a safety feature to prevent backfeeding, it is often a source of confusion for homeowners, who expect to continue drawing power from their solar panels even when the grid is down.

Off-grid inverters work independently of the grid and need to produce their own voltage and frequency. This type of inverter is common in the remote regions of Australia. Hybrid inverters incorporate the features of solar power, battery power, and grid power into one inverter. This is relevant in today’s scenario due to the increased usage of home batteries and cars.

What’s Happening Inside the Inverter

From an engineering perspective, one of the most important distinctions is that of voltage source inverters and current source inverters. This classification is based on the method used to supply the inverter and its control method on the DC side.

Voltage source inverters, abbreviated to VSI for simplicity, are by far the most popular type of inverter. Voltage source inverters use a stable voltage on the DC side and use switching elements to create an AC waveform. The reason for their prevalence is that they are simple to design and can be used for any application, including new control techniques.

The current source inverters, on the other hand, have a different design approach. They are supplied by a fixed current source, and the control is exercised over the current, not the voltage. This necessitates a different front-end arrangement, which usually involves large inductors. Even though the current source inverter is less popular, it is still used in some high-power industrial applications.

Single-Phase and Three-Phase Systems

Another basis for classifying inverters is that they can be either single-phase or three-phase. This is closely related to the system that is being supplied.

For instance, single-phase inverters are used in residential areas. This is because they can supply power to ordinary loads that are used in homes. On the other hand, three-phase inverters are used in commercial and industrial areas. This is because there are high-power loads that are used in commercial and industrial areas, and the inverter must be capable of supplying three-phase power for such loads.

How Modern Inverters Control Power

One of the most important developments in the technology used in inverters is the use of pulse width modulation. Instead of trying to create an actual waveform, the inverter rapidly switches its output on and off. By adjusting the width of the pulses, the waveform is effectively modulated into something similar to a sine wave.

This allows for the precise control of voltage and frequency, as well as reducing any harmonic distortion. It’s the reason why modern inverters are able to provide such good quality power with such compact designs.

The concept of multilevel inverters is an extension of this idea, in which rather than switching between two voltage states, several voltage states are employed. This produces a waveform that is even closer to the ideal sine wave, with less distortion and stress on the devices. This type of inverter is usually employed in high voltage applications.

Emerging Trends in Inverter Technology

As the energy landscape in Australia is still changing, there is an emergence of new inverters that can cater to these complex systems.

The grid-following inverter is the conventional type of inverter that is used in most solar systems today. These inverters depend on the grid to provide them with a reference signal. However, grid-forming inverters can provide their own voltage and frequency reference. This is an important feature in modern solar systems because of the high penetration of renewable energy sources, where stability is an issue due to the absence of grid support.

Another topology that is in development is the Z-Source inverter. This topology is unique in that it is able to step down as well as step up voltage levels. This gives it an advantage over other conventional topologies.

Choosing the Right Inverter

In reality, the selection of the right type of inverter ultimately boils down to understanding the needs of the system that you are dealing with. In the case of a residential solar system, as installed in Australia, the system will be a pure sine wave type, either grid-tied or hybrid, depending on whether a battery storage system is used. In the case of an independent system, the inverter should be able to operate independently and provide a clean sine wave under varying load conditions. In the case of a commercial system, the selection of the right type of inverter involves understanding the load requirements, the operating requirements, and the future needs.