Inverter is an electronic device or circuitry that changes DC to alternating AC. The input voltage, output voltage and frequency, and overall power handling, are dependent on the design of the specific device or circuitry. A power inverter can be entirely electronic or may be a combination of mechanical effects (such as a rotary apparatus) and electronic circuitry.
The arrangement of the working parts inside of aninverter is called its “topology”—the configuration of the various electronic components that allow it to produce an AC waveform from a DC source.
Some topologies can be used to make different types of AC output wave forms (square wave, modified square wave, or sine wave), or even work in an off-grid application with a battery, or a batteryless grid-tied RE application. The difference is in the details—the quantity, type, and arrangement of transistors, capacitors, transformers, and inductors; and the sophistication of the control system utilized.
Some topologies may be suitable only for certain applications due to safety requirements and performance limitations. Although this sounds like a serious limitation, it is more of an indicator of very specialized solutions for specific applications. Transformerless inverters, which are just starting to become available in the North American marketplace just for grid-tied PV applications are an example of this.
Each topology has its advantages and disadvantages. By understanding the trade-offs involved and by optimizing the components and design, the best topology for an application can be determined.
Square-Wave & Modified Square-Wave Inverter Topologies
The earliest inverters used in renewable energy applications produced a coarse square wave AC output—fairly easy to accomplish, and therefore much cheaper. Plus, they offered low losses. Early square-wave inverters were later replaced with an improved “modified” square-wave inverter design, which improved performance and appliance compatibility while using the same basic inverter topologies. Because of the low power quality, these inverters cannot be connected to a utility grid. While most current PV systems do not use these types of inverters, knowing how they work is important to understanding the evolution of inverter manufacturing.
Two different groups of square-wave inverter topologies are used to make essentially the same resulting modified square-wave AC output. There also are many additional variations within each of these two groups, but it is easiest to divide the topologies into either a low- or a high-frequency type.
Low-Frequency Modified Square-Wave Inverters. A set of transistors first converts the DC source into a low-voltage AC wave form. The transistors are switched on and off about 120 times per second during each AC cycle—also referred to as switching at 120 Hertz. A low-frequency transformer steps up the low AC voltage to the required 120 VAC. This topology is one of the simplest inverter designs, but is limited to producing square-wave and modified square-wave AC output waveforms.
These types of inverters are easily identified by their large size and weight. The relatively large low-frequency transformer makes them heavy, but it also makes the units rugged and reliable, since the transformer also provides DC-to-AC isolation and protects the transistors from damage, sort of like a heavy bumper on a truck.
Because of the simplicity of this topology and its low parts count, fairly high efficiencies—even at low power levels—can be attained since the low-frequency switching reduces losses in the transistors and the transformer. Since all of the switching is done at low frequencies, no elaborate DC or AC filtering is required to minimize interference with loads, although AM radio interference is often encountered.
This topology is only used in less-expensive, battery-based inverters in occasional-use applications such as RVs and cabin systems, as some common AC loads do not tolerate the non-sinusoidal modified square-wave form.
High-Frequency Modified Square-Wave Inverter. In a high-frequency inverter, the transistors are turned on and off about 20,000 times or more per second during each AC cycle, also referred to as switching at 20 kilohertz (kHz). This topology is more complex and can be used to produce a variety of AC output wave forms, including a true sine wave.
With this topology, the DC source is first stepped up to a higher-voltage AC wave form by a set of transistors switching at 20 kHz and a high-frequency transformer. Then, it’s rectified to an intermediate DC voltage (usually between 200 and 400 VDC), which is stored in a set of capacitors. An additional set of output transistors switching at low frequency (120 Hz) is then used to produce the modified square wave AC output from this high-voltage DC source.
These types of inverters are easily identified by their smaller size and lighter weight (compared to low-frequency units), since the large low-frequency transformer has been replaced with a much smaller high-frequency transformer. Because the output set of transistors is not isolated by a transformer, they also tend to be more sensitive to abuse and voltage surges and lightning, resulting in lower reliability.
Achieving high efficiencies (greater than 90%) with this topology can be challenging when working with low DC voltage systems, such as with battery applications. It also can be difficult to provide high “surge” currents for a long enough time period to start larger motors.