The arrangement of the working parts inside of an inverter 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.
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 (see “Inverter Basics” in HP134 for more information on AC wave forms).
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.
Common Brand Names: Dimensions Unlimited; Heart Interface; Magnum Energy ME series; Trace Engineering DR, RV, UX and U series; Tripplite
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.
Brand Names: Powerstar UPG series, Samlex SI & SPE series, Statpower Prowat & PortaWatts series, Xantrex Xpower series
Making a sine wave from a DC source is much more difficult than making a square wave or modified square wave form. It takes more parts, more design, and a much more sophisticated control system.
The biggest challenge with making a sine wave inverter is doing it in a way that provides high efficiency. The earliest sine wave inverters were not very efficient—particularly with low-power loads. Advances in transistors and high-speed digital control systems now allow modern sine wave inverters to provide high-quality power at higher conversion efficiencies—even at low power levels.
Sine-wave inverter topologies vary from simple to complex. Each has its benefits and drawbacks—there is no “best” topology. The range of applications for sine-wave inverters is too varied.
While many of the sine wave inverter topologies can be used for both off-grid and grid-tied applications, the construction and features utilized in a battery-based inverter are very different than for one designed to be used only with a PV array as a source.
Low-Frequency Ferro-Resonant Sine-Wave Inverter. The need for a higher-quality AC wave form prompted inverter engineers to figure out how to make a better sine-wave output from the rugged and simple low-frequency square wave inverter topology. The modified square-wave form was just not good enough to operate the more demanding electrical loads and also could not be used for grid-tied applications.
The first solution used was to add a filtering system to the output, to “round-off” the square edges of the modified square wave form. Several manufacturers offered ferro-resonant transformer-based output filters, which improved the compatibility with sensitive electronic loads such as laser printers, but had a big impact on the inverter’s efficiency, especially when operating at low power levels, making them unpopular for RE applications. But they have been used in high-end power supply and telecommunication markets to power sensitive loads when efficiency is not critical.
With these topologies, the quality of the sine wave form is very dependent on the characteristics of the AC load being powered, since there isn’t any feedback control of the ferro-resonant transformer, so the success of this solution was hit-and-miss. This topology does not readily allow the inverter to correct or adjust the shape of the output wave form, since the transformer operates passively and is not actively controlled.
Brand Names: Alpha Technologies, Eaton Sola, MGE Topaz, Shape Magnetronics Line Tamer
Low-Frequency Multistep Sine-Wave Inverter. Another solution that was developed to make the AC output closer to a true sine wave involves combining several low-frequency, inverters operating at different frequencies together in series. This allows multiple AC output voltage levels to be produced, creating a stepped sine-wave approximation of a sine wave form. This approach resulted in a surprisingly good sine wave, while still using low-frequency switching and transformers to maintain efficiency at low power levels while only modestly increasing the cost and complexity.
The AC output sine wave allows some inverters using this topology to be tied to the grid, although few manufacturers are currently using this topology due to the high parts count and resulting high manufacturing cost.
Brand Names: Trace Engineering SW & PS series; Sustainable Energy Technologies Sunergy series
Mixed Frequency Sine-Wave Inverters. This topology combines the benefits of both low-frequency and high-frequency inverters. High-frequency switching transistors convert the DC source to a lower-voltage AC waveform. The transistors are switched at high frequency—hundreds of times per AC cycle or about 20,000 times a second. An inductor then smooths the choppy, high-frequency square wave form into a smooth, low-frequency wave form—creating a low-voltage sine wave. Then, a low-frequency transformer steps up the AC voltage to the required 120 or 240 VAC. This type of inverter is able to produce a “true” sine wave like a high-frequency inverter, but it is simpler to build and more reliable.
Because the DC currents are being switched at high frequency, which causes electrical noise, a carefully designed output filter must be included to eliminate electrical interference with loads or clean up the power being sent into the utility grid.
Like the low-frequency modified square-wave inverters, these types of inverters are also large and heavy. When used in a batteryless grid-tied PV application with high DC input voltages, efficiencies as high as 96% can be attained since the DC currents being switched are much smaller than with low-voltage battery systems.
Brand Names: Apollo Solar TSW series; OutBack FX series; Xantrex GT and XW series; SMA Sunny Boy and Sunny Island series; PV Powered PVP series
High-Frequency Sine-Wave Inverters. Another way of producing a high-quality sine wave uses high-frequency power conversion. While significantly complex, it does allow a dramatic reduction in the size and weight compared to low-frequency sine wave inverters.
The challenge with this design is similar to the high-frequency modified sine-wave inverter—the complexity and high parts count can make reliability an issue, and efficiency at lower power levels can be unacceptable.
This topology is also used by many batteryless grid-tied PV inverters to send surplus power to the utility. In the two-step power conversion process, the control system dedicates the “front end” converter (which goes from DC to AC to DC and is comprised of transistors, and a transformer and rectifier) to extract the most power from the PV array (called maximum power point tracking), while the second converter comprised of transistors and an output filter (capacitors and inductors) is optimized to put the most power back into the utility grid.
Brand Names: Exeltech XP series; Fronius; Xantrex ProSine; Solectria PVI series; SMA SB HF series
High-Frequency Transformerless Sine-Wave Inverters. This topology is starting to become available as inverter manufacturers compete to offer the highest efficiency inverter at the lowest cost for grid-tied PV applications. Removing the transformer eliminates its losses, allowing inverter efficiencies up to 98%.
However, removing the transformer from the inverter introduces a new problem: the PV array’s DC output cannot be grounded. The PV array is connected to the utility grid at low frequency (60 times a second) in a “positive” and “negative” configuration—which requires that the PV array be “floated,” with neither the positive nor negative DC conductors grounded. In essence, the inverter’s transistor bridge “flips” the PV array with respect to ground 60 times a second. This scenario was not permitted under National Electrical Code guidelines until recently. In Europe, ungrounded electrical systems are commonplace, making the switch to transformerless inverters much easier.
Transformerless inverters are not likely to be used in battery-connected applications (e.g., off-grid systems), since the battery cannot be grounded with these types of inverters, which would increase hazards present on the serviceable connections of the battery. Also, the changes made to the NEC, which allow ungrounded systems, were limited specifically to on-grid PV array applications. Transformerless inverters can be utilized in an AC-coupled application with a transformer-equipped stand-alone inverter as this allows the battery to be grounded and the PV array to be ungrounded.
Brand Names: Power-One PVI series
Christopher Freitas has worked in the PV industry since 1986 as an electrical engineer. He has participated in the development of many UL, NEC, and IEEE standards and volunteers for developing-world RE projects with Sun Energy Power International (www.sunepi.org). He lives in an off-grid solar and microhydro-powered home in Washington state.