Back to the Future

With its Sunny Boy HF, SMA has reinvented the transformer inverter

The new Sunny Boy HF is more efficient, more flexible, and easier to install than comparable models on the market, primarily because it combines a concept SMA developed 19 years ago and state-of-the-art technology.

From today’s perspective, the PV-WR 1500’s efficiency was modest: The device, which was released in 1991, achieved almost 92 percent – at the time a real feat. Its basic technological concept combined with top technology from today is the inspiration for the new Sunny Boy HF. As the name suggests, the HF is a high-frequency transformer. “A typical 50-hertz transformer is the largest and heaviest component in an inverter,” explains Sven Bremicker, Head of Platform Development, speaking about the initial situation. “So there was good reason for SMA to press ahead with the development of a transformerless PV inverter and to be one of the first manufacturers to bring one to market." However, there is still a need for inverters with transformers, whether due to the grounding required by certain module types or the galvanic isolation that is mandatory in certain countries.

The best of two concepts

As a result, the requirements the new family of devices had to fulfill were clear: an inverter that combined the benefits of both concepts. One that is both compact and light, offers all the benefits of galvanic isolation, and at the same time is more efficient than previous devices with transformers in this power class. “We achieved this primarily through the advancement of the HF topology,” says Sven Bremicker. Its basic idea is relatively simple: a transformer with the same power output becomes smaller and lighter, the higher the frequency of the alternating current flowing through it. If the direct current from the solar modules is initially converted into high-frequency alternating current, a significantly more compact transformer can be used as a result. Afterwards, the current is then rectified in order to convert it into alternating current with the usual grid frequency of 50 hertz.

Whereas the intermediate high-frequency circuit requires a series of additional semiconductor circuits, thus making the inverter’s design more complicated and also results in additional losses. “With the current state of technology, the additional electronics are not a problem – neither with regard to cost nor reliability,” explains Sven Bremicker, “and to increase efficiency, we rely on a really outstanding technological feature – resonant circuits.” The secret behind these circuits is something everyone who has ever sat on a swing is familiar with: the swing has a certain resonant frequency (also known as period of oscillation), which is determined by the length of the rope. By periodically applying a small amount of energy at just the right moment, it’s easy to generate a powerful and continuous swinging motion. This is much more efficient than moving a nonresonant object in alternation – and it also works with the semiconductor circuits in an inverter.

Small, light, and highly efficient

This innovative circuit concept is also reflected in the compact Sunny Boy HF’s technical specifications: European efficiency of 95.5 percent and maximum efficiency of over 96 percent both set the standard for galvanically isolated inverters. Weighing in at only 17 kilograms, the SB 3000HF weighs about half as much as conventional devices in this power class. In addition, the new family of devices has significant benefits when it comes to planning and installation. The extremely wide DC-voltage range of 175 to 700 volts offers remarkable module compatibility, and because of this, most popular six-cell modules can be arrayed in one string.

Grounded in three seconds

The Sunny Boy HF is the world’s first inverter to offer the ability to ground PV plants in basically no more than a “flick of the wrist.” The only thing required is a small accessory, the SMA Plug-In Grounding unit. This is a pen-shaped plastic part about nine centimeters long, with a five-pin connector on one end. “The revolutionary thing about this system is the necessary wiring comes standard with the inverter. You no longer need to open the inverter. Instead you just need to connect the grounding wire – by simply plugging in the Plug-In Grounding unit” explains Sven Bremicker. That means grounding a PV generator takes no more than three seconds: simply pull down the ESS handle on the activated inverter, plug the SMA Plug-In Grounding unit into the designated connector from below, and that is it – you’re done.

On-site flexibility

But the new grounding concept isn’t just simple, it’s also extremely flexible. “Whether you need to ground the PV generator’s positive pole or negative pole is obviously an important distinction,” explains Mr. Bremicker. “That depends on the type of module you’re using. But you don’t have to worry about that aspect when installing the inverter on-site, since both are possible with SMA Plug-In Grounding.” The trick: you can plug in the Plug-In Grounding unit in two different directions. The inverter recognizes which direction the unit is plugged in using the five-pin connector, and ensures that your system is grounded accordingly, either positively or negatively. The installer can also use a menu setting to specify whether the device should disconnect from the grid or simply alert the operator in the case of a possible grounding error.

Incredibly easy to use, intelligent features, and a highly efficient circuit concept – “This device integrates several good ideas from our developers. The new Sunny Boy HF is simply the perfect inverter for sophisticated systems,” note s a happy Sven Bremicker.