Distributed inverters — the Sunvertec Innovation
Distributed inverters split the task of converting power across multiple units that work together. These units are connected to solar panels or battery cells, and they operate in a synchronised fashion to produce pure-AC. By splitting the hard task of converting power in to multiple simpler, smaller tasks the total size and weight of the inverter system can be reduced considerably.
A complete 5kW Xenos system takes up just 4 litres of space. Competing inverter solutions are in the 30-40L range, showing just how advantageous Sunvertec's approach is. With smaller size comes lower weight, simpler electronics, and lower prices.
How does it work? A Technical Description
A technical description of our distributed inverter technology follows, and assumes a high level of electronics understanding. Alternatively, see the video.
The Inverter Optimiser
Via a DC-DC converter, the Inverter Optimiser stores this charge into on-board local energy storage devices. The DC-DC converter typically does not alter the input voltage much at all, allowing each Inverter Optimiser to operate in excess of 99% efficiency, granting them huge power handling for their compact size. The local energy storage devices effectively "float" a certain charge level, experiencing low ripple currents and temperatures, giving them an ultra long life.
When commanded by the local Distriverter, an Inverter Optimiser places its energy storage across a series connection in either a positive or negative direction via the use of very low resistance MOSFETs. After a small period, the Distriverter returns the Inverter Optimiser to its bypassed state (the default state).
By switching between these three states, positive, bypassed, and negative, Inverter Optimisers can generate a "staircase" approximation of any low frequency signal, such as a mains signal. State changes occur infrequently (< 400Hz), meaning that switching losses involved in producing this "staircase" are negligible.
Whilst each Inverter Optimiser is only a low voltage part (typically < 80V), the series connection "adds" the output of each Inverter Optimiser to the next such that high total voltages can be produced. Just as two 12V batteries produce 24V when connected in series, strings of Inverter Optimisers (and their ability to switch "positive", "negative" or "bypassed") are able to output controlled voltages far above that of an individual unit.
Note the "staircase" produced by the series connected Inverter Optimisers. Just 4 Inverter Optimisers are required to produce mains level voltages, with the 5th (of the minimum system) aiding in flexibility for shaded systems and providing a level of redundancy.
Note also the few changes of state required, aiding in efficiency, and the small error between the staircase and the target waveform.
The Distriverter is a very small device placed at the start of an AC string. It is responsible for outputting the difference between the target waveform and the staircase produced by the Inverter Optimisers. When its output is applied to the same series connection, the staircase is "cleaned up" and pure AC is the result.
The Distriverter varies the waveform it's generating in real-time to target certain current and power factor profiles. It gathers its power from the series connection by spending roughly as long delivering charge as it does absorbing charge, meaning it requires no panel connection of its own. Inverter Optimisers with no or faulty solar panels gather power in a similar fashion.
Like all modules in a distributed inverter system, the Distriverter outputs only low voltages, allowing familiar low-voltage, long-life, electronics. Whenever the Distriverter finds itself near its voltage limits whilst "ramping", it simply requests an Inverter Optimiser to step in a supporting direction. This allows the Distriverter to step back away from its limits and then continue operating. By taking this approach the low voltage Distriverter can work in tandem with Inverter Optimisers to produce high voltages — up to 500VAC.
As the Distriverter is performing only a small, low-voltage, transformation its losses are very low. Also, as its losses do not increase with series voltage, it just gets more and more efficient at higher series voltage levels. Typical efficiencies of the Distriverter for grid-tie applications exceed 99.8%.
Excluding cabling losses, the Distriverter combined with a string of Inverter Optimisers can connect solar PV to the grid at > 99% efficiency.
It is clear from the description above that high speed communications and synchronisation are key to the distributed system. This is provided for by a designated line in our 6-core Optimiser Cable.
A custom transmission line provides for high speed communications down a single wire despite bidirectional data requirements and the high slew rates between modules in the system. A custom protocol both passes operational data and keeps devices synchronized on a nanosecond scale, reducing the size and cost of passive filtering components required.
A side effect of this high speed communications layer and recessed communications pin is that disconnecting any Optimiser Cable of an operating system leads to shutdown before the Active/Neutral and Earth pins have even separated, eliminating risk of unsafe arcing.
See the video
In 2015 a video was prepared explanation the operation of the Distriverter. There have been some minor changes since:
- The Distriverter is now a discrete module
- Optimisers are now referred to as "Inverter Optimisers"
But the underlying technology and principles remain the same.
For a very in-depth explanation of the system, please see Sunvertec's earliest pending patent, EP2601682A1.