5-level topology – the next logical step for inverters

Fri, 01/09/2017 - 10:16

Huawei Solar

Inverters work on the principle of switching between different levels of DC voltage. The output is thus not an ideal sine-shaped AC wave, but rather a pattern of rectangular pulses that are made to resemble the sine wave as accurately as possible by switching between specified at high frequency. Therefore, the ability to approximate an exact sine wave with rectangular pulses is restricted by the number of available voltage levels: the higher the number, the closer the approximation to the ideal sine shape. The non-ideal sine shape causes harmonics (currents and voltages at higher frequencies). Harmonic filters can be used to smooth the output by absorbing problematic harmonics, but these filters are both a cost factor and a cause of additional losses in energy conversion.

The simplest inverter is the two-level converter. It is called two-level because it can apply only two voltage levels: the DC supply voltage and the reverse of that voltage. The three-level neutral-point clamped (NPC) inverter is an extension of this concept that can additionally apply the neutral point voltage. This is the frequently encountered technology in today’s inverters.

Inverters could be (and have been) designed that take a step beyond this concept and increase the output to five voltage levels, i.e. positive, zero, negative, and two intermediate levels (a fraction between positive, respectively negative and zero). However, such circuits often come at the price of far higher complexity. For example, if the DC supply were to be built with five rather than three voltage levels, it would require additional clamping diodes and capacitors and the corresponding control and charging circuitry.

Huawei set out to address these issues of a 5-level-topology, and found a solution that can output five power levels without adding complexity to the DC supply by using so-called control circuits, which are controlling when to connect to positive, negative or zero levels. Huawei’s “more silicon, less copper” concept replaces electronic components such like capacitors and inductors by digital information components like chipsets and software. “more silicon, less copper” means to use more chipsets and software. The output will be smoother like a sine shaped wave, because of more precise power switching and controlling technology. Meanwhile, as it uses chipsets and software, each inverter truly becomes the “brain” of a PV plant. It enables smart O&M of PV plants, i.e. it can detect and process different faults intelligently of each string.

But using the multi-topology technology also reduces the quantity and size of “copper” as capacitors and inductors. Instead of working on physics to make bus bars thicker or breakers larger, Huawei seized the advantages that digitalization offers regarding efficiency and manageability of silicon without the expense of copper. As the quantity and size of “cooper” components are reduced greatly, the inverter size and weight could also be reduced greatly. With 5-level-topology and the use of digital information components, it will become easier to build inverters that are more cost effective by technology innovation and mass production. Meanwhile, as it uses the IC and software, each inverter truly becomes a “brain” of the PV plant. It enables smart O&M of PV plants, i.e. it can detect and process different faults intelligently of each string.

To summarize: The advantages of Huawei’s 5-level topology are:

  • higher peak and EU efficiency (98.8 % European Weighted Efficiency)
  • better grid compatibility
  • high power density due to smaller magnets
  • reduced LCL filter size, better reduction of harmonics and total harmonic distortion, resulting in a purer sinusoidal current
  • low stress in IGBTs, the switch that is used in order to allow power flow in the “On” state and to stop power flow when it is in the “Off” state
  • lower electromagnetic interference (EMI)
  • higher output voltage of 800 V, through the synthesis of 5 levels
  • saving copper costs
  • reduced inverter size and weight

Huawei has achieved these important improvements by building on its long legacy of technology innovation in the telecommunications business, not only being the world’s third largest smart phone manufacturer, but also supplying the underlying telecom infrastructure. Across its business divisions, nearly half of the company’s 180,000 headcount is specialized in R&D. As a consequence, Huawei is member of more than 360 standards organizations, industry alliances and open source communities.