Fraunhofer ISE has developed a grid-stabilising high-voltage (HV) inverter for the HV- silicon carbide (SiC) project under the Future Electricity Grids Funding programme financed by the German Federal Ministry of Education and Research. The inverter can regulate power currents of up to 10-15 kV, more than ten times higher than regular silicon inverters.
Fraunhofer’s three-phase inverter uses SiC transistors, enabling the inverter to be connected to the medium voltage grid without the need for an additional transformer. By regulating reactive power and filtering undesirable harmonics in the electricity grid, the inverter helps in the stabilisation of power grids with a greater share of renewables. While there are other ways of harmonics filtering, high voltage applications such as this are more effective than those working at lower voltages and require a 50 Hz transformer.
Currently, power electronics are coupled to the electricity grid mainly in a low voltage range. For grid stabilisation, STATCOMs (static synchronous compensators) supply continuous inductive or capacitive reactive power. Coupling to the medium voltage grid is thus affected by a 50 Hz transformer. Fraunhofer’s product can feed directly into the medium voltage grid, even without a transformer, through high voltage transistors made of SiC.
Commercially available silicon transistors have a blocking voltage of 6.5 kV, requiring a higher number of components and increased architectural complexity, resulting in more failure points. Another advantage of the new technology is that the transistors work with higher frequencies due to lower switching energy requirements. They also post lower power losses than regular silicon transistors, resulting in better control dynamics of the inverter. Due to the higher frequency, the inverter can act as an active filter to compensate for harmonics in the medium-voltage grid.
The use of high-blocking SiC transistors, however, presents new challenges. The transistors switch very fast. The steep rise in voltage during switching can cause faults or lead to partial or creeping discharges in the insulation. While developing the circuitry, efforts must be made to minimise these undesirable effects. Before the commercial implementation of the inverter, several technological improvements are needed in the power modules or in the inductive and capacitive components, etc.