In view of the ongoing energy transition, expanding electricity grids has become an increasingly important task. This task comprises, first and foremost, connecting renewable plants as well as energy storage systems to the grid – no mean feat considering that the power electronics of the existing grid infrastructure must now also perform additional grid-supporting tasks, alongside the mere feed-in or feed-back of electrical energy.
To tackle this problem, scientists at the Fraunhofer Institute for Solar Energy have now (2021) developed a 250-kW silicon-carbide (SiC) inverter that can be used, for example, in utility-scale PV projects connected to a medium-voltage grid. As part of the project "SiC-MSBat” a 250-kW inverter stack was designed for feeding energy into 3-kV AC grids. For the inverter stacks, novel 3.3-kV SiC transistors were used which showed significantly lower power losses than the commonly-employed silicon transistors. This enabled them to operate the inverter stack with a switching frequency of 16 kHz. With normal silicon transistors, much lower switching frequencies could be reached in this voltage class. The high switching frequency also facilitated savings on the passive components, as these can be dimensioned in a smaller format. Another innovative design of the inverter is its active liquid cooling with a synthetic ester as cooling medium which is pumped through the inverter and cools both transistors via a liquid heat sink and filter chokes encased in a closed tank. At the same time, the cooling medium for the filter chokes serves as an electrical insulation medium, allowing the filter chokes to be made even more compact. The inverter was built and tested at Fraunhofer ISE's laboratories, achieving a very high efficiency rate of 98.4 percent at rated power.
The Fraunhofer Institute has been carrying out extensive research involving the optimal design of inverters for power applications for several years. In 2018, for example, its scientists designed a three-phase inverter which used high-voltage silicon carbide (SiC) transistors, so that it could 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 was able to contribute to the stabilization of power grids with a high share of renewables. This inverter could feed directly into the medium voltage grid without a transformer due to the use of high voltage transistors made of silicon carbide (SiC). Component prototypes with a blocking voltage of 15 kV were used for the purpose.
In 2020 a project started that would, if successful, produce a battery inverter designed as a building block for modern power supply. The novel inverter would offer grid-forming controls based on statics when connected to a healthy grid. During a power supply disturbance, the device could switch into uninterrupted-power-supply mode and form an island grid from renewable energy sources nearby. The planned demonstrator will have 200 kVA capacity, but the technology will be adaptable to the multi megawatt-hour range.
The new SiC inverter design shows several advantages compared to common inverters: the use of new types of silicon carbide (SiC) transistors with very high blocking voltages enables operators to connect the inverters directly to the medium-voltage grid. Owing to the high control dynamics of SiC inverters, they perform grid-stabilizing tasks acting as active power filters to compensate for harmonics in the medium-voltage grid. Also, SiC inverters can achieve much higher power densities than conventional inverters. The inverter operates at 98.4% efficiency and can be installed as multiple inverter stacks, which makes it ideal for deployment at megawatt scale. Also, considering that normally an additional space for switchgear and a cooling unit would be needed, a volume saving of the inverter system of up to 40 percent can be achieved, compared to commercial inverter systems of this voltage class.
The scientists see many potential applications for the use of high-blocking SiC devices in the medium-voltage range. It can increase the potential for savings and improvements in the system concept of PV power plants. In addition to regenerative power plants and large battery storage systems, other areas of application for medium-voltage power electronics may include drive systems and rail technology.