Activities within a project are summarised below, concerning different technological areas, rather than directly respecting work packages:
- Direct Current Transformer topology: Motivated by extremely good performances in terms of efficiency, resonant LLC converter topology is selected, with a single 3L-NPC bridge and split DC link capacitors, on each side of the medium frequency transformer. Transfer characteristics have been analyzed and used to develop required algorithms for power reversal as well as reaction to faults in DC grids. The 1MW-rated demonstrator, with 10kV and 5kV on its terminals, is defined as for the demonstrator.
- Semiconductor devices: A lot of work is performed on the characterization of the IGCT for the resonant soft switching conditions, under very low turn-off current. Thorough analysis is performed about zero-voltage-switching (ZVS) versus zero-current-switching (ZCS), to derive and determine optimal operating conditions. Continuous operation at 5kHz is achieved with 4.5kV IGCT devices, setting the record in the field. Considering the need to operate at voltages, much higher than device-blocking voltages, similar investigations are performed with series connection of IGCTs, supported by simple capacitive snubbers, and again successfully operating at 5kHz. Finally, 10kV IGCT engineering samples are provided by HITACHI Energy, Switzerland, and are subject of ongoing testing. A novel gate driver is developed, optimized for soft switching and successfully used for the majority of experiments.
- Gate Driver for IGCT: To further optimize performances, an optimized gate driver for soft switching applications is developed, integrating several protection features. Optimized design offers significant size reduction and advanced features that go beyond the state-of-the-art designs.
- Medium Frequency Transformer: To realize a 1MW demonstrator, a selection of suitable technologies is defined and adequate models are developed. Nanocrystalline material is selected for the core material and custom-made cores have been manufactured by HITACHI Metals, Japan. For the windings, hollow copper tubes are selected, and insulation between the winding is realized using oil. Various models have been developed and integrated into a design optimization framework, used to explore design space and produce the final design. Currently, we are assembling the prototype transformer.
- Control: In addition to the 1MW prototype, another two low-voltage DC transformer units were developed, to support the development of the power reversal algorithms. They are successfully validated experimentally. Additionally, the scalability of the DC transformers is analyzed (input-output paralleling) and validated using the mentioned low-voltage prototypes.
- MVDC systems: Theoretical framework and models are developed, enabling investigations of the behavior of DC Transformer inside the large DC networks. Nodal impedance models and system identification techniques are utilized to explore interactions between different elements of the system, as well as to assist in planning future systems in various configurations. Real-Time Hardware-in-the-Loop system i is developed. It enabled investigation of the impact of the DC transformers on the dynamics of the power system, as well as the validation of the power flows that can occur.
