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Medium Voltage Direct Current Electronic Transformer

Periodic Reporting for period 2 - EMPOWER (Medium Voltage Direct Current Electronic Transformer)

Reporting period: 2020-12-01 to 2022-05-31

The main objectives of the project are related to development and demonstration of novel concept of Direct Current Transformer for high power Medium Voltage Direct Current (MVDC) applications. Nowadays, presence of power electronic converters in power distribution applications is only increasing and is considered a key technology for all future developments. This is mainly for the reasons that renewable energy generation (e.g. hydro, wind, photovoltaic) as well as new types of loads (e.g. electric vehicles), all require power electronics converters in order to be connected to existing Alternating Current (AC) power grid. A majority of these new technologies is actually DC by nature, there is great deal of interest into DC grids, motivated by various potentials and promises on the system level - generally being considered as better and more efficient solution overall. Yet, transition is not possible overnight, as AC grids have been perfected and cost optimised for more than a century.

Motivated by these undergoing changes, project focus is on a specific technology, presently not available, albeit judged as very important for future DC grids. That is a DC transformer, a power electronic converter that behaves as an equivalent transformer in AC world. Presence of such a device would allow for seamless integration and expansion of DC grids, providing simultaneously several functions: voltage adaptation, galvanic isolation and certain protection functions. Key differentiation, followed in the project, is the idea to explore operation of such an device without closed loop control and to explore how such a paradigm would impact the overall performances of the DC grids.

Considering that applications relevant for the project are characterised as medium/high voltage and high power, certain combination of technologies is considered and optimised for the desired performances. Namely, Integrated Gate Commutated Thyristor (IGCT) is exclusively considered as semiconductor device of choice, and pushed to its operating limits. Galvanic isolation is addressed through design optimisation of medium frequency transformers, using available technologies. System level investigations are performed through offline and real-time simulations, being widely accepted methodologies in power electronics.

Project results achieved so far, are disseminated, as much as it was possible under covid restrictions, and three patent applications have been filed with European Patent Office.
Activities within a project are summarised below, in relation to 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 single 3L-NPC bridge and split DC link capacitors, on each side of the medium frequency transformer. Transfer characteristics has been analysed and used to develop required algorithms for power reversal as well as reaction to faults in DC grids. 1MW rated demonstrator, with 10kV and 5kV on its terminals, is defined for the demonstrator.

Semiconductor devices: A lot of work is performed on the characterisation of the IGCT for the resonant soft switching conditions, under very low turn-off current. Thorough analysis is performed with regard to zero-voltage-switching (ZVS) versus zero-current-switching (ZCS), in order 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 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, optimised for soft switching and successfully used for majority of experiments.

Medium Frequency Transformer: To realize 1MW demonstrator, 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 realised by means of oil. Various models have been developed and integrated into design optimisation framework, used to explore design space and produce final design. Currently, we are assembling the prototype transformer.

MVDC systems: Theoretical framework and models are developed, enabling investigations of behaviour of DC Transformer inside the large DC networks. Nodal impedance models, system identification techniques are utilised to explore interactions between different elements of the system, as well as to assist planning of future system in various configurations. Real-Time Hardware-in-the-Loop modelling is ongoing, allowing for multitude of scenarios to be investigated.
Overally, project is well on track and it is expected that all goals are fulfilled within remaining period. Currently, all the activities defined in work packages, are coming together and project team is closely working on the realisation of the DC Transformer demonstrator during 2022. Already, several important achievement are demonstrated, such as: operation of IGCT at 5kHz, which is unprecedented for this kind of semiconductor; successful operation of soft-switched gate driver for IGCT; development of advance models to aid medium frequency transformer design optimisation; development of the scalable framework for the DC system level studies.
Test setup used for investigations related to IGCT devices