Periodic Reporting for period 4 - EMPOWER (Medium Voltage Direct Current Electronic Transformer)
Okres sprawozdawczy: 2023-12-01 do 2024-05-31
The main objectives of the EMPOWER project were related to the development and demonstration of a novel concept of a Direct Current Transformer for high power Medium Voltage Direct Current (MVDC) applications. Nowadays, the presence of power electronic converters in power distribution applications is only increasing and is considered a key technology for all future energy system 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 to be connected to existing Alternating Current (AC) power grid. A majority of these new technologies are DC by nature, there is a great deal of interest in DC grids, motivated by various potentials and promises on the system level - generally being considered as a better and more efficient solution overall. Yet, the transition is not possible overnight, as AC grids have been perfected and cost-optimized for more than a century.
Motivated by these undergoing changes, the project focus was set 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 the AC world. The 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, demonstrated in the project, is the idea to explore the operation of such a device without closed-loop control and how such a paradigm would impact the overall performances of the DC grids.
By taking a holistic approach, a DC transformer technology is ultimately developed and demonstrated, while simultaneously several underlying technologies are improved. The work conducted in the project has considered all elements of high-powerr converters, such as semiconductor devices and methods to drive them, galvanic isolation using medium frequency transformers, as well as operational algorithms suitable for the envisioned DC power distribution networks.
The EMPOWER project has addressed the evolutionary direction of the existing power systems. Large deployment of natural DC resources (sources and loads), will naturally lead to wider deployment of DC power distribution networks. This transition will not be quick, due to legacy AC systems, and it will require new technologies, such as the DC transformer developed in the EMPOWER project. For society as a whole, security of supply is an extremely important matter, and all results of the EMPOWER project are serving that need.
Motivated by these undergoing changes, the project focus was set 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 the AC world. The 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, demonstrated in the project, is the idea to explore the operation of such a device without closed-loop control and how such a paradigm would impact the overall performances of the DC grids.
By taking a holistic approach, a DC transformer technology is ultimately developed and demonstrated, while simultaneously several underlying technologies are improved. The work conducted in the project has considered all elements of high-powerr converters, such as semiconductor devices and methods to drive them, galvanic isolation using medium frequency transformers, as well as operational algorithms suitable for the envisioned DC power distribution networks.
The EMPOWER project has addressed the evolutionary direction of the existing power systems. Large deployment of natural DC resources (sources and loads), will naturally lead to wider deployment of DC power distribution networks. This transition will not be quick, due to legacy AC systems, and it will require new technologies, such as the DC transformer developed in the EMPOWER project. For society as a whole, security of supply is an extremely important matter, and all results of the EMPOWER project are serving that need.
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.
- 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.
Overall, the project stayed well on track and all goals were fulfilled. The realized DC Transformer demonstrator is operational and integrated into the research facility of the laboratory. Already, several important achievements have been demonstrated, such as: the operation of IGCT at 5kHz, which is unprecedented for this kind of semiconductor; the successful operation of a soft-switched gate driver for IGCT; the development of advanced models to aid medium frequency transformer design optimization; development of the scalable framework for the DC system-level studies. The high quality of results obtained in the project enabled us to present them in the form of tutorials at several IEEE conferences, as well as Keynote talks.