The research programme on Electronic Power Grid (eGrid) was initiated in 2018, aiming at developing a world-leading research cluster on the development of power-electronic-based infrastructure for modern power systems. The opportunities brought by the emergence of wide-bandgap (SiC and GaN) power semiconductor devices are exploited. The control, stability and electromagnetic compatibility of power electronic converters in modern power grids are also focus areas.
The research programme is organised in three groups:
- Packaging and Manufacturing of Wide Bandgap Power Modules
- Medium-/High-Voltage Megawatt Power Electronics
- Stability and Power Quality
MEDIUM-/HIGH-VOLTAGE MEGAWATT POWER ELECTRONICS
The newly emerging Silicon Carbide (SiC) devices in 10 kV voltage class are getting increased attention since it enables the utilization of the prevalent and simple two-level voltage source converter topology in medium voltage power conversion applications by eliminating the need for series connection of devices and utilizing multi-level converter topologies. In its present state the large-scale commercialization of this technology is constricted due to its lower market volume, higher production cost, scarce on-field reliability statistics as well as limited knowhow of dealing with engineering challenges in packaging and designing power electronics converters, however the trend is expected to shift with higher economies of scale and increasing adoption of these devices in industries.
This research programme is focused on the design and development of the megawatt scale medium voltage power electronics converter intended for marine, renewables and grid support applications. The 4.16 kV MV power electronic converters rated for 500 kVA with a two-level converter topology enabling switching frequency of up to 10 kHz will be demonstrated by utilizing the custom packaged half bridge 10 kV SiC MOSFET power modules. The proposed power electronics system based on the new semiconductor technology is expected to have system efficiency benefits in range of 1 % - 2 % compared to based state of the art Si semiconductor devices based solutions.
Fig. 1: Three-phase power stack using 10 kV SiC MOSFET power modules
In this research programme, the key research challenges imposed due to the high dv/dt switching transitions of SiC MOSFETs in regards to EMI/EMC, partial discharge and overall system efficiency will be addressed and overcome for the MV power electronics system building blocks such as: gate drivers, semiconductor power modules, MV filter inductors, converter control system.
Fig. 2: A 3D CAD model of medium voltage inductor for analysis using finite element simulation
PACKAGING AND MANUFACTURING OF WIDE BANDGAP POWER MODULES
The semiconductor die is the heart of any power electronic converter, enabling the control of power from energy sources, through the grid and to loads. Conventionally the die has been based on silicon as semiconductor material. Lately wide bandgap materials such as silicon carbide (SiC) and gallium nitride (GaN) has reached competitive prices and availability in the market.
Fig. 1: Bare semiconductor dies must be packaged and encapsulated to ensure safe operation.
A key property explored in the eGrid research programme is to utilize the high voltage operation capability of the SiC MOSFET. These are available in bare die form at voltage ratings of 10-15 kV, while traditional Si MOSFET are rated at 1.2-1.7 kV. The increase in voltage level has the potential to bring unprecedented performance for renewable energy sources, as it allows for high efficiency and direct integration to the grid. However, these properties can only be utilized if the dies are packaged properly. Research is focused on design of power modules, high voltage operation and insulation properties of materials.
Fig. 2: Developed 10 kV SiC MOSFET power module prototype
The research programme is determined to prove the concepts through rapid prototyping in the power module packaging laboratory facilities. Power modules are developed relying heavily on digital design and finite element simulations to predict the system behavior and reduce development time. Having accurate digital models including original 3D geometries, allows keeping the time of each design cycle iterations short. In this way, optimized power module designs are proposed faster. Finally, the power modules are built in the packaging laboratory and operation verified. Highlights of the in-house laboratory facilities include: etching of ceramic substrates, vacuum vapor phase soldering, heavy wire wedge bonder and ultrasonic terminal welder. Quality of developed prototypes is ensured using process control utilizing clean workstations, plasma cleaning, optical microscope inspection and scanning acoustic microscopy.
Fig. 3: In-house power module packaging laboratory
STABILITY AND POWER QUALITY
Power electronic converters are currently widely used as the interface for both renewables (e.g. solar and wind power) and modern loads (like LED and electronics). In this case, the future power system will naturally become a power electronic based power system, or electronic power grid. It has been widely recognized that power electronic converters can promise overwhelming flexibility, efficiency, and controllability in electrical power conversion over conventional equipment. However, the various control loops of different power electronic converters may have unexpected control interaction either with other control loops or due to the adverse grid condition. As a result, stability and power quality issues that differs from conventional understanding arise in electronic power grid scenario.
This research programme is dedicated to study stability and interoperability of power electronic converters to address abovementioned instability and power quality issues in electronic power grid. The major research activities are focused on advanced modeling methods, small- and large-signal stability analysis as well as impedance measurement for non-invasive black-box stability assessment.