Grid-level Power Electronics

Evolution of high voltage Power Electronics is transforming energy and power grid by means of wide-scale interconnection of renewable energy resources, as well as expansion of smart grids for flexible power flow. We investigate and report on latest devices, converters and components used in high voltage applications, such as in transmission and distribution energy networks.

Thermal Management
We investigate the potential performance of wide-bandgap power semiconductor devices, especially SiC power devices, in MMC-VSC converters used in HVDC and FACTS systems in terms of losses, junction temperature and its reliability consequences. HVDC-VSC systems are currently implemented in silicon IGBT technology. Proponents of SiC as a high voltage technology have long suggested that SiC has the potential to enhance the performance of HVDC-VSC systems by improving the energy conversion efficiency and less heat transfer. The topology of choice for the latest HVDC-VSC systems is modular-multilevel-converters (MMC) comprised of cascaded half or full bridge voltage units. However, the current state of the art SiC power devices are still a long way off the ratings required for HVDC. Furthermore, the low switching frequencies used in HVDC-VSC-MMC means that conduction losses dominate, hence, the fast switching capability of SiC power devices is not necessarily an advantage. State of the art high voltage silicon IGBTs/PiN diodes are comparable if not better than commercially available SiC power MOSFETs/Schottky diodes of similar ratings.  To this end, we are investigating the thermal management techniques suited to optimize the performance and reliability of these novel devices when used in such contexts.

‌Fault Management

IGBT/Diode pairs used at the heart of MMC-VSC converters are very vulnerable to high current transients and faults. So, it is critical that all the necessary protections are in place to ensure a cascade failure will not happen, which could be catastrophic from both technical and financial perspectives. To this end, we are working on fault management technologies and novel circuit breaker architectures that will enable a safe operation of converters. For example, we evaluate the impact of recent emergence of Silicon Carbide (SiC) power devices, i.e. thyristors, as protection units for MMC converters and to address the opportunities and challenges introduced by application of these devices.

‌Reliability Analysis

The newly emerged wide bandgap semiconductor devices, especially SiC power devices, have advantages which come mainly due to unipolar structure of these devices, enabling high switching rates. However, the combination of such fast switching transients with parasitic inductances in wirebonds and interconnecting passives are the source of electromagnetic strees, i.e. oscillations, in the output terminals of the device. This can have serious implications on reliability and control of the wide-bandgap-enabled power converters. We investigate main roots of such reliability concerns in SiC power modules. Some of these can be categorized as:

(i)   Ringing in the SiC MOSFET and SBDs due to high dI/dt.
(ii)  Ringing in the MOSFET’s body diode in hard commutation.
(iii) Ringing in device due to shoot-through by crosstalk.

Investigation of risk mitigation techniques and their effectiveness along with potential impact on the performance are also amongst our interests.

Converter Topologies

We investigate design of different converter topologies that could be used in grid-level application to enhance performance and reliability of converters. Half-bridge, full-bridge and hybrid converter architectures such as Alternate Arm Converter (AAC) and Series Bridge Converter (SBC) are amongst the topologies under research. These topologies can enable features such as fault blocking capability which would enhance reliability at a low expense compared with installation of DC circuit breakers. Design challenges of bespoke topologies for specialist applications, such as LVDC converters in distribution networks is another area of interest.