Supervisors: Prof. Stephen Finney, University of Edinburgh and Prof. Lie Xu, University of Strathclyde
PhD Student: Jiaqi Xu, University of Edinburgh
Status: Ongoing
This project will explore alternative solutions to HVDC network protection and evaluate any advantages these may have the use of HVDC hubs can facilitate the significant savings in the HVDC infrastructure required to connect post 2030 levels of offshore wind generation. The hubs will create multiterminal HVDC networks which will require the use of new protection techniques if they are
to match the security and reliability/security of today’s AC transmission networks. Current thinking is to achieve this using DC Circuit breakers capable the ultra-fast gault interruption needed to prevent fault propagation and network-wide voltage collapse. A variety of DCCB technologies have been proposed of which the Hybrid combination of fast mechanical isolators and semiconductor interrupters the most promising solution. The use of DCCB circuit breakers the most intuitive solution but the approach is not without problems, high over-voltage and breaking energies, high power semiconductor requirements, and additional current limiting reactors. This project will investigate how energy storage may be integrated into the HVDC hub to enhance the performance of reverse blocking converters
and reduce or eliminate the need for DCCB.
This project will explore the potential alternative approaches which coordinate the inherent controllability of the ‘power electronic rich’ HVDC may be a possible means of reducing or eliminating the need for DCCB.
1. Reverse blocking FB-MMC can remain connected to the AC network while blocking currents into the faulty network. In this mode the basic FB-MMC can continue to provide reactive power. This technology is attractive for use onshore OHL based HVDC where temporary line faults are common, and the value of fast fault interruption balances the additional losses of FB-MMC. The approach may also be applied to multiterminal HVDC where the reverse blocking converters can extinguish fault current allowing isolation to be completed by mechanical disconnectors without the need for additional power semiconductor-based interrupters. Whilst attractive for
network protection, this approach has the disadvantage of introducing a transient network-wide (albeit short) interruption of power flow, which must be accommodated by the wider power network.
2. It is predicted that 2030 and beyond will see the integration of high levels of electrical energy storage in the power network, some of which is likely to be located at HVDCHubs. Research has shown that storage can be integrated into the FB-MMC structure. This has the potential to allow real power to the AC network to continue while a DC fault on the DC network is isolated.
3. For offshore wind converters there may be a possible to avoid the use of FB-MMC. In such connections the offshore windfarm AC network is fed by the wind turbine converters capable of control of their output current. The contribution to DC fault current of such windfarm is inherently limited to near rated value and with appropriate control can be reduced to zero. This could remove the need for reverse blocking FB-MMC converters but would require new fault ride through control to manage the wind turbines during the DC fault and re-establish power flow once the DC fault is cleared and the DC network reenergised.