Integration of Lithium Batteries to Improve Reliability and Power Density of WBG-based Medium Voltage Direct Current Converters

The emerging wide bandgap semiconductors (WBG) offer an opportunity to significantly improve the performance of power electronics converters by increasing switching frequency and reducing losses. However, the required DC capacitors stay large and vulnerable even if fast-frequency WBG switches are used. This is because DC capacitors are sized mainly according to low-frequency voltage ripple. About one third of converter failures are caused by failed DC capacitors and it is often that more than half the size of power converters is occupied by DC capacitors, e.g., power modules in Modular Multi-level Converters (MMC).
This timely project is proposed to reduce DC capacitors by integrating high performance compact Lithium batteries into power converters. The integrated batteries can be used in a multi-functional way including to provide electric grid ancillary services, which improves the economic viability of such solution. It is accepted wisdom that future power systems will be dominated by power electronics converters, so the proposed study is expected to address a large new market for Lithium batteries.

The project will focus on battery-integrated AC-DC converters connected to Medium Voltage AC grid (11-132kV). Medium Voltage AC-DC (MVDC) converters are expected to be widely adopted in future distribution networks for system controllability. A study in Scotland has estimated a reduction of £1.7bn in the costs of the collector and connection circuits would be possible only in Scotland if they were connected to Medium Voltage DC. The proposed project is based on the Angle-DC project, which is being developed by Scottish Power Energy Networks (SPEN) to demonstrate the first point-to-point MVDC link (30 MW, 54 kV) for operation of a DC circuit. Cardiff University is the only academic partner of the Angle-DC project.

In detail, the project will (1) quantify benefits of integrating batteries into MVDC converters; (2) develop advanced MVDC converter topologies and control strategies to integrate batteries, considering efficiency, power density and reliability; (3) develop co-ordinated protection strategies of batteries and MVDC converters for safe operation; (4) co-ordinate system dynamics through distributed ledger technologies to provide ancillary services. The research will be verified on a lab-scale experimental platform, which has been established at Cardiff and has the same topology but down-scaled power ratings compared to the MVDC converters in the Angle-DC project.