Mitigating SSR in an HVDC– and renewable-rich grid at Cardiff University

Tibin Joseph, Carlos E. Ugalde-Loo, Jun Liang, et. al – Cardiff University

Cardiff University’s Centre for Integrated Renewable Energy Generation and Supply (CIREGS) is a world-leading research group with over 20 research projects on power system stability and control, DC grids, the integration of renewables, VSC-HVDC, and MTDC. Funded by the UK’s Engineering and Physical Sciences Research Council and the Higher Education Funding Council for Wales, their lab features a multi-terminal HVDC test-rig, RTDS Simulator, power amplifiers, DC grid emulator, wind turbine test-rig, and more.

In recent published work at CIREGS, a hardware-in-the-loop setup has been leveraged to investigate a wideband filter approach to damp the effects of subsynchronous resonance on a renewable- and HVDC-rich AC grid. The threat of SSR in Great Britain’s transmission system has increased due to the recent installation of series compensation near the Anglo-Scottish intertie. While traditional SSR mitigation schemes use filters placed at the generator terminals or integrated with network components, damping at transmission level is also common (via, for example, schemes embedded in HVDC and FACTS converter stations). However, most proposed controllers are designed for specific operating points and are sensitive to system configuration and loading conditions. The wideband filter based damper proposed by CIREGS, embedded in the VSC control system, damps multiple torsional modes and can be used regardless of system configuration and series compensation level. Instead of designing filters for each torsional mode, the wideband filter does not require re-tuning. Given the uncertainty around Great Britain (GB) system data, this flexible approach is highly relevant. Notably, damping via the embedded filter is provided without the need for overrating the converter—the dc voltage and power are within the converter’s existing capabilities.

Three-machine model of the GB transmission network with reinforcements

Performance was tested on a scaled down version of the GB transmission network. In the hardware-in-the-loop setup, the RTDS Simulator provided real-time representation of the three-machine model GB network (developed in consultation with National Grid and upgraded to include new infrastructure reinforcements).

 Via hardware-in-the-loop testing, it was found that SSRs were effectively damped by the operation of the wideband damper irrespective of the series compensation level. The damping scheme was tested for both strong and weak AC systems (varying effective short circuit ratio), and it was found that damping capability is preserved for weak systems. However, preservation of damping came at the expense of affected generators oscillating for a longer period.

The main limitation of this damping scheme is the location of the damper; the damping scheme relies on the use of local measurements and reacts slowly if the affected generator is far away. This issue could be alleviated by placing the damper nearby the generator most susceptible to SSR.

The SSR phenomenon will only become more common with increased integration of renewable sources and series compensation into existing AC grids. Complex solutions and extensive testing will be required in order to properly mitigate the issues that will arise from the incorporation of these new technologies into traditional systems. The CIREGS team was able to validate a scheme which effectively mitigated SSRs — via wideband filter-embedded HVDC links — using a real-time HIL setup which incorporated the RTDS Simulator.