Grid-Forming Inverter Capabilities for System Restoration in 100% IBR Power Systems

Australia’s National Electricity Market (NEM), the availability of traditional synchronousmachine black start providers is declining. This has prompted ongoing research into the potential of IBRs, particularly under scenarios of 100% IBR penetration, to support system restart.

The research presented in this work is part of the Commonwealth Science and Industrial

Research Organisation’s Australian Research in Power Systems Transition program, and focused on five key areas:

• Investigating the stability boundary conditions of restarted islands which are inclusive of multiple non-black start IBR support devices and analysis of the impact of and reasonable range for control system parameters of IBR during system restoration. • Dynamic modelling of Distributed Energy Resources (DER) control systems and energy source variations during a 100% IBR restart, using an Electric Power Research Institute

(EPRI)-developed model to assess potential destabilising effects when a grid-forming

(GFM) Battery Energy Storage System (BESS) acts as the black start source. • Evaluation of grid-following (GFL) IBR operational settings and strategies, including hybrid plants, to enhance stability during system restoration, with a focus on real-time operational adjustments. • Assessment of how IBR black start sources affect network protection relay performance, identifying potential issues that could hinder successful network restoration. • Development of an analytical tool to estimate the required GFM IBR capacity to replace the energisation role of retiring synchronous generators in the NEM. Using both manufacturer-specific and generic Electromagnetic Transient (EMT) models

(including representation of fast, inner control loops), the research has generated many shareable insights and models for the broader research community. A sample of key findings include:

• A GFM black start IBR to GFL support IBR device ratio of up to 1:10 is recommended as the upper limit for system restoration, neglecting network impedances. The 1:10

GFM to GFL ratio was demonstrable as stable for both system restoration and application of network faults during the restoration process. • DER active power variations due to energy source availability (e.g., cloud cover) along feeders do not destabilise the system, so long as it operates within black-starter import and export limits. This was true for both synchronous and grid-forming BESS black start sources, given that the variations were relatively slow compared to controller dynamics. • DER penetration exceeding 80% of the GFM BESS’s nameplate rating can cause phaselocked loop (PLL) instabilities, which are not observed with synchronous black start sources. • Instability in GFL IBR-heavy systems was more often due to poorly configured park power controllers (PPCs), not inverter fast-controller and PLL instability. • High internal power transfers in GFL AC-hybrid plants during disturbances or transformer energisation can rapidly destabilise the system due to different components thresholds for entering into fault ride-through mode. • Transformer differential protection may require lower harmonic blocking thresholds for successful energisation, but GFM BESS sources showed a greater ability to energise transformers without tripping differential protection when compared to same-sized synchronous machines. The research highlights the strong enablement role of GFM technology, especially GFM BESS, in future system restarts in areas of high IBR penetration. It demonstrates that GFM BESS may be viable as a primary restart source, and in several aspects, superior to synchronous machine technology.