Challenges and Mitigation Measures in Power Systems with High Share of Renewables—The Australian Experience

The paper addresses how modern power systems can operate securely with high penetration of inverter-based resources (IBRs), focusing on Australia’s National Electricity Market (NEM). It highlights challenges arising from renewable energy zones located in weak grid areas with low short-circuit levels and limited system strength. The purpose is to summarize common technical challenges (stability, protection, black-start) and practical mitigation measures while showcasing Australia’s experience and planning to reach up to 75% variable renewable generation by 2025 and ~90% by 2035. The study emphasizes the urgency for solutions that can be deployed faster than major transmission builds, such as synchronous condensers (SynCons), battery energy storage systems (BESSs), FACTS, and grid-forming controls, to maintain reliability, security, and cost-effectiveness.

The paper is a review synthesizing definitions and indices for grid strength (SCR, CSCR, WSCR), classes of stability affected by IBRs (traditional rotor angle, frequency, voltage; resonance/SSR; converter-driven stability), and protection/black-start challenges. It compiles international experience of IBR-related disturbances and mitigations (e.g., ERCOT, WECC, China), fault-induced PV tripping, subsynchronous oscillations in DFIG and PMSG wind farms, and controller/PLL-related issues. It catalogs mitigation practices: operational constraints, transmission upgrades (SynCons, FACTS), special protection schemes, and advanced inverter controls (grid-following vs grid-forming). A table of global case studies summarizes events, resources, mitigations, and recommendations, positioning Australia’s experience within the broader literature.

This is a comprehensive review and case-based analysis focused on the Australian NEM. The authors: (1) synthesize technical definitions and known challenges for high-IBR systems from standards, utility reports, and prior studies; (2) document Australian operational issues in weak areas (e.g., West Murray oscillations, transformer energization overvoltages in Queensland, voltage swings in Tasmania) from utility/TSO reports; (3) describe deployed and planned mitigation technologies (SynCons, BESSs, FACTS), including project characteristics and timelines; (4) present impact assessments via real operational data and case studies (e.g., South Australia’s SynCons and IBR dispatch envelopes; Hornsdale Power Reserve market and security services; Dalrymple grid-forming BESS performance under faults/islanding). Quantitative evidence is drawn from AEMO, ElectraNet, Transgrid, OpenNEM, and project documentation to illustrate outcomes on system strength, inertia, frequency control, costs, and renewable hosting capacity.

- High-IBR integration challenges: Reduced inertia and system strength increase risks in rotor angle, frequency, and voltage stability; resonance (SSR) and converter-driven stability issues can manifest from interactions between IBR controls and weak networks; protection sensitivity is reduced due to low IBR fault currents (~1.2–1.5 p.u.) compared to synchronous (~6 p.u.); most traditional IBRs lack black-start capability. - Grid strength metrics: SCR = SCCMVA/PMW; grids with SCR < 5 are considered weak. CSCR and WSCR account for multi-infeed IBR effects. - Australian operational issues observed: Sustained ~7 Hz, ~5% p–p voltage oscillations in West Murray during a planned line outage; resonance overvoltage on transformer energization in a weak Queensland area (short-circuit level ~1.8× transformer rating); large voltage rise (1.01→1.08 p.u. in ~40 s) in Tasmania due to reactive plant switching amid low fault levels. - Mitigation portfolio: Operational constraints (curtailments, minimum synchronous online); transmission upgrades (SynCons, FACTS); special protection schemes (e.g., transfer trip, RoCoF/differential protection); advanced inverter control—grid-forming (GFM) converters enabling voltage formation, synthetic inertia, higher fault current, and black-start (subject to energy buffer and overcurrent limits). - Synchronous condensers impact (South Australia): Four large SynCons (each 575 MVA fault capability at 275 kV) with flywheels providing 1100 MWs inertia per unit (total ~4400 MWs) installed at ~A$190 million capital cost. Operation of all four SynCons enables dispatching up to ~2500 MW of IBRs while requiring fewer synchronous units online under normal conditions; supports system strength and inertia, reduces renewable curtailment and reliance on high-priced gas units. - Record minimum demand and high renewables: On 21 Nov 2021, rooftop PV supplied ~92% (1220 MW) of underlying demand, with minimum operational demand of ~104 MW and negative scheduled demand of −38 MW momentarily; SA can at times exceed 100% renewable generation (exporting surplus), with observed maxima ~138% over a weekly window cited. - BESS services and benefits: • Hornsdale Power Reserve (HPR) 150 MW/194 MWh: Delivered arbitrage and FCAS; reduced average yearly regulation FCAS costs in SA by >91%; provided fast frequency response (e.g., 25 Aug 2018 separation event: frequency nadir 49.12 Hz, HPR response up to ~84.3 MW; 31 Jan 2020 Heywood trip: rapid transition to ~91 MW charging to contain frequency). • Dalrymple (ESCRI-SA) 30 MW/8 MWh grid-forming BESS: Provides synthetic inertia, high fault current, voltage support, FCAS, arbitrage, islanding of the Yorke Peninsula, and seamless black-start; successfully rode through multiple system faults while maintaining Wattle Point wind farm online; recorded 29 operational system events by late 2021. - Deployment status: Australia has multiple operational grid-scale BESS (e.g., Victorian Big Battery 300 MW/450 MWh) and >80 planned projects totaling ~18,660 MW; SynCons deployed/announced nationally (e.g., Darlington Point, Musselroe, Kiamal; large units at Davenport and Robertstown; further planned across NSW, SA, VIC).

The findings demonstrate that coordinated deployment of SynCons and grid-scale BESS, particularly with grid-forming controls, can directly address low-inertia and low-strength challenges in high-IBR power systems. SynCons contribute immediate inertia, reactive power, and fault current to stabilize weak areas and enable protective relays, expanding renewable hosting capacity without long lead-time transmission builds. Grid-forming BESS complement SynCons by providing fast frequency response, synthetic inertia, black-start, voltage support, and islanding capabilities, thereby improving resilience during contingencies and reducing ancillary service costs. Australian case studies show measurable system security improvements (stable frequency during separations, oscillation mitigation with constraints and strengthening) and economic benefits (FCAS cost reductions, reduced curtailment). However, continued reliance on some synchronous generation remains necessary to ensure active power support, ramping capability, and reactive reserves under certain conditions. The results are relevant to other systems seeking to safely increase IBR penetration by combining operational measures, targeted transmission-strengthening assets, and advanced inverter capabilities, supported by appropriate planning and market frameworks.

Australia’s NEM illustrates that modern power systems can integrate very high shares of inverter-based renewables by deploying a portfolio of enabling technologies. Synchronous condensers restore system strength and inertia, while grid-scale BESS—especially with grid-forming control—provide fast frequency response, black-start, and voltage support, improving reliability and lowering costs. These measures have enabled South Australia to operate with sustained high renewable fractions, set record low operational demands, and at times export surplus renewable energy. The paper consolidates definitions of system strength metrics, catalogs challenges observed globally and in Australia, and documents practical mitigations and their impacts. Future work should include: scaling grid-forming capabilities across IBR fleets; refining coordination between control objectives and converter physical limits; developing operational tools and processes fit for high variability and uncertainty; and advancing regulatory frameworks and market signals to incentivize timely deployment of enabling assets and practices.

- The review is based largely on Australian system experiences and publicly available reports; findings may not directly generalize to systems with different topology, market design, or regulatory frameworks. - Grid-forming IBR capabilities are constrained by energy buffer and overcurrent limits; coordination of multiple GFM units and control objectives requires further investigation and standardization. - Some solutions (e.g., SynCons) entail significant capital costs and site-specific engineering; benefits depend on local system conditions and may require complementary measures (e.g., reactive reserves, protection upgrades). - Data presented (e.g., capacities, costs, operational statistics) reflect the status up to late 2021/early 2022 and may evolve with ongoing deployments and policy changes.