Gigawatt-scale hybrid wind and battery energy storage system (BESS) plants are increasingly being deployed in weak, inverter-dominated networks where compliance with stringent grid-code requirements must be maintained across a wide range of operating conditions. While much of the existing grid-forming (GFM) literature focuses on inverter-level control behaviour, the dynamic performance of large hybrid plants is also strongly influenced by plant topology, electrical distance between generating assets and the point of connection (PoC), and the coordination of plant-level control functions.

This paper presents a plant-level coordination framework for a hybrid wind and GFM BESS power plant in which a centralized GFM BESS is integrated with grid-following (GFL) wind turbine generators (WTGs) through multiple power plant controllers (PPCs) operating at a common PoC. Particular focus is placed on the allocation of reactive-power duty between the wind farm and BESS subsystems and on the coordination of on-load tap changer (OLTC) target voltages.

The proposed architecture is evaluated using electromagnetic transient (EMT) simulations of a gigawatt-scale hybrid project connected to a weak grid. Performance is assessed against key Australian National Electricity Rules (NER) requirements, including disturbance ride-through, continuous uninterrupted operation, and system-strength capability. The results demonstrate that shifting reactive-power responsibility from the wind farm to the electrically closer BESS reduces wind-turbine terminal-voltage excursions and mitigates DC-link overvoltage tripping during fault clearance. Furthermore, coordinated adjustment of reactive-power sharing and OLTC target voltage enables successful ride-through under both reactive-power export and absorption extremes. Additional studies at progressively lower SCR conditions show that stable post-fault recovery can be restored by reducing wind-turbine fault reactive-current contribution, while the centralized GFM BESS maintains a well-damped response.

The findings indicate that reactive-power allocation and OLTC settings should be jointly optimized across the plant operating envelope rather than assigned solely in proportion to installed capacity. The proposed plant-level approach provides a practical pathway to weak-grid compliance for giga-scale hybrid plants.