Hybrid power plants combining photovoltaic generation, wind generation and battery energy storage are increasingly expected to provide controllable power exchange, voltage support and disturbance response, rather than only renewable energy production. Most present systems use voltage source converters and static transformers. This approach is mature, but it contributes little physical inertia, and its short circuit current is limited by power electronic converter rating. These limits become more important as renewable plants are connected to weaker distribution networks and conventional synchronous generation is displaced.

This paper investigates the use of a rotary transformer as a multifunctional grid interface for photovoltaic, wind and battery energy storage hybrid power plants. The proposed rotary transformer consists of a low voltage machine connected to the renewable and storage converter system, mechanically coupled to a medium voltage synchronous machine connected to the utility grid. In this arrangement, the coupled electromagnetic devices provide voltage transformation and bidirectional power transfer, while the grid connected synchronous machine also contributes electromechanical inertia, reactive power support and transient short circuit current.

The topology of the hybrid plant is a focus of this paper. Two renewable and storage connection structures are considered. In a DC coupled structure, the photovoltaic array, rectified wind generation system and battery storage are connected through a common DC link before supplying the rotary transformer through a shared voltage source converter. This may improve coordinated energy management and reduce conversion stages, but it places higher requirements on DC link control, protection and converter sizing. In an AC coupled structure, the photovoltaic, wind and BESS converters are connected on a local AC collection bus before the grid interface. This is more modular and easier to expand, but may introduce control interaction, reactive power circulation and power quality issues.

The paper then examines how these topologies affect the sizing of both the BESS and the rotary transformer machines. The rating of the rotary transformer cannot be determined only from the installed renewable capacity or the peak BESS power. It must also reflect the operating duty imposed by renewable system, i.e. generation variability, storage dispatch, voltage regulation, frequency support and short duration fault events. For example, during high wind generation and low demand, the BESS may absorb surplus power while the synchronous machine section mainly supports voltage and reactive power. During low renewable output or a frequency disturbance, the BESS and rotary transformer may jointly support active power injection. During a grid fault, the synchronous machine section of the rotary transformer provides transient current and assists voltage recovery, reducing the immediate burden on the converter system.

Operational cases including renewable smoothing, surplus charging, peak support, voltage regulation, frequency response and post fault recovery are analysed to clarify the interaction between battery storage and the electromechanical interface. Attention is given to whether the stored kinetic energy and transient overload capability of the rotary transformer can reduce instantaneous power stress on the BESS and its converter.