Frequency-Domain Multi-Objective Decoupled Control of a Harmonic Impedance Measurement Device for an Energy-Storage-Integrated Grid-Connected System

Energy-storage-integrated renewable energy stations contain both grid-following and grid-forming converters, which makes their wideband impedance behaviour difficult to predict. This paper studies a field-oriented harmonic impedance measurement method for such stations. A three-phase cascaded H-bridge converter is used to develop a harmonic impedance measurement device connected to the 35 kV bus. For the measured system, a generalized Norton equivalent model is established for the grid-following part, while a generalized Thevenin equivalent model is derived for the grid-forming energy-storage part. A frequency-domain multi-objective target-oriented decoupled control strategy is then designed for the measurement device. The strategy assigns fundamental power synchronisation, harmonic disturbance injection, and submodule capacitor voltage balancing to different frequency components. This design prevents the fundamental current loop from suppressing the injected harmonic current. Simulation studies are carried out under four conditions, including no harmonic injection and 5th-, 25th-, and 99th-order harmonic injections. The baseline case shows no clear commanded-frequency current component. Under harmonic commands, the device injects the corresponding current components while maintaining stable multilevel bridge-port voltage. The device also demonstrates robust impedance-identification performance under grid background harmonics, measurement noise, and equivalent-impedance variations. The results indicate that the proposed device has application capability for wideband impedance measurement and stability analysis in energy-storage-integrated grid-connected systems.