Combined Zonal and Local Control Using Grid-forming Inverters in a Complex Medium- and Low-voltage Island Grid: a Study Case in a Realistic German Network

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This webinar delves into a case study focusing on zonal control in a complex island grid hosting grid-forming (GFM) and grid-following (GFL) inverter-based resources (IBRs). The investigation addresses key questions relevant to stable operation after disconnection from the transmission network:

1) What is the minimum required GFM installed capacity for stable operation after disconnection?

2) Is it more effective to deploy multiple distributed GFM inverters at low voltage (400 V) or a few large GFM inverters at medium voltage (20 kV)?

3) Can inverter local control ensure long-term stability and adequate island operation, or is centralized control required?

Conducted in a combined medium- and low-voltage network representative of German grids, the study employs a highly complex model with 5483 buses, 569 distributed energy resources (DERs) at 400 V, and 6 large-scale IBRs at 20 kV. Leveraging SimBench datasets publicly available for power system analysis tools, the models include dynamic components and use the Western Electricity Coordinating Council (WECC) models for GFL inverters (specifically, the WECC_REGC_C IBR model for converter-driven oscillations in weak systems).

The paper addresses these questions through positive-sequence root-mean-square (RMS) simulations and frequency-domain analysis using the Fourier transform. The study proposes and validates a zonal control that coordinates DERs' efforts within its area by conducting hundreds of simulations to address uncertainties such as the geographical location of DERs in the distribution grid. This control sends necessary corrections to bring distribution voltages within adequate values. The webinar demonstrates how, within seconds, local control of inverters responds rapidly to disturbances, enhancing short-term dynamics. Simultaneously, the zonal control coordinates various DERs to refine local corrections within a timeframe of tens of seconds.