Transient stability analysis and modelling of systems with grid-forming converters and synchronous generators

The transient stability of a power system with both grid-forming converters (GFMs) and synchronous generators (SGs) under large faults is rarely reported. This paper finds out that even when a SG and GFM have the same frequency during a fault by setting the same inertia and damping coefficient for both, due to the impact of the dynamics of the SG field and amortisseur flux linkage, a parallel connected GFM with low capacity ratio between the GFM and SG outputs negative postfault active power. In other words, the GFM becomes synchronization unstable according to traditional definition. Since normally a GFM has much larger damping (to have strong robustness during small and large disturbances), this negative active power is even lower and lasts longer in practical cases. When conducting transient stability analysis, existing methods usually model a GFM in the same form as the classic SG model, and did not provide theoretical analysis. To simplify analysis and facilitate control design under large disturbances, this paper adopts a modal analysis-based model order reduction technique and finds out that while a GFM can be modelled as the classic form together with the virtual impedance current limiting control, the interacted SGs should not be modelled in the classic form, because the SG field and amortisseur flux linkage should also be included. Finally, this paper finds that for systems with only GFMs as voltage sources, reserving only the dynamics of the voltage angle control loop of GFMs is precise enough for representing the system low-frequency oscillations phenomenon.