The massive integration of power electronic converters into the power grid has led to a decrease in the mechanical inertia of the power system, causing an increase in the rate of change of frequency (RoCoF) that may lead to stability problems. The scientific community has focused on developing grid-forming control techniques, although their implementation implies a significant change in the firmware of the converter. Grid-supporting approaches, on the other hand, are an interesting alternative to add frequency support to the grid while preserving the original control structure of the converter. This paper proposes three new grid-supporting control techniques based on the dynamic behaviour of a synchronous machine (SM) and its equivalence with a second-order low-pass filter. They endow the converter with the capability of providing synthetic inertia, damping, droop-based p/f primary response and virtual reactance. The dynamics of the proposed implementations are compared with those of a reduced-order synchronous machine by means of time-domain simulations and in-depth state-space based small signal analyses. Besides, their operation is validated in a nine-bus low-inertia power system. Hardware in the loop (HIL) laboratory results are used to validate experimentally the proposed techniques.

This article makes a comparative analysis between the classic first-order filter-based inertia emulation method and the more recent second-order filter-based approach, which manages to emulate the electrodynamic behaviour of the synchronous machine. The dynamics of the interactions between the control techniques, the filter capacitance and the parasitic grid inductance are analysed assuming ideal and real phase-locked loops (PLL) and current control loops. The results reveal how electromagnetic oscillatory modes between the controllers and the LL elements of the grid are exited, which in the case of the first-order filter-based inertia emulation methods impose severe restrictions on the maximum inertia that can be emulated and in the bandwidth of the converter current controllers and the PLL. These restrictions are released in the case of using the second-order filter-based inertia emulation approach. The results of the analysis are validated through simulations on the IEEE 9-bus low inertia power system. Hardware in the loop experimental results are presented to support the investigation.