Frequency Dynamics with Grid Forming Inverters: A New Stability Paradigm

Traditional power system frequency dynamics are driven by Newtonian physics, where a synchronous generator (SG), the historical primary source of power, follows a deceleration frequency trajectory upon power imbalances according to the swing equation. Subsequent to a disturbance, an SG will modify pre-converter, mechanical power as a function of frequency; these are reactive, second order devices. The integration of renewable energies is primarily accomplished with inverters that convert DC power into AC power, and which hitherto have employed grid-following control strategies that require other devices, typically SGs, to establish a voltage waveform and elicit power imbalance frequency dynamics. A 100\% integration of this particular control strategy is untenable and attention has recently shifted to grid-forming (GFM) control, where the inverter directly regulates frequency; direct frequency control implies that a GFM can serve power proactively by simply not changing frequency. With analysis and electromagnetic transient domain simulations, it is shown that GFM pre-converter power has a first order relation to electrical power as compared to SGs. It is shown that the traditional frequency dynamics are dramatically altered with GFM control, and traditional second-order frequency trajectories transition to first-order, with an accompanying decoupling of the nadir and rate of change of frequency.