Interactive Power to Frequency Dynamics Between Grid-Forming Inverters and Synchronous Generators in Power Electronics-Dominated Power Systems

TL;DR

This paper investigates how grid-forming inverters influence frequency dynamics in power systems, revealing order reduction effects and proposing non-linear control methods to improve stability in inverter-dominated grids.

Contribution

It provides a rigorous analysis of power-frequency relationships in hybrid systems and introduces novel non-linear frequency droop control strategies based on order reduction insights.

Findings

Order reduction in frequency response due to grid-forming inverters.

Proper DC-side design has negligible impact on active power transfer.

Non-linear control improves frequency response and stability.

Abstract

With increased attention on grid-forming inverters as a power system stabilizing device during periods of high shares of inverter-based resource operations, there is a present need for a transparent and rigorous investigation of the inverted and direct power to frequency control capabilities, and associated impacts, of these devices on hybrid systems. Here, analysis of the frequency dynamics of the droop controlled grid-forming inverter and the synchronous generator illuminates the inverted active power-frequency relationship and the frequency response order reduction, forming the basis for novel, non-linear frequency droop control approaches. Device-level electromagnetic transient domain simulations corroborate the order-reduction findings, establish that a properly designed DC-side system has a negligible dynamical impact on active power transfer and will not impede frequency regulation, and confirm the frequency response improvement with non-linear control. Simulations of the 9- and 39-bus test systems validate the order reduction and associated decoupling of the nadir and rate of change of frequency in large networks. The primary system oscillatory mode confirms the correlation between high shares of grid-forming inverters and increased mode frequency and damping; a sharp decrease in damping is observed at shares above 80%, whether by grid-forming device quantity in large networks or rating variations in a small test system. Finally, simulation results on the Hawaiian island of Maui show a trend towards a first order frequency response with a grid-forming inverter, further corroborating the analytic findings and network impacts.