The transition from synchronous machine-based generation resources to inverter-based generation sources marks a significant shift in the dynamics of modern power systems. Traditionally synchronous generators, which are inherently capable of providing essential grid support services such as frequency regulation, voltage control, and inertia, have been the backbone of electrical grids for over a century. As the penetration of inverter-based renewable energy sources increases, the grid's dependency on these inverter technologies grows. This shift necessitates new strategies and technologies to ensure that grid support services traditionally provided by synchronous generators are adequately replicated or replaced by inverter-based resources.

One critical aspect of inverter-based generation is its ability to provide frequency regulation. Synchronous generators contribute to frequency stability through their rotational inertia, which acts as a mechanical buffer against sudden changes in load. Inverter, however, can be equipped with advanced control algorithms that allow them to rapidly respond to frequency deviations, effectively mimicking the inertial response of traditional machines. This capability, commonly known as frequency-droop or frequency-watt, is crucial as it helps maintain the balance between supply and demand.

The global trend in renewable generation resources is shifting towards distribution-connected systems, as opposed to traditional transmission-connected systems. The IEEE 1547-2018 interconnection standard outlines the required frequency-droop capability of distributed energy resources (DERs), which is essential for maintaining grid stability. This capability allows inverters to autonomously modulate their power output in response to changes in grid frequency. Specifically, under high-frequency conditions, inverters are required to reduce their power output, while under low-frequency conditions, they should increase their power output, if possible. This dynamic response helps balance supply and demand, preventing frequency deviations that could lead to grid instability.

This paper describes the frequency-droop characteristics of IEEE 1547-2018 certified solar PV and energy storage inverters. Residential-scale PV inverter’s responses to grid over- and under-frequency conditions under various solar irradiance and active power limiting conditions are explained with test results. Energy storage inverter’s response to similar grid-frequency conditions under different operating modes (charging, discharging, and floating) will be shown. Proper understanding of smaller scale DER inverters’ grid frequency support capability will help system operators to properly configure and utilize the millions of these smart devices installed on the distribution circuits for system frequency stability.

In addition, transmission and distribution co-simulation analysis will be presented to demonstrate the importance of autonomous frequency regulation support from DERs on system frequency recovery after a transmission fault causing sudden drop in system demand. The paper will also include field performance of a transmission connected solar PV plus energy storage hybrid inverter’s frequency-droop control and potential impacts of restrictive interconnection rules.