To mitigate carbon emissions, renewable energy has been widely adopted as an alternative to traditional fossil fuels. However, most renewable sources are integrated into the grid via converters, which inherently lack rotational inertia typically provided by synchronous generators. This gradual decline of system inertia renders power systems more vulnerable to frequency disturbances, potentially leading to undesirable consequences such as forced load disconnection, cascading outages, or widespread power failures during extreme frequency fluctuations. To overcome this challenge, this chapter introduces an inertia enhancement technique for battery energy storage systems. The presented method emulates inertia by establishing a proportional correlation between the frequency derivative and converter active power references. Particularly, a principal innovation involves employing a frequency-locked loop (FLL), which is used to precisely obtain the frequency derivative, circumventing high-frequency range noise introduced by conventional computational methods.