Grid ancillary services are a set of technical functions and operational support mechanisms used by electricity grid operators to maintain the stable and reliable operation of the power grid. These services include frequency regulation, voltage control, reactive power support, spinning reserve, black start capability, and load following. Each service ensures that the supply and demand of electricity remain balanced in real time and that the grid infrastructure remains within its operating parameters. Ancillary services can be provided by power plants, battery energy storage systems (BESS), and demand response units, and they are essential components in both regulated and deregulated power markets.