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Auxiliary services are services provided by parties to the electrical grid to facilitate and support the continuous flow of electricity so that the electrical energy demand is met in real-time. These services are used to ensure the stability, efficiency, and safety of the electricity grid.

There are two main categories of auxiliary services:

1.  Frequency Related Services: This includes Inertia, Frequency Containment Reserve(FCR), and Automatic Frequency Restoration Reserve (aFRR). These services are crucial to ensure that the main frequency remains within a specific range of the nominal frequency. Mismatches between electricity generation and demand cause variations in frequency, so control measures are required to bring the frequency back to the rated value and ensure that it does not vary out of range.
2. Non-Frequency Related Services: This includes reactive power and voltage control, and congestion management. These services are necessary to ensure that the voltage remains within a certain range and to compensate for voltage drops.

 Auxiliary services and the wholesale electricity market are two different but interrelated aspects of the electricity system. Here's how they compare:

• Wholesale Electricity Market: This is the market where electricity is bought and sold by various parties, such as utilities, energy suppliers, and large consumers. the market determines the price of electricity based on supply and demand. It includes energy trading for immediate delivery(spot market) and future delivery(future market).
• Auxiliary Services:  These are essential services required for the safe and reliable operation of the electricity transmission system. They support the basic services of energy generation and transmission, ensuring stability and efficiency in the electricity grid. This includes services such as frequency control, voltage control, and black start capabilities.