Which term describes the deviation of system frequency from its nominal value?

The term that accurately describes the deviation of system frequency from its nominal value is frequency variation. This refers to the fluctuations in the electrical frequency of the power system, typically measured in hertz (Hz), from its target standard (often 50 Hz or 60 Hz, depending on the region). Frequency variation occurs due to imbalances between electricity supply and demand, which can happen for various reasons, such as changes in load demand, unexpected generator outages, or renewable energy fluctuations. It's crucial for operators to monitor and manage frequency variations to maintain system stability and ensure reliable power delivery.

In contrast, the other terms relate to different aspects of power system operations. Frequency regulating reserve pertains to the reserves maintained to respond to frequency changes, whereas generation schedule refers to the planned generation output from power plants. Grid code involves the set of regulations for the operation of the electricity grid, addressing broader operational and reliability practices but not specifically focused on frequency deviations.