Operating Reserves

Every power plant must have a certain amount of reserve generating capacity to provide for overhaul of generating equipment, forced outage of equipment, minute-to-minute load variation, load forecasting error. We know that electricity power supplied must always be equal to power being consumed. To achieve this balance system controllers constantly monitor the demand in the province and match it with available supply. The system controller uses operating reserve to ensure that this supply-demand balance is achieved seamlessly. Thus, operating reserves are used to maintain system reliability when there is an unexpected imbalance between supply and demand due to various system conditions or contingencies and to ensure power is available when we need it.

Operating reserves are categorized as regulating reserves, spinning reserves and supplemental reserves, where each type of reserve performs a unique function.

Due to the size and complexity of the power grid, the balance between generation (supply) and consumption (demand) is not instantaneous—often there is a lag while generation is catching up to supply or while generation is decreasing in response to lower demand. Regulating reserves instantaneously provide the power difference between supply and demand required during that lag period. This can be achieved by the governors which provides a small boost to both the output frequency and the power of each generator. Hence, regulating reserves is also known as frequency-response reserve.

The spinning reserve ( = unused capacity) is extra generating capacity that is available by increasing the power output of generators that are already connected to the power system. For most generators, this increase in power output is achieved by increasing the torque applied to the turbine’s rotor. This reserve capacity meet minute–to–minute load variation and any extra load due to load forecasting errors.

The non-spinning reserve or supplemental reserve is the extra generating capacity that is not currently connected to the system but can be brought online after a short delay, usually within 10 minutes. In isolated power systems, this typically equates to the power available  from fast-start generators. However, in interconnected power systems, this may include the power available on short notice by importing power from other systems or retracting power that is currently being exported to other systems. This reserve capacity meet load variation due to forced outage (unexpected breakdown) of unit.

Spinning and supplemental reserves (collectively referred to as contingency reserves) are used to maintain the balance of supply and demand when an unexpected system event occurs. These reserves provide capacity the system controller can call on with short notice to correct any imbalance. These reserves can come from the supply side (generators) or from the demand side (load curtailment by reducing demand from large electrical consumers immediately).