Frequency response

Frequency response is the ability of a bulk electric system, or elements of that system, to react or respond to a change in system frequency. It is measured mathematically in units of MW per 0.1 Hz as:

(Change in demand + Change in generation) ÷ Change in frequency

Frequency response is managed by bulk system operators to ensure systems can maintain frequency within an acceptable range except in extreme circumstances. Various resources are used for frequency response. Some resources are controlled by the system operator while others respond automatically. 

Here are the various resources that provide frequency response:

• Inertia – the energy stored in large rotating generators that gives them the tendency to keep rotating at a specific speed, thus slowing a change in system frequency
• Generator governors – control systems on a generator that monitor the generator frequency and open or close valves whenever frequency deviates from set points, thus adjusting the generator real power output
• Loads with underfrequency relays (UFRs) – curtailable loads controlled by a relay that monitors system frequency and curtails loads when frequency drops below a specific value
• Regulating reserve – sources of supply that can ramp up or down within a few seconds in response to a control signal from the system operations energy management system
• Flexible ramp – sources of supply that have the ability to ramp up or down within five minutes in response to a dispatch order from the system operator
• Spinning reserve – sources of supply that are synchronized to the frequency of the grid and can ramp to full capacity within 10 minutes in response to a dispatch order from the system operator
• Non-spinning reserve – sources of supply that may not be synchronized to the frequency of the grid, but can be available within 10 minutes in response to a dispatch order from the system operator
• Replacement reserve – sources of supply that may not be synchronized to the frequency of the grid, but can be available within 30 minutes in response to a dispatch order from the system operator

Frequency response may be required during normal operations due to fluctuations in loads and changes in output of variable supply resources. It is also necessary following a contingency such as loss of a significant amount of supply following unexpected generation or transmission outages. The following diagram shows frequency response from various resources following a significant contingency:

1. Contingency: Loss of generation or transmission results in a sudden drop in system frequency. The amount of system inertia impacts the speed and depth of how far frequency falls. 
2. Arresting period: Primary frequency control including governor response and curtailment of loads with underfrequency relays slows, then stops the drop in frequency. As these resources ramp, the system frequency begins to rise.
3. Rebound period: Secondary frequency control provided by regulating reserve establishes a stable frequency that is still below the desired 60 Hz. As resources providing secondary frequency control are ramped up, sources of primary frequency control are returned to normal operation. 
4. Recovery period: Tertiary frequency control provided by spinning and non-spinning reserve returns the frequency to 60 Hz. As these resources are ramped up, regulating reserve is ramped back down so it is available to continue to regulate the frequency. Later in the hour, replacement reserve may be started as a replacement for spinning and non-spinning reserve so that the system will have these resources should a second contingency occur. 

Frequency response is critical to the reliable operation of a bulk electric system and planning for sufficient resources is an important function of the bulk system operator.