← Back to Live Frequency

Technical education

Frequency Regulation in Power Systems

Frequency regulation, or more broadly frequency control, is the set of actions used to continuously balance electrical power generation and electrical power demand. In a 50 Hz power system, this balance is essential for secure and reliable operation.

Terminology note: The broader engineering term is usually frequency control. In this portal, the term frequency regulation is used as an accessible way to describe the mechanisms that keep system frequency close to its nominal value.

1. Frequency as a measure of power balance

Power system frequency is a real-time indicator of the balance between generation and demand.

Balanced system

Generation = Demand

Frequency remains close to nominal.

Power deficit

Generation < Demand

Frequency tends to fall.

Power surplus

Generation > Demand

Frequency tends to rise.

Physically, this occurs because an imbalance between mechanical or electrical input power and electrical output power changes the stored energy in the system. In synchronous machines, this stored energy is mainly kinetic energy in rotating masses. In inverter-based resources, active power is adjusted through power electronic controls.

2. The physical principle

In a traditional power system, frequency is closely related to the rotational speed of synchronous machines. For a synchronous machine:

nₛ = 120 f / P

Where nₛ is synchronous speed in rpm, f is frequency in Hz, and P is the number of poles.

If electrical demand suddenly increases, the electrical torque on the generator increases. If turbine mechanical power does not immediately increase, the rotor slows down slightly and frequency decreases. If generation exceeds demand, the opposite occurs and frequency increases.

A simplified form of the swing equation can be expressed as:

(2H / f₀) · df/dt = ΔPₘ − ΔPₑ − DΔf
  • H — inertia constant.
  • f₀ — nominal frequency.
  • df/dt — rate of change of frequency.
  • ΔPₘ — change in mechanical/input power.
  • ΔPₑ — change in electrical demand/output power.
  • DΔf — load damping effect.

3. Power balance diagram

Generation
Power balance
Frequency response
Generation = Demand → Frequency stable
Generation < Demand → Frequency falls
Generation > Demand → Frequency rises

4. Primary frequency control

Primary frequency control is the first automatic response after a disturbance. It acts within seconds and is used to arrest the frequency decline or rise.

For synchronous generators, primary response is usually provided by governor droop control. If frequency decreases, the governor increases mechanical input power. If frequency increases, the governor reduces input power.

ΔP = −(1 / R) · Δf

Where ΔP is the active power change, R is the droop setting, and Δf is the frequency deviation.

Step 1

Disturbance

Step 2

Frequency deviation

Step 3

Primary response

Step 4

Frequency arrested

5. Secondary frequency control

Secondary frequency control acts after the initial primary response. Its objective is to restore frequency closer to the nominal value and rebalance the system more accurately.

In many systems, secondary control is implemented through Automatic Generation Control, or AGC. AGC sends control signals to participating generators, batteries or controllable resources to adjust active power output.

ACE = ΔPₜᵢₑ + BΔf

In interconnected systems, Area Control Error combines tie-line power deviation and frequency deviation. In islanded or isolated systems, the frequency deviation term becomes the dominant signal.

6. Tertiary frequency control

Tertiary frequency control acts over longer timeframes. It is associated with dispatch, reserve restoration, unit commitment, manual operator actions and market-based balancing services.

Typical tertiary actions

  • • Dispatch additional generation
  • • Start fast-start units
  • • Re-dispatch generation
  • • Restore operating reserves
  • • Manage BESS state of charge
  • • Activate demand response

Main objective

  • • Prepare the system for the next contingency
  • • Replace reserves used by primary and secondary control
  • • Maintain security over minutes to hours

7. Frequency control timeframes

Inertial response

Instantaneous to seconds

Natural energy exchange from rotating masses or fast inverter controls.

Primary control

Seconds

Arrests the frequency deviation after a disturbance.

Secondary control

Seconds to minutes

Restores frequency toward nominal value.

Tertiary control

Minutes to hours

Restores reserves and manages dispatch.

8. Contribution of different technologies

Synchronous generators

  • • Natural inertial response from rotating mass
  • • Governor-based primary frequency response
  • • AGC participation for secondary control
  • • Dispatch and reserve provision for tertiary control

Battery energy storage systems

  • • Fast active power injection during under-frequency events
  • • Fast charging or absorption during over-frequency events
  • • Primary frequency response and regulation services
  • • Performance depends on state of charge, converter capacity and control settings

Solar PV plants

  • • Over-frequency response by reducing active power output
  • • Frequency-watt control through inverter or plant controller
  • • Under-frequency support only if headroom is available
  • • No natural mechanical inertia in standard grid-following operation

Wind farms

  • • Active power control through turbine and plant controllers
  • • Over-frequency response by reducing output
  • • Upward response if headroom or suitable controls are available
  • • Ramp-rate control and frequency-sensitive active power response

Demand response

  • • Load reduction during under-frequency events
  • • Flexible demand can absorb excess energy during high-frequency periods
  • • Can complement generation-side frequency control

Inverter-based resources today

  • • Most installed resources are grid-following
  • • They track the existing grid voltage and frequency
  • • They contribute through active power control, frequency-watt functions and plant-level controls
  • • Grid-forming inverters are important, but are not the focus of this operational overview

9. Why this matters

As power systems transition toward higher levels of renewable and inverter-based generation, understanding frequency behaviour becomes increasingly important. Traditional synchronous machines provide inertia and governor response naturally, while modern inverter-based resources provide support through controlled active power response.

A secure power system requires a coordinated mix of technologies, control layers and operating reserves. The purpose of this portal is to make frequency behaviour visible and easier to understand using real measurements.

References