Synchronous and asynchronous energy: what they are and why they matter

The impact of synchronous and asynchronous energy on the electricity network

Asynchronous renewable energies – such as wind and solar – are two of the main contributors to Spain’s energy mix, helping drive the transition towards decarbonisation. However, energies such as nuclear and hydroelectric power, which are generated using synchronous generators, are essential to guarantee the stability and reliability of the electricity system.

Nuclear power plant in a mountain valley with two cooling towers releasing steam, surrounded by vegetation and industrial facilities.

Cofrentes Nuclear Power Plant, Valencia

What are synchronous and asynchronous energy sources, and how do they differ? Why are they becoming increasingly important to ensure the stability of the Spanish electricity system in the face of potential oscillations or disturbances?

In this article, we explain these concepts and the importance of companies such as Iberdrola España in ensuring a reliable and sustainable electricity supply.

What is synchronous energy and how does a synchronous generator work?

According to the Royal Spanish Academy (RAE), the word synchronous is “used to refer to something that develops in perfect temporal correspondence with something else”, while asynchronous refers to “something that does not take place in complete temporal correspondence with something else”.

When these terms are used in the energy sector to distinguish between synchronous and asynchronous energy, they relate to this meaning, but applied to the types of generators or alternating current machines used to produce electricity and to how they behave in relation to the electricity grid frequency (the rate at which electricity is transmitted).

In synchronous generators the mechanical energy supplied by a shaft (such as a turbine) is converted into electrical energy. The shaft maintains a constant rotational speed that is synchronised with the frequency of the electrical network. What’s more, these generators are machines with a large rotating mass, which allows them to provide inertia, a key property for dampening sudden changes in demand or in energy production.

Synchronous generators are mainly used in nuclear power plants, hydroelectric facilities and combined-cycle thermal power plants.

How does a synchronous generator work?

A synchronous generator consists of two main parts: a rotor (moving part) and a stator (fixed part).

The rotor (inductor) is supplied with a direct current for excitation (from a diesel engine, gas, steam or hydraulic turbines) and rotates at a constant speed. The rotor contains magnets or electromagnets.
The rotor’s rotating movement generates a rotating magnetic field that induces an alternating electromotive force in the stator windings (armature), in other words, a sinusoidal alternating electric current.
The electricity generated in the stator is perfectly synchronised with that of the electrical network through which it is transmitted, which is key to maintaining a stable grid frequency. In Spain the grid frequency of the electrical network is the same as that used in Europe: 50 hertz (Hz).

Parts of a synchronous generator

A synchronous generator is made up of several components, the most important of which are the stator and the rotor.

1 2 3 4 5 6 7 8 9 Illustration showing the parts of a synchronous generator

1 Shaft

Component that transmits mechanical motion from the turbine (hydraulic, steam or gas) to the inside of the generator.

2 Rotor

Moving part driven by the shaft that creates the generator’s main magnetic field.

3 Excitation winding

Set of coils on the rotor that, when supplied with direct current, produce a rotating magnetic field.

4 Excitation system

Device that provides direct current to the rotor or excitation winding using brushes, brushless systems or electronic control.

5 Rotating magnetic field

The magnetic field generated by the moving rotor passes through the stator, changing the magnetic flux through its coils.

6 Stator magnetic core

Component made of silicon steel sheets that channels the magnetic flux and reduces electrical losses.

7 Stator winding

Set of three-phase copper coils where the electricity that is sent to the grid is generated.

8 Bearings

Parts that support the shaft and allow stable rotation with minimal friction.

9 Housing

External structure of the generator that protects the internal components and provides insulation and stability.

These types of generators are equipped with control and protection systems that monitor voltage, frequency and temperature, safeguarding their operation against any anomalies.

They also include cooling systems to prevent overheating, dissipating heat through air, water or hydrogen.

What is asynchronous energy and how is it generated?

Unlike synchronous generators, in asynchronous or induction generators the rotor does not require direct current excitation. Instead, it depends on being connected to an electricity grid and rotates at a different speed – usually slower – than the magnetic field generated in the stator. This difference (known as slip) is what allows electricity to be produced in the stator. Asynchronous generators are not directly synchronised with the electricity grid frequency.

Asynchronous generators are mainly used in wind farms – although some use synchronous generators – photovoltaic plants and small hydroelectric facilities.

Differences between synchronous and asynchronous energy

Now that we have explained how synchronous and asynchronous generators work, here is a summary of their main differences in the following table:

Differences between synchronous and asynchronous energy

Synchronous energy

Illustration of an electric generator with a rotor and stator shown as concentric circles, connected to an electrical module

Uses a synchronous generator
Operates in synchronisation with the frequency of the electricity network
Provides frequency and voltage control
Offers a high level of inertia (network stability). Inertia is the kinetic energy stored in a rotating body such as a turbine. By rotating at a constant speed, synchronous generators produce electricity without frequency fluctuations and, if there are disturbances in the network (a sudden increase in demand or a drop in generation), this energy is released or absorbed, slowing the rate of change and allowing frequency controls to respond
Used in nuclear power plants, hydropower plants or combined cycle thermal power plants

Asynchronous energy

Illustration of an asynchronous electric generator shown with concentric circles, rotation arrows, and a connected electrical module

Uses an asynchronous generator
Does not operate in synchronisation with the frequency of the electricity network
Does not provide frequency or voltage control and must be connected to an existing electricity network for correct operation
Offers a low level of inertia (network stability). As they are connected to the network through inverters, these act as a ‘barrier’ that prevents them from contributing inertia to the electricity system
Used in wind farms, photovoltaic plants or small hydropower stations

The importance of synchronous energy for the stability of the Spanish electricity system

To understand the importance of synchronous energy in maintaining the stability of the electricity system and responding to disturbances or fluctuations in the grid, it is useful to briefly explain how the electricity system itself works.

When we switch on a light or connect an electrical appliance, a system is activated that begins at power generation plants, where electricity is produced. This energy is transformed into high voltage so it can be transmitted efficiently through the transmission network to distribution centres. There, it is adapted again to the voltage level required for each type of consumption (residential, industrial or services) and transported until it reaches the end consumer.

The electricity system is highly complex because it integrates multiple electricity generation sources that can complement or replace one another and must be coordinated at all times to ensure the system is continuously supplied. A constant balance between generation and consumption must be maintained because electricity cannot be stored on a large scale and must be generated in real time according to demand.

So what happens when there are disturbances in the electricity system? One increasingly common situation occurs during periods of high renewable generation, such as a day with strong sunshine and wind, when wind and photovoltaic production is very high. In these situations, rapid changes in generation may occur – for example, if the wind suddenly drops or clouds cover the sky – introducing variations in the balance between the electricity generated and consumed. The electricity system must be capable of absorbing these fluctuations immediately in order to avoid instability.

Maintaining a stable grid frequency is essential because it reflects this real-time balance. If generation and consumption are no longer aligned, the frequency deviates, which can affect equipment performance, compromise system stability and even cause blackouts.

In this context, synchronous energies – such as nuclear and hydroelectric power – play an essential role: because they are directly synchronised with the grid and provide inertia, they help dampen these changes and maintain frequency stability, thereby reinforcing the security and reliability of the electricity supply.

How does Iberdrola España help guarantee electricity system stability?

Spain has set itself the goal of decarbonising its energy model by 2050: by 2025, renewable energies account for 67% of the country’s total installed capacity thanks to companies such as Iberdrola España.

Iberdrola España is committed to the electrification of the economy in order to mitigate the effects of climate change and improve competitiveness by reducing dependence on gas. We have become the leading renewable energy company in the Spanish energy sector, and our electricity network extends across 10 autonomous regions and 25 provinces, covering an area of 270,000 km².

However, for electrification to become possible in a context where renewable energies – mainly asynchronous sources such as wind and photovoltaic power – are playing an increasingly important role, synchronous energies are essential to guarantee the stability of the electricity system.

In this regard, Iberdrola España is one of the companies in the sector with the greatest installed capacity in both synchronous and asynchronous technologies, contributing to a more balanced, secure and reliable electricity system.