What is an excitation system?

Excitation systems are crucial to the operation of modern synchronous machines as they are used to provide a continuous DC current to the machine’s field winding.

The amount of excitation needed depends on the type and speed of generator and rotor, load current, and power factor. For example, larger load currents necessitate more excitation.

 

Rotating exciters versus static exciters

There are two main types of excitation system: rotating exciters and static exciters. Rotating exciters use a small rotating machine to generate the DC current and can be further classified as brushed or brushless. Static exciters have no moving parts and rely on solid-state electronics to convert AC power from the generator itself into DC current.

Rotating exciter key points:

• Complexity – generally straightforward design using well-established technology
• Cost – lower costs than static exciters because of their simplicity
• Efficiency – have high energy losses because of the friction and windage in the rotating parts
• Maintenance – require regular maintenance of brushes, slip rings and bearings
• Response time – slower response time because of the inertia of the rotating components

Static exciter key points:

• Complexity – often more complex system designs that rely on electronic components and control systems
• Cost – more expensive than rotating exciters because of all the power electronic components involved
• Efficiency – more efficient because they have lower energy losses compared to rotating machinery
• Maintenance – generally need less maintenance because there is no wear and tear on moving parts to consider
• Response time – much faster response time than rotating exciters due to the absence of inertia from rotating parts

In terms of applications, rotating exciters are generally used in smaller generators, below 50 MW, because of their lower upfront cost and simple system design. Static exciters are typically used for larger generators, above 50 MW, due to their higher efficiency, faster response and lower maintenance needs.

 

The role of excitation systems in synchronous machines

Synchronous machines, such as generators in power plants, play a key role in maintaining stable and efficient power grids. Synchronous machines consist of a rotor and stator, with the rotor behaving as a rotating electromagnet and spinning inside the stator and supported by bearings.

A generator is used to turn mechanical energy from a prime mover into electrical energy for transmission to end users. The prime mover, which could be a hydro, gas, steam or wind turbine, controls the generator’s megawatt (MW) load.

Excitation systems are a critical part of a synchronous machine’s function as they provide the DC current to the field windings, which then creates a magnetic field in the generator’s rotor. The amount of magnetic flux generated controls the machine’s output voltage.

The strength of the magnetic field dictates the generator’s output voltage. By adjusting the DC current supplied to the field windings, the excitation system essentially controls the voltage produced by the generator, with stronger magnetic fields creating higher voltages.

An excitation system is needed to regulate excitation current for the prevailing load conditions.

As they regulate the output voltage and maintain a constant generator terminal voltage, excitation systems play a key role in maintaining power grid stability. Furthermore, as they control output voltages, effective excitation system control allows more responsiveness as they allow operators to adjust output voltages in accordance with changes to load on the grid. This helps to maintain the synchronism between different generators, preventing cascading outages.

Essentially, excitation systems are the silent guardians of power generation. They ensure synchronous machines, such as generators in power plants, to operate safely and efficiently, while contributing to a reliable and stable power grid.

 

What to do next

Excitation Engineering Services can help with all aspects of managing excitation systems, from designing and manufacturing new systems to maintenance of existing systems.

For more information about our design capabilities, which include standalone excitation systems or combined generator control packages, visit https://excitationengineering.co.uk/design/

To learn more about our preventative maintenance programmes that cover all types of excitation equipment, from legacy analogue systems to modern digital systems, visit https://excitationengineering.co.uk/maintenance/

Alternatively, contact a member of our team on +44 (0) 1827 286100.