Energy Explained: Frequency Control

01/07/2020
5 min

In our frequency explainer, we discussed what frequency is, and how it can help us understand the balance of supply and demand of electricity. In this explainer, we’ll be diving more into what we do if the system’s frequency starts to deviate from the required range (around 50 hertz).

To start us off, here are just some of the events that can disturb the delicate balance.

  • A large generator producing power has experienced a fault and disconnects from the system, reducing the amount of power available.
  • A large consumer using power has experienced a fault and disconnects from the system, reducing the amount of power demand.
  • A large storm has damaged major transmission towers, resulting in electrical separation of part of the interconnected network, preventing power from getting to where it needs to be.  
  •   Graphic showing a seesaw balancing act of frequency control
                                          Events that disturb this balance can impact our system’s frequency levels, pushing it out of the acceptable range.

What does AEMO do if the frequency levels deviate?

There are multiple lines of defence that provide barriers to frequency changing too much or too quickly. We have run through them for you below.

Inertia

Inertia is an object’s resistance to any change in its velocity. A good example to draw upon is a wombat running in a wheel. If the wombat stops running, the wheel will keep turning until friction makes them both stop. The wheel’s tendency to keep spinning even when the wombat stops running is the wheel’s inertia.

In the case of a power system, which has lots of large and fast spinning machinery, inertia reflects how easy it is to simultaneously speed up or slow down all those fast-moving parts.

Graphic demonstrating system inertia with a wombat

Why do we need inertia in our power system?

Inertia is the power system’s first in-built line of defence to a large disturbance. In a power system with higher inertia, the inertia slows down the rate at which frequency changes if there is a disturbance to the grid.

All things being equal, the more inertia in a power system, the easier it will be to ride-through some system disturbances and keep the power system going. A power system with more inertia means there is more time available to respond, react and counter a disturbance.

What provides inertia?

Inertia is provided by electricity generators with large spinning rotors. In Australia, this includes hydro, coal, or gas plants. Large synchronous condensers with flywheels can provide similar capability. These technologies are gradually forming a smaller part of the overall energy mix in Australia, reducing the levels of inertia in the system.

While inverter-based renewable energy (like wind and solar) do not provide inertia, there are ways that they can support the operation of a power system with lower inertia by providing fast frequency response. This means they can respond rapidly to changes in system frequency (much faster than the usual synchronous condensers), helping bring everything back to the required range. As the energy market transitions, we will need these technologies more and more.

Frequency control

Inertia is very important, but it does not stop or arrest the frequency change. Instead, other frequency control services stop the frequency changing and bring it back to 50 hertz. To get things back to the right range, we have two other methods of frequency control.

Primary frequency control

Primary frequency control is provided by generators that measure and respond to the frequency changes at their local connection points.

These generators have some room to ramp up or down as required and will act in response to these large disturbance events to try and correct the frequency. If there are enough power reserves available (additional power not being used), the frequency change can be stopped.

In the National Electricity Market (on Australia’s east and south-east coast), minimum levels of these reserves are ensured through existing markets, meaning a minimum level of reserve remains available to respond to unexpected disturbances. 

Secondary frequency control

Secondary frequency control services are also used to reset the system to balance. In the NEM, this reset is achieved by automatic generation control, which sends signals to generators to adjust their generation output. This is like using tiny movements to the steering wheel when you are driving a car to make sure you are driving straight down the middle of the road.

What happens if we can’t get the frequency back to the right level?

In some rare cases, frequency control services aren’t enough to get the frequency back to the required 50 hertz.

This can occur if there are simultaneous failures of multiple generators, or if there is an unexpected break-up of the grid into multiple smaller areas, each of which must suddenly control its own frequency by itself. While these events are uncommon, they can and do occur, and arrangements must be in place to deal with them.

If the frequency deviates too far, automated load-shedding or generation shedding can occur, which means that load (power being used) or generation is automatically disconnected from the system to try and restore the balance between supply and demand, and in some cases to protect itself from damage.

If this occurs, the energy sector has a wide range of emergency processes to restore everything back to normal. These processes are managed closely between AEMO, the Australian Energy Market Commission, the Australian Energy Regulator, State and Federal Governments, industry members and large consumers. 

If frequency correction does not occur at the right time, wider system instabilities might be inevitable where generators or consumers could face far longer and more expensive reconnection times, as the power system can totally collapse (also called a system black).

Want to know more about the basics of our energy system? Read the rest of the series, or contact media@aemo.com.au to let us know what explainers you would like to see! 

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