When we tune in to a radio station, we have to get the frequency spot on. Just 0.1 megahertz off, and the sound suffers badly. Well, the same principle applies to frequency in power systems around the world – but getting it wrong can have far bigger consequences than a bit of static over a song!
Australia’s power systems, including the interconnected National Electricity Market (NEM) power system, operate at a frequency range as close to 50 Hertz (Hz) as possible.
At 50 Hz, and within a few hundred millihertz either side of 50 Hz, the power system safely and securely transmits power from generators to consumers. AEMO is required to keep frequency in the range 49.85 Hz and 50.15 Hz, as the blue line on this chart shows.
If the frequency goes outside too far outside this range, the system doesn’t operate as it should. Generators, and large motors drawing power from the network, can start disconnecting to protect themselves against damage. If these disconnections are uncontrolled and create more imbalance between demand and supply in the system, affecting frequency further, consumers can experience a full or partial blackout.
So how does AEMO, as the power system operator, make sure frequency stays close to 50 Hz?
Balancing demand and supply
If the system has more power than it needs at any instant, its frequency will increase. If there is not enough power to meet demand at any time, the system frequency falls.
AEMO constantly monitors system frequency, and we co-ordinate generation coming into the system and demand going out so they are always balanced and frequency doesn’t go outside the set range.
In the energy market, demand is forecast and generation is dispatched on a 5-minute cycle, which means AEMO is taking active steps all the time to keep supply and demand balanced. Every five minutes, we combine the output of variable generation (like solar and wind), and small generators, with dispatched “scheduled” generation (like coal, gas, and hydro) to exactly meet forecast demand by consumers all across the power system.
Sometimes AEMO detects variations in frequency which must be managed at very short notice. We can’t wait for the next 5-minute generation dispatch cycle, because generation or load will start automatically disconnecting before then, potentially causing bigger problems.
These frequency variations can be caused by either demand or supply being suddenly higher or lower than expected.
To be ready for these times, AEMO contracts with some providers to have “ancillary services” on standby, to raise or lower frequency within seconds of being asked. These contracts are usually with generators that are technically able to increase or decrease output on direction, but frequency can sometimes also be stabilised by quickly disconnecting devices that are drawing energy from the network.
AEMO has arrangements for a range of ancillary services to respond to minor or larger variations in frequency.
One of these types of services, called “regulation” services, has to be available at all times. AEMO, through an automated control system, dispatches these regulation services every four seconds to keep the frequency steady at or close to 50 Hz.
Other services are available on call but are only used when major disturbances cause larger deviations from the normal operating frequency. These are called “contingency” services, because they are for situations which are possible, but are not expected to happen often.
This figure shows what happens in one of these “contingency” events. The frequency (the blue line) hovers close to 50 Hz during normal operation, then, after an unexpected event (marked here as T1), almost instantly falls outside the normal, accepted range (at T2).
So this event doesn’t cause damage and disruption for consumers, “contingency” ancillary services step in within seconds after T2 to arrest the fall. In this example, they then return frequency to the normal range in less than one minute.
In some rare cases, ancillary services aren’t enough to control a very severe event. Emergency schemes have also been set up to respond quickly to ‘high impact, low probability’ events like the loss of an interconnector, or multiple trips at the same time.
Whatever the size of the variation, AEMO’s goal is to arrest it and return the power system to the safe and secure 50 Hz frequency as quickly as possible while minimising any impact on consumers.