By Viduni Pathirana
The stability between generation and demand is essential in order to maintain a secure and reliable power system. An imbalance between generation and demand causes deviations in frequency. For instance, an off-balance may occur due to loss of a major generator, which will result in a frequency drop due to the shortfall of generation. In order to mitigate frequency variations and maintain the frequency of the power systems within the normal operating range (50 ± 0.15 Hz), the industry employs Frequency Control Ancillary Service (FCAS) to inject or consume energy to manage the balance in supply and demand through the Australian Energy Market Operator (AEMO). Currently with the recent increase of Distributed Energy Resource (DER) there are more participants in the FCAS market. One such participant is the South Australian Virtual Power Plant (VPP) which is registered provider for the FCAS market.
The frequency rise or fall is in line with the demand and generation. The two major scenarios are listed below.
1. If the demand is less than generation, then the frequency will increase above 50 Hz
as represented in Fig.1.
For example, such an event occurred in Australia in 2019, when the interconnection between Victoria and South Australia was disrupted which caused the South Australian grid frequency to increase due to the upsurge of generation.
2. If the demand is greater than generation, then the frequency will decrease below
50 Hz as represented in Fig.2.
For example, such an event occurred in Australia in 2019, the Queensland’s Kogan Creek generator tripped. This caused a deficit in generation causing the grid frequency to drop.
For both events mentioned above, South Australia’s grid used the aid of FCAS which responds by altering either the generation or the demand in order to correct the off-balance.
FCAS provides two types of services:
1. Regulation frequency control
Regulation frequency control is used on a regular basis in order to correct minor variations in generation or demand. For instance, one such variation may be due to discrepancy between the demand forecast observed in real-time. Regulation service consists of two components, regulating raise and regulating lower. These services are controlled by AEMO every 4 seconds via the Automatic Generation Control (AGC) system.
2. Contingency frequency control
Unlike Regulation services, Contingency frequency control is used only during a major contingency event such as loss of a major generating unit or industrial load. It consists of six components which are controlled locally. Initially, the Fast Raise and Fast Lower which are the 6 second responses to correct immediate frequency deviations. Subsequently, the Slow Raise and Slow Lower are the 60 second responses to maintain the frequency with the single contingency band. Finally, the Delayed Raise and Delayed Lower are the 5-minute responses to return the frequency to the normal operating range.
To conclude, it is evident that FCAS plays a vital role in the power system. In my next blog, I will be focussing on DER in the energy market.
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