Historically, as with other large grids, the NEM has relied on conventional coal, gas and hydro generators to provide mechanical inertia and respond rapidly to suppress rapid changes in frequency following a shock or disturbance. However, it is becoming clear that this can no longer be taken for granted as our energy mix moves away from conventional generators towards large-scale and distributed renewable energy and energy storage. Other generation sources and enabling systems in the grid must contribute alongside the conventional synchronous generators.
These technologies are available now and can be deployed to ensure the continued levels of security expected of a modern electricity grid. Failing to tap into them is a missed opportunity that could have significant ramifications for future energy security. Transforming our energy system is not a technology problem. However, it is important to ensure that regulatory and market settings provide the necessary frameworks that look past conventional grid design to bring new technologies into the market, rather than relying on outdated assumptions.
This article investigates the issue of inertia, how it works with the power grid and how renewables and storage can help using currently available technology.
‘Synchronous’ generators have large heavy electrical rotors and turbines that spin to generate power. Traditional power grid design assumes that the system’s inertia is created by the combination of these spinning masses.
Importantly, inertia only acts to manage the rate of change of frequency, or ROCOF. It slows the change, buying time for additional fast-acting protection schemes to act to restore the supply-demand balance thus arrest the change in frequency. The extent of inertia in a system becomes most apparent when a significant contingency event occurs. Lower inertia systems provide less time for the emergency controls to kick-in.
Because our grids were originally designed around these traditional solutions, it has been assumed that inertia is always sufficient. To date the rules that govern generator connection have not considered inertial contribution from any generator. We’ve taken it for granted. Despite this, it is also clear that our fleet of thermal generation is quickly ageing with an average age of 40 expected to be reached in the coming decade. Replacement of these assets is unlikely to take the same form, with wind and solar generation likely to dominate.
Inertia from conventional generators
While it is clear that the inherent mechanical inertia from traditional generators has played an important role in power systems, the performance of these generators during large disturbances is less well understood as they too may become unstable in low inertia conditions.
In an environment where new technologies are expected to support power system security it is essential to ensure that the existing system is very well understood. It is critical that all forms of generation work in concert to support power system security. Future expectations are for the closure of many of the existing synchronous generators so the market has to look beyond conventional technologies.
Fast frequency response from wind turbine generators
Wind turbine generators can draw upon the kinetic energy stored in the generator and rotating blades to provide a boost of power if triggered by a system disturbance or contingency. Quebec has been paving the way with wind turbines being used in this way. Although the grid and market is quite different, Québécoise standards have been in place requiring this capability since 2011 and ongoing improvements are pushing wind turbine generator suppliers to respond with even more sophisticated generator performance solutions.
Fast frequency response from inverter-based generation and storage
Solar PV and energy storage inverters are static devices so, unlike a wind turbine, have no moving parts from which extra energy can be drawn. However, the solid state nature of power inverters provides these systems with an ability to switch and change operation almost instantaneously. Where storage is included or where generation is available, this energy injection is highly controllable.
The technology works by continuously measuring frequency to detect the ROCOF. If triggered by a high ROCOF these devices can inject or absorb power very quickly in an attempt to slow the change in frequency and support the system’s inertia. Experience is rapidly growing with these technologies as they are deployed here in Australia and overseas. These solutions must form an integral part of the future design of Australian grids.
Integrating modern capabilities to build at 21st Century NEM
There are clearly opportunities to tap into new technologies that are being missed in Australia. Collectively, these opportunities and continued technological advancements show that it is not technology per se, but the failure to prepare for technological change that will impact power system security.
There is currently no standard for inertia or inertia substitutes like fast frequency response for generators in Australia. Nor have expectations been placed on new renewable energy generators or storage technologies to provide this in the past.
The following actions should be pursued to achieve a modern energy system that calls upon the diverse technological solutions available to support it:
- Establish appropriate standards for frequency conditions that apply to all technologies and focus on the speed and accuracy of their contribution to arresting the change in frequency following a disturbance.
- Accelerate trials of fast frequency response from wind turbines and inverter-based technologies to further prove this solution in the local context.
- Review the tuning of legacy synchronous generators and undertake testing to demonstrate the performance and capability of the fleet under high rates of change of frequency.