Australian Landscape of Grid-Forming Batteries

Australia is at the forefront of the transition of power systems away from large fossil-fuel-based generation to renewable generation. Recently, the Australian east coast power system (called the National Electricity Market, or NEM) reached an instantaneous renewable energy penetration of 68.7%, while the South Australian region of the NEM has operated with renewable generation sufficient to meet over 100% of customer load in this area on several occasions.

The majority of Australia’s renewables are interfaced with the grid via power electronics, which can present challenges in maintaining sufficient system security needs such as voltage and frequency control, synchronous inertia, system strength, fault current, and system restoration. Some of these system security needs can be provided by grid-forming inverters, when appropriately configured and deployed at suitable locations in the grid.

Existing and Near-Future Grid-Forming Batteries

Whilst the technology is relatively new and various grid-forming control philosophies exist, Australia is seeing a tremendous interest in the deployment of grid-forming batteries by developers, in part to demonstrate the capabilities offered by this technology. At the time of writing this blog, the world’s largest transmission-connected grid-forming battery, the 150 MW Hornsdale Power Reserve located in the NEM, is operating in grid-forming mode using Tesla’s Virtual Machine Mode control philosophy. This is not the first transmission-connected battery that uses grid-forming control in Australia. In 2018, a 30 MW battery with ABB’s grid-forming control was commissioned at the Dalrymple substation in South Australia.

Figure 1 shows the current and near-future landscape of grid-forming batteries in the NEM. At the time of writing, the total installed capacity of the grid-forming batteries in operation is 230 MW / 277 MWh.

ARENA’s Grid-Forming Battery Funding

The Australian Renewable Energy Agency (ARENA) supports the global transition to net-zero emissions by accelerating the pace of pre-commercial innovation. In early 2022, ARENA announced support of advanced inverter development through a competitive funding round. The funding support aims to:

• • accelerate demonstration of advanced inverter capabilities on projects at scale
  • overcome barriers that prevent projects from incorporating advanced inverter capabilities
  • improve industry understanding of the potential role of advanced inverters in supporting system stability during periods of very high inverter-based generation
  • reduce the reliance on synchronous generators and/or synchronous condensers for system stability
  • demonstrate the capability of advanced inverters (at scale) in multiple states and across multiple inverter types, such that they can be relied upon for critical system services in power system planning
  • inform the market regulatory bodies in order to facilitate the efficient delivery of services from grid-forming batteries

In December 2022, ARENA announced $176 million in conditional funding to eight grid-scale battery projects across Australia as the outcome of the funding round. Each of the batteries varies in scale between 200 MW and 300 MW. It is expected that these projects will commence operations in grid-forming mode between 2024 and 2026. Each battery will be equipped with grid-forming inverter technology, allowing them to provide essential system stability services traditionally provided by synchronous generation such as coal and gas. With a total project value of $2.7 billion and a capacity of 2.0 GW / 4.2 GWh, these projects represent a near ten-fold increase in grid-forming inverter capacity currently operational in the NEM.

Figure 1. Australian landscape of grid-forming batteries (source: AEMO)

With more installation of grid-forming batteries, it is vital to understand the performance of this technology and how it can contribute to power system security and reliability.

Demonstrated Capabilities of Grid-Forming Batteries

Since the deployment of grid-forming batteries in the NEM began, these devices have demonstrated various capabilities in response to real-time disturbances. During planned and unplanned outages of the upstream network, the Dalrymple grid-forming battery energy storage system (BESS) has successfully formed a stable electrical island supplying its downstream load, without a synchronous generator. During the operation of this stable island the frequency and voltage were tightly controlled. The frequency of an islanded network is shown in Figure 2. This grid-forming battery is capable of energization of the grid and load pick up during a system restoration process.

Figure 2. Frequency control provided by grid-forming battery during an island (source: AEMO)

The inertial response provided by the Hornsdale Power Reserve grid-forming battery is comparable to a typical inertial response provided by a synchronous machine. During a frequency event, the grid-forming battery provided its response to the rate of change of frequency like a synchronous machine. It should be noted that the magnitude of the response would depend on a number of factors such as amount of energy and head-room available prior to the disturbance.

As an example, Figure 3 compares the response from a single inverter in a grid-forming mode to a large fossil-fuel-based synchronous generator. The secondary Y-axis order is reversed to demonstrate a correlation between rate of change of frequency and active power change.

Figure 3. Inertial response – grid-forming inverter and synchronous generator (source: AEMO)

While industry is gaining confidence in some of the capabilities that have been practically demonstrated by grid-forming batteries on the large-scale grid, the technology and developers need ongoing support, guidance, and encouragement from utilities for large-scale deployment.

Grid-Forming Inverter Specification

A white paper on advanced grid-scale inverters published by the Australian Energy Market Operator (AEMO) in 2021 highlighted the need to progress the development and deployment of grid-forming technology in consultation and collaboration with industry. It also highlighted that detailed coordination between all relevant stakeholders is necessary when developing capability requirements and specification for grid-forming inverters.

Recently AEMO released a voluntary specification for grid-forming inverters, developed in collaboration with industry. This specification provides guidance to stakeholders while the regulatory environment around grid-forming technology develops.

What is a Grid-Forming Inverter?

While the international community is still establishing a definition for what constitutes a grid-forming inverter, with extensive discussion with stakeholders AEMO has proposed the following definition in the voluntary specification document:

A grid-forming (GFM) inverter maintains a constant internal voltage phasor in a short time frame, with magnitude and frequency set locally by the inverter, thereby allowing immediate response to a change in the external grid. On a longer timescale, the internal voltage phasor may vary to achieve desired performance.

The voluntary specification proposes two tiers of capabilities. Core capabilities are expected to be achievable with minimal modification to plant hardware when compared with a grid-following design, and mainly require modification to the control algorithm of the device. Additional capabilities might require material hardware upgrades or changes to operational practices.

Figure 4 provides a high-level view of these core and additional capabilities, with further details available in the specification document.

Figure 4. Voluntary grid-forming specification (source: AEMO)

Next Steps

AEMO is now preparing to develop a set of tests that could be applied to a grid-forming inverter design to evaluate its performance against the voluntary specification. This work seeks to provide designers and developers of grid-forming technology a quantitative way to inform their decision-making as the regulatory environment surrounding this technology continues to evolve.

Nilesh Modi, Australian Energy Market Operator
Chris Mock, Australian Energy Market Operator
Carl Christiansen, Australian Renewable Energy Agency