Large solar, wind farms, grid-scale batteries, and DER are now interconnecting within the National Electricity Market in volumes that have grown well beyond the retirement of conventional synchronous plants. Although the imperative of this change is core to the decarbonization imperative, as well as achieving sustainable costs, it has laid bare the particular technical issue that was hidden within the legacy electricity grid: the issue of system strength and voltage support.

Traditionally, the strength of the system has been “embodied” within the system. This has been achieved through the presence of conventional generators, which have offered inertia, fault current, and voltage stiffness through their mere presence. However, now that these conventional generators are leaving the system in large numbers, their attributes within the system are no longer assured. This has led to changes in the way the system is stabilized, how connection is provided, and also in the way that voltage and frequency are regulated.

It considers, in turn, why system strength has become such an important issue in Australia, how voltage stability is impacted by high inverter penetration, the distinction between shallow and deep system strength, and the technical pathways now being deployed to address these challenges, including synchronous condensers and grid-forming inverter strategies.

Why System Strength Matters More Than Ever

System strength is defined as the ability of a power system to withstand changes in voltage and phase-angle difference due to injections or removals of currents. A system is said to be strong if it maintains a constant level of voltage despite a change, whereas if it varies considerably due to a small change, it is called a weak system.

Traditionally, the main drivers of Australia’s grid flows have been large synchronous generators located close to the load centers or major nodes in the transmission grid. Such machines offered high short-circuit current, robust voltage regulation, and damping. The respective protection systems, inverter controls, as well as planning models, have all developed with these characteristics in mind.

The ever-growing role of inverters in the grid, as conventional synchronous machines are progressively substituted by inverters, introduces a radical shift in the workings of the grid. The inverter is naturally current-limited. The inverter neither injects considerable current into the grid in the case of faults nor depends upon inertia for the control of grid disturbances. When a large number of inverters are interfaced with the grid to supply the weak area, the grid becomes remarkably voltage-sensitive, control-sensitive, and fault-sensitive.

In Australia, such challenges are being faced in renewable energy zones and transmission corridors, where the presence of large radial connections, weak synchronous generation, and the fast pace of renewable development hinder the strength of the system. Such challenges are not mere theory. They have impacted the commissioning of projects, the connection to the network, as well as the reliability of the electricity network.

Voltage Stability in an Inverter-Dominated Grid

Voltage stability is defined as the ability of the power system to sustain satisfactory levels of voltage after various disturbances, as well as during steady-state conditions. A synchronous power system that maintains stability has excitation control systems, inertia, and large reactive reserves.

In inverter-based grids, however, the characteristics of voltage control change. Grid-following inverters, which constitute the dominating number of the current solar energy and wind energy fleet, also measure the grid voltage and inject current in line with the reference. They presume that the waveform of the voltage is already present and that it is stable. In weak grids, that assumption fails.

When operating at low system strength, slight variations in current injections may cause large differences in voltages. Phase-locked loops in grid-following inverters may have difficulties in reproducing the voltage signal accurately at low system strengths or during fault conditions. This might cause oscillations, premature recovery of voltages, or bulk disconnections of inverters.

Voltage instability in a high-IBR environment, from the perspective of the system operator, is not just a problem of steady-state voltages drifting outside their limits but also encompasses a range of dynamic phenomena such as poorly damped oscillations, delayed fault recovery, and unstable control interactions among multiple inverter plants connected to the same weak node.

Shallow Versus Deep System Strength

So, in the Australian situation facing the challenges of system strength, a working definition has arisen that contrasts between the concepts of shallow system strength and deep system strength. Although a definition has not been given in the text, a working understanding has provided a perspective regarding why certain solutions work in certain situations.

Shallow strength
“Shallow system strength” defines the local electrical stiffness at or near said particular connection point. Factors that affect shallow strength include the presence of other synchronous machines, fault levels on the bus, and the local network impedance. Inverter stability, both when operating normally and riding through faults, will be affected if shallow strength fails. Unstable or failing performance may occur if shallow strength in the system is not sufficient within grid-following inverters.

While the hive system strength represents how sound the electrical power infrastructure in the region or region group is, the deep system strength refers to the resilience of the power system as a whole. Deep system strength takes into account the spread of the effects of an event in the various regions and how the system behaves when faced with a severe event.

In the Australian system, the early focus of mitigation was typically the elevation of shallow system strength at particular points of connection for renewables. Although particular projects were thus able to secure connection, it was recognized that problems in the larger system were potentially leading to instabilities – particularly with the rising level of inverter penetration in the region.

This has led to the adoption of solutions from the perspective of system-wide change instead of treating each project separately.

The Role of Synchronous Condensers

One of the most noticeable ways in which the challenge of system strength has been addressed in Australia is through the use of synchronous condensers. The synchronous condenser is, in essence, a synchronous machine that lacks a prime mover. It does not supply real power but possesses most of the electrical characteristics that were lost with the closure of thermal power plants.

The synchronous condenser offers high fault currents, and this enhances the capabilities of the protection system and the short-circuit level. They are capable of providing dynamic support to the reactive power and hence improving the voltage stability both in the steady state and dynamic state. They are also capable of providing inertia and damping effects.

There are some regions within Australia where synchronous condensers have been put in place solely for the purpose of allowing renewable sources of power that had been previously limited by the system strength to come online. From an electrical perspective, these units are very effective. They are predictable, have easy interfaces, and are well-understood.

Nevertheless, there exist some disadvantages in synchronous condensers. The equipment has a significant capital intensity. Such equipment also has mechanical complexity, maintenance, and losses. That equipment, despite beingimportant in the current power system, does not scale well compared to the inverter-based system in the future power system dominated by power electronics.

As such, synchronous condensers are slowly being viewed as a bridging technology. Although they are very important in ensuring that the power grid today is stable, they are not very likely to remain the only option when Australia advances to even higher inverter-based power generations.

Grid-Following Inverters and Their Limitations

Most of the solar, wind, and battery installations in Australia today operate using grid-following control. These inverters synchronise to the grid voltage using a phase-locked loop and inject current based on active and reactive power setpoints.

Grid-following control works well in strong systems where the voltage waveform is stable, and disturbances are well damped by synchronous machines. In weak systems, it does not go that well with grid-following inverters. Their control systems depend on accurate voltage measurement, their response inherently being reactive rather than proactive.

In cases where many grid-following inverters are connected to the same weak point on the grid, coupling effects of control systems of the inverters might cause oscillations or instabilities. In the case of faults, the inverters’ low fault current support might impede voltage restoration, thus enhancing the stress on the grid, leading to the possibility of disconnection.

Such limitations have become more evident under high penetration of inverters and sparse synchronous generation resources in some areas of the Australian grid. Such limitations have triggered the review of control methods of inverters and encouraged the use of grid-forming solutions.

Grid-Forming Inverters as a Long-Term Solution

Grid-forming inverters signify the paradigm shift that occurs in the way inverters interact with the grid. Instead of synchronizing their operation with the existing voltage signal, grid-forming inverters set up the voltage and frequency at the grid interface autonomously. This is almost like the behavior of voltage-source converters, which resemble synchronous machines.

These inverters can easily work in a stable manner in a very weak power grid and even in an island mode because of creating a grid. They provide faster frequency response, synthetic inertia, as well as strong voltage control, independently of any external voltage reference. This is very helpful in a future power grid because there will not be enough synchronous power available.

Several underpinning grid-forming control strategies are under exploration and deployment in Australia, including large battery energy storage systems; these will be the droop-based control, virtual synchronous machine concepts, and other advanced control methods emulating generator-like behaviour.

Grid-forming inverters hold the potential, from a system strength perspective, to deal with shallow and deep strength issues. By offering voltage stiffness and dynamic stability, they might decrease dependence on synchronous condensers and allow more network operation flexibility.

Yet, the grid-forming technology is still in its development stage. For its practical implementation, it needs to be carefully coordinated among a number of grid-forming and grid-following devices with robust protection schemes and well-defined operational frameworks. The experience of Australia with large-scale batteries has put it on top in this learning curve, but wide deployment will surely require continued refinement of technical requirements and operational practices.

Implications for Grid Access and Network Planning

The emergence of system strength constraint, System Strength, as a binding constraint has made the interconnection of new generation to the Australian power system in a significantly different manner. The service providers, in association with the market operator, evaluate the thermal, voltages, and system strength constraints.

This has created new levels of uncertainty for developers as to what is involved in interconnecting their developments to a PNWE. This could involve funding work to improve system strength, modifying control settings, operating on constrained system conditions until new solutions can be implemented.

On a planning level, system strength considerations are increasingly integrated into transmission development and renewable zone design. Instead of responding to issues as they occur, planners would seek to get ahead of where strength is likely to be required and how that most effectively will be provided.

This represents a broader recognition that system strength is a system attribute that will be shared, rather than a purely local issue. It requires coordination across generation, networks, and control systems to address effectively.