“The pace of retirement of coal-fired plants, together with record levels of investment in wind, solar, storage, and transmission, is transforming the way the system is operated at its most fundamental level. Although this is necessary for decarbonization and the long-term affordability of energy, it has also highlighted a set of technical issues that had been invisible in the traditional power system model. One of the key technical issues is the way in which the unit commitment is affected as the percentage of invertor-based renewables in the overall mix continues to grow.”

For many years, grid stability was a natural consequence of the way in which power was produced. The presence of the large synchronous machines in coal, gas, and hydro power stations provided inertia, voltage, and fault current contribution simply through their connection to the grid. With the withdrawal of these power stations from the grid, this phenomenon will cease unless replaced with something that substitutes this quality.

How Traditional Power Systems Maintained Stability

Conventional Australian grid was designed around synchronous generation. Large rotating masses in such generators are directly coupled to the power system. In the event of a sudden mismatch between supply and demand, stored kinetic energy of those rotating machines opposes any frequency variations instantaneously. This inertial response is fully automatic and instantaneous; it does not require control systems or communications of any sort.

At the same time, synchronous generators provide strong voltage control through excitation systems and contribute high fault currents during network disturbances. Together, these characteristics created a power system that was inherently robust. Frequency and voltage stability were largely passive outcomes of generation technology choices rather than explicit design objectives.

The Impact of High Renewable Penetration

This inherent stability is progressively removed from the system as coal-fired power stations retire. The connection to the grid is through power electronic inverters rather than rotating machines for wind farms, solar farms, and battery energy storage systems. While these technologies are highly controllable and efficient, they do not provide inertia or high fault current naturally.

The result is a system that responds very differently to disturbances: frequency can change more rapidly following a fault, voltage recovery can be slower, and the grid becomes more sensitive to sudden changes in generation or load. Such effects become more pronounced as renewable penetration increases and synchronous generation is displaced.

Inverter-Dominated Grids and Control Challenges

In most cases, the mainly available sources of renewable energy utilize ‘grid following inverters.’ In these inverters, a predetermined voltage and frequency environment must be available in order for them to work properly. They use the available waveform in the generation of current.

However, as the use of synchronous generation reduces, the integrity of the grid reference itself is reduced. In such an environment, grid-following inverters can find it difficult to synchronize, especially during fault conditions or when the operating point suddenly changes. Additionally, control interactions among multiple inverters can result in oscillations that introduce Instabilities not seen in conventional power grids.

Understanding System Strength in the Australian Context

System strength is defined as the capability of the power system to maintain stable voltage during normal operation and following disturbances. The short-circuit current availability and the electrical impedance of the network are closely related to system strength. Strong systems absorb disturbances with a minimal voltage deviation, whereas weak systems have larger swings and slower recovery.

In Australia, system strength has emerged as a critical issue in regions with high renewable penetration combined with long transmission distances. Renewable energy zones distant from load centres invariably interface with the grid through weak networks. It is quite common that, even with spare transmission capacity, shortage of system strength can bound the renewable generation one can connect in these regions.

The Role of Synchronous Condensers

Synchronous condensers have appeared to be a significant answer to the issue related to system strength and inertia. In fact, a synchronous condenser can be defined as a type of synchronous generator, which is used without the production of active power. The condenser rotates as an autonomous machine and is always in synchronization with the grid. In addition, the machine acts similarly to a common generator, which consumes no fuel.

As far as the grid is concerned, a synchronous condenser is almost identical to a regular generator. It adds inertia through its weight, helps to control voltage by providing or draining reactive power, and supplies high levels of fault current during a grid fault. This device is therefore a extremely valuable asset for a system dominated by inverters.

Inertia and Frequency Stability Support

One of the most significant merits of synchronous condensers is their role in providing inertia. As the power disturbance takes place in the system, the kinetic energy stored in the condenser’s rotor is injected into the system. As a result, the rate of slowing down of the frequency is reduced. Therefore, the frequency does not drop too low. If that happens, the protective relays might act.

This initial slowing of the frequency deviation in low inertia systems is very important. This gives other systems that regulate frequency difference, for example battery startups or generator controls, an opportunity to act and stabilize the system. This makes the synchronous condenser help support other systems that act faster.

Voltage Control and Reactive Power Management

Another area where synchronous condensers are very valuable is in the provision of voltage stability. In the case of weak networks, there are chances of voltage variations in cases where there are fault currents or variations in the generator power output. This is where the synchronous condenser finds use as it can control the excitation currents.

Their response is smooth and continuous, and this is particularly important in a system with a high penetration of inverter-based sources. The role played by synchronous condensers in giving a strong voltage reference also relieves inverter control systems and improves system stability.

Fault Current Contribution and Protection Performance

Availability of fault current is vital to ensure reliable operation of network protection strategies. A good number of protection strategies rely on high values of fault current for faster detection and clearance of faults. Inverter-based generators usually cap the fault current to safeguard power electronics in the generator.

Synchronous condensers are known to furnish a large short-circuit current, thus aiding the existing protection system and enabling timely clearing of the fault. This is particularly important for a weak network, wherein low fault levels might pose system security issues.

Supporting the Renewable Energy Transition

The rise of the use of synchronous condensers in Australia is therefore symptomatic of the fact that, with the increased use of renewables, stability in the power sector in Australia is no longer assured and needs to be engineered.

Synchronous condensers make possible increased renewable production by restoring the grid properties on which inverter-based resources depend. In so doing, they make possible additional renewable supply and mitigate potential curtailment, thereby facilitating emissions reduction or energy affordability goals.

Relationship with Grid-Forming Inverters

However, grid-forming inverters are an innovative technology in the process of developing a capability for forming voltage and frequency, as opposed to following the voltage and frequency of the grid. This technology has the ability to fulfill all services associated with synchronous machines.

Nevertheless, the implementation of grid-forming inverters is still an emerging field, and the performance for extreme system conditions is still being tested. In the short term, synchronous condensers have been found to be an established, known technology for stability with immature grid-forming technology.

Economic and Planning Considerations

Synchronous condensers are major investments and entail careful consideration to ensure maximal value on the power system. The applicability of synchronous condensers relies largely on their position as well as synchronization with other support services. With proper positioning, their benefits may be reaped beyond the point of connectivity.

While enabling new connections of other renewables, optimizing system security, and addressing operational limitations, the system-level benefit of synchronous condensers is quite significant. Such system-level significance should be kept in mind while analyzing the importance of synchronous condensers in the energy transition.

A Transitional Technology for a Changing Grid

In view of the above, while the technology related to synchronous condensers is mature, their application in the modern power system is quite different. They are not a step back to fossil-fueled generation but rather an intermediate technology in the transformation to a low-emission power sector.

By being able to fill the gap that exists between the current synchronous generation and the future inverter-based grid, synchronous condensers make it possible for Australia to quickly transition into a high renewable system without affecting security and reliability of supply.