In 2022, Transgrid conducted a RIT-T to assess the preferred option to resolve the stability of south west NSW which was initiated to address the voltage stability due to the outage of Line 63 (see Figure 1)! Yes, the one that (has) caused a lot of curtailment for solar farms in the area, though line X5 stability is now a bigger deal! From the modelling in the RIT-T, Transgrid identified a preferred option consisting of a new 330 kV transmission line between Darlington Point and Dinawan together with short-term battery-based network support. Following the Project Assessment Conclusion Report (PACR) and the Australian Energy Regulator (AER) review (through its consultant), Transgrid published a notice of change to non-network option which indicated that the transmission upgrade was not the preferred option anymore.

Figure 1: South West NSW network and line 63

However, power systems evolve quickly. Only a few years later, Transgrid is going through a system strength RIT-T process and now proposing solutions such as additional synchronous condensers and grid-forming battery systems to meet increasing system strength requirements in NSW including in south west NSW and Darlington Point. This raises an important question: if system strength benefits had been explicitly considered at the time, would the 330 kV network upgrade have delivered broader benefits than originally recognised?

Figure 2: Transgrid’s system strength PACR – indicative map of new-build system strength solutions in portfolio option 2

This article does not attempt to replicates the modelling in either RIT-Ts. Rather, it revisits the engineering problem using today’s network conditions and asks whether transmission reinforcement may have delivered broader long-term benefits than originally recognised.

Today’s Planning Context

Transgrid’s recent Meeting System Strength Requirements in NSW PACR and a material change in circumstances (MCC) identify the need for a portfolio of grid-forming battery energy storage systems (BESS) and synchronous condensers across the NSW transmission network, including in the area of Darlington Point. In this area, the preferred solution comprises the installation of synchronous condensers together with grid-forming BESS to provide the required level of system strength as the power system transitions away from conventional synchronous generation.

Revisiting the Network

This raises the question: what if the 330 kV network upgrade had remained part of the planning assessment? To explore this, we conducted a fault level assessment for several major substations in south-west NSW under a range of network conditions. The four cases considered progressively strengthen the network through combinations of synchronous condensers and transmission reinforcement. Note that the MCC identified the need for one additional synchronous condenser in Dinawan, which we don’t consider that in the studies below.

• Case 1 – Project EnergyConnect (PEC) without Dinawan synchronous condensers

This case represents the weakest network configuration following completion of Project EnergyConnect without the proposed synchronous condensers as part of PEC.

• Case 2 – Project EnergyConnect with its Dinawan synchronous condensers (two 120MVA)

This represents the current planned approach in which synchronous condensers are installed at Dinawan.

• Case 3 – Case 2 plus an additional 120MVA synchronous condenser at Wagga

This case investigates whether further local synchronous generation provides additional improvement at Darlington Point 330kV.

• Case 4 – Case 2 plus a 330 kV upgrade between Dinawan and Darlington Point

Instead of introducing another synchronous condenser, this scenario strengthens the transmission network itself by reinforcing the electrical connection between the two substations.

Results

First, the disclaimer: note that these studies are conducted using a minimum grid strength case that we created for NSW, particularly in Southern NSW. Our fault level assessments follow AEMO’s approach for the system strength report, and could be different in a different study. Furthermore, the modelled parameters for synchronous condensers are EPEC’s assumptions which might be different to the actual parameters. While we have exercised reasonable skill, care and diligence in preparing the analysis, we don’t warrant the accuracy of the outcomes. We encourage the reader to follow the trend rather than absolute fault levels.

The fault level results demonstrate a clear trend across every study location. Introducing synchronous condensers produces meaningful improvements in system strength. However, reinforcing the transmission network delivers an even greater increase in fault level throughout the region. Let’s look at individual buses:

1. Buronga 330kV: exhibits relatively little sensitivity to either additional synchronous condensers or the Dinawan–Darlington Point reinforcement, indicating that it is electrically remote from the area where system strength is constrained.
2. Dinawan 330kV: obviously local synchronous condensers uplift the fault level both on N and N-1 conditions quite significantly. Interestingly the upgrade between Dinawan and Darlington Point increases the fault level at this bus, so as HumeLink.
3. Wagga 330kV: evidently the highest impact is by the local synchronous condensers. However, more importantly the impact of HumeLink is even higher for this location. The upgrade between Dinawan and Darlington Point has a marginal impact.
4. Darlington Point 330 kV: this is the key bus of interest. The Dinawan–Darlington Point reinforcement is expected to increase the fault level at this location by approximately 1,000 MVA, representing the largest uplift across the options studied.

Figure 3: Fault level assessments for different cases in south west NSW

Conclusions

Note that the findings do not imply that synchronous condensers are unnecessary. Nor does it imply that the original RIT-T reached the wrong conclusion based on the information available at the time. RIT-T assessments are necessarily undertaken using the best information available at a particular point in time.

When power systems are changing rapidly, however, some network benefits may only become apparent several years later. Transmission reinforcements often provide benefits that extend beyond their original investment need. And it is good to see that the recent AEMO’s Integrated System Plan (ISP) has considered system strength benefits in the cost benefits analysis.

And if system strength benefits were to be accounted for in the line 63 RIT-T, perhaps the 330kV upgrade from Darlington Point to Dinawan would now be under construction, and unlocking cheap reach solar generation in the area?!

About our Guest Authors

	Nadali Mahmoudi is the Director of Electricity Market & Network Modelling at EPEC Group. He has more than 15 years of experience across consulting, academia and modelling in electrical engineering and electricity markets. He also holds a PhD in Electrical Power Engineering from the University of Queensland. You can find Nadali on LinkedIn here.
	Liam Atkinson is a Power Systems Engineer at EPEC Group. He has more than 2 years of experience across consulting and modelling in electrical engineering and electricity markets. He also holds a Bachelor (Hons.) & a Masters of Electrical Engineering from the University of Queensland. You can find Liam on LinkedIn here.