Exploring the potential impacts of HumeLink and Snowy 2.0 on the Southern NSW electricity network

Snowy 2.0, a major generation project in the NEM, has been the subject of extensive discussion in recent years. As the largest storage project in the market, it offers significant benefits through its flexibility, firming capability, and long-duration storage during periods of low variable renewable generation. However, escalating costs and ongoing delays have also attracted frequent criticism.

At the same time, the transmission network in southern NSW is undergoing substantial investment, including projects such as Project EnergyConnect (PEC), HumeLink, and further south, VNI West. These developments are intended to unlock more efficient generation investment, enable access to lower-cost generation, improve utilisation of existing assets, and support projects such as Snowy 2.0.

In this article, we examine how these investments could impact the southern NSW transmission network, with a particular focus on key considerations that need to be addressed for Snowy 2.0.

Figure 1 — Existing and future southern NSW network.

Source: TransGrid TAPR 2025

Housekeeping

The results presented in this article are based on a modelling forecast, and actual outcomes that we’ll see in the real world may vary. The forecast was developed by EPEC’s Integrated PLEXOS & PSSE Model. In short, PSSE is used to create N and N-1 constraints, following AEMO’s constraint formulation guidelines, and PLEXOS is used to simulate half hourly market dispatch subject to these network constraints and many others.

Before we dive deep into the detail, let’s clarify a few assumptions:

• The results that are presented in this article are based on a single set of assumptions. Outcomes may vary under alternative market, policy, or system conditions.
• While EPEC has conducted detailed calculations to estimate the electrical parameters of the HumeLink transmission lines using publicly available data, we note that our estimates could be different to the final design of this network upgrade. Note also that we did not model the latest ISP actionable switching station at Wondalga which is expected to impact HumeLink.
• The modelling of the Snowy hydro scheme is based on a generic, publicly available Snowy scheme, optimised by EPEC’s Integrated PLEXOS & PSSE Model, which could be different to its real operation.

Some assumptions are highlighted in the table below.

In addition, the following table translates EPEC’s network constraints to their plain English names for reference throughout the article. It contains both N and N-1 thermal constraints for system normal conditions.

Snapshots in time

In this article we are going to examine the above network constraints for three different financial years. These financial years represent:

1. Pre HumeLink (FY 2028-29)
2. Post HumeLink but prior to commissioning of Snowy 2.0 (FY 2029-2030)
3. Post commissioning of Snowy 2.0 (FY 2030-31/ 2031-32)

Below we provide forecast congestion snapshots of these three future years. Each snapshot provides an overview of the forecast network performance in that particular year. We use the percentage of intervals binding as a measure of how much of the year that particular line or transformer was at its thermal limit.

Firstly, we can take a broad look at the annual binding percentages for each snapshot in the EPEC’s Client Portal map. EPECs congestion map shows all the transmission lines that are modelled (noting that only a subset of key lines is shown) and the colouring provides an indication of the amount of congestion that these assets experience. E.g. the red indicates high congestion risk, yellow is for medium and green is for low congestion risk.

For the year before HumeLink commissioning, the map clearly shows high congestion on the lines east of Wagga Wagga. These are the 132kV lines from Wagga to Yass. This is broadly consistent with what we see in the current market, where the 132kV lines between Wagga and Yass can limit high solar generation from south-west NSW and imports from Victoria.

Figure 2 — Pre HumeLink network conditions (annual average binding constraints heatmap)

Now, let’s look at the following year with HumeLink commissioned (note that Snowy 2.0 is not assumed to be operational yet). The obvious stand out here is that the congestion that was seen on the 132kV lines between Wagga and Yass has been alleviated. Positively, we also don’t see this congestion moving to anywhere else in the network.

Figure 3 — Post HumeLink, pre Snowy 2.0 network conditions (annual average binding constraints heatmap)

Lastly, we look at a year with Snowy 2.0 fully operational. Some orange and red coloured lines stand out here. The red line in the top right of the figure is line 39 (Bannaby to Sydney West 330kV), a major flow path into the Sydney load centre. Multiple factors are causing this line to bind more; the Waratah SIPS contract is assumed to end, some NSW coal is assumed to retire and as such Snowy 2.0 increases generation to cover the loss of capacity within Sydney. But this particular line is not the focus of this article and as such we will defer the conversation. In Figure 4 we also see an orange line on the 132kV path between Yass, Bango and Cowra. To understand what’s happening on this flow path we need to dig a bit further into the details. It is likely that the pumping load of Snowy 2.0 is causing day time flow from central NSW down through Cowra to Yass, increasing congestion during the day on this flow path. But to confirm this we need to dig a bit further into the details.

Figure 4 — Post Snowy 2.0 network conditions (annual average binding constraints heatmap)

Digging further

The charts in this section utilises the EPEC results dashboard to display the time-of-day distribution of binding hours for several key network constraints in the southern NSW region, across the four years between 2028-2032. Again, noting that in this model:

• 2030: HumeLink becomes operational
• 2031: Snowy 2.0 becomes operational

Central NSW and line 999 (Cowra to Bango)

Firstly, we look at constraints for line 94T (Molong to Orange) a notorious bottleneck in central NSW and line 999 (Cowra to Bango) as there is an interesting dynamic occurring here. Figure 5 shows that both lines are forecast to bind during the day across these four years. The model forecasts that HumeLink is expected to slightly alleviate congestion on line 94T, with further easing once Snowy 2.0 is online. While the opposite is true for line 999. What’s happening is that with Snowy 2.0 pumping load, solar generation in central NSW is pulled down towards Snowy causing some decreased pressure on line 94T, while increasing pressure on the path between Cowra and Bango towards Yass.

Figure 5 Time of day frequency of binding constraints – central to southern NSW

Southern NSW network

Now lets move our attention into the heart of the southern NSW network. In this study, the second most frequently binding constraint in the first year is line 990 (Wagga to Yass 132kV) on trip of line 51 (Wagga to Lower Tumut 330kV). From Figure 6 we see that in 2029 this constraint is forecast to bind heavily during daytime hours, but encouragingly, HumeLink is expected to fully alleviate this constraint once it is operational. By providing a lower-impedance pathway (not to be confused with line resistance), HumeLink effectively bypasses this 132kV corridor and several other 132 kV corridors between Wagga and Yass. A similar reduction is observed for the Yass transformer under the Line 51 trip, driven by the same network reconfiguration.

Turning to constraints primarily impacted by Snowy 2.0, the key one we see increasing after Snowy 2.0 is commissioned is Line 51 on trip of Gugaa–Maragle 500kV. Figure 6 shows that this constraint is forecast to bind during daytime hours. This is largely driven by Snowy 2.0 pumping with high solar output flowing from west and south of Wagga.

Figure 6 Time of day frequency of binding constraints – southern NSW

Conclusion – what needs to be explored further?

In conclusion HumeLink is expected to alleviate existing congestion on the 132kV network between Wagga and Yass. Humelink is also forecast to have a slight improvement on the already congested lines in central NSW such as line 94T by rerouting flows from southern NSW off the 132kV network. With Snowy 2.0 coming online, however, some considerations need to be taken. Our modelling forecast that Snowy 2.0 could cause some congestion in some parts of the NSW network. Specifically, congestion on the 132kV lines from central NSW towards Yass (Cowra to Bango), as well as congestion on line 51 (Wagga to Lower Tumut), was observed in the results, caused by Snowy 2.0’s pumping load.

A further note that over time with these changes as well as NSW coal retirements more congestion is expected on the flow paths into Sydney (i.e. line 39) which needs to be studied in more detail.