Calling an audible: Voltage control changing the playbook

Voltage instability concerns are becoming more relevant in the NEM as the system continues to incorporate higher levels of variable asynchronous generation in “electrically weak” areas of the network. Most recently, AEMO (on advice from TransGrid) implemented a new/revised system normal constraint equation in NSW for voltage collapse at Darlington Point on loss of the Darlington Point to Wagga 330kV line. The constraint equation includes generators in south-west NSW and north-west Victoria as well as MurrayLink.

There are multiple transmission augmentations that could address this, most notably EnergyConnect which proposed to install synchronous condensers at Darlington Point (planned for 2022-2024). Prior to this however, there is a chance that AEMO will need to intervene to restrict output in the area depending on the rate of connections and local demand.

Voltage instability, however, is not limited to “electrically weak” areas of our network. As power flow dynamics shift, there is an increased need to actively manage the transmission network. This has become clear in Victoria, where transmission line de-energisation has become a frequent occurrence to manage voltage.

In this chart of the week, we show the total hours that individual transmission lines have been out of service for voltage control as well as the equivalent percentage of time by month. The Hazelwood – South Morang 500kV lines have been frequently de-energised in response to voltage control. Across April 2020, line #1 was out for 21% of the month and #2 out for 27% of the month. This translates to almost half of the month where one of these lines was out of service. In fact, there was a 10 day stretch with at least one of these lines out of service to manage voltage.

These long outages can skew the averages, when stripped away, we see that the typical outage can range from a few hours (usually overnight) to two days. It highlights that as coal generation in the Latrobe Valley decreases, instances of de‑energisation for voltage control are likely to increase.

As AEMO have noted, “line-switching to control voltages is not a prudent operational measure in the long-term in the NEM, both in terms of system security and reliability”. New solutions to address this are necessary. Fortunately, there is a lot of activity in this space. We briefly mentioned the AEMC system strength framework investigation in Issue #30. However, the ESB have also released their thoughts on system service and ahead markets as part of the post-2025 market design project (Get in touch for more details). TransGrid (NSW TNSP) has also put forward a rule change request seeking to allow TNSPs to play an increased role in the planning and coordination of system strength services over the longer term.

There are many other examples however one thing is clear – there is no one-size-fits-all solution to tackling voltage, system security and reliability. Like most operations in the energy market, there are a range of tools/ technologies/ mechanisms that will work together to ensure security and reliability and ultimately deliver least cost energy to consumers.

We cover about this and more in our networks webinar next week where we will take an in-depth look at the transmission and distribution networks’ ability to facilitate the energy transition.