On February 25, 2025, AEMO market notice 125081 announced the removal of three constraints that were part of system normal sets.
MN125081
The notice informs us that an update to the Yass overload trip scheme is now managing flows on lines 990, 991 and 970 132kV in both directions.
We therefore understand that the constraints formerly used to manage those flows are now not needed in the system normal sets.
The three constraints
The three constraints were part of system normal sets, meaning they were always invoked.
Each one managed the flow on transmission connecting Yass to Wagga, protecting against overload in the event of a trip of the 051 line.
N>>NIL_970_051: avoid O/L Burrinjuck to Yass (970) on trip of Wagga to Lower Tumut (051) line
N>>NIL_990_051: avoid O/L Wagga to Yass (990) on trip of Wagga to Lower Tumut (051) line
N>>NIL_991_051: avoid O/L Wagga North 132 to Murrumburrah 132 (991) on trip of Wagga to Lower Tumut (051) line
Of note, the 051 line is 330kV and the ‘protected’ lines are 132 kV. See more about this in the “Further reading on why the 051 line is critical” section, below.
Transgrid network, annotating key lines (TAPR 2024)
Frequently bound
The news is of interest to the many participants impacted by these constraints.
By way of example, the Transgrid 2024 TAPR listed N>>NIL_970_051 as one of the most frequently bound constraints in all of NSW.
It ranked 7th highest by duration bound (Table A5.1) – with a duration totaling over 27 days in 2024.
It was ranked 3rd highest (Table A5.2) in terms of market impacts (measured by the marginal value).
It will be interesting to see what impact this has on the congestion and counter flow between Vic and NSW.