Impact of Tiwai exit

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Transpower, as System Operator, has completed analysis to determine system operation and security impacts of Tiwai’s exit following the initial announcement of an August 2021 closure. While Tiwai has signalled ongoing operation until at least 31 December 2024 and the outlook for its ongoing operation is positive. As a prudent System Operator we have continued to assess operational implications associated with a possible closure.

This webpage shares the findings of our analysis to date. 

Overall, our analysis show that the System Operator can take steps to maintain security and manage operational challenges created should TWI cease operation.

Our Approach

To understand the impacts we investigated the following to date:

  • Steady-state thermal limits under N-1 and N-1-1 conditions (covered in the 2020 System Security Forecast)
  • Steady-state high voltage management under light load conditions (covered in the 2020 System Security Forecast)
  • Steady-state voltage stability (refer reports below)
  • Transient angular stability (refer reports below)
  • Over frequency reserves (refer reports below)
  • Small signal stability (refer reports below)

These aspects will be considred closer to Tiwai's closure date once confirmed:

  • Review of automatic under frequency load shedding (AUFLS) adequacy
  • Updates to contingency and restoration planning:
    • Black start restoration process
    • Electrical islanding process
    • Regional contingency plans

Analysis Findings

  1. Operational impact assessment, Tiwai smelter closure - Summary of South Island stability [ pdf 1011.3 KB ]
  2. Operational impact assessment, Tiwai smelter closure - South Island transient angular stability [ pdf 2.33 MB ]
  3. Operational impact assessment, Tiwai smelter closure - South Island oscillatory stability [ pdf 682.02 KB ]
  4. Operational Impact Assessment, Tiwai Smelter Closure - Managing an HVDC bipole tripping [ pdf 2.46 MB ]


Links to Additional Information

Clutha and Upper Waitaki Lines Project Operational Information

System Security Forecast

Security of Supply Annual Assessment