Options to relieve Central North Island transmission constraints

In a series of reports published in March 2016, we highlighted that transmission capacity from Bunnythorpe to Whakamaru is constrained by the capacity of the 110 kV Bunnythorpe–Mataroa and 220 kV Tokaanu–Whakamaru circuits.  With the decommissioning of thermal generation in the Auckland region, the risk of these constraints binding increases. 

We have identified some low cost options to relieve these constraints in the short-term which include:

  • Upgrading the existing Tokaanu CB128 intertrip scheme
  • Installing a 24 ohm series reactor on the Bunnythorpe–Mataroa circuit (at Mataroa)
  • Installing a new special protection scheme on the Bunnythorpe–Mataroa circuit.

Tokaanu CB128 intertrip scheme

The Tokaanu CB128 intertrip scheme upgrade involves modifying existing protection systems to allow the full ratings of the Tokaanu–Whakamaru circuits to be used, including the 15 minute off-load ratings used in our operational tools.  We currently forecast commissioning the upgraded scheme in December 2017, subject to circuit outage availability on the Tokaanu–Whakamaru circuits.

Central North Island 110 kV constraints

The investment option with the highest net benefit to resolve the 110 kV Bunnythorpe–Mataroa circuit constraint is to install a 24 ohm series reactor and a special protection scheme on the Bunnythorpe–Mataroa circuit.

The reactor restricts power flow on the low capacity Bunnythorpe–Mataroa circuit preventing it from overloading pre-contingency.  This also has the benefit of reducing transmission losses as more power flows through the220 kV network.

In high north flow scenarios (due to high Lower North Island generation and/or high HVDC north transfers) or during outages on 220 kV lower North Island circuits, the Bunnythorpe–Mataroa circuit may still overload post-contingency even with the series reactor installed.  We are installing a special protection scheme to open the Mataroa–Ohakune circuit at the Mataroa end if an overload is detected on the Bunnythorpe–Mataroa circuit.  This scheme in conjunction with the series reactor will ensure the Bunnythorpe–Mataroa circuit does not constrain transmission capacity from Bunnythorpe to Whakamaru.

We are in the early design phases for the Bunnythorpe–Mataroa series reactor and special protection scheme installation.  The current expectation is to commission both installations by the third quarter of 2018 (calendar year).  We will provide updates on the expected commissioning date as the project progresses.