HVDC 2020 Outage FAQs

We have listed below common questions we have received from industry along with our responses. We will continue to update this section and share with participants any updates.


Can you please provide further clarification of the HVDC limits during the HVDC 2020 outages?

System Operator Response, 8 August 2019:

Please refer to the following document on limits:

HVDC Transfer Limits [ pdf 541.25 KB ]


Can you please provide descriptions for the following electrode outages in POCP?

System Operator Response, 8 August 2019:

  • HAY_THW_1A Outage Block Description: Haywards Te Hikowhenua Earth electrode 1A..
  • HAY_THW_1B Outage Block Description: Haywards Te Hikowhenua Earth electrode 1B.
  • BEN_BGR       Outage Block Description: Benmore Earth Electrode Circuit 1


Where can I find more information on the New Zealand Generation Balance assessment?

System Operator Response, 8 August 2019:

The System Operator produces a monthly report on the forecast generation balance. Our reports can be found on the NZGB website.

Currently our reports contain a section specifically on the HVDC 2020 outages and provide commentary on the current margins.


How does recent announced gas outages affect generation balance margins?

System Operator Response, 8 August 2019:

Currently our NZGB reports contain a section specifically on the HVDC 2020 outages and provide commentary on the calculated margins.


How will lake levels affect the generation balance margins?

System Operator Response, 12 December 2019:

As at mid December we are not forecasting security issues due to energy in normal operating conditions.


What HVDC risks and limitations are included in the NZ generation balance calculations?

System Operator Response, 8 August 2019:

Our calculations assume that the HVDC transfer is to the full capacity of each pole available. NZGB then calculates the reserves needed to cover the loss of the largest risk based on this assumption.  Our user guide available in NZGB provides more detail on our HVDC assumptions in sections 2.8 and 2.9, pages 24 onwards. 


What happens if P2 system testing finishes early. Will the start of the P3 outage be brought forward?

Grid Owner Response, 12 December 2019:

If testing is completed early the Grid Owner will return Pole 2 to service. Early starts to subsequent work are very unlikely as this would require re-planning and making resources available. This is difficult given the size of teams involved.


What happens if HVDC transfer North does not reach 400 MW at the time of Pole 2 testing?

Grid Owner Response, 8 August 2019:

Work to finalise the test programme with the System Operator is currently in progress, and as discussed we expect some tests may require HVDC transfers up to 400 MW. This requirement although desirable may not be essential. This work is ongoing and an update will be provided at the next industry briefing (30th October).


GO Update

Will future HVDC outages continue to occur in Jan/Feb period?

Grid Owner Response, 6 November 2019:

Yes, they are in POCP up to 2023.


GO Update

When will reconductoring of the deferred span (58A – 59A) take place?

What is the length of time required for this work? Will extra outages be required?

Grid Owner Response, 6 November 2019:

Planning for the deferred sections still needs to be completed.

It is intended the reconductoring will take place during existing planned outages in 2021,22 or 23. At this stage we do not expect any additional HVDC outages will be needed.


GO Update

Is the VBE work weather dependent?

Grid Owner Response, 6 November 2019:

All VBE work is inside-work so is not weather dependent.



When should participants put outages into POCP – when they are considering changes or until outages are confirmed?

System Operator Response, 6 November 2019:

We encourage participants to put outages in POCP as soon as possible. POCP allows you to mark outages as tentative or confirmed.



The NZGB scenarios do not show dips in margins during the bipole outages – why is that?

System Operator Response, 6 November 2019:

The bipole outages are at the weekends when we forecast significantly lower peak demands than weekdays.



What assumptions does NZGB consider around reserves?

System Operator Response, 6 November 2019:

Please refer to Q&A #6 on this page.



What is the basis of the assumption for reduced gas during the Ahuroa outage and have you considered the ‘draw down rate’ for Ahuroa?

System Operator Response, 6 November 2019:

NZGB scenarios for the period of the Ahuroa outage include:

  • 540 MW reduced gas risk if we face an unplanned Pohokura outage during the planned inspections;
  • 200 MW on top for the Ahuroa outage.

Both assumptions are based on discussions with participants and generator behaviour during the 2018 HVDC outages when Ahuroa storage was effectively at zero.


Security of Supply

Why is it more likely the HVDC outage will limit North transfer?

System Operator Response, 6 November 2019:

This is based on the time of year and the typical storage inflows in the South Island.


VBE Testing

During Pole 2 testing, will the HVDC provide self-cover?

System Operator Response, 6 November 2019:

During the test period the entire DC transfer will be considered as a single CE risk.

Pole 3 will cover Pole 2 during testing, so there is no reduction in Bi-pole power if Pole 2 trips.

Pole 2 will only be in service from time to time and under test conditions therefore it will not be used as reserve to cover the loss of Pole 3. During the testing the Bi-pole transfer will be limited to the capacity of Pole 3.


VBE Testing

Clarification of HVDC transfer level required for testing.

Grid Owner Response, 6 November 2019:

The test plan (set A) can be undertaken using HVDC transfer up to 780 MW. However we don’t expect the HVDC level to be more than 400 MW North based on North Island reserve provision (slide 21).

For test set A, tests can be completed with HVDC north transfer levels anywhere between 200 MW and 780 MW.


HVDC transfer levels

General questions seeking clarification on the HVDC limits during the briefing.

System Operator Response, 6 November 2019:

HVDC Transfer Limits during outage

Refer HVDC Transfer Limits under Q&A #1 on this page


NZGB assumes a maximum monopole transfer for Pole 2 and 3 of 500 MW and 700 MW, respectively. Note NZGB uses nominal ratings therefore for a Pole 2 outage this differs from the HVDC transfer limits in the link above.

During an electrode outage concurrent with monopole, a maximum transfer limit of 406 MW applies.

Security of Supply

The assumptions are 500 MW North and 270 MW South. 500 MW is the maximum north we believe is achievable assuming a dry North Island and therefore high levels of NI reserve offered.

Electricity Risk Curve assumptions are published on the Security of Supply webpage.

Grid Owner Response, 6 November 2019:

VBE System Testing

Two sets of test plans are proposed. 

  • Test Set A can be used with the HVDC transfer between 200 MW to 780 MW North
  • Test Set B can be used between 100 MW to 200 MW South.


Interisland price separation

Questions about interisland price separation during the monopole and bipole outages.

System Operator response, 10 Jan 2020:

Information explaining the price separation scenarios can be found in the Q and A section of the Market Insights page, here.


Pole 2 VBE Testing

What frequency deviations are expected/possible during Test 3 (Pole 3 Multiple Commutation Failure test between 12:00-13:00) on 22 February 2020.

System Operator response, 19 Feb 2020:

The range of expected disturbance depends on the bipole transfer at the time, the scale of the Pole 2 commutation failure (this is under test), and the Pole 2 recovery performance (also under test)

Possible ranges would be :

MW transfer

P2 interruption time (s)

P2 recovery time (s)

Hz deviation


0.1 - 0.2

0.15 - 0.2

0.075 - 0.15


0.1 - 0.2

0.15 - 0.2

0.1 - 0.2


0.1 - 0.2

0.15 - 0.2

0.15 - 0.3


0.1 - 0.2

0.15 - 0.2

0.175 - 0.35


0.1 - 0.2

0.15 - 0.2

0.2 - 0.4

For example, at a transfer of 150 MW, there may an interruption of some or all power transmission by the HVDC of between 100 and 200ms.  The commutation failure recovery and power pick up can take up to 150 to 200ms.  Removal of the 150 MW infeed for this period will result in a transient dip in frequency of around 0.075 - 0.15 Hz.

There is no permanent loss of power expected so frequency will stop falling during the Pole 2 recovery time and recover to the normal band within 0.5 seconds (from the start of the test).