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Definition of Core Grid (Revision 3)


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REF: S001-T5

Thursday 12 May 2005

TO: John Gleadow

Senior Adviser Transmission Electricity Commission CC: Victoria Coad

Concept Consulting

FROM: Ranil de Silva

System Studies Group NZ Ltd

Definition of Core Grid (Revision 3)

1. Introduction

The first limb of the Grid Reliability Standards (GRS) requires that the entire grid be designed to achieve a probabilistic economic reliability criterion through the application of the Grid Investment Test. In order to reduce the possible uncertainty of outcomes from applying the probabilistic techniques and the economic criterion, the second limb of the GRS applies a “safety net”

to the main elements of transmission system (the core grid). This safety net is the application of a minimum deterministic ‘N – 1‘ reliability criterion to the Core Grid.

This memo discusses some preliminary alternatives for the definition of a Core Grid in the New Zealand power system with respect to the Grid Reliability Standards.

2. Draft Rules Definition Based on Cascade Failure

The GRS include a requirement that the determination of the Core Grid should have regard to:

“avoiding the failure or removal from service of any asset forming part of the core grid, where the failure or removal from service of that asset may result in cascade failure”.


The application of this approach requires the definition of ‘cascade failure’.

‘Cascade failure’ generally implies the loss of parallel circuits, loss of

generation due to under-frequency, voltage collapse, and widespread loss of load.

2.1 Applying the Cascade Failure Test

The present system has been designed around an ‘N – 1’ criterion for the

“main interconnected transmission system”, therefore the loss of any single circuit will generally not cause a cascade failure or a loss of load. Thus the main interconnected transmission system probably satisfies the N-1 safety net. This makes any test to establish which elements of the transmission system should comprise the Core Grid for the purpose of the GRS


It is clear is that what was intended was a test that established a core grid comprising the transmission links which would need to have N-1 reliability in order to avoid cascade failures. However, applying this to the current

transmission system is not straightforward.

2.2 A Possible Application of the Test

A possible implementation of the concept of cascade failure may be to

consider the effect of the loss of an element (circuit, transformer, generator, or HVDC Pole) followed by the loss of another (typically parallel) element due to increased loading.

A common example of this would be two parallel overhead circuits supplying a load. The loss of one circuit will result in an increase in the load on the

second circuit. This may cause the second circuit to sag excessively, flashover to ground, and trip resulting in the loss of load.

The following steps define a method for implementing this approach : a) Consider the present 2005 network

b) Consider the median winter peak load forecast for 2010 (a 5 years ahead of the present is chosen to deliberately stress the network ).

c) Consider the present 2005 generation . (Only 3 x 33 MVAR compensators at Otahuhu)

d) Run all generation to meet the load

e) Trip each element and check the (typically parallel) elements that have a significantly increased loading

f) Identify the element with the highest per unit loading and also trip that element


Only trip a transformer if it is overloaded beyond 200% (this recognizes that transformers are capable of extreme overload without tripping but possibly some loss of life)

g) Again check elements that have an increased loading

h) Identify the element with the highest per unit overloading and also trip that element

Only trip an overhead transmission circuit if it is overloaded beyond 110%.

Only trip a transformer if it overloaded beyond 200%

i) Repeat from g) until no more elements are overloaded

j) Calculate amount of lost load for each scenario. (To help understand the future load distribution, Appendix 1 lists the forecast load in each region for 2010)

2.2.1 Core Grid for 150 MW Loss of Load

Figures 1A and 1B show the core grid resulting from this approach for a critical lost load of 150 MW (In 2010 3% of North Island or 7% of South Island). The core grid includes :

North Island : HVDC Pole 1&2

Huapai – Marsden 220 kV Huapai – Bream Bay 220 kV Henderson – Huapai 220 kV Albany – Huapai 220 kV Henderson – Albany 220 kV

Albany 220/110 kV interconnector T4 Otahuhu – Henderson 220 kV

Southdown – Henderson 220 kV Otahuhu – Southdown 220 kV

Otahuhu – Penrose 220 kV Circuits 5&6 Otahuhu B Generator

Huntly – Takanini – Otahuhu 220 kV Glenbrook – Takanini – Otahuhu 220 kV


Huntly – Glenbrook 220 kV

Whakamaru – Otahuhu 220 kV Circuit 3 Atiamuri – Tarukenga 220 kV Circuits 1&2 Wairakei – Whirinaki 220 kV

Wairakei – Redclyffe 220 kV Whirinaki – Redclyffe 220 kV

Mangere – Roskill 110 kV Circuits 1&2 Otahuhu – Mangere 110 kV Circuits 1&2 Otahuhu – Roskill 110 kV Circuits 1&2 Haywards – Takapu Road Circuits 1&2

South Island :

Kikiwa – Stoke 220 kV Circuits 1&2 Islington – Kikiwa 220 kV Circuits 1&2 Tekapo B – Islington 220 kV

Bromley – Islington 220 kV Twizel – Opihi – Islington 220 kV Livingstone – Islington 220 kV Ashburton – Bromley 220 kV Twizel – Opihi – Ashburton 220 kV

2.2.2 Core Grid for 800 MW Loss of Load

Figures 2A and 2B show the core grid resulting from this approach for a critical lost load of 800 MW (In 2010 17% of North Island or 35% of South Island). The core grid includes :

North Island :

Otahuhu – Henderson 220 kV Southdown – Henderson 220 kV Otahuhu – Southdown 220 kV


Otahuhu B Generator

Huntly – Takanini – Otahuhu 220 kV Glenbrook – Takanini – Otahuhu 220 kV Huntly – Glenbrook 220 kV

Whakamaru – Otahuhu 220 kV Circuit 3

South Island :

Tekapo B – Islington 220 kV Bromley – Islington 220 kV Twizel – Opihi – Islington 220 kV Livingstone – Islington 220 kV Ashburton – Bromley 220 kV Twizel – Opihi – Ashburton 220 kV

2.3 Cascade Failure Due to Loss of an Inter-regional Link

Another variation of the cascade failure definition is to consider links between geographical regions. These links can be considered to be arterial routes that carry most of the power between regions.

Figures 3A and 3B show the network divided into regions : a) Northland

b) Auckland c) Waikato d) Bay of Plenty e) Central f) Taranaki g) Hawkes Bay h) Bunnythorpe i) Wellington


j) Nelson / Marlborough k) West Coast

l) Christchurch m) Waitaki

n) Clutha o) Dunedin p) Southland q) Fiordland

Each group of circuits linking the regions forms an inter-regional link. Each inter-regional link may be sub-divided into different voltage levels (220 kV, 110 kV , or 66 kV) with the 220 kV links being the most significant.

The loss of some these 220 kV links is expected to cause cascade failure and widespread loss of load, depending on the generation and load pattern at the time. Consequently these links warrant a greater degree of reliability than other parts of the network. The term ‘widespread loss of load’ is somewhat arbitrary , however the loss of more than 50% load in either the North or

South Island may reasonably considered to be widespread and would typically require many hours to reconnect.

Under this definition it is expected that the following inter-regional links would be included in the Core Grid based on common generation and load patterns;

a) 220 kV circuits Waikato – Auckland b) 220 kV circuits Central – Waikato c) 220 kV circuits Waitaki – Christchurch d) 220 kV circuits Clutha – Waitaki e) 220 kV circuits Fiordland - Southland

Note again that the HVDC Link would be non-Core Grid because the loss of the bipole is treated as an extended contingency event which may result in extensive load shedding (up to 40% in the North Island) but still leave the system intact.

Also note that the Taranaki – Waikato, Taranaki – Bunnythorpe, and Central – Bunnythorpe inter-regional links are also likely to be non-Core because these links form a mesh and the loss of any single link is very likely to leave the system intact. Detailed modeling would be required to establish under what conditions these links would become part of the Core Grid.


3. Transpower Definition Based on the Meshed Network

It is convenient to discuss Transpower's proposed definition for the Core Grid as it is expected to be submitted as an alternative during the GRS

consultation process.

Transpower proposes that the Core Grid is based on elements of the market model that form a meshed network with loop flows. Most of the 220 kV, 110 kV, and 66 kV systems form a meshed network that would therefore be included in the Core Grid. The HVDC Link is also proposed to be part of the Core Grid. There are a few parts of the system that are radial networks and would be excluded from the Core Grid. A diagram showing Core and Non- Core parts of the system is included in Transpower's document 'Definition of Core and Non-Core Transmission Grid Assets'.

The rationale behind Transpower's proposal is that the meshed network is shared by all users of the grid because some portion of their power supply flows through the meshed elements. On the other hand the radial networks only supply power to certain loads and are therefore not shared by all users.

Transpower's proposed Core Grid is therefore based on elements of the network that are shared by a large number of users.

It should be noted that there is no obvious link between the shared meshed network and reliability . Just because network elements are shared does not imply that they are necessary to provide a reliable supply. Consequently Transpower's proposal for the Core Grid may be considered to be unjustifiable.

4. Definition Based on the Transmission of Bulk Energy

It is possible to take the view that the parts of the system associated with the transmission of bulk energy should be more reliable than the parts of the system that are associated with the transmission of small amounts of energy.

This leads to a definition of the Core Grid as being parts of the network that are associated with transmitting energy of more than some agreed value over a defined period (say 5 years). Call this agreed energy 'ECORE' (measured in GWh/ 5 years).

Transpower's SCADA/EMS system is likely to be capable of reporting on how much energy is carried by each network element during the 5 years. This information could then be directly used to define which elements form part of the Core Grid.


The information on bulk energy transfer is not yet available to us, however it is expected that a reasonable choice for the value of ECORE would result in the following being included in the Core Grid :

a) The HVDC Link

b) All of the 220 kV network

c) Some of the 110 kV network around Auckland, Bay of Plenty, and Wellington

d) Banks of interconnector transformers in Auckland, Hamilton, Taranaki, Wellington, Christchurch, and Dunedin

It is also expected that the following would be excluded from the Core Grid : a) The Hawkes Bay 110 kV network

b) The Nelson/Marlborough 110 kV and 66 kV network c) The West Coast 110 kV and 66 kV network

d) The Waitaki 110 kV network

e) The Clutha, Dunedin, and Southland 110 kV networks

5. Heuristic Definition Based on Voltage

It may be possible to define the Core Grid by selecting elements that meet some heuristic requirement.

For example the Core Grid could be defined as elements that operate at 220 kV or above, including interconnector transformers. The reasoning behind this being that the higher voltage parts of the system tend to carry more power and are more critical than lower voltage parts of the system.

This definition would result in the following being included in the Core Grid : a) All 220 kV circuits

b) The HVDC Link

c) 220/110 kV and 220/66 kV interconnectors d) 220 kV capacitors

6. Variability of Core Grid with Time


The Core Grid that follows from the application of these definitions may change with time.

The Core Grid based on ‘cascade failure and widespread loss of load’ may change if there is a significant change in the load distribution away from Auckland or Christchurch to other centres.

The Core Grid based on the ‘meshed network’ will change as new circuits are added to the mesh or some parts of the existing mesh become radial.

The Core Grid based on ‘transmission of bulk energy’ may change with changes in generation patterns which would tend to change the bulk power flow through the network.

The Core Grid based on ‘parts of the network operating at 220 kV or above’

will increase in size as more circuits are added at 220 kV or above.


7. Recommendation

A definition of the Core Grid similar to either

Option 2.2 ‘Cascade Failure Due to Loss of a Circuit and a Subsequent Circuit’


Option 2.3 ‘Cascade Failure Due to Loss of an Inter-regional Link’

appears to be most consistent with that proposed in the draft rules where the loss of Core Grid elements would cause a cascade failure.

Therefore it is recommended that one of these definitions or similar is used for the Core Grid.


Appendix 1. Forecast North and South Island Loads for 2010

North Isthmus 840 MW

Auckland 1405 MW

Waikato 567 MW

Bay of Plenty 476 MW

Central 348 MW

Hawkes Bay 301 MW

Taranaki 150 MW

Wellington 658 MW

North Island Total 4745 MW

Nelson / Marlborough 227 MW

West Coast 55 MW

Canterbury 835 MW

South Canterbury 86 MW

Otago / Southland 1074 MW

South Island Total 2277 MW


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