SSG
Definition of Core Grid (Revision 5)
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.
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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 problematic. 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 groups of parallel transmission circuits between major load and generation centres and the loss of interconnecting transformer banks at major substations. If the loss of these grouped elements causes a major loss of load then the group may be considered to be part of the Core Grid and therefore require an N – 1 level of reliability. Three levels of load loss were considered in this analysis : a) 150 MW b) 300 MW c) 600 MW
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In general, when the load is lost, it cannot be immediately reconnected. The typical restoration procedure involves progressively re-livening the disconnected area from the healthy system. As more of the transmission network is re-livened, load can be gradually restored by the distribution utilities. The restoration process requires communication and co-ordination between transmission, generation, and distribution operators. Table 1 shows some typical expected durations required to completely restore lost load and the consequent cost of the lost load based on VoLL.
Table 1. Amount of load lost
Amount and Cost of Lost Load Duration for complete restoration 40 - 80 min 60 - 120 min 90 - 150 min Cost of load lost @ $20,000/MWh 1 M$1 – 2 M$3 – 6 M$9 – 15
150 MW 300 MW 600 MW
Note 1. It is assumed that the load is gradually restored over the duration. The cost of load lost is based on the average duration which is half of the duration for complete restoration.
For the purpose of this analysis the 2005 network was considered but with loads corresponding to the median forecast for the winter peak of 2015 . The loads for 10 years ahead were chosen because the Core Grid is to be used for planning purposes. The worst case generation scenario from the SOO was considered when calculating the likely loss of load. Generation was assumed to be capable of 100% output except for wind generation and hydro generation with little storage which was assumed to run at about 30% output . The amount of lost load was calculated based on circuits tripping on thermal overload, or load shed due to under-frequency. Transient stability was not considered. It should be emphasised that the analysis was of an indicative nature only and that some judgment was used in determining the likely amounts of lost load.
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2.3 North Island Core Grid for 600 / 300 / 150 MW Loss of Load Figure 1 shows a single line diagram for the North Island with the resulting core grid corresponding to differing amounts of lost load :
Elements of North Island 600 MW Core Grid (Red) a) 220 kV circuits Otahuhu – Henderson and Otahuhu – Southdown – Henderson b) 220 kV circuits Huntly – Takanini – Otahuhu and Huntly – Glenbrook – Takanini – Otahuhu c) 220 kV circuits Otahuhu – Whakamaru and Otahuhu – Huntly – Hamilton – Whakamaru d) 220 kV circuits Bunnythorpe – Tokaanu – Whakamaru e) 220 kV circuits Bunnythorpe – Tangiwai – Rangipo – Wairakei – Pohipi – Whakamaru f) 220 kV circuits Haywards – Bunnythorpe and Haywards – Wilton – Linton – Bunnythorpe g) 220/110 kV interconnector transformers at Otahuhu h) 220/110 kV interconnector transformers at Haywards
Additional Elements of North Island 300 MW Core Grid (Green) i) 220 kV circuits Henderson – Huapai and Henderson – Albany – Huapai j) 220 kV circuits Otahuhu – Penrose k) 220 kV circuits Wairakei – Ohakuri – Atiamuri – Whakamaru l) 220 kV circuits Atiamuri – Tarukenga m) 220 kV circuits Bunnythorpe – Brunswick – Stratford n) 110 kV circuits Otahuhu – Mangere o) 220/110 kV interconnector transformers at Tarukenga
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Additional Elements of North Island 150 MW Core Grid (Blue) p) 220 kV circuits Huapai – Marsden and Huapai – Bream Bay – Marsden q) 220 kV circuits Wairakei – Redclyffe and Wairakei – Whirinaki – Redclyffe r) 110 kV circuits Marsden – Maungatapere s) 110 kV circuits Henderson – Hepburn Road t) 110 kV circuits Mangere – Roskill and Otahuhu – Roskill u) 110 kV circuits Wilton – Central Park v) 110 kV circuits Haywards – Takapu Road w) 220/110 kV interconnector transformers at Marsden, Albany, and Henderson x) 220/110 kV interconnector transformers at Penrose y) 220/110 kV interconnector transformers at Hamilton z) 220/110 kV interconnector transformers at Wilton
2.4 South Island Core Grid for 600 / 300 / 150 MW Loss of Load Figure 2 shows a single line diagram for the South Island with the resulting core grid corresponding to differing amounts of lost load :
Elements of South Island 600 MW Core Grid (Red) a) 220 kV circuits Twizel – Tekapo B – Islington b) 220 kV circuits Twizel – Ashburton – Bromley – Islington c) 220 kV circuit Twizel – Islington d) 220 kV circuit Livingstone – Islington e) 220 kV circuits Manapouri – Invercargill , Manapouri – North Makarewa – Invercargill, and Manapouri – North Makarewa
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Additional Elements of South Island 300 MW Core Grid (Green) f) 220 kV circuits Islington – Kikiwa g) 220 kV circuits Benmore – Ohau B – Twizel, Benmore – Twizel ,and Benmore – Ohau C - Twizel h) 220 kV circuits Benmore – Aviemore i) 220 kV circuits Clyde – Cromwell – Twizel j) 220 kV circuits Roxburgh – Clyde k) 220 kV circuits Roxburgh – Naseby – Livingstone l) 220 kV circuits Invercargill – Roxburgh m) 220 kV circuits Invercargill – Tiwai and North Makarewa – Tiwai n) 220/66 kV interconnector transformers at Islington
Additional Elements of South Island 150 MW Core Grid (Blue) o) 220 kV circuits Kikiwa – Stoke p) 220 kV circuits Roxburgh – Three Mile Hill, Three Mile Hill – Halfway Bush, and Three Mile Hill – South Dunedin – Halfway Bush q) 66 kV circuits Islington – Addington r) 220/66 kV interconnector transformers at Bromley
2.5 HVDC Link as Part of the Core Grid It is questionable whether the Benmore – Haywards HVDC Link should be considered to be part of the Core Grid. The South Island generally uses the HVDC Link to export power and therefore generally does not require the Link to meet peak load. Even in a dry year it is expected that the South Island peak load for 2015 can be met for some hours by running only South Island generation with no HVDC import. It is also expected that the North Island peak load for 2015 (minus 300 MW or minus 600 MW) can be met by North Island generation for all SOO scenarios, with zero HVDC injection. This suggests that the HVDC Link should only be part of the Core Grid corresponding to a 150 MW loss of load. On the other hand the loss of the bipole is likely to result in load shedding in excess of 600 MW in the North Island, this would suggest that the HVDC Link should be part of the Core Grid.
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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 NonCore 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.
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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
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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. Conclusion A definition of the Core Grid based on a defined loss of load (150 / 300 / 600 MW) appears to be most consistent with that proposed in the draft rules where the loss of Core Grid elements would cause a cascade failure (and consequent widespread loss of load). It is not clear whether the HVDC Link should be included in the Core Grid . It may be necessary to consider other criteria to determine this.
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