Electricity Asset Management Plan 2019-2029
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Vector Limited://
Warkworth South and Snells Beach battery systems), and Managed Integration, where we enable customer options by integrating and managing on-premise energy storage to defer, or even avoid, investment in traditional network solutions while enabling a much more targeted level of resilience to customers. 4.1.4 OUR EQUIPMENT CAPACITY ASSESSMENT STRATEGY The policy of using standardised equipment on our network minimises long-term costs and keeps a check on stocks of spare parts. The key factor in deciding the standard capacities (20 MVA and 10 MVA) of power transformers is the load density of the area being supplied. While economies of scale suggest the use of large capacity transformers, higher capacity zone substations result in a larger supply catchment area (for the same load density) and longer distribution feeders. Larger supply catchment areas also mean zone substations are further away from grid exit points (GXPs) so need longer subtransmission feeders. Deciding on the optimal economic capacity of standard urban transformers requires cable and transformer costs to be balanced, so as to achieve the lowest overall cost per MVA of network capacity. Our scenario analysis (of different transformer capacities and feeder lengths) considered a range of equipment costs and load densities. This supported by our decision to standardise urban transformer sizes at 20 MVA. Other factors considered include how the transformer will be used on the network to ensure resilience (for example, operating transformers in parallel to provide redundancy) and how this reflects back into the selection of the transformer parameters such as impedance, reactive power and tap changer/voltage control.
For power transformers used in rural zone substations, the voltage performance of the network is another important factor. Analysis of these rural areas indicated that 10 MVA was the optimal transformer capacity.
Apart from our variable capacity power transformers, most of our network equipment is standard. The list includes:
• • • • • • •
Protection and control equipment Zone substation buildings Subtransmission switchgear Distribution transformers Distribution switching equipment
Distribution cables
Poles
Asset capacities are specified at the equipment specification stage, before equipment is procured. It is verified through type-testing (for example, switchgear) or individual test results (for example, transformers). For existing equipment, we rely on equipment procurement records or name-plate parameters. Standard distribution cable sizes are nominated to ensure a relatively small range of cables is held in stock. We apply the same principle for overhead conductors, for which standard conductor sizes are nominated to ensure only a small range of stock need be held. Cable ratings are calculated using the CymCap cable-rating software. These are based on field-tested ground thermal conductivity results and standard cable installation practices. All new subtransmission cables are modelled, including their trench profiles, to ensure the target network rating can be achieved prior to procurement. Circuit ratings are the lowest of the individual asset ratings (e.g. cables, switchgear, transformers, etc.) in the circuit. Forecast demand is compared to the circuit ratings to confirm all equipment operates within its electrical, mechanical and thermal ratings, adjusted for seasonal variances and the impact of climate changes such as extended long periods of high temperatures, high winds and low rainfall. Where this is not the case equipment is uprated, or replaced. We are forecasting significant uncertainty around the impact of climate change on this process. We have put the necessary measures in place to mitigate what we have been observing to date but will review asset ratings and associated work practices on a continuous basis to manage climate change. 4.1.5 OUR LOAD DEMAND STRATEGY We will continue to use our load control system to manage peak load by remotely managing residential hot water. We intend to ensure existing load-control systems are maintained until retired at end-of-life and will continue to roll out our standard load-control systems in new developments. In the Northern network we will deploy a new radio-controlled hot water system.
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