Tim Divett

Optimization of multiple turbine arrays in a channel with tidally reversing flow by numerical modelling with adaptive mesh

At tidal energy sites, large arrays of hundreds of turbines will be required to generate economically significant amounts of energy. Owing to wake effects within the array, the placement of turbines within will be vital to capturing the maximum energy from the resource. This study presents preliminary results using Gerris, an adaptive mesh flow solver, to investigate the flow through four different arrays of 15 turbines each. The goal is to optimize the position of turbines within an array in an idealized channel. The turbines are represented as areas of increased bottom friction in an adaptive mesh model so that the flow and power capture in tidally reversing flow through large arrays can be studied. The effect of oscillating tides is studied, with interesting dynamics generated as the tidal current reverses direction, forcing turbulent flow through the array. The energy removed from the flow by each of the four arrays is compared over a tidal cycle. A staggered array is found to extract 54 per cent more energy than a non-staggered array. Furthermore, an array positioned to one side of the channel is found to remove a similar amount of energy compared with an array in the centre of the channel.

Long-term geomagnetically induced current observations in New Zealand: Earth return corrections and geomagnetic field driver

Transpower New Zealand Limited has measured DC currents in transformer neutrals in the New Zealand electrical network at multiple South Island locations. Near-continuous archived DC current data exist since 2001, starting with 12 different substations and expanding from 2009 to include 17 substations. From 2001 to 2015 up to 58 individual transformers were simultaneously monitored. Primarily, the measurements were intended to monitor the impact of the high-voltage DC system linking the North and South Islands when it is operating in “Earth return” mode. However, after correcting for Earth return operation, as described here, the New Zealand measurements provide an unusually long and spatially detailed set of geomagnetically induced current (GIC) measurements. We examine the peak GIC magnitudes observed from these observations during two large geomagnetic storms on 6 November 2001 and 2 October 2013. Currents of ~30–50 A are observed, depending on the measurement location. There are large spatial variations in the GIC observations over comparatively small distances, which likely depend upon network layout and ground conductivity. We then go on to examine the GIC in transformers throughout the South Island during more than 151 h of geomagnetic storm conditions. We compare the GIC to the various magnitude and rate of change components of the magnetic field. Our results show that there is a strong correlation between the magnitude of the GIC and the rate of change of the horizontal magnetic field (H′). This correlation is particularly clear for transformers that show large GIC current during magnetic storms.

Modeling Geoelectric Fields and Geomagnetically Induced Currents Around New Zealand to Explore GIC in the South Island's Electrical Transmission Network

Transformers in New Zealands South Island electrical transmission network have been impacted by geomagnetically induced currents (GIC) during geomagnetic storms. We explore the impact of GIC on this network by developing a thin-sheet conductance (TSC) model for the region, a geo-electric field model, and a GIC network model. (The TSC is comprised of a thin-sheet conductance map with underlying layered resistivity structure) Using modeling approaches that have been successfully used in the United Kingdom and Ireland, we applied a thin-sheet model to calculate the electric field as a function of magnetic field and ground conductance. We developed a TSC model based on magnetotelluric surveys, geology, and bathymetry, modified to account for offshore sediments. Using this representation, the thin-sheet model gave good agreement with measured impedance vectors. Driven by a spatially uniform magnetic field variation, the thin-sheet model results in electric fields dominated by the ocean-land boundary with effects due to the deep ocean and steep terrain. There is a strong tendency for the electric field to align northwest-southeast, irrespective of the direction of the magnetic field. Applying this electric field to a GIC network model, we show that modelled GIC are dominated by northwest-southeast transmission lines, rather than east-west lines usually assumed to dominate.

Assessment of GIC Based On Transfer Function Analysis: GIC Risk

Transfer functions are calculated for periods between 2 and 1000 minutes between geomagnetically induced currents (GIC) measured at three transformers in the South Island of New Zealand and variations in the horizontal components of the geomagnetic field measured at the Eyrewell Observatory near Christchurch. Using an inverse Fourier Transform, the transfer functions allow the GIC expected in these transformers to be estimated for any variation of the inducing magnetic field. Comparison of the predicted GIC with measured GIC for individual geomagnetic storms shows remarkable agreement, although the lack of high frequency measurements of GIC and the need for interpolation of the measurements leads to a degree of underestimation of the peak GIC magnitude. An approximate correction for this is suggested. Calculation of the GIC for a magnetic storm in November 2001 which led to the failure of a transformer in Dunedin suggests that peak GIC were as large as about 80 A. Use of spectral scaling to estimate the likely GIC associated with a geomagnetic storm of the magnitude of the 1859 Carrington Event indicate that GIC of at least 10 times this magnitude may occur at some locations. Although the impact of changes to the transmission network on calculated transfer functions remains to be explored, it is suggested that the use of this technique may provide a useful check on estimates of GIC produced by other methods such as thin-sheet modelling. Plain Language Summary Rapid changes in Earth’s magnetic field, such as occur during a magnetic storm, induce electric currents in the ground. These currents, known as geomagnetically induced currents (GIC), are able to enter a power transmission network through the ground connection of a substation transformer. Not only can such currents cause damage to transformers, but in extreme situations they may cause failure of the entire power transmission network. We relate measurements of GIC in the New Zealand power transmission network to variations in the magnetic field at a local magnetic observatory. This allows us to construct mathematical relationships between GIC and magnetic field variations which enable us to predict the magnitude of GIC that might occur in the event of a magnetic storm such as the so-called Carrington Event of 1859 – the largest such storm ever recorded. It is found that GIC of almost 1000 A might occur.