Tim Divett | University of Otago (original) (raw)

Papers by Tim Divett

43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 2021

The optimum global blockage for maximum power per turbine decreases as rows are added to the arra... more The optimum global blockage for maximum power per turbine decreases as rows are added to the array. .. . 3.3 Tuned total power in uniform rows .

Renewable Energy, 2016

Large arrays of tidal turbines are critical to realise the potential of tidal current power. This... more Large arrays of tidal turbines are critical to realise the potential of tidal current power. This study is a systematic exploration of large tidal array optimisation in channels with numerically modelled array layouts in 2-D. Crucially, flow along channels is driven by head loss leading to significantly more realistic results than previous models which assume constant velocity. The 2-D adaptive mesh approach bridges the gap between large- and small-scale array models. Hundreds of layouts and turbine tunings have been simulated using LES of turbulent flow in tidally reversing currents to explore channel-scale optimisation and tuning of large arrays. Simulations show that total power capture increases as rows are added to the array although there are diminishing returns on additional turbines. Each turbine in 1 (7), optimally blocked row in a small channel captures 2.5× (0.5×) the power of an isolated turbine. There is an optimum blockage for maximum power per turbine which decreases linearly from 1.0 as the number of rows increases. As array size increases individual turbine wakes become less important than stepped head loss across each row. Free-stream velocity reduces linearly with total power capture, with the gradient increasing with channel size.

Renewable and Sustainable Energy Reviews, 2015

ABSTRACT Much of the global tidal current energy resource lies in the accelerated flows along nar... more ABSTRACT Much of the global tidal current energy resource lies in the accelerated flows along narrow channels. These channels have the potential to produce 10s -1000s of MW of electricity. However, realizing 100 MW of a channel’s potential is much more complex than installing 100 one MW turbines because large scale power extraction reduces tidal currents throughout the channel, changing the resource. This synthesis and review gives an overview of the issues and compromises in designing the layout of the large tidal turbine arrays required to realize this potential. The paper focuses on macro- and micro-design of arrays. Macro-design relates to the total number of turbines and their gross arrangement into rows, while micro-design adjusts the relative positions of the turbines within a grid and the spacing between rows. Interdependent macro-design compromises balance the total number of turbines, array power output, the power output of each turbine, the loads turbines experience, turbine construction costs, maintaining navigability along the channel and any environmental impacts due to flow reduction. A strong emphasis is placed on providing physical insights about how channel-scale dynamics” and the “duct-effect” impact on the compromises in array design. This work is relevant to the design of any “large” array which blocks more than 2%-5% of a channel’s cross-section, be it 2 turbines in a small channel or 100 turbines in a large channel.

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2013

At tidal energy sites, large arrays of hundreds of turbines will be required to generate economic... more 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 e...

see.ed.ac.uk

Abstract—At tidal energy sites large arrays of hundreds of turbines will be required to generate ... more Abstract—At tidal energy sites large arrays of hundreds of turbines will be required to generate economically significant amounts of energy. Due to wake effects within the array, the placement of turbines within will be vital to capturing the maximum energy from the ...

Continental Shelf Research, 2012

Abstract Successful extraction of tidal stream energy will require a good understanding of flow a... more Abstract Successful extraction of tidal stream energy will require a good understanding of flow at a range of scales, including those relating to average energy, variability in energy supply and fatigue. Current and turbulence measurements from the Karori Rip area of Cook Strait, the prime focal region of open-water tidal stream electricity generation in New Zealand, are described. A key issue is that a significant portion of the energy is contained in waters deeper than normally considered for energy extraction. Here we compare shallow and deep sites. Velocity data were derived from acoustic Doppler current profiler moorings, as well as spatial surveys and show flow magnitudes reaching 3.4 m s −1 in the shallow regions. The maximum speeds in both shallow and deep sites were typically located in the upper part of the measured water column although moored acoustic Doppler current profiler (ADCP) observations showed some complexity in this regard. Benthic boundary-layers were resolved in the bottom ∼20 m of the water column. Measured turbulent kinetic energy dissipation rates e exceeded 10 −5 m 2 s −3 and estimated maximum e is a factor of 10 greater. This was not distributed evenly through the water column, with stratification and velocity shear clearly persisting, especially around the turn of the tide. The implications for tidal stream energy are that (i) there is sufficient energy resource in the region for a moderate sized array of turbines, (ii) the vertical variability in the flow speed suggests turbines that can operate near the surface would be more effective at accessing the resource, (iii) stratification may persist and influence the scales of turbulence and (iv) wave–current interaction effects will influence any near-surface structure as well as vessel operations.

Junctures-the Journal for Thematic Dialogue, 2018

Swirling structures in calmly flowing water inspire a deep, primal sense of peace and well-being.... more Swirling structures in calmly flowing water inspire a deep, primal sense of peace and well-being. At the same time, images of Poe’s maelstrom in turbulent oceans inspire a sense of terror.1 Throughout the duration of my PhD, modelling the flow through ocean channels full of tidal turbines, I experienced both of those feelings. The mathematical beauty in my work is involved in the equations that I use to describe the ocean flowing through a tidal channel full of turbines. A von Karman vortex street is the repeating pattern in parallel rows of swirling eddies that form in the wake of an obstruction in flowing fluid. The beauty and terror that eddies inspire in humanity is mirrored by the blessing and curse that these cause for engineers designing tidal turbines. While the fast-flowing water provides the power to drive the turbine, the turbulent vortices in the wake of a turbine put stress on downstream turbines by bending and twisting the blade as a vortex moves past the turbine. Und...

Tidal stream electricity generation is embryonic with a few test deployments of fixed orientation... more Tidal stream electricity generation is embryonic with a few test deployments of fixed orientation devices which are simpler and potentially more reliable than other turbines which are free to rotate passively into the direction of instantaneous maximum current. These fixed orientation devices ideally suit bi-directional flow. However, tidal flow in large tidal straits, where large energy resources are focused, is usually a complex mix of a mean component and several tidal ellipse components. For example eddies formed behind headlands on each tidal cycle can result in higher harmonic tidal constituents around a headland in a tidal strait. This theoretical study investigates the potential of fixed orientation turbines to capture the energy from flows that deviate from strictly bi-directional flow by calculating the maximum energy that can be captured by a fixed orientation turbine from synthetic flows composed of a mean component and tidal ellipse components. Fixed orientation tidal s...

Transformers in Transpower New Zealand Ltd's electrical grid have been impacted by geomag... more Transformers in Transpower New Zealand Ltd's electrical grid have been impacted by geomagnetically induced currents (GIC) during geomagnetic storms, for example in November 2001. In this study we have developed an initial model of the South Island's power grid to advance understanding of the impact of GIC on New Zealand's (NZ) grid. NZ's latitude and island setting mean that modelling approaches successfully used in the UK in the past can be used. Vasseur and Weidelt's thin sheet model is applied to model the electric field as a function of magnetic field and conductance. However the 4 km deep ocean near NZ's coast compared to the UK's relatively shallow continental shelf waters restricts the range of frequency and spatial grid that can be used due to assumptions in the thin sheet model. Some early consequences of these restrictions will be discussed. Lines carrying 220kV, 110kV and 66kV make up NZ's electrical transmission grid with multiple earthing nodes at each substation. Transpower have measured DC earth currents at 17 nodes in NZ's South Island grid for 15 years, including observations at multiple transformers for some substations. Different transformers at the same substation can experience quite different GIC during space weather events. Therefore we have initially modelled each transformer in some substations separately to compare directly with measured currents. Ultimately this study aims to develop a validated modelling tool that will be used to strengthen NZ's grid against the risks of space weather. Further, mitigation tactics which could be used to reduce the threat to the electrical grid will be evaluated. In particular we will focus at the transformer level where the risk lies, and not at the substation level as has been commonly done to date. As we will validate our model against the extensive Transpower observations, this will be a valuable confirmation of the approaches used by the wider international community.

"Tidal stream electricity generation is embryonic with a few test deployment... more "Tidal stream electricity generation is embryonic with a few test deployments of fixed orientation devices which are simpler and potentially more reliable than other turbines which are free to rotate passively into the direction of instantaneous maximum current. These fixed orientation devices ideally suit bi-directional flow. However, tidal flow in large tidal straits, where large energy resources are focused, is usually a complex mix of a mean component and several tidal ellipse components. For example eddies formed behind headlands on each tidal cycle can result in higher harmonic tidal constituents around a headland in a tidal strait. This theoretical study investigates the potential of fixed orientation turbines to capture the energy from flows that deviate from strictly bi-directional flow by calculating the maximum energy that can be captured by a fixed orientation turbine from synthetic flows composed of a mean component and tidal ellipse components. Fixed orientation tidal stream turbines miss out on capturing a significant proportion of the total energy that is available to turbines that are free to rotate into the instantaneous maximum current and the angle of orientation into the flow is significant. "

Space Weather, 2018

During space weather events, geomagnetically induced currents (GICs) can be induced in high volta... more During space weather events, geomagnetically induced currents (GICs) can
be induced in high voltage transmission networks, damaging individual transformers
within substations. A common approach to modeling a transmission
network has been to assume that every substation can be represented by a
single resistance to Earth. We have extended that model by building a transformerlevel
network representation of New Zealand’s South Island transmission network.
We represent every transformer winding at each earthed substation
in the network by its known DC resistance. Using this network representation
significantly changes the GIC hazard assessment, compared to assessments
based on the earlier assumption. Further, we have calculated the GIC
flowing through a single phase of every individual transformer winding in the
network. These transformer-level GIC calculations show variation in GICs
between transformers within a substation due to transformer characteristics
and connections. The transformer-level GIC calculations alter the hazard assessment
by up to an order of magnitude in some places. In most cases the
calculated GIC variations match measured variations in GIC flowing through
the same transformers. This comparison with an extensive set of observations
demonstrates the importance of transformer-level GIC calculations in
models used for hazard assessment.

Geomagnetically induced current (GIC) observations made in New Zealand over 14 years show inducti... more Geomagnetically induced current (GIC) observations made in New Zealand over 14 years show induction effects associated with a rapidly varying horizontal magnetic field (dBH/dt) during geomagnetic storms. This study analyzes the GIC observations in order to estimate the impact of extreme storms as a hazard to the power system in New Zealand. Analysis is undertaken of GIC in transformer number six in Islington, Christchurch (ISL M6), which had the highest observed currents during the 6 November 2001 storm. Using previously published values of 3,000 nT/min as a representation of an extreme storm with 100 year return period, induced currents of ~455 A were estimated for Islington (with the 95% confidence interval range being ~155–605 A). For 200 year return periods using 5,000 nT/min, current estimates reach ~755 A (confidence interval range 155–910 A). GIC measurements from the much shorter data set collected at transformer number 4 in Halfway Bush, Dunedin, (HWB T4), found induced currents to be consistently a factor of 3 higher than at Islington, suggesting equivalent extreme storm effects of ~460–1,815 A (100 year return) and ~460–2,720 A (200 year return). An estimate was undertaken of likely failure levels for single-phase transformers, such as HWB T4 when it failed during the 6 November 2001 geomagnetic storm, identifying that induced currents of ~100 A can put such transformer types at risk of damage. Detailed modeling of the New Zealand power system is therefore required to put this regional analysis into a global context.

43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 2021

The optimum global blockage for maximum power per turbine decreases as rows are added to the arra... more The optimum global blockage for maximum power per turbine decreases as rows are added to the array. .. . 3.3 Tuned total power in uniform rows .

Renewable Energy, 2016

Large arrays of tidal turbines are critical to realise the potential of tidal current power. This... more Large arrays of tidal turbines are critical to realise the potential of tidal current power. This study is a systematic exploration of large tidal array optimisation in channels with numerically modelled array layouts in 2-D. Crucially, flow along channels is driven by head loss leading to significantly more realistic results than previous models which assume constant velocity. The 2-D adaptive mesh approach bridges the gap between large- and small-scale array models. Hundreds of layouts and turbine tunings have been simulated using LES of turbulent flow in tidally reversing currents to explore channel-scale optimisation and tuning of large arrays. Simulations show that total power capture increases as rows are added to the array although there are diminishing returns on additional turbines. Each turbine in 1 (7), optimally blocked row in a small channel captures 2.5× (0.5×) the power of an isolated turbine. There is an optimum blockage for maximum power per turbine which decreases linearly from 1.0 as the number of rows increases. As array size increases individual turbine wakes become less important than stepped head loss across each row. Free-stream velocity reduces linearly with total power capture, with the gradient increasing with channel size.

Renewable and Sustainable Energy Reviews, 2015

ABSTRACT Much of the global tidal current energy resource lies in the accelerated flows along nar... more ABSTRACT Much of the global tidal current energy resource lies in the accelerated flows along narrow channels. These channels have the potential to produce 10s -1000s of MW of electricity. However, realizing 100 MW of a channel’s potential is much more complex than installing 100 one MW turbines because large scale power extraction reduces tidal currents throughout the channel, changing the resource. This synthesis and review gives an overview of the issues and compromises in designing the layout of the large tidal turbine arrays required to realize this potential. The paper focuses on macro- and micro-design of arrays. Macro-design relates to the total number of turbines and their gross arrangement into rows, while micro-design adjusts the relative positions of the turbines within a grid and the spacing between rows. Interdependent macro-design compromises balance the total number of turbines, array power output, the power output of each turbine, the loads turbines experience, turbine construction costs, maintaining navigability along the channel and any environmental impacts due to flow reduction. A strong emphasis is placed on providing physical insights about how channel-scale dynamics” and the “duct-effect” impact on the compromises in array design. This work is relevant to the design of any “large” array which blocks more than 2%-5% of a channel’s cross-section, be it 2 turbines in a small channel or 100 turbines in a large channel.

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2013

At tidal energy sites, large arrays of hundreds of turbines will be required to generate economic... more 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 e...

see.ed.ac.uk

Abstract—At tidal energy sites large arrays of hundreds of turbines will be required to generate ... more Abstract—At tidal energy sites large arrays of hundreds of turbines will be required to generate economically significant amounts of energy. Due to wake effects within the array, the placement of turbines within will be vital to capturing the maximum energy from the ...

Continental Shelf Research, 2012

Abstract Successful extraction of tidal stream energy will require a good understanding of flow a... more Abstract Successful extraction of tidal stream energy will require a good understanding of flow at a range of scales, including those relating to average energy, variability in energy supply and fatigue. Current and turbulence measurements from the Karori Rip area of Cook Strait, the prime focal region of open-water tidal stream electricity generation in New Zealand, are described. A key issue is that a significant portion of the energy is contained in waters deeper than normally considered for energy extraction. Here we compare shallow and deep sites. Velocity data were derived from acoustic Doppler current profiler moorings, as well as spatial surveys and show flow magnitudes reaching 3.4 m s −1 in the shallow regions. The maximum speeds in both shallow and deep sites were typically located in the upper part of the measured water column although moored acoustic Doppler current profiler (ADCP) observations showed some complexity in this regard. Benthic boundary-layers were resolved in the bottom ∼20 m of the water column. Measured turbulent kinetic energy dissipation rates e exceeded 10 −5 m 2 s −3 and estimated maximum e is a factor of 10 greater. This was not distributed evenly through the water column, with stratification and velocity shear clearly persisting, especially around the turn of the tide. The implications for tidal stream energy are that (i) there is sufficient energy resource in the region for a moderate sized array of turbines, (ii) the vertical variability in the flow speed suggests turbines that can operate near the surface would be more effective at accessing the resource, (iii) stratification may persist and influence the scales of turbulence and (iv) wave–current interaction effects will influence any near-surface structure as well as vessel operations.

Junctures-the Journal for Thematic Dialogue, 2018

Swirling structures in calmly flowing water inspire a deep, primal sense of peace and well-being.... more Swirling structures in calmly flowing water inspire a deep, primal sense of peace and well-being. At the same time, images of Poe’s maelstrom in turbulent oceans inspire a sense of terror.1 Throughout the duration of my PhD, modelling the flow through ocean channels full of tidal turbines, I experienced both of those feelings. The mathematical beauty in my work is involved in the equations that I use to describe the ocean flowing through a tidal channel full of turbines. A von Karman vortex street is the repeating pattern in parallel rows of swirling eddies that form in the wake of an obstruction in flowing fluid. The beauty and terror that eddies inspire in humanity is mirrored by the blessing and curse that these cause for engineers designing tidal turbines. While the fast-flowing water provides the power to drive the turbine, the turbulent vortices in the wake of a turbine put stress on downstream turbines by bending and twisting the blade as a vortex moves past the turbine. Und...

Tidal stream electricity generation is embryonic with a few test deployments of fixed orientation... more Tidal stream electricity generation is embryonic with a few test deployments of fixed orientation devices which are simpler and potentially more reliable than other turbines which are free to rotate passively into the direction of instantaneous maximum current. These fixed orientation devices ideally suit bi-directional flow. However, tidal flow in large tidal straits, where large energy resources are focused, is usually a complex mix of a mean component and several tidal ellipse components. For example eddies formed behind headlands on each tidal cycle can result in higher harmonic tidal constituents around a headland in a tidal strait. This theoretical study investigates the potential of fixed orientation turbines to capture the energy from flows that deviate from strictly bi-directional flow by calculating the maximum energy that can be captured by a fixed orientation turbine from synthetic flows composed of a mean component and tidal ellipse components. Fixed orientation tidal s...

Transformers in Transpower New Zealand Ltd's electrical grid have been impacted by geomag... more Transformers in Transpower New Zealand Ltd's electrical grid have been impacted by geomagnetically induced currents (GIC) during geomagnetic storms, for example in November 2001. In this study we have developed an initial model of the South Island's power grid to advance understanding of the impact of GIC on New Zealand's (NZ) grid. NZ's latitude and island setting mean that modelling approaches successfully used in the UK in the past can be used. Vasseur and Weidelt's thin sheet model is applied to model the electric field as a function of magnetic field and conductance. However the 4 km deep ocean near NZ's coast compared to the UK's relatively shallow continental shelf waters restricts the range of frequency and spatial grid that can be used due to assumptions in the thin sheet model. Some early consequences of these restrictions will be discussed. Lines carrying 220kV, 110kV and 66kV make up NZ's electrical transmission grid with multiple earthing nodes at each substation. Transpower have measured DC earth currents at 17 nodes in NZ's South Island grid for 15 years, including observations at multiple transformers for some substations. Different transformers at the same substation can experience quite different GIC during space weather events. Therefore we have initially modelled each transformer in some substations separately to compare directly with measured currents. Ultimately this study aims to develop a validated modelling tool that will be used to strengthen NZ's grid against the risks of space weather. Further, mitigation tactics which could be used to reduce the threat to the electrical grid will be evaluated. In particular we will focus at the transformer level where the risk lies, and not at the substation level as has been commonly done to date. As we will validate our model against the extensive Transpower observations, this will be a valuable confirmation of the approaches used by the wider international community.

"Tidal stream electricity generation is embryonic with a few test deployment... more "Tidal stream electricity generation is embryonic with a few test deployments of fixed orientation devices which are simpler and potentially more reliable than other turbines which are free to rotate passively into the direction of instantaneous maximum current. These fixed orientation devices ideally suit bi-directional flow. However, tidal flow in large tidal straits, where large energy resources are focused, is usually a complex mix of a mean component and several tidal ellipse components. For example eddies formed behind headlands on each tidal cycle can result in higher harmonic tidal constituents around a headland in a tidal strait. This theoretical study investigates the potential of fixed orientation turbines to capture the energy from flows that deviate from strictly bi-directional flow by calculating the maximum energy that can be captured by a fixed orientation turbine from synthetic flows composed of a mean component and tidal ellipse components. Fixed orientation tidal stream turbines miss out on capturing a significant proportion of the total energy that is available to turbines that are free to rotate into the instantaneous maximum current and the angle of orientation into the flow is significant. "

Space Weather, 2018

During space weather events, geomagnetically induced currents (GICs) can be induced in high volta... more During space weather events, geomagnetically induced currents (GICs) can
be induced in high voltage transmission networks, damaging individual transformers
within substations. A common approach to modeling a transmission
network has been to assume that every substation can be represented by a
single resistance to Earth. We have extended that model by building a transformerlevel
network representation of New Zealand’s South Island transmission network.
We represent every transformer winding at each earthed substation
in the network by its known DC resistance. Using this network representation
significantly changes the GIC hazard assessment, compared to assessments
based on the earlier assumption. Further, we have calculated the GIC
flowing through a single phase of every individual transformer winding in the
network. These transformer-level GIC calculations show variation in GICs
between transformers within a substation due to transformer characteristics
and connections. The transformer-level GIC calculations alter the hazard assessment
by up to an order of magnitude in some places. In most cases the
calculated GIC variations match measured variations in GIC flowing through
the same transformers. This comparison with an extensive set of observations
demonstrates the importance of transformer-level GIC calculations in
models used for hazard assessment.

Geomagnetically induced current (GIC) observations made in New Zealand over 14 years show inducti... more Geomagnetically induced current (GIC) observations made in New Zealand over 14 years show induction effects associated with a rapidly varying horizontal magnetic field (dBH/dt) during geomagnetic storms. This study analyzes the GIC observations in order to estimate the impact of extreme storms as a hazard to the power system in New Zealand. Analysis is undertaken of GIC in transformer number six in Islington, Christchurch (ISL M6), which had the highest observed currents during the 6 November 2001 storm. Using previously published values of 3,000 nT/min as a representation of an extreme storm with 100 year return period, induced currents of ~455 A were estimated for Islington (with the 95% confidence interval range being ~155–605 A). For 200 year return periods using 5,000 nT/min, current estimates reach ~755 A (confidence interval range 155–910 A). GIC measurements from the much shorter data set collected at transformer number 4 in Halfway Bush, Dunedin, (HWB T4), found induced currents to be consistently a factor of 3 higher than at Islington, suggesting equivalent extreme storm effects of ~460–1,815 A (100 year return) and ~460–2,720 A (200 year return). An estimate was undertaken of likely failure levels for single-phase transformers, such as HWB T4 when it failed during the 6 November 2001 geomagnetic storm, identifying that induced currents of ~100 A can put such transformer types at risk of damage. Detailed modeling of the New Zealand power system is therefore required to put this regional analysis into a global context.

Transformers in Transpower New Zealand Ltd's South Island electrical grid have occasionally been ... more Transformers in Transpower New Zealand Ltd's South Island electrical grid have occasionally been impacted by near-DC geomagnetically induced currents (GIC) during geomagnetic events, for example November 2001. In this study an initial model of the South Island's grid has therefore been developed to advance understanding of the impact of space weather on New Zealand's grid.

New Zealand's latitude and island setting, mean that modelling approaches successfully applied in the UK in the past can be used. However, deep water (4000 m) near the coast means even stronger spatial gradients of conductance can occur around New Zealand compared to the situation in the UK's shallow continental shelf. This strong gradient poses challenges for the thin-sheet model (of Vasseur and Weidelt) used to model the electric field as a function of magnetic field and conductance.

The NZ electrical transmission grid consists of lines carrying 220kV, 110kV and 66kV with multiple earthing nodes at each transformer substation. The relative importance of the 66kV network is explored in this study, in relation to currents induced in the higher voltage lines. Transpower have measured DC earth currents at 17 substations at selected locations in the South Island grid for up to 15 years, including through multiple transformers at the same substation. Different transformers at the same substation can experience quite different GIC during space weather events. Therefore, in this work, each transformer at each substation is modelled separately to compare directly with the measured currents. The sensitivity of induced current in the grid to the direction of an imposed electric field is also explored.

Our model will eventually be used as an operational and validated tool to explore the risk to the New Zealand grid from geomagnetic storms. Further, mitigation tactics which could be used to reduce the threat to the electrical grid will be evaluated. Ultimately this study aims to develop a modelling tool that will be used to strengthen New Zealand's grid against the risks of space weather. In particular we will focus at the transformer level where the risk lies, and not at the substation level as has been commonly done to date. As we will validate our model against the extensive Transpower observations, this will be a valuable confirmation of the approaches used by the wider international community.

Transformers in Transpower New Zealand Ltd's electrical grid have been impacted by geomagneticall... more Transformers in Transpower New Zealand Ltd's electrical grid have been impacted by
geomagnetically induced currents (GIC) during geomagnetic storms, for example in November
2001. In this study we have developed an initial model of the South Island's power grid to advance
understanding of the impact of GIC on New Zealand's (NZ) grid.
NZ's latitude and island setting mean that modelling approaches successfully used in the UK in the
past can be used. Vasseur and Weidelt's thin sheet model is applied to model the electric field as a
function of magnetic field and conductance. However the 4 km deep ocean near NZ's coast
compared to the UK's relatively shallow continental shelf waters restricts the range of frequency
and spatial grid that can be used due to assumptions in the thin sheet model. Some early
consequences of these restrictions will be discussed.
Lines carrying 220kV, 110kV and 66kV make up NZ's electrical transmission grid with multiple
earthing nodes at each substation. Transpower have measured DC earth currents at 17 nodes in
NZ's South Island grid for 15 years, including observations at multiple transformers for some
substations. Different transformers at the same substation can experience quite different GIC
during space weather events. Therefore we have initially modelled each transformer in some
substations separately to compare directly with measured currents.
Ultimately this study aims to develop a validated modelling tool that will be used to strengthen NZ's
grid against the risks of space weather. Further, mitigation tactics which could be used to reduce
the threat to the electrical grid will be evaluated. In particular we will focus at the transformer level
where the risk lies, and not at the substation level as has been commonly done to date. As we will
validate our model against the extensive Transpower observations, this will be a valuable
confirmation of the approaches used by the wider international community.