Abstract
Geomagnetically Induced Currents (GICs) manifest in power systems as the
result of disturbances on the Sun’s surface, specifically coronal mass ejections, which travel to Earth and distort the geomagnetic field. The changing
geomagnetic field on the Earth’s surface induces currents in long transmission lines (i.e., GIC), passing through earthed transformers. The effect of
GICs upon power transformers can be significant due to the resulting generation of even orders of the fundamental harmonic of Alternating Current
(AC) in a phenomenon known as half cycle saturation. This process can
lead to the damage and failure of transformers due to overheating, as well
as operational problems caused by the harmonics themselves. Together this
makes study of even order harmonics during geomagnetic disturbances very
relevant for energy providers across the world. Our research is intended to
help Transpower NZ Ltd further understand the impact of GIC on the New
Zealand power network, hopefully leading to mitigation efforts.
In this study we make use of Transpower’s Even Total Harmonic Distortion
(ETHD) nationwide data records spanning 2013 to the present day, made
at 139 substations across New Zealand. The ETHD data is examined, in
combination with Transpower’s GIC records as well as local geomagnetic
field data from INTERMAGNET. ETHD indicates how much of a signal
is made up of even order harmonics, for which GIC is one of the only
production routes, and so fluctuations in ETHD are very often correlated
with GIC, which are in turn well correlated with the times of geomagnetic
field perturbations.
During this study we undertook significant processing and analysis of Transpower’s ETHD data set in order to achieve this research. This has led to
improved understanding of ETHD production, and produced a data set
which can be built on in the future as new geomagnetic disturbances take
place (especially when the number of events is limited).
Our work demonstrates that, in New Zealand, earthed single-phase transformers are the primary source of ETHD and that three-phase transformers
generate much smaller levels of ETHD, often none. We also identify five key
substations of interest, those being Halfway Bush (Otago region), New Plymouth (Taranaki region), Islington (Canterbury region), Robertson Street
(West Coast region), and Henderson (Auckland region). Each of these have
been the primary generators of ETHD in their respective regions, although
Halfway Bush is generating significantly reduced harmonics since late 2017
with the removal of an earthed single-phase transformer and New Plymouth
was decommissioned in late 2019. Another result of this work is that we
observe repeating patterns of ETHD amplitude propagation between substations in regions around each of the five substations of interest. The
propagation patterns observed are approximately linear decay of ETHD
along transmission lines, which are consistent over time for each of the
individual regions, although the pattern varies between regions.
We have good understanding of ETHD generation during geomagnetic disturbances for four of the five regions, the exception being the Robertson
Street substation on the South Island’s West Coast. All the information
we can find indicates that there is no single-phase transformer in that substation. The fall-off in ETHD amplitude with propagation distance is also
faster from Robertson Street than for other locations. We suspect, but at
this time cannot prove, that the ETHD generation at this location is due to
a network resonance, with the ETHD source being a different South Island
transformer. Modelling is currently underway with the help of researchers
at the University of Canterbury in order to assess this.
A final key result of this thesis is the confirmation of GIC hot-spot modelling undertaken in MacManus et al. (2022). In that paper, New Zealand
substations are ranked in terms of GIC production during simulated extreme storm events, and our observations of ETHD during less extreme
storms confirm the validity of this modelling.