validating modelled transformer-level gic flow in new

Validating modelled transformer-level GIC flow in New Zealand’s South Island with extensive observations

3 year MBIE funded project Solar Tsunamis: Mitigating Emerging Risks to New Zealand's Electrical Network

Photo: Benmore HVDC main AC side transformer

Tim Divett1, Craig J Rodger1, Daniel Mac Manus1, Malcolm Ingham2, Michael Dalzell3, Ciaran Beggan4, Gemma Richardson4, Ellen Clarke4, Yuki Obana5, and Alan W P Thomson4

1. Department of Physics, University of Otago, Dunedin, NZ

2. School of Chemical and Physical Sciences, Victoria University of Wellington, NZ

3. Transpower New Zealand Limited

4. British Geological Survey, Edinburgh, United Kingdom

5. Osaka Electro-Communication University, Osaka, Japan

A large geomagnetic storm started on 6 November 2001 at ~2:53pm LT (1:53am UT). At this time HWB T4 (Dunedin) tripped, as did systems at ISL (Christchurch). Alarms occurred at multiple locations across the South Island, including OHA, OHB, and CYD.

The transformer at Dunedin / Halfway Bush (HWB T4) suffered a major internal flashover. A subsequent internal inspection found the transformer was beyond repair - it was subsequently written off (~$2 million value in 2016 NZD).

Effects of GIC on power systems - NZ

HWB T4 (written off)

ISL SVC (tripped)

= Alarms

Extensive GIC observationsTranspower NZ Ltd measures and archives transformer neutral currents at - 35 transformers in 2001- 64 transformers in 2015In many substations currents are measured in several transformers

Mac Manus et al., Space Weather, 10.1002/2017SW001635

Rodger et al, Space Weather, 10.1002/2017SW001691

Method: Modelling in New Zealand’s South Island


Divett et al., Space Weather, 10.1002/2017SW001697

Observations St Patrick’s Day Storm 2015

Representative storm

We had 41 transformers monitored for GIC – 27 with reliable measurements

Observed ~50A at HWBT4 at sudden commencement

Observed several peaks up to 20A for ~30 min for next 14 hours

+ EYR

. HWB

1st attempt at Validation

Try what Beggan et al. did for the UK

- input time series of B field

- linear fit to geomagnetic latitude (not SECS b/c not enough east-west points)

- calculate E field for T = 10 min

- calculate timeseries of GIC

- compare with 10 min mean of observations as a low pass filter of observations

- looks more like B than GIC which looks more like dB/dt

2nd attempt at ValidationTry what Bailey et al. (2018) did for Austria

- Time series of dB/dt field

- linear fit of dB/dt to geomagnetic latitude

- calculate E for T = 10 min

- compare with 10 min mean of observations

- multiply modelled GIC by 500 to get on same scale

- sudden commencement about right

- doesn’t reproduce any of the later GIC peaks at all

3rd attempt at Validation - do it in the frequency domain

Time domain Frequency domain

Single pointB observations

Spatially varying B field

Spatially varying E field

GIC

FFT

Linear fitamplitude and phase

Thin sheet model

Network modelFilter

& IFFT

Validation in frequency domainMagnetic field 27/3/2015

+ EYR+ TEW

+ MDM

+ MCQ

Results: Efield for period of 23.6 min

Validation: GIC in frequency domain

Validation: GIC in frequency and time domain

Validation: GIC converted to the time domain

Sudden commencement not as high as raw but similar to filtered observations

Later peaks match timing of filtered observations

Amplitude of later peaks not always perfect but not too bad

Validation: GIC in frequency domain19/27 ‘Good’ matches (70%)

8/27 ‘Bad’ matches

Exclude transformers where <1A measured

18/21 ‘Good’ matches (86%)3/21 ‘Bad’ matches HWBT4L, ISLT6H, OHAT7H

Comparing spatial variation with 2001 storm events

SDN

TWI

KIK

ISL

HWB

}

}}

HWB T4 (written off)

ISL SVC (tripped)

= Alarms

+KIK

+TWI

+ SDN

+TKB+OHBOHA +

} OHBOHA

TKB

single period = 26.3 min

Conclusions and Future Work

Adapt BGS’s Thin Sheet and Network Model’s to NZ

Build conductance model Modify network model to calculate

transformer-level GIC Validate model with Transpower’s

GIC data in frequency domain Validate model in time domain

- Range of frequencies we can simulate is limited at high and low T- Transformer-level detail is very important - KIK may be more affected by GIC than realised in the past

- The transformers with by far the highest observed GIC are modelled worst HWBT4 and ISLT6 modelled GIC is lower than measured

- Better understanding of spatial variation of B field would improve modelling

- More detail of conductance, particularly around Otago, might improve modelling- Seems that the thin sheet model should be used in the frequency domain

validating modelled transformer-level gic flow in new

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