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Evaluating Transformer Heating due to Geomagnetic Disturbances

Presented by:Brian Penny, American Transmission Company

53rd Annual Minnesota Power Systems Conference November 7, 2017

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• Solar Activity and GMD• NERC TPL-007• Transformer GIC Susceptibility• Transformer GIC Effects• GIC Heating Analysis• Summary of Main Points• Questions

Presentation Summary

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Geomagnetic Disturbance (GMD)

• Interaction of the sun and earths magnetic fields

• Solar Activity Cycle

• Measured using Kp - Index

Solar Activity and GMD

The Space Weather Environment © NASA

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KpNOAA Level Description Comments

0 – 4 - Inactive No fluctuations observed on power grid

5 G1 Minor Weak power grid fluctuations can occur

6 G2 Moderate May cause voltage alarms at high latitudes

7 G3 Strong May require voltage corrections and trigger false alarms on some protection devices

8 G4 Severe Possible widespread voltage control problems and some assets tripped by false protective system actions

9 G5 Extreme Widespread voltage control and protective system problems with possible blackout and transformer damage

Kp - Index: GMD Storm Severity

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The Space Weather Environment © NASA

Geomagnetic Induced Current (GIC) Interactions with the Electrical System:

• GMD induced GIC

• Interacts with electrical system (quasi DC)

• Part-cycle core saturation in transformers (DC offset)

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Regional Geoelectric Field Peak Amplitude

• Epeak = E x α x β (volts/km)

o E – Reference geoelectric field amplitude

o α – Scaling factor for local geomagnetic latitude

o β – Scaling factor for local earth conductivity structure

NERC TPL-007

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Benchmark Event (TPL-007-1)

• E = 8 volts/km at reference geomagnetic latitude of 60°

• α = 0.001 x e(0.115 x L) or Table II-1 L is the geomagnetic latitude in degrees

• βB = Table II-2

• Thermal assessment for GIC > 75 amps per phase

NERC TPL-007

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Supplemental Event (TPL-007-2)

• E = 12 volts/km at reference geomagnetic latitude of 60°

• α = 0.001 x e(0.115 x L) or Table II-1 L is the geomagnetic latitude in degrees

• βS = Table II-2

• Thermal assessment for GIC > 85 amps per phase

NERC TPL-007

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Table II-1: α for Benchmark and Supplemental

NERC TPL-007

Geomagnetic Latitude

(Degrees)Scaling Factor

(α )> 60 1.059 0.958 0.857 0.756 0.654 0.550 0.345 0.2

< 40 0.1

Sheet1

Geomagnetic Latitude (Degrees)Scaling Factor (a)

> 601.0

590.9

580.8

570.7

560.6

540.5

500.3

450.2

< 400.1

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NERC TPL-007 USGS Earth

Model

Benchmark Scaling Factor

(β B)

Supplemental Scaling Factor

(β S)

AK1A 0.56 0.51AK1B 0.56 0.51AP1 0.33 0.30AP2 0.82 0.78BR1 0.22 0.22CL1 0.76 0.73CO1 0.27 0.25CP1 0.81 0.77CP2 0.95 0.86FL1 0.74 0.73CS1 0.41 0.37IP1 0.94 0.90IP2 0.28 0.25IP3 0.93 0.90IP4 0.41 0.35NE1 0.81 0.77PB1 0.62 0.55PB2 0.46 0.39PT1 1.17 1.19SL1 0.53 0.49SU1 0.93 0.90BOU 0.28 0.24FBK 0.56 0.56PRU 0.21 0.22BC 0.67 0.62

PRAIRIES 0.96 0.88SHIELD 1.00 1.00

ATLANTIC 0.79 0.76

Table II-2:

• βB - Benchmark• βS - Supplemental

Sheet1

USGS Earth ModelBenchmark Scaling Factor (bB)Supplemental Scaling Factor (bS)

AK1A0.560.51

AK1B0.560.51

AP10.330.30

AP20.820.78

BR10.220.22

CL10.760.73

CO10.270.25

CP10.810.77

CP20.950.86

FL10.740.73

CS10.410.37

IP10.940.90

IP20.280.25

IP30.930.90

IP40.410.35

NE10.810.77

PB10.620.55

PB20.460.39

PT11.171.19

SL10.530.49

SU10.930.90

BOU0.280.24

FBK0.560.56

PRU0.210.22

BC0.670.62

PRAIRIES0.960.88

SHIELD1.001.00

ATLANTIC0.790.76

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Transformer GIC Susceptibility

Transformer in AC operation subjected to DC:

• Unidirectional DC flux in core– Additive for one half of cycle– Subtractive for other half of cycle

• Reluctance of flux path• Amplitude and duration of GIC• Result is a flux density shift

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Designs strongly susceptible to GIC:


Three phase Shell type

Three phase 5 Leg Core type

• Single Phase Shell and Core Form

• Three Phase 5 Leg Core Form• Three Phase Shell Form

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3 Leg Core type

Designs weakly susceptible to GIC:

• Three Phase 3 Leg Core Form

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Transformer GIC Effects

Transformer Impacts:

• Generation of harmonics• Increased reactive power consumption• Winding hotspots• Hotspots in steel structures

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Part cycle core saturation:

• DC offset of AC Sine Wave• Stray flux• Audible noise• Vibration


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Heating of materials:

• Long thermal time constant (hours)– Oil

• Short thermal time constant (minutes)– Windings and leads– Steel clamps and structures


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Analysis to be performed by manufacturer:

• Design information is proprietary• Tools, models and expertise to perform analysis• Historical archives of old designs• Design records for defunct manufacturers

GIC Heating Analysis

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Specifying GIC Requirements:

• Ambient Temperature• Load Condition• DC Current

– Amplitude– Duration

• Temperature Limits– Oil– Core & Windings– Tank & Steel structures


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ATC’s EHV transformer GIC requirements:

• Conditions– 40 °C ambient temperature– 70% of maximum MVA load– 0 amps dc prior to and after event

• GIC magnitude: time and duration– 5 amps dc (neutral) - 30 minutes– 100 amps dc (neutral) - 2 minutes– Repeated cycle over 8 hours

• Temperature Limits– 110 °C: Top Oil– 140 °C: Winding or Metallic Hot Spot


40 50 60

Time (minutes)

0 10 20 30

100

10

20

30

40

50

60

70

80

90

0

Neu

tral

GIC

Cur

rent

(am

ps)

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Example 1:3 Leg Core Form Autotransformer

• 300/400/500 MVA• HV – 345 kV• LV – 138 kV

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Harmonic Currents

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Reactive Power Consumption

Neutral GIC

(Amps)

Reactive Power (MVA)

0 0.65 0.9

100 14.1

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Top Oil Temperature vs. Time

Maximum Top Oil Temperature = 75.7 °C vs. 110 °C Limit

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Winding Hot Spot Temperature vs. Time

Maximum Hot Spot Temperature = 90.5 °C vs. 140 °C Limit

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Flitch-Plate Hot Spot Temperature vs. Time

Maximum Hot Spot Temperature = 80.1 °C vs. 140 °C Limit

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Example 2:5 Leg Core Form Autotransformer

• 300/400/500 MVA• HV – 345 kV• LV – 138 kV

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Reactive Power Consumption = 0.20 MVA

Exciting Current Waveform:0 Amps/phase GIC

Harmonic Currents

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Harmonic Currents

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Harmonic Currents

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Thermal Results:• Maximum Top Oil Temperature = 87.5 °C vs. 110 °C Limit• Maximum Hot Spot Temperature = 103.6 °C vs 140 °C Limit

Upper Core Clamp

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• GIC and effect on electrical system is not consistent• Heating effects on transformers are design specific• System instability from VAR consumption• Reference IEEE C57.163: IEEE Guide for Establishing

Power Transformer Capability while under Geomagnetic Disturbances

Summary of Main Points

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Questions

Brian PennyConsultant Engineer & Transformer Subject Matter Expert

for American Transmission [email protected]

262-832-8706

Thank you for your attention

Questions?

mailto:[email protected]

Evaluating Transformer Heating due to Geomagnetic DisturbancesPresentation SummarySolar Activity and GMDSolar Activity and GMDSolar Activity and GMDNERC TPL-007NERC TPL-007NERC TPL-007NERC TPL-007NERC TPL-007Transformer GIC SusceptibilityTransformer GIC SusceptibilityTransformer GIC SusceptibilityTransformer GIC EffectsTransformer GIC EffectsTransformer GIC EffectsGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisGIC Heating AnalysisSummary of Main PointsQuestions

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