Benchmark GMD Event. Project (GMD Mitigation) Standard Drafting Team. GMD Task Force In-person meeting March 18-19, 2014

Transcription

1 Benchmark GMD Event Project (GMD Mitigation) Standard Drafting Team GMD Task Force In-person meeting March 18-19, 2014

2 Topics General Description Luis Marti, Hydro One Statistical Considerations for Determining the Reference Peak Geoelectric Field Antti Pulkkinen, NASA / Emanuel Bernabeu, Dominion Accounting for Geomagnetic Latitude and Earth Conductivity- Randy Horton, Southern Company / Luis Marti, Hydro One Examples 2

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4 Background The Benchmark GMD Event describes the design basis for GMD vulnerability assessment and planning studies It defines the geoelectric field values used to compute GIC flows It is used in conjunction with Reliability Standards in development for Project (GMD Mitigation) as directed in FERC Order No. 779 Whitepaper will be posted on the GMD Standard Project Page for comment as part of the Standard Development Process Project page: Disturbance-Mitigation.aspx 4

5 General Characteristics The Benchmark GMD Event takes into account known characteristics of a severe GMD event and impact on the system Geomagnetic latitude Earth conductivity Transformer electrical response Transformer thermal response Geoelectric field waveshape Wide-area geomagnetic phenomena 5

6 Benchmark GMD Event Description The Benchmark GMD Event is described by: Reference geoelectric field amplitude o Determined statistically from geomagnetic field measurements for a reference earth model o Used for GIC studies and load-flow simulations that account for transformer Reactive Power absorption Reference geomagnetic field waveshape o March GMD event selected from recorded GMD events o Used for time-domain analysis on equipment such as transformer thermal impact assessment Scaling factors for geomagnetic latitude and local earth conductivity can be used to adjust geoelectric field amplitude 6

7 GMD Benchmark Geoelectric Field E peak = E benchmark x α x β (in V/km) where, E peak = Benchmark Geoelectric field amplitude at System location E benchmark = Benchmark Geoelectric field amplitude at reference location (60 N geomagnetic latitude, resistive ground model) α = Factor adjustment for geomagnetic latitude β = Factor adjustment for regional Earth conductivity model 7

8 Reference Geoelectric Field Amplitude (E benchmark ) E benchmark is determined statistically from geomagnetic field measurements using a reference earth model Reference earth model (Quebec) Scaled to the reference geomagnetic latitude of 60 North 8 V/km is conservative value for E benchmark Frequency of occurrence in the range of 1 in 100 years 8

9 Reference Geomagnetic Waveshape Selected after analyzing recorded GMD events March 13-14, 1989 (NRCan observations) 2003 Halloween storm (Nurmijarvi and Memanbetsu observations) NERC Interim Report reference storm NRCan s Ottawa 10-second data for March 1989 event selected Provides conservative results for transformer thermal analysis 9

10 Reference Geomagnetic Waveshape Benchmark geomagnetic field waveshape. Red Bn (Northward), Blue Be (Eastward). 10

11 Reference Geoelectric Field Waveshape Benchmark geoelectric field waveshape at 60 North / Quebec ground model. E E (Eastward). 11

12 Reference Geoelectric Field Waveshape Benchmark geoelectric field waveshape at 60 North / Quebec ground model. E N (Northward). Use UTC in the time axis. 12

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14 Introduction Objective: characterize the occurrence rates geoelectric field. We need to address the following key characteristics of the extreme geoelectric fields: i. Amplitude. ii. Spatial structure including directionality and appropriate spatial scale lengths. iii. Temporal waveform. NERC interim report 2012: Localized peak geoelectric field wide area geoelectric field. Same data source: IMAGE 10-sec resolution. 14

15 Technical Considerations The geomagnetic induction process that generates the geoelectric field is dependent on external and internal factors: The effect of the geomagnetic latitude needs to be taken into account. The local ground conductivity dictates the ground response. The intent of the Benchmark GMD Event is to address widearea effects on the power system: Voltage stability. Thermal response of power transformers. Spatial structure of storm-time high-latitude geoelectric fields can be very complex. What we believe are ionospheric hotspots are seen commonly at times of peak geoelectric field amplitudes. 15

16 Spatial Structure March 1989 storm (1-min global data) 16

17 Spatial Structure October 2003 storm (1-min global data) 17

18 Spatial Structure Peak geoelectric fields in localized regions of a few hundred kilometers or less may be 2-3 times larger than neighboring locations 18

19 Spatial Structure Most earlier statistical efforts have focused on characterizing the occurrence of the geoelectric field at individual stations no reflection of spatial scale beyond one single point. Since wide-area effects caused by a severe GMD are of main interest, we need to approach the statistics from a new angle. Spatial structure: Geoelectric field amplitudes using 10-s IMAGE magnetometer array observations. Reference ground model was selected. Spatial averaging over four different station groups spanning an area of approximately 500 km in diameter. 19

20 Analysis Method Group 1 Group 2 Group 3 Group 4 20

21 Computed Statistics E benchmark in 3-8 V/km at 60⁰ N geomagnetic 8 V/km to be conservative Statistical occurrence of spatially averaged high-latitude geoelectric field amplitudes and visual extrapolation to 1-in-100 year occurrence 21

22 Extreme Value Statistics While visual extrapolation with understanding of the physics of the phenomenon is reasonable, we have studied the statistics also by means of rigorous Extreme Value Theory (EVT) analyses. 22

23 Extreme Value Statistics Objective: estimate 95% confidence interval for 100 years return levels. Asymptotic Extreme Value Theorem (similar to CLT). Three statistical models: Generalized Extreme Value (GEV). Group data into sequence of blocks maxima of length n. This sequence of maxima has a GEV distribution. ( ) G z Point over threshold (POT). r-largest values GEV. = e z µ 1+ ξ σ 1 ξ Enhance sample size to improve efficiency 23

24 Extreme Value Statistics 3000 Station Station E-field [mv/km] E-field [mv/km] E-field [mv/km] Year σ ( t) Station 3 E-field [mv/km] Year µ ( t) Station Year RELIABILITY Year ACCOUNTABILITY

25 Extreme Value Statistics Example: POT. Data has been declustered. Threshold 1 V/km. Data size: 83. Profile Likelihood Return level e-01 1e+01 1e+03 Profile Log-likelihood years Return ~ 3.45 V/km CI = [2.61, 7.5] 25 Return period (years RELIABILITY Return Level ACCOUNTABILITY

26 Statistical Considerations Recap Statistical occurrence of extreme geoelectric field amplitudes is characterized considering spatial scales: Same data source as NERC interim report. Localized peak 20 V/km is mapped to a wide area 8 V/km. Any impacts associated with localized geomagnetic activity? Temporal waveform: 1989 GMD storm. 26

27 Impact of Localized Geoelectric Field Analysis was performed on a 500 km x 500 km section of the North American transmission system subdivided into 100 km by 100 km sections - 25 sections in total. The geoelectric field is assumed uniform within each section. Geoelectric field in one section increased by 2.5 and computing the corresponding GIC flows in the network, resulting in a total of 25 GIC distribution simulations. 500 km 27

28 Impact of Localized Geoelectric Field 28 Number of transformers that experience a GIC increase greater than 10 Amps (in red). Reduction in GIC of more than 10 Amps (in blue). Essentially the same (in green)

29 Impact of Waveshape on Transformer Hot-spot Heating Waveshape has a strong impact on transformer hot spot heating of windings and metallic parts. Thermal effects in power transformers are not instantaneous. Time constants in the 5-20 minute range. Thermal analysis was performed on a large and diverse test network. The March 1989 GMD event was found to be a conservative choice when compared to other events of the last 20 years. Halloween storm GMD Report reference storm. 29

30 Thermal Analysis Calculated peak metallic hot spot temperature for all transformers in a test system with a temperature increase of more than 20 C. for different GMD events scaled to the same peak geoelectric field 30

31 Thermal Analysis Calculated peak metallic hot spot temperature for the top 25 transformers in a test system for different GMD events scaled to the same peak geoelectric field. 31

32 Statistical Consideration Conclusions Statistical occurrence of extreme geo-electric field amplitudes years) is characterized considering spatial scales: Localized peak 20 V/km is mapped to a wide area 8 V/km. Temporal waveshape: 1989 GMD storm. The reference geoelectric field (8 V/km) and temporal waveshape must be scaled as a function of latitude and ground conductivity. 32

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34 GMD Benchmark Event Electric Field E peak = E benchmark x α x β (in V/km) where, E peak = Benchmark Geoelectric field magnitude at System location E benchmark = Benchmark Geoelectric field magnitude at reference location (60 N geomagnetic latitude, resistive ground model) α = Factor adjustment for geomagnetic latitude β = Factor adjustment for regional Earth conductivity model 34

35 Geomagnetic Latitude Scaling Preliminary analysis of IMAGE magnetometer data with spatial averaging conducted by Pulkkinen Analysis also shows approximately one order of magnitude drop from 65 deg to 40 deg of geomagnetic latitude 35

36 Data Analysis Plot shows the geomagnetic latitude distribution of spatially averaged geoelectric field amplitudes for the October 2003 storm Averaging was done for station pairs 36 Distances varied between 300 km to about 700 km Order of magnitude drop across deg is similar to results obtained from analysis of peak e-fields Geomagnetic Latitude Distribution of Maximum Spatially Averaged Geoelectric Fields

37 Scaling Factors Tabular α scaling factors are being determined Sample α scaling factors for geomagnetic latitudes 1.0 at 60⁰ N Juneau; Winnipeg; Churchill Falls, NL 0.3 at 50⁰ N New York ; St Louis; Salt Lake City 0.1 at 40⁰ N Jacksonville; New Orleans; Tucson Geomagnetic Latitude Chart. Application for converting geographic latitude to geomagnetic latitude is available from NOAA website 37

38 Scaling the Geoelectric Field The peak geoelectric field, E peak, is dependent upon geomagnetic field waveshape and local Earth conductivity Plane wave method may be used to calculate E peak, or a scaling factor can be applied to account for Earth conductivity model Conductivity scaling factor (β) calculated as the ratio of local peak geoelectric field to the reference peak value of 8 V/km 38

39 Earth Conductivity Scaling Earth conductivity model factor (β) Scaling Factor: 0.81 Atlantic Coastal (CP-1) (analysis ongoing) 0.30 Columbia Plateau (CO-1) Based on data from US Geological Survey (USGS) and Natural Resources Canada (NR Can) 39

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41 Example 1 Transmission service territory that lies at a geographical latitude of 45.5 (geomagnetic latitude of 55 ) α = (using formula α=0.001 exp(0.115 L)) Same earth conductivity as the benchmark β=1 E peak = = 4.5V/km Geomagnetic Latitude (Degrees) Scaling Factor1 (α) If territory spans more than one physiographic region (i.e. several locations have a different earth model) then the largest α can be used across the entire service territory for conservative results. Alternatively, the network can be split into multiple subnetworks, and the corresponding geoelectric field amplitude can be applied to each subnetwork

42 Example 2 Transmission service territory that lies at a geographical latitude of 45.5 (geomagnetic latitude of 55 ) USGS α = (using formula α=0.001 exp(0.115 L)) Earth model Earth conductivity NE1, β=0.81 Scaling Factor (β) AK1A 0.56 AK1B.0.56 E peak = = 3.6V/km If the utility has a technically supported conductivity model or models and the tools to calculate the geoelectric field from the geomagnetic field then E peak can be calculated directly using the reference geomagnetic field waveshape scaled by α AP AP BR CL CO CS IP IP IP IP NE OTT