Safety Grounding Update

1
Safety Grounding Update

• July 25, 2006
• Phoenix, AZ

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Power System Safety Manual (PSSM)

• 2005 PSSM did not have Grounding Tables for
  Regions
• 2006 PSSM has new Grounding Tables for Regions
• Appendix K for DSW Grounding Tables

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2006 PSSM Appendices
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3 Tables for DSW Data

• Table 1 Substation Grounds for both the lines
  and substation equipment
• Table 2 Line Grounds for entire line
• Table 3 Line Grounds with specific section
  breaks

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2006 PSSM Appendix KTable 1 pages 173 through
183
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Table 1 Explanation

• Faults placed on all buses in cases with DSW and
  connected systems
• 3 Phase and SLG faults maximum of both
• 3 Grounding conductor sizes
• SBS for substation
• LNS for lines

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Table 1 Calculations

• Calculations use the maximum fault current
• Table 10.1 in 2006 PSSM Cable Limits

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Substation and Line Calculations

• 15 Second time used for Substation Bus grounding
  cables
• 30 Second time used for Line grounding cables
• 90 derating is used for 2 or more cables

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Example 1 Calculation
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Example 2 Calculation
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2006 PSSM Appendix KTable 2 pages 184 through
190
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Table 2 Explanation

• Same calculations as Table 1 for each substation
  line
• Home Bus and Home Bus kV
• Remote Bus
• Maximum for entire line
• 2/0 cables used
• 30 second time

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Example 3 Calculation
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Each Line Entered Twice
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Example 4 Calculation
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Example 4 Calculation
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Table 3 2006 PSSM Appendix Kpages 192 through 193

• 2 Pages
• Spreadsheet with data is 2 pages wide
• Match pages for the full line entry
• Most lines have 2 entries with home bus at each
  end
• Not all lines in Table 3
• Use Table 2 for max grounding if line not in
  Table 3

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Page 1 of Table 3
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Page 1 of Table 3

• Contains the data for the substations
• Home and Remote Buses
• Home Bus Grounding for substation
• 15 Second Clearing for Buswork
• 27,000 Amps / 2/0 ground cable
• 90 used for multiple ground threshold
• Three phase and SLG fault current at home bus
• Maximum fault current at home bus used

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Page 2 of Table 3
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Page 2 of Table 3

• Contains the data for the lines
• Home bus line grounding given
• Length of line given
• 4 distances from home bus for ground changes

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Home Bus Line Grounds

• 30 Second Clearing for Lines
• 20,000 Amps / 2/0 ground cable
• 90 used for multiple ground threshold
• Three phase and SLG fault current at home bus
  (Page 1 of Table 3)
• Maximum fault current at home bus used

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Line Distance Grounds

• Calculated in 2 increments along Line
• Change conductors at distance listed for
  breakpoint
• Distance and grounds start with home bus ground
• Calculations based on 30 second clearing

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Example 5 Calculation
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Example 5
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Example 5
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Example 5
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Example 6 Calculation
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Example 6
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Example 7 Calculation
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Example 7
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Example 8 Calculation
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Example 8
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Example 8

• 2 increments are 4.8 miles
• Higher conductor number to lower conductor change
  uses end of 2 increment (conservative)
• Lower conductor number to higher conductor change
  uses beginning of 2 increment (conservative)

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Future Work

• Put lines in Table 3 for all lines with 1 or more
  break points on grounding cable number
• Update values for lines from previous study
• Remove buses in Table 1 that are not WAPA buses
• Remove PTI bus numbers in Tables

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Questions?