Electric Power Distribution Handbook, T. A. Short

13. Lightning Protection

Questions

  1. Which of the following can cause damage to distribution equipment? Mark all that apply.

    1. Negative cloud-to-ground direct stroke
    2. Positive cloud-to-ground direct stroke
    3. Negative cloud-to-ground stroke 40 ft from a line
    4. Positive cloud-to-ground stroke 40 ft from a line
    5. Intracloud lightning, 2000 ft up

  2. Lowering the pole ground resistance at a capacitor bank will help with which of the following (mark all that apply)?

    1. Damage to the capacitor cans
    2. Damage to the capacitor controller
    3. Damage to the capacitor switches
    4. Damage to the arrester

  3. Lowering the pole ground resistance at a transformer will help with which of the following (mark all that apply)?

    1. Damage to the transformer
    2. Damage to the arrester
    3. Damage to customer equipment
  4. Which is better for transformer protection?
    1. Tank mounted lightning arrester
    2. Arrester on the crossarm
    3. No arrester

  5. Which of the following will be improved with better grounding? Mark all that apply.

    1. Line flashovers
    2. Transformer failures
    3. Induced voltage flashovers
    4. Fuse operations

  6. On which factor do you select the surge arrester rating? Mark all that apply.

    1. Surge duty
    2. Grounding
    3. Nominal system voltage

  7. Better line insulation helps with which of the following? Mark all that apply.

    1. Induced voltage flashovers
    2. Direct strike flashovers
    3. Lines with a shield wire
    4. Underbuilt circuits
    5. Lines with line-protection arresters

  8. External gaps are recommended to help reduce arrester failures.

    1. True
    2. False

  9. Per IEEE C62.22-2009, what is the minimum MCOV for an arrester used on a 13.8-kV multigrounded system?

    1. 7.65 kV
    2. 8.4 kV
    3. 10.1 kV
    4. 12.7 kV
    5. 15.3 kV

  10. Which type of insulation is most dependent on the voltage waveshape and duration?

    1. Air
    2. Oil
    3. Cable insulation

  11. Which type of insulation is least dependent on the voltage waveshape and duration?

    1. Air
    2. Oil
    3. Cable insulation

  12. For arresters at the open point of a cable, which type of surge rise time is most damaging to the cable system?

    1. Slow rise time
    2. Fast rise time

  13. On a three-phase line recloser, how many arresters are needed on the structure for adequate protection?

    1. 3
    2. 6

Problems

  1. What is the probability of a negative first stroke that is greater than 45 kA? Use the EPRI (1992) approximation on p. 664.

  2. What is the probability of a negative first stroke that is greater than 45 kA? Use the Anderson-Eriksson model in Table 13.1 on p. 664.

  3. In open ground, find the average strikes to an overhead line using the Eriksson model. Assume 5 flashes/km^2/year and a line height of 40 ft.

  4. For an underground cable in open ground, find the average strikes that attach to that cable per year using the Sunde model. Assume 5 flashes/km^2/year and 1000 ohm-m soil.

  5. On a distribution transformer, calculate the protective margins for both BIL and CWW for the following parameters: system voltage = 12.5 kV, transformer size = 50 kVA, lead length = 4 ft, arrester duty cycle rating = 10 kV, arrester LPL = 34 kV. Assume 8 kV/ft of lead length.

  6. On a distribution transformer with the following parameters, calculate the lead length necessary to maintain a protective margin for BIL of 50%: system voltage = 34.5 kV, transformer size = 50 kVA, lead length = 4 ft, arrester duty cycle rating = 10 kV, arrester LPL = 34 kV. Assume 8 kV/ft of lead length.

  7. Using the enhanced version of the Rusck model proposed by Darveniza, find the induced voltage on an overhead line based on the following parameters: line height = 40 ft, lightning current = 25 kA, distance from the line = 50 ft, and earth resistivity = 1000 ohm-m.

Projects

  1. Visit overhead distribution lines in your area. Find and photograph five examples of lightning protection issues, possibly including the following:

    • Weak links with lower-than-normal insulation
    • Long arrester lead lengths
    • Other arrester issues like blown disconnects

  2. Use EMTP or another transient program to model a lightning strike to a distribution line as in Figure 13.38. Evaluate the impact on a pole-mounted controller. Include the following:

    • Model both first strokes and subsequent strokes
    • Model results as a function of the distance between the controller and the power transformer (vary between 50 and 500 ft).
    • Evaluate the impact of both grounds.
    • Evaluate the impact of secondary arresters at the controller.
    • Evaluate the impact of secondary arresters at the control power transformer.

  3. Write a spreadsheet that calculates protective margins for overhead and underground systems. Include all appropriate inputs needed.



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To the extent possible under law, Tom Short has waived all copyright and related or neighboring rights to these study questions. This work is published from the United States. Please use this material however you want.