Minimum Approach Distances (MAD) The final rule updated OSHA s default values for transient over voltages (TOV), selecting far more conservative values. As TOV is one of the major factors for calculating MAD, this change leaves utilities little option other than to calculate of system specific TOV values. A summary of the impacts follows: By January 31, 2016 the employer shall: Use the default TOV values in the final rule for calculating MAD for voltages above 72.5 kv; or Determine specific TOV values though engineering analysis (MAD for voltages 72.5 kv and below have minor changes).
500 kv Target TOV is 2.4 (per unit) The longest line meets existing TOVs and MAD when considering the existing installed mitigation [2.4] If all lines have line arresters or pre-insertion resistors, then all lines would meet old OSHA MAD even with high speed reclosing [1.8 to 2.3] If some lines do not have mitigation installed, then the high speed reclosing case would exceed old OSHA TOV values and require considerably larger MADs. (Consider one MAD for mitigated lines versus another for unmitigated lines.) [3.6] If a breaker restrike is considered, the TOV climbs and significantly exceeds the old OSHA values as well as new OSHA default rules [4.5] Shorter line lengths have little effect on the TOV for the high speed reclosing.
345 kv The target TOV is 3.0 (per unit) For the longest line, with no mitigation present (for example, line arresters or breaker pre-insertion resistors), at maximum system voltage, clearing a fault with no reclosing yields the worst case TOV of well under 3.0 [1.9] If high speed reclosing is considered with fault clearing, then for the no mitigation case, the TOV exceeds 3.0 [3.6]. This calculates to an additional MAD of 2-0 resulting in a MAD of 11-2 compared to the present MAD of 9-2 Shorter line lengths have little effects on the TOV for the high speed reclosing. Considering the existing mitigation that is in place on this longest line (arresters but no pre-insertion resistors) then the worst case TOV is less than 3.0 [2.4] If just line arresters are considered for TOV mitigation, then even for the high speed reclosing case, the TOV is less than 3.0 [2.4] If just pre-insertion resistors are considered for TOV mitigation, then even for the high speed reclosing case, the TOV is less than 3.0 at [1.9] If a breaker restrike is considered, the TOV climbs and significantly exceeds the old OSHA values as well as new OSHA default rules [4.7]
500 kv & 345 kv System 500 KV System 58 lines Arrestors at both ends = 44 No mitigation = 7 Unknown mitigation = 7 345 KV System 88 lines Arrestors at both ends = 41 No mitigation = 35 Unknown mitigation = 12 Recommendations Reclosing held-off for live line work Investigate unknowns (including foreign TO s) Create two MAD tables (mitigated and unmitigated lines) Assess impact on substation (breaker control cabinet) and tower climbing
MAD Tower Climbing MAD of 9 6 The NESC phase-toground work MAD used in this drawing was determined from NESC Rule 441.A of the 2007 NESC edition and was calculated to include an altitude adjustment (per NESC Rule 441.A.6.b) for elevations up to 4000 feet. The base value used in the calculation (9'-4") was taken from NESC Table 441-2, using an overvoltage factor 'T' of 2.0 and a maximum phase-to-phase voltage of 550 kv, and an additional 2 was added for the increased elevation requirements.
230 kv Target TOV is 3.0 (per unit) For the longest line, with no mitigation present (for example, line arresters or breaker pre-insertion resistors), at maximum system voltage, clearing a fault with no reclosing yields the worst case TOV of well under 3.0 [1.6] If high speed reclosing is considered with fault clearing, then for the no mitigation case, the TOV is less than 3.0 [2.9]. This requires no change to the current MADs. Considering the existing mitigation that is in place on this longest line (arresters but no pre-insertion resistors) then the worst case TOV is less than target [2.9] If just line arresters are considered for TOV mitigation, then even for the high speed reclosing case, the TOV is less than target [2.9] If a breaker restrike is considered, the TOV climbs and significantly exceeds the old OSHA values as well as new OSHA default rules [4.1]
138 kv The target TOV is 3.0 (per unit) For the longest line, with no mitigation present (for example, line arresters), at maximum system voltage, clearing a fault with no reclosing yields the worst case TOV of well under 3.0 [1.7] If high speed reclosing is considered with fault clearing, then for the no mitigation case, the TOV exceeds 3.0 [4.1]. This calculates to an additional MAD of 0-7 resulting in a MAD of 4-5 compared to the present MAD of 3-10. Shorter line lengths have little effects on the TOV for the high speed reclosing. Considering the existing mitigation that is in place on this longest line (rod gaps, arrestors, no reinsertion resistors) then the worst case TOV is less than 3.0 [2.4] If just line arresters are considered for TOV mitigation, then even for the high speed reclosing case, the TOV is less than 3.0 [2.4] If a breaker restrike is considered, the TOV climbs and significantly exceeds the old OSHA values as well as new OSHA default rules [4.3]
115 kv The target TOV is 3.0 (per unit) For the longest line, with no mitigation present (for example, line arresters), at maximum system voltage, clearing a fault with no reclosing yields the worst case TOV of well under 3.0 [1.5] If high speed reclosing is considered with fault clearing, then for the no mitigation case, the TOV exceeds 3.0 [4.1]. This calculates to an additional MAD of 7 resulting in a MAD of 3-11 compared to the present MAD of 3-4. Shorter line lengths have little effects on the TOV for the high speed reclosing. Considering the existing mitigation that is in place on this longest line (arrestors at one terminal only) then the worst case TOV is less than 3.0 [2.5] If a breaker restrike is considered, the TOV climbs and significantly exceeds the old OSHA values as well as new OSHA default rules [3.7]