Lightning & Its Effects on the Florida Power & Light Distribution System & Other Related Issues LARRY VOGT
Lightning – Its Effects on the Florida Power & Light Distribution System and related issues – Larry Vogt Note: Actual interruption data from 2007
Lightning is a major contributor to customer interruptions and equipment failures on the FPL Distribution System. On average, there are more than 300,000 lightning strikes on the FPL service territory every year. Lines in areas with high ‘Isokeraunic’ levels (# of strikes/sq. km) could experience 2-3 direct strikes per mile Flashes per sq. km 2
YTD - Customers Interrupted by Major Categories - All Types 700,000 YTD 6/06 600,000 Weather, including lightning, is the second highest 546,161 YTD 6/07 cause of customer interruptions (CC 01 – Lightning) GAP 500,000 374,387 400,000 264,158 300,000 260,953 192,922 173,550 169,731 166,050 200,000 149,325 110,276 100,000 43,728 0 EQUIP Weather Accidents Unknown VEG OTHER Wire Cable Imp Animals Request -100,000 Process -200,000 -300,000 Note: Many of the equipment related interruptions may also have been initiated by lightning 3
Current Situation Lightning – Typical lightning can deliver Monthly Correlation - Lightning Strikes vs Storm Interruptions (Codes 01 & 02) currents ranging from 10,000 - 200,000 5000 amps. Lightning currents also induce 4500 voltages exceeding one million volts. 4000 3500 Storm Interruptions When lightning hits a distribution line, a 3000 high-energy traveling wave is created on 2500 either side of the strike point, as the 2000 lightning charge travels to the nearest 1500 1000 ground. FPL’s distribution system has 500 insulation levels between 95 kV and 150 0 0 20000 40000 60000 80000 100000 120000 140000 kV BIL. While a surge arrester can clamp Lightning Strikes voltages at 40 to 50 kV, a flashover can still An obvious correlation exists between occur before the wave reaches the lightning strikes and customer service arrester. interruptions 4
Surge Arresters: FPL has over 1.5 million arresters installed on its distribution system, and still experiences lightning flash-overs on all typical distribution framing, Better Framing for overhead lines: The FPL distribution system contains approximately 30% cross-arm construction; 25% triangular; 25% Modified vertical; 15% Vertical; and, 5% Vertical with overhead ground wire. Modified Vertical Modified vertical framing has been the Framing standard for over 30 years. 5
After a lightning storm, crews typically find obvious lightning related damage … shattered insulators, blown fuses, ruptured transformers, etc. Lightning, however, causes many problems that don’t show up right away. Compromised transformer insulation, cracked insulators, pitted contacts, damaged blocks on arresters, etc…..these could show up as equipment failures, much later on. 6
Analysis - Operation and Limitations of Lightning Arresters To line • Modern Lightning Arresters are designed to have a high resistance at normal line voltage, essentially acting like an insulator. • Under surge conditions, as the voltage rises on the distribution line, the resistance of the arrester drops. This allows surge current to be diverted through the arrester to ground. Disconnector Arrester energy limitation To Ground LS • Arresters are designed to dissipate a Most Lightning Strikes Energy limited amount of energy. • If an arrester is subject to energy levels higher than its rating, the internal metal oxide blocks short, causing the disconnector and ground terminal to # of LS separate from the body of the arrester. Direct strokes in Not to scale. For demonstration purposes only. this area 7
There are approximately 50 feeder interruptions related to arresters, annually Estimated Population on the FPL system FEEDER INTERRUPTIONS (with exclusions) – 1,545,000 ARRESTER - 12MOE Porcelain – 390,000 70 # Polymer – 1,155,000 60 o f Expected life: 20-25 Years 50 I Usage (2007): 83,000 N T 40 E Failures: 7000 annually R 30 R Porcelain – 4000 U P 20 T Polymer - 3000 I O 10 N S Arresters typically fail for one of two 0 Sep-05 Dec-05 Mar-06 Jun-06 Sep-06 Dec-06 Mar-07 Jun-07 reasons: • Lightning - While arresters can handle some strokes effectively, their energy dissipation capacity is sometimes exceeded. • Moisture Intrusion - In older porcelain arresters, the top seal often fails with age, allowing a great deal of moisture inside. Polymer arresters also have moisture intrusion. 8
The manner in which the arrester was originally installed, accounts for ARRE STE R FAILURE MODE S (12 MOE) 66% of the feeder interruptions. (1 st causing FE E DE R INTE RRUPTIONS (with exclusions) two bars on chart) 50 100% 48 46 88% 90% N=50 44 42 • Ground lead too close - The ground 40 80% 38 36 lead is formed too close to arrester 70% # of FDR INTERRUPTIONS 34 66% 32 base (An oversized or stiff lead makes 30 60% 28 26 matters worse) 50% 24 22 20 40% • Ground or Primary lead too long - 20 40% 18 16 The ground lead was left too long 30% 13 14 11 12 from the last staple on the pole. 10 20% 8 6 • Tracked bracket – 6 10% 4 2 This is the result of degradation of the 0 0% Framing - Gnd lead Too close Gnd/prim. lead into phase Tracked Bracket Failure Modes glass-filled polyester bracket. . 9
Analysis - Scenarios where a Surge Arrester may be identified as the cause of a Feeder Interruption Scenario 1: Ground lead does not allow arrester ground terminal to separate 1. High-energy lightning strike hits line and exceeds arrester energy ratings, causing disconnector operation. 2. Ground lead is too stiff at the bottom of the arrester. Often the case in 3 Ph triangular installations, where one ground lead is used for all arresters. The non-failed arresters keeps the ground lead in place. 1 0
Analysis - Scenarios where a Surge Arrester may be identified as the cause of a Feeder Interruption Scenario 2: Arrester ground lead is too long and makes contact with primary conductor 1. Ground lead was too long from the last staple on the pole (Note: Long leads also reduce the degree of protection). 2. High-energy lightning strike hits line and exceeds arrester energy rating, causing the ground terminal stud to separate from body of arrester. 3. Ground lead makes contact with another phase (phase- to-ground fault). 1 1
Analysis - Scenarios where a Surge Arrester may be identified as the cause of a Feeder Interruption Scenario 3: Tracking on Mounting Brackets 1. Ground lead is attached according to Construction Standards. 2. High-energy lightning strike hits line and exceeds arrester energy rating, causing the ground terminal to separate from the arrester body. 3. Arrester is not replaced or removed promptly and remains attached to the primary. 4. With the ground lead removed, full line voltage is now present across the bracket. 5. A combination of time and contamination leads to tracking across the bracket. When the leakage current increases to a significant level over time, an interruption occurs (phase-to- ground fault). 1 2
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Analysis - Scenarios where a Surge Arrester may be identified as the cause of a Feeder Interruption Scenario 4: Porcelain Arrester shatters; primary Line lead makes contact with another phase 1. Line lead is installed correctly, but may have been too long. 2. Water leakage shorts blocks, and gas builds up, causing the arrester to shatter. All distribution arresters installed prior to 1987 were porcelain. Because of seal problems, some tend to LA get water inside. If this happens, they may fail violently, causing the primary lead to contact another phase or ground . We are addressing this problem by removing all porcelain arresters after they have been de-energized. 3. When the arrester shatters, the line lead propels into the air and makes contact with another primary line or ground, causing either a phase-to-phase or phase-to-ground fault. 1 4
Countermeasures- Surge Arresters 1 5
Analysis – Framing & Lightning study To better understand the capability of our distribution line framing, we joined forces with University of Florida and their Triggered Lightning Experiments at Camp Blanding. Our conclusions follow: • Modified vertical framing (our present standard) flashes over 90 % of the time for Average 5 return direct strikes to the line. Note: Direct strikes strokes per strike. range from 10% to 20% of all strikes Arrester energy depending on line location. i.e. Rural areas exceeds 84 KJ have highest exposure, while buildings and trees provide more shielding in urban areas. Continuing currents flow There is an average of 5 return strokes per • between each return stroke and damage strike, with continuing current flowing arresters. These currents between return strokes . The energy created are low frequency and average a few hundred from the continuing current is a major factor in amps. arrester failure. 1 6
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