Electrical Clearance

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<p>K N Srinivasan</p> <p>What is clearance</p> <p>Number telling a lengthDefined asminimum distance required between live parts and earthed material -Earth Clearance between live parts - Phase Clearance.</p> <p>CategoriesElectrical Clearance in air Electrical Clearance in gas/oil</p> <p> Lightning Impulse withstand Network-open to sky Clouds-300 to 600 MV 1.2 X 50 3 lakh Km/Sec</p> <p>Typical LI Wave</p> <p>BIL</p> <p>-Ve Polarity 1 Km 300 kJ 5 to 20 shots</p> <p>L I - Experimental Value</p> <p>Point Plane</p> <p>a-safe clearance</p> <p>SUBSTATIONMaximum Clearances Between Bushing Transformer &amp; Lightning Arresters System Voltage (kV) 11 33 66 132 220 Max distance between LA &amp; Transformer bushing ( meter) 12 18 24 42 64</p> <p>Switching surge 250/2500 micro sec. Beyond 220kV</p> <p>C u r v e a : A p p x . S a fe c le a r a n c e v a lu e s C u r v e b : E x p e r im e n ta l w it h -s t a n d v a lu e s</p> <p>F ig . 7 .1 .5 b .</p> <p>S a fe c le a ra n c e fo r 3 5 0 /2 3 0 0 m ic r o -s e c S w itc h in g</p> <p>S u rg e s</p> <p>Corona</p> <p>Blue Halo 16 kV /m Power loss Interference</p> <p>Partial discharge</p> <p>Imperfect insulation Voids Voltage buildup and equalization Pico q</p> <p>Environment</p> <p>Pollution Sea City Pre monsoon showers Dry band-Flashover</p> <p>Pollution &amp; Creepage distance</p> <p>Degree of contamination</p> <p>Examples</p> <p>Minimum creepage distance mm/ kV 16 20 25 31</p> <p>I) slight</p> <p>Predominantly rural areas without industry and far from sea air, II moderate Areas in which little severe pollution is expected III severe Industrial areas with relatively severe pollution, sea air, etc., IV very severe Areas with heavy industry and much dust, fog sea air.</p> <p>Live Maintenance Ease of maintenance</p> <p>Measurement Leakage capacitance Grading rings</p> <p>Thumb rulea.c Power frequency Voltages (rms)/m 200 kV</p> <p>D.C.Voltage Impulse voltages</p> <p>275 kV/m 500 kV/m</p> <p>Switching Surges</p> <p>d= (2V)2</p> <p>d in meters, V in MV</p> <p>primary sub transmission voltage33 kV derived either directly from 220 kV substations or 132 kV substations except in few states where 66 kV substations are still existing.</p> <p>secondary sub transmission voltage is 11 kV derivedfrom 33 kV sub stations. This is further stepped down to low tension voltage of 415 V 3 Phase and 230 V Single Phase and reaches consumer end, either through over head line or under ground cables</p> <p>SUBSTATIONComponents of substation a) Structures like Incoming - Outgoing gantries Mounting Structures for Breakers, Isolators, Fuses, Insulator supports, CTs, PTs and Bus bar structures. b) Power Transformers. c) Circuit Breakers. d) HT fuses only upto 33 kV sub station e) Isolating Switches f) Bus bar Arrangement g) Insulators h) Earthing Switches</p> <p>SUBSTATIONComponents of substation contd... p) Batteries q) Fencing Arrangement. r) Fire Extinguishers. s) Earthing Pit.</p> <p>SUBSTATIONComponents of substation contd... i) Lightning Arresters. j) Current Transformers. k) Potential Transformer. l) Measuring Instruments like, A/ m, V/ m, F, TVM m) Varieties of protection relays. n) Control Room. Power Cables o) Cables Controls Cables</p> <p>T able 7.1.2 - Standard insulation levels for range I (1 kV &lt; U m 245 kV) High est voltage for equipm en t Um kV (r.m.s. value) 3.6 7.2 12 Rated sh ort duration power frequen cy with stan d voltage kV (r.m.s. value) 10 20 28 Rated lightn ing im pulse with stand voltage kV (peak value) 20 40 40 60 60 75 95 75 95 95 125 145 145 170 250 325 450 550 (450) 550 650 (550) 650 750 (650) (750) 850 950 1050</p> <p>17.5 24</p> <p>38 50</p> <p>36 52 72.5 123</p> <p>70 95 140 (185) 230 (185) 230 275 (230) 275 325 (275) (325) 360 395 460</p> <p>145</p> <p>170</p> <p>245</p> <p>Note - If values in brackets are con sidered in sufficien t to prove that the required ph ase to ph ase with stan d voltages are m et, addition al phase-to-ph ase with stan d tests are n eeded.</p> <p>E le c tric a l c le a ra n c e s fo r o p e n - ty p e o u td o o r s w itc h g e a r o f 2 2 k V to 8 8 k V R a te d V o lta g e a n d fo r N o n - e f fe c tiv e ly E a rth e d S y s te m o f 1 1 0 k V a n d a b o v e Im p u ls e v o lta g e w ith s ta n d le v e l (p e a k v a lu e s ) 1 150 200 250 350 450 550 650 750 1050 R a te d v o lta g e M in im u m c le a ra n c e to e a rth M in im u m c le a ra n c e b e tw e e n p h a s e s in a ir o r b e tw e e n c o n n e c tio n s o f th e s a m e p h a s e s e p a ra b le e le c tric a lly fro m e a c h o th e r 4 in m m 13 330 17 432 22 558 31 786 39 989 48 1219 58 1473 67 1702 94 2388</p> <p>2 22 33 44 66 88 110 132 165 220 in 11 15 19 27 34 42 50 58 82</p> <p>3 m m 279 381 482 685 863 1068 1270 1473 2082</p> <p>N o te : W h e re th e m e th o d o f m o u n tin g in su la to rs is n o t s u c h a s to p r e v e n t lo d g e m e n t o f b ir d s o r v e r m in , it m a y b e n ecessary to e m p lo y c le a ra n c e s in excess o f th e m in im a g iv e n fo r ra te d v o lta g e s o f 2 2 k V a n d 3 3 k V .</p> <p>SUBSTATIONClearances in Air for Indoor Switchgear Rated Voltage Minimum Clearance Between Phases or Between Connections of the same phase separable Electrically from each other Distance (mm) 19.0 19.0 50.8 88.9 127.0 165.1 241.3 355.6</p> <p>415 V 600 V 3.3 kV 6.6 kV 11.0 kV 15.0 kV 22.0 kV 33.0 kV</p> <p>SUBSTATIONPost Insulator Minimum Creepage Distances Power Frequency Withstand Voltage (kV) 27 35 55 75 140 185 230 275 395 460 680 Minimum Creepage Distance ( mm) 130 230 380 580 1100 1600 1850 2250 3400 3800 6400</p> <p>Bus bar clearances in Oil or compound Rated voltage Minimum between phases of clearance to earth poles kV inches Up to and including 0.6 3.3 6.6 1 11 1 22 2 Minimum clearance between phases inches 1 1 2 3</p> <p>ELECTRICAL CLEARANCES IN AIRfor assemblies of outdoor switchgears upto 800 kV class as well as Indoor switchgears upto 33kV class in India i) Safety Clearance to Conductors (Section Clearance) minimum distance required between an unscreened live conductor and the limits of a work section ii) Safety Clearance to Insulation (Ground Clearance) minimum distance required between any exposed insulator which supports or contains live conductor and the limits of a work section</p> <p>GROUND CLEARANCES1) No conductor of over head line erected across a street shall be at any part there of be at a height less than a) For LT : 5.791 mtr</p> <p>b) For HT upto 33 kV : 6.096 mtr Clearance from Building Structure etc., a) The vertical clearance from the heighest point of the building on the basis of maximum slag shall not be less than 2.439 (8 ft) meter for LT lines upto 650 V and 3.64 (12ft) meter for 11 kV &amp; 33 kV lines. b) The horizontal clearance is to be 4 ft (1.219 m) upto 11 kV &amp; 6 ft (1.820 m) upto 33 kV line.</p> <p>CLEARANCE BETWEEN POWER AND TELECOM LINESMinimum clearances between, lines of various voltages LT lines (400 / 230 V) - 4 feet - 1.22 meter 11 kV lines 33 / 66 kV lines 132 kV lines 220 kV lines Note:Guard rings are to be provided at crossings of telecom lines with power lines upto 33 kV - 6 feet - 1.83 meter - 8 feet - 2.44 meter - 10 feet - 3.05 meter - 15 feet 4.88 meter</p> <p>110kV Substation Layout</p> <p>a) Line trap suspended. Capacitor Standing</p> <p>b) Line trap and capacitor suspended in portal structure</p> <p>c) Line trap mounted on capacitive Voltage transformer</p> <p>1. Circuit Breaker 2. Feeder Isolator 3 Current Transformer 4. Inductive voltage Transformer 5. Capacitive Voltage Transformer 6. Capacitor 7. Line Trap</p> <p>110kV Load Centre Station</p> <p>1. Bus bars 2. Bus bar Isolator 3. Circuit Breaker 4. Current Transformer 5. Voltage Transformer 6. Feeder Isolator 7. Lightning Arrester.</p> <p>220 kV Outdoor switchyard with double bus bars of tubular type</p> <p>1. Bus bar system I 2. Bus bar System II 3. Bus bar Isolator 4. Circuit Breaker 5. Current Transformer 6. Feeder Isolator 7. Line Trap 8 Capacitive Voltage Transformer</p> <p>110kV Outdoor switchyard with double bus bar low rise layout 1. Bus bar system I 2. Bus bar system II 3.Bus bar Isolator 4. Circuit Breaker 5. Current Transformer 6. Voltage Transformer 7. feeder Isolator 8. Lightning Arrestor</p> <p>M in im u m E le c tric a l C le a ra n c e fo r O u td o o r V o lta g e Im p u ls e M in im u m M in im u m ra tin g w ith s ta n c le a ra n c e to c le a ra n c e e a rth b e tw e e n d le v e l phases</p> <p>1 k V (rm s ) 36 7 2 .5 123 145 245</p> <p>*420</p> <p>2 kV (P e a k ) 170 200 325 350 450 550 550 650 825 900 1050 1300 1425 1550</p> <p>3 mm 320 375 630 675 920 1150 1150 1380 1800 1960 2300 2550 3070 3640</p> <p>4 mm 400 480 750 810 1065 1350 1350 1600 2000 2300 2700 3400 3200 5200</p> <p>s w itc h g e a r M in . c le a ra n c e fro m a n y p o in t w h e re th e m an m aybe re q u ire d to s ta n d to th e n e a re s t u n s c re e n e d c o n d u c to r in a ir (S e c tio n a l c le a ra n c e ) 5 mm 2920 2970 3230 3270 3520 3750 3750 3980 4400 4550 4900</p> <p>M in im u m E le c tric a l C le a ra n c e fo r O u td o o r S w itc h g e a r V o lta g e ra tin g Im p u ls e w ith s ta n d le v e l M in im u m c le a ra n c e to e a rth M in im u m c le a ra n c e b e tw e e n ph ases M in im u m c le a ra n c e fro m a n y p o in t w h e re th e m a n m a y b e re q u ire d to s ta n d to th e n e a re s t u n se c re a n e d c o n d u c to r in a ir (S e c tio n a l c le a ra n c e ) 5 m m 2920 2970 3230 3270 3520 3750 3750 3980 4400 4550 4900</p> <p>1 kV (rm s) 36 7 2 .5 123 145 245</p> <p>2 k V (P e a k ) 170 200 325 350 450 550 550 650 825 900 1050 1300 1425 1550</p> <p>3 m m 320 375 630 675 920 1150 1150 1380 1800 1960 2300 2550 3070 3640</p> <p>4 m m 400 480 750 810 1065 1350 1350 1600 2000 2300 2700 3400 3200 5200</p> <p>*420</p> <p>R e p ro d u c e d fro m IS :3 0 7 2 -1 9 7 5 c o d e o f p ra c tic e fo r in s ta lla tio n a n d m a in te n a n c e o f s w itc h g e a r . * R e p ro d u c e d fro m m a n u a l o n la y o u t o f s u b s ta tio n s C B IP te c h n ic a l R e p o rt N o . 3 .</p>