Electrical Clearance
Transcript of Electrical Clearance
K N Srinivasan
• Number telling a length
Defined as
minimum distance required
between live parts and earthed material -Earth Clearance
between live parts - Phase Clearance.
Categories
Electrical Clearance in air
Electrical Clearance in gas/oil
What is clearance
• Lightning Impulse withstand
Network-open to skyClouds-300 to 600 MV1.2 X 503 lakh Km/Sec
Typical LI Wave
-Ve Polarity1 Km300 kJ5 to 20 shots
BIL
L I - Experimental Value
Point Plane a-safe clearance
SUBSTATIONSUBSTATIONMaximum Clearances Between Bushing Transformer
& Lightning Arresters
System Voltage (kV) Max distance between LA & Transformer bushing ( meter)
11
33
66
132
220
12
18
24
42
64
Switching surge250/2500 micro sec.Beyond 220kV
F ig . 7 .1 .5 b . S a fe c le a r a 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 S u r g e s
C u r v e a : A p p x . S a f e c le a r a n c e v a lu e sC u r v e b : E x p e r im e n t a l w it h - s t a n d v a lu e s
Blue Halo16 kV /mPower lossInterference
Corona
Imperfect insulation
Voids
Voltage buildup and equalization
Pico q
Partial discharge
PollutionSeaCityPre monsoon showersDry band-Flashover
Environment
Degree ofcontamination
Examples Minimumcreepagedistancemm/ kV
I) slight Predominantly rural areaswithout industry and far fromsea air,
16
II moderate Areas in which little severepollution is expected
20
III severe Industrial areas withrelatively severe pollution,sea air, etc.,
25
IV very severe Areas with heavy industryand much dust, fog sea air.
31
Pollution & Creepage distance
Live MaintenanceEase of maintenance
Measurement
Leakage capacitance
Grading rings
Thumb rule
a.c Power frequency Voltages 200 kV (rms)/m
D.C.Voltage 275 kV/m
Impulse voltages 500 kV/m
Switching Surges d= (2V)2
d in meters, V in MV
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.
secondary sub transmission voltage is 11 kV derived from 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
SUBSTATIONSUBSTATIONComponents 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
SUBSTATIONSUBSTATIONComponents of substation contd...
p) Batteries
q) Fencing Arrangement.
r) Fire Extinguishers.
s) Earthing Pit.
SUBSTATIONSUBSTATIONComponents 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.
o) Cables
Power Cables
Controls Cables
Table 7.1.2 - Standard insulation levels for range I (1 kV < Um ≤ 245 kV)
Highest voltagefor equipment
Um
kV(r.m.s. value)
Rated short durationpower frequencywithstand voltage
kV(r.m.s. value)
Ratedlightning impulsewithstand voltage
kV(peak value)
3.6 10 2040
7.2 20 4060
12 28 607595
17.5 38 7595
24 50 95125145
36 70 145170
52 95 25072.5 140 325123 (185)
230450550
145 (185)230275
(450)550650
170 (230)275325
(550)650750
245 (275)(325)360395460
(650)(750)850950
1050
Note - If values in brackets are considered insufficient to prove that the required phase to phase withstand voltages are met, additional phase-to-phase withstand tests are needed.
E l e c t r i c a l c l e a r a n c e s f o r o p e n - t y p e o u t d o o r s w i t c hg e a r o f2 2 k V t o 8 8 k V R a t e d V o l t a g e a n d f o r N o n - e f f e c t i v e ly
E a r t h e d S y s t e m o f 1 1 0 k V a n d a b o v e
I m p u l s ev o l t a g e
w i t h s t a n dl e v e l ( p e a k
v a l u e s )
R a t e dv o l t a g
e
M i n i m u mc l e a r a n c e t o
e a r t h
M i n i m u m c l e a r a n c eb e t w e e n p h a s e s i n a i r o rb e t w e e n c o n n e c t i o n s o f
t h e s a m e p h a s es e p a r a b l e e l e c t r i c a l l y
f r o m e a c h o t h e r3 41 2
i n m m i n m m1 5 0 2 2 1 1 2 7 9 1 3 3 3 02 0 0 3 3 1 5 3 8 1 1 7 4 3 22 5 0 4 4 1 9 4 8 2 2 2 5 5 83 5 0 6 6 2 7 6 8 5 3 1 7 8 64 5 0 8 8 3 4 8 6 3 3 9 9 8 95 5 0 1 1 0 4 2 1 0 6 8 4 8 1 2 1 96 5 0 1 3 2 5 0 1 2 7 0 5 8 1 4 7 37 5 0 1 6 5 5 8 1 4 7 3 6 7 1 7 0 2
1 0 5 0 2 2 0 8 2 2 0 8 2 9 4 2 3 8 8
N o t e : W h e r e t h e m e t h o d o f m o u n t i n g i n s u l a t o r s i s n ot s u c ha s t o p r e v e n t l o d g e m e n t o f b i r d s o r v e r m i n , i t m a y b en e c e s s a r y t o e m p l o y c l e a r a n c e s i n e x c e s s o f t h em i n i m a g i v e n f o r r a t e d v o l t a g e s o f 2 2 k V a n d 3 3 k V .
SUBSTATIONSUBSTATIONClearances 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)
415 V600 V3.3 kV
19.019.0 50.8
6.6 kV
11.0 kV
15.0 kV
22.0 kV
33.0 kV
88.9
127.0
165.1
241.3
355.6
SUBSTATIONSUBSTATIONPost Insulator Minimum Creepage Distances
Power Frequency Withstand Voltage (kV)
Minimum Creepage Distance ( mm)
27355575
140
130230380580
1100185
230
275395
460
680
16001850
2250
34003800
6400
Bus bar clearances in Oil or compound
Rated voltagebetween phases of
poles kV
Minimumclearance to earth
inches
Minimumclearance between
phases inchesUp to andincluding
0.63.36.61122
½½¾1
1 ¾2 ½
½¾1
1 ½2 ½3 ½
ELECTRICAL CLEARANCES IN AIRELECTRICAL 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
GROUND CLEARANCESGROUND CLEARANCES
1) 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
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 &
33 kV lines.
b) The horizontal clearance is to be 4 ft (1.219 m) upto 11 kV &
6 ft (1.820 m) upto 33 kV line.
CLEARANCE BETWEEN POWER AND CLEARANCE BETWEEN POWER AND TELECOM LINESTELECOM LINES
Minimum clearances between, lines of various voltages
LT lines (400 / 230 V) - 4 feet - 1.22 meter
11 kV lines - 6 feet - 1.83 meter
33 / 66 kV lines - 8 feet - 2.44 meter
132 kV lines - 10 feet - 3.05 meter
220 kV lines - 15 feet 4.88 meter
Note:-
Guard rings are to be provided at crossings of telecom lines with power lines upto 33 kV
110kV Substation Layout
c) Line trap mounted on capacitive Voltage transformer
a) Line trap suspended. Capacitor Standing b) Line trap and capacitor suspendedin portal structure
1. Circuit Breaker 2. Feeder Isolator 3 Current Transformer 4. Inductive voltage Transformer 5. Capacitive Voltage Transformer 6. Capacitor 7. Line Trap
110kV Load CentreStation
1. Bus bars 2. Bus bar Isolator 3. Circuit Breaker 4. Current Transformer5. Voltage Transformer 6. Feeder Isolator 7. Lightning Arrester.
220 kV Outdoor switchyard with double bus bars of tubular type
1. Bus bar system I 2. Bus bar System II 3. Bus bar Isolator 4. Circuit Breaker5. Current Transformer 6. Feeder Isolator 7. Line Trap 8 Capacitive Voltage
Transformer
220 kV Outdoor switchyard with double bus bars of tubular type
1. Bus bar system I 2. Bus bar System II 3. Bus bar Isolator 4. Circuit Breaker5. Current Transformer 6. Feeder Isolator 7. Line Trap 8 Capacitive Voltage
Transformer
110kV Outdoor switchyard with double bus bar low rise layout1. Bus bar system I 2. Bus bar system II 3.Bus bar Isolator 4. Circuit Breaker5. Current Transformer 6. Voltage Transformer 7. feeder Isolator 8. Lightning
Arrestor
M in im um E lec tr ica l C lea rance fo r O u tdoo r sw itchgearV o ltagera ting
Im pu lsew ithstand leve l
M in im umc learance to
earth
M in im umclearancebe tw eenphases
M in . c lea rance fromany po in t w here the
m an m ayberequ ired to stand to
the nearestunsc reened
conduc to r in a ir(S ec tiona lc lea rance)
1kV (rm s)
2kV
(P eak )
3m m
4m m
5m m
36 170200
320375
400480
29202970
72 .5 325350
630675
750810
32303270
123 450550
9201150
10651350
35203750
145 550650
11501380
13501600
37503980
245 825900
1050
180019602300
200023002700
440045504900
*420 130014251550
255030703640
340032005200
M i n i m u m E l e c t r i c a l C l e a r a n c e f o r O u t d o o r S w i t c h g e a r
V o l t a g er a t i n g
I m p u l s ew i t h s t a n d
l e v e l
M i n i m u mc l e a r a n c e
t o e a r t h
M i n i m u mc l e a r a n c eb e t w e e np h a s e s
M i n i m u m c l e a r a n c ef r o m a n y p o i n tw h e r e t h e m a n
m a y b e r e q u i r e d t os t a n d t o t h e n e a r e s t
u n s e c r e a n e dc o n d u c t o r i n a i r
( S e c t i o n a lc l e a r a n c e )
1k V
( r m s )
2k V ( P e a k )
3m m
4m m
5m m
3 6 1 7 02 0 0
3 2 03 7 5
4 0 04 8 0
2 9 2 02 9 7 0
7 2 .5 3 2 53 5 0
6 3 06 7 5
7 5 08 1 0
3 2 3 03 2 7 0
1 2 3 4 5 05 5 0
9 2 01 1 5 0
1 0 6 51 3 5 0
3 5 2 03 7 5 0
1 4 5 5 5 06 5 0
1 1 5 01 3 8 0
1 3 5 01 6 0 0
3 7 5 03 9 8 0
2 4 5 8 2 59 0 0
1 0 5 0
1 8 0 01 9 6 02 3 0 0
2 0 0 02 3 0 02 7 0 0
4 4 0 04 5 5 04 9 0 0
* 4 2 0 1 3 0 01 4 2 51 5 5 0
2 5 5 03 0 7 03 6 4 0
3 4 0 03 2 0 05 2 0 0
R e p r o d u c e d f r o m “ I S : 3 0 7 2 - 1 9 7 5 c o d e o f p r a c t i c e f o r i n s t a l l a t i o na n d m a i n t e n a n c e o f s w i t c h g e a r ” .
* R e p r o d u c e d f r o m m a n u a l o n l a y o u t o f s u b s t a t i o n s CB I Pt e c h n i c a l R e p o r t N o . 3 .