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 .