OPERATION & MAINTENANCE OF OF OVER HEADLINES.ppt

OVERHEAD LINES- OPERATION AND MAINTENANCE

1. O&M OF OVER
HEADLINES
R.V.Chalapathi
Senior Faculty
Former Chief Engineer,APSPDCL

2. Operation of over head lines
The declared voltage at the consumer premises
are 415/240 V
Permissible limits of variation in voltage
1.Not greater than 250v +/-6%
2.Medium voltages not greater than 650v +/-6%
3.HV not greater than 33KV +6 to -9%
4.EHV above 33 KV +/-12.5%

3. The voltage regulation is expressed as % drop
w.r.t the receiving end voltage
% Regulation= Es-Er/ Er
REC has standardized the sizes of the
conductor for
33 kv,11kv, andLT lines

4. VoltageSize of conductor Type
33kv
lines
7/3.35mm/50sqmm
7/4.09MM/80sqmm
6/4.72+7/1.57mm(100s.m
ACSR/AAAC
,,
,,
11kv
lines
7/2.11mm/20sqmm
7/2.59mm/30sqmm
7/3.35mm/50sqmm
,,
,,
,,
LT lines7/2.11mm/20sqmm
7/2.59mm/30sqmm
7/3.35mm/50sqmm
7/2.21mm/25sqmm
7/3.10mm/50sqmm
,,
,,
,,
AAC/AAAC
,,

5. Size of the conductor
All lines have inherent resistances, inductances and
capacitances, resulting in voltage drop in the line.
The main basis of the selection of the size of a
distributor is the voltage drop.
The distributor also be capable of carrying the
required current without excessive temp rise
ACSR conductors are preferred to AAAC for long
spans owing to their greater tensile strength
Conductor size to be selected based on current
carrying capacity, tensile strength of the conductor

6. Length of 11 KV line corresponding
to different loads.
Size of
conductor
KVA-KM for
8% voltage
drop.
Length of line
(KM)
.maximum
Load (KVA)
50 sq.mm
ACSR.
RABBIT
10,640 30 355
30 sq.mm
.ACSR.
WEASEL
7200 20 360
20 sq.mm.
ACSR.
SQUIRREL
5120 15 341

7. Length of 415 V line corresponding
to different loads.
Size of the
conductor
KVA-KM for 8%
voltage drop
Length of line
KM.
Maximum.
Load (KVA)
30 sq.mm.
ACSR.Rabbit
11.76 1.6 7.35
20s.mACSR .
WEASEL
7.86 1.6 4.86
13s.m.ACSR 5.58 1.0 5.58
30s.m.AAC
ANT
12.06 1.6 7.54
16s.m.AAC
GNAT
6.96 1.0 6.96

8. Conductors (ACSR)
The Generally used condutctors in over head systems are
Power utility organizations are also procuring all aluminium alloy conductors
equivalent to the ACSR conductors mentioned above
The benefits are i) Higher mechanical strength
ii) Reduction in Line losses
iii) Prevent of theft of conductor
The only disadvantage is costlier than ACSR conductor
Sno Code No. Of
Aluminium
wires
No. Of Steel
Wires
Dia of wire
(mm)
Current
carrying
capacity
1 SQUIRREL 6 1 2.11 107A
2 WEASEL 6 1 2.59 139 A
3 RABBIT 6 1 3.35 193 A
4 RACOON 6 1 4.09 260 A
5 DOG 6 7 4.72 300
6 PANTHOR 30 7 3.00 482 A

9. MEASURES TO BE TAKEN
•Reconfigurationismerechangeoffeed
ofloadsusingexistingnetwork
•Reconductoring isenhancing the
conductorsizeorchangingsingleto
doublecircuit
•Ifnecessarylayingsmallinterlinking
linestofacilitatechangeoverofloads
•Socalledtailendmayturnouttobea
loadnearertosource.

10. Network reconfiguration
Formation of new links within a feeder to form a tree
structure
Erection of interlinking lines to change the area of
feed from one substation to another and balance the
load among the substations.
Bifurcation of existing feeder to form parallel paths
of power flow.
Reconfiguration among feeders is effective only if
there is unequal loading among the feeders of the
network.

11. Reconductoring
•Replacingexistingconductorbyhighersize
orchangeovertodoublecircuit
•Asannualgrowth ratesarehighin
developingcountries,planningtobedone
forfiveyearshorizon
•Maybenecessaryonlyatthebeginningof
linefromemanatingpointandnotthrough
outtheline
•Reinforcingthefirstfewmainsectionswith
conductorofoptimalsize,thelossescan
beminimized.
•Costintensiveandhencecarefulstudyis
needed

12. Operation of overhead lines
The distribution system should be aimed at
reliable supply to the consumers.
The areas affected are to be minimised and
defective section should be isolated, in case
of permanent fault.
This can be achieved by employing
sectionalisers in conjunction with station auto
reclose breakers

13. Sectionalisers : Consists of 3 components
A line switch (S), fault detecting or control relay box
(R), voltage transformer
The switch is mounted on the line and is controlled by
R and it is closed or opened through a solenoid. The
voltage transformer provides power source for S
operation and voltage detection for R.
The sectionaliser opens only when supply fails and
recloses after a preset time of resumption of supply

14. Auto reclosures
The majority of faults occuring on feeders are transient in nature.
It is desirable to get the feeders reclosed automatically for pre-set
number of times at preset intervals.
The low rupturing capacity PMR provides protection against over
current and earth faults.
It has microprocessor controlled circuitry providing the above
protection making use of over current and earth fault electronic
relays.
It is also possible to pre-set the duration of delay in the operation.

15. In order to maintain voltage profile switched
capacitor banks, AVBs are employed and the
location of these is to be optimised to derive max
benefits.
Switched capacitor banks are switched on and off
automatically to put them into service when needed
only and avoid over compensation during light load
periods, by sensing current (load)
AVBs: The taps are automatically selected based on
voltage levels.Voltage can be boost up to 10% in
steps

16. Maintenance of overhead lines
Overhead lines maintenance is required to minimize
interruptions and improve the efficiency of power
supply
They should be inspected periodically to detect any
fault which may lead to B/D or interruption of electric
supply and necessary repairs should be
programmed.
All OH lines should be patrolled periodically at
intervals not exceeding one month at ground level,
while the line is live and is called ground patrol or
pole to pole inspection

17. A special inspection of the lines just before the
monsoon period is to be arranged. This is normally
carried out without supply to enable close inspection of
the line and is called as pre monsoon inspection.
When an OH line trips on a sustained fault, it should
be inspected to find out the nature of fault such as
loose sag, snapping of conductor, tree branches
touching the line conductor, falling on cross arms and
insulator failure or puncture
To avoid these type of faults pre monsoon inspection
is to be arranged and repairs carried out

18. 33KV Lines
Pre-monsoon inspection of all 33kv lines have to be completed
between January and March every year under the supervision of
ADE, duly obtaining the approval for prearranged shut downs for
the entire program.
The tree clearance shall be done, and all the minor defects like
1) damaged insulators, 2)improper pin binding 3)loose jumpering,
4)loose stays shall be rectified during the inspection itself. All the
insulators are to be cleaned. All AB switches shall be lubricated
and defective blades replaced

19. The defects which may take considerable time for
rectification shall be noted down( such as insertion
of poles, replacement of damaged conductor,
damaged supports) and attended within the next
one week.
The staff carrying out the PMI should carry all the
T&P like ropes, petroleum jelly, cotton waste and
sufficient O&M materials like insulators, discs, nuts
for the pins, binding wire etc.

20. Periodical patrolling of 33 kv lines are to be
done monthly by the area LI. Whenever the
line trips on fault, the patrolling shall also be
done and suspected defects rectifi ed by the
area LI.
The 33 kv b/ds are to be attended under the
personal supervision of the ADE.

21. 11 KV LINES
The PMI of 11kv lines are to be carried out under the
personal supervision of the section officer
All the steps taken in 33 kv lines are to be carried
out here also
The periodical patrolling every month and whenever
trips on fault, shall be carried out by the area LM
under the supervision of LI
The 11kv b/d are to be attended under the personal
supervision of the section officer

22. LT lines
The PMI of the LTlines are to be carried out by the area LM, duly
monitored by LI
All the steps mentioned in 33 kv lines to be taken
The LT lines are to be patrolled and defects rectified by the area
LM/ALM/JLM whenever the section fuse/HG fuses blow off
frequently
The area LM/ALM/JLM is responsible for providing spacers,
wherever loose spans are observed.
Rectification of loose stays, insertion of poles in the lengthy
spans etc have to be programmed by the LI/SO and completed
in a limited frame of time

23. Road crossings are to be checked
clearance of at least 1.2 M shall be maintained with
telephone line
Damage to LT conductor such as damage to
strands, black spots shall be checked
Bird nests etc. shall be removed on cross arms
Tree clearence should be done once in a month
Loose spans stringing and providing PVC spacers
should be done

24. Ground patrolling (Pole to Pole)
a) Poles: Leaning of poles, sinking of poles around the pole,
corrosion of metal at ground level (RSJ poles), cracks on the
poles (PSCC poles)
b) Cross arms: Tilting of cross arms, Rusting of cross arms,
observing of any bird nest or creeper on the cross arm
c) Bindings: Looseness and cutting of bindings
d) Conductors: Observing of cut strands, burnt marks and corrosion
etc., Looseness of conductors, Observing of kites, green keepers
on the conductors, Sufficient clearence over roads, rivers,
railways telecommuni-cations etc., Guarding and earth for
conductors are intact

25. Guys: Corrosion of guy rod and stay wire, Guy wire tightness,
and insulator intact, Any creeper on the stay wire, Sleeve
concreting is in order any guy pits sleeving
A.B.Switches: Defect in closing of the A.B.Switch, Missing of the
lock, Damage of earth wire, Dust accumulation on the insulators,
Blades/ Contact burnings
11KV Cable and cable box: Proper supporting of cable and cable
boxes, Damage to insulators and compound leakage from the
box, Intactness of terminal connections with overhead lines and
earthing lead from the cable box
Insulator discs; Insulator, Flash over marks

26. For special inspections (pre-monsoon) and emergency
inspections (B/D), the following points to be checked
in addition to pole to pole inspection
Lightning Arrestors: Damage to porcelain, intactness
of line and earth connections, Any external indication
to show the LAs that have been punctured
Earthing system: Earthing connections of the metal
supports are intact
The patrolling party should write inspection reports and
pass them on to the concerned section officer for
arranging necessary repairs. Side by side with this
inspection, the repairs obviously necessary are also
carried out simultaneously to the extent possible.