Modern distribution systems and its OH lines-its construction
and regulations
INDEX
Introduction to Distribution Sector
.Distribution System
Distribution System Consideration
Voltage regulation
Load characteristics
Reactive Power Control
Distribution Line-Construction Practices
Route Survey & Fixing
Selectionj of Supports
Stringing
IE Rules-Road Crossings, Private Lands, Railway
Crossing, Telephone Crossing, Crossings over buildings etc.
Substation
Distribution Line-regulations for clearance
Distribution Sector-APDRP
Aims & Initiatives
Power Systems in the country have grown both in size and complexity and the growth has been phenomenal in
the last two decades. The size of a single largest generating unit which was 30 MW has now increased to 500
MW. The transmission voltages have increased from 66-132 kv to 400-800 kv.
Thrilling technological advancements have taken place in generation and transmission but the same in
Distribution Sector are not equally spectacular. Still many of the old conventional, manual Systems are
continuing and 'Distribution Automation is at the introduction stage.
The engineers in charge of Distribution deal with comparatively low quantums of power and energy at HT and LT
voltage but have to provide supply to a very large number of consumers with assured power supply quality at
affordable costs.
The investiments in Generation, transmission & Distribution shall be in the ratio of 50:25:25 for homogeneous
development of the power sector as a whole. However for a couple of decades in the past, focus was on
capacity additions. Development of Transmission was just enough to evacuate the power. Commensurate
investments were not made in the Distribution Sector.
Unbundling of SEBs : The erstwhile State Elecftricity Boards are unbundled into functionally distinct entities as
Gencos, transcos and Discoms for generation transmission & Distribution as their names indicate.
The growing demand for electricity in terms of increasing growth rates and high load densities warrant proper
planning to meet increased system capacity, higher distribution voltages, achieve greater control through
sophisicatioc like SCADA and Distribution Automation in order 'to ensure service continuity and reliability and
minimisation of power system losses.
Load management at customer level and methods for ENCON (Energy Conservation) and reduction of losses at
various stages of power flow is a cheaper alternative to cdapacity additions.
The advent of computer aided tools for optimal system planning to achieve maximum LRVI (Loss Reduction and
Voltage Improvement) with least cost so as to obtain high benefit cost ratio and low pay back period shall be
used for prioritising the system improvement works.
APDRP
Accelerated Power Development
& Reforms Programme
Aims & Initiatives
Power sector reforms were initiated in 1991 and the focus was initially on Generation Sector. The
main thrust is now on distribution the real interface between power utilities and millions of their
consumers.
The objectives of reforms are to provide reliable quality power at affordable rates to the consumer
and to improve the commercial viability of the power sector as a whole.
The works identified in distribution sector reforms are system upgradation, reducing losses,
control theft of energy, bring in consumer friendly orientation of the employees and introducing
competitive environment to ensure improved customer satisfaction. It is also envisaged that various
initiatives shall be taken to use Information Technology as a tool to replace human interface wherever
possible. It is decided that at every energy exchange point proper metering shall be done so that energy
accounting and energy audit is possible. Measures towards "Encon" (Energy Conservation) are to be
implemented as a measure to reduce capacity additions needed which is more costly.
The Government.. of India has identified certain short term and long term measures to achieve the
above objectives and has drawn up a clear plan to implement for accelerating the power development
through implementation of reforms.
Areas of high commercial losses are first concentrated as the reduction of these losses can be
achieved with practically no investment and the revenues get boosted up.
Government of India has launched a strategic scheme APDRP with a two-fold objective : to
reduce the commercial gap (initially and eliminate it finally) and improve the quality of power supply.
As per this scheme the Govt. provides 50 percent of the cost of the project (25% as grant and 25% as
loan. The balance 505 will be counterpart funding proposed to be provided from financial institutions
like PFC & REC. For certain states specially identified 100% financial assistance is proposed with 90%
as grant and 10% as loan. A State will become eligible for APDRP if that State Comes forward to enter
into MOU for restructuring SEBs and establish state Electricity Regulatory Commissions enter into a
Tripartite Agreement (TPA) in respect of outstanding dues to central power sector utilities and also sign
a MOA (Memorandum of Agreement) agreeing to take initiatives for 100% consumer metering, energy
accounting & energy audit, reduce gap between costs of power supply and realization of revenue, reduce
T & D losses & financial losses to reach approved benchmark. 35 States have signed MOUs, 19 have
entered into TPAs and 20 have signed MOAs.
The areas where interventions are necessary for taking initiatives to achieve the objectives are also
identified.
Goi Level
:
Legal frame work for distribution reforms, standardization of accounting
practices.
State Level
:
To unbundle state utilities, Rationalization of tariffs, removal of Cross
Subsidies, better management of distributions.
SEBs Level
:
Fix up accountability, Switch over to commercial accounting practices,
English on line information systems, achieve higher customer satisfaction,
implement Grid Code, adopt TOD metering.
Distribution
:
Reduce outages, reduce losses, 11kv feeder to be made a profit cdentre,
implement 100% metering, install capacitors at all levels, computerize
billing, reconfigure lines and DTRs, Meters at DTRs for energy audit. Usage
of IT as a tool to replace human interface wherever possible.
Feeder Level
:
Reduce HT/LT ratio, capacity building exercises, reduce losses and make
feeder as profit centre.
Consumer Levels :
Implementation of DSM measures. Energy conducting Conservation, public
awareness programmes and eradication of theft.
This book contains chapters dealing with distribution System Planning. Standard Construction
Practices, Earthing Practices, Modern Trends in the systems. I.T. initiatives in Distribution Sector.
Model functions and Duties to fix up accountability, and Important Technical Tables frequently required
for reference by Distribution Engineers.
Organisation like NTPC, Power Grid, CPRI are appointed as Advisors cum Consultants to help
Discoms and this book is a compilation of information from different sources to serve as a ready
reference for various activities in Distribution Sector.
DISTRIBUTION SYSTEM CONSIDERATIONS
The design of the distribution systems mainly depends on the chosen classification of single or
three phase, radial or loop network, overhead fine or underground cables.
The essential factors to be kept in mind while designing a distribution system are :
1.
Safety : The safety factor requires the distributors to be laid following
(i) Proper clearances
(ii) Voltage safe enough to be used for consumers gadgets.
2.
Smooth and Even flow of power : A steady, uniform, non-fluctuating flow of power is
necessary to feed loads of all categories of consumers.
3.
Economy : The third factor is economy. this usually calls for use of higher voltages to
ensure minimum losses while distributing power.
Types of Electric Systems :
(i)
AC single-phase two-wire system. It consists of two conductors between which a relatively
constant voltage is maintained, with the load connected between the two conductors.
(ii)
AC Single phase, 3-wire system : It is a combination of two two-wire systems with a single
wire serving as a neutral of each of the two wire systems as shown in figure below.
If the load is balanced in the two (two wire) systems, the common neutral conductor carries no
current and the system acts as a two-wire system at twice me voltage of the component system.
Alternating currents 3 Phase Systems :
The most widely used system is 3 phase 3 wire for HT and 3 phase 4 wide for IT. The voltage of
each phase is 120 degrees out of phase with the voltages of the other two phases. The neutral is g
rounded and single -phase loads are connected between one phase wire and the neutral. 3 phase loads
have each of the separate phases connected to the 3 phase conductors and the neutral.
Power delivered is equal to the sum of the powers in each of the 3 phases. Power loss is equal to
the sum of the I2R looses in all 4 wires.
Three Wire System :
If the load is equally balanced on the three phases of a four-wire system the neutral carries no
current.
Indian Electricity Rules ;
The IE Rules among other things deal with the following important points.
1. Preliminary definitions.
2. Inspection of Electrical Installations.
3. General safety precautions.
4. General conditions relating to supply and use of energy
5. Electric supply lines, systems and apparatus for low and medium voltages.
6. Electric supply lines, systems and apparatus for high and extra high voltages.
7. Over head lines.
Determination of size of conductor for a Distribution System :
11kv feeders carry comparatively bulk power from secondary sub station (33/11kv) to distribution
substation DTRs and distributors carry power from DTRs through service lines which deliver power
from suppliers nearest support to consumer's premises upto energy meter, through a weather proof
service wire.
All lines have inherent resistance, inductances and capacitances, resulting in a voltage drop in the
line. The declared voltage at the consumer premises are 15/240 V. All appliances and motors give good
performance for long duration if this voltages is maintained. In case of large variation in the voltages
(more than + 6% of declared voltage) appliances will get damaged or will give poor performance.
Therefore, regulations are laid down fixing up limits of variation in voltage as permissible at the
consumers terminals. The question regarding the voltage drop in lines, thus, assumes importance and
must be considered while designing the lines.
The main basis of the selection of the size of a distributor is the voltage drop. Obviously, the
distributor must also be capable of carrying the required current without excessive temperature rise.
However, it is found in practice that a distributor is of ample size if its voltage drop is within required
limits. This is also important so as to limit the technical losses to the minimum.
ACSR and AAAC conductors are used for secondary distribution systems. ACSR conductors are
preferred to AAC for long spans owing to their greater tensile strength. In selection of conductor size,
the following should be considered.
1. Current carrying capacity.
2. Tensile strength of the conductor
The size of conductor for a distributor is determined in the following manner.
Based on the load incident on the conductor (including anticipated load growth) the current that
the distributor has to carry is calculated. From standard tables the conductor size, capable of carrying
this current at the ambient temperature of the area is selected. the voltage drop as per example given in
calculated taking products of loads and their distances.
Current carrying capacity of conductors:
Squirrel 7/2.11-115A
Weasel 7/2.59-150 A
Rabbit 7/3.35-208 A
VOLTAGE REGULATION
Declared voltage of supply to consumers
1. Not greater than 250 Volts ± 6%
2. Medium voltage not greater than 650 Volts ± 6%
3. H. V not greater than 33,000 Volts
± 610-9%
4. Extra High Voltage above 33,000 Volts
± 125%
Procedure
The voltage regulation is usually expressed as a percentage drop with reference to the receiving
end voltage.
Percentage regulation= 100 (Es-Er)/ Er
Where as
Es=Sending end voltage
Er=Receiving end voltage
Sample Calculation:
Voltage Drop calculation for 11 KV lines
Let us consider a 11kv feeder emanating from a 33 11kv S/S with 7/2.59mm ACSR for the main
feeder and 7/2.11mm ACSR for tap lines with the connected distribution transformers and distances as
indicated below:
Total connected transformer capacity on the 11kv line is:
1) 10x100 =
1000 KVA
2) 6x63
=
378 KVA
3) 5x25
=
125 KVA
Total 1503 KVA
For calculating the voltage regulation of the main feeder, it is assumed that the loads on the tap
lines are concentrated at the point of tapping and taking moments about the section we have
OA
=
150x1
=
1503.0
AB
=
1403x1.5
=
2104.5
BH
=
1215x1.5
=
1822.5
HC
=
1115x0.5
=
557.5
CI
=
927x1
=
927.5
IO
=
764x1
=
764.0
OR
=
601x2
=
1202.0
RE
=
413x2
=
826.0
EF
=
288x2.5
=
720.0
FG
=
100x1
=
100.0
Total KVA KM
10526.5
% Regulation =Total KVA KM x Regulation constant per 100 KVA KM / 100 x DF
Assuming a diversity factor of 2.5 Regulation constant for 7/2.59 ACSR at 0.8 Power factor is
0.08648 (See Table 5)
% Regulation= 10526.5 x 0.08648/100 x 2.5=3.64%
Similarly the regulation of the tap lines also can be calculated.
Let us consider the farthest tap lines and find out the regulation taking moments in KVA KM for
the main feeder with 7/2.59 mm ACSR; we have
1503 x 1
=
1503.0
1403 x 1.5
=
2104.5
1215 x 1.5
1822.5
1115 x 0.5
=
557.5
927 x 1
=
927.5
764 x 1
=
764.0
601 x 2
=
1202.0
413 x 2
=
826.0
288 x 2.5
=
720.0
Total KVA KM
10,2426.5
For tap line FM with 7/2.11 mm ACSR we have
1.
188 x 1.0
=
188
2.
88 x 0.5
=
44
3.
25 x 0.5
=
12.5
Total KVA KM
244.5
2) % Regulation on 11kv main feeder = 10426.5 x 0.08648 / (100 x 2.5) = 3.61
3) % Regulation 11kv tap line = 244.5 x 0.12115 / 100 x 2.5 = 0.118 %
% Regulation at the point M of tap line = 3.61 + 0.118 = 3.728%
THE LOAD CHARACTERISTICS
Connected Loads :
The aggregate capacity of all electric devices (lamps, applicances, equipments etc. connected by a
consumer to the supply system is called connected load.
Maximum demand :
The total of loads used at a time energy and this various from hour to hour and the maximum that will
be incident some point of time is called the instantaneous maximum demand. However, the maximum
demand in a continuous period of 15 or 30 minutes is taken as "Maximum Demand" for billing purpose
where MD is a billing parameter.
Demand factor :
The ratio of the maximum demand to the total connected load is called the demand factor.
Load factor :
The load factor is the ratio of the average load or demand for a period of time to the maximum demand
during that period.
Diversity :
Load diversity is the difference between the sum of the maximum demands of two or more individual
consumers' loads and the maximum demand of the combined loads.
Diversity factor :
The diversity factor is the ratio of the sum of maximum demands of each of the component loads to the
maximum demand of the load as a whole. This is the most important factor for economical planning and
design of distribution facilities.
Coincidence factor :
The coincidence factor is the ratio of maximum coincident total demand of a group of consumers to the
sum of the maximum demands of each of the consumers.
Utilisation factor :
The ratio of the maximum demand of a system to the rated capacity of the system is known as the
utilisation factor. The factor indicates the degree to which a system is being loaded during the load peak
with respect to its capacity. The rated capacity of a system is usually determined by thermal rating
coupled with the voltage drop.
Power factor :
The ratio of power (in watts) to the product of voltage and current (in volt amperes) is called the power
factor. It is a measure of the relation between current and voltage out of phase with each other brought
about by reactance in the circuit (including the device served). Infrastructure must be designed to carry
the full current and the losses vary with square of the current. Voltage drop is proportional to the current.
Sizes of Transformer, conductor cable, fuses, switches etc., are all based on values of current. Each type
of loads operate at different power. For example incandescent lamps and heaters work at unity power
factor where as induction motors run at about 0.7 power factor.
Consumer classification : Consumers are classified into different categories such as Residential,
Commercial, Industrial and Agriculture. As a parameter for planning, load sensitive can be expressed in
KVA per square kilometer or MW/Sc.Km.
REACTIVE COMPENSATION FOR DISTRIBUTION FEEDRS
Introduction :
An unloaded transmission or distribution line is capacitive in character and a fully loaded line has
inherent inductive and resistive characteristics. On main transmission lines operating at near unity
power factor, the power which can be transmitted is determined largely by the system stability limit in
which the phase angle between the voltage at the sending and receiving ends does not exceed a certain
critical value. On distribution system, especially those operating on low power factors, the inductive
reactance makes a major contribution to the voltage drop, and it is the voltage drop consideration which
frequency dictate the amount of power which can be distributed.
The effect of inductance in power network is to cause :
q
A phase shift between the different paths of the network tending towards instability at
undesirably low power level.
q
Excessive voltage drops at tail ends of feeders.
q
Unequal sharing of the load betwen parallel feeders, thus limiting the total power which can
be permitted.
Overall improvements in operating condition will be brought about by means of reducing the
system reactance or by reducing the phase angle between the system current and voltage, In distribution
system this can be done by installing static equipment namely :
* Shunt Capacitors.
*
Series Capacitors
CHOICE OF SERIES OF SHUNT CPACTIROS :
1.
With series capacitors the regulation or reductioning voltage drop achieved depends mainly
on reactive power of the load and this type of capacitor is of little use unless the conditions
are such that reactive power is consumed by the load.
2.
With shunt capacitors the regulator achieved depends mainly on the reactance of the system,
and a useful increase in voltage will be obtained only if the reactive power is substantial.
3.
If continuous and automatic voltage regulation is the main objective a series capacitor
installation is more likely to satisfy the need.
4.
A shunt capacitor bank with means of automatically increasing or decreasing the number of
sections of the bank in service, will provide a measure of regulation albert in steps and with
some delay.
5.
A capacitor connected in series with a line must have a current rating equivalent to that of
the line, the output of reactive power from the capacitor being dependent upon the line
current.
Output = I x C = KVAR/Phase
6. With a shunt connected capacitor the output of the bank is independent of the current
compensation being determined by the applied voltage.
Table below gives direct multiplication factors to obtain reactive power compensation required
for various power factors.
Compensation to Cos
Cos
1.0
0.95
0.90
0.85
0.80
0.20
4.90
4.75
4.42
4.28
4.15
0.22
4.43
4.10
3.95
3.81
3.68
0.24
4.04
43.72
3.56
3.43
3.30
0.26
3.71
3.39
3.23
3.09
2.06
0.28
3.43
3.10
2.94
2.81
2.68
0.30
3.18
2.85
2.70
2.56
2.43
0.32
2.96
2.63
2.48
2.34
2.21
0.34
2.00
2.44
2.28
2.15
2.02
0.36
2.59
2.26
2.11
1.97
1.84
0.38
2.13
2.11
1.95
1.81
1.68
0.40
2.20
1.96
1.81
1.67
1.54
0.42
2.16
1.83
1.68
1.54
1.41
0.44
2.04
1.71
1.56
1.42
1.29
0.46
1.93
1.60
1.45
1.31
1.18
0.48
1.13
1.0
1.34
1.21
1.09
0.50
1.23
.40
1.25
1.11
0.98
0.52
1.04
1.31
1.16
1.02
0.89
0.52
1.56
1.23
1.07
0.94
0.81
0.56
1.48
1.15
1.00
0.86
0.73
0.51
1.41
1.08
0.92
0.79
0.65
0.61
1.33
1.00
0.78
0.65
0.52
0.62
1.20
0.87
0.72
0.78
0.45
0.61
1.4
0.81
0.65
0.52
0.39
0.62
1.27
0.94
0.78
0.65
0.52
0.62
1.20
0.87
0.72
0.58
0.45
0.63
1.14
0.81
0.65
0.52
0.39
0.64
.08
0.75
0.59
0.46
0.33
0.71
1.20
0.69
0.54
0.40
0.27
0.72
1.96
0.64
0.48
0.34
0.21
0.73
0.91
0.58
0.43
0.29
0.16
0.74
0.86
0.53
0.37
0.24
0.10
0.75
0.80
0.47
0.32
0.18
0.05
0.00
0.75
0.42
0.27
0.13
-
0.00
0.70
0.37
0.21
0.08
-
0.00
0.65
0.32
0.16
0.03
-
0.00
0.59
0.27
0.11
-
-
0.00
0.54
0.21
0.06
-
-
0.00
0.48
0.16
-
-
-
0.00
0.43
0.10
-
-
-
0.00
0.36
0.03
-
-
-
0.00
0.29
-
-
-
-
0.00
0.29
-
-
-
-
0.00
0.20
-
-
-
-
DISTRIBUTION LINES CONSTRUCTION PRACTICES
DISTRIBUTION LINES - CONSTRUCTION
Introduction :
Before construction of any new sub-transmission and distribution lines is taken up, one should get
acquaint himself with Indian Electricity Rules. 1956 as amended from time to time. As per Rule No. 29,
all electric lines and apparatus shall be sufficient in size and of sufficient mechanical strength for the
work they may be required to do, and shall be constructed, installed, protected, worked and maintained
in such a manner as to prevent break down in supply or accidents.
Sub-transmission and Distribution Line Voltages :
REC has standardized the following voltages
(1)240-415 volts
Low and Medium Voltages
(2)11,000 volts
Primary Feeders
(3)33,000 voplts
Sub-transmission Lines.
Survey of Lines :
The area is to be surveyed to determine the route along which the line is to be laid. The survey of
the line should be carried out accurately. The proposed route of the line should be the shortest
practicable distance. The first thing to do is to select the route and plot it on a map. The second step is to
conduct a walk out survey to determine the topography of the area and identify places where right of
way is difficult. This is followed by a detailed survey from which the quantity of material required for
the construction of line is determined.
Choice of Route :
The proposed route shall be of the shortest practicable distance. However, attention should be kept
on the possibility of taking the line as close as possible to the roads for easy approach and maintenance
during emergency. The following locations should be avoided as far as possible.
(a) Rough and difficult countryside
(b) Urban where right of way is a problem areas.
(c) Restricted access for transport vehicles to move
(d) Abrupt changes in the line route.
(e) Difficult crossing like Rivers, Railway Crossing etc.
(f) Proximity to aerodromes
(g) Prone to natural hazards like proximity of Nalls, tanks etc.
Survey for Low and Medium Voltage Lines :
The low and medium voltage lines are directly tapped for consumer services in towns, cities and
villages. As such they are confined within the limits of the towns and cities and within small distances
say 2 to 3 KM. in rural areas.
The Steps in survey work are :
1.
To locate the position of agricultural consumers and mark the route of the distribution
line.
2.
In case of villages mark the position of the distribution line along the lanes of the
villages.
3.
In case of cities and towns, the position of the distribution lines are marked along with
streets.
After marking the route of the line on the map as above, the lines and poles are plotted to scale on
the map by carrying out the walk-in-survey of the route, i.e. the survey party walks along the route of the
line as marked on the map and finalises the position of the poles by making physical measurements
using survey charts or tapes. Also obstructions to right of way are to be eliminated.
Lines in Rural Areas :
For conducting a survey of distribution lines in villages, a scaled map of the village is taken and
following are marked on the map.
1. The nearest HT lines from which the tapping will be taken for the distribution transformer.
2. The location of distribution transformer after taking into consideration the distribution of
load.
3. The layout of the LT lines network with locations of angle poles and road crossings.
Lines in Urban Areas :
The power lines should be pointed on the map of city of town after a consultation with the
municipal authorities. Proposed road widening works in the near future are to be kept in mind.
The following should be taken into consideration.
1. Road crossings.
2. Private lands
3. Railway crossings
4. Telephone line crossings
5. Crossing over buildings.
Agricultural Field Area :
While surveying for laying power lines for agricultural loads it should be borne in mind that the
transformer should be at the load centre; so as to reduce the length of LT lines and consequently voltage
drop.
Survey for HT Lines (11 & 33 KV) :
The survey of HT lines is done in two stages viz. (a) preliminary survey and (b) detailed survey.
the steps to be followed for the preliminary survey of the route of HT lines is the same as described for
the LT distribution lines. Having marked the position of the line on the map, on the basis of the walk in
survey, a detailed survey is carried out to mark the exact position of the poles.
Right of Way :
Before taking up the erection of lines both HT/LT along the roads the concerned authorities should be
contracted and approval obtained for location of all poles, crossings, tree cuttings, or trimming and guy
locations. In some cases, it may be necessary-also to contact local town planning authorities for
approval if the lines are to be drawn in the urban areas. Telephone line crossings etc are to be planned
and it may be better to carry out the survey in consultation with telephone authorities.
Tree Clearances :
The following minimum clearances may be adhered to
Voltage of line
Tree Clearance Required
Relaxation
1. Low & Medium Voltages
240 & 415 Volts
The growth of the branches of
trees and the height up to which
each tree can grow are to be
kept in mind
Nil
2. 11KV (Normal)
All growth within 4.572 MTt.
(15 Ft.) on wither side from
the center line of the support
and all trees which may fall
and foul with the lines
In case of betel leaf
garden all growth
within 3.048 Mt
(10 Ft.) on either
side of the line.
3. 11 KV/33 KV Trunk lines
All growth within 6.096 Mt
(20 Ft.) from the centre line of
support and all trees which
may fall and foul with the lines
Nil
Railway & Road Crossings :
The railway and road crossing should be as minimum as possible. The crossings of the railway line
should be done beyond the outer signal of the nearest railway station and the structures and clearances
above the railway track should conform to the latest railway regulations in this regard.
Crossing of Telecommunication Line by Power Lines :
Approval of state level PTCC has to be obtained before taking up the construction work.
Construction, Testing and commissioning of 11KV & LT Lines
1. Construction of 11KV & LT overhead lines may be divided into the following parts.
2. Erection of supports.
3. Providing guys to supports
4. Mounting cross arms, pins and insulators
5. Stringing line conductor.
6. Jointing of conductors
7. Sagging and tensioning of conductors
8. Earthing
9. Testing and commissioning REC has standardised the following sizes and types of supports for
11 KV and LT lines.
REC has standardized the following sizes and types of supports for 11 KV and LT lines.
Type
Length
Voltage
Max. Span
Type of
(Mts)
With or without
earth wire
PSCC
8.0 Mt.
11 KV
107
Vertical
Without earth wire
PSCC
8.0 Mt.
415/240V
67
Vertical
- do -
Alignment of the Line ;
A detailed route survey for the line has to be made and alignment of the line should be done before
excavation of pits.
Erection of Supports :
After the final survey of the line and after marking of the pole locations with per excavation work
has to be commenced. The excavation of pits for the supports in the direction of the line will facilitate
easy erection of supports in addition to giving lateral stability. The depth of the foundation shall be
1500mm.
After the excavation of pits is completed, the supports / poles are to be transported and erected.
For erection of support we may use, either X-shaped structure made of two wooden cross arms or Bipod.
Before the pole is placed into the pit, suitable base plate shall be fixed at the bottom of the pole to
increase contact between the pole and soil. The padding will distribute the density of the pressure due to
weight of the pole on the soil.
Having lifted the pole, the same should be kept in a vertical position with the help of manila rope
of 20/25 mm dia using the rope as temporary anchor.
As the poles are being erected, say from one angle point to next angle point, the alignment of the
poles are to be checked and set right by visual check. The verticalities of the poles are to be checked
with a spirit level on both transverse and longitudinal directions. Having satisfied that the verticalities
and alignment are all-right, earth filling is to be done.
After the poles have been set, the temporary anchors are to be removed.
Erection of Double Pole Structure for Angle Locations:
Forengles of deviations more that 10o, double pole structures may be erected. The poles are to be
excavated along the bisection of the angle of deviation. The concreting of the poles shall be done with
cement, granite chips of size 2030 mm and sand in the ration of 1:3:6. Before lifting the pole in the pit
concern padding of not less than 75 mm thickness shall be put for the distribution of the road of the
support on the soil. After concreting is done, the pit shall be filled with earth after curing of the concrete
is completed.
Stays are to be provided for the support with 7/3.15 mm or 7/2.5 mm GI stay wire and 16 mm stay rods.
Four stays along the line, two in each direction and two stays along the bisections of the angle of
deviation, or as required depending on the angle of deviation are to be provided. Stay concreting may be
done with 1:3:6 concrete mixture.Special care has to be taken where black cotton soil is encountered.
If such locations, mass concrete foundations are to be adopted to avoid collapse of foundations in
black cotton soil.
Anchoring and Providing Guys for supports :
Normally the guys are provided to the supports at the following places :
1. Angle locations
2. Dead and locations
3. Tee-off points
4. Steep gradients location to avoid uplift on the pole.
Sometimes storm guys are provided along the straight run at equal intervals where wind pressure
is more than 50kg/M2.
The installation of guy will involve the following works.
1. Excavation of pit and fixing of stay rod
2. Fastening of guy wire to the support
3. Tightening guy wire and fastening to the anchor.
The marking of the guy pit for excavation, the excavation of pits and setting of the anchor rod
must be carefully carried out. The stay rod should be placed in a position such that the angle of
inclination of the rod with the vertical facr of the pit is 35o/45o as the case may be. The concreting of the
stays at the bottom should then be carried out. The back filling and ramming must be well done
thereafter and allowed to set for at least 7 days. The free end of the guy wire is passed through the eyes
of the anchor rod, bent back parallel to the main portion of the guy and bound after inserting the GI
thimble. The loop is projected by the GI thimble where it bears on the anchor rod where the existence of
the guy wire proves hazardous, it should be protected with a suitable asbestos pipe, fitted with concrete
about 2 Mt. length above the ground level, duly painted with white and black stripes so that it may be
visible at night. The turn buckle shall be mounted at the pole end of the stay and guy wire so fixed that
the turn buckle is half way in the working position, thus giving the maximum movement for tightening
or loosening.
Guy insulators are placed to prevent the lower part of the guy from becoming electrically
energised by a contact of the upper part of the guy when the conductor snaps and tail on them or due to
leakage. Guy insulator shall be located at 2/3 of its length from the ground. The minimum breaking load
of guy insulator shall be 9900 Kv. for 11 KV and 4000 kg for LT lines.
Fixing of cross Arms and Insulators :
After the erection of supports and providing guys the next step would be to amount the cross arms on the
support. The practice of fixing of cross arm before the pole is erected is followed in some states. In case
the cross arm is mounted after the support is erected, the lineman should climb the support having
requisite tools with him. The cross arm is then tied to a hand line and pulled up by the ground man
through a pully till the cross arm reaches the lineman. The ground man should station himself well to
one side so that if any material
drops from the top of the pole it may not strike him. All the materials required should be lifted or
lowered by means of the hand line. In no case the material or the tools should be dropped or thrown
from the pole top.
Insulators :
The pin insulators are fixed in the holes provided in the cross arms and the pole top brackets. The
insulators are mounted in their places over the pins and tightened In the case of strain or angle supports,
where strain fittings are placed over the cross arm. The nut of the straps is so tightened that the strap can
move freely in horizontal directions, as this is necessary to fix the strain insulator.
Stringing of the Line Conductor :
REC has standardized the following sizes of conductors for 33 KV, 11 KV and LT lines.
Voltage Class
33 KV lines
11 KV lines
LT line
No. and Diameter of Wire
Type of Conductor
i)
7/3.35 mm (50 mm2)
ACSR (AAAC)
ii)
7/.09 mm (80 mm2)
ACSR (AAAC)
iii)
6/4.72 mm + 7/1.57 mm(100 mm2)
ACSR (AAAC)
i)
7/2.11 mm (20 mm)
ACSR (AAAC)
ii)
7/2.59 mm (30 mm)
ACSR (AAAC)
iii)
7/3.35 mm (50 mm)
ACSR (AAAC)
i)
7/2.11 mm (20 mm)
ACSR (AAAC)
ii)
7/2.59 mm (30 mm)
ACSR (AAAC)
iii)
7/3.35 mm (50 mm2)
ACSR (AAAC)
iv)
7/2.21 mm (25 mm2) Gnat
AAC (AAAC)
v)
7/3.10 mm (50 mm2) Ant
AAC (AAAC)
Conductor Erection :
The erection of overhead line conductor is very important phase in construction.
The erection of conductor can be divided into 4 parts.
2. Paving and stringing of conductors
3. Tensioning and sagging of conductors and
4. Jointing of conductors
At important crossings of roads, canals navigable rivers, railways etc. flagmen should be in attendance
to ensure that normal services are not unduly interrupted. these crossings should only be carried out in
conjunction with and with the approval of the proper authorities concerned.
Having transported the conductor drum to the tension point, the drum should either be mounted on
the cable drum supports or jacks or hung by means of chain pully of suitable capacity suspended from a
tripond. The conductor should be passed over the pole or wooden or aluminum snatched pully blocks
provided with low friction bearings. The conductor shall be raised to a minimum height of 5 mt. above
ground by rough sagging.
The mid span joints of conductors can be carried out by twisting joint or compression joints. The
twisting joints for aluminum conductors and ACSR consist of relatively thin walled aluminum sleeves,
the end of the wire should project a few centimeters beyond the end of the sleevees. The projected wires
are given a sharp bend to keep them from slipping out of the sleeve. Twisting Tongues are preferably to
be used for joints and jumpers.
Sagging and Tensioning :
On completion of the paving of conductors and making mid span joints if any, tensioning
operation will commence. Temporary guys will have to be provided for both anchoring supports. At the
tensioning end one of the conductors is pulled mannually up to a certain point and then come-along
clamp is attached to double sheave pully block or the pull lift (TIRFOR) machine and gradually
tensioned.
The conductor should then be sagged in accordance with the sag temperature chart for a particular
conductor and span. The sag should then be adjusted in the middle span of the section.
The stretch of the conductor has to be taken out before strining in order to avoid the gradual
increase in sag due to setting down of the individual wires. there are two ways of accomplishing this
1. Prestressing
2. Over Tensioning
1.
Prestressing : In this method, the conductor is pulled upto a tension considerably above the EL
correct figure, but never exceeding fifty percent of breaking load for a short period of say 20
minutes. As this method requires more time and involves the use of stronger tackle to secure high
tension, the other method of over -tensioning is commonly adopted.
2.
Over-tensioning: The method consists of pulling up the conductor to a tension a little above the
theoretical tension for the prevailing temperature and fixed it up at that tension with a
correspondingly reduced sag. After a certain time the conductor will settle down to the correct sag
and tension. A tension of 5% to 8% more than the theoretical value has been found to be suitable
for the sizes of ACSR and AAC conductors. The ambient temperature during the sagging may
recorded correctly.
Conductors can be sagged correctly only when the tension is the same in each span throughout the entire
length of the section. Use of snatch blocks reduces the friction and chances of inequality of tension in
various spans.
Sagging can be accomplished by different methods, but most commonly used method for the
11KV line is "SIGHTING" by use of targets placed on the supports below the cross arms. The targets
are light strips of wood clamped to the pole at a distance equal to the sag below the conductor when the
conductor is placed in snatch blocks. The lineman sees the sag from the next pole. The tension of the
conductor is then reduced or increased ; until the lowest part of the conductor in the span coincides with
the lineman's line of sight.
When sagging is completed, the tension clamps shall be fixed. The clamps can be fitted on the
conductor without realising the tension. A mark is made or the conductor at a distance from the cross
arm equal to the length of complete strain insulators. Before the insulator set is raised to position all nuts
should be free. A come along clamp is placed on the conductor beyond the conductor clamp and
attached to the pulling unit. The conductor is pulled in sufficiently to allow the insulator assembly to be
fitted to the clamp. after the conductor is clamped to the insulator assembly unit may be released
gradually. After the stringing is completed all poles, cross arms, insulators, fittings etc are checked up to
ensure that there have been no deformities.
The next step is to place the conductor on the top of the pin insalator from the snatch block and
removing snatch blocks. Conductors are then fastened to insulator by the use of aluminum wires.
Before tying the conductor to the insulator, two layers of aluminium taps should be wrapped over
the conductor in the portion where it touches the insulator.
Normally in straight lines the conductros are run on the top of the insulators. When there is a small
angle of deviation the conductor is placed inside groove and binded.
Earthing of Distribution Lines :
1.
Metal Supports - All metallic supports shall be earthed.
2.
RSC/PCC poles - The metal cross arms and insulator pins shall be bonded together and
earthed at every pole for HT lines and a every 5th pole at for LT lines.
3.
AM special structures on which switches transformers, fuse etc. are mounted should be
earthed.
4.
The supports on either side of a road railway or river crossing span should be earthed.
5.
All supports (metal, PSCC) of both HT & LT lines passing through inhabited areas; road
crossings and along such other places, where earthing of all poles is considered desirable
from safety considerations should be earthed.
In special locations, railways, telegraph line crossings special structures etc. pipe/rod type earthing
should be done.
In other locations, coil earthing may be done.
The earth resistance should be allow as possible and should not exceed 10 Ohms.
Anticlimibing Devices :
In order to prevent unauthorised persons from climbing supports of HT and LT lines suitable
anticlimbing devices should be provided to the supports (such as barbed wires)
Testing and Commissioning :
When the line is ready for energisation, it should be completely inspected, and after visual
inspection, the conductor is tested for community / ground by means of a megger. At the time of testing
by megger, no person should climb on the pole or touch the conductor, guarding or guy wire etc.
Before charging any new line, it should be ensured that the required inspection fees for the new
line is paid to the Electrical Inspector and approval obtained from him for changing the line.
The line should be energised in the presence of the authorized officer.
Before energising any new line, the officer incharge shall notify to the workmen that the line is
being energised and that it will be no longer safe to work on line. Acknowledgment of all workmen in
writing should be taken in token of having intimated them.
Vide publicity should be arranged in all the localities through which the line (to be energised)
passes intimating the time and date of energising.
The officer-in-charge of the line shall personally satisfy himself that the same is in fit state to be
energised.
Pole Schedule of Overhead Lines
(Bill of Materials)
Before the construction of 11 KV line and LT lines is taken up, the field Engineer is required to
prepare a detailed list of material for completion of the job.
Bill of material for 1KM of the 11 KV line and 1 KM of LT line are given below as a guide.
Bill of Materials for 1 KM of 11 Lines (100 Mt. Span)
Sl. No.
Description of Material
Quantity
1.
PSCC poles (8 Mt. Long)
11 Nos.
2.
ACSR 20 M2 size
3.1 KM
3.
11 KV Pin insulator with pins
27 Nos.
4.
11 KV Disc insulators
6 Nos.
5.
11 KV Strain insulators with hardwarw
6 Nos.
6.
Jointing Services for ACSR
3 Nos.
7.
PG clamps for 20 Sq. mm ACSR Conductor
6 Nos.
8.
Danger Plates
11 Nos.
9.
Earthing Sets
3 Nos.
10.
Guy sets (HT)
6 Nos.
11.
G.I. wire 7/8 SWG
40 Kgs.
12.
M.S. Flat 50 x 10 mm
40 Kgs.
13.
M.S. Channel 75 x 40 x 6 mm
140 Kgs.
14.
Binding wire for HT
1 Kg.
15.
Binding Tape for HT
1 Kg.
16.
G.I. wire 8 SWG
13 Kg.
17.
Nuts & Bolts of required dizes
25 Kg.
18.
Barbed Wire
22 Kg.
19.
Aluminum paint
2 Lts.
20.
Cross arms one for each plle
11 Nos.
21.
Top jumpers for intermediate poles
9 Nos.
Bill of Materials for 1 KM LT Lines 3 Phase (Span 53 m approx)
Sl. No.
Description of Material
Quantity
1.
PSCC poles 8 (Mt. Long)
20 Nos.
2.
ACSR Conductor 30 mm2 A1.
3.15 Km.
3.
ACSR Conductor
1.05 Km.
4.
LT Shackle insulator with hardware
80 No.s
5.
Jointing services
6 Nos.
6.
PG Clamps
6 Nos.
7.
Earthing set, LT
5 sets.
8.
Complete guy set
8 sets
9.
Guy wire 7/10 SWG
40 Kg.
10.
MS Flat 50 x 60 mm
35 Kg.
11.
Al. Binding Wire
2 Kg.
12.
Al. Binding Wire
2 Kg.
13.
GI Wire 8 SWG
12 Kg.
14.
Nuts & Bolts of required sizes
40 Kgs.
15.
Aluminium paint
2 Lts.
DISTRIBUTION OVER HEAD LINES - CROSSINGS - RAILWAYS /
RIVERS / P&T LINES / ROADS - REGULATIONS.
Railway Crossings :
Before commencing work on any crossing, approval of the Railway and the Electrical Inspector in
writing shall be obtained for the purpose location and detailed design of the crossing. For this propose,
data, designs and drawings relating to the crossing shall be submitted in duplicate by the owner to the
Railway and the Electrical Inspector.
Classification of Crossings :
For the purpose of these regulations, electric overhead lines crossings are classified in accordance
with the clearances required under the following categories.
1. Category 'A'
:
Tracks electrified.
2. Category 'B'
:
Tracks already electrified or likely to be converted to or electrified
on 25 KV AC system within the foreseeable future.
3. Category 'C'
:
Tracks not likely to be electrified in the foreseeable future.
2.
The crossing shall be constructed in accordance with the approved design and drawings; with good
materials and workmanship.
3.
The owner shall notify his intention to bring the crossing into use at least 15 days in advance to
railways.
4.
A certificate shall be submitted to the Railways indicating that the works have been constructed in
full compliance with the regulations and in conformity with design / drawings approved by the
Railways.
5.
Methods of Crossings, overhead or under-ground :
(a)
For tracks already electrified or to be electrified in the foreseeable future (Categories A & B)
all low and medium voltage crossings (upto 650V) shall be by means of under ground
cables, for high voltage crossings above 650V upto 11KV, the use of under ground cable is
recommended. For higher voltages crossings may be overhead or underground as preferred
by the owner.
(b)
For non-electrified tracks, (Category C), the crossing may be either over head or under
ground as preferred by the owner. For low and medium voltage (upto 650V) underground
crossings are recommended.
6.
No work involving removal, alteration or maintenance of any crossings shall be under taken
without obtaining the consent in writing from Railways.
7.
Each installation shall be inspected by the owner periodically to determine ins fitness for service.
Defects noticed or pointed out by the Railways shall be rectified by the owner as soon as possible. The
crossing shall be maintained so as to reduce hazards to life and property.
8.
The crossing spans and two adjoining spans shall be kept free by the owner as far as practicable
from over hangings, or decayed trees which might fall on the spans.
9.
If steel poles or fabricated steel structures are not galvanised, they shall be painted periodically:
10.
Earthing arrangement shall be inspected and tested annually. Where earth resistance is higher than
10 Ohm, the owner shall state necessary steps to improve the earth resistance. (i.e. lowering the
values to within the limits.
11.
If shifting of, or modifications to, or dismantling of the crossings are required for the proper
functioning of Railway, such works shall be carried out by the owner expeditiously and to the
satisfaction of Railways.
The cost of such works shall be borne by the Railways except in those works where the need for
such works on account of Railways anticipated developments was foreseen and the owner had
agreed in writing prior to the construction of the crossing to meet the cost of such works."
12.
Normally an overhead crossing shall be effected at right angles to the railway track and a deviation
to the extent of 30o may be permitted.
13.
Structures:
(i)
The minimum distance of the supporting structures for the crossing span from the centre of
the nearest Railway track shall normally by the height of the structure above ground level
plus 3 meters.
(ii)
Steel poles, fabricated steel structures, or reinforced, or pre stressed concrete poles either of
the self supporting or guyed type conforming to the latest IE Rules shall be used on either
side of the track at the crossing span steel structures shall preferably be galvanised.
(iii) The crossing span shall not be more than 80% of the normal span for which the structures
are designed.
(iv) Wind pressure: Maximum wind pressure on the structure shall be determined from the
recommended maximum wind pressure for the area.
14.
The factor of safety of all conductors, guards, guys and ground wire in the crossing shall be in
accordance with IE Rules 1956.
15.
No conductor in the crossing span having a breaking strength less that 560 Kgs. shall be used.
16.
All Guard Wires shall be of galvanised steel of not less than 6 SWG for bearer wires and 10 SWG
for cross wires.
17.
All gue wires shall be galvanised steel and shall not be less than 7/41 SWG.18. No jointing shall
be permitted in the crossing span.
19.
Minimum clearance between the over head lines and Railway Tracks:
(i)
Crossing over tracks already electrified shall normally be located remind spans of the
overhead traction conductors, but in any case scall not be less than 6 meters from the nearest
traction mast.
(ii)
The minimum height above rail level of the lowest portion of any conductor under
conditions of maximum sag shall be as follows :
(i) For category A & C :
Voltage
Broad Gauge
Inside Station
limits
Upto and
including 11KV
Above 11 KV
including 33 KV
Metre / Narrow Gauge
Outside Station
limits
Inside Station
limits
Outside Station
limits
10.0m
7.6m
8.6m
6.3m
10.0m
7.6m
8.8m
6.4m
Note : Low and Medium voltage (upto 650V) crossing of category A only will be obligatory
by means of underground cables, (m-metre)
(ii) For Category B :
Voltage
Upto 650 Volts
Above 650 volts upto 33 KV
For Broad, Meter and Narrow Cauges
Inside Station limits
Only by Cable
Outside Station limits
Only by Cable
12.5m
10.5
Note : If the crossing is located on a metre gauge or a narrow gauge section, likely to be converted
to broad gauge, clearance applicable to broad gauge shall be adopted.
20.
Minimum clearance between conductros and any Railway Structure :
The minimum vertical and horizontal clearances between any conductros and any Railway
buildings and structures other than traction supports and overhead equipment under the most
adverse conditions shall be in accordance with Rule 80 of IE Rules, 1956.
21.
Minimum clearance between owners and Railway conductor :
The minimum clearance between any of the owner's conductor or guard wiress and the railway
conductor shall not be less than 2 metres.
22.
Insulators :
Categories A & B : double set of strain insulator strings shall be used in the crossing span in
conjunction with a yoke plate where necessary. It is recommended that each string of such strain
insulator shall have one insulator disc-more than that used in normal span of the overhead line.
Categories A & B : Double set of strain insulator strings shall be used in the crossing span in
conjunction with a yoke plate where necessary. It is recommended that each string of such strain
insulator shall have one insulator disc-more than that used in normal span of the overhead line.
Category 'C' : Strain insulatos, suspension insulatos or pin insulators may used as required.
BGuarding : The minimum height above the rail level to the lowest level of any cradle guard and
guard wires under conditions of maximum sag shall be as follows :
Category 'A & B' : The same clearances as given in clause 19(1) shall apply.
Category 'C' : The minimum height of 6.9 m shall apply for broad gauge and 6.1 m, for metre
and narrow gauges.
23.
The minimum height between any guard wire and live crossing conductor under the most adverse
conditions shall not be less that 1.5 mts.
24.
Anti climbing Devices and warning notices :
Where the voltage exceeds 650V, the supporting structures on railway land shall be provided with
approved anti climbing devices and warning notices shall be erected at appropriate locations.
25.
Protection from Moving Road Vehicles :
Supporting structures including guys', adjacent to road ways shall be so located that the danger of
being struck by moving road vehicles it avoided or reduced to the minimum. Wherever required
guardrails suitable painted to make them conspicuous shall be provided for the purpose.
A.
Earthing : AE supporting structures, guarding and guy wires of the crossing span shall be
efficiently earthed. A separate earth shall be provided at each supporting structure.
B.
RIVERS ROSSING :
For the rivers on which the crossing is to be done the dates of high flood level of at least the
previous 20 years is to be obtained from the revenue authorities and the structures are to be
erected so that in rainy season also they will be approachable under the flooded conditions of
the river. In case of navigable rivers, the structures should be so designed as to give
sufficient clearance between the lowest conductor and the highest flood level. Consultation
which Navigation authorities is necessary. In case of non-navigable rivers, the structures
should be designed in such a way that the lowest conductor should be 3 mts above the
maximum flood level. This is necessary because floodwater may carry trees and their
branches.
C.
CROSSINGS OVER TELEGRAPH OR TELEPHONE LINES AND P.T.C.C.
Communication circuits coming under the influence of electric field of high voltage power
lines experience extraneous induction, which may introduce noise in communication
equipment and cause danger to the equipment and persons handling them.
Therefore, it is very essential that whenever power lines and communication lines pass close to each
other suitable measure are taken. Approval of PTCC (Power Telecommunications Coordination Committee) may be obtained.
Further, while crossing the telecom lines, the following points may be observed.
(1)
The power lines cross over the telecom lines because the Dia of power line conductros
is generally greater than telecom line conductors.
(2)
The angle of crossing shall be as far as possible 900, but shall not be less than 600
(3)
A safe clearance between the communication lines and overhead power lines has to be
maintained in accordance with the PTCC code of practice.
(4)
Power contact protectors are to be installed on all conductors of telecom lines at
crosings with HV power lines.
(5)
The telecom lines shall be erected close to the support of the OH power lines for
increased clearances.
D.
ROAD CROSSINGS :
The ground clearance of the guard wire and strength of the supports on either side of the
road have to be considered in installing a cradle guard at the road crossings. The supports on
either side of the road on which the cradle guard terminates are earthed separately and the
line should not cross at an angle less than 600. The safe minimum distances between the
support of OH lines and road had to be kept in view while planning road crossing.
The clearances above ground level of the lowest conductor shall be in accordance with Rule
No.77 of IE Rules 1996.
See Also Tables for
1.
Permissible Spans
2.
Clearances at Railway Xings
3.
Clearances at Road Xings.
4.
Clearances & Spacings from Buildings
5.
Clearances at Highways
6.
Clearances between power and Telephone Lines etc.
33 KV Substations
I.
Site :
The mention of a 33 KV Substation has to be decided after studying necessary studies as per
IOSP (Integrated Optimal System Planning) and were loss reduction and voltage improvement
substantiates erre.... of a Substation taking anticipated loads upto horizon year of next five years. It shall
be ensured that cheaper options like reconfiguration, reconductoring, erection of a capacitor bank
locating at NB or a combination of two or more of these will not achieve the required LRVI.
The site shall be selected ensuring
(a)
It is at load center
(b)
Existence of right of way for 33 KV incoming & 11 KV outgoing lines.
(c)
Easy nessibility by road for transport of heavy equipment like power transformers.
(d)
Low revel areas susceptible for water logging and places near calls canals etc., shall be
avoided.
(e)
Shall be far away from Slaughter houses to avoid bird faults
II.
(f)
Shall be away from ground where firing practices are done by police military
(g)
There shall be a water source like municipal water connection or a bo..... for good
water supply.
(h)
Atleast 3 KMs away from Airports and not in the take-off or 'Lam... corridors of
Aircraft.
(i)
Availability of sufficient area for enhancing the size of the submission for additional
incoming and outgoing lines.
(j)
Station switchyards shall have enough elevation for satisfactory and quick run off and
drainage facility.
Structures
The structures at the Substations shall be designed ensuring
(a)
Adequate electrical clearances
(b)
Sufficient height for statutory ground clearances
(c)
Shall have adequate mechanical strength
(d)
Bay sizes to have statutory widths
(e)
Provision for enough working place for maintenance and operating staff.
Statutory Requirements : Outdoor Substations
33 KV
11 KV
Bay widths
4.7 metres
3.5 metres
Spacing for Bus
1400 mm
1300 mm
Minimum clearance from
live part to ground
3700 mm
3700 mm
2.8 metres
2.6 metres
Sectional clearance for
subject of inspecting staff
III. Power Transformers (IS 2026) - Impedance rating
Minimum percentage impedance for transformer of 33/11 KV rating are given below :
Up to
1 MVA
-
5%
3 MVA
-
6%
—
5 MVA
-
7%
7.5 MVA
-
8%
8 MVA to 12 MA
-
9%
Maximum permissible temperature rise over ambient while delivering full load
continuously.
Oil-45oC
—
Impulse strengths : 33KV - 170 KV
Winding-550C
11 KV-75 KV
IV. The CTs, PTs, relays, switchgear (breakers) and protective gear (relays) are chosen as per
requirements (not included in this book).
V.
Battery : A station battery of adequate Amp hrs capacity or individual batteries for each
switchgear (or a pair of them) are provided at 33 KV Substations.
VI. The preventive maintenance schedules for power transformers are provided in the tables. In
respect of Circuit Breakers the schedules as per Manufacturer's manuals are to be followed. An
exclusive chapter on earthing practice at the substations is included in this book. Standards for fire
protection are also appended which can be redefined by every power utility.
Typical layouts for outdoor and indoor 33/11 KV substations are provided herewith (Drawing
enclosed).
Section of Conductor Size and Impact on losses
Like in any other system, the power system should be efficient. This means that the ratio of the
power utilised by the ultimate consumers of electric power to the power produced at the generating
stations must be as high as possible. In other words the losses occurring in carrying electric power
from the generator to the consumers must be kept at the minimum. These losses are called "Line
Losses" and occur in the transmission and sub-transmission lines, Step up and step down
transformers primary and secondary distribution line and distribution transformers.
These losses are I2R losses in the resistance of the conductor and equipment in the line, iron losses
in the transformers etc.
Low line losses result in a low cost per unit to the consumer. For the country as a whole, low line
losses mean better utilisation of the sources of energy.
In India, line losses vary form region to region from 15-30%. This is extremely high compared to
about 8-10% in the developed countries of Europe, America and Japan.
This shows that the percentage losses need to be brought down to reasonable level in order to improve
the efficiency of the distribution system.
This assumes great urgency and importance in view of the acute shortage of power.
Factors contributing towards Increased Line Losses :
The major amount of losses in a power system are in primary in secondary distribution lines; while
transmission and sub-transmission lines account for only about 30% of the total losses. Therefore
the primary and secondary distribution systems must be properly planned to ensure losses within
acceptable limits.
The factors contributing to the increase in the line losses in the primary and secondary systems :
1.
Largely Distribution Lines :
The primary and secondary distribution lines in rural areas; by and large radially laid, usually
extend over long distances. This results in high line resistance and therefore high I2R losses in the
line.
Because of the above reason, the rural loads contribute towards increased line losses.
2.
Inadequate Size of Conductor :
Rural loads are usually scattered and generally fed by radial feeders. the conductor size of these
feeders should be properly selected.
The size of the conductor should be selected on the basis of KVA x KM capacity of standard
conductor for required voltage regulation.
Table provided below indicate the length of lines for 11 KIV and 415 volts corresponding to
different loads for the voltage regulation prescribed by IE Rules; for different sizes of conductors
respectively.
Table -1
Length of 11KV line corresponding to different loads
Size of
conductor with
Coe No.)
KVA - KM for 8%
voltage drop at 0.8
PF
Maximum of length
of line (KM)
Load that can be
connected (KW)
50 MM2 ACSR Rabbit
10,460
30
355
30 MM2 ACSR Weasel
7,200
20
360
20MM2 ACSR Squirrel
5,120
15
341
The figures are for a conductor temperature of 60oC For a conductor temperature of 50oC,
the above figures shall be about 3% higher and for a temperature of 70oC about 3% lower.
Table -2
Length of 415 line corresponding to different loads
Size of conductor
with Coe No.)
KVA - KM for 8%
voltage drop at 0.8
PF
Maximum of length Load that can be
of line (KM)
connected (KW)
30 MM2 ACSR Rabbit
11.76
1.6
7.35
20 MM2 ACSR Weasel
7.86
1.0
4.86
13MM2 ACSR
5.58
1.0
5.58
30mm2 AAC ANT
12.06
1.6
5.58
16 MM2 AAC Gnat
6.96
1.0
6.96
The figures are for a conductor temperature of 600C. For a conductor temperature of 500 C, the
above figures shall be about 3% higher and for a temperature of 700C about 3% lower.
3.
Distribution Transformers not Located at Load centre on the Secondary Distribution
System:
Often DTs are not located centrally with respect to consumers Consequently, the farthest
consumers obtain an extremely low voltage even though a reasonably good voltage level is
maintained at the transformers secondaries. This again leads to a higher line losses. (The reason
for the line losses' increasing as a result of decreased voltage a the consumers terminally are
explained in para - 5)
Therefore in order to reduce the voltage drop in the line to the furthers consumers, the distribution
transformer should be located at the load centre to keep voltage drop within permissible limits and
thus minimize the looses.
4.
Over-rated Distribution Transformers and Leading to Under-Utilisation :
Studies on 11 KV feeder have revealed that often the rating of DTs is much higher than the
maximum KVA demand on the feeder. Over rated transformers draw an unnecessarily high iron
losses. In addition to this iron losses in over rated transformers the capital costs locked up is also
high.
From the above it is clear that the rating of DT should be judiciously selected to keep the losses
with in permissible limits.
For an existing distribution system the appropriate capacity of distribution transformer may be
taken as very nearly equal to the maximum KVA demand at good PF (say 0.5) Such an exercise has
been carried out for a number of distribution systems and transformers with capacity of 25, 63, 100, 160,
315 KVA and and standardised for different systems with power factors and diversity factors.
5.
Low Voltage (less than declared voltage) at Transformers and Consumers Terminals :
Whenever the voltage applied to induction motor varied from rated voltage. Its performance is
affected. Within permissible voltage variation of +/- 6% in practice, the supply voltage various by
more than 10% in many distribution systems. A reduced voltage in case of induction motor results
in higher currents drawn for the same output.
For a voltage drop of 10%, the full load current drawn by the induction motors increase by about
10% to 15% the starting torque decreases by nearly 19% and the line losses in the distributor
increases by about 20%.
As the bulk toad of rural areas and small scale industrial areas consists of induction motors, the
line losses in the concerned distribution systems may even touch 20%.
The above situation is corrected by operating an "on-toad-tap changing in the power transformer
situated at high voltage substations 66/11 KV and 33/11 KV sub-stations and providing on the 11
KV feeders a combination of switched capacitors and automatic voltage regulators.
Further, the "off load tap changing" in distribution transformers is adjusted prior to the
commencement of agricultural load season which is readjusted before the onset of monsoons when
the rural load is small if the off-load tap changing gear is available.
6.
Low Power Factor :
In most of the LT distribution circuits, it is found that the PF ranges from 0.65 to 0.75. A low PF
contributes towards high distribution losses. For a given load, if the PF is low, the current drawn is
high. Consequently, the losses proportional to square of the current, will be more.
Thus, line losses owing to the poor PF can be reduced by improving the PF. This can be done by
installing of shut capacitors.
Shunt capacitors can be connected in the following ways :
(i)
Shunt capacitors are connected on the secondary side (11 KV side) of the 33/11 KV power
transformers.
Table 3 shows from the studies carried out on 11 KV lines, how the improvement of power
factor results in considerable reduction of losses.
Table-3
Reduction of Line Losses with Improvement in Powers Factor
(ii)
Load (KW)
PF
KVA
Current (A)
Line Loss
Remarks
300
0.7
428
38.9
27.2
Before
300
1.0
300
27.2
13.4
After
Line losses in LT distribution lines may also be considerably reduced by installing shunt
capacitors of optimum rating at vantage points as decided during load stations.
The optimum rating of capacitors banks for a distribution system is 2/3rd of the average
KVAR requirement of that distribution system. The vantage point is at 2/3rd the length of
the main distributor from the transformer.
A study made in an urban distribution system fed from a 200 KVA 11 KV/41 volts
transformer with 300 HP CL of more than 200 consumers having small loom loads showed
the rage of PF of the distribution system varied from 0.65 to 0.70. The reactive power
requirement was 135 KVR. The inductive loads occurred on the distribution system between
7 AM and 8 PM on all the working days with almost constant load - cycle. The capacitor
bank rating 42.5 KVAR were installed at 2/3rd the distance from the transformer on the
main distributor and were switched on and off manually at 7 AM and 8 PM respectively on
all working days. The loss reduction was thus found to be 6-7%. The released capacity in
this particular study was 42 HP, which could be supplied to the additional consumers.
(iii) A more appropriate manner of improving this PF of the distribution system and thereby
reduce the line losses is to connect capacitors across the terminals of the motors (inductive
load). The extent of reduction of line losses in this manner depends mainly on the extent to
which the PF of consumer is improved. In this case, the capacitor is connected in parallel
directly to the terminals, the capacitor being switched on and off together with the
equipment itself.
Many electricity supply authorities are including a clause in terms & conditions of supply
making it compulsory for the consumers to provide capacitors of adequate rating for all
types of installations with connected loads of 5 HP and above.
By connecting the capacitors across all individual inductive loads it is observed that 10%
voltage improvement 20% reduction in current and reduction of losses upto 9% can be
achieved depending upon the extent of PF improvement.
7.
Bad Workmanship Resulting in Poor Contracts at Joints and Connections :
Bad Workmanship contributes significantly towards increase in distribution losses. In this context
the following points should be borne in mind.
(i)
points are a source of power loss. Therefore the number of joints should be kept to a
minimum. proper jointing techniques should be used to ensure firm connections.
(ii) (ii) Connections to the transformer bushing-stem, drop-out fuse, isolator, LT switch etc.
should be periodically inspected and proper joint ensured to avoid sparking and heating
of contacts.
(iii)
(iii)Replacement of deteriorated wires and services should also be made timely to avoid
any cause of leakage and loss of powers.
(iv) Pilferae of Energy :
(v)
In addition to the above, pilferage of energy through manipulation of metering, direct
tapping, application of wrong multiplying factors, non performing meters, under
performing meters etc cause commercial losses. Since it is often not possible to measure
the stolen energy and, therefore, cannot be charged to any one. Stolen energy is,
therefore, considered as a part of T & D losses.
(vi)
Unserupulous consumers extract energy illegally either by passing the energy meter or by
connecting leads directly to the distribution lines. Electricity supply authorities take
various measures to stop theft of energy as below :
(vii) (1)Surprise inspections are carried out by vigilance squads.
(viii)
(2)
The energy meter is houses in a separate box sealed and made
inaccessible to the consumers. The fuse cut-outs are provided after the meter.
(ix)
(3)Multicoe PVC cables are used as service mains instead of single core VIR wires.
(x)
(4)The energy meter manufacturers are asked to provide the potential link inside the body
of the energy meter and not inside terminal cover. This prevents the potential link to be
disconnected by the consumer.
(xi)
(5)Heavy fines are imposed on consumers found committing theft of energy including
imprisonment.
(xii) It must be stated that the consumer stealing energy is imposing extra financial burden on
honest law abiding consumers. Further high line losses result increased consumption of
fuel oil etc. used in the generation of electric power. Therefore, it is necessary that line
losses (including pilferage of energy) are to be kept down to minimum.
(xiii) A unit saved is one and a half units saved for the system and this slogan is well worth
remembering.
(xiv)
(1)
To enable the designing and operating personnel in each power supply
undertaking to determine the specific causes of interruptions and take suitable remedial
measures to improve the standard of supply.
(xv) (2)To determine whether any improvement in service continuity is necessary to any
consumer or group of consumers and to ensure that expenditure on improvements is
incurred when it is most needed.
(xvi)
(3)
To keep a Discom informed of the performance of its various operating
systems of divisions.