WELCOME WELCOME
TOTO
PRESENTATIONPRESENTATION
ON ON
CONCEPT OF SUBCONCEPT OF SUB--STATION ENGINERINGSTATION ENGINERING
2
Contents of PresentationContents of Presentation
�� PURPOSEPURPOSE
�� CLASSIFICATIONSCLASSIFICATIONS
�� VOLTAGE CLASS & RATINGSVOLTAGE CLASS & RATINGS
�� PLANNING OF SUB STATION INSTALLATIONPLANNING OF SUB STATION INSTALLATION
�� SUBSUB--STATION ENGINEERINGSTATION ENGINEERING
�� SUBSTATION EQUIPMENTSSUBSTATION EQUIPMENTS
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1.0 PURPOSE OF ESTABLISHING A SUBSTATION1.1 The substations are very much essential to
• Evacuate power from generating stations.
• Transmit to the load centers.
• Distribute to the utilities & ultimate consumers.
1.2. The Electrical power generation from Hydel, Thermal, Nuclear and
other generating stations has to be evacuated to load centers. • The generation voltage is limited to 15/18 KV due to the limitation of
the rotating machinery. • This bulk power has to be stepped up to higher voltages depending on
quantum of power generated and distance to the load centers. • Again the power has to be stepped down to different lower voltages for
transmission and distribution.
1.3 In between the power houses and ultimate consumers a number of Transformation and switching stations have to be created. These are generally known as sub-stations
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2.0 CLASSIFICATIONS
Accordingly the substations are classified as
a) Generating substations called as step up substations
b) Grid substations
c) Switching stations
d) Secondary substations.
2.1. The generating substations are step up stations as the generation voltage
needs to be stepped up to the primary transmission voltage so that huge
blocks of power can be transmitted over long distances to load centers.
2.2 The grid substations are created at suitable load centers along the primary
transmission lines.
2.3 Switching stations are provided in between lengthy primary transmission lines • To avoid switching surges.• For easy segregation of faulty zones.• For providing effective protection to the system in the A.C. network.• The switching stations also required wherever the EHT line are to be tapped and line
to be extended to different load centers without any step down facility at the switching stations.
• The number of outgoing lines will be more than the incoming lines, depending on
the load points.
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2.4. Secondary substations are located at actual load points along the
secondary transmission lines where the voltage is further stepped
down to sub transmission & primary distribution voltage.
2.5. Distribution substations are created where the sub-transmission
voltage and primary distribution voltage are stepped down to supply
voltage and feed the actual consumers through a network of
distribution and service lines.
3.0. VOLTAGE CLASS AND RATINGS.
Generally the following voltage class substations prevailing in India
• 6.6 KV, 11 KV, 22KV, 33 KV ---------- High Voltage
66KV, 110/132KV,
� 400 KV and above 220/230KV ---------- Extra high Voltage
3.1 Sub station rating is defined as the capacity of power transformers installed.
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4.0 PLANNING OF SUBSTATION INSTALLATION
The process of planning sub-station installations consists in
• Establishing the boundary conditions.
• Defining the plant concept, type, & Planning principles.
4.1 The boundary conditions are governed by following environmental
circumstances & availability of the land in the required place.
• Local climatic factors
• Influence of environment
• The overall power system voltage level
• Short circuit rating
• Arrangement of neutral point
• The frequency of operation
• The required availability or reliability
• Safety requirements
• Specific operating conditions
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4.2. Boundary conditionsThe following boundary conditions influence the design concept and measures to be considered for different parts of substation installations.
Boundary conditions Concept and measuresOutdoor / indoor
Conventional / GIS
Equipment utilization
Construction
Protection class of enclosures
Creepage, arcing distances
Corrosion protection
Earthquake immunity
Short circuit loadings
Protection concept
Lightning protection
Neutral point arrangement
Insulation coordination
Environment, climate conditions
Net work data / Net work form
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Boundary conditions Concept and measures
Bus-bar concept
Multiple in-feed
Branch configuration
Standby facilities
Un-interruptable supplies
Fixed/draw out apparatus
Choice of equipment
Network layout
Scope for expansion
Equipment utilization
Instrument transformer design
Automatic/conventional control
Remote/local control
Construction/configuration
Network layout
Arcing fault immunity
Lightning protection
Earthing
Touch protection
Step protection
Fire protection
Ease of operation
Safety requirements
Availability and abundance of power
supply
Power balance
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4.4. Type of sub4.4. Type of sub--stationsstations
4.4.1. The types of Sub Stations depends upon:
• The availability of the land in the required place.
• Environmental conditions.
4.4.2. Sub-Station types are:
• Out door
• In door
• Compressed Air insulated
• GIS
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4.5 Sub-Station Engineering
•The Sub Station Engineering comprises:
� Sub-station site selection
� Switching scheme.
� Bus-Bar.
� Safety clearances.
� Phase to phase clearances.
� Phase to ground clearances.
� Sectional clearance.
� Ground clearance.
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4.5 Sub-Station Engineering(Contd)�Yard levels.
�Single line diagram & Layout.
�Bus levels.
� First level ---- Equipment interconnection level.
� Second level ---- Bus levels.
� Third level ---- Cross Bus / Jack Bus level.
�Bay widths
�Lightning protection.
�Earth mat.
�Civil Engineering works.
�Electrical Installation works.
�Main electrical equipments.
�Auxiliary supplies
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4.5.1. Sub station site selection
• The aspects are to be considered for site selection
� Fairly level ground
� Right of way around the sub station yard for incoming & out
going transmission & distribution lines
� Preferably of soil strata having low earth resistance values
� Easy approach & accessibility from main roads for Heavy
equipment transportation and routine O & M of sub station
� Economy / Cost
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4.5.2. Switching schemes:• The factors considered for selection of switching schemes
� Reliability factor
� Availability of the space
� Economics (project cost)
� There can be several combinations in which the equipments, bus-
bars, structures etc. can be arranged to achieve a particular
switching scheme.
� The switching schemes can be made more flexible
by making minor modifications like providing sectionalisers using
bye-pass path etc.
�The various types of switching schemes along with its advantages
and disadvantages are:
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Switching SchemesSwitching SchemesSwitching Scheme Advantages Disadvantages
Bus fault or breaker failure causes station outage
Maintenance is difficult
No station extension works without complete
shutdown
For use only where loads can be disconnected
or supplied from another substation.
Single busbar with
sectionaliser
Shut down on the part of
the Bus can be availed Aditional cost for the isolator
Higher flexibility as
compared to single bus
Maintenance of main bus will involve outage of
substation.
One breaker can be taken
for maintenance at a timeAdditional cost for the Transfer Bus & Breaker
High flexibility with two
busbars of equal merit
Expensive for additional bus and BC breaker and
associated equipments and also extra space is
required
Each busbar can be
isolated for maintenance One Breaker maintenance possible at a time.
Each branch can be
connected to either of the bus
with bus tie breaker
There will be a time delay for restoration of the
circuit in case of breaker outage
The two buses can be
individually operated in case
of island operations
Single main and transfer
bus
Single busbar Least cost
Double main busbar
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16
LinesLines
Lines LinesLines
Main Bus
Transfer
Bus
Main Bus1
Main Bus2
Transformer Transformer Transformer Transformer
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Switching Schemes (Switching Schemes (contdcontd))
Switching Scheme Advantages Disadvantages
High flexibility with 3 buses
and 2 tie breakers
One breaker is available at a
time for maintenance
No time delay for restoration
of the circuit in case of breaker
outage.
Greatest operational flexibility
High reliability
Breaker fault on the busbar
side disconnects only one
branch
Each main bus can be
isolated at any time
All switching operations
executed with circuit-breakers
Bus fault does not lead to
branch disconnections
Greatest operational flexibility
Each branch has two circuit
breakers
Connection possible to either
bus bar
Each breaker can be serviced
without completely disconnecting
the branch
High reliability
Flexibility for breaker
maintenance
Each breaker removable without
disconnecting load
Only one breaker needed per
branch
Each branch connected to
network by two breakers
All change-over switching done
with circuit-breakers & hence
flexible
Area required will be more
2 breaker system Most expensive method
Ring bus
Double main bus with
transfer bus
Greater outlay for protection and auto-reclosure,
as the middle breaker must respond independently
in the direction of both feeders
Three circuit-breakers with associated equipments
required for two branches
Auto-reclosure and protection fairly complicated
Breaker maintenance and any faults interrupt the
ring
1½ Breaker system
Expensive consequent to additional two buses
and two breakers with associated equipments and
additional space is required.
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Line
Main Bus1
Transfer Bus
Transformer
Main Bus2
Line
Transformer
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20
21
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4.5.3.4.5.3. Bus Bus -- BarsBars• Selection of bus-bars
�Type of Bus Bar
� Sizes of Bus Bar
• Types of Bus –Bars
� Strung Bus / Flexible Bus
� Rigid Tubular Bus
• Strung Bus:
The various Types of conductors used for Strung Bus are
� All Aluminum conductor (AAC)
� All Aluminum alloy conductor (AAAC)
� Aluminum conductor with aluminum alloy reinforced (ACAR)
� Aluminum conductor with steel reinforced (ACSR
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• RIGID TUBULAR BUS.
� Rigid tubular conductors are also used in substations.
� Rigid tubular buses are more advantageous than the
flexible conductors.
• Sizes of Bus Bar
The factors to be considered for selection of the Bus-Bar sizes are:
�Normal current carrying capability
�Short circuit heating with stand capability
�Surface gradient
�Corona free performance
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• Selection Criteria for Bus Sizes
� Electrical & Mechanical Stresses:
� The bus-bars must be designed for:
• The operating current.
• To withstand short-circuit fault currents.
• The anticipated stresses on the bus-bars and their supports
in the event of a short circuit must therefore be calculated.
�Thermal stresses
� Bus bars including clamps and connectors are also stressed
thermally under short circuit conditions.
� The bus bar conductors/tubes are suitably sized / designed to
with stand the short circuit currents not only mechanically, but
also thermally.
Case Case –– Study for 1000 MVA 400/220 S/SStudy for 1000 MVA 400/220 S/S
•• REQUIREMENTS:REQUIREMENTS:
� Normal full load current for 1000 MVA (2 X 500) capacity.
400 KV -- 1445 amps220 KV -- 2625 amps.
�Short circuit heating withstanding capability: Minimum cross sectional aluminum area required to with stand one KA for one second is 15.29 .
For 40 KA for 1 sec --- 610.7 sq mmFor 31.5 KA for 1 sec --- 481 sq mm
� Maximum Permissible conductor surface gradient -21 KV/cm.
� Permissible radio interference level --- 40 to 50 db
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Rph Yph Bph
1.
Single Moose 830 Amps *34.6 KA
Aluminum area for
with standing 1 KA
/sec is 15.29 sq
mm
400 Kv system
ph–ph 7 mtrs
Ph–gr 8.0 mtrs
a) Single moose
15.82 20.81 14.2 47.84
220 KV system
Ph-ph 5.0 mtrs.
Ph-gr 5.5 mtrs
a) Single Moose
34.54 41.34 24.4 139.11
11.94 14.16 13.32 38.76
3.
For 1000 MVA
Transformer
2665 Amps
four moose is
required
For 31.5 KA S.C.w ith
standing capacity for 1
sec single moose is
required for 220 KV
The characteristics of the ACSR Moose conductor are as follows.
Conductor Surface
gradient at KV/CM
Radio
interfere
nce level
db
2.
For 1000 MVA
Transformer
1445 Amps
Tw in moose is
required
For 40 KA S.C.w ith
standing capacity for 1
sec tw in moose is
required for 400KV
b) Tw in moose w ith 450
mm conductor spacing
Sl
no
Voltage system in
KV
Normal
Current
carrying
capacity
at 85 C
Short circuit heating
withstand capacity
for 1 sec having
cross sectional area
of 529 mm
Considering the Example of Moose A.C.S.R. Characteristics.
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REQUIREMENTSREQUIREMENTS
• Normal full load current for 1000 MVA ( 2 X 500 )capacity
400 KV -- 1445 amps
220 KV -- 2625 amps
• Short circuit heating withstanding capability
Minimum cross sectional aluminum area required to with stand one KA for one
second is 15.29 sq mm.
For 40 KA for 1 sec --- 610.7 sq mm
For 31.5 KA for 1 sec --- 481 sq mm
• Maximum Permissible conductor surface gradient --- 21 KV/cm
• Permissible radio interference level --- 40 to 50 db
By the above it is found
� Twin moose conductor is required for 400 KV.
� Quadruple moose conductor is required for 220 KV main bus, bus coupler bay.
� Twin moose conductor is required for 220 KV transfer bus, transformer & line bays.
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The characteristics of 100 mm and 75 mm IPS aluminum tube are as follows:
* 400 KV system: conductor height 8 mtrs, phase to phase spacing 7 mtrs.** 220 KV system: Conductor height 5.5 mtrs, phase to phase spacing 4.5 mtrs.By the above it is observed that
For 400 KV system 100 mm IPS tubes are required For 220 KV system 100mm IPS tubes are required
*400 KV **220 KV
1 100 mm 114.2 97.18 2825 2665 18.08 11.63
2 75 mm 88.9 77.93 1428 1775 21.89 13.98
Surface voltage
gradient KV rms/cmSl.
No.
Size of
IPS
Outer dia.
mm
Internal dia.
mm
Aluminium
area sq mm
Normal current
carrying capacity
at 850C
• Rigid conductor selection.Rigid conductors are selected based on the following criteria.
� Normal current carrying capacity
� Short circuit heating withstand capability
� Surface voltage gradient
� Fiber stress in tube &Vertical deflection
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4.5.3.4 Fiber stress in tube & Vertical deflection
• Aluminum tube should be capable to with stand the gravitational wind & short circuit forces.
• The vibrations in aluminum tube are caused due to study wind blowing across the bus at right angles to aluminum tube span.
• The fiber stress/bending stress of Aluminum tube depends upon the span of the Aluminum tube between two supports.
• The vertical deflection also depends upon the span of tube and type of supports [i.e. Whether two ends are pinned (simple supported) orfixed, or whether one end is fixed and other is pinned].
• The safe vertical deflection should be less than the half of the outer dia. of Aluminum tube.
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4.5.3.5 The maximum allowable span lengths are as follows
Size of Aluminum
tube.
Two ends pinned or
simply supported.
Permissible Span
Both ends Fixed
permissible Span length
in mtrs100 mm 11 **12.5
75 mm 9 **12.5
** Maximum permissible to limit the fibre stress.
� The adequacy of span of Aluminium tubes has to be verified depending upon sub-station layout arrangement.
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4.5.3.5 The standard sizes of aluminum tubular bus and conductors generally used for different substations are as follows
SL.
No.
Voltage
referenceAl. Tube ACSR conductor
1 33 KV 50 mm Coyote / Drake
2 66 KV 63 mm Falcon / Twin Drake
3 110KV 75 mm Falcon / Twin Drake
4 220 KV100/75
mm
Single / Twin Falcon Twin
/ Quadruple Moose
5 400 KV 100 mm Quadruple MOOSE
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4.5.4 Electrical safety clearances.
The electrical , safety clearances to be adopted in substation are governed by following parameters.
• Basic Impulse Insulation levels (BIL).
• Basic Switching Impulse level (BSL).
• IE Rules.
• Allowances in tolerance in dimensions of structural work.
• Safety margins for unforeseen errors.
Based on the above, certain minimum clearances are defined for
a given voltage class and the same are applied in substation
• The various clearances need to be defined.
� Phase-to-earth clearance.
� Phase-to-phase clearance.
� Sectional clearance.
� Ground clearance.
� Equipment to equipment spacing
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4.5.4.1. Phase to earth and phase to phase clearancesThe minimum phase to phase and phase to earth clearance for 400 KV, 220 KV and other voltage classes are based on the BIL & BSL values.
The above mentioned clearances do not include clearance between the live and ground parts of equipments including bus post insulators for which insulation is prescribed as per relevant standards and guaranteed by the manufacturers and confirmed by type tests.
Sl.
No.
Voltage
classBIL BSL
Phase to phase
clearance in mm
Phase to ground
clearance in mm
1. 400 KV 1425 KVp 1050 KVp 4000 3500
2. 220 KV 1050 KVp 460 KVp 2400 2100
3. 110 KV 550 KVp 230 KVp 1100 1100
4. 66 KV 325 KVp 140 KVp 630 630
5. 33 KV 170 KVp 90 KVp 320 320
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4.5.4.2Section clearance
• Section clearance is the distance between two sections of
substation, which enables a person to work on one section
of a substation in a safe manner, while the other section is
charged.
• Section clearance is chosen in such a manner that phase to
earth clearance is maintained between the live point and the
approach of the working personnel with adequate margin.
• In case of 400 KV:
� The phase to earth clearance of 3.5 meters.
� The approach of man is considered as 2.5 meters.
� Margin of 0.5 meters for unforeseen reasons like errors in
erections, dimensions of tools and platforms etc.
� Thus the section clearance is taken as 6.5 meters.
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The section clearance for all voltage classes shall be:
Voltage
class in
KV
Highest
system
voltage
in KV
Minimum safety
working
clearance followed as
per the design
Minimum safety working
clearance as per rule no 64 of I.E.Rule1956
400 420 6500 mm 6000 mm
220 245 5000 mm 4300 mm
110 123 4000 mm 3500 mm
66 72.5 3500 mm 3000 mm
33 36 3000 mm 2800 mm
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4.5.4.3. Ground clearance
• The ground clearance is the distance between ground level and bottom of any
insulator in an out door substation.
• This ensures that any person working in the area cannot touch or damage the
insulators accidentally.
• This clearance is kept as 2.5 meters for all voltage levels.
• However in cases, where the vehicles and cranes are allowed inside a
substation, the ground clearance for the equipment falling on both sides of the
road are to be enhanced as the vehicles and cranes height is generally 3.5
meters.
• The minimum ground clearances between the live point & ground at the
substation for the different voltage classes as per rule no 64 of I.E.Rule 1956
are as follows
� 400 KV 8000 mm
� 220 KV 5500 mm
� 110 KV 4600 mm
� 66 KV 4000 mm
� 33 KV 3700 mm
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4.5.4.4. Bus levels
Generally in all the substations two / three level bus arrangements are necessary.
� The first level is the equipment interconnection.
� The second level is main buses 1 & 2, which may be Rigid / Strung Bus.
� The third level cross bus / Jack Bus , required in few large sub stations.
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40
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4.5.4.5. First level : Equipment inter connection levelThe first level height is fixed based on the following considerations
• The bottom part of the insulator or top most part of the earth metal
position should have the minimum ground clearance i.e. the height of the
man standing on the ground with shoes on holding the tools and
extending the arms upwards, which is already prescribed as 2.5 meters
in all voltage class substations.
• The insulator height / length as per I.E.C / I.S.S i.e., phase to earth
clearances as prescribed for different voltage classes.
• Live metal part height of the various equipments.
• Maximum value of electrical field at a height of 1.8 meters i.e. height of
an average person level.
� The electrical field is the deciding factor not only for the height of the
bus level but also for conductor configuration and phase spacing.
� It is generally considered that 10 KV per meter as safe design value of
an electrical field for a period of 180 seconds.
� The effect of electrical field reduces with increase in bus level.
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• 400 KV System.� The calculated electrical field at a level of 1.8 meters from
ground level with a bus level of 7 meters height is 12.11 KV per meter.
� The calculated electrical field at a level of 1.8 meters from ground level with a bus level of 8 meters height is 9.4 KV per meter.
• 220 KV System.
� The calculated electrical field at a level of 1.8 meters from
ground level with a bus level of 5.5 meters height, is 9.4 KV per
meter..
• The minimum bus level height for 400 KV is calculated as
� As per IEC & ISS ---------- 2500 + 3500 = 6000 mm.
� As per live metal part Ht of eqpt --- less than 6000 mm
� As per electrical field ------------ 8000 mm
� As per I.E. Rule also -------------- 8000 mm
• For 400 KV minimum first bus level shall be 8000 mm
• For 220 KV minimum first bus level shall be 5500 mm.
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4.5.4.6. Second level:
• Cross Bus levels are generally called second levels in a sub-
station switchyard.
•The height of this bus is decided / designed based on the
following.� Height of the equipment inter connection level i.e., first level.
� The extension of top level live metal part of Bus post insulator / isolator with an expansion clamp.
� The maximum sag of the conductor if it is a strung bus.
� Phase to phase clearance.
� Half of conductor / rigid bus diameter.
� Some minimum factor of safety ie some adequate marginparticularly to maintain
� minimum phase to phase clearance between main bus & cross bus.
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4.5.4 Standard Bus levels (First levels) / Equipment inter connection level and second level ie cross Bus / main Bus levels for different voltage classes in a sub-station designed as per above principles are follows.
* Third level or Jack bus level in 400 KV stations
SlSlSlSl. . . .
No.No.No.No.Voltage classVoltage classVoltage classVoltage class
First level First level First level First level
mmmmmmmmSecond level mmSecond level mmSecond level mmSecond level mm
1. 400 KV 8000 15000/22000 *
2. 220 KV 5500 11000
3. 110 KV 4500 9000
4. 66 KV 4250 8500
5. 33 KV 4000 8000
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4.5.4.5 Equipment to equipment spacing.
The equipment to equipment spacing is decided based upon following factors.
• Adequate clearances (phase to earth, phase to phase, section and
ground clearances).
• Convenience of erection and security.
• Adjacent equipments should not foul physically while installing
terminal clamps.
• Equipment foundations should not foul with each other and cable
trenches.
• Technical requirements.
� Location of surge arrestors with respect to protected equipments such as
transformer and reactors.
� Position of CVT, wave-trap and shunt reactor approaching from line side.
� Maintenance flexibility
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4.5.5 Bay widthsThe bay widths are chosen in such a way that the minimum clearances are maintained
even when the isolator is kept under fully open condition with one end energised. The
different types of the isolators like horizontal center brake, horizontal double brake,
pantograph and vertical break has a great impact in deciding the Bay widths.
• Vertical brake isolator
The bay width can be reduced, but the bus height increases.
Hence this type of isolator is not generally used.
• Pantograph isolator
It requires fine adjustment of sag and too expensive. Bay widths & Lay out sizes can be
reduced considerably. These type of isolators will be used in critical lay outs where
space is criteria.
• Horizontal center break isolator
This type is most commonly used isolator due to it’s low cost, and it will never be placed
under the gantry as it will intrinsically demands higher clearances and the bay width has to be
increased beyond the rational value.
• Horizontal double break center rotating isolator
It is very rigid, good performance, less bay widths and lay out sizes can be reduced. But
costlier compared to H.C .B.
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48
49
50
4.5.5.1 The factor considered for computing bay widths are
• Phase to Ground / Earth clearance
� The distance between nearest point of tower from extreme phases
� Minimum Phase to earth clearance: as per IE Rule
� Maximum horizontally protruding live metal part from the center of the equipment
� Tolerances for execution
� Length of the isolator blade
� The movement due to swing of conductor and insulator string in case of strung
bus only and not applicable in case of rigid bus.
• Phase to phase clearance� Length of the isolator blade
� Horizontally protruding live metal part of adjacent equipment
� Phase to phase clearance as per IE rule
� Tolerances for execution
� The movement due to swing of conductor and insulator string in case of
strung bus only and not applicable for rigid bus system.
• Considering tolerances for execution etc, the phase to phase and phase to
ground clearances for 400 KV is as follows
� Phase to ground ---- 6500 mm� Phase to phase ----- 7000 mm� Bay width will be 6.5 + 7.0 + 7.0 + 6.5 = 27 meters
51
4.5.5. The standard bay widths, ground & sectional clearances based on above analogy for rigid and strung buses for different voltage classes of substations are as follows
Voltage
class
KV
Bay
width
in mtrs.
Phase
to
phase
clearan
ce in
mtrs
Phase
to earth
clearan
ce in
mtrs.
Bay
width
in mtrs.
Phase
to
phase
clearan
ce in
mtrs
Phase
to earth
clearan
ce in
mtrs.
1. 33 4.5 1.25 1 5.5 1.5 1.25 3.8 2.5
3. 110 8.2 2.1 2 10.5 2.75 2.5 4.6 3.5
4. 220 14 3.65 3.35 17 5 3.5 5.5 4.3
5. 400 27 7 6.5 27 7 6.5 8 6.5
3
Sectio
nal
clearan
ce in
mtrs.
2. 66 7.6 2 1.8 8.6 2.3 2 4
Sl.
No.
Rigid bus Strung bus
Ground
clearanc
e in mtrs.
52
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4.6 Yard leveling4.6 Yard leveling
• Complete switchyard shall be generally maintained at the same level
to have sufficient ground clearances and easy for execution,
operation & maintenance.
• But in some cases leveling of the complete switchyard is too
expensive.
• There will be huge cutting and filling if there are large undulations of
the site.
• In such cases 2 to 3 different levels can be maintained for different
voltage classes viz. 400 KV, 220 KV, control room etc.
• Generally the levels of the switchyard will be mainly decided by
balancing volume of earth cutting and volume of earth filling for
economical considerations.
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4.7. Single line diagram & lay out design.• Draw a single line diagram also called key diagram, before the locations of
various equipments in the substation are decided.
• This diagram indicates the proposed bus bar arrangement and relative
positions of various equipments. There are numerous variations of bus bar
arrangement.
• The choice of a particular arrangement depends on various factors viz.
System voltage, position of the substation in the system, flexibility, expected
reliability of power supply and cost.
• The following technical consideration must be borne in mind while deciding
upon any one arrangement.
� Simplicity is the key note of a dependable system
� Maintenance should be easy with minimum interruption of supply
� Safety to the operating personnel
� Alternative arrangement should be available in the event of an outage on
any of the equipments or sections of sub station
� The layout should not hinder for expansion and/or augmentation at a later
date, to meet the future load growth
� The installation should be as economical as possible keeping in view of the
requirements and continuity of supply
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56
57
58
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4. 8. LAYOUT DESIGN
The first task which a substation designer has to undertake after finalizing the single line
diagram, bus switching scheme, bay widths, section & ground clearances, is to translate
the selected scheme into a layout so as to physically achieve the feeder switching
required for ease in erection and maintenance.
4.8.1. BROAD PARAMETERS
Following are the broad parameters, which change from one substation to another.
a) Nature of bus bars i.e. Rigid or flexible ( Strung Bus)
b) Orientation of bus bar
c) Location of equipments
d) Manner of inter-connections
e) Structural arrangement
f) Direct stroke lightning Protection
4.8.2. FACTORS INFLUENCING THE CHOICEThe factors need to be considered while choosing a type of layouta) Reliability
b) Ease of construction and provision for extension
c) Ease of operation and maintenance
d) Safety of operating personnel
e) Land requirement
f) Safety of Equipment and installation
g) Aesthetic look.
h) Economy
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4.9. SAFETY MEASURES:4.9. SAFETY MEASURES:
4.9.1.Power Distribution.
• The distribution of power and current is checked and the currents occurring in
the various parts of the station under normal and short circuit conditions are
determined.
• The power flow to be balanced to an extent possible by properly locating th
incoming / out going lines & Transformer bays.
4.9.2 Safety MeasuresThe safety measures for the substation and its components are to be designed in
respect of
a) Insulation co-ordination.
b) Lightning Protection system.
c) Safe Clearance.
d) Thermal and mechanical stresses
62
4.9.3.INSULATION – CO-ORDINATIONInsulation coordination is the total of all measures taken to restrict flash over or break down of the insulation caused by over voltages at places with in an installation at which the resulting damage is as slight as possible. This is achieved by using lightning arresters to limit over voltages.
The equipments are also to be designed to withstand lightning and switching surges. The nominal lightning impulse withstand voltage and power frequency withstand voltage for various voltage classes are as follows.
Nominal
lightning
impulse
Nominal power
frequency
withstand voltage
Peak value RMS value
36 KV 170 KVP 90 KV
72.5 KV 325 KVP 140 KV
123 KV 550 KVP 230 KV
245 KV 1050 KVP 460 KV
420 KV 1425 KVP 1050 KVP*
Maximum
voltage of
equipments
* Nominal Switching impulse with stand voltage
63
4.9.4 LIGHTNING PROTECTION:
In H.V.& EHV substations, the protection from the lightning is done either by shield wire or
lightning mast (high lattice structure with a spike on top) and sometimes combinations of
both depending upon type of layout of substation.
• Shield wire
Shield wire lightning protection system will be generally used in smaller sub stations of:
� Lower voltage class, where number of bays are less, area of the substation is
small, & height of the main structures are of normal height.
� The major disadvantage of shield wire type lightning protection is, that it
causes short circuit in the substation or may even damage the costly
equipments in case of its failure (snapping ).
• Lightning masts (LM)
This type of protection will be generally used in large, extra high voltage sub stations
where number of bays are more. It has the advantages,
� It reduces the height of main structures, as peaks for shield wire are notrequired
� It removes the possibility of any back flashover with the near byequipments/structure, etc.during discharge of lightning strokes
� Provides facility for holding the lightning fixtures in the substation forillumination purposes
� Aesthetic look.
64
4.9.4.1 PROTECTION ZONE BY SHIELD WIRE.
• The height of equipment to be protected by shield wires depends upon the
height of the earth wire and distance between them.
• As per experiment it has been found that, the displacement of the electrode
from shield wire at a distance of “B”=2h where ‘h’ is the height of the shield
wire, all the discharges will strike the shield wire, and protects all the
equipments from lightning discharges in the zone.
• There fore for two shield wires at a distance of “S”= 4 x h between them
(“h” is the height of the shield wire), the point situated on ground surface
mid way will not be struck by lightning.
• Similarly for protection of any equipment of height “h0”, the distance
between shield wire “S”, shall not be more than 4 times effective height ( h-
h0 ), i.e., the difference of height between. shield wire and the object to be
protected
“S” = or < 4(h-h0)
ho = or < (h-S/4)
ho the height of equipments shall be = or < h – S/4
65
• 400 KV switch yard.
� 400 KV bay width = 27 mtrs.
� Hence Shield wire distance ‘S” is 27 mtrs, apart..
� Height of shield wire “h” = 23.5 mtrs.
� Maximum height of equipments which can be protected by these two shield
wires are (23.5 – 27/4) = 16.75 Mtrs
� The height of the main bus level in the 400 KV station is15.00 Mtrs and all
the equipments will be with this level only. Hence the shield wires provided
on the peaks of the bus structures will protect all the equipments in the
respective bays.
• 220 KV switchyard.
� Bay width ------ 17 Mtrs.
� Height of shield wires ------ 19 Mtrs
� Maximum height of equipments which can be protected by these two shield
wires are (19 – 17/4) = 14.75 Mtrs
� The height of the main bus level in the 200 KV station is13.50 Mtrs and all
the equipments will be with in this level only.
� Hence protects all the equipments in the bay.
66
67
68
69
4.9.4.2 SELECTION OF LM HEIGHT
The factors to be considered
• The height of the LM will be decided, depending upon the height of equipment
to be protected
• The protection zone or coverage area of LM increases with the increase of its
height
• Hence LM’s height depends upon the height of equipment to be protected
• The protection zone of same LM would be more if the equipment height to be
protected is less
• The numbers of lightning masts in substation can be reduced by increasing
the height of LM, but this will cause increase in cost of structure and civil
foundations.
• The detailed analysis and experience revealed that 30 mtr. LM height is
economical proposition & hence to be limited to this height.
70
4.9.4.3. LOCATION & NUMBER OF LIGHTNING MASTS
The exact number and locations of LMs will be calculated for complete protection of equipment in substation, by considering the following aspects.
• The protection zone of one LM is very limited.
• In case of two LMs, the protective zone is considerably more than
the sum of protective zones of two single LMs.
• A point of height ho situated midway between two lightning masts
of height h can be protected if the distance ‘a’ between LMs is not
more than a seven times of active height ( i.e. difference of height
between a LM height h and height of equipments to be protected
ho ) or a = or < 7(h-ho).
• In case of 3 LMs forming triangle or 4 LMs forming rectangle, of
height h can protect the object of height ho situated inside the
triangle or rectangle if diameter D of the circle passing through
the tips of LMs is not more than 8 times the active height i.e. D
< = 8 (h-ho).
71
72
4.10. EARTH MAT REQUIREMENT
The main objectives of earthing system in the substation are:
• To ensure that a person in the vicinity of substation is not exposed to
danger of electrical shock
• To provide easy path for fault currents into earth under fault
condition without affecting the continuity of service
• Hence intentional earthing system is created by laying earthing rod
of mild steel in the soil of substation area.
• All equipments/structures which are not meant to carry the currents
for normal operating system are connected with main earth mat
• The earthing system in a substation serves :
� Protects the life and property from over-voltage
� To limit step & touch potential to the working staff in substation
� Provides low impedance path to fault currents to ensure prompt and
consistent operation of protective device
� Stabilizes the circuit potentials with respect to ground and limit the
overall potential rise
� Keeps the maximum voltage gradients within safe limit during ground
fault condition inside and around substation
73
4.10.1 SELECTION OF EARTHING CONDUCTOR SIZE FOR MAIN EARTH MAT
The selection of earthing conductor is based on
• The thermal stability criteria
• Jointing method
� Welded with maximum temperature rise of 6200C
� Bolted with maximum temperature rise of 3100C
• Magnitude of short circuit fault current & its duration
74
4.10.2. FORMATION OF SUBSTATION EARTHING:
• The main earth mat shall be laid horizontally at a regular spacing in both X & Y direction based upon soil resistivity value and substation layout arrangement.
• The main earth mat shall be designed to limit the following;
� Touch Potential – The potential difference between two points, one on the ground where a man may stand and any other point which can be simultaneously touched by either hand.
� Step Potential – The potential difference between any two points on ground surface which can be simultaneously touched by feet.
�Maximum ground mat resistance shall be less than 1.0 ohm for substations of 220kV class and below, and shall be 0.5 ohms for 400kV and above voltage class.
� The earth rods shall be capable of with standing short circuit current for specified period.
� For I KA SC current for 1 second the minimum cross sectional area of M.S. Rod / Flat shall be 12.16 sq mm with welded joints.
75
• The crushed rock (Gravel) of 15 mm to 20 mm size shall be
used as a surface layer of 150 mm in the substation for the
following reasons:
� To provide high resistivity for working personnel
� To minimize hazards from reptiles
� To discourage growth of weed
� To maintain the resistivity of soil at lower value by retaining
moisture in the under laying soil
� To prevent substation surface muddy and water logged.
• The main earth mat shall be laid at a depth of 600 mm from
ground.
• The earth mat shall be connected to the following in substation
i. Lightning down conductor, peak of lightning mast
ii. Earth point of S A, CVT
iii. Neutral point of power Transformer and Reactor
iv. Equipment framework and other non-current carrying parts.
v. Metallic frames not associated with equipments
vi. Cable racks, cable trays and cable armour
i.
76
5.0. INSULATORS.• Types
� Disc type� Post type.
a) Disc type
These are required for stringing the ACSR conductor for main bus and
jack bus/cross bus. The individual units are rated for 11 KV and string of
these units will be used for deferent voltage classes. The number of units
per string depends on following parameters.
i) System voltage.
ii) Insulation level
iii) Power frequency withstand level
iv) Tensile strength
v) Purpose – tension string or suspension string.
77
Typical details of string used for various system voltages
Tension
string
Suspen
sion
string
400 160 25 -
400 120 - 23
220 120 16 -
220 90 - 14
110 90 8 8
66 90 5 5
33 70 2 2
System
voltage
in KV
Tension
strength
in KN
No. of units per
string
78
b) Post type insulators
These are used for supporting ACSR conductor or Rigid
Aluminum tube for connecting main bus to equipment or
forming main bus. These are also used as supporting insulators
for isolators.
There are two types
i. Pedestal post or stacking type
ii. Solid core type
The solid core type are preferred
The design considerations are,
i) The phase to earth clearance which determines the height
ii) Insulation level
iii) Power frequency withstand level
iv) Mechanical strength i.e., mainly cantilever strength
v) Minimum creepage dimensions
79
Typical parameters for various voltage levels
System voltage in
KV
Height of stack
(mm)
Nol of units per
stack
Minimum
creepage
dimension in mm
at 25 mm/KV
Cantilever
strength KN
400 3650 3 10500 8
220 2300 2 6125 6
110 1220 1 3075 4.5
66 770 1 1815 4.5
33 380 1 900 4
80
6.0 Steel structures:
i) Towers and Beams:Required for stringing the ACSR conductor for main busand cross bus/jack bus
ii) Lightning masts:Required for providing protection against lightning andinstalling the luminaries fitting for illumination ofswitchyard.
iii) Support structuresThese are required for supporting the equipments andpost insulators to maintain the live point heights andother clearances as per the statuary clearancerequirements.
81
6.1. The steel structures can be classified broadly into two groupsi) Lattice type
Formed by mild steel angle sections/plate sections etc by fastening the various sections by bolts, nuts or by welding.
ii) Tubular typeFormed by using mild steel pipes. These are preferable for support structures for lightning arrestors, post insulators, & instrument transformers. etc.
6.2 Protection against corrosionThe steel structures are generally made of mild steel, which are galvanised / painted to protect against corrosion. Galvanising by applying zinc coating is preferable as the protection achieved is superior to painting & maintenance free. The coastal areas where due to saline weather conditions – corrosion phenomenon occurs very fast and hence only galvanising is recommended.
6.3 Design considerationsa)Towers
i) Wind loadii) Reactions of loads on the beams to which conductor is (along with
insulator) strungiii) Vertical loads on the beams like under strung isolatorsiv) Tension of conductor (if strung directly ) and ground wire.v) Short circuit forcesvi) Type of foundation – Stub type or anchor bolt type
82
b) Beams
i) Tension of conductor
ii) Wind load / weight of conductor and insulator string
iii) Vertical loads due to under strung isolators /post insulator etc.
iv) Short circuit forces
v) Configuration of conductors.
c) Lightning masts (25.0 to 30.0 meters height)
i) Wind load
ii) Weight of luminaries fittings.
d) Support structure
i) Weight of equipment/post insulator
ii) Wind load on conductor
iii) Load due to aluminum pipe and cantilever strength of post insulator
83
7.0. ILLUMINATIONThe indoor & out door areas of sub station are to be properly illuminated. The minimum lux levels to be maintained in the different areas are follows.
Sl No Location in sub station Minimum lux levels to be
provided
1 Control Room 350
2 L.T.Room. 150
3 CableGallery 150
4 Battery Room 100
5 Entrance Lobby 150
6 Corridor Landing 150
7 Conference Room &
Display Room
300
8 Rest Room 250
Main Equipment -- 50
Balance area. -- 30
10 Street / Road 30
9 Out Door Switch Yard
84
7.1. The aspects to be considered are
• The illumination design to be done by using the software
program to achieve specified levels of illumination most
economically
• The energy conservation methods are to followed by using CFL
fittings etc. where ever feasible with out compromising on
required illumination levels
85
8.0 Classification of the works to be executed in a sub-station
a) Civil engineering works
b) Electrical works8.1. Civil engineering works:
The Civil works comprise of
1. Buildings
i. Residential
ii. Non Residential – Office, control room, repair bay etc.
2. Design & construction of foundations for structures and equipment
structures and transformer plinth.
3. Cable trenches
4. Fencing around switchyard
5. Water supply
6. Drainage & Sewerage
7. Roads & paths
86
8.2. Electrical works comprise of:a) Choice of:
i. Switching schemes.ii. Bus bars.iii. Preparation of key diagram / single line diagram.iv. Preparation of Lay outs
b) Design & layout of earthing grids and protection against directlightning strokes.
c) Auxiliariesi. D.C. supply
• Battery set.• Battery Charger• D.C. Panel
ii. A.C. supply• Auxiliary Distribution Transformer.• Diesel Generator set.• A.C.Panel.
iii. Control cable & power cable schedule.iv. Switchyard lightingv. Fire fighting equipment.
Major SubMajor Sub--station Equipmentsstation Equipments
a)a) Power Transformers.Power Transformers.
b)b) Circuit breakers.Circuit breakers.
c)c) Instrument Transformers:Instrument Transformers:i.i. Current Transformers.Current Transformers.
ii.ii. Voltage transformers.Voltage transformers.
iii.iii. Capacitor voltage transformersCapacitor voltage transformers
d)d) Isolators / Disconnects.Isolators / Disconnects.
e)e) Lightning Arrestors.Lightning Arrestors.
f)f) Control & Relay panels.Control & Relay panels.
g)g) Shunt Capacitor Banks.Shunt Capacitor Banks.
h)h) Reactors.Reactors.
Technical parametersTechnical parameters
a)a)The principle points to be considered for The principle points to be considered for
selecting subselecting sub--station equipments arestation equipments are
•• Standards.Standards.
•• Principle parametersPrinciple parameters
•• Ratings & their choice.Ratings & their choice.
•• Technical requirements.Technical requirements.
•• Tests.Tests.
StandardsStandards -- Power TransformersPower Transformers
Sl.No Standards Title
1. IS – 10028 (Part 2 & 3)
Code of practice for selection,
installation & maintenance of
transformers (P1:1993), (P3:1991)
2. IS – 2026
3. IEC –76 (Part1 to Part 5)
4. IS-3347 (Part 1 to Part 8)
Dimensions for porcelain
transformer bushings for use in
lightly polluted atmospheres.
5. IS-3639(1991)Fittings and Accessories for power
transformers
6. IS – 6600 (1991)Guide for loading of oil immersed
transformers
7. IEC-354 (1991)Loading guide for oil immersed
power transformers
8. IEC-214 (1989) On-load tap – chargers
9. NEMA – TR – 1Transformers, Regulators and
reactors10.
Power transformers
CBIP Manual on Transformers
Principle Parameters Principle Parameters --Power TransformersPower TransformersSl. No. Item1. Type of power
transformer/installation
2. Type of mounting
3. Suitable for rated system
frequency
Rated voltage
Voltage ratio HV/IV/LV
5. No. of phases
6. No. of windings
7. Type of cooling
Maximum rating MVA 220 KV
winding HV
110/66 KV winding IV 100
11 KV tertiary winding LV
MVA rating corresponding to
cooling system.
a) ONAN cooling 60
b) ONAF cooling
c) OFAF cooling
11.
Connection symbol (vector
group)
4.
Examples of Technical requirements3 phase, auto winding
interconnecting transformer
suitable for outdoor installation.
3 phase core type winding
transformer suitable for out
door installation-----On wheel mounted on rails----
50 Hz 50Hz
245 KV class
220/110/11 KV
245 KV class
220/66/11 KV
Three Three
Auto Tr. With tertiary Three winding with tertiary
OFAF OFAF
8.
100
100
100
9.
80
100
80
100
10.
Winding connection HV Star
IV Auto
LV Delta
HV Star
IV Star
LV Delta
YNaod 11 YNynod 11
60
Principle Parameters Principle Parameters --Power TransformersPower TransformersSl. No. Item1. Type of power
transformer/installation
12.
System earthing
Percentage impedance voltage
on normal tap and at rated MVA
tolerance as per IS-2026.
a) HV-IV
b) HV-LV
c) IV-LV
Anticipated continuous loading of
windings
a) HV and IV
b) Tertiary
Tap changing gear :
a) Type
b) Provided on
c) Tap range
d) Step voltage
e) No. of steps
17.
Max. flux density in any part of
core and yoke at rated MVA,
frequency and normal voltage
(tesla)
18.
Current density of HV/IV/LV
winding
Insulation levels for windings:
a) 1.2/50 micro-second
wave shape impulse withstand
(KVP)
HV IV LV
950 325 170
HV IV LV
395 140 70
Type of winding insulation:
a) HV winding
b) IV winding
c) LV winding
Examples of Technical requirements3 phase, auto winding
interconnecting transformer
suitable for outdoor installation.
3 phase core type winding
transformer suitable for out
door installation
10
-----Effectively solidly earthed-----
13.
10
The tertiary winding is for stabilising purpose without loading.
The impedance shall be designed in confirmity with BIS, to
with stand the short circuit currents for a specified period.14.
------ Not to exceed 110 % of its rated capacity --------
15.
On load, suitable for bi-directional power flow
--------Neutral end of HV winding -------
+5% to –15% +5% to –15%
1.25% of 220 KV 1.25% of 220 KV
16 16
16.
Over voltage operating capability
& duration
i) 115% of rated voltage continuously
ii) 125% of rated voltage for 60 seconds
iii) 140% of rated voltage for 5 seconds
----------------1.60-----------------
-----------not exceeding 3 Amps per sq. mm----------
19.
HV
950
b) power
frequency voltage
withstand (KV rms)
HV
395
20.
Graded
Graded
Full
Graded
Graded
Full
-------------------- Un loaded teritiary ----------------
Principle Parameters Principle Parameters --Power TransformersPower TransformersSl. No. Item1. Type of power
transformer/installation
21.
System short circuit level &
duration for which the
transformer shall be capable to
withstand thermal and dynamic
stress (KA rms/sec)
Permissible temperatures rise
over ambient temp. of 500C
i) Of top oil measured by
thermometer
ii) Of winding measured by
resistance method
Minimum clearance in air (mm).
a) H.V.
i) HVPhase to Phase
b) I.V.
i) Phase to Phase
ii) Phase to ground
c) L.V.
i) Phase to Phase
ii) Phase to ground
Bushings
a) HV winding Line end
c) HV/IV winding neutral end
(for solid grounding)
-------------40 KA for 3 seconds ------------
Examples of Technical requirements3 phase, auto winding
interconnecting transformer
suitable for outdoor installation.
3 phase core type winding
transformer suitable for out
door installation
22.
Noise level at rated voltage and
frequencyLess than 83 db
---- as per table 01 of latest NEMA std. TR-1------
550C
500C
23.
500C
550C
ii) Phase to ground 1820 1820
20002000
350 350
700
1270 660
1430
320 320
25.
245 KV class OIP condenser 245 KV class OIP condenser
b) IV winding line end
24.
145 KV class OIP condenser
bushing
72.5 KV class OIP condenser
bushing
--------36 KV porcelain bushing-------
d) LV winding --------36 KV porcelain bushing-------
Principle Parameters Principle Parameters --Power TransformersPower TransformersSl. No. Item1. Type of power
transformer/installation
26.
Insulating medium
Terminal current rating
HV
IV
LV
HV/IV Winding neutral
28.
Max. Radio interference voltage
level at 1 MHz and 1.1 times
max. rms phase to ground
voltage for HV winding
Cooling equipments.
a) Number of Banks
c) No. of fans
Insulation level of bushings:
a) Lightning impulse withstand
(KVP)b) 1 minute power frequency
withstand voltage (KV rms)
c) Creepage
distance (mm)
a) Bushing current transformers
for tertiary provided in each
phase
i) Current Ratio (A/A)
ii) Accuracy class
iii) VA Burden
Examples of Technical requirements3 phase, auto winding
interconnecting transformer
suitable for outdoor installation.
3 phase core type winding
transformer suitable for out
door installationOIL OIL
27.
800 Amp 800 Amp
800 Amp 1250 Amp
2000 Amp 2000 Amp
800 Amp 1250 Amp
------------ 5000 Micro volts--------------
29.
Two nos. of 50% Bank Two nos.of 50% Bank
One 100% pump & one 100%
standby pump in each bank
One 100% pump & one 100%
standby pump in each bank
Adequate number of fans 18”/24” sweep with one stand by fan
in each group
25 mm per KV of highest
system voltage
25 mm per KV of highest
system voltage
HV IV LV
1050 325 170
HV IV LV
1050 550 170
HV IV LV
460 230 70
b) No. of pumps
30.
31.
--------- To be provided for LV bushings-------
1000/1 1000/1
�-----------5P – 20------------�
�------------15--------------�
HV IV LV
460 140 70
Standard RatingStandard Rating --Power TransformersPower Transformers
Sl.
No.
KV CLASS RATING
1 33/11 KV 5 MVA
8 MVA
12.50MVA
16/20 / 31.5 MVA
a. 110/11 KV 10 MVA
16/ 20 / 31.5 MVA
b. 110/33-11KV 10 MVA
16/20 / 31.5 MVA
a. 220/110/11 KV 100 / 150 MVA
b. 220/66/11 KV 100 / 150 MVA
5 a. 400 / 220 /33 KV 315 MVA
b. 400 / 220 / 33 KV
3 Units of single phase
transformers.166 MVA
2 66/11KV
3
4
500 MVA
TestsTests --Power TransformersPower Transformers
TESTS:TESTS:
a) a) Type tests.Type tests.
b) Routine / Acceptance Testsb) Routine / Acceptance Tests
Type TestsType Tests
i.i.Temperature rise testTemperature rise test
ii. ii. Vacuum test on transformer tankVacuum test on transformer tank
iii.iii.Relief device testRelief device test
iv.iv.Short circuit testShort circuit test
v v Impulse test on principle tap. Impulse test on principle tap.
vi. vi. IPIP--55 test for OLTC cabinet and cooler control cabinet. 55 test for OLTC cabinet and cooler control cabinet.
TestsTests --Power TransformersPower TransformersRoutine tests:Routine tests:
i.i.Operation and dielectric test of OLTCOperation and dielectric test of OLTC
ii.ii.Magnetic circuit testMagnetic circuit test
iii iii OC & SC testsOC & SC tests
iv. iv. Oil leakage test on transformer tank after complete assembly.Oil leakage test on transformer tank after complete assembly.
v. v. Measurement of zero sequence and reactance Measurement of zero sequence and reactance
vi vi Measurement of acoustic noise levelMeasurement of acoustic noise level
vii vii Measurement of power consumption by fans and oil pumpsMeasurement of power consumption by fans and oil pumps
viii. viii. Measurement of harmonic level on noMeasurement of harmonic level on no--load currentload current
ix.ix.Measurement of capacitance and tanMeasurement of capacitance and tan--delta to determine capacitancedelta to determine capacitance
between winding and earth before and after series of between winding and earth before and after series of didi--electric tests.electric tests.
x. x. Insulation resistance testInsulation resistance test
xi. xi. Ratio and polarity test Ratio and polarity test
xii.xii.DiDi--electric and PPM test on oilelectric and PPM test on oil
xiii xiii TanTan--delta test on bushingdelta test on bushing
xiv. xiv. Measurement of copper and iron losses.Measurement of copper and iron losses.
xvxv Leakage tests for radiators / cooler tanksLeakage tests for radiators / cooler tanks
xvi xvi Weld test Weld test
97
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