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SWITCHYARD
OF
A 500MW POWER PLANT
ISOLATED PHASE BUSDUCT
3 NOS.GEN TRF 1  BANK
400 / 21 kV 200 MVA
FOR GENERATOR AND TRFS.
GCB
UNIT TRF
21 / 11.5 KV
50 MVA
A
11 kV 1 BA SWGR
UT - B
LINE
CT’s
GENERATOR
400 KV
SC
VT 1,2,3
11 kV 1 BB SWGR
EARTH SWITCH OF
GENERATOR
NEUTRAL
CT’s
21 KV
11 KV
LA
VT 4
CT
EARTH
NGT & NGR
21KV / 220 V
175 KVA
0.212 OHMS 800 A
Salient Features of the Project
•Total Capacity
•Generating Voltage
•Transmission Voltage•400kV Switchyard
-
•220kV Switchyard
-
500MW
21kV
400kV & 220kV
One & Half Breaker
Scheme
Two Main one Transfer
Bus Scheme
What is a Switchyard ?
It is a switching station which has the following credits :
(i) Main link between Generating plant and Transmission system,
which has a large influence on the security of the supply.
(ii) Step-up and/or Step-down the voltage levels depending upon
the Network Node.
(iii) Switching ON/OFF Reactive Power Control devices, which
has effect on Quality of power.
Switchyard Type
 Conventional Air Insulated Type.
 Gas Insulated type.
 Outdoor Gas Insulated type.
Switchyard layout
Objective:
Substation layout consists essentially in arranging a number
of switchgear components in an orderly pattern governed by
their function and rules of spatial separation as described in
electrical single line diagram.
Pre-requisites:
1) single line diagram
2) general layout plan of power plant
3) orientation of line evacuation
4) control room building
LAYOUT CONTD…

Options / Alternatives
The layout will vary for the following:
1) Switching schemes
2) Type of insulation - Air Insulated/Gas Insulated.
SWITCHYARD EQUIPMENTS
Equipments commonly found in switchyard :
1.
Lightening arrestor
2.
Current transformer
3.
Voltage transformer
4.
Power transformers / I.C.T.
5.
Bus bar and clamp fittings
6.
Support structure
7.
Isolators
8.
Circuit Breaker
9. Wave traps
10. Earthing switch
Functions of various equipment :
* Transformers :
- Transforms the voltage levels from higher to lower level
or vice versa, keeping the power constant.
* Circuit breakers :
- Makes or automatically breaks the electrical circuits under
Loaded condition.
* Isolators :
- Opens or closes the electrical circuits under No-load
conditions.
* Instrument transformers :
- For stepping-down the electrical parameter (Voltage or
Current) to a lower and safe value for Metering and
Protection logics.
* Earth switch :
- Used to connect the charged body to ground to discharge the
trapped charge to have a safe maintenance zone.
* Lightning arrestors :
- Safe guards the equipment by discharging the high currents
due to Lightning.
* Overhead earth wire :
- Protects the O/H transmission line from Lightning
strokes.
* Bus bar :
- Conductors to which a number of circuits are
connected.
* Wave Traps/Line traps :
- Used in PLCC circuits for Communication and
telemetering.
* Reactive Power control devices :
- Controls the reactive power imbalance in the grid by switching
ON/OFF the Shunt Reactors, Shunt Capacitors etc.,
* Current Limiting Reactors :
- Limits the Short circuit currents in case of faulty conditions.
EXECUTION SEQUENCE:
Execution sequence for a substation
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Tower foundation
Equipment foundation
Laying of Cable trench
Laying of Earthmat
Support structure installation
High level stringing
Equipment installation
Equipment interconnection
Cabling layout
Commissioning
TABLE I: INSULATION LEVELS & CLEARANCE
REQUIREMENTS AT DIFFERENT VOLTAGE LEVELS
NOMIN
AL
SYSTE
M
VOLTA
GE
KV
33
66
132
220
400
765
INSULATION LEVELS
LIGHTNING
IMPULSE
LEVEL
(kVp)
SWITCHIN
G SURGE
LEVEL
(kVp)
POWER
FREQUE
NCY
IMPULSE
LEVEL
(kVrms)
170
325
650
1050
1425
2100
1050
1550
275
460
630
830
HIGHES
T
SYSTEM
VOLTAG
E KV
36
72.5
145
245
420
800
MINIMUM CLEARANCE
BETWE
EN
PHASE
AND
EARTH
(MM)
BETW
EEN
PHAS
ES
(MM)
320
630
1300
2100
3500
--
320
630
1300
2100
4000
--
GROUND
CLEARA
NCE
(MM)
SECTION
AL
CLEARA
NCE
(MM)
HEIGHT
OF
SUPPOR
TS (mm)
3700
4000
4600
5500
8000
--
2800
3000
3500
4300
6500
10300
2500
2500
2500
2500
2500
2500
Clearance contd…
5) Equipment spacing
a)
Ease of maintenance/removal of
equipment.
b)
Equipment foundation & their
cable trenches.
c)
Distance between LA and
equipment based on the
protection reach of LA.
d)
The spacings are generally kept
in
order to achieve
various
clearances specified
Clearance contd…
6) Bus bars:
The bus bars of 400 kV switchyard are generally made up 4 “IPS
aluminum tube or Quad Moose rated for 3000 A”.
The bus bars of 220/132kV switchyard are generally made up of 3 “IPS
aluminum tube or quad/ twin moose conductor”. Bus bars are placed at right
angles to the feeders for tapping the power.
7) Equipment Interconnection
8) Spacer spans and locations
9) Connection Level
10) Land & Road Layout
11) Sequence and mounting of line traps
Clearance contd….
12) Control Room Layout
13) Lighting System
14) Cabling Philosophy
15) Gravel Filling
16) Earthing System
17) Lightning Protection System
Selection of Bus Switching Scheme

PRE-REQUISITES
1)System security
2)Operational flexibility
3)Simplicity of protection arrangements
4)Ability to limit short circuit levels (ease
of
sectionalizing)
5)Maintenance – Its effect on system
security
6)Ease of extension
7)Total land area
EVOLVING A SUBSTATION
LAYOUT
LAYING OUT A SUBSTATION INVOLVES STEP-BY-STEP
PROCEDURE. MOST IMPORTANT POINTS TO BE CONSIDERED
ARE BRIEFLY DESCRIBED BELOW:
THE IMPORTANT ELECTRICAL PARAMETERS ARE ESTABLISHED
BY THE SYSTEM DESIGN. THE MAIN PARAMETERS ARE:
1) THE VOLTAGE AND BASIC INSULATION LEVEL OR
SWITCHING SURGE LEVEL., THE SITE AND CLIMATIC
CONDITIONS, THE METHOD OF CIRCUIT CONNECTION,
AND SWITCHING OVER-VOLTAGE CONDITIONS.
2) THE BUS BAR SYSTEM DIAGRAM, THE NUMBER OF
CIRCUITS AND THEIR PURPOSE I.E. THE CONTROL OF
GENERATORS, TRANSFORMERS, FEEDERS,
ETC.
THE DIAGRAM SHOULD INCLUDE DETAILS
EXTENSIONS
AND FUTURE CONVERSION TO
DIFFERENT BUS BAR
SYSTEM, IF INTENDED.
OF
A
EVOLVING A SUBSTATION
LAYOUT
1)
THE CONTINUOUS CURRENT RATING OF THE BUS BARS AND
CIRCUITS.
2)
THE SHORT CIRCUIT RATING OF BUS BARS AND EQUIPMENTS.
3)
PARTICULARS OF REACTORS, NEUTRAL EARTHING EQUIPMENT
AND REACTING, Interconnecting Transformers REQUIRED.
4)
METHOD OF CONNECTION OF CIRCUITS, WHETHER BY
OVERHEAD LINES OR BY CABLES.
5)
DETAILS OF LIGHTNING PROTECTION EQUIPMENT.
6)
DETAILS OF PROTECTIVE EQUIPMENT, DETERMINING THE
INSTRUMENT TRANSFORMERS REQUIREMENTS, CARRIER
CURRENT EQUIPMENT ETC.
EVOLVING A SUBSTATION
LAYOUT
THE EXTENT TO WHICH CIRCUIT AND BUSBAR OUTAGES FOR
MAINTENANCE WILL BE POSSIBLE.
SOME PARAMETERS WHICH INFLUENCE THE FORM OF THE LAYOUT ARE
DETERMINED BY THE LOCAL CONDITIONS. THESE ARE:
1)
THE AVAILABLE LAND AREA, SITE AND CLIMATE CONDITIONS,
PLANNING AUTHORITY REQUIREMENTS AND AESTHETIC
CONSIDERATIONS DETERMINE THE TYPE OF SUBSTATION.
2)
THE DIRECTION OF OVERHEAD LINE ENTIRES POSITION AVAILABLE FOR
TERMINAL TOWERS, LOCATION OF TRANSFORMERS AND REACTORS,
ETC.
3)
THE AVAILABILITY OF MATERIALS AND THE TRANSPORT AND ACCESS
FACILITIES.
4)
THE CAPABILITY AND SKILL OF THE MAINTENANCE STAFF DETERMINES
THE IMPORTANCE OF CLARITY OF LAYOUT AND SIMPLICITY OF
MAINTENANCE ZONING.
GUIDELINES FOR MAINTENANCE OF
OIL PIT FOR
TRANSFORMERS AND REACTORS
 1.0 INTRODUCTION:
 The layout for a transformer or reactor is planned in such away that there is
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adequate oil drainage facilities from underneath the equipment. This is essential in
order to prevent catastrophic damage to nearby building/ equipments, if the
transformer fire takes place and the oil is accumulated below the equipment due to
explosion of the transformer/ reactor tank. The oil pit needs to be cleaned at a
regular interval so that the oil drainage path is not blocked and in case of explosion,
the oil is freely drained to the main oil pit. This regular cleaning is essential because
at one of the site, although the oil pit was there below the transformer tank, its
drainage was chocked and the transformer fire was accelerated since the
accumulated oil in the pit also caught fire.
2.0 TYPES OF OIL PIT:
There are two types of oil pits in practice which are made below transformers/
reactors depending upon location, size and oil quantity etc. These are:
Soak Oil pit.
Drain and Retention Oil Pit.
SOAK OIL PIT:
If the oil pit provided below the transformer/reactor is not connected with the oil
pit of any other equipment or main oil pit. it is classified as soak oil pit. The total
volume of this individual soak oil pit is designed in such a way that volume of soak
oil pit up to gravel filling level minus the volume of gravels should at least be equal
SOAK OIL PIT
DRAIN AND RETENTION OIL PIT:
 For the transformer or reactors located in the
transformer yard i. e. unit aux.
transformers/station aux. transformers/generator
transformers and other transformers of 25 MVA
and above rating, individual oil drain pits are
provided and these individual oil pits are
connected to one common retention oil pit for
oil/water separation as these transformers are
provided with mulsifire system and in case of fire,
the mulsifire system will spray water, which will
occupy the empty volume available in retention oil
RETENTION OIL PIT
 The retention oil pit has two interconnecting
chambers. The first chamber is called main chamber
and the second chamber is called separation chamber.
These two chambers are interconnected with a pipe. In
case of fire, the oil-water mixture comes to the main
chamber where the pipes from drain oil pit of
individual transformers are connected. From here, by
virtue of difference in specific gravity, the water is
separated and flows to separation chamber. The
separation chamber is connected to surface water
drain to which the water is drained. The arrangement
is shown in figure-3.
RETENTION OIL PIT
 After the water is removed, the waste oil is pumped
out to waste oil tankers. Under normal conditions,
the retention oil pit is filled with water.
 The total volume of main and separation chamber
is sized to contain total oil volume of the largest
transformer plus the volume of water sprayed
during 10 minutes of mulsifire operation.
 RECOMMENDATIONS:
 Due to oil seepage and accumulation of dust in the
oil pit, it becomes sticky substance like paste and
has a tendency to clog the loose gravels in such a
way that the oil/water cannot flow freely from the
drain oil pit to retention oil pit. Further, due to
Design Philosophy / Practice

For designing a switchyard layout, various aspects are
considered which are described here under:
 Space around the switchyard. Adequate space
should be provided for extension of the switchyard
facilities when generating units or transmission lines
are added in the future. The immediate surroundings
should permit the routing of lines to the switchyard
area from at least one direction without the need for
heavy dead-end structures in the yard.
 Switchyard location. The switchyard should be
sited as near to the main plant as space permits, in
order to minimize the length of control circuits and
power feeders and also to enable use of service
facilities located in the main plant.
CLEARANCE:
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Clearance. The placement of equipment in EHV switchyard is
greatly affected by the air clearances to be adopted. They are as
follows:
Earth Clearance: this is the clearance between live parts and earthed
structures, walls, screens and ground. A minimum height of live
conductors above ground must also be maintained as per IE rules.
Also there should be a certain minimum height of supports of
various equipment (depending upon the fact that the bottom of any
insulator has to be 2.5 meters above the ground level as statutory
clearance).
Phase Clearance: this is the clearance between live parts of different
phases.
Isolating Distance: this is the clearance between the terminals of an
isolator and the connections thereto.
Section Clearance: this is the clearance between live parts and the
terminals of a work section. The limits of this work section, or
maintenance zone, may be the ground or a platform where a
personnel has to carry out work.
Mounting of Line traps:
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Sequence and mounting of line traps
The sequence of installation of line traps, lightning
arresters, coupling capacitors is given below for any
line feeder:From line end: BPI, LA, CVT, Line Trap.
A bus post insulator is installed at line end
to avoid mechanical forces on LA.
Mounting of the line trap shall be of
pedestal type
Control room layout&lighting
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Control Room Layout
A Control room building has to be built in the switchyard. Its
location should be such as to minimize control cables between
switchyard and main plant. Also it should have easy approach from
main road and switchyard view should be possible from the control
room. The control of the switchyard is either form the switchyard
control room or the main plant control room. Al the panels for the
purpose are to be located in this control room. For control of the
switchyard please refer separate guidelines.
Lighting System
Switchyard lighting consist of outdoor lighting of the
yard and lighting of Switchyard control room building. The outdoor
lighting is to be done with 400W HPSV lamps. The various lighting
fixtures complete with lamps and accessories shall be mounted only
on Lightning Masts or Lighting poles. There should be no Lamp
suspended on gantries or any live structure of the switchyard as per
IE rules. The lighting along the road shall be achieved by providing
suitable fixtures on lighting poles. Different illumination levels and
type of lighting fixures and lighting design in Switchyard shall be as
 cable trenches. All cables used in switchyard are
armoured type due to induced voltages. There are
two types of cables used in a Switchyard viz. Power
cables and control cables. The sizes for power
cables vary from 3X 35mm2 to 2cX 6 mm2 and for
control cables it varies from 5c X2.5 mm2 to
27cX2.5 mm2. Refer guideline on cable routing in
switchyard for more details.
 Gravel Filling

Gravel or surface material coverings,
usually upto 150mm in depth, are useful in
retarding the evaporation of moisture and thus in
GRAVELS IN SWITCHYARD
 1.The gravels with voids from a good insulating layer
above the soil(Earthing) so that step potential is easily
achieved.
 It slows weeding in the soil
 Gravels acts as a flame retardant in case of flaming oil
being dropped from CT/CVT
 It avoids snakes and other raptiles.
PREPARATION OF BASIC
LAYOUT
 WHILE MEETING ALL THE NEEDS ESTABLISHED THE
FOLLOWING IDEALS SHOULD BE AIMED AT IN MAKING THE
BASIC CIRCUIT LAYOUT.
 MINIMUM GROUND AREA
 MINIMUM QUANTITIES OF CONDUCTOR, JOINTS AND
STRUCTURE
 MINIMUM NUMBER OF INDEPENDENT INSULATORS,
ESPECIALLY IN THE BUS BAR ZONE.
 AFTER HAVING DETERMINED THE ELECTRICAL CLEARANCE BE
USED A ROUGH CIRCUIT LAYOUT IS MADE. SEVERAL POSSIBLE
ALTERNATIVES ARE PREPARED FROM WHICH THE MOST
SUITABLE ONE WILL BE SELECTED. SOME VARIATION IS
NEEDED, TO MEET THE REQUIREMENTS OF DIFFERENT TYPES
OF CIRCUIT.
 IT IS ALSO NECESSARY TO CALCULATE SHORT CIRCUIT AND
OPTIONS/ALTERNATIVES
 OPTIONS/ALTERNATIVES
1)Single sectionalised bus
2)Main and transfer bus
3)Sectionalised Main bus with transfer bus
4)Sectionalised double main and transfer
bus
5)Double Bus Scheme
6)Ring bus
7)One and a half breaker
8)Double bus, double breaker
Single Sectionalized
Bus-bar system
I/C Feeders
CB
Bus-bar
Isolators
O/G Feeders
Double main bus & transfer bus system
Merits
Demerits
Remarks
1.
High cost due to
three buses
Preferred by
some utilities for
400kV and
220kV
important
substations
2.
Most flexible in
operation
Highly reliable
Either main bus can be
taken out
of service at any time for
maintenance
ONE & HALF BREAKER DESCRIPTION
BUS-1
3. THEY ARE DESIGNATED AS 1-52 CB, 2-52 CB, 3-52 CB.
1-52 CB
BUS-1
BUS-2
1-52 CB
3-52 CB
2-52 CB
2-52 CB
3-52 CB
BUS-2
ONE & HALF BREAKER DESCRIPTION
BUS-1
4. LINE - 1 IS CONNECTED IN BETWEEN 1-52 CB & 2-52 CB.
5. LINE - 2 IS CONNECTED IN BETWEEN 3-52 CB & 2-52 CB.
1-52 CB
LINE-1
BUS-1
BUS-2
1-52 CB
3-52 CB
2-52 CB
LINE-2
2-52 CB
3-52 CB
LINE-1
BUS-2
LINE-2
400KV One and half Breaker Scheme
400 KV BUS SECTIONALIZER
220KV two main Bus one transfer Bus Scheme
One & half breaker scheme
Merits
1.
Flexible operation for
breaker in breaker
maintenance
2. Any breaker can be
removed
from maintenance without
interruption of load.
3. Each circuit fed by two
4. All switching by breaker.
5. Selective tripping
Demerits
1)One and half
breakers per
circuit,
hence higher cost
2) Protection and
auto-reclosing
more
complex since
middle breaker
must
be responsive to
both associated
circuits
Remarks
1. Used for 400kV
& 220kV
substations
Imp. considerations in substation
design

Safety of personnel and equipment
 Reliability and Security
 Adherence to
 Statutory obligations
 – I.E. rules, Environmental aspects
 Electrical design considerations
 Structural design considerations
 Ease of maintenance
 Possibility to Expand