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Bus Switching Scheme& Substation Layout
K K Sarkar (E_mail:[email protected])
Chief Design Engineer (Engg-s/s)
Power Grid Corporation of India
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Important considerations in layout..
Reliability and Security
- Selection of Bus Scheme
- Ease of Maintenance
- Operational Flexibility
Short Circuit Level
Shape of the land
Altitude of the land above mean sea level
Feeder orientation
Safety of Equipment and personnel
Possibility of future expansion
Cost
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Bus Switching Schemes…
Single Main Scheme
Double Main Scheme
Single Main & Transfer Scheme
Double Main with by-pass isolator scheme
Double Main & Transfer Scheme
One & Half Breaker Scheme
Double breaker Scheme
Ring Bus Scheme
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Simplest and cheapest bus bar scheme
Maintenance and extensions of busbars are not possible without shutdown of the substation.
Operation & maintenance of bus bar is easy.
SINGLE BUS SCHEME
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Individual CB can be taken out for maintenance on-load at a time.
The transfer bus coupler acts as the breaker for the circuit under by pass.
Individual circuits have a bypass isolator to connect to the transfer bus and this isolator will be closed during bypass operation of that particular circuit.
SINGLE MAIN AND TRANSFER SCHEME
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Load will be distributed on both the buses and the bus coupler shall be normally closed.
For maintenance & extension of any one of the buses the entire load will be transferred to the other bus.
On load transfer of a circuit from one bus to the other bus is possible through bus isolators provided the bus coupler is closed and thereby two buses are at the same potential.
On load bypassing of any circuit for breaker maintenance is not possible.
DOUBLE BUS SCHEME
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This bus arrangement provides the facilities of a double bus arrangement & a main and transfer bus arrangement.
The bus to which the transfer bus isolator is connected can be used as a transfer bus also.
During the time a circuit is under bypass, the bus coupler will act as the breaker for the bypassed circuit.
DOUBLE BUS WITH BY-PASS SCHEME
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In this bus scheme, in addition to the two main buses there will be a separate transfer bus also.
Since separate transfer bus is available there will be no need of transferring the load from one bus to the other bus unlike in a double main cum transfer bus arrangement.
Other features are similar to the one described in double bus with by pass arrangement.
DOUBLE MAIN AND TRANSFER SCHEME
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In this scheme, two circuit have three breakers, the middle breaker ties the two circuits and hence is called the tie breaker.
Breaker or bus maintenance is possible without any shut down of the feeder
Even if both the buses are out of service, power can be transferred from one feeder to another feeder through tie breaker
ONE AND HALF BREAKER SCHEME
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Each feeder is controlled by two breakers.
This arrangement is comparatively costlier than other scheme and hence followed in very important circuit only.
In this arrangement breaker maintenance for any feeder circuit is easily possible without any shutdown.
DOUBLE BUS TWO BREAKER SCHEME
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As long as the ring is closed load has two sources of supply and any circuit breaker can be taken out of service without affecting the supply.
Extension of ring scheme is difficult.
No bus bar protection required.
RING BUS SCHEME
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Selection of Switching Schemes…
No reliability in Single Main, Double Main, Single Main & Transfer Scheme w.r.t bus fault, feeder fault & breaker maintenance
Double Main & Transfer Scheme, One & Half Breaker Scheme & Double breaker Scheme are characterized by reliable and interruption free supply.
One & half breaker scheme can be selected for EHV substations due high reliability, operational flexibility, ease of maintenance, ease of expansion, due consideration of cost
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Important considerations in layout..
Reliability and Security
- Selection of Bus Scheme
- Ease of Maintenance
- Operational Flexibility
Short Circuit Level
Shape of the land
Altitude of the land above mean sea level
Feeder orientation
Safety of Equipment and personnel
Possibility of future expansion
Cost
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Bus Bar Design, Selection of conductor levels & Bay width calculation.. Selection of conductor (AAC, ACSR, Tube)
Current Carrying capacity with temperature rise of 35 deg.C over ambient of 50deg.C ambient (IEEE-738)
Temperature Rise during short circuit
Stresses in tubular bus
Cantilever Strength of post insulator
Deflection of the tube
Natural frequency of tubular bus bar
Aeolian Vibration
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Bus Bar Design & Selection of conductor levels..
Electrical Clearances (IEC-60071)
Corona
Electric Field (10kV/m)& Magnetic Field (500μT)
Short Circuit Forces (IEC-60865)
Sag-Tension Calculation
Normal Tension (Factor of safety 2.0) and Short Circuit Tension (Factor of Safety 1.5)
Height of conductor levels
Bay width & Phase to Phase spacing
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Minimum Clearances for Layout (at altitude <1000m above mean sea level)…
Voltage Level
(Rated)
Ph-Ph
(m)
Ph-E
(m)
Sectional
Clearance
(m)
BIL
(kVp)
SIL
(kVp)
765 kV 7.6
(cond-cond)
9.4
(rod-str)
4.9
(cond-str)
6.4
(rod-str)
10.3 2100 1550
400 kV 4 3.5 6.5 1550 1050
220 kV 2.1 2.1 5 1050 650
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Minimum Clearances for Layout (at altitude <1000m above mean sea level)…
Voltage Level (Rated)
Ph-Ph
(mm)
Ph-E
(mm)
Sectional
Clearance
(mm)
132 kV 1300 1300 4000
110 kV 1100 1100 3800
66 kV 630 630 3500
33 kV 320 320 2800
Altitude corrections w.r.t clearances, insulation levels, creepage and oil
temperature rise of the equipment shall be considered for altitudes more
than 1000 m above mean sea level.
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Bay widths & levels…
Voltage Level
Bay width
First Level
Second Level
Third level
BIL SIL
400 kV 24m 8m 15m 22m 1550 1050
220 kV 16m 5.9m 11.7m 16.2m 1050 650
132 kV 12m 4.6m 8m 12m 650 NA
66 kV 7.6m 4m 6m 9.5m 325 NA
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Type of Isolator & Structure in Layout
Type of Isolator Horizontal Centre Break Isolator (HCB)
Horizontal Double Break Isolator (HDB)
Pantograph Isolator (Panto)
Vertical Break Isolator (VB)
Staggered
Type of Structure Pie (╥) structure
Enclosed (Π) structure
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Height of shield wire, Height & Location of LM & Location of Fence..
DSLP Calculation to decide the height of shield wire and/or Height & location of LM Rolling Sphere Method (IEEE-998)
Razevig Method
Earthmat Design (IEEE-80/CBI&P Report No. 302) – Location of switchyard fence Touch Potential
Step Potential
Grid Resistance
Earth Potential Rise (EPR)
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Location of other buildings, auxiliaries..
Control Room
Fire fighting pump house (FFPH)
DG set
LT station placement
Roads & rail tracks
Switchyard Panel Room
Open Store
Colony and other infrastructures
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N 4
775.
0
N 4
800.
0
N 4
825.
0
N 4
850.
0
N 4
875.
0
N 4
900.
0
N 4
925.
0
N 4
950.
0
N 4
975.
0
N 5
000.
0
N 5
025.
0
N 5
050.
0
N 5
075.
0
N 5
100.
0
N 5
125.
0
N 5
150.
0
N 5
175.
0
N 5
200.
0
N 5
225.
0
N 5
250.
0
N 5
275.
0
N 5
300.
0
N 5
325.
0
N 5
350.
0
N 5
375.
0
5000.0 E
5025.0 E
5050.0 E
5075.0 E
5100.0 E
5125.0 E
5150.0 E
5175.0 E
5200.0 E
5225.0 E
5250.0 E
5275.0 E
5300.0 E
5325.0 E
5350.0 E
5375.0 E
5400.0 E
N 4
750.
0
4975.0 E
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1. ALL DIMENSIONS ARE IN METRE UNLESS SPECIFIED.
2. LOCATION OF ALL BUILDINGS ARE INDICATIVE.
3. ROUTE OF APPROACH ROAD IS INDICATIVE ONLY. THE SAME SHALL BE DECIDED BY SITE.
400 kV M
AIN
BU
S - II
PRESENT SCOPE
FUTURE
FOR TENDER PURPOSE ONLY
400 kV M
AIN
BU
S - I
GA Drawing of 400/132kV Substation
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A. DSLP by Lightning Mast
0.2h
h
2h/3
hx rx
0.75h 0.75h
1.5h 1.5h
rx
Fig. 2 (a) : Zone of Protection for single lightning mast
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Zone protected by two lightning mast around themselves
R
a c
b 0.2h
h
hox rx
hx
a 0.75h
1.5h
rx
Box
rx
Fig. 2.(b): Zone of protection for two lightning masts
Where R is the circumradius of the triangle formed by a, b & c.
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Calculation of overlappings (Bx) :
Bx=1.5.hox.px( 1-(hx/0.8hox)) if hx<2/3rd of hox
Bx=0.75.hox.px.( 1-(hx/hox)) if hx>2/3rd of hox
where,
h is the height of the lightning mast/tower including peak
hox is the maximum hight protected is given by, hox=h-(a/7p)
a is the distance Lightning Masts / Tower Peaks
hx is the maximum height of the objects to be protected from side strokes
px= 5.5/sqrt(hox) if hox>30.0 m
px= 1.0 if hox<30.0 m
DSLP Calculation
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Zone protected by three(3) lightning masts :
The condition that the area among the three (3) lightning masts
at a level 'hx' will be protected is given as :
D <= 8(h-hx)p
where,
D is the circumdiameter of the triangle formed
by the three lightning masts.
D=a1 /sin{arccos((a22+a3
2-a1
2)/2a2a3)}
DSLP Calculation
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LIGHTNING PROTECTION BY OVERHEAD SHIELD WIRES:
0.2h
h
2h/3
hx bx
0.6h 0.6h
1.2h 1.2h
2bx
Fig : Protective Zone of a ground/ shield wire
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The breadth of the protective zone offered by a single shield
wire on the ground level in a plane perpendicular to the shield
of wire is equal to 1.2 h , where h is the height suspension
of the shield wire.
Half the breadth of the protective zone "bx" at level hx is given by:
bx=1.2 h ( 1-(hx/0.8h)) if hx<2/3rd of h
bx=0.6 h ( 1-(hx/h)) if hx>2/3rd of h
where,
h is the height of the tower including peak
hx is the height of the objects to be protected
from side strokes
DSLP Calculation
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The radius of protective zone offered by a lightning mast at height "hx"
from ground level is given by:
rx=1.5hp( 1-(hx/0.8h)) if hx<2/3rd of h
rx=0.75hp( 1-(hx/h)) if hx>2/3rd of h
where,
h is the height of the lightning mast/tower including peak
hx is the height of the objects to be protected from side strokes
p= 5.5/sqrt(h) if h>30.0 m
p= 1.0 if h<30.0 m
DSLP Calculation
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POWERGRID 400 kV STANDARD LAYOUT
Bus Scheme adopted : One & Half Breaker SchemeLayout : I-TypeFirst (Equipment Level)- 8mSecond Level (Main Bus) - 15mThird Level (Jack Bus) – 22mBay Width – 24mMain Bus : Quad ACSR Bersimis/Quad AAC BULLMain Bus Span: 48mEquipment Interconnection: 4” or 4.5” IPS Al tubeNormal Tension for gantry Structure: 4T/phaseNormal Tension for O/H shield wire: 0.8T, 10.98mm dia GS
wire (7m peak)Fault Level: 40kA/50kA/63kA (1 sec)Cantilever Strength of Post Insulator : 800 kg
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POWERGRID 220 kV STANDARD LAYOUT
Bus Scheme adopted : Double Main & Transfer Scheme Scheme
First (Equipment Level)- 5.9m
Second Level (Main Bus) – 11.7m
Third Level (Jack Bus) – 17.2m
Bay Width – 16m
Main Bus : Quad ACSR Moose
Main Bus Span: 48m
Equipment Interconnection: 4” IPS Al tube
Normal Tension for gantry Structure: 4T/phase or 2T/Phase
Normal Tension for O/H shield wire: 0.8T, 10.98mm dia GS wire (5m peak)
Fault Level: 40kA/50kA (1 sec)
Cantilever Strength of Post Insulator : 800 kg
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