ÖUnderground
Distribution
System
Outline
Introduction
Network Architecture
Padmounted Transformer
Padmounted Switchgear
Cables
Terminations and Splices
Direct Burial vs Duct System
Cable Ampacity Rating Factors
Calculation of Pulling Tension
Code Requirements
UG Accessories
Introduction
WHY GO FOR UG DISTRIBUTION?
1. Congestion of Overhead Facilities
2. Aesthetics
Aesthetics
3. Safety
4. Reliability
OTHER REASONS FOR ADOPTING
UG DISTRIBUTION?
Ö ROW constraints
Ö Government regulations
Ö Customer requirement
DISADVANTAGES OF UG SYSTEM
1. Costs more than the traditional OH
system
2. Requires longer time to maintain
Overhead
vs.
Underground
Distribution System
Overhead Residential Customers
Network Architecture (OH)
Overhead Lines
Substation1
Distribution
Transformers
Loadbreak Switch
Distribution
Transformers
Overhead Distribution System
Substation2
Underground Residential Customers
Underground Residential Customers
TERMINAL POLE
CONCRETE
SECONDARY
PEDESTAL
CONCRETE
PEDESTAL
SERVICE
BOX
FINISH
GRADE LINE
PADMOUNTED
TRANSFORMER
PRIMARY
CONDUIT
MANHOLE
SECONDARY
CONDUIT
Typical URD Lay-out
T Y P IC A L
STREET A
STREET B
U R D
T R A N S F O R M E R
S E R V IC E
B O X
M A N H O L E
P R IM A R Y
C A B L E
S E C O N D A R Y
C A B L E
U R D
L A Y O U T
Network
Architecture
1. Primary Radial System
2. Alternate Feed System
3. Loop Feed System
4. Loop Feed with Radial Tap System
5. Loop-On-Loop System
Network Architecture (UG)
Open-Point
Overhead Line
Substation
Fuse
Padmount
Transformers
Padmount
Transformers
Underground Distribution System Tapped
from an Overhead w/o Switchgear
Network Architecture (UG)
Overhead Line
Open-Point
Fuse
Substation
Switchgear2
Switchgear1
Padmount
Transformers
Padmount
Transformers
Underground Distribution System Tapped
from an Overhead w/ Switchgear
Padmounted Transformer
Single-Phase Padmounted Transformer
Flip-top Cover
Transformer tank & cabinet
Bushing with elbow
Parking stand bracket
Low voltage terminal
Source: Cooper Power Systems
Typical Primary Connections on
Deadfront Pad-Mounted Transformer
1-Phase Pad-Mounted Transformers
Cable Entrance Zone
Three-Phase Padmounted Transformer
Transformer tank & cabinet
Bayonet fuse
Deadfront primary bushing
Parking stand bracket
Secondary bushing & terminal
Drain valve
Source: Cooper Power Systems
Three-Phase Padmounted Transformer
Three-Phase Padmounted Transformer
PRIMARY
COMPARTMENT
Bayonet Fuse
Pressure Vacuum Gage
Off Load Tap
Changer
T-Blade Loop-feed
Switch
Parking Stand
High Voltage
Bushing
High Voltage
Neutral Bushing
Three-Phase Padmounted Transformer
SECONDARY
COMPARTMENT
Liquid Level Gage
w/ Alarm Contacts
Phenolic Board Support
Pressure Release Device
Nameplate
Low Voltage Bushing
Current Transformer
Mounting Plate
Padmounted Transformer Construction
Partial Range CLF
Bayonet Expulsion
Fuse
Tank
Transformer Winding
and Core
T-blade sectionalizing
switch
Typical Primary Connections on a Live
Front Pad-Mounted Transformer
Submersible Transformer
Padmounted Switchgear
Padmounted Switchgear
• Latest technology
• Low profile
• Low maintenance
• Fast service restoration
• For flooded areas and
high contaminant sites
Basic Switchgear Configuration
Single-Line Diagram
Padmounted Switchgear
Tap-Side
flip-top door
full range CLF
fuse oil drip tray
200A loadbreak bushings
copper grounding rod
lifting provision
Source: Cooper Power Systems
Cables
Types of Underground Cable
Conductor
Semiconductive Conductor Shield
XLPE Insulation
Semiconductive Insulation Shield
Copper Concentric Neutral Wires
Copper Tape Shield
PVC Jacket
Concentric Neutral Cable
Tape Shielded Cable
Solid Dielectric Cable
Voltage Classes
• 5 kV
• 15 kV
• 25 kV
• 35 kV
Insulation Level Category
• 100% Insulation - fault will be cleared within one (1) minute
• 133% Insulation - fault will be cleared within one (1) hour
• 173% Insulation - fault clearing is indefinite
Insulation Wall Thicknesses for
Shielded URD Cables
INSULATION THICKNESS
VOLTAGE RATING
PHASE-TO-PHASE
VOLTS
CONDUCTOR SIZE
(AWG-Kcmil)
100%
LEVEL (Mil)
133%
LEVEL (Mil)
2001-5000
8-1000
90
90
5001-8000
6-1000
115
140
8001-15000
2-1000
175
220
15001-25000
1-1000
260
345
25001-28000
1-1000
280
---
28001-35000
1/0-1000
345
---
Since Dec 1987, REA now requires 220 mils for 15kV cable
and 345 mils for 25kV cable.
Electro-Chemical Treeing
URD Secondary Cables
Terminations
and
Splices
Cable Termination
Electric Field Flux-Lines on URD Cable
Terminators Used for URD Construction
Cable Splice
Types of Terminations and Splices
Ö Taped
Ö Pre-molded
Ö Cold-Shrink
Ö Heat-Shrink
Ö Slip-on
Direct Burial
vs
Conduit System
Direct Burial vs. Duct System
ADVANTAGES
DIRECT BURIAL
A.
B.
LOWER INITIAL COST
RELATIVELY EASY
INSTALLATION
DISADVANTAGES
A.
B.
C.
DUCT SYSTEM
A.
B.
C.
D.
RELATIVELY EASY TO REPLACE
DAMAGED CABLES OR TO INCREASE
CABLE SIZE.
MECHANICALLY AND
ENVIRONMENTALLY SUPERIOR TO
DIRECT BURIED SYSTEM.
PROVIDES PROTECTION FOR CABLE
BOTH DURING INSTALLATION AND
FOR ACCIDENTAL DIG-INS LATER.
HIGH RELIABILILTY.
A.
B.
C.
RELATIVELY DIFFICULT
TROUBLESHOOTING AND
MAINTENANCE.
VERY COSTLY CABLE
REPLACEMENT.
SELECT BACKFILL IS
REQUIRED IN ROCKY
TERRAIN.
HIGHER INITIAL COST
FAULT LOCATION IS
SOMETIMES MORE
DIFFICULT WITH SOME
TYPES OF FAULT
LOCATING EQUIPMENT.
CANNOT BE SPLICED,
ENTIRE SECTIONS MUST
BE REPLACED.
Direct Burial Cable System
RESTORED PAVEMENT
0.3M (TYP.)
WARNING TAPE
0.9M (MIN.)
EARTH BACKFILL
CONCRETE SLAB
SAND BACKFILL
(0.075M – 0.15M COVER ALL-AROUND)
0.15M - 0.2M
PRIMARY CABLE
Buried Duct System
RESTORED PAVEMENT
0.3M (TYP.)
WARNING TAPE
0.9M (MIN.)
EARTH BACKFILL
CONCRETE SLAB
SAND ENVELOPE
(0.075M – 0.15M COVER ALL AROUND)
DUCT
0.025M – 0.1M
PRIMARY CABLE
Concrete-Encased Duct System
RESTORED PAVEMENT
0.3M (TYP.)
WARNING TAPE
0.76M (MIN.)
EARTH BACKFILL
CONCRETE ENVELOPE
(0.075M – 0.1M COVER ALL AROUND)
0.025M – 0.1M
PRIMARY CABLE
Manhole, Pulling Box,
and
Service Box
Manhole
PRIVATE
PROPERTY
LINE'
0.3 M
SIDEWALK
2.72 M to 2.85 M
ROADWAY
MANHOLE
DUCT RUN
W
0.6 M
WIDTH VARIES ACCORDING
TO TYPE/APPLICATION
• STRAIGHT MANHOLE - 2.30 M
• 3-WAY MANHOLE
- 2.42 M
• 4-WAY MANHOLE
- 3.40 M
Typical Installation Along Narrow Sidewalks
2300
1270
1150
2420
STRAIGHT
3-WAY
950
3400
4-WAY
3400
950
3250
3300
Types of Manhole
Above Grade Service Pedestal
Below Grade Service Pedestal
Service Box
Cable Ampacity
Rating Factors
Rating Factor for
10kV-90kV 35mm2 Screen
CONDUCTOR, mm2
COPPER SCREEN, mm2
Al.
Cu.
16
35
50
95
150
300
300
185
1.01
1
0.99
0.98
0.97
0.95
500
300
1.01
1
0.99
0.97
0.95
0.92
800
500
1.02
1
0.99
0.95
0.92
0.88
1200
630
1.02
1
0.98
0.94
0.90
0.84
2000
800
1.03
1
0.98
0.92
0.87
0.80
-
1200
1.04
1
0.97
0.89
0.83
0.75
-
2000
1.06
1
0.96
0.86
0.78
0.69
Rating Factor for
100kV-400kV 95mm2 Screen
CONDUCTOR, mm2
COPPER SCREEN, mm2
Al.
Cu.
16
35
50
95
150
300
300
185
1.03
1.02
1.01
1
0.99
0.97
500
300
1.04
1.03
1.02
1
0.98
0.95
800
500
1.06
1.04
1.03
1
0.97
0.92
1200
630
1.08
1.06
1.04
1
0.96
0.90
2000
800
1.11
1.08
1.06
1
0.94
0.87
-
1200
1.16
1.11
1.08
1
0.93
0.84
-
2000
1.23
1.16
1.11
1
0.91
0.80
1 mm2 COPPER SCREEN IS EQUIV. TO : 1.66 mm2 ALUMINUM SHEATH
12.40 mm2 LEAD SHEATH
Rating Factor for
Laying Depth
LAYING DEPTH, m
L
LOW VOLTAGE
HIGH VOLTAGE
0.50
1.05
1.10
0.70
1.00
1.05
0.90
0.97
1.01
1.00
0.95
1.00
1.20
0.93
0.98
1.50
0.91
0.95
Rating Factor for
Ground Temperature
GROUND TEMPERATURE, OC
CONDUCTOR
TEMPERATURE,
OC
10
15
20
25
30
35
40
45
90
1.07
1.04
1
0.96
0.93
0.89
0.84
0.80
65
1.11
1.05
1
0.94
0.88
0.82
0.74
0.66
Rating Factor for
Ground Thermal Resistivity
THERMAL
RESISTIVITY, K*m/W
0.7
1.0
1.2
1.5
2.0
2.5
3.0
RATING FACTOR
1.14
1.00
0.93
0.84
0.74
0.67
0.61
Rating Factor for
Phase Spacing
ONE GROUP IN FLAT FORMATION WITH CROSS-BONDED
OR SINGLE-BONDED SCREENS
SPACING s, mm
De
De+70
200
250
300
350
400
RATING FACTOR
0.93
1.00
1.03
1.05
1.07
1.08
1.10
De
s
s
Rating Factor for
Groups of Cables in the Ground
ONE THREE-CORE CABLE IS EQUIVALENT TO ONE
GROUP OF SINGLE-CORE CABLES
DISTANCE cc
BETWEEN
GROUPS, mm
1
2
3
4
5
6
7
8
9
100
1
0.76
0.67
0.59
0.55
0.51
0.49
0.47
0.46
200
1
0.81
0.71
0.65
0.61
0.58
0.56
0.53
0.52
400
1
0.85
0.77
0.72
0.69
0.66
0.64
0.63
0.62
600
1
0.88
0.81
0.77
0.74
0.72
0.71
0.70
0.69
800
1
0.90
0.84
0.81
0.79
0.77
0.76
0.75
0.75
2000
1
0.96
0.93
0.92
0.91
0.91
0.91
0.90
0.90
NUMBER OF GROUP
Rating Factor for
Groups of Cables in the Ground
cc
Rating Factor for Cables in Pipes
In the Ground
SINGLE-CORE
CABLES PARTIALLY
INSTALLED IN
SEPARATE PIPES
0.94
SINGLE-CORE
CABLES IN
SEPARATE
PIPES
SINGLECORE CABLE
IN A
COMMON
PIPE
THREE-CORE
CABLE IN A
PIPE
0.90
0.90
0.90
Calculation of
Pulling Tension
Equations for Max. Permissible
Cable Pulling Tension (in Pounds)
1. PULLING EYE ATTACHED TO CONDUCTOR
8 * kcmil (Cu) ; 6 * kcmil (Al)
2. CABLE GRIP OVER LEAD SHEATH CABLE
4712 * t * (D – t)
3. CABLE GRIP OVER NON-METALLIC SHEATH CABLE
8 * kcmil (Cu) ; 6 * kcmil (Al)
(BUT NOT TO EXCEED 1,000 lbs.)
4. FOR BENDS (SIDE WALL PRESSURE)
300R
Equations for Max. Permissible
Cable Pulling Tension (in Pounds)
Where :
D
=
dia. of cable in inches
t
=
lead sheath thickness in inches
R
=
radius of duct bend in ft.
Calculation of Pulling Tension for
Straight Section
PULLING
END
FEEDING
END
DIRECTION OF PULL
L
Where :
T
=
L
=
w
=
f
=
v
=
=
T = L*w*f*v
total pulling tension in lbs.
duct length in ft.
cable wt. in lbs/ft
coefficient of friction
occupancy factor
1, for 1 cable per duct
Calculation of Pulling Tension
For Curved Section
L1
DIRECTION OF PULL
R
T = T1e0.0174*f*θ*v + T2
Where :
T
=
T2
=
T1
=
total pulling tension in Newton (N)
tension for straight section at pulling
end in Newton (N)
tension for straight section at feeding
end in Newton (N)
Ø
L2
FEEDING
END
PULLING
END
Calculation of Pulling Tension
For Curved Section
e
f
θ
v
=
=
=
Napierian logarithmic base
2.718
coefficient of friction
=
=
=
angle of bend in radians
occupancy factor
1, for 1 cable per duct
The occupancy factor (v) for 3 cables per duct triplexed
v = [ 1 – ((D1/D)-1)-2 ] -0.5
Calculation of Pulling Tension
For Curved Section
The occupancy factor (v) for 3 cables per duct random lay
v = 1 + 4/3 * ((D1/D)-1)-2
Where :
D1
=
inside diam. of duct in inches
D
=
dia. of cable in inches
Sample Computation
FEEDING END
4 5
.5M
2
M
6
PULLING END
3 .0
5M
9 0°
30
61
M
3 .0 5
30°
1
30.5M
3
a. Let Ls = max. straight pulling length
22,240 = Ls (15.34)(0.5)(1)
Ls = 2,899M
b. T12 = Lwfv = 234N
T13 = T12 * Bend Factor = 512N
T14 = 512 + Lwfv = 980N
T15 = T14 * Bend Factor = 1274N
T16 = 1274 + Lwfv = 1,508N
At curve 4-5, due to SWBP
T15/R45 = 1274/3.05
= 418 N/m < 8,896 N/m
Since T16 , 22,240N, a pulling grip can be used.
Coefficient of Friction (f)
DUCT TYPE
NON-LEADED
CABLE
LEADED CABLE
POLYETHYLENE
AND PVC
0.25
0.25
FIBER
0.28
0.25
CONCRETE
0.48
0.40
STEEL
0.50
0.48
Allowable Percent Fill of Duct by Cables
NUMBER OF CABLES
CABLE TYPE
1
2
3
4
OVER 4
RUBBER
COVERED
CABLE 600V &
OVER
53
31
40
40
40
LEAD COVERED
CABLES
55
30
40
38
35
ALL OTHER
CABLES
35
25
25
25
25
Percent Fill of Duct by Cables
• When triplex cables are used, they should be considered
as equivalent single conductor. The equivalent single conductor
outside diameter (OD) is obtained by calculating 1.05 times the
diameter of the circle enclosing the 3 conductors.
• For triplex cable the diameter of circumscribing circle,
D = 2.26 * d
d = diameter of individual cable
Code Requirements
General Requirements
• Effectively grounded shield or sheath for systems above 2 kV to
ground
• Supply, control, and communication cables not to be installed
in the same duct unless maintained or operated by the same
utility
• Supply and communication cables should be racked from
separate walls
• Where racked from the same wall, the supply cables should be
racked below the communication cables
Typical Siting Requirements for
Pad-Mounted Transformer
Min. Separation between Supply &
Communication Facilities in Joint-use
Manholes & Vaults
Ø-Ø SUPPLY VOLTAGE
MM (SURFACE TO SURFACE)
0 TO 15,000
155
15,001 TO 50,000
230
50,001 TO 120,000
305
120,001 AND ABOVE
610
Clearances
† Conduit systems to be occupied by communication conductors shall
be separated from conduit systems to be used for supply systems by:
1. 77 mm of concrete
2. 100 mm of masonry
3. 305 mm of well tamped earth
Direct-Buried Cable
• Minimum horizontal clearance of 305 mm from other UG structures.
Conditions for Random Separation between
Supply & Communication Cables
• VOLTAGE
a. Grounded supply systems not in excess of
22 kV to ground
b. Ungrounded supply systems not in excess of
5.7 kV to ground
• CABLE
a. With bare or semi-conducting jacketed grounded
conductor with continuous contact with the earth
b. Conductor in contact with the earth of suitable corrosion
resistant material
c. Radial resistivity of semi-conducting jacket not more than
20 m-ohms.
d. Adequate bonding between the supply and communication
cables shield at intervals which should not exceed 305mm
Underground
Accessories
UG Components and Accessories
600 A Deadbreak
Elbow Connector
RTE 35 kV Class Type BOL-T
UG Components and Accessories
600 A Deadbreak Elbow Connector with 200 A Interface
RTE 35 kV Class Type T-OP II
UG Components and Accessories
200 A Loadbreak Elbow Connector
RTE 35 kV Class Elbow
Connector
UG Components and Accessories
Elbow Arrester
RTE M.O.V.E.
UG Components and Accessories
Grounding Elbow
RTE 200A 35 kV Class
Grounding Elbow
UG Components and Accessories
Portable Feedthru
RTE 200A 35 kV Class
Portable Feedthru
1-Phase Deadfront Pad-Mounted
Transformer w/ 1-Phase Tap
Separable Splices
UG Components and Accessories
Insulated Standoff Bushing
RTE 600A 35 kV Class
Standoff Bushing
UG Components and Accessories
Insulated Standoff Bushing
RTE 200A 35 kV Class
Standoff Bushing
UG Components and Accessories
Insulated Protective Cap
RTE 600A 35 kV Class
Insulated Protective Cap
UG Components and Accessories
Insulated Protective Cap
RTE 200A 35 kV Class
Insulated Protective Cap
UG Components and Accessories
Fault Indicator
Cooper Power Systems’ TPR Voltage Reset Fault Indicator
UG Components and Accessories
Fig: Typical Application of Fault Indicators
Location of Fault Indicators
On Primary UG Cables
Recommended Methods of Concentric
Neutral Primary Cable Preparation
UG Components and Accessories
UG Components and Accessories
Unshielded Cable
Conductor
Electrostatic Flux Lines
Equipotential Lines
Shielded Cable
Insulation Shield
Insulation
Conductor
Electrostatic Flux Lines
Conductor Shield
Equipotential Lines
Series II 3-Phase Secter
Cable Splice
Cable Splice
COLD SHRINK RUBBER BODY
SHIELD WIRE
UG Components and Accessories
SERVICE CONNECTORS
Typical Underground System
Typical Underground System
Single-Phase Padmounted Transformer
T-BLADE
SWITCH
A
PARTIAL
RANGE CLF
EXPULSION
FUSE
TRANSFORMER
WINDING
B
C
Three-Phase Padmounted Transformer
UG Components and Accessories
200 A Loadbreak Elbow Connector
RTE 35 kV Class Elbow
Connector
1-Phase Deadfront Pad-Mounted
Transformer w/ 1-Phase Tap
Insulation Wall Thicknesses for
Shielded URD Cables
INSULATION THICKNESS
VOLTAGE RATING
PHASE-TO-PHASE
VOLTS
CONDUCTOR SIZE
(AWG-Kcmil)
100%
LEVEL (Mil)
133%
LEVEL (Mil)
2001-5000
8-1000
90
90
5001-8000
6-1000
115
140
8001-15000
2-1000
175
220
15001-25000
1-1000
260
345
25001-28000
1-1000
280
---
28001-35000
1/0-1000
345
---
Since Dec 1987, REA now requires 220 mils for 15kV cable
and 345 mils for 25kV cable.
Suggested Minimum Burial Depths
For Direct Buried Cables
VOLTAGE
BURIED DEPTH
600 AND BELOW
24”
601 TO 22,000
30”
22,001 TO 40,000
36”
40,001 AND ABOVE
42”
UG Components and Accessories
Portable Feedthru
RTE 200A 35 kV Class
Portable Feedthru
Margins Provided by Distribution-Class
Arresters at Pole Tops
URD
SYSTEM
MARGIN
ARRESTER
RATING
(KV)
IR
at
10KA
OVERHEAD
SYSTEM
MARGIN
95
9
36
164%
32%
14.4/24.9
125
18
66
89%
-5%
19.9/34.5
150
27
96
56%
-22%
DISTRIBUTION
VOLTAGE
EQUIPMENT
BIL
(KV)
7.2/12.5