EE 741
Spring 2014
Sub-transmission Lines and
Distribution Substations
Overview
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Types of sub-transmission layouts
Substation equipment
Substation bus schemes
Substation location
Substation rating (voltage drop calculations.)
Substation application curves
SCADA
Substation Automation
Grounding
One-Line Diagram of Distribution System
Radial-Type Sub-transmission
• Simple and low cost
• Low service continuity
• Usually not employed due to poor service reliability
Improved Radial-Type Sub-transmission
• Simple but with higher cost
• Allows relatively faster service restoration when a fault
occurs on one of the lines
Loop-Type Subtransmission
• Higher service reliability
• No single fault on any circuit will interrupt service to a
distribution substation
Network-Type Subtransmission
• Multiple bulk power sources
• Greatest service reliability
• Costly control of power flow and relaying
Distribution Substation Equipment
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Power Transformers
Circuit Breakers
Disconnecting switches
Station buses
Insulators
CTs and PTs
Lightning arrestors
Protective relaying
Instrumentation
Station batteries
Communications equipment
Grounding systems
Shunt capacitors
Other apparatus…
Single-Bus Scheme
• Lowest cost
• Failure of bus or circuit breaker results in shutdown of
entire substation
Double-Bus-Single-Breaker Scheme
• Either bus may be isolated for maintenance
• Four switches are required per circuit
Double-Bus-Double-Breaker Scheme
• Any breaker or bus can be taken out of service
without power interruption
• Most expensive
Main-and-Transfer-Bus Scheme
• Any breaker can be taken out for service
Ring-Bus Scheme
• Requires only one breaker per circuit
• Protective relaying circuitry can be complex
Breaker-and-Half Scheme
• Most flexible operation
• Relaying and automatic re-closing is somewhat involved
Substation Location
• Rules of selecting an ideal substation location:
– As close as possible to the load center
– Proper voltage regulation can be obtained without
expensive measures
– Easy access to incoming subtransmission lines and
outgoing distribution feeders
– Enough space for future expansion
– Location should not be opposed by neighbors, local
ordinances, and land use regulations
– Minimize the number of customers affected by service
discontinuity
– Easy access (for service and emergency).
Voltage Drop Calculations on Main Feeder: lumped load
d
R = dr, X = dx
Sr = 3-phase load
VD  Vs  Vr  IR cos   IX sin  
VD pu
dS r
(r cos   x sin  )
3Vr
1000(r cos   x sin  )
 dKS r , K 
3VrVB
K is a function of conductor size, spacing, power factor and operating voltage.
VB : base phase voltage
d: effective length of the feeder (r and x are in Ω/mile)
Constant K:% Voltage Drop per kVA.mi
Spacing = 37 in
Voltage Drop (VD): Uniformly Distributed Load
d
½d
S  A D (kVA),
VD (%) 
D: load density
VD: voltage drop on at end of feeder.
1
dKS
2
Voltage Drop (VD) in Feeder with Linearly
Increasing Load Density
S 4  A4 D  l42 D (kVA),
2
VD (%)  l4 KS 4  0.667 KDl43
3
l (↔ d): main feeder length
D: load density
VD: voltage drop on primary feeder.
Voltage Drop (VD) in Feeder with Linearly
Increasing Load Density
S 6  A6 D 
1 2
l6 D (kVA)  0.578l62 D (kVA)
3
2
VD (%)  l6 KS 6  0.385KDl62
3
Voltage Drop (VD) in Feeder with Linearly
Increasing Load Density
S n  An D  ln2 tan( )D (kVA)
o
2
360
VD (%)  ln KS n  0.667 KDln3 tan(
)
3
2n
Voltage Drop (VD) in Feeder with Linearly
Decreasing Load Density
S 2  A2 D  l22 D (kVA),
1
VD (%)  l2 KS 2  0.333KDl23
3
Example
VD = sKSn =1x0.01x500 = 5%
VD = sKSn =(1/2)x0.01x500 = 2.5%
VD = sKSn =(2/3)x0.01x500 = 3.33%
VD =3%
Spacing: 37”
PF = 0.9
#4/0 AWG Cu
VD =6%
Spacing: 37”
PF = 0.9
#4/0 AWG Cu