S&C Vista® Underground Distribution Switchgear
Outdoor Distribution
S&C Vista Underground Distribution Switchgear features
load-interrupter switches and resettable, vacuum fault
interrupters in series with disconnect switches . . . all in a
submersible, SF6-insulated, welded-steel tank. The
switchgear includes clearly visible open gaps and integral
visible grounds to simplify operations, enhance safety,
and minimize the duration of an outage. With the optional
voltage-indication feature, all routine tasks—switching,
voltage testing, and grounding—can be accomplished by a
single person without cable handling or exposure to high
voltage.
The load-interrupter switches provide three-pole
switching for 600-ampere main feeders, and the fault
interrupters provide single-pole switching and single-pole
Overcurrent Control
or three-pole fault protection for 600-ampere main feeders
and 200-ampere taps, laterals, and subloops. Fault interruption is initiated by a microprocessor-based overcurrent control which is housed in a watertight enclosure
mounted on the gear. The control features innovative
time-current characteristic (TCC) curves with selectable
instantaneous and definite-time delay attributes for superior coordination with upstream protective devices and
downstream power fuses. Standard “E” speed curves can
also be selected. The minimum total clearing time (from
initiation of the fault to total clearing) is 40 milliseconds
(2.5 cycles) allowing for easier coordination with
upstream circuit breakers. Integral current transformers
provide power and current sensing.
Watertight enclosure
Adapter cable
Overcurrent control—current transformers
(not shown) provide power and signal input
for the control
User-supplied personal computer
is used to program the overcurrent
control. Allows selection of TCC
curves and single-pole or three-pole
tripping for the fault interrupters
Figure 1. User-supplied personal computer is attached to the overcurrent control for programming of the control in the field.
Selectable TCCs include E-speed, coordinating-speed tap, and coordinating-speed main curves. Coordinating-speed curves
can be tailored to the application using various instantaneous and definite-time settings.
Supersedes Photo Sheet 681-703 dated 9-3-96
©2004
PHOTO SHEET
S&C ELECTRIC COMPANY
681-703
Page 1 of 8
February 23, 2004
S&C Vista® Underground Distribution Switchgear
Outdoor Distribution
Available TCCs
The overcurrent control was designed to meet specific
coordination needs. Existing pad-mounted gear with fuses
only coordinates with upstream relays and downstream
current-limiting fuses through 5,000 or 6,000 amperes,
even though underground distribution systems often see
higher fault currents. In response, S&C has developed
special new “coordinating” speed TCC curves that provide
complete coordination with upstream circuit-breaker
relays as well as downstream current-limiting fuses
through the entire range of typical fault currents . . . all the
way to 12,500 amperes.
The control features phase-overcurrent curves as well
as ground-overcurrent curves to coordinate with sourceside breakers or reclosers with ground-trip settings. Separate curves were designed for main and tap protection to
provide complete coordination between fault interrupters
applied on main feeders and those applied on associated
subloop taps.
Five families of curves are available as follows:
• “E” speed curves—from 25E amperes through 200E
amperes.
• “K” speed curves—from 25K amperes through 200K
amperes.
• “Coordinating” speed tap curves—with minimum
pickup settings from 50 amperes through 400 amperes.
These curves are used in conjunction with fault interrupters feeding subloop taps and have been specifically
designed to optimize coordination with load-side weaklink/backup current-limiting fuse combinations and
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Overcurrent Control
source-side relays with low time-dial settings. As such,
the curves are faster than “E” speed and “K” speed
power fuse TCC curves.
• “Coordinating” speed main curves—with minimum
pickup settings from 100 amperes through 800
amperes. These curves are used in conjunction with
fault interrupters on main feeders and have longer minimum response times and different shapes to coordinate with tap-interrupter curves.
• Time-overcurrent relay curves conforming to
IEEE C37.112-1996 IEEE Standard Inverse-Time
Characteristic Equations for Overcurrent Relays
—U.S. Moderately Inverse Curve U1, U.S. Inverse
Curve U2, U.S. Very Inverse Curve U3, U.S. Extremely
Inverse Curve U4, U.S. Short-Time Inverse Curve U5,
I.E.C. Class A Curve (Standard Inverse) C1, I.E.C. Class
B Curve (Very Inverse) C2, I.E.C. Class C Curve
(Extremely Inverse) C3, I.E.C. Long-Time Inverse
Curve C4, and I.E.C. Short-Time Inverse Curve C5.
The coordinating-speed TCCs can be customized into
hundreds of different curves using a variety of instantaneous (1 ka through 8 ka) and definite-time delay (32 ms
through 96 ms) settings. The minimum total clearing time
is 40 milliseconds (2.5 cycles) to allow proper coordination with typical upstream circuit-breaker relays.
If activated, the definite-time delay attribute adds an
intentional time delay to allow coordination with conventional power fuses located downstream. For even more
specialized shapes, a definite-time delay setting can be
combined with an instantaneous trip setting.
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S&C ELECTRIC COMPANY
Control Settings
The parameters for the TCC curves are set using a
personal computer connected to the data port of the overcurrent control. Programming of the control can be done
in the shop or in the field. Refer to Figure 1. Unlike other
controls that use knobs and dials, the settings cannot be
inadvertently or haphazardly changed by unauthorized
persons. Only qualified technicians have the ability to
alter the settings. There is no need to purchase and install
new circuit cards or entirely new controls to make
changes. For maximum reliability, there are no mechanical parts such as switches and potentiometers to wear out
or malfunction.
The control allows a choice of single-pole or three-pole
fault interruption, and 50- or 60-hertz operation. Input
parameters for the TCCs include minimum pickup levels,
instantaneous pickup levels, and definite-time delays for
both phase- and ground-overcurrent curves. The groundovercurrent circuit, and the instantaneous and definite-
time delay attributes can be disabled if desired. When an
E-speed curve is selected, only the minimum pickup level
for phase overcurrents is programmed into the control. As
shown in Figure 2, the settings programmed into the computer can also be viewed for verification.
When the overcurrent control is programmed for
single-pole tripping of the fault interrupter, only those
poles that have faulted will trip. The control will not
respond to ground overcurrents, but will respond to phase
overcurrents whether or not a ground fault is involved.
For three-pole tripping of the fault interrupter, all three
poles will operate regardless of which pole has faulted. In
this case, the control will operate the fault interrupter in
response to ground overcurrents given that a ground-fault
setting has been programmed into the control.
The ability to program the control for three-pole tripping allows for three-pole protection of three-phase transformers and other three-phase loads.
Figure 2. Overcurrent-control settings.
PHOTO SHEET
S&C ELECTRIC COMPANY
681-703
Page 3 of 8
February 23, 2004
S&C Vista® Underground Distribution Switchgear
Outdoor Distribution
Applications
Overcurrent Control
1000
Phase
Overcurrent Relay
Type: CO-9
Time Dial: 3
Min. Pickup Current: 720A
CTI: 0.15 sec.
10
1
.1
Miscoordination
100 000
1 000
10 000
.01
100
As shown in Figure 3, the first application involves an
industrial park with 1,000- and 2,000-kva transformers
served by a single loop. Due to the kva sizes of the transformers, power fuses are used at the transformers. It is
necessary to limit the loop to two transformers of this size
because of coordination problems between the 100E fuse
at the transformer, the 200E fuse in the pad-mounted gear,
and the utility’s phase relay at the substation which is
set to pick up at 720 amperes. The coordination plot in
Figure 4 shows how this design will only coordinate
through 5,600 amperes, which is not always high enough
since many industrial parks are located at the beginning
of a circuit where fault currents are high.
When Vista switchgear is applied in the same situation,
at least twice as many transformers can be served by two
Subloop Fuse
(200E, Standard
Speed)
100
10
Increased Load-Carrying Capability and
Improved Coordination
TransformerPrimary Fuse
(100E, Standard
Speed)
TIME IN SECONDS
Two applications of S&C Vista Underground Distribution
Switchgear are described below. Each shows how the
switchgear improves coordination and reliability using
the same or fewer units of gear than required with traditional equipment.
CURRENT IN AMPERES
Figure 4. Miscoordination between 100E transformer fuse
and 200E subloop fuse.
B
CO-9 Relay
Phase: 720A
200E
200E
N.O.
100E
50E
1000 KVA
2000 KVA
Figure 3. Subloop protection provided by conventional (fused) pad-mounted switchgear.
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S&C ELECTRIC COMPANY
Improved Circuit Reliability and Reduced
Equipment Needs
1000
100
Tap Interrupter (Phase)
Min. Pickup Current: 400A
Def. Time Delay: 4 cycle
TransformerPrimary Fuse
(100E, Standard
Speed)
Phase
Overcurrent Relay
Type: CO-9
Time Dial: 3
Min. Pickup Current: 720A
CTI: 0.15 sec.
TIME IN SECONDS
10
1
.1
10 000
1 000
100
.01
10
As shown in Figure 7, the second application involves the
use of conventional pad-mounted gear to tap a main
feeder to serve two commercial parks. The settings of the
phase- and ground-overcurrent relays at the utility’s substation are 720 amperes and 480 amperes, respectively.
Three 1,500-kva transformers, each protected by an internal current-limiting fuse rated 100C (which is the largest
internal fuse available), are used to serve three buildings.
The largest tap fuse that will carry the load of two transformers and coordinate with the substation relays is 140K.
Refer to Figure 8. Thus, it is necessary to establish two
loops to serve the load.
The total demand of the commercial park is 3,000 kva,
and three such commercial parks could be connected to
one main feeder without exceeding the capacity of a
13.2-kv circuit. However, the reliability of this design may
100 000
units of gear. See Figure 5. Full coordination is also
achieved—through 12,500 amperes—by using a 400ampere coordinating-speed curve. As shown in Figure 6,
the tap-interrupter curve includes a 4-cycle definite-time
delay to coordinate with the 100E standard-speed fuse
protecting the 2,000-kva transformer.
CURRENT IN AMPERES
Figure 6. Coordinating-speed tap curve with definite-time
delay coordinates completely with 100E transformer fuse.
B
CO-9 Relay
Phase: 720A
400A
400A
N.O.
50E
1000 KVA
100E
2000 KVA
50E
1000 KVA
100E
2000 KVA
Figure 5. Vista switchgear provides increased load-carrying capability in a single subloop.
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S&C ELECTRIC COMPANY
681-703
Page 5 of 8
February 23, 2004
S&C Vista® Underground Distribution Switchgear
Outdoor Distribution
1000
100
Subloop Fuse
(140K)
Transformer
Weak-Link Fuse
(100A)
Phase
Overcurrent Relay
Type: CO-9
Time Dial: 3
Min. Pickup Current: 720A
CTI: 0.15 sec.
10
TIME IN SECONDS
be questionable, depending on the length of the main
feeder and the history of cable failures. The solution until
now has been to bring a second circuit into the area to
serve one of the commercial parks . . . obviously very
expensive.
With Vista switchgear, there is a better solution involving the installation of a “main” fault interrupter with a
phase-overcurrent rating of 450 amperes and ground-overcurrent rating of 400 amperes. Refer to Figure 9. When a
main interrupter is used on the main feeder, not only is an
additional circuit not required but the reliability of the
system is substantially improved through increased
sectionalization. Due to better coordination and the
higher continuous-current capability of the switchgear,
only two units of gear are required to serve the entire load
at each commercial park where previously three units
were required. Figures 10 and 11 show the improved
coordination for the phase current and ground current,
respectively.
Overcurrent Control
1
.1
Ground
Overcurrent Relay
Type: CO-9
Time Dial: 2.5
Min. Pickup Current: 480A
CTI: 0.15 sec.
100 000
10 000
1 000
10
100
.01
CURRENT IN AMPERES
Figure 8.
Coordination between 140K subloop fuse,
upstream relay, and 100A (C16) transformer weak-link fuse.
CO-9 Relay
Phase: 720A
Ground: 480A
B
140K
140K
140K
140K
140K
COMMERCIAL
PARK
N.O.
100C
1500 KVA
N.O.
100C
100C
1500 KVA
1500 KVA
COMMERCIAL PARK
Figure 7. Commercial parks supplied by conventional (fused) pad-mounted switchgear.
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February 23, 2004
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MAIN PROTECTOR
Phase: 450A
Ground: 400A
CO-9 Relay
Phase: 720A
Ground: 480A
B
TAP PROTECTOR
Phase: 350A
Ground: 300A
COMMERCIAL
PARK
COMMERCIAL PARK
N.O.
100C
100C
100C
1500 KVA
1500 KVA
1500 KVA
Figure 9. Addition of a fault interrupter on the main feeder provides increased reliability.
1000
1000
Main Interrupter (Phase)
Min. Pickup
Current: 450A
Tap Interrupter
(Phase)
Min. Pickup
Current: 350A
Tap Interrupter
(Ground)
Min. Pickup
Current: 300A
100
10
1
10
100 000
10 000
.01
1 000
.01
100
.1
10
.1
1 000
1
100
TIME IN SECONDS
10
Ground
Overcurrent Relay
Type: CO-9
Time Dial: 2.5
Min. Pickup Current: 480A
CTI: 0.15 sec.
100 000
Phase
Overcurrent Relay
Type: CO-9
Time Dial: 3
Min. Pickup Current: 720A
CTI: 0.15 sec.
Transformer
Weak-Link Fuse
(100A)
10 000
100
TIME IN SECONDS
Main Interrupter (Ground)
Min. Pickup
Current: 400A
CURRENT IN AMPERES
CURRENT IN AMPERES
Figure 10. Complete coordination between upstream phase
relay, feeder interrupter (phase TCC), and subloop tap interrupter (phase TCC).
Figure 11. Complete coordination between upstream
ground relay, main feeder interrupter (ground TCC), and
subloop tap interrupter (ground TCC).
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S&C ELECTRIC COMPANY
681-703
Page 7 of 8
February 23, 2004
S&C Vista® Underground Distribution Switchgear
Outdoor Distribution
Event Recording
Overcurrent Control
have faulted, the magnitude of the fault, and whether or
not a ground overcurrent was involved. This information
can be accessed via a personal computer connected to the
data port of the control.
The overcurrent control features an event log that captures the last twelve operations of a fault interrupter. As
shown in Figure 12, the event log indicates which poles
View Menu INTERRUPTER 1: <select number>
1. View event report (last 12 events)
2. View load current
3. View relay settings
4. View relay self-test
9. Switch to Main Menu
= View FI 1 > 1
View FI 1> 1
*Eve*
#
1
2
3
**Faulted Poles**
1
2
X
X
3
X
X
**** * Amperes *****
G
X
X
X
1
4
24
2010
2
2040
1926
22
3
1829
18
18
G
1908
1908
1992
Elapsed
**Time**
(Seconds)
0.019
Press <ENTER> for menu
Printed in U.S.A.
Figure 12. Event log showing last three events that caused the fault interrupter to operate.
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February 23, 2004
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