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TABLE OF CONTENTS:
Description
Standard
Padmount Switchgear Application
Unique
PVI Advantages
Special
Padmount Applications
Vault
PVI Switchgear Applications
Summary
OVERVIEW
This guide provides a brief description of G&W's Puffer
Vacuum Interrupter (PVI) SF6 switch and it possible applications
on distribution systems. The guide looks at the use of the
PVI as a replacement for air or oil insulated padmount gear
and identifies unique application benefits.
DESCRIPTION
The Puffer Vacuum Interrupter (PVI) provides great flexibility
in meeting the requirements of underground distribution systems
rated 5kV through 38kV. It is designed to eliminate many of
the shortcomings of conventional air or oil insulated switchgear
that has been used by both industry and utilities for many
decades. While many of these problems associated with other
gear have become accepted as obstacles that must be worked
around, the PVI supplies an attractive alternative that tackles
these limitations head-on.
Vacuum Interrupter
Fault Protection
The fault interrupting mechanism of the PVI is unique. Unlike
typical fusing, the PVI makes use of Vacuum Interrupters that
combine vacuum bottles and electronic controls to clear faults
with a wide variety of time-current characteristic curves
to protect the downstream distribution system. The resettable
Vacuum Interrupter means that fuses are no longer required
when the PVI is used in place of most padmount equipment.
In addition, the electronic controls can be set for either
single or three phase trip, thereby preventing single phasing
conditions common to fusing constraints. Finally, the Vacuum
Interrupter is rated up to 600 amps continuous current, so
fault protection above 200 amps is much more readily available
using the PVI. See Figure 1 for basic operation sequence.
Figure 1: PVI operation
sequence
In addition to providing fault protection,
the vacuum bottle can be operated manually to act as a load
break switch. Many utilities require a separate switch on
the tap (fuse) side in order to change out fuses. With the
PVI, this requirement is no longer necessary. It is important
to note that even though the vacuum bottles are surrounded
by SF6, any high energy fault or load break interruption is
done within the vacuum bottle.
Puffer Load break
Interruption
The 600 amp load break switch technology used for source
switching offers some unique features above standard load
break switches. The PVI uses Linear Puffer load break switches
for the source side of the switchgear. The Linear Puffer design
concentrates a burst of SF6 gas in the arc zone of the switch
that minimizes contact wear and maximizes the life of the
switch without maintenance. See Figure 2. In addition, the
switch can be rated up to a 40kA momentary or close-into-
fault condition, far above most switch ratings. A Rotary Puffer
design is also available offering ratings through 27kV, 20kA
asymmetric momentary. The Puffer switch is an ideal choice
for either automatic transfer control or possible distribution
automation schemes.
Figure 2: Linear Puffer
Load break Principle.
One common requirement of load break switches
is that the opening of the switch provide a visible disconnect
for operating crews to identify when a switch is isolated
from line voltages. The Linear Puffer switch in the PVI provides
this visible disconnect through viewing windows on the unit
by the operating handle. All three phases can be seen through
the window to insure the switch is open prior to working on
the switch or downstream devices.
The PVI unit is built inside of a fully
sealed, dead-front tank that is filled with SF6 (Sulfur Hexafluoride)
gas. The SF6 gas provides the dielectric strength to the electrical
components and supplies two primary advantages above other
dielectric compounds. First, like oil, SF6 gas has a dielectric
strength, roughly 2 to 3 times higher than air, which allows
the tank to be compact and built with a minimum of insulating
barriers. Fortunately, SF6 gas is not combustible like oil
so has no danger of the dielectric becoming flammable and
causing safety problems. Second, unlike air, the SF6 gas does
actually have arc- extinguishing properties that minimize
maintenance and maximize equipment life. The PVI equipment
is anticipated to have a 30+ year lifespan with virtually
no maintenance required.
STANDARD
PADMOUNT SWITCHGEAR APPLICATIONS
The PVI can be used as a substitute for other types of padmount
gear in almost all applications. The wide variety of available
standard industry configurations allows the PVI to be a direct
replacement for any deadfront switchgear. The unit may be
used for a livefront replacement by changing the cable terminations
to elbows. Note however that the PVI enclosure footprint will
be larger than most livefront gear, so pad dimensions may
need to be examined prior to a change-out.
The single most important decision to make
in selecting the correct PVI is the proper configuration for
the specific location. The available configurations are shown
in Figure 3. Determination of the configuration is based on
the type of distribution system that is being installed.
Figure 3: PVI one-line
configurations
Distribution Systems
Design Methods
Distribution systems are generally broken down into either
radial or loop systems for general discussion. In many cases,
the two types of systems will both be used within a company
and often within the same distribution feed.
Nevertheless, the difference between a loop
and radial feed is key in determining the proper configuration
required for the PVI. A loop system is almost always installed
with two load break switches involved in the design. With
the two load break switches, the loop can be opened from either
direction, which allows great flexibility in selecting the
open point of the loop and in isolating the switch when work
is required. Therefore, a PVI-6, 9, 10, 11, 13, or 14 is most
commonly used in loop designs.
In the case of radial feeds, a single load
break switch is required to provide the visible disconnect
associated with working on the switch after determining that
the switch is isolated from line voltage. Therefore, common
configurations of the PVI used for radial feeds are the PVI-3,
5, 7, and 12. Note that a radial feed that will be converted
into a loop feed in the future should normally be installed
as a loop-style switch (two or more load break switches) as
opposed to adding a second switch in the future. The engineering
trade-off in determining whether to use a loop or radial system
is primarily cost versus reliability. The radial feed is less
costly but also provides less reliability, since the source
can only provide power along a single path. If the radial
path has a faulted conductor or outage, the load can not be
served until the problem is repaired. This repair may take
hours or days depending on severity and accessibility of the
location.
The loop design has a higher cost associated
with the installation, but also offers a higher degree of
reliability. In most cases, the loss of power through one
source can be compensated for by feeding all loads through
the second source. Also, a faulted conductor can be compensated
for in a short time period by switching the load break switches
so that the faulted cable is isolated from the sources and
loads. Under most circumstances, all customer loads can be
re-energized and served by isolating the faulted line with
the sectionalizing switches and repairing the line after customers
are back in service.
Figure 4A shows the situation that occurs
when a radial feed experiences a fault, while Figure 4B shows
how a loop system can be re-configured to isolate the faulted
line and bring customers back into service quickly. In general,
utility engineers are moving towards loop systems for at least
the major lines to maintain higher reliability levels.
Figure 4A: The radial feed design is a good
method of supplying power to a load where reliability is not
critical. However, Figure 4.A.2 shows that a fault in the
radial line will remove the load from service until the fault
is repaired.
Figure 4B: Looped systems offer far more
versatility in re-establishing service to customers under
fault conditions. In order to restore power to all loads after
the fault occurred in Figure 4.B.2, only one switch needs
to be opened by the crews prior to restoring power. Other
fault locations would require closing the normally open switch
and opening two other load break switches to again restore
power to all loads. The fault can now be repaired at a later
time by fault locating specialists since no customers are
impacted.
Basic Radial System
Designs
Common placement of switchgear and possible configurations
are shown in the following one-line diagrams. Figure 5A shows
a simple radial feed system and how the PVI gear may be utilized.
In this simple system, the three phase distribution lines
feed a single load through a PVI-5 unit. Figure 5B shows a
more complex radial system where a combination of PVI-5, PVI-11
and PVI-7 units feed loads from a single radial feed.
Figure 5: Examples of typical
radial feed line designs.
The PVI-11 allows the single incoming source
to be split into a VI tap near the switch, plus provide sectionalizing
capability for two other cabinets on the distribution feed.
The PVI-5 unit provides a simple Puffer load break switch
in series with a VI protected tap for a single load. The PVI-7
design adds a second tap for multiple load protection.
Note that the equipment can be used to protect
either single phase loads (denoted "1 phase") or
3- phase loads (denoted "3-phase") by selecting
the proper setting on the electronic control panel. Unlike
normal fusing, the VI"s with the electronic controls
set in "3-Phase" mode will trip all three phases
at once for protecting 3-phase loads. This capability provides
single phasing protection which assists in protecting both
3-phase motors and 3- phase transformers against problems
caused by single phasing.
Basic Loop Design
Systems
Figure 6A shows a basic loop design that consists of two sources
feeding through two PVI-9 units into a loop with three sectionalizing
cabinets. Loads are fed from the VI's for overcurrent protection.
Figure 6B shows a similar system that is fed from a single
utility source. The advantage of the loop in this case is
that the loads can be fed from two points along the source
in case a portion of the source line becomes faulted.
Figure 6: Dual sources provide higher reliability
(6A), but are normally more costly. A single source (6B),
feeding a loop at two different points decreases the effect
of losing a load due to cable failure.
Hybrid System Designs
In many cases, the utility will design a hybrid system that
consists of both loop and radial feeds. The major source feeds
(backbone) will be configured as a loop, while the tap points
off of the loop may be either radial or loops. Figure 7 shows
an example of a hybrid system that contains a backbone loop
with a secondary loop in one area and radial feeds for the
rest of the system. Note that the secondary loop is actually
protected by VI's rated at 600A continuous current,
which is difficult to accomplish with standard padmount
switchgear. This capability offers the advantage of not
only having sectionalizing capability at the two source switches,
but also limiting outages due to possible faults in the 600
amp cable feeding the secondary loop. This is accomplished
by limiting the outage to the first VI instead of taking out
a much larger service as typically is required by using load
break switches on the backbone of the system.
Figure 7: A hybrid system where the major
feed (backbone) is a loop system and most other feeds are
radial. Note in the dashed section that there is a secondary
loop that feeds (SEC. 7.A) 2 other switches. This second loop
is protected by VI's that are set to 600A protective curves
which is normally difficult to accomplish with standard padmount
switchgear.
Naturally, this capability is also available
for radial loads being fed from 600 amp rated VI taps as well.
In this case, a fault on the single line feeding the radial
load would clear at the VI instead of going back to the breaker.
This capability will improve reliability by limiting outages
to smaller sections of the system, which is a major advantage
in many cases.
Figure 8 contains a more complex system
that has a loop for a backbone and combination of loops and
radial feeds to handle customer loads based on reliability
requirements. In Section 8A, the secondary loop is fed from
the Puffer load break switch as is more typical in standard
padmount installations.
Figure 8: Hybrid system with a backbone
loop fed by two sources. A secondary loop (SEC. 8A) is also
fed from the main loop as shown in the dotted section at the
right of the diagram. All other circuits are radial feeds
using a variety of PVI configurations.
Automatic Transfer
Design Systems
A level of reliability that exceeds that of the loop is to
have two sources feeding a load that will automatically transfer
to an energized circuit when the primary feed is interrupted.
The advantage to the customer is that the transfer between
sources can happen in seconds or even cycles instead of waiting
minutes or hours for a crew to make their way to the site
and manually throw over the switches. A common one-line diagram
is shown in Figure 9. The PVI-6 or 9 configuration is typically
supplied for this application, although any PVI unit that
has 2 or more Puffer switches can be used.
Figure 9: The addition of automatic transfer
control (ATC) package dramatically reduces outage time for
a customer that requires continuous power by automatically
switching to the second source if an outage occurs on the
primary source.
A number of options are available when ordering
the Automatic Transfer Control (ATC) package. Decisions need
to be made on open versus closed transition, time delay settings
for both initial and return switching, and if the switch may
be monitored via SCADA in the future. Contact your G&W
sales representative to resolve these issues.
UNIQUE
PVI ADVANTAGES
In addition to the PVI being used as a substitute for standard
padmount gear, the design offers special features that can
alleviate problems inherent to older designs incorporating
the more traditional switch and fuse concepts. Most of the
advantages are in eliminating the fuse and replacing it with
the electronically controlled vacuum bottle, but the Puffer
load break switch offers advantages as well.
Three
Phase Tripping
The electronic controls for the PVI fault protection system
allow the user to select the proper settings as required for
the specific location. These include setting the trip points
(similar to setting the fuse size in standard gear) within
a range of 12 available pre-selected points. As important,
a switch for selecting single phase or three phase trip is
available on the control panel. See Figure 10A for a layout
of the standard Type 1 panel.
Figure 10A: PVI electronic
control standard panel layout
Single phase trip may be used
for all single phase loads which are typical in residential
areas. This is equivalent to a fuse tripping and opening up
the single phase that contains the fault on a three phase
circuit. For this circumstance, only the customers fed from
this phase of the circuit are out of service until the repair
is complete.
However, many industrial and large commercial
loads may be better served by selecting a three phase trip
setting. Three phase trip will interrupt all three phases
even if only a single phase has a fault, similar to a circuit
breaker. Three phase trip is typically used to protect three
phase transformers and motors that can be damaged by single
phasing. In particular, even if the motors have three phase
protection built into the motor starters, the three phase
transformers that feed these loads are subject to ferro-resonance
conditions that often damage equipment. The PVI provides a
cost-effective way of providing protection to both motors
and transformers in these conditions.
Standard Electronic
Controls Capability
All PVI units are supplied with standard electronic controls
for each Vacuum Interrupter. The standard controls are supplied
with a Single Phase / Three Phase selector switch and a selector
switch for each phase that allows the trip set point to be
selected from one of twelve pre-selected settings as shown
in Figure 10A.
Trip level ranges are 15 to 300 or 30 to
600 amp settings that correspond to the time current characteristic
(TCC) curve selected during the order process. Each unit is
programmed with multiple TCC curves, e.g. E-slow, E-standard,
EF, K, NX, Oil Fuse Cutout, QA and selected overcurrent relay
curves. A dip switch allows the user to choose the TCC curve
to match individual coordination requirements. Vellums are
available upon request to assist in coordination studies.
The electronic controls are powered through
the current transformers that sense the current flowing through
the load side of the vacuum bottles. A minimum current of
approximately half of the minimum trip setting is required
to keep the electronics energized and protecting the load.
If this current level is not available prior to a fault, the
TCC curve that is protecting the load may be slowed by a maximum
of the greater of .5 cycles or 10% of the trip time.
Note that if a protection package is selected
that exceeds 300 amps, or if anticipated load growth may push
the load to over 300 amps continuous in the future, the PVI
should be ordered with 600 amp bushings on the Vacuum Interrupter
side of the equipment. The vacuum bottle and all associated
buswork is rated to 600 amps, but the 200 amp bushings will
de-rate these ways to 200 amps continuous unless the 600 amp
bushings are selected. Many conditions can change over the
anticipated 30+ year lifespan of a padmount unit, so the PVI
will allow the change out of the electronics if required.
If a new TCC curve set is needed, the controls can be easily
changed in the field.
Optional
Electronic Control Schemes
A number of options are available to extend the features to
the PVI electronic control schemes if required. An enhanced
electronic package, Type 2 Figure 10B (three phase only) is
available and offers the following features:
Figure 10B: PVI electronic
control
Phase Selector/Manual Trip optional package layout.
A Ground Fault control provides a
three phase comparison of current values to determine when
a ground fault is present on the load side of the Vacuum Interrupter.
If a ground fault or an overcurrent condition is detected,
the controls will trip all three phases of the Vacuum Interrupter.
The Ground Fault control panel provides a maximum percent
of phase imbalance selector switch and a timer to set the
ground fault parameters.
The enhanced package also provides a selector
switch for an Adjustable Phase Time Delay. This permits
coordination of other protection devices which may be in series
with the switch. A delay time from 0 to 0.15 seconds (up to
0.5 seconds for later models) is provided.
In addition, a Manual Trip Button
is provided permitting electronic tripping of all three phases
from the controller.
An External Contact control is included
for connecting an external relay or sensing device to trip
the electronic controls. The standard TCC curves are also
available if desired for tripping on overcurrent conditions.
An example of how the External Contact package may be useful
is where a transformer sudden over-pressure relay can be tied
into the Vacuum Interrupter controls to automatically trip
the VI if an over-pressure condition occurs. Many external
devices can be used as well based on the application. These
situations would also call for the External Power capability,
which allows an external power supply to feed the control
package to make certain that the controls will operate independent
of current flow.
For situations where the TCC coordination
is very tight, the External Power is included to ensure
that the controls will be continuously powered even if there
is no load on the Vacuum Interrupter. This option does require
an external 120V AC or 24V DC power supply, 1mA current maximum
output.
Superior TCC Curve
Matching
Since the electronic controls do not depend on a melting element
to clear, they maintain a much more consistent Time-Current-Characteristic
(TCC) curve. Most fuses are specified as meeting the TCC within
a plus-or-minus 10% range. The electronic controls in the
PVI are rated to plus-or-minus 3%, which can be helpful in
situations where the coordination scheme is tight.
In addition, common fuse curve problems
are eliminated by the electronic controls. “Sneakouts”
are fairly common in fuses that are subjected to high levels
of current relative to their ratings for short time periods,
e.g. during motor start-up. Fuses subjected to these conditions
often fail during near nor-mal operation and never at a convenient
time. Fortunately, the VI unit eliminates sneakouts by tripping
based on the actual current exceeding pre-defined TCC curves
independent of prior overcurrent conditions.
Vacuum Interrupter
Reset vs. Fuse Replacement
The Vacuum Interrupter is a resettable device that clears
faults by opening up the vacuum bottle contacts to break the
fault current. Testing proves that the bottle will last through
a large number of fault break operations, dependent on the
actual fault levels available. Under reasonable circumstances,
the Vacuum Interrupter will last over 30+ years with-out need
for maintenance.
Since the vacuum bottle clears faults and
can be reset, the need for fuse replacement is eliminated.
Fuse replacement may be costly, is never timely, often results
in improper fuse sizing being installed at a site, and may
be a safety issue for the crews that are called on to replace
failed fuses by closing a new fuse into a possibly still faulted
conductor. The Vacuum Interrupter eliminates these problems.
Frequently Switched
Padmount Units
The Linear Puffer load break switch offers the strong advantage
of having been tested to over 1200 operations at full 600
amp load without failing. As a result, the PVI should be considered
for any location that requires frequent switching. Common
applications include using the PVI for automatic transfer
control schemes and distribution automation where the load
break switch may be operated frequently through automation.
SPECIAL
PADMOUNT APPLICATIONS
The PVI has a number of unique features
that allow it to be used for situations that are not easy
to accomplish with standard gear. These include using the
front-accessible PVI switchgear in situations where either
normal switchgear would require two separate padmount units
or where space is tight and a standard front and back accessible
unit is difficult to place. In addition, some special switchgear
designs are available for specific problem areas, e.g. requirements
for a bus tie switch in a single tank unit.
Front-Accessible
PVI Designs
A fairly common situation that occurs during distribution
system design in congested areas is a requirement for more
than two taps feeding from a loop system. Because of the need
for two load break source switches for a loop, the common
solution is to use at least two padmount units to meet the
requirements for three or more fused tap compartments. This
design may end up looking something like that shown in Figure
11A.
Figure 11A is the normal method of meeting
a requirement that calls for more taps than are available
in a standard 4 bay padmount unit. Figure 11B shows the design
using the front-accessible PVI. Since the PVI is expanded
to six bays with four protective taps, there is no longer
a need for two padmount units and the associated real estate,
cabling, two pads and vaults and extra terminations otherwise
required.
Figure 11: Comparing normal
padmount requirements for multi-tap applications to the use
of front-accessible PVI switchgear.
In addition, the front-accessible gear is
wider than standard designs, but significantly more shallow
so that overall space required is minimized. One other feature
that is attractive in the front-accessible gear is that the
equipment can be placed against a wall with no impact to operating
access. In a situation where aesthetics are important or real
estate is particularly valuable, the front-accessible PVI
be-comes a very attractive option to either standard padmount
gear due to smaller space requirements (only front access
is required, so no side or back work access space is needed)
or to placing the switchgear in sub-surface vaults. In comparing
the front-accessible gear to placing all equipment in vaults
or basements, the front-accessible gear is significantly less
expensive.
All of the front-accessible PVI designs
are shown in Figure 3. The more common standard numbering
system is used for all units that have four or less bays,
e.g. PVI-6F or PVI-9F (See Figure 12). Since industry standards
are not available for more than four bays, G&W adopted
a numbering system that first has the number of total bays,
then supplies the number of load break switches in the unit.
For example, the PVI-62F has a total of six bays, with two
Puffer load break switches and four Vacuum Interrupter protective
taps.
Figure 12: Drawing of a
standard front-access PVI-9F.
Bus Tie Applications
Another situation that is difficult to handle with standard
padmount gear is achieving a bus-tie scheme. In most cases,
this is used in industrial applications that require two sources
feeding two or more taps with a center switch that can be
opened to prevent the sources from paralleling or preventing
an overload condition on a single source unknowingly. This
is frequently accomplished with the use of metal-enclosed
switchgear.
An alternate to this bus-tie scheme can
be accomplished with a PVI unit that incorporates a center
switch. In the case of a simple bus-tie with two VI taps,
the PVI can accomplish the same function in a single tank
that is much less obtrusive. Multiple Vacuum Interrupters
can be incorporated if required by using additional front-
accessible PVI's as needed. Contact your G&W field sales
representative for additional information.
Integral Ground
Position Applications
For applications requiring system grounding at the switch,
G&W PVI switches can be supplied with an integral third
switching position permitting safe and easy grounding without
having to disconnect elbow or other cable entrance connections.
Switches can be designed for front access only, or with front/back
access to cable compartments and operating apparatus. The
puffer source operating mechanisms provide ground stops with
padlocking provisions to help assure proper operation in the
desired position.
G&W three position Triad Series switches
are rated through 38kV, 25kA asymmetric momentary and incorporate
a rotary puffer style contact system. Rotary puffer style
contacts are also available for two position PVI applications
where the maximum short circuit current is below 20kA asymmetric.
Rotary puffer style switches permit a smaller switch construction
and should be considered, were ratings permit, for confined
space requirements.
VAULT
PVI APPLICATIONS
The PVI gear is also designed for placement
in vault or subsurface locations. The major change in construction
versus the padmount design is that the PVI is designed for
total submersion by using all watertight control housings
and cables, plus the padmount enclosure is not supplied with
the unit. Most customers prefer to use the front-access style
design due to the tight space available in vaults. As a result,
all of the designs shown in Figures 4-9 are also available
in the front-access style, although front and back designs
are available for vaults if required.
Use of the vault PVI (VPVI) switches is
normally similar to the uses listed in the padmount sections.
Again, loop versus radial decisions must be made prior to
determining the proper configuration based on number of load
break switches required. The other important decision is regarding
the number of taps required to provide overcurrent protection.
Since most of the vault installed units are supplied with
front-accessible designs, going from one VI tap to four or
five is normally a matter of increasing the width of the tank
by an additional two feet per tap (approximate).
If the VPVI is going into a vault with a
manhole opening, the opening size may be a concern. The smallest
round manhole opening that will allow a VPVI unit to fit is
46 inches. Many vaults also provide the capability of removing
the top for placement of large items which may lower this
concern for many applications.
Refer to Figures 4 through 9 for a description
of common design scenarios. Any of these designs can be used
with the VPVI units to provide sound distribution systems
that will meet most customer requirements. Contact your local
G&W representative to discuss specific system requirements
that may not be listed above.
SUMMARY
The PVI allows
the distribution system designer to incorporate many advantages
that have not been present with past switchgear products.
In addition to the standard switchgear applications of switch
and fuse capabilities, the PVI offers many features that add
flexibility for both customer and designer requirements. Primary
advantages are in the VI design and operation. The Vacuum
Interrupters supply the capability of having a resettable
fault interrupting device as opposed to a replaceable fuse.
Also, the electronic controls supply either single or three
phase trip capability by making field selectable adjustments.
The VI also provides the ability to protect continuous loads
above 200 amps (up to a maximum of 600 amps) by specifying
600 amp bushings and the proper electronic control TCC curves
at time of order.
In addition to the advantages supplied by
the VI, the Puffer load break switch supplies a far larger
number of operations without maintenance required than other
switch designs on the market. As a result, the Puffer switch
is ideal for applications involving frequent switching, e.g.
automatic transfer or automating the distribution system equipment.
Finally, in addition to padmount and vault
switchgear, G&W offers an extensive range of overhead
switchgear as well. Sectionalizing switches are available
that use Puffer load break switches, and fault protection
can also be supplied with overhead VI designs. In combination
with automation control schemes, automatic fault sectionalizing
can be set up to increase reliability on the overhead system
by using G&W equipment.
In short, the PVI offers the next generation
of distribution switchgear designs available today. After
examining the information in this product guide, please contact
your local G&W representative to discuss specific applications
of the PVI switchgear for use on your system.
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