THE LINE
Cooper Power Systems
October 1996
page 3 Multi-Stress Environmental Testing: Comparison of Different Polymer Arrester Designs
page 5
Distribution
and Protection
in One Compact
Package
page 7
Harmonic Filters
Key to Plant
Reliability
Cooper Power Systems
THE LINE
October 1996
Cooper Industries
Cooper Power Systems Division
THE LINE
P.O. Box 1640
Waukesha, WI 53187
October, 1996
Published by Cooper Power Systems
Editorial Board
Cooper Power Systems
W. D. Martino
President
Editor-in-Chief, Steve Benna
Marketing Manager Components &
Protective Equipment
Executive Editor, Patrick Taugher
Manager, Marketing Communications
Bob Jozwowski, Apparatus Engineer
Welcome to the Fall ‘96 issue of The Line magazine.
Jack McCall, Product Marketing
Manager, Power Capacitors
The name of the magazine was not chosen lightly. Some of you may remember
an earlier Line, published by the McGraw Edison Company for many years,
and by Line Materials Company before that. In over 50 years of publication,
The Line earned a reputation for providing important and useful information
to electrical utility industry professionals. We hope to continue that tradition
with the revival of this magazine.
Jim Quinn, Sales Director
This is an example of Cooper Power Systems’ paying attention to our customers’
challenges, and providing cost effective business solutions. Whether it’s new
product development, innovative solutions to complicated application problems,
or continuing industry education, our efforts are focused on helping you and
your company achieve lower operating costs and higher system reliability.
If you have some information you’d like to share with your fellow industry
professionals, we’d like to hear from you. Contact us at the editorial address
listed on the right, or talk to your Cooper Power Systems sales engineer.
We hope you find The new Line useful to you and your company.
Sincerely,
Ron Willoughby, Manager,
Systems Engineering
Jim Byrnes, Marketing Images
Contributing Engineers
Jeff Lindgren
Brian Steinbrecher
Daniel Wycklendt
Dave Brucker
Craig Befus
Ronald Willoughby
Reginald Mendis
Craig Wahlgren
Address questions, inquiries and
letters to:
Patrick Taugher
Cooper Power Systems
2800 Ninth Avenue
South Milwaukee, WI 53172
(414) 768-8431
FAX (414) 768-8334
Photocopy permission: Photocopy
permission extended to single copy
only. Permission for all other reprints
must be obtained from The Line editor.
©1996 Cooper Power Systems
All Rights Reserved
Design and Production by
Marketing Images, Inc.,
Waukesha, WI
Printed in USA
2300 Badger Drive
Waukesha, WI 53188-2400
414-896-2401
Systems Engineering Group
Cover Story
7
Multi-Stress Environmental Testing:
Comparison of Different
Polymer Arrester Designs
Transformer Products Group
5
Bob Schmac, Utility Marketing
Manager, Transformers
Web Site Address:
http://www.cooperps.com/
Bill Martino
President
3
Gavin McFarlane, Sales Director
Harmonic Filters
Key to Plant
Reliability
Component Products Group
9
Distribution and
Protection in One
Compact Package
New U-OPTM 600 A
Connection System
Improves Safety,
Makes Operation
Easier
Systems Engineering 1996-97 Workshop Schedule on page 4
2
THE LINE / October 1996
Component Products Group
Multi-Stress Environmental Testing:
Comparison of Different
Polymer Arrester Designs
by Jeff Lindgren
Overvoltage Protection Equipment Specialist
and Brian Steinbrecher
Manager, Overvoltage Protection Equipment
I
n recent years, an increasing percentage of surge arresters have been
produced with various polymer
housings rather than porcelain housings. Polymers, being relatively new
insulating materials for arresters, do
not have the extensive field perfor-
UltraSILTM testing
mance history that porcelain does. As
a result, accelerated aging tests were
devised to try to predict the long term
service reliability of these ‘new’ polymer materials.
Accelerated aging tests are used to
over-stress polymer insulating materi-
Figure 1
als in an attempt to predict the long
term reliability of the insulation system. Typical stresses applied in this
type of testing include: voltage, heat,
UV radiation, moisture, and contamination. Historically, accelerated aging
testing involved only a few of these
stresses not always applied concurrently. In contrast to these accelerated
aging tests, a new type of test has been
developed which involves exposure to
all of the above mentioned stresses
applied repeatedly over a continuous
time period. This testing (called multistress testing) is the most realistic and
severe type of test developed to simulate the long term exposure and stresses seen by arresters when installed and
in service.
Multi-stress testing can help predict
which polymer arrester insulating systems will provide more reliable field
performance.
This is important,
because arrester manufacturers produce polymer-housed units incorporating different housing materials. So, will
these arresters perform the same over
time? (The answer is no.) More importantly, are the tests which were used to
test porcelain insulators suitable for
Daily cycle chart
testing polymer housed arresters?
(Again, the answer is no, they do not
apply the full range of stresses seen in
the field.)
When comparing the different
polymers, all exhibit a tendency to
repel water (hydrophobicity) when
new. However, some polymer materials
lose this ability over time (just as a
waxed car tends to stop beading water
over time). This is a concern, because
if the housing does not repel water and
instead allows it to coat the surface,
significantly elevated leakage currents
(measured as increased watts loss)
flow across the housing. The excessive
leakage current leads to surface degradation that accelerates under long term
exposure and eventually leads to
arrester failure. Unlike other polymer
materials, silicone rubber retains this
hydrophobic characteristic much more
completely over time. This results in
longer service life, higher withstand
voltage levels (less flashovers), and the
lowest surface leakage currents.
Cooper Power Systems uses multistress testing to simulate the effects of
different environmental conditions on
several commercially available arrester
designs and to compare the relative performance of the different housing materials over time. By
over-stressing the
insulation system of
each arrester design
and monitoring the
surface leakage current,
multi-stress
testing shows that
excessive increases
in surface leakage
current (measured
in watts) will lead to
polymer
material
degradation
and
Weekly cycle chart eventual failure.
THE LINE / October 1996
3
Weekly Duration [h]
Severity
Voltage
SYSTEMS ENGINEERING
WORKSHOPS 1996-1997
Salt Fog
47.5
40 g /l - 0.133 l/hm3
on
Steam Fog
25.0
demineralized water
33 g /hm3
on
Rain
20.0
demineralized water
1.5 mm/min - 100 Ωm
on
Solar Radiation
48.0
1.5 kW/m2
on
Distribution OverCurrent
Protection
Pause with Voltage
25.0
on
2.1 CEU
Pause without Voltage
2.5
off
Table 1
DAT E S
Description of test cycle
&
L O C AT I O N S
Feb. 11-13, 1997
April 8-10, 1997
May 6-8, 1997*
May 6-8, 1997
Oct. 14-16, 1997*
Dallas,TX
Jackson, MS
Baltimore, MD
Atlanta, GA
Milwaukee,WI
Power Capacitor Application
Daily Average Watts/kV MCOV
1.7 CEU
Nov. 19-21, 1996
Sept. 30-Oct. 2, 1997
Dec. 2-4, 1997
Atlanta, GA
Greenwood, SC
Atlanta, GA
Distribution OverVoltage
Protection
1.7 CEU
Nov. 12-14, 1996
Oct. 28-30, 1997
Nov. 4-6, 1997
Atlanta, GA
Atlanta, GA
Milwaukee,WI
Power Quality
& Harmonic Analysis
2.1 CEU
Oct. 29-31, 1996
Oct. 28-30, 1997
Milwaukee,WI
Milwaukee,WI
Transformer Application
& Protection
2.1 CEU
March 25-27, 1997
April 15-17, 1997
Oct. 7-9, 1997
Figure 2
We tested four different arrester
housing designs: our UltraSIL™-housed
VariSTAR® arrester with its exclusive
silicone rubber housing; an Ethylene
Propylene Diene (EPDM)/silicone alloyhoused arrester, an Ethylene Propylene
Copolymer (EPM)-housed arrester and
an Ethylene Vinyl Acetate (EVA)housed arrester.
As an initial measure of leakage current, the watts loss of each arrester
while energized at MCOV was recorded
with the housing clean and dry. The
arresters were then placed into the
multi-stress test chamber simulating a
sea coast environment and energized at
MCOV. The test cycles were then run
for over 6000 hours with watts loss
measurements taken periodically. The
daily and weekly cycles involved in this
test are shown in Figure 1 on page 3.
The testing proved that the UltraSIL
housed arrester had the overall lowest
level of leakage current (watts loss) of
the designs tested. The graph shown in
4
THE LINE / October 1996
Total leakage current
Figure 2 illustrates the total leakage
current (both internal and external) for
the various arresters taken throughout
the test. The dramatic increase in
watts loss during the test for the nonsilicone rubber-housed arresters is
mainly due to leakage currents flowing
across the outside of the wet and contaminated housing surface. Housing
surface leakage current leads to degradation of the housing, which accelerates under long term exposure and
eventually leads to failure.
When selecting a polymer housed
arrester it is important to realize that
not all housing materials will perform
equally well over time. Multi-stress
testing is the best test designed to
gauge how different materials will perform over time. This testing has confirmed the superior nature of silicone
rubber for long term performance, with
no loss of insulating properties and
resistance to degradation. THE LINE
Milwaukee,WI
Atlanta, GA
Raleigh, NC
Distribution Analysis
2.8 CEU
March 11-14, 1997
April 22-25, 1997
May 13-16, 1997
Sept. 23-26, 1997
Oct. 21-24, 1997
Atlanta, GA
Denver, CO
Cincinnati, OH
Milwaukee,WI
Portland, OR
Distribution Voltage Regulation
2.1 CEU
June 10-12, 1997
Sept. 9-11, 1997
Sept. 30-Oct. 2, 1997
Toronto, ON
Milwaukee,WI
Atlanta, GA
For registration information on
Systems Engineering Workshops,
call Marilyn McGair at 414-523-3940
or 800-523-9307, or e-mail at
marilyn@marketingimages.com
* A one-day Symmetrical Components Tutorial
will precede this workshop.
Transformer Products Group
Distribution and Protection
in One Compact Package
Three-Phase Compartmental Transformer with Vacuum Fault
Interrupter (VFI) Protection Provides Competitive Advantage
by Daniel Wycklendt, Senior Market Specialist,
Dave Brucker, Apparatus Engineer
and Craig Befus, Apparatus Engineer
U
tilities have always been looking for
ways to work smarter, safer, and
cheaper. This is even truer today as
they prepare for the changing utility
marketplace.
Many have embraced
this new environment and are proactively pursuing practices and strategies
that will make them more competitive.
Competition usually begins with a singular focus — price — and then evolves
to include reliability, responsiveness,
quality, customer-focused services,
innovation and safety.
The following three cases are examples of how utilities are using new technology to meet or exceed the needs of
their customers and accomplish this in
a cost effective, competitive manner.
City of Banning
A utility preparing for the future is
the City of Banning, California, located
east of Los Angeles at the beginning of
high desert country. The municipal
utility purchases power from the
Southern California Edison grid, and
has residential, small commercial and a
few large industrial customers on their
system. One industrial customer, concerned about improper overcurrent
protection coordination and the costs
incurred from short- and long-term outages, was pressing the utility to either
eliminate the coordination problem
that was causing these outages or furnish a secondary feed to their facility.
Coordinating the overcurrent protection of the three aging banks of 4 kV
transformers was not possible with a
conventional transformer fusing and
substation relay protection scheme,
because the transformer fuses would
not operate before the substation relay
tripped. This miscoordination would
mean that half of the loop would be deenergized due to a fault on the load side
of the transformer. Meeting the customer’s demand for reliability in a traditional method would be an expensive
proposition, since it would require the
utility to build a new substation.
It was at this point that the use of
a Three-Phase Compartmental Transformer with Vacuum Fault Interrupter
(VFI) Protection was proposed to
Tim Trewyn, City of Banning Electrical
Operations Engineer.
The VFI
Transformer combines a three-phase
transformer with a three-phase vacuum
fault interrupter to provide distribution
and protection in one compact package. The major factors considered
when making this proposal were coordination, reliability, flexibility, and cost.
The VFI Transformer was able to meet
or exceed the requirements for all.
Substation
City of Banning
Overcurrent
Relay
Circuit
Breaker
VFI
XFMR
VFI
4160Y/2400
1500 kVA
480Y/277
4160Y/2400
1000 kVA
480Y/277
4160Y/2400
1000 kVA
480Y/277
Figure 1
4160Y/2400
1000 kVA
480Y/277
City of Banning
Protection coordination and reliability were achieved via the VFI TriPhase Breaker Control. The Tri-Phase
electronic breaker control is designed
so that it can be equipped to approximate one of five separate Time-CurrentCharacteristic curves. TCC curves can
be chosen to approximate either an
S & C E fuse, a McGraw-Edison K or T
link fuse, or resemble a recloser or
relay curve. The KF TCC curve was
chosen because it provided the proper
coordination with the substation relays
protecting the loop. Although each
phase of the VFI breaker can be individually set at one of over 100 minimum
trip settings, a common trip setting was
chosen. In addition, an instantaneous
trip setting could also have been chosen, thereby providing maximum coordination with the upstream protection
devices while still providing the needed
transformer protection.
The VFI breaker operation is initiated by the Tri-Phase Control when an
overcurrent condition is detected on
any of the three phases. All three interrupters are electronically controlled
and mechanically ganged together, so
that a trip signal on any one phase will
trip all three phases. This feature eliminates single-phasing of three-phase
loads and the associated ferroresonance or motor damage problems. The
VFI breaker is resettable, which makes
restoring service faster and easier.
This helps improve the availability of
service, which is an important component of reliability.
Flexibility was also a requirement.
The VFI breaker, in conjunction with
the two loadbreak switches, provides
the utility and the customer with various options for feeding and sectionalizing the loop. These options permit the
outage area and outage duration to be
minimized when a fault condition is
present.
Cost was also a driving force in the
selection of the VFI Transformer over
other alternatives. There are four factors that need to be considered when
calculating the cost of alternatives: real
estate, installation, maintenance and
operating costs.
Usable real estate was at a premium
at the site and therefore it was important to maximize its use. Because the
combination VFI breaker and transformer has a footprint basically the
same size as a conventional transformer, the space usually required for a
THE LINE / October 1996
5
separate piece of switchgear is eliminated.
Because the VFI breaker is integral
to the transformer, installation is simplified and the costs to locate and connect a separate piece of equipment are
eliminated.
Maintenance costs are reduced
because there is only one piece of
equipment, and that piece requires less
maintenance when compared with
fused switchgear or other alternatives.
Operating costs are reduced by the
use of a vacuum fault interrupter in lieu
of fuses to clear the fault. This eliminates the need to buy, stock, and
replace fuses when a fault occurs. In
addition, current trip settings can be
adjusted without de-energizing customers. This allows maintenance crews
to revise the interrupter trip settings to
accommodate load or circuit changes
without having to schedule an outage.
The VFI Transformer met the City of
Banning’s needs for protection coordination, reliability, flexibility, and cost.
In short the VFI Transformer provided a
clean, inexpensive, fast installation that
exceeded the customer requirements
for service and fulfilled the utility’s
budget constraints.
Vera Water and Power
Vera Water and Power in Spokane,
Washington, recently was given an
opportunity to provide electric power
to a new shopping center. Vera was
able to secure this customer because
they could provide them with a higher
level of reliability. Brian Dilts, Vera’s
engineer, found a creative way to use
the VFI Transformer to provide
improved sectionalizing and better
feeder protection coordination for this
important new customer, while eliminating the need to purchase additional
switchgear, vaults, and real estate.
The customer is served by an
underground feeder from the substation to the VFI transformer, the feeder
then continues underground to serve
the other customers on the loop. The
VFI Transformer specified by Vera uses
the VFI breaker to protect the downstream section of the 600A feeder
rather than protecting the transformer.
In this configuration, if a fault occurs
downstream, the VFI breaker will trip
and isolate the fault, leaving electric
service intact for the shopping center.
The VFI breaker control was
equipped with a type H TCC curve
because it provided coordination with
6
THE LINE / October 1996
the upstream portion of the circuit that
was protected by a three-phase electronic recloser at the substation and
fusing that protected the downstream
portion of the loop. Because the VFI
breaker is resettable, restoring service
for the customers affected by an outage
will be easier and faster to complete.
Substation
Vera Water
& Power
that there was no way to properly coordinate fusing on the high side of the
transformer with power fuses on the
riser pole of the 25 kV system.
Clay found that the VFI Tri-Phase
breaker control with type EF TCC curve
provided proper transformer protection and easily coordinated with both
the riser pole fuses and the substation
overcurrent relays, eliminating the
need to change the fuses and the overcurrent relay settings.
Overcurrent
Relay
Substation
El Paso Electric
Recloser
Overcurrent
Relay
VFI
Circuit
Breaker
VFI
XFMR
13,200GY/7620
750 kVA
480Y/277
Figure 2
Riser
Pole
Fusing
Vera Water and Power
If a fault condition is present when
the circuit is re-energized, the trip-free
operation feature of the VFI breaker will
prevent the breaker mechanism from
being held in the closed position. This
feature provides added operator safety
when fault locating is necessary.
In addition to being able to offer
higher reliability, reduced outage duration and outage area, Vera Water and
Power was able to provide a safer operating environment for its workers. It
was also able to save thousands of dollars when compared to conventional
construction which incorporates
switchgear separate from the transformer.
The savings came from
reduced labor, material, and equipment
costs. When the additional costs of
real estate, operating costs, maintenance and customer outages costs are
included, the VFI Transformer is an
even better choice.
EL Paso Electric
El Paso Electric provides electric
power to El Paso, Texas as well as the
city of Las Cruces, New Mexico. A
major hosiery factory in Las Cruces
recently expanded and increased electric load significantly. The factory preferred a new service drop in lieu of running a 4 kV bus through the plant to
serve
newly
installed
knitting
machines. A big problem that El Paso
Electric’s engineer Clay Doyle faced
was how to properly coordinate overcurrent protection between the transformer and the substation. It appeared
VFI
VFI
XFMR
23,900
5000 kVA
4160Y/2400
Figure 3
El Paso Electric
The VFI breaker provided the added
benefit of eliminating ferroresonance
and motor problems associated with
single-phasing of the three-phase deltawye transformer connection. Singlephasing of loads is eliminated because
a trip signal on any one phase will open
all three phases.
The VFI Transformer provides convenience, safety, and transformer or
loop overcurrent protection coordination in one compact enclosure. When
compared with two separate pieces of
equipment, the VFI Transformer has
many advantages. Many of these result
in lower Total Owning Costs due to
reduced installation, maintenance, and
operating expenses. The unit is ideal
for distribution systems, because it
meets the need for reliable service,
easy maintenance, flexibility for
load expansion, and protection against
faults and other abnormal currents.
Best of all, it simplifies installation and
requires less real estate than conventional distribution transformer/switchgear packages. The VFI Transformer is
a competitive solution that enables utilities to meet or exceed the needs of
their customers while still meeting
their own budget constraints.
THE LINE
Systems Engineering Group
Harmonic Filters
Key to Plant Reliability
by Ronald D. Willoughby, PE
Manager, Systems Engineering
and S. Reginald Mendis
Power Systems Staff Engineer
Filter Design
Harmonic currents may be prevented from flowing into the power system
by one of two methods: 1) Use of a
high series impedance to block them;
or 2) diverting them by means of a low
impedance shunt path. The most practical approach is usually shunt filters.
The most common shunt filters, the single-tuned and high-pass filters, are the
simplest to design and the least expensive to implement. Their general layout
is shown in Figure 1.
C
C
L
C
L
C1
R
L
IMPEDANCE MAGNITUDE
(POWER OF 10)
FREQUENCY
Filter Alone
IMPEDANCE MAGNITUDE
(POWER OF 10)
C1
C2
R
R
L
C2
R
A
B
A)
B)
C)
D)
E)
Figure 1
FREQUENCY
Filter and System
R
C
D
E
Single-tuned filter
First order high-pass filter
Second order high-pass filter
Third order high-pass filter
C-Type high-pass filter
Common shunt filters
The single-tuned or notch filter is
probably the most common shunt filter.
A typical frequency response plot is
shown in Figure 2.
The high-pass filter is so named
because of its characteristic low
impedance above a corner frequency
as shown in Figure 3. This filter will
shunt a large percentage of all harmonics at or above the corner frequency.
Frequently, one high-pass filter whose
corner frequency is located at the lowest harmonic to be eliminated is used
for all filtering. However, two factors
may discourage such an application: 1)
The minimum impedance of the highpass filter in its passband never
achieves a value comparable to that of
the single-tuned filter at its notch frequency; and 2) the shunting of a per-
Figure 2
Typical frequency response of a
notch filter
centage of all the system harmonics
through one filter may require that
filter to be significantly overrated
when compared to the fundamental
frequency.
IMPEDANCE
MAGNITUDE
(POWER OF 10)
H
armonics have been shown to have
detrimental effects on equipment
including transformers, rotating
machinery, switchgear, capacitors,
fuses,
and
protective
relays.
Transformers, motors, and switchgear
may experience increased losses and
excessive heating. Induction motors
may refuse to start or may run at subsynchronous speeds. Circuit breakers
may fail to interrupt currents from
improper coil operation. Capacitors
may prematurely fail from increased
dielectric stress and heating. Time-current characteristics of fuses can be
altered, and protective relays may
experience erratic behavior.
Electronic power converters have
become one of the major sources of
harmonics in industry today. These
converters operate at a low power factor, usually making it necessary to also
apply shunt capacitors for reactive
compensation.
Harmonic filters are effective in
minimizing harmonic voltage distortion
caused by nonlinear loads. However,
different configurations should be considered before making the final design
decision. Among the performance criteria are current and voltage ratings of
the filter components, and the effect of
filter and system contingency conditions.
The general procedure in analyzing
any harmonic problem is to identify the
worst harmonic condition, design a
suppression scheme and recheck for
other conditions.
A good computer program for harmonic analysis is essential for most
investigations. It should be able to
model nonlinear loads, perform multiple-source solutions, and execute frequency scans. An example of such a
program is the V-HARMTM harmonics
analysis program by Cooper Power
Systems.
FREQUENCY
Figure 3
Typical frequency response of a
high-pass filter
Design Equations
The impedance of the filter branch
is given by:
THE LINE / October 1996
7
Resonance occurs when the imaginary part is equal to zero, at which time
the impedance is limited by the value R.
The frequency for which the filter is
tuned is given by the value of f that
results in series resonance. This frequency is given as:
The quality factor, Q, of a filter is a
measure of the tuning sharpness and is
represented by:
IMPEDANCE
MAGNITUDE
(POWER OF 10)
where the reactances at the resonance
frequency are given by XLr and XCr.
Q=100
Q=7
Q=0
Q=4
FREQUENCY
Figure 4
Scan plots for a 4.7th harmonic
notch filter with various values of Q
Design Considerations
Filter interaction with the source
impedance results in a parallel resonant peak.
For inductive source
impedances ( LS ), this point occurs at
a frequency below that at which the filter is tuned and is given by:
For multiple parallel single-tuned filters, one resonance peak will exist for
each filter. The resonance peak also
has its own value of Q as given by:
The proximity of the parallel resonance peak and the filter notch is
dependent entirely on the source
inductance. This in turn is a function of
the available three-phase fault MVA.
Figure 5 shows how the response for
Figure 5
8
the same 4.7th harmonic filter varies
for different values of available shortcircuit MVA.
The problem associated with these
adjacent resonance points is one of filter detuning. If a filter is tuned exactly
at the frequency of concern, then an
upward shift in the tuned frequency
will result in a sharp increase in
impedance as seen by the harmonic.
Should the resonance peak shift
upward enough to coincide with the
harmonic of concern, the resulting voltage amplification may be disastrous.
Therefore, it is advantageous to
tune a filter to a frequency somewhat
below the desired frequency. This will
provide sufficient harmonic filtering
action, yet will allow acceptable operation in the event detuning takes place.
Typically, filter banks are tuned to
approximately 3-to-10 percent below
the desired frequency.
Response of a 4.7th harmonic
filter for different values of MVAsc
THE LINE / October 1996
Filter Component Ratings
Capacitors
The allowable overload limits of
capacitors based on standards are:
kvar
rms voltage
sum of peak voltages
rms current
135 %
110 %
120 %
180%
All of these parameters should be
checked when applying capacitors in a
harmonic environment, especially if
they are parts of a filter. The use of an
inductor in series with a capacitor
results in a voltage rise at the capacitor
terminals according to the following:
n = tuned harmonic of the filter
When calculating the maximum
voltage rise, worst conditions should
be used: maximum system voltage
together with maximum capacitance
tolerance (typically 8 %) and maximum
inductor tolerance (typically 5%).
Taking these tolerances into account
will yield the maximum voltage rise
across the reactor.
If a capacitor is used in a system
with voltage lower than the capacitor
rating, the following formula should be
used to determine the effective kvar:
The presence of a filter reactor
changes the effective kvar output as
follows:
If reactive compensation is required
from the filter, the designer will likely
perform several iterations before finally deciding on the capacitor ratings.
The current limit may be lower than
standards (180% by standards)
because individual capacitor units are
usually fused at 125-165% of their rating.
When designing a filter, the limits on
the rms voltage and currents, and the
arithmetic sum of the peak voltages on
the capacitor bank, should be close to
100% of rating for normal conditions.
Tuning Reactor
Reactors used for filter applications
are usually built with an air core, which
provides linear characteristics with
respect to frequency and current. A
±5% tolerance in the reactance is usually acceptable for industrial power system applications. The 60-Hz X/R ratio
is usually between 25 and 150. A series
resistor may be used to lower this ratio
if desired.
The reactor should be rated to
withstand a short circuit at the point
between the reactor and the capacitor.
The reactor insulation (BIL) should be
similar to that of power transformers
connected at the same voltage level.
Parameters to include when specifying a reactor are the following: fundamental current; harmonic current
spectrum; short-circuit current; X/R
ratio; system voltage; and BIL.
Filter Selection and
Performance Evaluation
Before any filter scheme is specified, a power factor study should be
done to determine if any reactive compensation requirements are needed. If
power factor correction is not necessary, then a minimum filter can be
designed; one that can handle the fundamental and harmonic currents and
voltages without consideration for
reactive output. Sometimes, more than
one tuned filter is needed. If so, the filter system must be designed for the
possibility of having specific filter
branches out of service.
While the effectiveness of a filter
installation ultimately depends on the
degree of harmonic suppression, it also
involves consideration of alternate system configurations. As the supplying
utility reconfigures its system, the
continued on page 11
Component Products Group
New U-OPTM 600 A
Connection System
Improves Safety,
Makes Operation Easier
A new 15 and 25 kV class, 600 A deadfront connector provides
increased safety and easier operation for field personnel —
the U-OPTM Visible Break Connector System.
by Craig Wahlgren
Molded Rubber Products Specialist
T
he U-OP connector system was
designed with safety in mind.
Safety in a 600 A connector system
requires that you be able to easily
achieve a visible break and visible
ground before beginning repair work on
underground cable. Maximum safety
requires that the visible ground be in
place while the visible break is
achieved. The U-OP connector satisfies
these requirements without having to
move heavy 600 A underground cable.
The U connector is constructed so that
one leg is 3” longer than the other. This
allows the U-OP connector system to be
used with different types of apparatus
bushings, some of which may protrude
different distances from the equipment
frontplate than others. To assure that
the U connector is properly seated
when installed, the deadbreak junction’s bracket and the parking stand are
installed on studs that are welded to
the frontplate of the equipment.
The U-OP connector system kit
consists of:
• A U connector (Figure 1).
The U connector has EPDM rubber
molded over an aluminum or copper
buss. There are tulip contacts inside
each rubber leg of the U that mate with
probes mounted on the bushings on
which the U is installed. The U connector has a stainless steel shaft between
the two rubber legs that threads onto a
mating stud attached to the apparatus,
securing the U connector in place. The
shaft has a handle for shotgun stick
operation for installation or removal.
Figure 2
• A two-bushing 600 A deadbreak
junction.
• A parking stand.
Figure 1
U connector
Connector system
Typical U-OP Application/Operation
The U-OP connector can be used in
many different applications. In a deadfront switchgear application, the 600 A
apparatus bushing and a T-OPII™ 600 A
connector are typically used with the UOP connector kit to form the complete
connector system (see Figure 2). The
T-OPII connector consists of a
T-Body, compression lug, and loadbreak
reducing tap plug (LRTP). The LRTP
provides a 200 A interface on which a
200 A protective cap, 200 A loadbreak
elbow, or Metal Oxide Varistor (MOV)
elbow arrester can be installed during
normal system conditions.
In a typical padmount switchgear
application, the current path during
normal system operation is through the
apparatus bushing, through the U connector, through the two way junction,
through the T-OPII connector, and into
the underground cable.
When isolating underground cable
that needs repair, the equipment’s
switch must first be opened to de-energize the unit. The switch or other
device at the far end of the cable should
also be opened, so that the cable is
completely de-energized. The protective cap, elbow, or arrester mounted on
the T-OPII’s LRTP is then removed. A
test probe can then be inserted into the
LRTP to verify that the system is deenergized. A fault close rated (to 10 kA)
grounding elbow can be installed onto
the LRTP.
At this point, the cable can be isolated, using a protective cap or arrester, or
it can be grounded with a regular
grounding elbow.
The visible ground will be in place
while the visible break is being
obtained, providing maximum safety to
operating personnel.
An insulated or grounded standoff
bushing can be installed in the parking
stand. A shotgun stick is then used to
grasp the U connector’s handle and
rotate it to remove the U connector
from connecting the deadbreak junction to the apparatus bushing. The
U connector can then be reoriented 90°
and re-installed to connect the apparatus bushing to either the insulated or
grounded standoff bushing (see Figure
3). A 600 A U-OP protective cap should
then be installed on the upper bushing
of the deadbreak junction to cover it
and protect the interface. When a safe
visible break and visible ground have
been achieved on the other end of the
cable, repair work can safely be performed on the cable.
THE LINE / October 1996
9
Figure 3
An insulated standoff bushing or a
grounded standoff bushing may be
used in the parking stand in the process above. In either case, all bushings
will either be grounded or covered with
insulated protective caps or the U connector before work is performed on the
system. Tag out procedures can then
be followed to ensure that the cable or
equipment cannot be re-energized accidentally.
Once the cable has been repaired,
the operation steps are reversed to
make the U-OP and switchgear ready to
be energized again.
Rotating the U connector for visible break
bushing to the grounded standoff bushing, an external visible break is
achieved. Some types of deadfront
equipment rely on a viewing window to
detect opened switch contacts internal
to the equipment. With this equipment,
a visible break isolating the switchgear
There are many features to the U-OP
connector that make it the safest 600 A
connector system available.
Visible Ground
The use of the T-OPII connector in conjunction with the U-OP provides maximum safety and protection for line
crews. The LRTP not only provides a
location for a 200 A protective cap, tap,
or elbow arrester, but it also provides
access to the cable via a test probe to
ensure that the system is de-energized
before removing the U connector and
subsequently performing repair work
on the cable. It also provides a 200 A
interface on which a 200 A grounding
elbow or fused grounding elbow can be
installed (see Figure 4). This achieves
the desired visible ground for the cable,
not only while the cable is being
repaired but also while the U connector
is being removed.
Visible Break
When the U connector is removed from
its normal operating position and reinstalled to bridge from the apparatus
10
THE LINE / October 1996
Figure 4
While many utilities are successful in
using connector systems to move feeder cable to obtain a visible break, the
U-OP connector eliminates the need to
move the feeder cable, which is often
heavy and stiff. This ergonomic benefit
greatly simplifies lineperson operating
practices. Florida Power Corporation
and other utilities have specified U-OP
on their pad mounted switchgear
because of the safe operation it provides. They believe it gives them the
best method for getting a visible break
without having to move 1000 MCM
cable. They also like the fact that they
can test to ensure the system is deenergized and they can get a visible
break with the LRTP. A Florida Power
Corporation line foreman says that his
line personnel have been very pleased
with the initial installations of U-OP and
their operation.
Easy Operation
The easy operation of the U-OP connector also increases the safety of the
system. The mechanical advantage provided by the threaded
U-OP shaft easily separates the rubber interfaces, minimizing the
possibility of problems
associated with having
to separate stuck interfaces. Not having to
physically
separate
stuck interfaces also
decreases the time
needed to operate the
Installing grounding elbow on T-OP II connector connector system.
is not obtained. There are also other
connector systems available that do
provide an external visible break, but
the components between which there
is supposed to be a visible break are
held together with metal brackets.
Cooper Power Systems believes that an
external visible break using components that are not bracketed together is
the best alternative.
The U-OP connector system allows
the visible ground to be in place while
the visible break is being achieved.
Other connector systems are available
which do not allow a visible ground to
be in place while the connector system
is being operated and the visible break
is being obtained.
The visible break is provided by the
U-OP connector without having to
move the 600 A underground cable.
Deadfront Construction
The U-OP connector is a deadfront system used on deadfront apparatus.
Deadfront equipment is inherently
dependable because all current-carrying parts are enclosed in insulated
EPDM rubber during normal operation.
The highly conductive ground shield,
when connected to ground with a drain
wire, keeps the exterior surfaces of the
connector system at ground potential,
minimizing exposure to shocks or discharges. Also, deadfront equipment is
very reliable - it is very unlikely that
faults will be caused by animals or
floods bridging between phases or from
phase to ground. Most importantly,
safety is maximized by prevention of
accidental contact with conductors,
which could result in equipment failure
and possible bodily harm.
Cable Termination Tolerances
When underground cable is terminated
into a T-Body, the cable may be cut so
that it is slightly too long or too short.
It can then be more difficult to install
the T-Body onto the apparatus bushing
if it is not perfectly aligned. This problem is exacerbated with connector systems that have to be moved often to
provide a visible break or that rely on
the removable portion of the connector
to be directly connected to the T-Body.
The U-OP minimizes the dependency on precise cable termination
because the cable never has to be
moved.
Also, the effect of cantilever force
on the bushing is minimized because
the U-OP is never directly connected to
the T-Body. In the switchgear application discussed above, the T-Body is
installed onto one bushing of a two
bushing 600 A junction while one side
of the U connector is installed onto the
other bushing of the junction. The
EPDM rubber will flex if forces are
applied on one bushing of the junction,
minimizing the effect on the positioning
of the other bushing. The U connector
will remain easy to install even if the TBody is putting cantilever force on the
other bushing of the 2 way junction.
Reliability Increased Due
to Clean Rubber Interfaces
While the underground cable is being
repaired, the U connector is installed
so that it bridges from the apparatus
bushing to an insulated or grounded
standoff bushing. During both normal
operation or de-energization, the U connector is always installed onto mating
bushings, keeping interfaces free of
contaminants, and therefore increasing
reliability.
Other connector systems with
removable pieces are not installed on
mating bushings during de-energization, making additional care necessary
to keep the interfaces clean.
Tag Out Procedures
The U-OP connector allows tag out
operation of the system because the
disconnection,
isolation,
and/or
grounding is under the control of the
on-site operators. The full array of
operating devices that can be used with
the U-OP connector, such as grounding
elbows, operating and test tools, insulated protective caps, insulated standoff bushings, and grounded standoff
bushings make it easy for the operator
to follow traditional safety practices
and existing operating rules.
Application Versatility
There are many configurations in which
the U-OP can be installed. If the phase
to phase spacing between apparatus
bushings does not allow for the U-OP to
be configured as in Figure 2, it is possible to install the junction, apparatus
bushing, and parking stand either vertically or in other ways to fit it to the
apparatus.
The U-OP connector system can be
installed on various types of equipment, including:
•Pad mounted switchgear including
many retrofit applications
•Heavily loaded 3Ø transformers
•Pad mounted capacitors
•Separable splicing in vault
applications
•Above ground sectionalizing
Using a 3 way junction instead of a two
way junction, a bushing extender can
be used with a meter to provide SCADA
access.
The U-OP connector system provides
all of the advantages of existing 600 A
connector systems plus much more —
it is the safest 600A connector system
available.
It provides:
•Visible ground
•External visible break (with the
visible ground already in place)
•No moving of 600 A cable
•Mechanical advantage for easy,
quick operability.
Contact your Cooper Power Systems
Sales Engineer for more information
about the U-OP connector system.
THE LINE
Harmonic continued from page 8
impedance, looking back to the source
from the plant’s standpoint, will
change. Similar effects will be seen
with the plant running under light versus heavy loading conditions, with
split-bus operation, etc. Therefore, the
filtering scheme must be tested under
all reasonable operating configurations.
First, the connected utility should
be contacted to determine the minimum and maximum available threephase fault MVA at the point of connection to the plant. This will allow the calculation of minimum and maximum values of source impedances. Second, a
list should be drawn up consisting of all
reasonable operating contingencies.
Frequency scans of these contingencies
should be made. A frequency scan
should be made at each problem node
in the system, with harmonic injection
at each point where harmonic sources
exist. This allows easy evaluation of
the effects of system changes on the
effective tuning.
Of particular importance is the variability of parallel resonance points
(peaks) with regard to changing system
parameters. For example, if for maximum system load a resonance peak
exists at the 6th harmonic, but at 50%
load it exists at only the 4.8th harmonic, then at some loading in between resonance will occur exactly at the fifth, a
characteristic harmonic for variablespeed drives. This will require either a
redesign of the filter or special operating rules that will minimize the effect of
this resonance point.
Abnormal system conditions such
as frequent transformer energizations,
back-to-back switching of capacitors or
filters should also be analyzed to check
the filter component ratings.
Traditional performance criteria are
the total harmonic distortion (THD)
factor and the telephone influence factor (TIF). THD is an rms value of the
distortion component of the fundamental frequency voltage wave due to harmonics. TIF measures the likelihood of
high-frequency harmonics on the
power lines inducing noise on adjacent
telephone lines.
The duties that all filter components see must be checked. If a
device’s rating is exceeded, then that
device must be reselected or the filter
must be redesigned. An unbalanced
protection scheme is generally
required to detect filter unbalances and
trip the bank to prevent damaging overvoltages. THE LINE
THE LINE / October 1996
11
Together We Can Take A
Blue Sky Idea And Bring
It Down To Earth.
Every day at Cooper, our customers
are the inspiration for our innovation.
Here are just a few examples.
They began as what-if’s and I-wish’s from our customers.
What if you could combine a distribution transformer and the
overcurrent protection of a vacuum fault interrupter into one safe,
convenient, space-saving package? (We did.)
I wish someone would find an alternative to oil or SF6
insulation for switchgear applications. (Presenting our new
maintenance-free solid dielectric VCS epoxy-encapsulated
vacuum switch.)
How to get visible break on pad-mounted switchgear without moving heavy 600-Amp cable? (Spec our U-OP™ connector
system.)
Isn’t there a way to design polymer arresters which are
safe, reliable, virtually indestructible, and also simple enough to
allow for quick customization and just-in-time delivery? (Let us
tell you about our revolutionary new UltraSIL™.)
How about a transformer that eliminates problems with
harmonics? (Ask for Cooper’s R-Tran K-Plus™ transformer.)
A high-energy distribution class arrester with current-limiting
fuse protection to mount at the customer’s meter? A latch indicator insert that gives a visible indication that the elbow is properly
latched? And, please, an indicator for the indicator to distinguish
which have indicators? (Consider it done.)
The list of customer-driven innovations from Cooper goes
on. All of which work to reduce downtime. Enhance power quality. Improve safety. Ensure reliability. Lower maintenance costs.
And save you money. That’s not just down-to-earth. It’s all the
way down to your bottom line. And it’s why our customers tell us
they can count on Cooper Power Systems down the line.
Share some of your own ideas.
You Can Count On Cooper Power Systems Down The Line
Web Site Address: http://www.cooperps.com
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