Component Products
The Cooper
Posi-Break™
Solution to
Separable Connector Switching Problems
at Wisconsin Electric Power Company
by Kevin Fox
Senior Product Specialist
S
ince the late 1980s, many utilities
noticed an increased number of
flashovers occurring on their
25 kV underground systems. This happened only during the break operation
and involved 25 kV, 200 A loadbreak
elbows and protective caps. These
events would usually cause a protective device to operate, resulting in a
customer outage and damage to
associated equipment.
In 1994, Cooper Power Systems’
Components & Protective Equipment
Group, with the cooperation of several
utilities including Northern States
Power, Public Service of Colorado,
Missoula Electric and most notably,
Wisconsin Electric, began research to
find a solution to this flashover problem. A synthetic circuit that replicated
field conditions was built to monitor
circuit parameters. Testing conducted
by Cooper’s Systems Engineering
Group at the Thomas A. Edison
Technical Center began in early 1995
and lasted through the winter of 1997.
The result of this testing and research
revealed several common factors:
• The flash to ground occurred before
actual contact separation between
the two components.
• Majority of the flashovers involved
little to no load current (there is no
load present when removing
protective caps).
Armed with this new data, a process of elimination was started to
determine the cause of the flashovers.
Several system and operator variables
were tested to evaluate their influence
on the flashover phenomena. Most
were eliminated, however the discovery of a partial vacuum effect justified
further study.
The Partial Vacuum Effect
The partial vacuum effect occurs
only on the break operation of separable connectors and would usually
involve stuck interfaces. In colder
weather when the rubber and silicone
lubricant is stiffer, or in warmer
climates where the grease has
evaporated or “bled” off, the elbow
or cap will be harder to remove.
• Regionally, most failures occurred in
the northeastern and north central
United States. However, there were
occurrences in the southeast and
western United States.
• Failures occur only on a “break”
operation.
• Most failures occurred below 35° F
ambient.
• Stuck interfaces increased the
likelihood of a flashover.
• A failure can occur on any component with a 25 kV, 200 A interface or
35 kV small interface, from all
manufacturers.
the component, the air space located at
the back of the elbow or cap (by the
bushing nosepiece) expands, creating a
partial vacuum. A partial vacuum environment is a very poor insulator. The
relationship of the decreased voltage
withstand of air, due to a drop in pressure is known as “Paschen’s Law.”
Figure 1 Paschen’s Law curve showing the
relationship between air pressure and its corresponding dielectric strength
During a break operation, as the
lineman applies more force to unlatch
Figure 2 Strike path of a standard 25 kV
elbow and insert during a partial vacuum
flashover
As this air space continues to
expand and the vacuum increases, the
energized copper probe and conductive
insert becomes surrounded by the
partial vacuum, thus reducing the
dielectric strength. When enough force
is applied to “unlatch” the component
and the interfaces between the elbow
or cap and the bushing insert separate,
a flashover occurs from the exposed
probe and conductive insert to the
nearest ground, usually the collar of
the bushing. This will occur within the
first 0.4” of travel.
The POSI-BREAK Solution
Once the cause was determined,
Components and Protective Equipment
set out to find a solution. Working with
actual utilities, several design parameters were set as “must have”, including:
• Must meet current IEEE-386
standards.
• Must be completely interchangeable
with all old and new components.
• Must be retrofittable without a
customer outage.
• Must maintain the stress relieving
Faraday cage.
This was ultimately accomplished
by encapsulating the molded conductive insert inside the elbow and cap,
in insulating rubber. This isolates the
conductive components away from
the insulated interface of the bushing.
In a standard elbow and cap this
conductive insert is in direct contact
with the bushing interface.
Next, a rigid insulating sleeve
was added to the top portion of the
copper probe of the elbow and cap.
These two efforts increased the strike
distance from an energized component to ground from approximately
Figure 3 The increased strike distance
of the POSI-BREAK elbow and cap is
achieved by insulating the molded
conductive insert and the copper probe
3.2” to 5.9”, an increase of over 84%.
The name Cooper POSI-BREAK™ was
selected because it reflects the higher
degree of positive switching performance that these products provide.
By the fall of 1997, only three
months after the design was conceptualized, we had limited production
parts ready for field evaluation. With
the cooperation of Wisconsin Electric
and Missoula Electric, trial POSIBREAK units were installed on their
two systems. These two utilities
agreed to install and periodically
switch units throughout the winter of
1997-98 and to keep detailed records
to assist in the product evaluation.
In the meantime, Cooper continued their own testing of the POSIBREAK components. One of the main
objectives was to determine how
much additional margin this design
provided over the existing product.
In “real world” applications, transient
overvoltages can result in a rise of
more than 43 kV peak (30 kV rms)
during switching operations.
Consequently, switching tests were
conducted at 30 kV L-G for both the
elbow and cap. This is well in excess
of the 26.3 kV currently required by
the IEEE standard. After over 60 operations, the POSI-BREAK components
passed without a single failure. This
extra margin assures reliable operations under the worst-case situations
seen in the field.
By the end of April, both
Wisconsin Electric and Missoula
Electric had completed their trials.
Wisconsin Electric had installed the
POSI-BREAK components on two
separate, existing 14.4 kV circuits and
switched in temperatures from -5° F to
+39° F. They successfully switched the
components in excess of 200 times
without a single failure.
Missoula constructed a circuit
especially to test the POSI-BREAK
components. They designed the
circuit to have both the standard and
new design installed side by side for a
direct comparison. After switching all
winter, they too had a 100% success
rate with the POSI-BREAK design.
However they did have a standard
design cap fail under the same conditions that they had just successfully
switched with the POSI-BREAK design.
Both utilities had previously
reported a relatively high number of
“unexplained” flashovers, especially in
the winter months. Wisconsin Electric
stated they would have from 10 to 15
of these flashes during the months
between October to March. As a
temporary solution, they implemented
preheating the elbows in pad-mounted
equipment with a portable kerosene
heater. They did this for approximately 20 minutes before they tried pulling
the elbow or cap off the bushing.
While the number of flashovers
dropped, the manpower cost was
prohibitive for a long-term solution.
Missoula Electric was experiencing up to a 40% flashover rate of caps
in one feeder alone and was considering making their 25 kV system deadbreak. This was a safety and reliability
concern that would also have a
significant economic impact on their
operating practices.
No design eliminates the partial
vacuum. The increased strike distance
this design provides, increases the withstand margin of the component that
effectively negates any adverse effects
that can be created by the partial vacuum. By virtue of this increased margin,
the POSI-BREAK design has broader
applications than just solving the
partial vacuum puzzle. The increase
in dielectrics also increases the components’ tolerance to other system and
operator issues, including transient
overvoltages caused by ferroresonance
and circuit capacitance and interface
contamination that can occur during
installation or switching.
Figure 4 The POSI-BREAK is setting a new
standard in the utility industry
Working closely with utility users
has helped Cooper develop a new product concept that not only addressed
their concerns but also increased their
system reliability in ways not previously
considered. This solution was conceived
by using feedback from actual users in
the field, not just lab data. Voltages used
in the test procedure were taken from
actual field measurements and the
vacuum analysis was based on readings
observed on elbows and caps that were
really stuck on bushing interfaces in the
middle of winter.
Utilizing all these resources helped
Cooper create a solution that met all
the criteria set forth by the utility user,
including complete compliance to IEEE386, fully retrofittable without an outage,
and completely interchangeable with all
available mating components. Used in
combination with the Cooper exclusive
latch indicator insert that provides
immediate reliable feedback on the
quality of the installation, the Cooper
POSI-BREAK 25 kV elbow and cap
represent the most dependable, costeffective loadbreak system available.
Bulletin Number 98065 • © 1998 Cooper Power Systems • MI 10/98 5M