Download 97043

Component Products Group
Current-Limiting Fuses
and Power Quality for
Georgia Power Customer
by Craig Price, Staff Engineering Associate
Georgia Power Company
and Karen Leix, Manager, Overcurrent Protection Equipment
Cooper Power Systems
P
ower quality is more than a buzz
word in today’s competitive utility
marketplace. Customers, be they
commercial, industrial or residential
consumers, demand and have a right
to expect that the power they are provided will be beneficial to them and
support their endeavors. Utilities are
faced with an on-going problem of
providing an increasing power load to
customers for increasingly sensitive
equipment from computers to variable
speed drives. Voltage sags or blinks
on electrical distribution systems can
cause this sensitive equipment to
malfunction, costing customers time
and money.
Georgia Power has a large manufacturing customer with a regional
headquarters and pilot assembly line
where they test out new assembly
line processes, using computers and
other sensitive equipment. They were
accustomed to reliable service from
Old Alabama Substation. As the area
became more developed with a shopping mall and other commercial establishments, continuous construction
was going on, causing dig-ins on
underground feeders. Expulsion fuse
operations due to the faults on the
underground feeders caused breaker
operations on feeders parallel to the
manufacturing customer. This resulted in voltage dips on the feeder going
to the customer.
As part of their underground
development, Georgia Power had,
over the years, put in place livefront
pad-mounted switchgear with expulsion-type, power fuses. The industrial
customer was becoming critical of the
power quality being supplied, and its
effect on the customer’s susceptible
equipment, because they were seeing
voltage dips from faults not associated
with trouble in their facility.
Expulsion fuses are zero-awaiting
devices. Before the system is faulted,
the expulsion fuse acts as part of
the line. When the line is faulted, the
element heats to the melting point,
then breaks apart at its hottest point
of the fusible portion. The current
continues to flow through the parti-
A= System Voltage
B= Feeder experiencing fault
C & D= Feeders on same substation bus
re-ignite, due to the system’s recovery
voltage rising across the severed ends
of the link faster than the dielectric
strength growing between the melting
fuse ends.
Eventually, there is final extinction
and removal of the fault from the system through the now electrically open
(blown) fuse. All expulsion fuses
require at least one-half cycle before
they can clear a fault, and in many
cases take several cycles to clear.
During this clearing process, the voltage at the fuse drops to zero.
On underground systems the
impedance of the fault is very low,
typically on the order of one ohm.
The entire power available at the substation flows to the fault. This causes
the substation bus voltage to collapse.
As a result, the other feeders on the
same substation bus also experience a
dip in voltage.
Figure 1 Expulsion fuse
Figure 1 shows that the voltage of the
expulsion fuse at location B collapsed to
zero and stayed at zero for 8.1 ms until
the current was interrupted. At this
point, the system was subjected to a
transient recovery voltage (TRV) which
can range from 1.2 to 1.7 pu. Voltages
at locations C and D decrease due to
the fault current induced voltage drop
across the source impedance. This voltage drop may be enough to interrupt
sensitive industrial loads.
cles of vaporized element and ionized
gases. Heat from the arc will burn
back the remaining element and
release large quantities of gas from the
surrounding tube wall. When the alternating waveform of current reaches its
zero point, the arc is momentarily
extinguished. After passing through
zero, the arc may re-establish or
The duration of the voltage
dip due to the operation of an expulsion fuse is a growing concern to utilities today. When a fault occurs at a
transformer, an expulsion fuse at the
transformer can isolate the fault and
de-energize the transformer. However,
because it must take a minimum onehalf cycle to clear, a voltage dip at
the transformer and on all parallel
feeders can last 8-12 milliseconds or
longer. This is sufficient time to have
a negative effect on motor contactors
and electro-mechanical relays, high
intensity discharge lamps, adjustable
speed motor drives, and programmable logic controllers.
Current-limiting fuses can typically interrupt current levels up to
50,000 A RMS symmetrical and, in the
process, limit peak current magnitude
when operating in current-limiting
mode. In contrast to expulsion fuses,
a current-limiting fuse will reduce the
fault duration of these normally high
underground faults, and will support
system voltage through much of the
clearing process.
Typically, a current-limiting fuse
has a silver ribbon element, surrounded by fine granular silica sand housed
in a fiberglass tube. Before the system
fuse to increase dramatically. The
resistance that the fuse adds to the circuit changes the circuit power factor
to near unity, shifting the time at
which current crosses zero to be at the
same time as system voltage zero.
This allows the current-limiting fuse to
clear the fault before the natural prefault current zero and typically within
a half cycle.
Before the fuse initially melts,
the voltage drops to zero for a short
period of time, typically 1-4 milliseconds, not long enough to negatively
affect equipment connected to parallel
feeders. During the fault clearing
process, the current-limiting fuse and
the system combine to produce an arc
voltage which supports the system
voltage on all parallel feeders.
A= System Voltage
B= Feeder experiencing fault
C & D= Feeders on same substation bus
Figure 2 Current-limiting fuse
Figure 2 shows that the voltage at the
current-limiting fuse location B collapses
to zero for only 1.2 ms and then rises to
an overvoltage of 1.85 times peak system voltage at location B. The voltages
at locations C and D will be supported.
This fuse induced overvoltage exceeds
normal system peak voltage for about 1
ms. The fault is cleared after 2.5 ms
total and the system voltage returns to
normal. The analysis shows that for the
same high current fault conditions, current-limiting fuses have significant
advantages in supporting system voltage and in improving power quality
when compared to expulsion fuses of
the same rating.
is faulted, the fuse acts as part of the
line. For low fault currents, the fuse
TCC is similar to an expulsion fuse.
But when these typically high fault
currents flow on underground circuits,
the silver ribbon element senses the
fault current, and quickly heats and
vaporizes along much of its entire
length. The high temperature of the
resulting arc melts the sand around
the arc, forming a glass-like structure
called fulgurite. The fulgurite restricts
the arc, causing the resistance of the
At a Power Quality workshop
conducted by Georgia Power, and
attended by the customer experiencing
problems, the topic of currentlimiting fuses and how they could
improve power quality and help
prevent voltage dips was discussed.
Concurrently, Georgia Power was discussing current-limiting fuses during
an Overcurrent Protection workshop
conducted by Cooper Power Systems.
The manufacturing customer originally
proposed that Georgia Power designate a separate substation transformer
bank for them, at a cost of over one
million dollars. Needless to say,
Georgia Power was very anxious to
find a current-limiting fuse that could
be applied in their existing pad-mounted switchgear, as a much more cost
effective solution to accomplish the
same goal.
The Components and Protective
Equipment group at Cooper Power
Systems worked with Georgia Power to
modify Cooper’s X-Limiter fuse to fit
the S & C SML-4Z mount. Georgia
Power provided the Components
group with a sample of the fuse so
they could evaluate the end fittings.
Georgia Power also gave Cooper a
video of the inside of the switching
cubicle, showing clearances in the
switchgear and removal and installation of the fuse. Cooper Power
Systems developed specifications
including voltage, current and timecurrent characteristics. When Georgia
Power received mechanical samples to
test in the field, only minor modifications were required. Over sixty, 100 A,
X-Limiter current-limiting hinge fuses
were installed at critical areas served
by Old Alabama Substation.
This solution is ideal because no
modifications are necessary to install
Cooper’s current-limiting fuses in the
switching cubicle, and the cubicle can
be left energized at all times. Because
of the interchangeability of these fuses
with expulsion fuses, Georgia Power
can still use expulsion fuses in the
future if they choose to.
From a cost standpoint, Cooper’s
current-limiting fuses are considerably
less costly than competitive brands
and require no alterations to the cubicle to install. Georgia Power can
expand their direct buried cables
instead of encasing them in concrete. If
cables are accidentally cut by a dig-in,
the chances of affecting service to critical customers has been diminished.
Most importantly, Georgia Power
improved the quality of service to
their customer and saved at least
$900,000 from the original request by
the customer for a dedicated substation transformer bank for their service.
With calculated available fault
currents at the substation bus of over
5000 A with short lines, it’s only a matter of time before the X-Limiter fuses
at Georgia Power are called on for high
fault current duty. Since the installation of the X-Limiter current-limiting
fuses, there have been low current
faults, all of which were cleared by the
fuses, with no complaint calls from
Georgia Power’s critical customers.
SML® is a registered trademark of S & C Electric Co.
X-Limiter® is a registered trademark of Cooper
Industries, Inc.
Bulletin Number 97043 • © 1997 Cooper Power Systems • MI 5/97 8M