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Capacitors
September 2000 — Greenwood SC
General Capacitor Fusing Criteria
KILOVAR BRIEFS
9
Introduction
The purpose of this issue of Kilovar Briefs is to present
the fundamental principles for effective fusing of the
Cooper Power Systems all-film capacitor. This criteria
can be applied to other types of shunt capacitors;
however, the degree of protection may not be as effective.
Figure 1. Group Fusing.
The basic objectives in selecting capacitor fuses are twofold:
1. The fuse must be capable of withstanding steady
state and transient currents in order to avoid spurious
fuse operations.
2. The fuse must effectively remove a failed or failing
capacitor unit from service without causing further
damage or disruption to the system.
These objectives are accomplished through two different
fusing methods — group fusing and individual fusing.
Group Protection is defined as one fuse protecting more
than one capacitor. This usually involves one fuse on
each phase protecting all the capacitors on that phase
(see Figure 1). Group Protection is generally used for
protecting pole-mounted distribution capacitor racks. In
this type of application, the fuse links are installed in
cutouts and mounted on a cross arm above the capacitor
rack. Individual Protection is defined as each capacitor in
a bank being protected by its own individual fuse (see
Figure 2). This type of protection is commonly used in
outdoor substation capacitor banks. Fuses are the busmounted type.
Withstanding Steady State and
Transient Currents
The continuous current and the transient current duties
determine the minimum acceptable fuse size so as to
avoid spurious fuse blowing under normal conditions.
The requirements for group fused and individually fused
applications are the same for the continuous current duty
but differ for the transient duty.
September 2000 • Supersedes 3/87
Printed in USA
Figure 2. Individual Fusing.
The fuse link is chosen to have a minimum rating of at
least 135% of rated capacitor current. This overrating is
due to these three allowances — 10% for overvoltage
conditions, 15% for capacitance tolerance and 10% for
harmonics.
Fuses can be damaged due to high magnitude, high
frequency currents. If possible, it is desirable to minimize
spurious fuse operations by selecting an appropriately
large fuse link so as to withstand these transient currents.
The two sources of system-generated transient current
are capacitor bank switching and lightning surges.
Switching is typically only a concern when capacitor
banks are switched on the same bus; i.e., back-to-back
switching. This is seldom the case for pole-mounted
group fused capacitors. However, these fuses are
subject to high frequency transients due to lightning
surges. Low current rated links are especially susceptible
to these surges.
1
General Capacitor Fusing Criteria
Unlike pole-mounted capacitor racks, individually fused
substation capacitor banks are generally not exposed to
significantly high magnitude, high frequency lightning
surges. Transient currents due to switching are also of no
significant concern unless capacitor banks are switched
back-to-back. Even in that case if the switchgear is
applied within the ANSI standards for inrush current
frequency and magnitude, the fuse duty is generally
acceptable.
The fuse link and capacitor must be able to adequately
handle the available fault current. When capacitors are
connected grounded wye in a single series group application, a capacity failure (terminal-to-terminal short) will
cause system fault current to flow. The capacitor must be
able to withstand the fault current until the fuse interrupts
the circuit. Additionally, the fuse must be able to successfully interrupt the available fault current.
Figure 3. Energy Discharge into Failed Unit and
Energy Outrush from Remaining Good Capacitors.
When a capacitor unit goes to a complete short, other
series groups within the capacitor banks are subject to a
60 Hz overvoltage until the fuse clears. The fuse should
clear fast enough so as not to damage the good units due
to this overvoltage.
In an individually fused application, there is one additional transient consideration. When a capacitor unit fails,
i.e., goes to a short circuit, the remaining good capacitors
will discharge into the failed capacitor. The fuses on the
good capacitors should be able to withstand this high
frequency outrush current to avoid multiple fuse operations whenever a single unit fails. See Figure 3.
Effectively Removing a Failed or
Failing Capacitor Unit
In removing a failed or failing capacitor unit from service,
it is desired that this occur without causing any further
damage or disruption to the system. It is important that
the clearing fuse and the capacitor unit be able to withstand the available 60 Hz current and the high frequency
energy discharge from the parallel capacitors. In addition, the fuse must clear fast enough to limit the voltage
on the remaining healthy capacitors to acceptable levels
and to coordinate with an unbalance detection scheme.
2
The maximum clearing TCC curve for the fuse link must
coordinate with the tank rupture curve for the capacitor
(see Figure 4). This coordination is necessary to ensure
that the fuse will clear the circuit prior to tank rupture
occurring. The fuse maximum clear TCC must fall to the
left of the tank rupture TCC curve at and below the level
of available fault current. In the case of high fault
currents, the tank rupture curve should be compensated
for asymmetry.
The fuses and the capacitor must be capable of handling
the available parallel stored energy. When a capacitor
failure occurs, all of the stored energy of the parallel
connected capacitors can discharge through the failed
capacitor and its fuse. The total calculated parallel stored
energy should not exceed the energy capability or joule
rating of the capacitor unit and fuse. Exceeding their
ratings would result in probable fuse failure and rupture
of the capacitor unit.
When a fuse operates in a capacitor bank, an increase in
the fundamental frequency voltage occurs on the
remaining units in that series group. An unbalance detection scheme is employed to monitor such conditions and
to take action as required. The settings of this protection
scheme should be coordinated with fuse TCCs so that
the fuses will be allowed to clear a failed capacitor unit
before the unbalance detection scheme trips the capacitor bank. If the bank is tripped before the fuse operates
there will be no visible indication of the cause of the bank
tripping.
KILOVAR BRIEFS
Improper Coordination
Proper Coordination
1000
1000
Tank
Rupture
Tank
Rupture
100
(Time) Seconds
100
(Time) Seconds
9
10
1
10
1
Fuse
Fuse
0.1
0.1
0.0
0.0
5
50
500
5000
(Current) Amps
5
50000
50
500
5000
(Current) Amps
50000
Figure 4. Tank Rupture Coordination.
Summary
Group and individual fusing will be dealt with in greater
detail in future Kilovar issues, at which time recommendations will be included.
A summary of the key criteria in choosing the appropriate
fusing for a shunt capacitor application is given in the
following table. In comparing the need for slow clearing
and fast clearing fuses, it sometimes is not reasonably
possible to meet all of these criteria. In those cases, trade
offs must be made and some risks taken as to those
conditions when the fuses and capacitors may not
operate in a desirable manner.
TABLE 1
Summary of Shunt Capacitor Fusing Criteria
Key Criteria
Group
Individual
Protection
Protection
Withstanding Steady State and Transient Currents:
Continuous Current
External Transient Currents
— Lightning
— Switching
Outrush Current
Effectively Removing Failed or Failing Capacitor Unit:
Fault Current
Tank Rupture Curve Coordination
Voltage on Good Capacitors
Energy Discharge Into Failed Unit
Coordinate with Unbalance Detection Scheme
X
X
X
X
X
X
X
X
X
X
X
X
Fuse Characteristic
Desired
Slow
Fast
Clearing
Clearing
X
X
X
X
X
*
*
*
X
X
*
X
*This criteria helps to determine whether expulsion or current limiting fuses are required.
3
P.O. Box 1640
Waukesha, WI 53187
http://www.cooperpower.com
©2000 Cooper Industries, Inc.
Printed on Recycled Paper