Ingeniero

Capacitors

I ower Systems
COOPER P
September 2000

General Capacitor Fusing Criteria

- Greenwood SC

KlLOVAR 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 ofprotection 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
thistype 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
TransientCurrents
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
Printed In USA

Supersedes 3/87

Figure 2. I ndividualFusing.

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 selectirrg anappropriately
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
subjectto high frequency transients due to lightning
surges. Low current rated links are especially susceptible
to these surges.

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 nosignificant 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.

A

Figure 3. Energy Discharge into Failed Unit and
Energy Outrush from Remaining Good Capacitors.

In an individually fused application, there is one additionaltransient 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...