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Abstract—Large capacitor banks at medium-voltage levels are
finding applications and greater acceptability in industrial distribution
systems. However, these can give rise to current and voltage
transients, stress the switching devices and insulation systems, and
can be detrimental to the sensitive loads, i.e., drive systems. The
paper discusses the methodology of the analysis and control of the
switching transients. The results of the Electromagnetic Transients
Program (EMTP) simulation are presented where applicable
INTRODUCTION
HISTORICALLY, capacitor-switching transients have
caused problems that have been studied in existing literature.
It is not the intention of this paper to go into the
details of the historical perspective, yet a brief mention is
appropriate. During the period covered by the late 1970s to the
1980s, switching of transmission capacitor banks caused high
phase-to-phase voltages on transformers and the magnification
of transients at consumer-end distribution capacitors. Problems
with switchgear restrikes caused even higher transients
[1]–[6]. Problems were common in plants with capacitors and
dc drive systems. The advent of pulse-width-modulated (PWM)
inverters created a whole new concern for capacitor switching.
Wagner et al. [7] describes an investigation of the failure of
a drive system due to frequent cycling of a capacitor bank in
the utility system, which caused the dc link voltage of a drive
system to trip.
Transient studies for an industrial distribution system are
not normally performed, and when needed, capacitor switching
accounts for most of these investigations consequent to the
failure of an equipment or the shutdown of a process. Even
in these study cases, analysis can be indirect, without rigorous
calculations or computer modeling.
The switching transients originate from:
1) switching of a shunt-capacitor bank, which may include
switching on to a fault;
2) back-to-back switching, i.e., switching of a second capacitor
bank on the same bus in the presence of an alreadyenergized
bank;
3) tripping or deenergizing a bank under normal operation
and under fault conditions;
4) possible secondary resonance when the capacitors are
applied at multivoltage level (i.e., at the 13.8-kV level as
well as at the 480-V level) in a distribution system; and
5) restrikes and prestrikes in the switching devices.
A transient, from its point of origin, will be propagated in
either direction in the distribution system and will be transferred
through the transformer inductive/capacitive couplings
to other voltage levels. Transformer part-winding resonance can
occur [8].
The application of shunt capacitors can lead to the following
additional side effects:
1) bring about severe harmonic distortion, and resonance
with load-generated harmonics;
2) increase the transient inrush current of power transformers
in the system, create overvoltages, and prolong
its decay rate [8];
3) capacitors themselves can be stressed due to switching
transients;
4) increase the duty on switching devices, which may
be stressed beyond the specified ratings in American
National Standards Institute (ANSI)/IEEE standards
[9], [10];
5) discharge into an external fault, and produce damaging
overvoltages across current-transformer (CT) secondary
terminals; and
6) impact sensitive loads, i.e., drive systems, and bring about
a shutdown.
The scope of the paper is limited to the analysis and control
of the switching transients.
II. ENERGIZING TRANSIENT
On connecting to a power source, a capacitor is a sudden
short circuit, because the voltage across the capacitor cannot
change suddenly. The voltage of the bus to which the capacitor
is connected will dip severely. This voltage dip and the transient
step change are a function of the source impedance behind the
bus. The voltage will then recover through a high-frequency
oscillation. In the initial oscillation, the transient voltage can
approach two per unit of the bus voltage. The initial step change
and the subsequent oscillation are important. As these are
propagated in the distribution system, these can couple across
Surge Arresters
Gap-type surge arresters in series with nonlinear resistors can
be exposed to high stresses when protecting capacitor banks. If
a transient triggers the arrester, the capacitors will discharge
totally and the energy is dissipated in the arresters. Conversely,
gapless metal-oxide arresters will not discharge the capacitors
below the system-rated voltage, as there is a smooth transition
from the conducting to the insulating condition. The parameters
of consideration are
1) system-rated voltage and maximum operating voltage;
2) system grounding;
3) amplitude and duration of possible overvoltages (phaseto-phase
overvoltages should not be ignored);
4) energy dissipation through the arrester (a digital computer
model using EMTP can be used for analysis).
VIII. CONCLUSION
A rigorous computer model of the distribution system, simulated
on electromagnetic transients program (EMTP) without
lumping of the system elements, will give reliable results of the
surge transference, propagation, decay, magnification, and secondary
resonance throughout the distribution system. However,
such an analysis will be fairly involved. In most distribution
systems, the transients can be estimated and controlled by
following the procedures outlined in the paper, i.e., preinsertion
resistor, series inrush-current-limiting reactors, and limiting the
application of capacitors to one voltage level.