24-09-2013, 02:30 PM
Considerations for the Application of Series Capacitors to Radial Power Distribution Circuits
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Abstract
This paper summarizes many of the considerations
in the application of series capacitors on radial distribution cir-
cuits. The effects of series capacitors on receiving end voltages are
illustrated via phasor diagrams and power-voltage characteristics.
The unique voltage profile of the compensated distribution circuit
is presented. Discussed are the various types of distribution cir-
cuit problems that series capacitors typically used to solve. Ap-
plication guidelines are presented to help minimize oscillatory in-
teractions with transformers and motors. The various protection
schemes used in practical series capacitor banks are discussed. The
information required to specify distribution series capacitor banks
is outlined.
Introduction
SERIES compensation is a means of improving the per-
formance of transmission as well as distribution circuits.
Today, many series capacitors are being installed on radial
distribution circuits, especially those with industrial customers.
The application of series compensation makes the line or cir-
cuit appear electrically shorter because it reduces the total ef-
fective reactance. Some of the positive effects provided by se-
ries compensation are improved circuit voltage profile, reduced
voltage fluctuations, higher circuit capacity, and a reduced de-
mand for reactive power from the power system.
The objective of this paper is to provide a better under-
standing of series capacitors for radial distribution circuits.
Discussed are the basic theory, typical applications, the re-
sulting benefits, application precautions and typical equipment
schemes.
Voltage Profile on a Compensated Circuit
The voltage profile on a radial circuit; (i.e., the voltage magni-
tude versus distance from the source end of the circuit), depends
on the circuit parameters, the power factor of the loads and the
load magnitude, as noted above. This voltage profile will vary
from hour to hour as the steady-state load changes and may vary
from second to second as loads are switched and motors are
started. Usually, the peak load with maximum inductive current
is the one of greatest interest, since it represents the maximum
voltage change from the source to the end of the circuit and may
occur when the source voltage is minimum.
A typical voltage profile for a radial circuit without and with
a series capacitor is shown in Fig. 12. No-load condition and
maximum load condition are shown. Exciting currents of trans-
formers as well as currents due to the circuit capacitance are ne-
glected. Note that the voltage profile curve has a discontinuity
or jump at the location of the series capacitor. Downstream from
the series capacitor there is a large rise in the voltage. Also, note
the small rise in the voltage upstream from the series capacitor
because the increased voltage on the circuit downstream from
the series capacitor reduces the current drawn by the motor loads
located there.
Increase in Fault Current Due to Series Compensation
At first sight, it seems plausible to conclude that the series
capacitor in a radial circuit increases the fault current due to
the reduction of the reactance from the source to the location of
the fault. However, this is only partially true. The reason is that
the series capacitor always is provided with an overvoltage pro-
tection (bypass system), which will instantaneously bypass the
series capacitor in case of high current line faults downstream
from the series capacitor. For low current line faults, however,
which do not cause the bypass system of the series capacitor
to operate, an increased short-circuit capacity is experienced.
This can be valuable to ensure correct fuse operation in the low
voltage networks connected downstream from the series capac-
itor, i.e., in the networks connected to the low voltage side of
the load step-down transformers. The magnitude of the fault
currents for which the bypass will occur can be tailored for the
application.
APPLICATIONS
The most common application for series capacitors in radial
power distribution circuits is to reduce voltage drops due to
load current and improve the voltage profile along the circuit
thereby increasing the loadability of the circuit. The benefit is
the greatest on circuits with rapidly varying loads.
Since the voltage across the series capacitor and the radial
circuit reactance is proportional to the circuit current, the series
capacitor provides continuous and virtually instantaneous
voltage regulation in case of relatively rapid variations of
the load. Thus, the series capacitor gives a continuous and
instantaneous voltage control determined by the actual load
current. This action provides very effective voltage regulation
if the major portion of the voltage drop is associated with the
circuit reactance.
OPERATION OF THE SERIES CAPACITOR BANK
When the series capacitor bank is first energized, the bypass
switch is in the closed position and the bypass disconnect switch
is opened. Alternately, if there is no bypass disconnect switch,
then the series capacitor is energized with the power circuit by
its circuit breaker. In this case the bypass switch is in the closed
position. Then after a short delay, the bypass switch is opened
and the capacitor is inserted.
If there is a loss of voltage on the circuit, the bypass switch
is automatically closed and it opens after some time delay when
the voltage is restored. As a result, the bypass switch is always
in the closed position when the circuit is energized. This greatly
reduces the probability of exciting ferroresonance.
CONCLUSION
1) The application of series capacitors can reduce the voltage
variations associated with variations in load on radial dis-
tribution circuits. Series capacitors are most effective on
ratio and for load variations
circuits with a higher
involving a high reactive content.
2) Series capacitors must be applied with care. Application
with certain types of loads requires study. Overcompen-
sation should be avoided. If the series capacitor bank is
bypassed prior to the energizing or reclosing distribution
circuit, the possibility of oscillations with the downstream
loads and transformers is reduced.
3) Series capacitors have been applied on distribution cir-
cuits for over fifty years. Many of these applications have
been to reduce voltage variations associated with saw
mills, rolling mills, crushers, mines and ski lifts. Motor
starting is often one of the causes of the voltage variations.
4) The series capacitor bank typically includes the capaci-
tors, overvoltage protection equipment, a bypass switch
and a control system. Modern overvoltage protective
schemes now include metal oxide varistors and, in some
cases, triggered bypass gaps.