Page 901 - Industrial Power Engineering and Applications Handbook

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the ground directly or through a current limiting reactor
Size of a three phase reactor Size of a single-phase reactor
its neutral current may be considered for a short-time
duration only, i.e. until the ground fault exists assuming
650 x 1 .O
VAr= &xl.Ox650 VAr = ~ that the ground fault protective scheme will isolate the
6 faulty circuit promptly.
= 1125 kVAr = 375 kVAr But when the neutral is grounded through an arc-
suppression coil (reactor) the current through the grounded
neutral may be of a limited amplitude, say, up to its
continuous rating (Section 20.5) and it may exist for
1 x 1000
and X,/phase = ~ longer.
fi x 650 These transformers are three-phase and may be
= 0.89 Wphase connected for zig-zag or staddelta connections (Section
20.9.1). The delta may also be made open type by inserting
27.4.3 Dampening reactors a resistor across it to help adjust the zero-sequence
impedance, if required.
These are meant to limit the inrush currents occurring
during a switching operation of a capacitor. They are 27.4.6 Arc suppression or Petersen coil (reactor)
connected in series with the capacitors and may be short-
time rated for the values of the inrush currents and These are meant to compensate the ground capacitive
continuously rated for normal line currents. They are current on a ground fault in the system, which may be
almost the same as the series reactors with fixed reactance. grounded naturally or artificially (Section 20.5). They
are connected between the neutral of the system and the
27.4.4 Neutral grounding reactors ground and are single-phase and may be short-time or
continuously rated, depending upon the system
These are meant to limit the ground fault current and are requirement. If it is being used as a ground fault neutralizer
used between the neutral of the system and the ground. it may have to be continuously rated. It may be of variable
They are single-phase and may be short-time rated, type to help tuning with the system ground capacitance.
otherwise they are the same as the current limiting reactors
(Figure 27.12). Their ratings can be calculated by
27.4.7 Tuning or filter reactors
These are meant to be used with a capacitor to tune a
filter circuit, with resonances in the audio frequency range
where for reducing and filtering the harmonics or communication
X, = reactance - R frequencies. They provide a near short-circuit for the
VI = rated voltage of the system required harmonics to filter them out of circuit. They
(5- 1 series or parallel of the capacitor circuit and may have a
may be single-phase or three-phase and connected in
rated voltage of the reactor
fixed or variable reactance, rated continuously with
Ig = current through the ground (rated current of saturated magnetic characteristics. They may incur heavy
the reactor) losses.
X, = zero sequence reactance (or impedance) - R
= 3.x, 27.4.8 Smoothing reactors
These are meant to provide high impedance to harmonic
27.4.5 Grounding transformer or neutral currents and block their entry or reduce their amplitudes
couplers and are therefore also known as blocking reactors. They
These are meant to provide a neutral to an ungrounded may have any of the magnetic characteristics shown in
system. Figure 27.2 and have a fixed reactance, rated continuously.
When the ground transformer neutral is connected to
Example 27.2
Consider a distribution system as illustrated in Figure 27.1 3(a)
B Y being fed by two power sources in tandem:
A transformer 5000 kVA, 33/11 kV, having a reactance of
7.15% and connected to a power grid.
A generator 2500 kVA, 11 kV, having a steady-state
reactance, xd of 25 %.

A three-phase fault somewhere in the bus system, without
reactive compensation and ignoring the line impedance, can
reach a level of
Figure 27.12 Neutral grounding reactor or a Petersen coil Fault MVA = base (equation(l3.6))
Ze,