[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 2.9.23 34-kV, 25-MVAR (per phase) thyristor-controlled reactors.
FIGURE 2.9.22A TCR current and voltage waveforms.
FIGURE 2.9.22B TCR harmonic-current spectra as a percentage of fundamental current.
power and disturbance mitigation on the power system may require the installation of SVCs. A typical
example of an industrial load that can cause annoyance to consumers, usually in the form of flicker, is
the extreme load fluctuations of electrical arc furnaces in steel works. A typical installation at a steel mill
is shown in Figure 2.9.23. The thyristor-controlled reactors are rated at 34 kV, 710 A, and 25 MVAr per
phase.
2.9.2.8 Filter Reactors
The increasing presence of nonlinear loads and the widespread use of power electronic-switching devices
in industrial power systems is causing an increase of harmonics in the power system. Major sources of
harmonics include industrial arcing loads (arc furnaces, welding devices), power converters for variablespeed
motor drives, distributed arc lighting for roads, fluorescent lighting, residential sources such as TV
sets and home computers, etc.
Power electronic-switching devices are also applied in modern power-transmission systems and include
HVDC converters as well as FACTS (flexible ac transmission system) devices such as SVCs. Harmonics
can have detrimental effects on equipment such as transformers, motors, switchgear, capacitor banks,
fuses, and protective relays. Transformers, motors, and switchgear can experience increased losses and
excessive heating. Capacitors can fail prematurely from increased heating and higher dielectric stress. If
distribution feeders and telephone lines have the same “right of way,” harmonics can also cause telephone
interference problems.
In order to minimize the propagation of harmonics into the connected power distribution or transmission
system, shunt filters are often applied close to the origin of the harmonics. Such shunt filters, in
their simplest embodiment, consist of a series inductance (filter reactor) and capacitance (filter capacitor).
Figure 2.9.24 shows a typical filter-reactor connection. If more than one harmonic is to be filtered, several
sets of filters of different rating are applied to the same bus. More-complex filters are also used to filter
multiple harmonics. More background information can be found in the IEEE paper, “Selecting Ratings
for Capacitors and Reactors in Applications Involving Multiple Single Tuned Filters.” [5]
2.9.2.9 Reactors for HVDC Application
In an HVDC system, reactors are used for various functions, as shown, in principle, in Figure 2.9.25.
The HVDC-smoothing reactors are connected in series with an HVDC transmission line or inserted in
the intermediate dc circuit of a back-to-back link to reduce the harmonics on the dc side, to reduce the
current rise caused by faults in the dc system, and to improve the dynamic stability of the HVDC
transmission system.
Filter reactors are installed for harmonic filtering on the ac and on the dc side of the converters. The
ac filters serve two purposes simultaneously: the supply of reactive power and the reduction of harmonic
currents. The ac filter reactors are utilized in three types of filter configurations employing combinations
of resistors and capacitors, namely single-tuned filters, double-tuned filters, and high-pass filters. A singletuned
filter is normally designed to filter the low-order harmonics on the ac side of the converter. A
double-tuned filter is designed to filter multiple discrete frequencies using a single combined filter circuit.
A high-pass filter is essentially a single-tuned damped filter. Damping flattens and extends the filter
response to more effectively cover high-order harmonics. The dc filter reactors are installed in shunt with
the dc line, on the line side of the smoothing reactors. The function of these dc filter banks is to further
reduce the harmonic currents on the dc line (see Figure 2.9.24 and Figure 2.9.25).
PLC (power-line carrier) and RI (radio interference) filter reactors are employed on the ac or dc side
of the HVDC converter to reduce high-frequency noise propagation in the lines.
2.9.2.10 Series Reactors for Electric-Arc-Furnace Application
Series reactors can be installed in the medium-voltage feeder (high-voltage side of the furnace transformer)
of an ac electric-arc furnace (EAF) to improve efficiency, reduce furnace electrode consumption,
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC