Page 778 - Industrial Power Engineering and Applications Handbook

Power capacitors: behaviour, switching and improvement of power factor 231735
4 Use of filter circuits in the power lines at suitable to block its entry into the system through its neutral
locations, to drain the excessive harmonic quantities from another system. For other frequencies, particularly
of the system into the filter circuits. at power frequency, it will offer a very low impedance
and permit them to flow into the system. The filter circuit
may now be designed for a notch frequency of
Filter circuits
A filter circuit is a combination of capacitor and series
1
reactance, tuned to a particulrv harmonic frequency (series fh = 2n. JLC (23.6)
resonance), to offer it the least impedance at that frequency
and hence, filter it out. Say, for the fifth harmonic, Xc5 =
XLS. At this frequency, the filter circuit will offer a very high
impedance and block the entry of that harmonic to flow
Since,fh, = 5 .f into the circuit through its neutral. For other frequencies,
it will provide a very low impedance and permit them to
.. =27r.Sf-L flow into the circuit. Thus for any value offh the product
2n-5f.C of LC can be determined, and hence the blocking
x, circuit can be designed. To achieve a narrow bandwidth
or = 5X,- of attenuation, at near ,fh. the blocking circuit may be
S designed separately for each major harmonic frequency
Cf,). To ensure a high Xch, it is advisable to keep the
01' X, = 5' ' X, value of L low and hence limit the inflow of charging
currents.
Generalizing, X, = N; . X, for the N,,th ordinal number Application of such circuits is also provided in the
of the harmonic disorder. form of a line trap, which is a blocking circuit and is
A capacitor is susceptible to variations in the system used at each end of the line to which the carrier relaying
voltage, frequency and operating temperature. Failure of is connected as discussed later. A bypass or filter circuit
capacitor elements may also cause a variation in capacitive is also provided to drain the power frequency component
values in the three phases of the filter circuit. Under of the current when present in the communication network.
these conditions, it is possible that a filter circuit may One end is connected to the main line through a coupling
fall out of tune during normal service and cause excessive capacitor or CVT and the other to the ground through
currents in one or more phases due to sub-synchronous the PLCC (Power Line Carrier Communication), as
or ferro-resonance effects with the impedance of the main illustrated in Figure 23.9(b). Leading manufacturers
system. If we consider the reactance of the detuned circuit combine the coupling equipment and PLCC coupling
as X, (considering it to be capacitive after detuning, as device into one unit for ease of application and installation.
X, will rise when C of the capacitor units decreases),
which may fall in phase opposition to the system reactance
X, (considering it to be inductive), as shown in Figure
23.15(b) then it will lead to such a situation and cause Use of tertiav (auxiliary) winding
excessive harmonic currents through the filter circuit, When a power transformer is y/)' or y/y connected
due to its low effective impedance, 2, since and is feeding a system on which harmonics are not
desirable, it is generally provided with a tertiary winding.
Z = R,, + XL -Xc(R,, is the resistance of the main system) A tertiary winding, which is also called an auxiliary
winding, is an additional winding connected in A. and
= very low sandwiched between the main primary and the secondary
windings of the transformer. It thus has the same magnetic
It is therefore recommended that a small resistance of circuit. This winding provides a closed path for the third
a low I?. R loss be introduced into the filter circuits as harmonic quantities to circulate and prevents them from
shown in Figure 23.15(a) to limit such an excessive flow appearing on the load side. It may also be used for the
of currents through them. Knowledge of the system purpose of metering, indication and protection of ground
parameters (resistance and reactance) is also essential to fault currents.
design an appropriate filter circuit to avoid a possible Other than the system harmonics, electrical interferences
resonance in the first instance. If this occurs the resistance are also caused by line disturbances, which may be caused
thus introduced will limit the excessive flow of current. by lightning, switching, sparking or a fault. As discussed
5 Another remedial measure is the use of blocking in Chapter 17, line disturbances occur at very high
circuits by providing a high-impedance path in the ground frequencies but some may coincide with the audio
circuit when a phase-to-ground circuit is used for the frequency of telephone lines. and cause disturbance in
com-munication network, to block the entry of the third the audio quality of the telephone system. AI1 these
harmonic quantities into the system from other systems. disturbances are referred to as inductive interferences.
We provide more details on inductive interferences in
Blocking circuits the box below, to make the subject of electrical interference
These are parallel resonant L-C circuits, and are tuned to more informative. We also provide a passing reference
offer a high impedance to a particular harmonic frequency to communication systems being adopted worldwide.