Page 150 - Industrial Power Engineering and Applications Handbook

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capacitive switching (Section 17.7). They also produce A high value of L will limit the rate of rise of fault
harmonics. However, a power diode converter unit having current for the same voltage and save the circuit
no switching sequence is devoid of such a phenomenon. components.
A thyristor (SCR) switched phase-controlled converter 2 A low dildt will also help to smooth the d.c. link
unit produces large quantities of harmonics on the supply current waveform.
(a.c.) side and also in the d.c. link and also voltage and 3 It has a disadvantage in that it may have a sluggish
current surges on the incoming supply side. An IGBT, response to the control circuit demands due to its
on the other hand, as used in an inverter circuit, causes high time constant (z = L/R)
only moderate harmonics during switching, but does
produce voltage surges on the load side. All these factors Current harmonics on the incoming
are not desirable and must he suppressed or tamed at the ax. supply side
point of occurrence to save the connected equipment
and the devices. Below we discuss these phenomena, The presence of harmonic quantities in the electronic
particularly for thyristor circuits and their possible circuit distorts the sinusoidal incoming supply system to
remedies. a non-sinusoidal one the magnitude of which will depend
upon the configuration of the converter circuit and the
6.13.1 Suppressing the harmonics (in phase- variation in the connected load. The line side converter
controlled rectifier units) unit draws a somewhat squarish waveform current from
the mains, as analysed in Figure 23.7. It may adversely
Phase-controlled rectifier circuits generate excessive odd influence the power equipment operating on the incoming
harmonics such as 5th, 7th, llth, 13th etc., depending supply side of the system, which may be a motor, a
upon the pulse number, n, of the circuit configuration transformer or a generator, due to higher no-load losses
adopted, as discussed in Section 23.6(h). These harmonics as a consequence of high harmonic frequencies (equations
add to the inductive loading of the circuit since XL 0~ fh (1.12) and (1.13)). It may also cause overloading of the
and diminish the p.f. of the system, although they hardly capacitor banks connected on the incoming side and
influence an induction motor, (Section 23.5.2(B)). The subject all this equipment to higher voltage stresses. The
3rd harmonic3 are totally absent because mostly six pulse higher inductive loading also diminishes the p.f. of the
thyristor converters are employed, which eliminate all system. To contain the influence of these features, the
the 3rd harmonics from the voltage and the current output use of filter circuits to suppress the harmonics and power
waveforms. Thyristors in other configurations such as capacitors, to improve the system p.f. on the incoming
12, 18 and 24 pulses are also possible, which can eliminate side are mandatory to maintain a healthy supply system,
most of the harmonics from the output waveforms. The particularly when it is feeding a few phase-controlled
higher the pulse number, the closer it approaches the converter units, handling large machines and generating
mean and effective (r.m.s.) values of the rectified voltage high harmonics. Figure 6.34 shows the use of an inductor
and the voltage approaches a near peak value (Section in the incoming circuit to suppress the harmonics and
23.6(b)). (See also Figure 23.10.) However, higher pulse limit current overshoots. Power capacitors are not shown,
thyristor converters become very expensive and are which can be provided for the whole system at a centralized
employed only for very large power applications. location. The design of filter circuits and the size of
power capacitors, to adopt a more appropriate corrective
Current harmonics in a d.c. link method, will require a meticulous network analysis to
determine the actual numbers and magnitudes of such
To limit the current harmonics generated in the d.c. link, harmonics present in the system. The subject is dealt
series smoothing reactors are inserted on the d.c. side as with in more detail in Section 23.5.2. In Figure 6.24(a)
shown in Figure 6.24(a). They are large iron core un- we have shown a few more common types of thyristor
saturable reactors (L). (For details on reactors see Chapter configurations, their voltage and current wave-forms and
27.) They provide high impedance paths to the different the application of reactors to suppress the harmonics and
harmonic quantities and suppress the more prominent smooth ax. ripples.
of these at the source, and provide a near smooth d.c.
output voltage waveform. For large power applications,
requiring a near-constant d.c. output, more accurate L-C
circuits (even more than one) may be provided in the 6.14 Protection of semiconductor
d.c. link to suppress the more prominent of the harmonic devices and motors
quantities.
A large inductor in the d.c. link may also play the 6.14.1 Overvoltages and voltage surges caused
following roles: by disturbances in an LT system

I In the event of a fault in the d.c. link it will add to the Semiconductor devices are irreparable after a failure and
circuit impedance and limit the rate of rise of fault hence require extra precautions for their protection.
current, since under a transient condition Although a voltage transient generally is a phenomenon
of HT systems, as discussed in Chapter 17, moderate
di long-duration switching surges (voltage spikes), other
v= L-
dt than lightning and the transference of surges, are noted