Page 789 - Industrial Power Engineering and Applications Handbook

Power capacitors: behaviour, switching and improvement of power factor 23/745

load, and hence further distort an already distorted source reactance, an inductor equivalent to 6% value of
voltage waveform. the capacitive reactance can be provided in series with
Harmonics will mean the shunt capacitors.
- Higher voltage and current than apparent If there are many large or small consumers that a
- Adding to line loading and losses, and distribution line is feeding, it is possible that the voltage
~ Reducing the actual load p.f. of the network may be distorted beyond acceptable limits.
Harmonic effects are reduced by the use of shunt In this case it is advisable to suppress these harmonics
capacitors installed to improve the p.f. of the system from the system before they damage the loads connected
or an individual load, as they provide a very low in the system. Preferable locations where the series
impedance path to harmonic quantities and absorb inductor or the filter-circuits can be installed are:
them. But using shunt capacitors alone is not sufficient
as at certain harmonic frequencies, the capacitive
reactance of these capacitors, along with leakage 1 At the supply end and/or
capacitance C,, of the line, if it is an HV or EHV line 2 At the receiving end of the consumer. using power at
(Table 24.l(b)) may tune up with the line inductive HT and generating high harmonics (e.g. rolling mills,
induction and arc furnaces, electrolysis plants,
reactance L and cause a series or parallel resonance
and develop high voltages. This phenomenon will be chemicals and paper mills). Electricity companies will
always prefer the consumers to suppress the harmonics
more apparent on an HV and EHV system than an
at their end. Even if this practice is adopted by some.
LT system. On LT systems, the line resistance itself small harmonics generated by many small consumers,
is enough to provide an in-built dampening effect to who may not take any preventive steps to contain the
a possible resonance (z = R,) at least). To avoid harmonics generated by their plant, may still exist in
resonance, filter-circuits are used. Filter-circuits may the system. As a result, the distribution network may
bc tuned for the lowest harmonic content or for each contain some harmonics which affect the performance
harmonic separately as discussed later, when the of other loads in the system. If the level of harmonics
system is required to be as near to sinusoidal waveform is unacceptably high (Tables 23.2 and 23.3) they must
as possible. The system may contain second, third be suppressed at the supply end.
and fifth and higher harmonics, but it may be sufficient
to tune it to just below the fifth harmonic, which will
be able to suppress most of the other harmonics also
to a great extent. as it will make the circuit inductive 23.9 Filter circuits: suppressing
for all harmonics. harmonics in a power network

In an HT system, either the star is not grounded or it The use of a reactor in series with the capacitors will
is a delta-connected system and hence the third harmonic reduce the harmonic effects in a power network, as well
is mostly absent, while the content of the second harmonic as their effect on other circuits in the vicinity, such as a
may be too small to be of any significance. For this telecommunication network (see also Section 23.1 I and
purpose, where harmonic analysis is not possible, or for Example 23.4). The choice of reactance should be such
a new installation where the content of harmonics is not that it will provide the required detuning by resonating
known, it is common practice to use a series reactor of below the required harmonic, to provide a least impedance
6% of the reactive value of the capacitors installed. This path for that harmonic and filter it out from the circuit.
will suppress most of the harmonics by making the circuit The basic idea of a filter circuit is to make it respond to
inductive, up to almost the fourth harmonic, as derived the current of one frequency and reject all other frequency
subsequently. Where, however, second harmonics are components. At power frequency, the circuit should act
significant, the circuit may be tuned for just below the as a capacitive load and improve the p.f. of the system.
second harmonic. To arrive at a more accurate choice of For the fifth harmonic, for instance, it should resonate
filters, it is better to conduct a harmonic analysis of the below 5 x 50 Hz for a 50 Hz system, say at around 200-
system through a harmonic analyser and ascertain the 220 Hz, to avoid excessive charging voltages which may
actual harmonic quantities and their magnitudes present lead to
in the system, and provide a correct series or parallel
filter-circuits for each harmonic.
As noted from general experience, except for specific 0 Overvoltage during light loads
large inductive loads such as of furnace or rectifiers, the 0 Overvoltage may saturate transformer cores and
fundamental content of the load current is high compared generate harmonics
to the individual harmonic contents. In all such cases, it Failure of capacitor units and inductive loads connected
is not necessary to provide a filter-circuit for each harmonic in the system.
unless the current is required to be as close to a sinusoidal
waveform as possible, to cater to certain critical loads or It should be ensured that under no condition of system
instruments and devices or protective schemes operating disturbance would the filter circuit become capacitive
in the system, where a small amount of harmonics may when it approaches near resonance. To achieve this, the
lead to malfunctioning of such loads and devices. filter circuits may be tuned to a little less than the defined
Otherwise only the p.f. needs be improved to the desired harmonic frequency. Doing so will make the Land hence
level. Also to eliminate a parallel resonance with the X, always higher than Xc, since