Page 777 - Industrial Power Engineering and Applications Handbook

23/734 Industrial Power Engineering and Applications Handbook
The rating of a capacitor unit will thus vary in a square C Effects of harmonics on telephone lines
proportion of the effective harmonic voltage and in a
direct proportion to the harmonic frequency. This rise in In earlier years, to reach a remote area, where separate
kVAr, however, will not contribute to improvement of telephone lines had not been laid it was normal practice
the system p.f. but only of the overloading of the capacitors to run them through the same poles as the HT power
themselves. distribution lines (generally 11-33 kV). This was
particularly true of internal communications of the
electricity companies for ease of operation and to save
costs and time. This communication was known as the
When determining the actual load current of a capacitor unit in magneto-telephone system. But the proximity of telephone
operation, a factor of 1.15 is additionally considered to account Iines to power lines adversely affected the performance
for the allowable tolerance in the capacitance value of the of the telephone lines due to generation of overvoltages
capacitor unit (Section 26.3.1( I)): (Chapter 20) and electrical interferences (conductive and
:. Effective kVArh = 1.3 x 1. IS inductive interferences, discussed later) on the telephone
lines by the power lines. Some of these interferences,
- 1.5 times the rated kVAi
particularly system harmonics, had the same frequency
and for which all switching and protective devices must be as the audio frequency of the telephone lines and affected
selected. It may, however, sometimes be desirable to further their audio quality.
enhance the overloading capacity of the capacitor and so also The running of telephone lines through power lines is
the rating of the current-carrying components if the circuit long discontinued. They are now run on separate structures
conditions and type of loads connected on the system are prone within a city and nearby areas at audio frequency (=
to generate excessive harmonics. Examples are when they are 0.3-3.4 kHz), and maintain enough distance from HT
connected on a system on which we operating static drives and
arc furnaces. power distribution lines. They are therefore almost
It is desirable to contain the harmonic effects as far as practicable unaffected from such disturbances. Nevertheless,
to protect the capacitors as well as inductive loads connected interferences must be kept in mind when installing these
on the system and the communication network, if running in lines so that they are out of the inductive interference
the vicinity. zone of the power lines. The latest method in the field of
communications to avoid disturbances is to use
underground optical fibre cables, where possible, as
Influence of harmonics on the performance of discussed later. Optical fibre cables are totally immune
an inductive load to such disturbances.
In an inductive circuit the presence of harmonics is not
felt so much as the higher harmonics tend to enhance the Remedies for electrical interferences
harmonic reactance, which causes a dampening effect:
The following measures are recommended to mitigate
XL = 277 .f. L these problems:
For a harmonic order n and harmonic frequency n the
harmonic reactance will become Use of screened cables in the communication network
and grounding the screen effectively. Metallic-sheath
XLh = 277. n .f. i.e. n .f or armoured cables are not recommended.
L
Transposing the overhead communication lines, i.e.
At higher harmonic frequencies, X,, will rise pro- reversing the respective positions of thc two sides of
portionately and reduce the harmonic currents and the lines every I km or so, to avoid continuous
provide a dampening effect. The harmonic quantities parallelism (due to electrostatic and electromagnetic
present in the system thus do not have any significant inductions), as illustrated in Figure 23.8. See also
effect on the performance of an inductive load, which Section 28.8.4(3) on phase transposition.
can be a generator, transformer, motor, cables and It is common practice to leave the star-connected
overhead lines etc. In an LT system, such effects are capacitor banks ungrounded when used in the system
highly dampened due to high system impedance. No or use delta-connected banks to prevent the flow of
special efforts are therefore generally made to suppress third harmonic currents into the power system through
the harmonic effects in an LT system. the grounded neutral.
The magnetic core of an electromagnetic equipment
(generator, transformer, reactor and motor etc.) will,
however, generate additional no-load iron losses at
higher harmonic frequencies (equations (1.12) and A 0
(1.13)). The content of iron losses, being low, will rise
only marginalIy and can be ignored for all general
applications.
Electronic appliances that are highly susceptible to
such effects are, however, provided as standard practice
with harmonic filters in their incoming circuits. Figure 23.8 Transposition of overhead communication lines