ABB A43, User Manual

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Technical Description
A43/A44 50
2CMC48001M0201
User Manual Revision: C
5.3 Harmonics
General The presence of harmonics in voltages and currents may cause a number of un-
wanted problems. This chapter describes the origin of harmonics, how the nega-
tive effects of harmonics can be eliminated and how harmonics is measured.
The harmonics data can be read via communication or directly in the display with
the help of the buttons.
Generation of harmonics
Generators in the power system produce a nearly pure sinusoidal voltage with a
frequency near the stated system frequency, normally 50 or 60 Hz. Linear loads,
consisting of pure resistors, capacitors and inductors, draw a pure sinusoidal cur-
rent if the voltage over the load is pure sinusoidal.
A non-linear load, however, draws non-sinusoidal current resulting in a current
consisting of several frequencies. One example of a common non-linear load are
power supplies in electronic equipment which normally contains rectifier diodes
which rectifies the incoming voltage and charges a capacitor. The power supply
only draws current at the top of the sine wave when the rectified voltage exceeds
the voltage over the capacitor. Another example of a non linear load is a thyristor
controlled load, where the current normally is turned on at the voltage zero cross-
ing and turned off sometimes during the sine wave.
These currents are all non-sinusoidal and can be divided into a fundamental part,
which is the same as the mains frequency, and high frequency parts, that is har-
monics, which have frequency that are integral multiples of the mains frequency.
Harmonics in the current will in turn cause harmonics in the voltage since the
mains wires and the generator have an impedance causing a voltage drop that is
proportional to the current. It should also be pointed out that if there is
harmonics in the voltage, also a linear load will cause harmonics in the current,
that have the same magnitude as the voltage harmonics. However, the origin of
harmonics in the voltage are non-linear loads.