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Explain how Hormonics will effect the Electrical System?
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Answer Posted / mgalalkhedr
A pure sinusoidal voltage is a conceptual quantity produced
by an ideal AC generator built with finely distributed
stator and field windings that operate in a uniform
magnetic field. Since neither the winding distribution nor
the magnetic field are uniform in a working AC machine,
voltage waveform distortions are created, and the voltage-
time relationship deviates from the pure sine function. The
distortion at the point of generation is very small (about
1% to 2%), but nonetheless it exists. Because this is a
deviation from a pure sine wave, the deviation is in the
form of a periodic function, and by definition, the voltage
distortion contains harmonics.
When a sinusoidal voltage is applied to a certain type of
load, the current drawn by the load is proportional to the
voltage and impedance and follows the envelope of the
voltage waveform. These loads are referred to as
linearloads (loads where the voltage and current follow one
another without any distortion to their pure sine waves).
Examples of linear loads are resistive heaters,
incandescent lamps, and constant speed induction and
synchronous motors.
In contrast, some loads cause the current to vary
disproportionately with the voltage during each half cycle.
These loads are classified as nonlinear loads, and the
current and voltage have waveforms that are nonsinusoidal,
containing distortions, whereby the 60-Hz waveform has
numerous additional waveforms superimposed upon it,
creating multiple frequencies within the normal 60-Hz sine
wave. The multiple frequencies are harmonics of the
fundamental frequency.
Normally, current distortions produce voltage distortions.
However, when there is a stiff sinusoidal voltage source
(when there is a low impedance path from the power source,
which has sufficient capacity so that loads placed upon it
will not effect the voltage), one need not be concerned
about current distortions producing voltage distortions.
Examples of nonlinear loads are battery chargers,
electronic ballasts, variable frequency drives, and
switching mode power supplies. As nonlinear currents flow
through a facility's electrical system and the distribution-
transmission lines, additional voltage distortions are
produced due to the impedance associated with the
electrical network. Thus, as electrical power is generated,
distributed, and utilized, voltage and current waveform
distortions are produced.
Power systems designed to function at the fundamental
frequency, which is 60-Hz in the United States, are prone
to unsatisfactory operation and, at times, failure when
subjected to voltages and currents that contain substantial
harmonic frequency elements. Very often, the operation of
electrical equipment may seem normal, but under a certain
combination of conditions, the impact of harmonics is
enhanced, with damaging results.
Motors
There is an increasing use of variable frequency drives
(VFDs) that power electric motors. The voltages and
currents emanating from a VFD that go to a motor are rich
in harmonic frequency components. Voltage supplied to a
motor sets up magnetic fields in the core, which create
iron losses in the magnetic frame of the motor. Hysteresis
and eddy current losses are part of iron losses that are
produced in the core due to the alternating magnetic field.
Hysteresis losses are proportional to frequency, and eddy
current losses vary as the square of the frequency.
Therefore, higher frequency voltage components produce
additional losses in the core of AC motors, which in turn,
increase the operating temperature of the core and the
windings surrounding in the core. Application of non-
sinusoidal voltages to motors results in harmonic current
circulation in the windings of motors. The net rms current
is [I.sub.rms] = [square root of [([I.sub.1]).sup.2] +
[([I.sub.2]).sup.2] + [([I.sub.3]).sup.2] +] ..., where the
subscripts 1, 2, 3, etc. represent the different harmonic
currents. The [I.sub.2]R losses in the motor windings vary
as the square of the rms current. Due to skin effect,
actual losses would be slightly higher than calculated
values. Stray motor losses, which include winding eddy
current losses, high frequency rotor and stator surface
losses, and tooth pulsation losses, also increase due to
harmonic voltages and currents.
The phenomenon of torsional oscillation of the motor shaft
due to harmonics is not clearly understood, and this
condition is often disregarded by plant personnel. Torque
in AC motors is produced by the interaction between the air
gap magnetic field and the rotor-induced currents. When a
motor is supplied non-sinusoidal voltages and currents, the
air gap magnetic fields and the rotor currents contain
harmonic frequency components.
The harmonics are grouped into positive (+), negative (-)
and zero (0) sequence components. Positive sequence
harmonics (harmonic numbers 1,4,7,10,13, etc.) produce
magnetic fields and currents rotating in the same direction
as the fundamental frequency harmonic. Negative sequence
harmonics (harmonic numbers 2,5,8,11,14, etc.) develop
magnetic fields and currents that rotate in a direction
opposite to the positive frequency set. Zero sequence
harmonics (harmonic numbers 3,9,15,21, etc.) do not develop
usable torque, but produce additional losses in the
machine. The interaction between the positive and negative
sequence magnetic fields and currents produces torsional
oscillations of the motor shaft. These oscillations result
in shaft vibrations. If the frequency of oscillations
coincides with the natural mechanical frequency of the
shaft, the vibrations are amplified and severe damage to
the motor shaft may occur. It is important that for large
VFD motor installations, harmonic analyses be performed to
determine the levels of harmonic distortions and assess
their impact on the motor.
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