HARMONICS & EFFECTS OF
HARMONICS IN ELECTRICAL
SYSTEM
Harmonics
Defined as deviations from the fundamental frequency sine wave,
expressed as additional sine waves of frequencies that are multiple of
generated frequency. They are expressed as In a 50 Hz electrical system,
150Hz is the 3rd Harmonic, 250 Hz is the 5th harmonic etc.
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Harmonics Content
A measure of the presence of harmonics in the wave form
expressed as a percentage of the fundamental frequency. The
total harmonic content is expressed as the square root of the
sum of each of the harmonics amplitudes, expressed as
percentage of the fundamental
THD (I) =
Ih/If x 100
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Classification of Electrical Load
According to Load Nature there are two types of electrical Load:1 Linear Load Example:- Power Factor Improvement Capacitors , incandescent
lamps, Heaters etc.
2 Non Linear Load Example:- VFD, UPS, Electronic Ballast, Computers, Rectifiers, SMPS
etc.
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Linear Load & its Characteristics
A load where the wave shape of the steady-state current will follow the
wave shape of the applied voltage.
•Sinusoidal in nature
• Fixed Impedance
•Uniform frequency(V & I)
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Linear Load Characteristics
A “linear” load connected to an electric power system is defined as a load which
draws current from the supply which is proportional to the applied voltage (for
example, resistive, incandescent lamps etc). An example of a voltage and current
waveforms of a linear load is shown in below Figure:
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3 Phase current wave Linear Load
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Nonlinear Load & its Characteristics
Nonlinear Electrical Load is a load where the wave shape of the steadystate current does not follow the wave shape of the applied voltage.
•Non sinusoidal wave in load
•Non-linear loads create harmonic currents in addition to the original (fundamental
frequency) AC current causing distortion of the current waveform leads to distortion
of the voltage waveform. Under these conditions, the voltage waveform is no longer
proportional to the current.
•Non-linear loads’ impedance changes with the applied voltage.
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Nonlinear Load Characteristics
A load is considered “non-linear” if its impedance changes with the applied
voltage. Due to this changing impedance, the current drawn by the non-linear
load is also non-linear i.e., non-sinusoidal in nature, even when it is connected to
a sinusoidal voltage source (for example computers, variable frequency drives,
discharge lighting etc.). These non-sinusoidal currents contain harmonic currents
that interact with the impedance of the power distribution system to create
voltage distortion that can affect both the distribution system equipment and
the loads connected to it.
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3 Phase current wave Nonlinear
Load
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Current harmonics
Current harmonics are caused by non-linear loads. When a non-linear load, such as
a rectifier, is connected to the system, it draws a current that is not necessarily
sinusoidal. The current waveform can become quite complex, depending on the type
of load and its interaction with other components of the system. Regardless of how
complex the current waveform becomes, as described through Fourier series analysis,
it is possible to decompose it into a series of simple sinusoids, which start at the
power system fundamental frequency and occur at integer multiples of the
fundamental frequency.
Further examples of non-linear loads include common office equipment such as
computers and printers, Fluorescent lighting, battery chargers and also variable-speed
drives.
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Voltage harmonics
Voltage harmonics are mostly caused by current harmonics.
The voltage provided by the voltage source will be distorted
by current harmonics due to source impedance. If the source
impedance of the voltage source is small, current harmonics
will cause only small voltage harmonics.
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Third-order harmonics
In power systems, Harmonics are multiples of the fundamental
wavelength. Thus, the third order harmonic is the third multiple of the
fundamental wavelength. This type of harmonics is generated in nonlinear loads. Examples of nonlinear loads include transistors, electrical
motors, and the non-ideal transformer. Nonlinear loads create
disturbances in the fundamental harmonic, which produce all types of
harmonics. However, in this section we focus on the 3rd order harmonic
due to its certain special characteristics in the context of powers
systems
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Harmonics fundamentals
Harmonics provides a mathematical analysis of distortions to a current or voltage
waveform. Based on Fourier series, harmonics can describe any periodic wave as
summation of simple sinusoidal waves which are integer multiples of the fundamental
frequency.
Harmonics are steady-state distortions to current and voltage waves and repeat every
cycle. They are different from transient distortions to power systems such as spikes,
dips and impulses.
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Harmonics fundamentals
An example of a current wave affected by harmonic distortion on a 50Hz electrical
distribution system. The distorted signal is the sum of a number of superimposed
harmonics:
The value of the fundamental frequency (or first order harmonic) is 50 Hz,
The 3rd order harmonic has a frequency of 150 Hz,
The 5thorder harmonic has a frequency of 250 Hz, etc.
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Total harmonic distortion
Total harmonic distortion, or THD is a common measurement
of the level of harmonic distortion present in power systems.
THD is defined as the ratio of total harmonics to the value at
fundamental frequency.
where Vn is the RMS voltage of nth harmonic and n = 1
is the fundamental frequency.
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Effects
Problems caused by harmonic currents:
•overloading of neutrals
•overheating of transformers
•nuisance tripping of circuit breakers
•over-stressing of power factor correction capacitors
•skin effect
Problems caused by harmonic voltages:
•voltage distortion
•induction motors
•zero-crossing noise
•Problems caused when harmonic currents reach the supply
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Neutral Overloading
• In a three-phase system the voltage waveform from each phase to the
neutral so that, when each phase is equally loaded, the star point is
displaced by 120 combined current in the neutral is zero.
• When the loads are not balanced only the net out of balance current flows
in the neutral.
• The effective third harmonic neutral current is shown at the bottom. In
this case, 70% third harmonic current in each phase results in 210%
current in the neutral
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Effects on transformer
The effect of harmonic currents at harmonic frequencies causes increase in core
losses due to increased iron losses (i.e., eddy currents and hysteresis) in
transformers. In addition, increased copper losses and stray flux losses result in
additional heating, and winding insulation stresses, especially if high levels of
dv/dt
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Effects on Motor
Harmonics distortion raises the losses in AC induction motors in a similar
way as in transformers and cause increased heating, due to additional
copper losses and iron losses (eddy current and hysteresis losses) in the
stator winding,
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Effects in Cable
Cable losses, dissipated as heat, are substantially increased when carrying harmonic
currents due to elevated I2R losses, the cable resistance, R, determined by its DC
value plus skin and proximity effect. The resistance of a conductor is dependent on the
frequency of the current being carried. Skin effect is a phenomenon whereby current
tends to flow near the surface of a conductor where the impedance is least. An
analogous phenomenon,
proximity effect, is due to the mutual inductance of conductors arranged closely parallel
to one another. Both of these effects are dependent upon conductor size, frequency,
resistivity and the permeability of the conductor material. At fundamental frequencies,
the skin effect and proximity effects are usually negligible, at least for smaller
conductors. The associated losses due to changes in resistance, however, can
increase significantly with frequency, adding to the overall I2R losses.
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Circuit Breakers and Fuses
Premature trip due to heat
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Lighting
One noticeable effect on lighting is the phenomenon of “flicker” (i.e., repeated
fluctuations in light intensity). Lighting is highly sensitive to rms voltage changes;
even a slight deviation (of the order of 0.25%) is perceptible to the human eye in
some types of lamps.
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REMEDY
1.Over sizing Neutral Conductors
2. Using Separate Neutral Conductors
3. Passive harmonics filtering
5. Active harmonics Filtering
6. Hybrid Filtering
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Passive Harmonic Filtering
Passive filters are series capacitor and reactor resonant
circuits ‘tuned’ to present a high impedance path to the
fundamental frequency and low impedance path to higher
specific frequencies (i.e. 5th - 250Hz, 7th - 350Hz).
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Active Harmonic Filtering
Active harmonic filtering (AHF) is the process by which harmonic current
produced by the load is continuously monitored and an adaptive waveform
is then generated which corresponds to the exact shape of the non linear
portion of the load current.
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Hybrid Harmonic Filtering
Hybrid harmonic filtering is the combination of passive and
active harmonic filtering. Hybrid harmonic filtering combines the
two solutions in situations where the use of passive harmonic
filters can be used reliably for static loads of an electrical
installation and a smaller active filter can be used to mitigate
harmonics generated by the other variable loads. This solution
can be both cost and application effective.
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THANKS
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