HARMONICS
Harmonics always originate as current harmonics and voltage harmonics are the results of current harmonics.
Current harmonics originate because of the presence of non-linear loads like variable speed drives, inverters, UPS,
television sets, PCs, semiconductors circuits, welding sets, arc furnaces in the system. They act as harmonic current
sources. The resulting current waveform can be quite complex depending on the type of load and its interaction with
other components of the system.
Symmetrical waves contain only odd harmonics but odd
and even both harmonics are produced due to
asymmetrical waves.
In odd harmonics both positive and negative parts of the
wave are same but in asymmetrical waves both positive
and negative parts are different. Asymmetrical wave is the
result of half wave rectifier.
HARMONICS PHASE SEQUENCE
In order to describe a physical three phase system, Power Engineers have adopted a technique of balanced
machineries which is based on Fortescue’s theorem.
An unstable set of 𝑛 phasors may be resolute into (𝑛 − 1) stable n-phase systems of diverse phase sequence on
one zero phase sequence system.
Locating Harmonic Sources
On radial utility distribution feeders and industrial plant power systems, the main tendency is for the harmonic
currents to flow from the harmonic-producing load to the power system source. This is illustrated in Fig.4.20. The
impedance of the power system is normally the lowest impedance seen by the harmonic currents. Thus, the bulk of
the current flows into the source.
This general tendency of harmonic current flows can be used to locate sources of harmonics. Using a power quality
monitor capable of reporting the harmonic content of the current, simply measure the harmonic currents in each
branch starting at the beginning of the circuit and trace the harmonics to the source.
Power factor correction capacitors can alter this flow pattern for at least one of the harmonics. For example, adding a
capacitor to the previous circuit as shown, may draw a large amount of harmonic current into that portion of the
circuit. In such a situation, following the path of the harmonic current will lead to a capacitor bank instead of the
actual harmonic source. Thus, it is generally necessary to temporarily disconnect all capacitors to reliably locate the
sources of harmonics.
It is usually straightforward to differentiate harmonic currents due to actual sources from harmonic currents that are
strictly due to resonance involving a capacitor bank. A resonance current typically has only one dominant harmonic
riding on top of the fundamental sine wave
Another method to locate harmonic sources is by correlating the time variations of the voltage distortion with specific
customer and load characteristics. Patterns from the harmonic distortion measurements can be compared to particular types
of loads, such as arc furnaces, mill drives, and mass transits which appear intermittently. Correlating the time from the
measurements and the actual operation time can identify the harmonic source.
In power systems, the response of the system is equally as important as the sources of harmonics. In fact, power
systems are quite tolerant of the currents injected by harmonics producing loads unless there is some adverse
interaction with the impedance of the system.
Identifying the sources is only half the job of harmonic analysis. The response of the power system at each harmonic
frequency determines the true impact of the nonlinear load on harmonic voltage distortion. There are three primary
variables affecting the system response characteristics, i.e., the system impedance, the presence of a capacitor bank,
and the amount of resistive loads in the system.
Voltage harmonics do not originate directly from non-linear loads. The current harmonics (distorted waveform) flow
through system impedance (source and line impedances) and cause harmonic voltage drop across the impedances. This
will distort the supply voltage waveform. Thus voltage harmonics are generated. Long cable runs, high impedance
transformers, etc. contribute to higher source impedance and hence, higher voltage harmonics.
Usually, grid impedances are very low and hence, the harmonic voltage distortions are also low there. However,
they may be unacceptably higher on the load side as they are subjected to full system impedance there. Hence, it
becomes important where the harmonics measurements are done.
However, in case of DG sets, the source impedance is large resulting in high voltage harmonics despite small
current harmonics. Thus, a clear distinction between current and voltage harmonics becomes important here
An industry, say industry A, that has large non-linear loads will generate huge current harmonics in its system. A
nearby industry, say industry B, connected to the same grid may not have non-linear loads, yet, it may be subjected to
high voltage harmonics. These voltage harmonics are the result of high current harmonics of industry A and
impedance of grid & transformer. Thus, industry B despite small current harmonics, has high voltage harmonics.
However, if industry B goes for power factor correction, then, due to the presence of capacitors, current harmonics
may also appear in the system, magnifying voltage harmonics further.
Current harmonics increase the rms current flowing in the circuit and thereby, increase the power losses. Current
harmonics affect the entire distribution all the way down to the loads. They may cause increased eddy current and
hysteresis losses in motor and transformers resulting in over-heating, overloading in neutral conductors, nuisance
tripping of circuit breakers, over-stressing of power factor correction capacitors, interference with communication etc.
They can even lead to over-heating and saturation of reactors.
Voltage harmonics affect the entire system irrespective of the type of load. They affect sensitive equipment throughout
the facility like those that work on zero-voltage crossing as they introduce voltage distortions.
The harmonic distortion is evaluated by the “Total Harmonic Distortion” (THD ). This is separated into THDv (or
THDu ) for voltage distortion and THDi for current distortion.
IEEE 519 Guidelines
The purpose of harmonic limits in a system is to limit the harmonic injection from individual customers to the grid so
that they do not cause unacceptable voltage distortion in the grid. IEEE 519 specifies the harmonic limits on Total
Demand Distortion (TDD) and not Total Harmonic Distortion (THD). TDD represents the amount of harmonics with
respect to the maximum load current over a considerable period of time (not the maximum demand current), whereas,
THD represents the harmonics content with respect to the actual load current at the time of measurement.
It is important to note here that a small load current may have a high THD value but may not be significant threat to the
system as the magnitude of harmonics is quite low. This is quite common during light load conditions. TDD limits are
based on the ratio of system's short circuit current to load current (Isc / IL ). This is used to differentiate a system and its
impact on voltage distortion of the entire power system. The short circuit capacity is a measure of the impedance of the
system. Higher the system impedance, lower will be the short circuit capacity and vice versa.
Systems with higher ISC / IL have smaller impedances and thus they contribute less in the overall voltage distortion
of the power system to which they are connected. Thus, the TDD limits become less stringent for systems with
higher ISC / IL values. In other words, higher the rating of transformer used for the same amount of load, higher will
be the allowable current distortion limits.
The limits on voltage are set at 5% for total harmonic distortion and 3% of fundamental for any single harmonic at
PCC level. Harmonics levels above this may lead to erratic functioning of equipment. In critical applications like
hospitals and airports, the limits are more stringent (less than 3% V ) THD as erroneous operation may have severe
consequences. As discussed already, the harmonic voltage will be higher downstream in the system.
Interharmonics
The IEEE defines interharmonics as:
“A frequency component of a periodic quantity that is not an integer multiple of the frequency at which the supply
system is operating (e.g., 50 Hz or 60 Hz).”
The IEC defines interharmonics as:
“Between the harmonics of the power frequency voltage and current, further frequencies can be observed which
are not an integer of the fundamental. They can appear as discrete frequencies or as a wide-band spectrum.”
One characteristic of all periodic signals is that they can be represented by their fundamental component and a
Fourier series of harmonics of various magnitudes, frequencies and angles. By definition interharmonics are not
periodic at the fundamental frequency, so interharmonics can be thought of as a measure of the non-periodicity of a
power system waveform. Similarly, any waveform that is non-periodic on the power system frequency will include
interharmonic distortion.
INTERHARMONIC SOURCES
Power system interharmonics are most often created by two general phenomena. The first is rapid non-periodic
changes in current and voltage caused by loads operating in a transient state (temporarily or permanently) or when
voltage or current amplitude modulation is implemented for control purposes. These changes can be quite random or,
depending on the process and controls utilized, quite consistent. Changes in current magnitude or phase angle can
also create sidebar components of the fundamental frequency and its harmonics at interharmonic frequencies.
Oscillations between series or parallel capacitors or when transformers or induction motors saturate can also produce
interharmonics. Some specific sources of interharmonics include arcing loads, induction motors (under some
conditions), electronic frequency converters, variable load drives, voltage source converters and power line
communications.
Arcing Loads include arc furnaces and welding machines. For most large industrial arc furnaces, transient loading
and interharmonics vary throughout the melt cycle. Load variability and the resulting interharmonics are usually
greater at the beginning of the melt cycle. Welding machines produce a more continuous frequency spectrum that
includes many interharmonic frequencies.
Induction Motors have slots in their stator and rotor iron that can produce interharmonics when their magnetic
circuits saturate. These interharmonics can increase during startup. Motor asymmetry can also produce
interharmonics. Variable torque loading can also cause motors to produce interharmonics.
Electronic Frequency Converters use a dc link to convert one frequency to another. Filtering on the dc bus is used to
decouple the voltage and current on each side of the link, but this filtering is never perfect and interharmonic
distortion can pass between the two ac systems.
Variable Load Dives such as traction system power supplies that use IGBTs or experience sudden load changes can
produce interharmonics, usually at fixed frequencies.
Light Flicker is caused by variations in rms voltage magnitude. The perceptibility of flicker varies with the frequency
and magnitude of these voltage variations.
The potentials of passive filters:
(1) Well designed passive filters can be implemented in large sizes of Mvars of ratings and provide almost
maintenance free service.
(2) These are more economical to implement than the synchronous condensers.
(3) A single installation can serve many purposes, like reactive power compensation and power factor
improvement, reducing THD, voltage support on critical buses in case of source outage, reducing starting impact
and voltage drop of large motors
• It acts as very low impedance at the frequency for which it is tuned, as such effectively shunts most harmonic line
quantities at that frequency.
• When the source impedance is inductive, there is a resonance peak, which occurs at a frequency lower than the
frequency for which the filter is tuned.
• The impedance rises with frequency for frequencies above that at which the frequency is tuned.
The concept of series connected filter is parallel resonating electrical circuit, which offers very high impedance at
tuning frequencies. Most commonly the filters are shunt connected to then AC system. This type of filters use series
resonating electrical circuit offering negligible impedance compared to the AC system harmonic impedance at
tuning frequencies.
PASSIVE FILTERS:
• Passive filters are inductance, capacitance, and resistance elements configured and tuned to control
harmonics. They are commonly used and are relatively inexpensive compared with other means for
eliminating harmonic distortion. However, they have the disadvantage of potentially interacting adversely
with the power system, and it is important to check all possible system interactions when they are designed.
They are employed either to shunt the harmonic currents off the line or to block their flow between parts of
the system by tuning the elements to create a resonance at a selected frequency.
• SHUNT PASSIVE FILTERS:
• The most common type of passive filter is the single tuned “notch” filter. This is the most economical type
and is frequently sufficient for the application. The notch filter is series-tuned to present low impedance to a
particular harmonic current and is connected in shunt with the power system.
• Thus, harmonic currents are diverted from their normal flow path on the line through the filter. Notch filters
can provide power factor correction in addition to harmonic suppression. In fact, power factor correction
capacitors may be used to make notch filters.
SERIES PASSIVE FILTERS
Unlike a notch filter which is connected in shunt with the power system, a series passive filter is connected in
series with the load. The inductance and capacitance are connected in parallel and are tuned to provide high
impedance at a selected harmonic frequency. The high impedance then blocks the flow of harmonic currents at
the tuned frequency only. At fundamental frequency, the filter would be designed to yield low impedance, thereby
allowing the fundamental current to follow with only minor additional impedance and losses.
Series filters are used to block a single harmonic current (such as the third harmonic) and are especially useful in
a single-phase circuit where it is not possible to take advantage of zero-sequence characteristics. The use of the
series filters is limited in blocking multiple harmonic currents. Each harmonic current requires a series filter
tuned to that harmonic. This arrangement can create significant losses at the fundamental frequency.
FORMULA FOR COMPONENT VALUE CALCULATION
Double tuned high pass filters
ADVANTAGES: - (1) Quality factor of the filter is low which makes it effective for a range of harmonics around the
first and/or second tuning frequency depending on the connection of the damping resistor. Because of high bandwidth
maximum attenuation is obtained for a range of harmonics.
(2) There is only one high voltage capacitor and one high voltage reactor because of which it is cheaper than two
second order high pass filters connected in parallel
(3) Less sensitive to variation in the fundamental frequency and component values.
(4) Improved filter redundancy
DISADVANTAGES:
(1) It requires larger installed MVAR rating than multiple second order high pass filters to meet same specified
performance.
(2) Higher losses in the filter because of presence of damping resistor
(3) Complex filter because of many components.
(4) Rating of low voltage components is decided mainly by transient behavior of the filter circuit
(5) Additional requirements of protection equipment e.g. protection panels, current transformers etc.
(6) Surge arresters are required for both low voltage and high voltage section to limit insulation levels