Power Quality Seminar Report ‘03 1. INTRODUCTION Since last 25 years there has been an increase in the use of solid state electronic technology. This new, highly efficient, electronic technology provides product quality with increased productivity. Today, we are able to produce products at costs less than in the years passed, with the introduction of automation by using the solid state electronic technology .This new technology requires clear electric power. The conventional speed control systems are being replaced by modern power electronic systems, bringing a verity of advantages to the users. Classic examples are DC $ AC drives, UPS, soft stators, etc. Since the thrusters converter technology is rapidly gaining in the modern industrial plants, the power supply systems are contaminated as the ideal sinusoidal current and voltage waveforms are getting distorted. This is in turn is 1
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Power Quality Seminar Report ‘03
1. INTRODUCTION
Since last 25 years there has been an increase in the use of solid state electronic
technology. This new, highly efficient, electronic technology provides product quality
with increased productivity. Today, we are able to produce products at costs less than in
the years passed, with the introduction of automation by using the solid state electronic
technology .This new technology requires clear electric power.
The conventional speed control systems are being replaced by modern power
electronic systems, bringing a verity of advantages to the users. Classic examples are
DC $ AC drives, UPS, soft stators, etc. Since the thrusters converter technology is
rapidly gaining in the modern industrial plants, the power supply systems are
contaminated as the ideal sinusoidal current and voltage waveforms are getting
distorted. This is in turn is affecting the performance of the equipment in the electrical
network.
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Power Quality Seminar Report ‘03
2. WHAT IS POWER QUALITY?
Adequate to superior power quality is essential for the smooth functioning of
critical industrial processes. As industries expand, utilities become more interconnected
and usage of electronically controlled equipment increases, power quality is
jeopardized. Most large industrial and commercial sites are served by overhead lines
with feeders that are subject to unpredictable and sporadic events, e.g. lightning and
contact with tree limbs. Most distribution circuits have resoling devices that clear
temporary faults through a timed series of trip and close operations.
This minimizes the possibility of long-term outages but leads to a number of
minor power disturbances. These typically occur several times a month. Many electric
utilities have increased the voltage at which they distribute power. This allows a single
circuit to serve more customers or deliver higher loads, and reduces energy losses in the
system. But it often means the overhead distribution circuit is longer, with more
exposure to disturbances. And disturbances travel farther because of lower system
impedances associated with higher voltage circuits. Sophisticated new systems are
providing vastly increased efficiency and control in critical processes. But with their
high sensitivity even to brief variations in electric power quality, today's computer-
driven devices fail when power is disturbed for even a few milliseconds.
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Power Quality Seminar Report ‘03
3. HARMONICS-BASIC CONCEPTS
A pure sinusoidal voltage is conceptual quantity produced by an ideal AC
generator build with finely distributed stator and field windings that operate in a
uniform magnetic field. Since neither the winding distribution nor the magnetic field is
uniform in a working AC machine, voltage waveform distortion is created, and the
voltage time relation-ship 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 wave
form .These loads are referred to as linear loads (loads where the voltage and current
follow one another without any distortion to their pure sine waves).examples of
nonlinear 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 non sinusoidal containing distortions
where by 50 Hz waveform has numerous additional waveforms superimposed upon it
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Power Quality Seminar Report ‘03
creating multiple frequencies within the normal 50 Hz sine wave .The multiple
frequencies are harmonics of the fundamental frequency.
Normally current distortion produce voltage distortions .However when there is
a stiff sinusoidal voltage source there is a low impedance path from the power source
which has sufficient capacity so that loads placed upon it will not affect the voltage one
need not be concerned about current distortions producing voltage distortions Examples
of non linear loads are battery chargers, electronic ballasts; variable frequency drives,
and switched mode power supplies.
As nonlinear currents flows 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 waveforms distortions are produced.
Power systems designed to function at the fundamental frequency which is 50
Hz in India are prone to unsatisfactory operation and at times failure when subjected to
voltages and currents that contains 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.
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Power Quality Seminar Report ‘03
4. THE AFFECTS
The actual problems of any building/industry will vary depending on the type
and number of installed harmonics producing loads. Most electrical network can
withstand nonlinear loads of up to 15% of the total electrical system capacity without
concern but when the nonlinear loads exceed 15% some non expected negative
consequences can be expected. .for electrical networks , they have on linear loading of
more than 25% particular problems can be apparent.
The following is a short summery of most problems caused by harmonics:
Blinking of incandescent lights-transformer saturation
Capacitor failure-harmonics resonance
Circuit breaker tripping-inductive heating and over loading
Computer malfunctioning-voltage distortion
Transformer failure-inducting
Motor failure-inductive heating
Fuses blowing for no apparent reason-inductive heating & over load
Electronic component shut down- voltage distortion
Flickering of florescent lights-transformer saturation
The heating effects of harmonic currents can cause destruction of equipment,
conductors, and fires. The results can be unpredictable legal and financial ramifications
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Power Quality Seminar Report ‘03
apart from safety risks. Voltage distortions can lead to over heating of equipment
failure, expensive down time and maintenance difficulties. Harmonic currents and
voltage distortions are becoming the most severe and complex electrical challenge for
the electrical industry .The problems associated with nonlinear loads were once limited
to isolated devices and computer rooms, but now the problem can appear through the
entire network and utility system
The point at which the harmonic limits are applied is called the point of
common coupling (PCC). When the input transformer is the point of measurement then
the PCC refers to this point where the facility electrical system is common to the
facility of additional consumers. If there is a distortion present on the electrical power
system at this point it may be experienced by the neigh boring facilities as well. So we
need to avoid this situation
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5. SOLUTION
Users of variable frequency drives often have strict demands placed on them to
mitigate harmonic distortion caused by the nonlinear loads. Many choices are available
to them including line reactors, harmonic traps, 12 pulse rectifier, 18 pulse rectifiers,
and low pass filters.
5.1 LINE REACTORS
The input harmonic current distortion can be reduced by simple addition of
input line reactance. The inductive reactance of an input line reactor allows 50 Hz or 60
Hz currents to pass easily but presents considerably higher impedance to all other
harmonic frequencies. Harmonic currents are thus attenuated by the reactance offered
by the line reactor.
These reactors are also used to solve the problems in variable frequency drive
installations.Eg: harmonic attenuation , drive tripping .The line reactors are always used
in the line side or input of the variable frequency drives. Thus they are called the line
reactors. The line reactors cannot be used at the output of the variable frequency drives
Because the reactors are over heated due to the harmonic content of the output
waveform of the VFD Harmonic compensated reactors can be used on the either side of
the variable frequency drives .Due to the introduction of the Harmonic compensated
reactors the following problems are eliminated: motor noise, low efficiency of the
motors, temperature rise in motors and variable frequency drives short circuit problem.
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5.2 HARMONIC FILTERS
In some cases, reactors alone will not be capable of reducing the harmonic
current distortion to the desired levels. In these cases, a more sophisticated filter will be
required. The common choices include shunt connected, tuned harmonic filters
(harmonic traps) and series connected low pass filters (broad band suppressors). They
consist of a capacitor and an inductor which are tuned to a single harmonic frequency.
Since they offer very low impedance to that frequency, the specific (tuned) harmonic
current is supplied to the drive by the filter rather than from the power source. If tuned
harmonic filters (traps) are selected as the mitigation technique, then multiple tuned
filters are needed to meet the distortion limits which are imposed.
When employing tuned harmonic filters, we need to take special precautions to
prevent interference between the filter and the power system. A harmonic trap presents
a low impedance path to a specific harmonic frequency regardless of its source. The
trap cannot discern harmonics from one load versus another. Therefore, the trap tries to
absorb that entire harmonic which may be present from all combined sources (non-
linear loads) on the system. This can lead to premature filter failure.
Since harmonic trap type filters are connected in shunt with the power system,
they cause a shift in the power system natural resonant frequency. If the new frequency
is near any harmonic frequencies, then it is possible to experience an adverse resonant
condition which can result in amplification of harmonics and capacitor or inductor
failures. Whenever using harmonic trap type filters, one must always perform a
complete system analysis. You must determine the total harmonics which will be
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absorbed by the filter, the present power system resonant frequency, and the expected
system resonant frequency after the filter (trap) is installed. Field tuning of this filter
may be required if adverse conditions are experienced.
5.3 12 PULSE RECTIFIERS
12 Pulse drives are frequently specified by the engineers for heating, ventilating
and air conditioning applications because their ability to reduce harmonic current
distortion. In the mid 1960s when power semiconductors were only available in limited
ratings, twelve-pulse drives provided a simpler and more cost effective approach to
achieving higher current ratings than direct paralleling of power semiconductors.
A typical diagram of a large twelve-pulse drive appears in figure the drive's
input circuit consists of two six-pulse rectifiers, displaced by 30 electrical degrees,
operating in parallel. The 30-degree phase shift is obtained by using a phase shifting
transformer. The circuit in figure simply uses an isolation transformer with a delta
primary, a delta connected secondary, and a second wye connected secondary to obtain
the necessary phase shift. Because the instantaneous outputs of each rectifier are not
equal, an inter phase reactor is used to support the difference in instantaneous rectifier
output voltages and permit each rectifier to operate independently. The primary current
in the transformer is the sum of each six-pulse rectifier or a twelve-pulse wave form.
Theoretical input current harmonics for rectifier circuits are a function of pulse
number and can be expressed as:
h = (np + 1) where n= 1, 2, 3, and p = pulse number
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Power Quality Seminar Report ‘03
For a six-pulse rectifier, the input current will have harmonic components at the
following multiples of the fundamental frequency.
5, 7, 11, 13, 17, 19, 23, 25, 29, 31, etc.
For the twelve-pulse system shown in figure 1, the input current will have
theoretical harmonic components at the following multiples of the fundamental
frequency:
11, 13, 23, 25, 35, 37, etc.
Note that the 5th and 7th harmonics are absent in the twelve-pulse system. Since
the magnitude of each harmonic is proportional to the reciprocal of the harmonic
number, the twelve-pulse system has a lower theoretical harmonic current distortion.
12 PULSE RECTIFIERS
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Figure shows the actual measurement of input current harmonic distortion for 12
pulse rectifier supplied from a balanced 3 phase voltage source while operating at full
load conditions. For test purpose transformer has delta primary and delta,wye secondary
windings. To obtain the best results, the bridge rectifier is connected in series so equal
dc windings. To obtain the best results, the bridge rectifier is connected in series so
equal dc
The data shows when the current through both sets of the rectifiers is equal,
harmonics can be as low as 10% to 12% total harmonic current distortion, at full load.
Current sharing reactors will help parallel connected bridge rectifiers to share current
equally. Even with balanced current harmonic current distortion can increase
appreciably at light loaded conditions. Even with perfectly balanced line voltages, the
resultant % total harmonic current distortion increases as the load increases. As the load
reduced, that is 23% total harmonic current distortion at 20% load.
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5.4 18 PULSE RECTIFIER
A typical diagram of a series connected eighteen pulse drive constructed from a
standard six-pulse drive, two external rectifiers and a conventional 18 pulse isolation
transformer appears in figure 1. The drive has terminals available to connect a DC link
choke. These terminals are used to connect the two external rectifiers in series with the
drives internal rectifier. The eighteen pulse transformer is designed to provide one third
the normal input voltage to each of the three rectifiers at a 20 degree phase
displacement from each other. The 20-degree phase shift is obtained by phase shifting
the transformers secondary windings. The circuit in figure 1 simply uses an isolation
transformer with a delta primary, and three delta connected secondary windings, one
shifted + 20 degrees, one shifted -20 degrees and one in phase with the primary.
The primary current in the transformer is the sum of each six-pulse rectifier or
an eighteen-pulse wave form.
Theoretical input current harmonics for rectifier circuits are a function of pulse
number and can be expressed as:
h = (np ± 1) where n= 1, 2, 3,... and p = pulse number
For a six-pulse rectifier, the input current will have harmonic components at the