Problems Created by Harmonics
- Excessive heating and failure of capacitors & capacitor fuses,Transformers,Electric motors, fluorescent lighting ballasts, etc.
- Nuisance tripping of circuit breaker or blown fuses
- Presence of the third harmonic & multiples of the 3rd harmonic in neutral grounding systems may require the derating of
neutral conductors
- Noise from harmonics that lead to erroneous operation of control system components
- Damage to sensitive electronic equipment
- Electronic communications interference
Any device with non-linear operating
characteristics can produce harmonics in your power system. If you are
currently using equipment that can cause harmonics or have
experienced harmonic related problems, capacitor reactor or filter bank
equipment may be the solution. The following is a discussion of
harmonics; the characteristics of the problem; and a discussion of
our solution.
Origins of Harmonic Distortion
The ever increasing demand of industry and commerce for stability,
adjustability and accuracy of control in electrical equipment led
to the development of relatively low cost power diodes,
thyristors, SCRs and other power semi-conductors. Now used widely in
rectifier circuits for UPS systems, static converters and A.C.
& D.C. motor control, these modern devices replace the mercury
arc rectifiers of earlier years and create new and challenging
conditions for the power engineer of today.
Although solid state devices, such as the
thyristor, have brought significant improvements in control designs and
efficiency, they have the disadvantage of producing harmonic
currents. Harmonic currents can cause a disturbance on the supply
network
and adversely affect the operation of other electrical equipment
including power factor correction capacitors. We are concentrating
our discussions on harmonic current sources associated with solid
state power electronics but there are actually many other sources
of harmonic currents. These sources can be grouped into three main
areas:
- Power electronic equipment: Variable speed drives
(AC VFD's, DC drives, PWM drives, etc.); UPS systems, rectifiers,
switch
mode power supplies, static converters, thyristor systems, diode
bridges, SCR controlled induction furnaces and SCR controlled systems.
- Arcing equipment: Arc furnaces, welders, lighting (mercury vapor, fluorescent)
- Saturable devices: Transformers, motors, generators, etc. The harmonic amplitudes on these devices are usually insignifi cant
compared to power electronic and arcing equipment, unless saturation occurs.
Waveform
Harmonics are sinusoidal waves that are integral multiples of the
fundamental 60 Hz waveform (i.e., 1st harmonic = 60 Hz; 5th harmonic =
300 Hz).
All complex waveforms can be resolved into a series of sinusoidal
waves of various frequencies, therefore any complex waveform is the sum
of a number of odd or even harmonics of lesser or greater value.
Harmonics are continuous (steady-state) disturbances or distortions on
the
electrical network and are a completely different subject or
problem from line spikes, surges, sags, impulses, etc., which are
categorized
as transient disturbances.
Transient problems are usually solved by
installing suppression or isolation devices such as surge capacitors,
isolation
transformers or M.O.V.s. These devices will help solve the
transient problems but will not affect the mitigation of low order
harmonics or
solve harmonic resonance problems.
Harmonic Content
Thyristor and SCR converters are usually referred to by the number
of DC current pulses they produce each cycle. The most commonly used
are 6
pulse and 12 pulse. There are many factors that can infl uence the
harmonic content but typical harmonic currents, shown as a percentage
of the
fundamental current, are given in the below table. Other harmonics
will always be present, to some degree, but for practical reasons they
have
been ignored.
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Harmonic Overloading of Capacitors
The impedance of a circuit dictates the current fl ow in that
circuit. As the supply impedance is generally considered to be
inductive, the
network impedance increases with frequency while the impedance of a
capacitor decreases. This causes a greater proportion of the currents
circulating at frequencies above the fundamental supply frequency
to be absorbed by the capacitor, and all equipment associated with the
capacitor.
In certain circumstances, harmonic currents can
exceed the value of the fundamental (60 Hz) capacitor current. These
harmonic problems can also cause an increased voltage across the
dielectric of the capacitor which could exceed the maximum voltage
rating
of the capacitor, resulting in premature capacitor failure.
Harmonic Resonance
The circuit or selective resonant frequency is reached when the
capacitor reactance and the supply reactance are equal. Whenever power
factor
correction capacitors are applied to a distribution network, which
combines capacitance and inductance, there will always be a frequency
at
which the capacitors are in parallel resonance with the supply.
If this condition occurs on, or close to, one
of the harmonics generated by solid state control equipment, then large
harmonic currents can circulate between the supply network and the
capacitor equipment. These currents are limited only by the damping
resistance in the circuit. Such currents will add to the harmonic
voltage disturbance in the network causing an increased voltage
distortion.
This results in a higher voltage across the capacitor and
excessive current through all capacitor components. Resonance can occur
on any
frequency, but in general, the resonance we are concerned with is
on, or close to, the 5th, 7th, 11th and 13th harmonics for 6 pulse
systems.
See Figure. 2.
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Overcoming Resonance
If resonance cannot be avoided, an alternative solution is
required. A reactor must be connected in series with each capacitor such
that the
capacitor/reactor combination is inductive at the critical
frequencies but capacitive at the fundamental frequency. To achieve
this, the
capacitor and series connected reactor must have a tuning
frequency below the lowest critical order of harmonic, which is usually
the 5th.
This means the tuning frequency is in the range of 175 Hz to 270
Hz, although the actual frequency will depend upon the magnitude and
order
of the harmonic currents present.
The addition of a reactor in the capacitor circuit increases the fundamental voltage across the capacitor. Therefore, care
should be taken when adding reactors to existing capacitors. See Figure. 4.
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Reduction of Harmonic Distortion
Harmonic currents can be signifi cantly reduced in an electrical
system by using a harmonic filter. In its basic form, a filter consists
of
a capacitor connected in series with a reactor tuned to a specifi c
harmonic frequency. In theory, the impedance of the fi lter is zero at
the tuning frequency; therefore, the harmonic current is absorbed
by the filter. This, together with the natural resistance of the
circuit,
means that only a small level of harmonic current will fl ow in
the network.
Types of Filters
The effectiveness of any fi lter design depends on the reactive
output of the filter, tuning accuracy and the impedance of the network
at
the point of connection. Harmonics below the filter tuning
frequency will be amplified. The filter design is important to ensure
that
distortion is not amplified to unacceptable levels. Where there
are several harmonics present, a filter may reduce some harmonics while
increasing others. A filter for the 7th harmonic creates a
parallel resonance in the vicinity of the 5th harmonic with
magnification of
the existing 5th harmonic; therefore, a 7th harmonic filter
requires a 5th harmonic filter. See Figure. 5.
Consequently, it is often necessary to use a multiple filter design where each fi lter is tuned to a different frequency.
Experience is extremely important in the design of such fi lters to ensure:
(a) the most efficient and cost effective solution is selected;
(b) no adverse interaction between the system and the filter.
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Load Alteration
Whenever load expansion is considered, the network is likely to
change and existing fi lter equipment should be evaluated in conjunction
with
the new load condition. It is not recommended to have two or more
fi lters tuned to the same frequency connected on the same distribution
system. Slight tuning differences may cause one filter to take a
much larger share of the harmonic distortion. Or, it may cause
amplification
of the harmonic order which the equipment has been designed to
reduce. When there is a need to vary the power factor correction
component of
a harmonic filter, careful consideration of all load parameters is
necessary.
Harmonic Analysis
The first step in solving harmonic related problems is to perform
an analysis to determine the specifi c needs of your electrical
distribution
system. To determine capacitor and fi lter requirements, it is
necessary to establish the impedance of the supply network and the value
of
each harmonic current. Capacitor, reactor and fi lter bank
equipment are then specifi ed under very detailed and stringent computer
analysis
to meet your needs.
Your ABB Solution to Harmonics
ABB is the world's largest manufacturer of dry type low voltage
capacitors! ABB Control Inc. utilizes this experience in recommending
three
options to solve the problems associated with applying capacitors
to systems having harmonic distortion:
- Apply the correct amount of capacitance (kvar) to the network
to avoid resonance with the source. This may be diffi cult, especially
in automatic systems as the capacitance is always changing. This
solution usually means connecting less capacitance to the system than
is actually needed for optimum power factor correction.
- Install reactors in series with capacitors to lower the
resonance below critical order harmonics; i.e., 5th, 7th, 11th &
13th.
This design tunes the resonant frequency of the system well below
the critical harmonic and is called an anti-resonance bank. This
solution allows the capacitors to operate in a harmonic
environment.
- Filters are recommended if a problem exists with harmonic
distortion before the application of power factor correction, or if the
harmonic distortion is above the limits recommended in IEEE 519,
"Guide for Harmonic Control and Reactive Compensation of Static Power
Converters". (The recommended limits for voltage distortion in
IEEE 519 are presently 5% for general applications.) Tuned filters
sized to reduce the harmonic distortion at critical frequencies
have the benefi ts of correcting the power factor and improving the
network power quality.
With our knowledge of harmonics, ABB provides a
complete range of products from individual capacitors, fixed banks and
automatic banks, to power filter systems. All these products
utilize dry type low voltage ABB power factor correction capacitor
elements
which are self-healing for internal faults.
To maintain stringent quality control
standards, most control components found in automatic and anti-resonance
filter
bank products are also ABB products. These products include
contactors, circuit breakers, control relays, disconnect switches, power
factor
relays and pushbutton devices.
ABB Capacitor Features & Services
Every ABB Control low voltage capacitor product incorporates our
unique dry type design. Therefore, environmental and personnel concerns
associated with leakage or flammability of conventional oil-filled
units are eliminated. Other features include:
- Patented Sequential Protection System includes dry, self-healing design; internally protected elements; and dry, non-flammable
vermiculite filler
- Individual units, fixed and automatic capacitor bank designs, 208-600V
- Automatic and fixed tuned or anti-resonance capacitor banks
- Power factor and harmonic studies
- UL and CSA
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