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Power factor correction with
Harmonic Filters
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Modern electrical equipment demands Power Quality
Modern electrical equipment imposes stringent demands
on voltage stability and power quality. The power
network has to be free from harmonics and other
electrical disturbances.
By installing harmonic filters several benefits are
obtained:
- Higher power factor, improved voltage stability and
network losses
- Filtering of harmonics in the system
- Avoidance of resonance problems and amplification of
electrical disturbances.
A clean network also imposes much less strain on
equipment and lengthens its life span. This means lower
maintenance cost and lower cost for replacing worn-out
equipment.
What problems can harmonics cause?
- Increased losses, e.g. machines will operate at
increased temperature and can be overheated
- Resonance problems between the inductive and
capacitive parts of the power network
- Malfunctioning of control systems since electronic
meters, relays, etc. are matched to the fundamental
frequency
- Overloading of capacitors, leading to malfunctioning
and premature ageing
- Interference with telecommunications and computers
- Disturbances in ripple control systems
- High currents in neutral conductors.
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Harmonics become apparent when a distorted sinus
curve is mathematically analysed. Through Fourier
analysis, an arbitrary periodic function can be dividedinto a number of sine waves. The example below shows
that the red distorted curve consists both of the
fundamental frequency (e.g. 50 Hz) and of super-
imposed 5th (250 Hz) and 7th (350 Hz) harmonicfrequencies.
What are harmonics?
= + +
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Where do harmonics come from?
Most types of equipment common in electrical systems
today generate harmonics:
- Equipment containing electronics that control other
apparatus, e.g. variable speed drives, soft starters,
static compensators, rectifiers, etc.
- Arc furnaces
- In certain cases, transformers, reactors and rotatingmachines.
Harmonics are not only found in industrial networks.
They can also spread into the distribution network and
cause problems for other power users.
Variable speed drives...
...rolling mills... ...and paper machines are examples of industrial equipment generating
harmonics.
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Each plant needs its own solution
Thorough know-how and experience is required to design
harmonic filters properly. Each plant is unique and needs
its own special technical solution. Both system and
component know-how are essential to find the optimum
solution in each individual case. An important component
in the filter is the capacitor. Since it copes with the whole
harmonic content, it must be designed with the greatest
care.
From analysis to commissioning
ABB Capacitors has the competence to support filter
projects from beginning to end. During the entire course
of the project we concentrate on cooperation and dialogue
with our customer, so that the best solution possible is
selected and attained.
Analysis
We analyse the plant using as inputs single-line diagrams,
type and size of loads and mode of operation. The
analysis culminates in a measurement plan.
Measurement
We carry out measurements in the plant under differing
operating conditions. The type of activity pursued
determines the scope of measurements. Complex
processes can take several days to measure, whereas
static processes do not normally involve much time. We
utilize sophisticated harmonic analysis equipment and
store information for subsequent processing.
Calculation
The measurements are then mathematically evaluated,
using computer programs. Various filter configurations
and operation modes are simulated. The calculations
result in a report containing a recommendation as to how
the filter installation should be designed to achieve the
best results and where it should be located. We also
calculate the distortion level anticipated after the
proposed installation.
In simple terms one might say that the filter absorbs
harmonics.
The filter is built up from passive RLC components, i.e.
resistors, reactors and capacitors. They are designed to
form a tuned circuit. The tuning frequency is matched to
the harmonics in question; normally of the 5th, 7th and
11th order. To avoid resonance, the tuning frequency is
usually placed lower than the harmonics occurring in the
network.
What is a harmonic filter?
There are basically three types of filter:
- The band-pass filter is tuned to one frequency. A
special case primarily occurring in low-voltage
systems is the de-tuned filter.
- The double-tuned filter is tuned to two frequencies.
- The high-pass filter filters frequencies above thetuning frequency. In certain cases a high-pass filter can
be installed as a C-type filter, which is used when
compensating arc furnaces or cyclo converters.
A filter installation normally consists of 1 - 3 band-pass
filters that filter one frequency each. Combinations with
high-pass filters, which is open to higher frequencies, can
also be applied.
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Design
ABB Capacitors calculates and designs the filter. Our
own production plant manufactures the capacitor units
and we enjoy close cooperation with prominent producers
of other components such as reactors and resistors.
We can also supply the relay protection for the filter, so
that the correct protective level is attained.
Commissioning
We supervise erection and commissioning. Our under-
taking can also include verification measurements to
check that the performance the equipment was designed
for has been achieved.
The filter design is arrived at with the aid of modern computer equipment.
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Band-pass filter Doubble-tuned filter High-pass filter
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High power quality pays!
A tough competitive climate leads to stringent availability
demands being imposed on all industrial installations.
Profits are dependent on the avoidance of downtime and
breakdowns as far as possible. A reliable and consistent
power supply is an important condition for this. The
expression high power quality is of vital concern to
utilities and industrial plants.
A harmonic filter represents a dependable method of
achieving high power quality in the form of stable voltage
and low distortion. This ensures reliable power supplies
and a high degree of availability.
Highly automated industry...
...and sensitive computer equipment require power supplies that are free
from harmonics for operations to run smoothly.
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...switchgear...
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We have unique experience
ABB Capacitors has been manufacturing power
capacitors since the early 1930s. We have considerable
experience in the design of harmonic filters for different
plants and industrial sectors. Further, we have
comprehensive know-how from all other types of
capacitor applications, e.g. HVDC, SVC, shunt and
series compensation etc.
Our capacitors are of the very highest quality. They areusually equipped with internal fuses, i.e. each capacitor
element has an individual fuse. In this way a stable tuning
frequency, high availablity and long service life are
achieved.
Capacitor filter projects imply close collaboration on our part with thecustomer since filters must be specially matched to each plant.
The maximum acceptable voltage distortion and total
harmonic distortion (THD) have been specified in
various national and international standards and
recommendations. In the table below, for example, the
values recommended by IEC for industrial plant are
given.
Recommendations and standards regarding power network distortion
ABB Capacitors designs and manufactures filters
according to various specifications. We will discuss with
and advise our customers on the choice of suitable
requirements. Differing national requirements and
technical considerations may determine the optimum
choice.
Odd order Odd order
non-multiple of 3 multiple of 3 Even ordern Dn (%) n Dn (%) n Dn (%)
5 6 3 5 2 2
7 5 9 1,5 4 1
11 3,5 15 0,3 6 0,5
13 3 21 0,2 8 0.5
17 2 >21 0,2 10 0,5
19 1,5 >10 0,2
23 1,5
25 1,5
>25 0,2 + 12,5/h
Compability levels for harmonics in MV industrial networks (class 2)
Maximum THD 8% (IEC 61000-2-4)
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ABB Capacitors AB
SE-771 80 LUDVIKA, Sweden
Phone +46 240 78 20 00
Fax +46 240 78 36 77
www.abb.com/powersystemsSECAP A03.en
Edition 2, 2000-05
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For more than seventy years, ABB Capacitors has
manufactured power capacitors for small and large
installations all over the world. ABB Capacitors in
Ludvika bears overall responsibility for the capacitorbusiness of the ABB Group.
ABB Capacitors is certified according to ISO 9001
(quality) and ISO 14001 (environment).
The product range of ABB Capacitors covers most types
of capacitors for power applications, for instance:
- High voltage shunt banks
- Harmonic filters
- Series compensation
- Static var compensators
- High voltage direct current
(HVDC) installations
- Capacitive voltage dividers (CVDs).
Cover photo:
5:th Harmonic Filter,
36 Mvar, 145 kV,
Sweden.