Download Harmonic Distortion

VARIABLE SPEED DRIVES AND FILTERS
CLASSIFICATION AND APPLICATION
Variable Speed Drives and Filters
APPLICATION SCOPE
INDEX
1. Variable Speed Drives. Distortions.
2. Variable Speed Drives. Harmonic
distortion.
3. Variable Speed Drives. Electric
interferences (noise).
4. Annex I. Norm UNE-EN 618003/A11:2002
5. Annex II. Filters summary.
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1
Variable Speed Drive
DISTORTIONS
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Variable Speed Drives
DISTORTIONS
DISTORTIONS PROVOKED BY VARIABLE SPEED DRIVES
The distortions produced by the Variable Speed Drive can be classified in the
following way:
1. Harmonic distortion: Produced by the rectifier bridge.
2. Electric noise: Produced by the inverter bridge.
We analyze what do they mean and the different ways to reduce them, specially thanks to
the use of different filter types.
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2
Variable Speed Drive
HARMONIC DISTORTION
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Variable Speed Drives
DISTORTIONS
HARMONIC DISTORTION
What are “the harmonics”?
It can be demonstrate that any
periodic waveform (squared,
triangular, …) can be represented
as the sum of several sinusoidal
waves with different frequencies and
phases. All those waves constitute
the harmonic spectrum of the wave.
» Harmonic content of a squared wave
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Variable Speed Drives
DISTORTIONS
HARMONIC DISTORTION
Which are the effects of the input harmonic currents?
The increment of the RMS value of the current through the conductors and
transformers. This “extra” current is not real (it will not produce work) but
nevertheless due to this current, transformers and wires must be overdimensioned to prevent over-heating effects.
Distortion of the input voltage wave. That could implicate important effects:
 It could affect to the correct functioning of those equipments with
single-phase power supply such us computers (PCs) and other devices
of low consumption.
 Non desired resonance effects can be produced affecting to the
distribution system.
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Variable Speed Drives
DISTORTIONS
HARMONIC DISTORTION
Which are the effects of the input harmonic currents?
 Power factor degradation.
 Overload of transformer, wiring, components, …
 Overload of power factor correction capacitors.
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Variable Speed Drives
DISTORTIONS
HARMONIC DISTORTION
How compensate the input harmonic distortion?
1. Incorporating inputs coils, as well called choke inductances. These passive
filters can be placed in the rectifier bridge input, realizing a double mission:
• On the one hand, they protect to the rectifier from voltage
variation of the main power supply.
• On the other hand, they filter the produced harmonics making
softer the sinusoidal wave of current.
They can also be placed in the DC bus. The rectifier bridge will not be as
protected as in the previous configuration, but this is always a low cost
option.
2. Using Active filters of harmonics.
This system allows analyzing the harmonics generated by the drive and they
can be annulled by mean of the generation of the opposite harmonics.
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Variable Speed Drives
DISTORTIONS
HARMONIC DISTORTION
How compensate the input harmonic distortion?
3. Installing a 12 pulses distribution system.
If we divide the rectifying input of the inverter bridge into two rectifier bridges
and the input voltage of each rectifier bridge is 30º diphase each other, in
theory the harmonic smaller than 11 order can be suppressed.
• To do that, a transformer with double secondary is required. One
secondary in star connection and the other secondary in delta
connection.
• Half a load will be supplied by the first secondary and the rest will be
supplied by the second one, getting their currents with 30º diphase.
• It is possible to demonstrate then that harmonic 5th and 7th has been
erased.
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Variable Speed Drives
DISTORTIONS
HARMONIC DISTORTION
12 pulses rectifying system.
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Variable Speed Drives
DISTORTIONS
HARMONIC DISTORTION
12 pulses rectifying system.
Moment where the thyristors of the
rectifier bridge are in conduction: The
input voltage is higher than the Bus
voltage.
The input inductances avoid the
complete discharge and make the
waveform softer, getting it similar to a
square wave.
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Variable Speed Drives
DISTORTIONS
HARMONIC DISTORTION
12 pulses rectifying bridge.
We can suppose a quasi ideal system applied to the rectifier, considering squared
wave signal as reference. Having into consideration that the double secondary,
explained before, enters a 30º diphase in the currents applied to each rectifier, the
result is a waveform much more sinusoidal in the inverter bridge:
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3
Variable Speed Drives
ELECTRIC INTERFERENCES (NOISE)
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Variable Speed Drives
ELECTRIC INTERFERENCES (NOISE)
ELECTRIC NOISE
The electric noise is produced by the inverter bridge.
It is due to the interruption of the current signal when the thyristors commutate
their status (switch over ON and OFF and vice versa).
For this reason the electric noise is a high frequency current signal which is
coupled to the current that is flowing in the drive to the motor and that
additionally can be emitted..
It is necessary to deal with two kind of emissions:
•
Conduced
•
Radiated
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Variable Speed Drives
ELECTRIC INTERFERENCES (NOISE)
RADIATED EMI
The RADIATED electric noise will be attenuated
considering:
•
The use of metallic conductions.
•
The use of shielded wires.
•
The own metallic cabinet of the drive
will help to minimize this effect.
» Power wires screened
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Variable Speed Drives
ELECTRIC INTERFERENCES (NOISE)
CONDUCED EMI
The CONDUCED electric noise can be attenuated in the following ways:
•
Noise coupled to the input signal of the drive.
 Recommended use of RFI Input Filters (Radio Frequency Interferences)
as well called EMC Filters (Electro-Magnetic Compatibility).
 Regulation which controls the selection of these filters is
UNE-EN 61800-3/A11:2002. See details on Annex I.
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Variable Speed Drives
ELECTRIC INTERFERENCES (NOISE)
CONDUCED EMI
The CONDUCED electric noise can be attenuated in the following ways :
•
Noise coupled in the current flowing to the motor.
 Using Output Common Mode Ferrites, in case of the couple noise is
common mode noise, that means, noise signal coupled to the capacitances
existing between the phases and the earth and also between the motor
windings and the earth. This noise is the responsible of the bearing
damages.
 Using dV/dt Output Filters, in case of the coupled noise is differential
noise, that means, noise signal coupled to the capacitances existing between
phases. This noise will produce isolation drillings and additionally will
increase the dV/dt factor. These filters can be:
• Output Inductance (output coils in series, one per phase)
• Iron Dust Toroids, in all output phases.
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Variable Speed Drives
ELECTRIC INTERFERENCES (NOISE)
FUNCTION OF THE dV/dt FILTER
It is possible to observe that the output waveform of the drive is as follow:
This is the result of the inverter
bridge action.
Show film
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Variable Speed Drives
ELECTRIC INTERFERENCES (NOISE)
FUNCTION OF THE dV/dt FILTER
If the waveform is amplified, it is possible to observe that the angle is not 90º exactly:
Drive
132kW
132kW
dV/dt
800V/µs
4000V/µs
Losses
1380W
1100W
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Variable Speed Drives
ELECTRIC INTERFERENCES (NOISE)
FUNCTION OF THE dV/dt FILTER
By incrementing the dV/dt ramp is possible to reduce the drive losses, that allows to the
drives to dissipate less power and consequently they can be smaller.
Drive
132kW
132kW
dV/dt
800V/µs
4000V/µs
Losses
1380W
1100W
The main disadvantage of this
method is the appearance of brusque
over-impulses in the drive output
which will be higher at motor input.
It is possible to check this in the
measurement realized using a
competitor drive:
» Actual measurement on competitor drive of 200A on load 21
Variable Speed Drives
ELECTRIC INTERFERENCES (NOISE)
FUNCTION OF THE dV/dt FILTER
To solve this problem, Power Electronics
works over the gate resistor of the IGBTs,
guaranteeing that those over-impulses do
not overcome a concrete value.
» Actual measurement on a drive of 200A on load: Competitor
Rg is the gate resistor and it controls the
load of the “capacitor” which conform the
IGBT.
» Actual measurement on a drive of 200A on load: POWER ELECTRONICS
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Variable Speed Drives
ELECTRIC INTERFERENCES (NOISE)
FUNCTION OF THE dV/dt FILTER
3rd Level: Filters
As a complement of that design, the
drives of Power Electronics, integrate
additionally the dV/dt output filters. These
filters, as has already been explained,
Inductances
consist of coils placed in serial connection
with the output, which complete the
attenuation of those non-desirable effects
before exposed. The result:
RFI Filters
» Actual measurement on a drive of
200A on load: POWER ELECTRONICS
2nd Level: Cooling
1st Level: Electronic
Power Supply
dV/dt Filter
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Variable Speed Drives
ELECTRIC INTERFERENCES (NOISE)
CONDUCED EMI
Finally, there is a filter type
which deal with the problem
by converting the chopped
signal into a sinusoidal signal:
this is the Sinusoidal Output
Filter (LC circuit in the drive
output).
The noise depend on the
capacitance and on the dV/dt
factor.
The capacitance is variable on
each installation.
The dV/dt depends on the
drive. If the dV/dt factor is
reduced up to non-considering
values, the noise will almost
disappear. The result:
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Annex I
Norm UNE-EN 61800-3/A11:2002
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Annex I
ELECTRO-MAGNETIC COMPATIBILITY
NORM UNE-EN 61800-3/A11:2002
Limits application in the measurement of the Conduced Perturbations
Nowadays exist several norms about EMC according to the 89/336/EEC directive that refer
to the variable speed drive or to those equipments incorporating drives.
Specific norms for drives
UNE-EN 61800-3/A11:2002: Adjustable speed electrical power drive systems. Part 3: EMC
product standard including specific test methods.
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Annex I
ELECTRO-MAGNETIC COMPATIBILITY
NORM UNE-EN 61800-3/A11:2002
Norm
Classifying Criteria
First environment
Non restricted distribution
Restricted distribution
Second environment
Input current  100A
Input current > 100A
Application Limit
Limit 1
Limit 2
Limit 3
Limit 4
Where:
First Environment: Includes domestic or residential use. It includes also, places directly
connected, without intermediate transformers, to a power supply distribution system of low
energy which additionally gives supply to buildings used for domestic uses (cinemas,
theatres, shopping centres, hospitals,…).
Second Environment: (Named also industrial). It includes all places different from those
which are directly connected to a power supply distribution system of low energy which
additionally gives supply to buildings used for domestic uses (factories and facilities
supplied with transformer of medium voltage to low voltage).
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Annex I
ELECTRO-MAGNETIC COMPATIBILITY
NORM UNE-EN 61800-3/A11:2002
Norm
Classifying Criteria
First environment
Non restricted distribution
Restricted distribution
Second environment
Input current  100A
Input current > 100A
Application Limit
Limit 1
Limit 2
Limit 3
Limit 4
Where:
Non-Restricted Distribution: Marketing modality where the power supply of the drive does
not depend on the customer or user regarding to EMC issues for the application of
operation.
Restricted Distribution: Marketing modality where the manufacturer limits the supplying of
the drive to those customers or users which, in an independent or together way, have
technical competence on the EMC requirements for the application of operation.
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Annex II
Filters Summary
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Annex II
FILTERS SUMMARY
FILTER TYPES
Summarizing:
Harmonic Distortion
Passive filter: Input coils (Choke
Inductances)
DC Bus Inductance
Three-phase Input Inductance
Active filters
Metallic conductions
Radiated
Screened cables
Metallic cabinets
Electric Noise
Conduced
EMC Input filter (or RFI filter). For noise coupled to the inputs
signal
Output Common Mode Ferrites. For noise coupled in the signal
flowing to the motor.
Output Inductances in series
(1 per phase)
dV/dt Output Filter
Iron Dust Toroids
(differential mode noise)
(1 per phase)
Sinusoidal Output Filter (LC circuit)
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Thanks for your attention
Presentation
VARIABLE SPEED DRIVES AND FILTERS
Classification and Application
Realized
Pilar Navarro
Organized
Marketing Department
www.power-electronics.com
©2006 Power Electronics España, S.L.