Chapter 7 - EMI.pdf

Chapter 7 – Electromagnetic Interference of
Power Electronics
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Advanced Power Electronics (EE4007A/B/D)
19/11/2019

Outline
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Chapter 7 – Electromagnetic Interference
• Electromagnetic interference (EMI) generation and transmission
• International standards about EMI
• EMI suppression methods
• EMI related power quality issues

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Electromagnetic Interference (EMI)
• EMI - Electromagnetic disturbance leads to degradation of the performance
of devices, equipment or systems
• EMI is not a varying voltage or current. It is the harmful effect caused by
electromagnetic waves or fast changing voltage/current

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Electromagnetic Interference (EMI)
• In power electronics, electromagnetic interference (EMI) is generated
internally and externally. EMI can be transmitted by electrical conduction,
electromagnetic induction and radiation to and from a power electronic circuit.
❖ Electrical conduction – EMI transmitted through electrical current in
physical channel such as cable.
❖ Electromagnetic induction – without physical contact
❖ Radiation - emission or transmission of
energy in the form of waves through space

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Spectrum of Electromagnetic Waves
• Electromagnetic interference (EMI) is also called radio-frequency interference
(RFI).
Sunlight
Main focus of EMI in
power electronics
-- Wikipedia
• This chapter is about how the EMI affects the electrical circuits, devices, and systems and
how to suppress EMI

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Sources of Electromagnetic Interference
❑ Natural sources
▪ Lightning striking on conductor (eg.
overhead power line)
• Producing large surge
• Inducing electric field in frequency with
50MHz to 100MHz
▪ Solar radiation
• Cosmic ray producing electromagnetic
waves in frequency with 100MHz to
1000MHz

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❑ Man-made Sources
▪ Electrostatic discharges (ESD)
• Sudden flow of electricity between two electrically charged objects
caused by contact
▪ Electromagnetic Pulse (EMP)
• From nuclear explosions or any systems or weapons with this
function (eg. EMP Bombs)
▪ Variations in mains supply voltage
• From malfunction of power systems
• From tripping of circuit breakers
• From sudden change of output of power generators, critical loads
and reactive power (VAR)

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❑ Man-made Sources
▪ Electrical and electronic sub-systems producing noise from
• Systems and parts in automotive such as ignition systems,
alternators and electric machines
• Power distribution systems including power lines, static VAR
compensators (SVC) and generator stations
• Industrial equipment such as welding machines, induction heaters,
circuit breakers, variable speed drives and oscillators
• Radio transmitters including all mobile communication systems
• Emitted waves generated by switching transient of high frequency
systems such as SMPS and inverters
• Harmonics of input current of appliances from SMPS, motor drives
and any appliances without power factor correction

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❑ Media of EMI transmission from power electronics
EMI from power electronics
❑ Switching of transistors
▪ Rapid changed of current (di/dt)
• Generating high transient voltage with parasitic inductor
▪ Rapid change of voltage (dv/dt)
• Generating high leakage current through stray capacitance
(electrostatic coupling)
▪ EMI generated internally in a power electronic circuit may be spread to
the line and the load by electrical conduction, and to the surrounding by
electromagnetic induction, electrostatic coupling and radiation.

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Chapter 7– Electromagnetic Interference
EMI from power electronics
❑ Harmonics of input current
▪ EMI transmitted by conduction
▪ Degrading power quality
▪ Generating EMI
▪ Examples of power electronics
• Bridge rectifiers, Static VAR compensators (SVC), controlled
rectifier, switched mode converters without good input filtering

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EMI to power electronics
❑ From power network by conduction
▪ Poor power quality
• Harmonics
▪ Sudden change of power generators and
appliance
• Voltage surge and sags (dv/dt)
▪ Noise in power line
❑ Noise through electromagnetic induction and radiation
▪ Electrical machine, transmitters, mobile phone and remote
signal transmission equipment

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Definition of Terms of EMI and Harmonics
❑ Electromagnetic Interference (EMI)
▪ Electromagnetic disturbance degrading performance of devices,
equipment or system
▪ Low EMI is preferred
❑ Electromagnetic Compatibility (EMC)
▪ Ability of an equipment or system to function satisfactorily in its
electromagnetic environment without introducing intolerable
electromagnetic disturbances to anything, in that environment
▪ High EMC is preferred

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❑ Emission
▪ Phenomenon which which electromagnetic energy emits from a source
▪ Divided into conductive emission and radiated emission
❑ Immunity (what is the difference to EMC?)
▪ Ability of a device, an equipment or a system to perform without
degradation in presence of an electromagnetic disturbance. In other
words, the ability to withstand the EMI

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❑ Susceptibility (the inverse of immunity)
▪ Inability of a device, an equipment or a system to perform without
degradation in the presence of an electromagnetic disturbance
❑ Total Harmonic Distortion (THD)
▪ RMS addition of all harmonics, except the fundamental, as compared to
fundamental component
▪ THD can be higher than 100%
▪ Good power quality if THD < 15%
Harmonics is one of the sources causing EMI

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International Standards
❑ The British Standards Institute (BSI) -UK
❑ The Federal Communications Commission (FCC) -USA
❑ Verband Deutscher Elektrotechniker (VDE) -Germany
❑ The International Electrotechnical Commission (IEC) -EU
❑ The International Special Committee on Radio Interference
(CISPR) -IEC
▪ Subcommittee of IEC
▪ Firstly proposing EMC standards
❑ CE Mark -EU
❑ Why are Standards needed?

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❑ IEC standards for emission

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❑ IEC standards for emission

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❑ IEC standards for Immunity

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IEC Standard -EN61000-3-2 (Emission)
❑ Class A
▪ 3-phase equipment except the following classes
❑ Class B
▪ Portable tools
❑ Class C
▪ Lighting equipment including dimming devices
❑ Class D
▪ Equipment having input current with special wave shape and P< 600W
▪ Measured under test conditions given in EN61000-3-2-Annex C

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IEC Standard -EN61000-3-2 (Emission)

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IEC Standard -EN61000-3-2
❑ Harmonic standard of Class A

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❑ Harmonic standard of Class C
*λ is the circuit power factor

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❑ Harmonic standard of Class D

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Harmonics from Bridge Rectifiers
❑ Bridge rectifier without an output filter capacitor
▪ Input current is sinusoidal without harmonic, but the output voltage
presents large ripple

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▪ Higher output capacitance decreasing output ripple but increasing
input harmonic current
❑ Bridge rectifier with an output filter capacitor

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▪ Higher output capacitance decreasing output ripple but increasing
input harmonic current
❑ Bridge rectifier with an LC low-pass filter capacitor

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Power Factor Correction Converters
❑ Power factor correction (PFC) converters can be used for voltage
rectification with regulated DC output voltage to solve the above input
harmonic current and power factor problem.
❑ AC/DC converter with very high PF and regulated output voltage
❑ Closed-loop controlled for I/P current and O/P voltage
▪ Shape of input current regulated to be half sinusoidal
▪ Phase of input current regulated to be in phase with I/P voltage
❑ Peak current mode control or average current mode control
❑ Size of output filtering capacitor NOT able to be reduced by increasing
switching frequency

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❑ Single phase PFC converter with boost topology
▪ Shape and phase of iL regulated to be the same as vDC

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❑ 3-phase full bridge PFCC

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Active Harmonic Compensation
❑ Harmonic active filters eliminating a range or whole range of harmonics
❑ Compensating displacement VAR
❑ Fast response
❑ For shunt type, shunt connected to the line
▪ Switched mode converter (Inverters)
▪ Current mode closed-loop control
▪ Similar to STATCOM
▪ Series type active filter similar to SSSC

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❑ Constructed with a bidirectional 3-phase full-bridge inverter
▪ 3-ph Inverter →6 boost converters sharing 3 input inductors
❑ Large capacitor as energy storage device and power source
Diagram of a shunt active filter

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❑ When the power line current is higher than the reference current, the large capacitor provides
power to the inverter to generate the amount of exceeding current to the load through the
power line instantaneously.
❑ When the power line current is lower than the reference current, the bidirectional inverter acts
as a boost converter (similar topology of 3-phase power factor correction converter with boost
topology). The active power filter inputs the amount of current under the reference current
instantaneously to the converter to charge the capacitor.
Control objective: grid current are sinusoidal currents
Operation principle: i_load (ila, ila, ila) = i_grid (ia, ib, ic) + i_filter (i1, i2, i3)
Grid current

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Types of Electromagnetic Interference
❑ Differential mode noise
❑ Common mode noise
▪ EMI conducting through conductors to and from input lines
▪ EMI conducting through stray capacitors by electrostatic coupling
between power lines and ground
❑ Both types of EMI in AC current or voltage manner
❑ Suppressing EMI necessary for meeting standard
▪ EMI suppression filters commonly used
▪ Different types of filters for different types of EMI

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Types of Electromagnetic Interference
Noise from converter
Noise to converter

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Differential Mode Noise Suppression
❑ Differential mode chokes
▪ Using magnetic components (a pair of inductors)
❑ Class X capacitors
▪ Shunt connected capacitor at the input of the converter

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❑ Constructed with a pair of inductors
▪ Connected to input of converter
▪ Providing high reactance of noise (high frequencies)
▪ AC current of noise reduced

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❑ Varistor, Also called voltage dependent resistor
❑ Connected in the input of the whole converter circuit
❑ With nonlinear and variable resistance characteristic with different voltage
▪ Very high resistance during normal operation
▪ Resistance decreasing while increasing voltage
▪ Resistance decreasing drastically while voltage beyond rated voltage

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❑ Resistance of varistor is very low during surge occurring
❑ Current returning to the power lines
▪ Circuit protected from differential mode voltage surge
▪ Especially surge during switching on the converters
❑ Selecting varistors with different varistor voltage, maximum clamping
voltage and maximum energy (Joule, J)

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Other Methods for Reducing EMI
❑ Good design of transformers and inductors
▪ Using toroid cores to reducing leakage magnetic flux
❑ Electrical isolation of power converters isolating differential mode
noise by the transformer
❑ Applying snubber circuits for decreasing dv/dt but this decreasing
efficiency of the converters
❑ Applying soft-switching techniques to reduce switching loss, di/dt and
or dv/dt of transistors

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Other Methods for Reducing EMI
❑ Good PCB tracks and components arrangement
▪ Decreasing inner area of PCB tracks of current loop ()
▪ Decreasing length of tracks
❑ Using double layer PCB with ground on the top layer
▪ The ground on the top layer absorbing external and internal EMI
• The routine of current forms a current loop. Decreasing inner
area of the current loop or decreasing the length the tracks can
reduce parasitic inductance and EMI

Chapter 7 - EMI.pdf

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Chapter 7 - EMI.pdf