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ECE636 B2
Dynamics and Control of Voltage Source
Converters (VSCs)
Winter 2013
Instructor:Dr. Yasser MohamedElectrical and Computer Engineering Dept.
University of AlbertaOffice W2-014 ECERFemail: [email protected]
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1. Electronic Power Processing
Overview of Electronic Power Processing
Power Electronic Switches
Classification of Power Converters
Basic Configurations and Switching Strategies in
VSCs 2
Outline
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Overview of Electronic Power Processing
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Power Electronic Switches
Power Electronics Switches
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Uncontrollable
Switch
Semi-controllable
Switch
DiodeThyristor
1- Gate-Turn-Off Thyristor (GTO)
2- Integrated Gate-Commutated Thyristor (IGCT)
3- Insulated-Gate Bipolar Transistor (IGBT)
4- Metal-Oxide-Semiconductor Field-Effect
Transistor (MOSFET)
In ON state, the switch conducts current in one direction only
(unidirectional switch).
In OFF state, the switch blocks only positive voltages (unipolar) or both
positive and negative voltages (bipolar), depending on the switch type.
On/OFF states are
controlled
by the power circuit.
On state iscontrolled; off state
is controlled
by the power
circuit.
Controlled Switches
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Power Electronic Switches
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Uncontrollable Switch : Diode
Diode is a 2-terminal pn-junction device.
Diode conducts when forward biased, i.e., when VAK>VF. VF1 to 4 V and is called
forward voltage drop.
When reverse biased, diode conducts a very small leakage current in reverse
direction.
If the reverse bias voltage exceeds reverse breakdown voltage, the device breaks
down and conducts dangerously high reverse currents. This situation has to be
avoided.
On and OFF states of diode are controlled by the power circuit, not a control
signal. That is why it is completely uncontrollable switch.
3 kV, 3 kA Fast
recovery diode
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Power Electronic Switches
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Semi-controllable Switch : Thyristor (Silicon Controlled Rectifier (SCR))
ON state of thyristor is controlled by a control signal, but its OFF state is controlled by
the power circuit; thats why it issemi-controllable.
When in OFF state, thyristor can block forward positive voltages (forward blocking
voltage) below forward breakdown voltage andreverse voltages below reversebreakdown voltage. Thats why it is a bipolar unidirectional switch.
Thyristor can be turned on by applying a positive gate current pulse when the device is f
forward biased. To assume ON state, thyristor current must reach a certain level called
latching current.
To turn the thyristor off, its current has to brought below a certain level called holding
current and a negative voltage has to be maintained across its terminals for longer thana specified period of time.
4 kV, 3 kA
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Power Electronic Switches
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Controllable Switch : Gate-Turn-Off Thyristor (GTO)
When forward biased, GTO can be turned on by injecting a positive current pulse of
specified amplitude into the gate.
When ON, the switch can be turned off by withdrawing a current pulse of specified
magnitude from the gate.
GTO is a Current Controlled Switch. A high negative gate current pulse of up to 1/3 ofthe anode current is required to turn off the GTO. This implies large power consumption
in the gate drive circuit (high losses in the drive circuit).
GTO is capable of withstanding reverse voltages (bipolar unidirectional switch).
Typical GTO ratings are 6 kV, 6 kA, at switching frequency 500 Hz, and Von= 2-3 V.
Very popular in 80s and 90s; now it is almost obsolete due to IGBT and IGCT!
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Power Electronic Switches
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Controllable Switch : Integrated Gate-Commutated Thyristor (IGCT)
IGCT is a newer semiconductor device (introduced in 1997) in the thyristor family that
is replacing GTO in high power applications due to its superior characteristics. IGCT is a Voltage Controlled Switch. This implies very small power consumption in the
gate drive circuit.
IGCT has reverse voltage blocking capability (suitable for current-source inverters).
IGCT is superior to GTO in that
IGCT can operate without dv/dt snubbing at high current density, thanks to its improved
switching characteristics.
IGCT has low ON-state and turn-off losses as a result of minimized Silicon thickness.
IGCT has low gate drive requirements especially during conduction.
IGCT has low ON-state voltage drop.
Typical IGCT ratings are 3 kA, 4.5kV, 500-2000 Hz, Von= 2.4 V, maximum turn-on di/dt=
3000 A/s, and maximum turn-off dv/dt= 4000 v/s.
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Power Electronic Switches
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Controllable Switch : Insulated-Gate Bipolar Transistor (IGBT)
When forward biased, IGBT can be turned on by applying a positive voltage of specified value
between the gate and source or emitter. The gate voltage has to be maintained for as long as the
switch is to be ON.
When ON, the switch can be turned off by applying a negative voltage of specified value between the
gate and source or emitter. The gate voltage has to be maintained for as long as the switch is to be OFF.
Note that only at the turn-on and turn-off, a current pulse will be input to or withdrawn from the gate.
The gate current during ON and OFF states is practically equal to zero.
IGBT is a Voltage Controlled Switch. This implies very small power consumption in the gate drive circuit. IGBT combines the advantages of MOSFET (low power consumption at the gate), low ON-state
voltage drop, and GTO (reverse voltage blocking capability).
IGBTs reverse voltages blocking capability is lower than that of positive voltage blocking.
New generation IGBTs called Non-Punch-Through (NPT IGBTs) are capable of blocking higher reverse
voltages, but are slower in switching.
IGBT ratings can reach 3300 V, 1200 A, and switching times as low as 1 s with switching frequency up
to 2 kHz for high power applications (Silicon IGBT).
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Power Electronic Switches
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Controllable Switch : Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)
When forward biased, MOSFET can be turned on by applying a positive voltage of specified value
between gate and source. The gate voltage has to be maintained for as long as the switch is to be ON.
When ON, the switch can be turned off by applying a negative voltage of specified value between gate
and source. The gate voltage has to be maintained for as long as the switch is to be OFF.
MOSFET is a Voltage Controlled Switch. This implies very small power consumption in the gate drivecircuit.
MOSFET is NOT capable of withstanding reverse voltages (Unipolar unidirectional switch).
MOSFET is a Positive Temperature Coefficient device. In other words, as the device temperature rises, its
ON-state resistance Ron rises as well.
Typical MOSFET ratings are from 100 A and small voltage to 1000 V and small current.
Switching frequency of MOSFETs can reach as high as 1 MHz for small power ratings.
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Power Electronic Switches
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Switching Losses in Controlled Switches
The figure shows a typical situation for a controllable switch in a power electronic
converter. Vd is the dc source voltage and Io represents the inductive load current.
When the switch in ON, the dc voltage source supplies power to the load through the
switch.
When the switch is OFF, the diode freewheels the load current, as the inductive loadcurrent cannot be interrupted. Note that the interruption of an inductive current can cause
dangerously high Ldi/dt voltages, unless an alternate path for the flow of current is
provided to avoid this situation.
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Power Electronic Switches
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Switching Losses in Controlled Switches
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Power Electronic Switches
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Switching Losses in Controlled Switches
The energy loss during turn-on and turn-off processes
in the switch is
The energy loss during the on-state is
The switching power losses (averaged over one
switching period Ts
) will be
The conduction power loss (averaged over one
switching period) will be
wherefsis switching frequency, Von is the on-
state voltage of the switch, and dis switch duty
ratio.
with fs increasing Ploss inc
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Power Electronic Switches
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Switching Losses in Controlled Switches
Switching waveforms of a Silicon Carbide (SiC)
MOSFET (Cree) at 225C.
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Power Electronic Switches
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Switching Losses in Controlled Switches
1.5 MW Full scale wind turbine
@ 3 kHz switching frequency, rated power
Several MOSFETs are used to meet thecurrent rating of 1.5 MW wind turbine.
siliconcarbid
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Power Electronic Switches
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Switching Losses in Controlled Switches
1.5 MW Full scale wind turbine
Several MOSFETs are used to meet thecurrent rating of 1.5 MW wind turbine.
constant efficiency over wide temperature range
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Power Electronic Switches
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Future trend
2
kW
cm
kHz
700
400
150
51 10
125 15 kV
SiC IGBT
225 15 kV
SiC IGBT
Si
SiC IGBT and IGCT will dominate mediumand high power applications.
Higher switching frequencies with low
switching losses will be feasible.
Higher switching frequencies will yield
higher power quality, smaller filter size,
lower cost, and more importantly higherdynamic capabilities.
Many topologies proposed for high power
converters under limited switching
frequencies will be obsolete. VSCs will be
commonly used even in high power
applications.
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Power Electronic Switches
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How to give a unidirectional switch a reverse
conduction capability?
In ON state, the switch conducts current in
one direction only (unidirectional switch).
In OFF state, the switch blocks only positive
voltages (unipolar) or both positive and
negative voltages (bipolar), depending on the
switch type.
(a) Generic schematic diagram of a switch cell. (b)
Symbolic representations
of a switch cell.
If the switch is reverse biased by a fewvolts, the diode conducts and provides a
path for the reverse current.
VSCs require switching cells to provide
two-way power flow medium.
bidirectional diode
l f f
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Classification of Converters
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According to Commutation (the process of current transfer from outgoing switch
(switch turned of) to incoming switch (switch turned on)
line-commutated (or naturally-commutated converter): The commutation process
depends on the AC voltage polarity
- 6-pulse line-commutated converter, widely
used in conventional HVDC transmissionsystems.
A-
1-
l ifi i f
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Classification of Converters
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According to Commutation (the process of current transfer from outgoing switch
(switch turned of) to incoming switch (switch turned on)
Forced-commutated (or self-commutated converter) Gate-Turn Off Switches
control the current transfer process
three-wire, three-phase, two-level VSCfull-bridge, single-phase, two-level VSC
three-phase current-source converter
2-
note that CSC is single direction
Cl ifi i f C
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Classification of Converters
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According to DC-Source Characteristics
Three-phase voltage-source converter
+
Vd
_
Three-phase current-source converter
Id
Three-phase impedance-source (Z-source) converter
bidirectional converteunidirectional converter
B-
Cl ifi i f C
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Classification of Converters
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According to DC-Source Characteristics
Three-phase voltage-source converter
+
Vd
_
- AC voltage is less than dc voltage
Buck (step-down) dc/ac converter (buck inverter)
Or
Boost (step-up) ac/dc converter (boost rectifier)
- Low number of passive components (low losses)
- Switching cells are used. IGBTs offer fast switching,
low losses and high ratings. SiC IGBTs will
remarkably extend the range power ratings and
switching frequencies
- DC-link voltage has fixed polarity. Film-type DC
capacitors offer high density, smaller size, and
higher reliability.
-AC voltage is pulsed. Reflecting waves can be yielde
with long cables between the converter the load (e.
motor)
- Dominating in broad band of applications (up toHVDC light) with increasing capacity.
Output voltage and load current in a 3-phVSC
Cl ifi ti f C t
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Classification of Converters
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According to DC-Source Characteristics
- Three-phase VSC motor drive with
long cable. Unmatched impedance causes
high voltage reflected wave that can damage
winding insulation.
- Even in short cables, this can be a problem
when the frequency of oscillation matches
internal high frequency resonant modes in
the motor.
Typical line-to-ground voltage oscillation in a
PWM-based induction motor drive with 40mcable due to the reflected-wave phenomenon.
One solution to this problem is usematching and damping filter (e.g. RLC filter)
Pulse at themotor
Pulse at theinverter
the other solution for reflecting wave problem (resonance) is using CSC
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Classification of Converters
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According to DC-Source Characteristics - AC voltage is greater than dc voltage
Buck (step-down) ac/dc converter (buck rectifier)
Or
Boost (step-up) dc/ac converter (boost inverter)
- Bulky dc-inductor is usually needed (high losses)
- Switching cells cant be used. Switches should offer
unidirectional current flow and bipolar voltage
blocking. Although bipolar versions of the GTO andthe IGCT are commercially available, they are
limited in terms of switching speed and are mainly
tailored for very high-power
electronic converters.
- DC-link current has fixed polarity.
- AC voltage is sinusoidal. This is a good feature for
large motors with long cables (e.g. submerged ac
motors).
- Less frequently used in the industry.
Three-phase current-source converter
Id
Cl ifi ti f C t
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Classification of Converters
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According to Source Characteristics
Three-phase impedance-source (Z-source) converter
-The impedance network at the input givesthe converter a buck-boost nature in both
rectifier and inverter modes.
-Higher utilization of passive components
increases the losses.
-Not highly accepted in the industry due to
higher losses
B i C fi ti d S it hi St t i i VSC
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Basic Configurations and Switching Strategies in VSCs
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Multi-module VSCs
Square-Wave VSCs
Sinusoidal Pulse-Width Modulated (PWM)
Multi-level VSCs
Selective harmonic Elimination
Voltage Cancellation (for single phase
VSCs)
Space Vector Pulse Width Modulation
(SVPWM)
Sinusoidal PWM with 3rd-Harmonic
Injection
Switching StrategiesBasic Configurations
Single
Phase Half-bridge2-level VSC
SinglePhase Full-bridge
2-level VSC
3-Phase Full-Bridge 2-Level VSC
R f
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References
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- Mohan, Undeland and Robbins, Power Electronics: Converters, Applications, and
Design, 3rd Edition, John Wiley & Sons, Inc., 2003.
-Yazdani and R. Iravani, Voltage-sourced converters in power systems, Modeling,
Control, and Applications, John Willy, 2010.
- Hui Zhang, and Leon M. Tolbert, Efficiency impact of silicon carbide power
electronics for modern wind turbine full scale frequency converter, IEEE Transactions
on Industrial Electronics, vol. 58, no. 1, pp. 21-28, 2011.
- F. Peng, Z-Source inverters,IEEE Transactions on Industrial Applications, vol. 39, no.,
2, pp. 504 510, 2003.