VSC Course Lecture2

7/28/2019 VSC Course Lecture2

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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 Electronics Switches

4

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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5

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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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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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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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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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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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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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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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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Switching waveforms of a Silicon Carbide (SiC)

MOSFET (Cree) at 225C.


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15


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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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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17

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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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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20

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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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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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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- 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

Cl ifi ti f C t


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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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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.

VSC Course Lecture2

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