tste19 power electronics · • controlled rectifier and inverters – single phase – three phase...

TSTE19 Power ElectronicsLecture 4

Tomas JonssonISY/EKS

Outline

• The thyristor

• Controlled rectifier and inverters

– Single phase

– Three phase

2015-11-10 2TSTE19/Tomas Jonsson

Thyristors

• Only possible to turn on

• Pulse on gate whenforward blocking turns on thyristor

• Current reversal followed by reverseblocking turns off thyristor

• 3 modes:

– Forward blocking

– On-state

– Reverse blocking


Actual thyristor characteristics2015-11-10 4TSTE19/Tomas Jonsson

Thyristor, example circuit

• Thyristor can be triggered in interval0 < t < T/2


Thyristor turn-off process

Succesful turn-off:

• Current reversal

– reverse recovery as adiode

• Reverse blocking for t ³ tq


Thyristor presspack• Active part on a

single silicon wafer.


Thyristor

Capacitor

Resistors

TCU

Thyristor, junction structure2015-11-10 8TSTE19/Tomas Jonsson

Power Semiconductorsby Stefan LinderÓEPFL press 2006

Thyristor triggering to latched on-state• =

– b: common emitter currentgain

• =• =• =• = +• Thyristortriggerat

≥ 1

• = where a: common

base current gain

• ⇒ + ≥ 1



Thyristor, non-ideal characteristics

• IH = Holding currentVH = holding voltage

• IBO = Breakover currentVBO = Breakover voltage

• iG can be a current pulse

• VRW = Reverse workingvoltage

– Thyristors found with VRW > 7kV

• Forward voltage drop only a few volts even at IA > 2kA


Thyristor turn-off timing

• Turn-off similar to diode

• Thyristor turn-off requirenegative VAK longerthan tq (otherwise still on)

• dVF/dt limit to keepthyristor off(fast forward voltageincrease may turn onthyristor!)


dVA/dt triggering

• Forward blocking

• Junction J2 isdepleted and has acapacitance

• High dVA/dt willcharge CJ2.

• Charging currentthrough pn-junctionJ3 may trigger thethyristor


G


Improved dVA/dt capability

• Emitter shorts

• Provide a currentpath bypassing thepn-junction J3

• On-state voltagedrop increasedrelated to



Thyristor types• Phase-control

– Used for rectifying line-frequency voltages and currents, lowon-state voltage and large voltage and current range

– > 4 kA, > 7 kV, 1.5 - 3 V on-state voltage

• Inverter-grade

– Small turn-off time (1 us < tq < 100 us) and low on-statevoltages

• Light-triggered

– Trigger by light pulse instead of current

– Used in high-voltage applications, where series connectedthyristors used to support high voltages

– ~ 5 mW light pulse to trigger


Datasheet

Support current in both directions: Triac

• Function as antiparallell thyristor

• Enable triggering on both positive andnegative part of the cycle


Lecture 4Thyristor operation• Gate control• Single phase• Three phase• Rectifier vs inverter mode

2015-11-1016TSTE19/Tomas Jonsson

How to control gate

• IC circuits available


α = 180 ˆ

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Thyristor triggering in HVDC applications

Electrically triggered thyristor(ETT)

n The ETT is triggered by the thyristorcontrol unit (TCU).

n The TCU is energized by the main circuit,and triggering is initiated by an opticalpulse

n The TCU protects and monitors the ETT

Light-triggered thyristor (LTT)n The LTT is triggered directly by an optical pulse

n The LTT is self-protected against overvoltage

n Separate recovery protection is provided

n The TMU monitors the LTT

TMUFP IPTCUFPIP

FP = Firing pulse

IP = Indicatingpulse

Full bridge thyristor converters

• Includes source inductance Ls and load inductance Ld

• Source inductance


Single-phase thyristor converter

• Either thyristor T1 and T2, or T3 and T4 conducting


Single-phase thyristor converter waveforms

• Id constant => thyristordoes not turn off unlessother thyristor turns on(current commutation)

• Vd and P controlled by α


=1π ∫ 2 sinω ω =

2 2π cosα = 0.9V cosα

=1∫ = = 0.9V cosα

DC voltage level

• Power from DC to AC side when α > 90 degrees


Line current properties2015-11-10 23TSTE19/Tomas Jonsson

= 0.9= (rms)

= 48.3%

Power, power factor, reactive power2015-11-10 24TSTE19/Tomas Jonsson

= cosΦ = cosα

= = 0.9cosα

= cosΦ = 0.9 cosα= sinΦ = 0.9 sinα

=

Ls effect2015-11-10 25TSTE19/Tomas Jonsson

= 2ω cos α + = cosα −2ω2

= 0.9 cosα −2πω

Inverter mode• Negative dc-side voltage, vd, Ed

• Always positive current. Thyristorconducts in one direction only

• P<0: (vd<0)(id>0)

• Power flows from DC to AC side

• Vs controls switching of currentbetween thyristors


Inverter thyristor voltage• Extinction angle γ = 180 – (α + u)

• Thyristor require tγ = γ/ω > tq (turn off time)

• Failing requirement leads to retriggering of the thyristor tooearly (commutation failure)=> large currents (all thyristors conduct for long time)!


,

Three-phase thyristor converter

• One thyristor active in top group and one in bottom group


Thyristor rectifier operation

Star

winding

6-pulse Graetzrectifier bridge

To load

From load

0

Phase Angle

Thyristor rectifier operation

Star

winding

6-pulse Graetzrectifier bridge

To load

From load

0

Phase Angle increased èReduced average

Converter waveforms2015-11-10 31TSTE19/Tomas Jonsson

=3 2π cosα

= 1.35 cosα

DC side voltage

• AC ripple frequency sixtimes the line frequency


Input line current2015-11-10 33TSTE19/Tomas Jonsson

THD =∑ , , , …..

= 31%

=1π 6 = 0.78

: =23

Line current vs alpha2015-11-10 34TSTE19/Tomas Jonsson

• Current phase shift equal toa

= cosΦ = cosα= = cosα

Exercise 4-100

• In the ideal three-phasethyristor rectifier circuit, thefiring angle a=30 deg

a) Construct the thyristorT1 voltage and current

b) Construct the waveform of the dc-outputvoltage

c) Calculate the averagedc-output voltage forVLL=400V


Active and reactive power vs alpha2015-11-10 36TSTE19/Tomas Jonsson

= cosΦ = cosα

= =3

cosα

= 3 cosΦ = 3 0.78 cosα = 1.35 cosα= 3 sinΦ = 3 0.78 sinα = 1.35 sinα

Converter with source inductance2015-11-10 37TSTE19/Tomas Jonsson

=3 2π cosα −

3ωπ

Input line current waveform

• DPF » cos (α + ½ u)


cos − cos + =22

Inverter waveform2015-11-10 39TSTE19/Tomas Jonsson

Inverter voltage over thyristor

• Extinction angleγ = 180 – (α + u)

• Thyristor requiretγ = γ/ω > tq(turn off time)

• Failing requirement leadsto retriggering of thethyristor too early(commutation failure)=> large currents (boththyristors in the samephase conducts)!


Exercise 4-101• In the three-phase thyristor

rectifier circuit with thefollowing data:VLL = 400 V at 50 Hz, Ls = 7 mH,Id = 10A

a) What firing angle a shallbe used to get an averagedc-voltage of 500V(rectifier mode)

b) What firing angle a shallbe used to get an averagedc-voltage of -500V(inverter mode)

c) Calculate g. Whatminimum tq is required?


12-pulse converter

Star

winding

Delta

winding

6-pulse Graetzbridge

12-pulse groupThe Delta winding shifts thephase with 30º electricalversus the Star winding

Primary

winding

Control & ProtectionSystem

T/4 T /2 3 T / 4

[%]

[%]

105

5

5 7 11 13 17 19 23 25

11 13 23 25

n

n

i1

i2

i + i1 2

S Ii + i

n1 2

IIn1

Phasecurrent

Y

Y

Y

D

i1

i2

i + i1 2

Harmonic currents on the AC side of aconverter

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tste19 power electronics · • controlled rectifier and inverters – single phase – three phase...

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