TSTE25 Power Electronics. Lecture 4 Tomas Jonsson ISY/EKS

Transcription

1 TSTE25 Power Electronics Lecture 4 Tomas Jonsson ISY/EKS

2 Outline The thyristor Controlled rectifier and inverters Single phase Three phase

3 Thyristors Only possible to turn on Pulse on gate when forward blocking turns on thyristor Current reversal followed by reverse blocking turns off thyristor 3 modes: Forward blocking On-state Reverse blocking

4 Actual thyristor characteristics

5 Thyristor, example circuit Thyristor can be triggered in interval 0 < t < T/2

6 Thyristor turn-off process Succesful turn-off: Current reversal reverse recovery as a diode Reverse blocking for t t q

7 Thyristor presspack Active part on a single silicon wafer. Capacitor TCU Resistors Thyristor

8 Thyristor, junction structure Power Semiconductors by Stefan Linder EPFL press 2006

9 Thyristor triggering to latched on-state I C2 = β 2 I G : common emitter current gain I B1 = I C2 I C1 = β 1 I B1 I B2 = I C1 I C1 = β 1 β 2 I G + I C1 Thyristor trigger at β 1 β 2 1 β = α where : common 1 α base current gain α 1 + α 2 1 Power Semiconductors by Stefan Linder EPFL press 2006

10 Thyristor, non-ideal characteristics I H = Holding current V H = holding voltage I BO = Breakover current V BO = Breakover voltage i G can be a current pulse V RW = Reverse working voltage Thyristors found with V RW > 7kV Forward voltage drop only a few volts even at I A > 2kA

11 Thyristor turn-off timing Turn-off similar to diode Thyristor turn-off require negative V AK longer than t q (otherwise still on) dv F /dt limit to keep thyristor off (fast forward voltage increase may turn on thyristor!)

12 dv A /dt triggering Forward blocking Junction J2 is depleted and has a capacitance High dv A /dt will charge C J2. Charging current through pn-junction J3 may trigger the thyristor G Power Semiconductors by Stefan Linder EPFL press 2006

13 Thyristor types Phase-control Used for rectifying line-frequency voltages and currents, low on-state voltage and large voltage and current range > 4 ka, > 7 kv, V on-state voltage Inverter-grade Small turn-off time (1 us < tq < 100 us) and low on-state voltages Light-triggered Trigger by light pulse instead of current Used in high-voltage applications, where series connected thyristors used to support high voltages ~ 5 mw light pulse to trigger Datasheet

14 Support current in both directions: Triac Function as antiparallell thyristor Enable triggering on both positive and negative part of the cycle

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

16 How to control gate IC circuits available α = 180 v control Vˆst

17 ABB Power Technologies Power Systems DC Thyristor triggering in HVDC applications IP FP TCU FP TMU IP FP = Firing pulse IP = Indicating pulse Electrically triggered thyristor (ETT) The ETT is triggered by the thyristor control unit (TCU). The TCU is energized by the main circuit, and triggering is initiated by an optical pulse The TCU protects and monitors the ETT Light-triggered thyristor (LTT) The LTT is triggered directly by an optical pulse The LTT is self-protected against overvoltage Separate recovery protection is provided The TMU monitors the LTT

18 Full bridge thyristor converters Includes source inductance L s and load inductance L d Source inductance

19 Single-phase thyristor converter Either thyristor T1 and T2, or T3 and T4 conducting

20 Single-phase thyristor converter waveforms I d constant => thyristor does not turn off unless other thyristor turns on (current commutation) V d and P controlled by α π+α V dα = 1 π α 2V s sinωtd ωt = 2 2 π V scosα = 0.9V s cosα T 1 P = I d T v d dt = I d V d = 0.9V s I d cosα 0

21 DC voltage level Power from DC to AC side when α > 90 degrees

22 Line current properties I s1 = 0.9I d I s = I d (rms) THD i = 48.3% DPF = cosφ 1 = cosα

23 Power, power factor, reactive power DPF = cosφ 1 = cosα PF = I s1 I s DPF = 0.9cosα P = V s I s1 cosφ 1 = 0.9V s I d cosα Q 1 = V s I s sinφ 1 = 0.9V s I d sinα S 1 = V s I s1

24 L s effect A u = 2ωL s I d cos α + u = cosα 2ωL si d 2V s V d = 0.9V s cosα 2 π ωl si d

25 Inverter mode Negative dc-side voltage, v d, E d Always positive current. Thyristor conducts in one direction only R d I d = V d ( E d ) R d P<0: (v d <0)(i d >0) Power flows from DC to AC side V s controls switching of current between thyristors

26 Inverter thyristor voltage Extinction angle γ = 180 (α + u) Thyristor require t γ = γ/ω > t q (turn off time) Failing requirement leads to retriggering of the thyristor too early (commutation failure) => large currents (all thyristors conduct for long time)! α 1,2

27 Three-phase thyristor converter One thyristor active in top group and one in bottom group

28 Thyristor rectifier operation Phase Angle To load Star 0 winding 6-pulse Graetz rectifier bridge From load

29 Thyristor rectifier operation Phase Angle increased Reduced average To load 0 Star winding 6-pulse Graetz rectifier bridge From load

30 Converter waveforms V dα = 3 2 π V LLcosα = = 1.35V LL cosα

31 DC side voltage AC ripple frequency six times the line frequency

32 Input line current RMS current: I s = 2 3 I d I s1 = 1 π 6I d = 0.78I d THD i = σ h=5,7,11,13.. I 2 sh = 31% I s1

33 Line current vs alpha Current phase shift equal to DPF = cosφ 1 = cosα PF = I s1 I s DPF = 3 π cosα

34 Exercise In the ideal three-phase thyristor rectifier circuit, the firing angle =30 deg a) Construct the thyristor T 1 voltage and current b) Construct the wave form of the dc-output voltage c) Calculate the average dc-output voltage for V LL =400V

35 Active and reactive power vs alpha DPF = cosφ 1 = cosα PF = I s1 I s DPF = 3 π cosα P = 3V LL I s1 cosφ 1 = 3V LL 0.78I d cosα = 1.35V LL I d cosα Q 1 = 3V LL I s1 sinφ 1 = 3V LL 0.78I d sinα = 1.35V LL I d sinα

36 Converter with source inductance V d = 3 2 π V LLcosα 3ωL s π I d

37 Input line current waveform with source inductance DPF cos (α + ½ u) cos α cos α + u = 2ωL s 2V LL I d

38 Inverter waveform

39 Inverter voltage over thyristor Extinction angle γ = 180 (α + u) Thyristor require t γ = γ/ω > t q (turn off time) Failing requirement leads to retriggering of the thyristor too early (commutation failure) => large currents (both thyristors in the same phase conducts)!

40 Exercise In the three-phase thyristor rectifier circuit with the following data: V LL = 400 V at 50 Hz, L s = 7 mh, I d = 10A a) What firing angle shall be used to get an average dc-voltage of 500V (rectifier mode) b) What firing angle shall be used to get an average dc-voltage of -500V (inverter mode) c) Calculate. What minimum t q is required?

41 12-pulse converter TSTE25/Tomas Jonsson Star Primary winding winding Delta 6-pulse Graetz bridge winding The Delta winding shifts the phase with 30º electrical versus the Star winding 12-pulse group Control & Protection System

42 Harmonic currents on the AC side of a converter TSTE25/Tomas Jonsson i 1 Y Y i 1 + i2 i 1 T /4 T /2 3 T / 4 Phase current i 2 Y D i 2 i + i 1 2 In I1 [%] n In i 1 + i2 [%] n

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