Thyristor Converters Chapter 6 In some applications

Thyristor Converters Chapter 6 • In some applications (battery charger, some ac/dc drives), the dc voltage has to be controllable • Thyristor converters provide controlled conversion of ac into dc • Primarily used in three-phase, high power application • Being replaced by better controllable switches 6 -1

Thyristors (Review Class) • Semi-controlled device • Latches ON by a gate-current pulse if forward biased • Turns-off if current tries to reverse 6 -2

Thyristor in a Simple Circuit (Review Class) • For successful turn-off, reverse voltage required 6 -3

Thyristor Converters • Fully controlled converter shown in Fig. 6 -1 a • Average dc voltage Vd can be controlled from a positive maximum to a negative minimum on a continuous basis • The converter dc current Id can not change direction • Two-quadrant operation • Rectification mode (power flow is from the ac to the dc side): +Vd & +Id • Inverter mode (power flow is from the dc to the ac side): : -Vd & +Id • Inverter mode of operation on a sustained basis is only possible if a source of 6 -4 power, such as batteries, is present on the dc side.

• Basic thyristor circuits: Line-frequency voltage source connected to a load resistance • In the positive half cycle of vs, the current is zero until wt=a, at which a gate pulse of a short duration is applied • With the thyristor conducting, vd = vs • vd becomes zero at wt = p • By adjusting the firing angle a, the average dc voltage Vd and current Id can be controlled 6 -5

o Basic thyristor circuits: Line-frequency voltage source connected to a RL load o Initially, the current is zero until wt=a, at which the thyristor is fired during the positive half cycle of vs o With the thyristor conducting, current begins to flow, vd = vs o Voltage across the inductor: v. L=vs-v. R o During a to q 1, v. L is positive, and the current increases o Beyond q 1, v. L is negative, and the current begins to decline o q 2 is the instant at which current becomes zero and stays at zero until 2 p+a at which the thyristor is fired again 6 -6

o Basic thyristor circuits: The load consists of L and a dc voltage Ed o The thyristor is reverse biased until q 1 o The thyristor conduction is further delayed until q 2 at which the thyristor is fired o With the thyristor conducting, vd = vs o Between q 2 to q 3, v. L is positive, and the current increases o Beyond q 3, v. L is negative, and the current begins to decline o When A 1 is equal to A 2, current goes to zero at q 4 6 -7

Thyristor Gate Triggering • Generation of the firing signal • The sawtooth waveform (synchronized to the ac input) is compared with the control signal vcontrol, and the delay angle a with respect to the positive zero crossing of the ac line voltage is obtained in terms of vcontrol and the peak of the sawtooth waveform Vst. 6 -8

Full-Bridge (Single- and Three-Phase) Thyristor Converters 6 -9

Single-Phase Thyristor Converters • One thyristor of the top group and one of the bottom group will conduct • If a continuous gate pulse is applied then this circuit will act like a full bridge diode rectifier and the web forms are as shown below • a=0 for 1 and 2 and a=p for thyristors 3 and 4 6 -10

1 -Phase Thyristor Converter Waveforms • Assumptions: Ls=0 and purely dc current Id • a: delay angle or firing angle • Prior to wt=0, current is flowing through 3 and 4, and vd = -vs • Beyond wt=0, thyristors 1 and 2 become forward biased, but cannot conduct until a. • vd becomes negative between 0 and a as a consequence of the delay angle • At wt=a, gate pulse applied and current commutation from thyristors 3 and 4 to 1 and 2 is instantaneous (Ls = 0), and vd = vs • Thyristors 1 and 2 will keep conducting until 3 and 4 are fired 6 -11

Average dc Voltage as a Function of the Delay Angle The expression for the average voltage Vd: Let Vd 0 be the average dc voltage with a=0, Then, drop in average voltage due to a, The average power through the converter, With a constant dc current (id=Id), 6 -12

Average dc Output Voltage The variation of Vd as a function of a: Average dc voltage is positive until a=90 o: this region is called the rectifier mode of operation Average dc voltage becomes negative beyond a=90 o: this region is called the inverter mode of operation 6 -13

1 -Phase Thyristor Converter o AC side inductance is included, which generally cannot be ignored in practical thyristor converters. o For a given delay angle, there will be a finite commutation interval o Commutation process is similar to that in diode bridge rectifiers o During the commutation interval, all four thyristors conduct, and therefore, vd=0, and the voltage v. Ls=vs. 6 -14

1 -Phase Thyristor Converter o During the commutation interval, all four thyristors conduct, and therefore, vd=0, and the voltage v. Ls=vs. 6 -15

1 -Phase Thyristor Converter: with and without Ls with Ls o Voltage drop due to the inclusion of Ls. 6 -16

Example In the converter circuit, Ls is 5% with the rated voltage of 230 V at 60 Hz and the rated volt-ampere of 5 k. VA. Calculate the commutation angle m and Vd/Vd 0 with the rated input voltage, power of 3 k. W, and a=30 o. 6 -17

Solution 6 -18

Thyristor Converters: Inverter Mode (Vd is negative) • Average value of vd is negative for 90 o90 o • Inverter mode of operation is possible because there is a source of energy on the dc side • ac side voltage source provides commutation of current from one pair of thyristors to the others 6 -19

3 -Phase Thyristor Converters • Current Id flows through the one thyristor of the top group and one of the bottom group • If a continuous gate pulse is applied then this circuit will act like a threephase full bridge diode rectifier and, as a result, 6 -20

3 -Phase Thyristor Converter Waveforms 6 -21

Average Output DC Voltage 6 -22

vdc-side voltage waveforms as a function of a v. Vd repeats at six times the line frequency 6 -23

Conclusions • Thyristor converters provides controlled transfer of power between the line frequency ac and adjustable-magnitude dc • By controlling a, transition from rectifier to inverter mode of operation can be made and vice versa • Thyristor converters are mostly used at high-power levels • Thyristor converters inject large harmonics into the utility system 6 -24