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

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

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

Thyristor Converters Fully controlled converter shown in Fig. 6-1a 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 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

Basic thyristor circuits: Line-frequency voltage source connected to a RL load Initially, the current is zero until wt=a, at which the thyristor is fired during the positive half cycle of vs With the thyristor conducting, current begins to flow, vd = vs Voltage across the inductor: vL=vs-vR During a to q1, vL is positive, and the current increases Beyond q1, vL is negative, and the current begins to decline q2 is the instant at which current becomes zero and stays at zero until 2p+a at which the thyristor is fired again

Basic thyristor circuits: The load consists of L and a dc voltage Ed The thyristor is reverse biased until q1 The thyristor conduction is further delayed until q2 at which the thyristor is fired With the thyristor conducting, vd = vs Between q2 to q3, vL is positive, and the current increases Beyond q3, vL is negative, and the current begins to decline When A1 is equal to A2, current goes to zero at q4

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.

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

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

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

The expression for the average voltage Vd: Average dc Voltage as a Function of the Delay Angle The expression for the average voltage Vd: Let Vd0 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),

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

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

1-Phase Thyristor Converter During the commutation interval, all four thyristors conduct, and therefore, vd=0, and the voltage vLs=vs.

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

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 kVA. Calculate the commutation angle m and Vd/Vd0 with the rated input voltage, power of 3 kW, and a=30o.

Solution

Thyristor Converters: Inverter Mode (Vd is negative) Average value of vd is negative for 90o<a<180o. Average power Pd is negative (Pd=VdId) and thus power flows from the dc to the ac side On the ac side, Pac=VsIs1cosf1 is also negative because f1>90o 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

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 three-phase full bridge diode rectifier and, as a result,

3-Phase Thyristor Converter Waveforms

Average Output DC Voltage

dc-side voltage waveforms as a function of a Vd repeats at six times the line frequency

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