Thyristor Commutation Techniques 100403040622 Phpapp02

Thyristor Commutation Techniques*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

IntroductionCommutation Process of turning off a conducting thyristor.Current CommutationVoltage Commutation*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Methods of CommutationNatural Commutation

Forced Commutation*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Natural CommutationOccurs in AC circuits*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Natural Commutation of Thyristors takes place in AC voltage controllers.Phase controlled rectifiers.Cyclo converters.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Forced CommutationApplied to dc circuitsCommutation achieved by reverse biasing the SCR or by reducing the SCR current below holding current value.Commutating elements such as inductance and capacitance are used for commutation purpose.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Methods of Forced CommutationSelf commutation.Resonant pulse commutation.Complementary commutation.Impulse commutation.External pulse commutation.Load Commutation.Line Commutation.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Forced Commutationis applied toChoppers.

Inverters.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Self Commutation Or Load Commutation Or Class A Commutation (Commutation By Resonating The Load)*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Circuit is under damped by including suitable values of L & C in series with load.Oscillating current flows.SCR is turned off when current is zero.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Expression for Current

Fig. shows a transformed network*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Expression For Voltage Across Capacitor At The Time Of Turn Off *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

ProblemCalculate the conduction time of SCR and the peak SCR current that flows in the circuit employing series resonant commutation (self commutation or class A commutation), if the supply voltage is 300 V, C = 1F, L = 5 mH and RL = 100 . Assume that the circuit is initially relaxed.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

ProblemFigure shows a self commutating circuit. The inductance carries an initial current of 200 A and the initial voltage across the capacitor is V, the supply voltage. Determine the conduction time of the SCR and the capacitor voltage at turn off.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

The transformed circuit of the previous figure is shown in figure below*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

ProblemIn the circuit shown in figure V = 600 volts, initial capacitor voltage is zero, L = 20 H, C=50F and the current through the inductance at the time of SCR triggering is IO = 350 A. Determine Peak values of capacitor voltage and currentConduction time of T1. *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

To calculate conduction time of T1The waveform of capacitor current is shown in figure.When the capacitor current becomes zero the SCR turns off.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Resonant Pulse Commutation (Class B Commutation)*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Series LC circuit connected across thyristor T.Initially C is charged to V volts with plate a as positive.Current in LC oscillates when SCR is ON.T turns off when capacitor discharges through thyristor in a direction opposite to IL*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Expression For tC , the Circuit Turn Off Time *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

The LC Circuit During The Commutation Period

LC Circuit Transformed Circuit *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Expression For Conduction Time Of SCR*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Alternate CircuitFor Resonant Pulse Commutation*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Initially C charged with polarity as shown in figure.T1 is conducting & IL is constant.To turn off T1, T2 is fired.iC(t) flows opposite to IL & T1 turns off at iC(t) = IL*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Expression For tC*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Resonant Pulse CommutationWith Accelerating Diode *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Diode D2 connected as shown to accelerate discharge.T2 turned on to turn off T1.Once T1 is off at t1. iC(t) flows through D2 until current reduces to IL at time t2.From t = t2 , C charges through load, T2 self commutates.But thyristor recovery process low hence longer reverse bias time.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

ProblemThe circuit in figure shows a resonant pulse commutation circuit. The initial capacitor voltage VC(O)=200V, C = 30F and L = 3H. Determine the circuit turn off time tC, if the load current IL is 200 A and 50 A.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Problem*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

ProblemFor the circuit shown in figure calculate the value of L for proper commutation of SCR. Also find the conduction time of SCR.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

ProblemFor the circuit shown in figure given that the load current to be commutated is 10 A, turn off time required is 40sec and the supply voltage is 100 V. Obtain the proper values of commutating elements.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

ProblemIn a resonant commutation circuit supply voltage is 200 V. Load current is 10 A and the device turn off time is 20s. The ratio of peak resonant current to load current is 1.5. Determine the value of L and C of the commutation circuit. *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Complementary Commutation (Class C Commutation,Parallel Capacitor Commutation)*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Two SCRs are used, turning ON one SCR turns off the other.T1 is fired, IL flows through R1.At same time C charges towards V through R2 with plate b positive.To turn off T1, T2 is fired resulting in capacitor voltage reverse biasing T1 and turning it off.When T2 is fired current through load shoots up as voltage across load is V+VC*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Expression For Circuit Turn Off Time tC*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Problem In the circuit shown in figure, the load resistances R1 = R2 = R = 5 & the capacitance C = 7.5 F, V = 100 volts. Determine the circuit turn off time tC *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

ProblemCalculate the values of RL and C to be used for commutating the main SCR in the circuit shown in figure. When it is conducting a full load current of 25 A flows. The minimum time for which the SCR has to be reverse biased for proper commutation is 40sec. Also find R1, given that the auxiliary SCR will undergo natural commutation when its forward current falls below the holding current value of 2 mA. *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

In this circuit only the main SCR carries the load and the auxiliary SCR is used to turn off the main SCR. Once the main SCR turns off the current through the auxiliary SCR is the sum of the capacitor charging current iC and the current i1 through R1, iC reduces to zero after a time tC and hence the auxiliary SCR turns off automatically after a time tC, i1 should be less than the holding current.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Impulse Commutation(Class D Commutation)*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

C charged to a voltage VC(O) with polarity as shown.T1 is conducting and carries load current IL.To turn off T1, T2 is fired.Capacitor voltage reverse biases T1 and turns it off.C Charges through load.T2 self commutates.To reverse capacitor voltage T3 is turned ON.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Expression For Circuit Turn Off Time tC*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

T1 is turned off by applying a negative voltage across its terminals. Hence this is voltage commutation.tC depends on load current. For higher load currents tC is small. This is a disadvantage of this circuit.When T2 is fired, voltage across the load is V+VC; hence the current through load shoots up and then decays as the capacitor starts charging.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

An Alternative Circuit For Impulse Commutation *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Initially C is charged to VC with top plate positive.T1 is fired, load current IL flows.C discharges at the same time & reverses its polarity.D ensures bottom plate remains positive.To turn off T1, T2 is fired.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

ProblemAn impulse commutated thyristor circuit is shown in figure. Determine the available turn off time of the circuit if V = 100 V, R = 10 and C = 10 F. Voltage across capacitor before T2 is fired is V volts with polarity as shown. *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

When T2 is triggered the circuit is as shown in figure *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Writing the transformcircuit, we obtain*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

The waveform of vC(t) is shown in figure *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

ProblemIn the commutation circuit shown in figure C = 20 F, the input voltage V varies between 180 and 220 V and the load current varies between 50 and 200 A. Determine the minimum and maximum values of available turn off time tC.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

External Pulse Commutation (Class E Commutation) *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

T1 is conducting & RL is connected across supply.T3 is fired & C is charged to 2VAUX with upper plate positive.T3 is self commutated.To turn off T1, T2 is fired.T2 ON results in a reverse voltage VS 2VAUX appearing across T1.*Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Load Side CommutationIn load side commutation the discharging and recharging of capacitor takes place through the load. Hence to test the commutation circuit the load has to be connected. Examples of load side commutation are Resonant Pulse Commutation and Impulse Commutation. *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Line Side CommutationIn this type of commutation the discharging and recharging of capacitor takes place through the supply. *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Thyristor T2 is fired to charge the capacitor C. When C charges to a voltage of 2V, T2 is self commutated. To reverse the voltage of capacitor to -2V, thyristor T3 is fired and T3 commutates by itself. Assuming that T1 is conducting and carries a load current IL thyristor T2 is fired to turn off T1. *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

The turning ON of T2 will result in forward biasing the diode (FWD) and applying a reverse voltage of 2V acrossT1. This turns off T1, thus the discharging and recharging of capacitor is done through the supply and the commutation circuit can be tested without load. *Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Power ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower ElectronicsPower Electronics

Thyristor Commutation Techniques 100403040622 Phpapp02

Documents

Transcript of Thyristor Commutation Techniques 100403040622 Phpapp02