Power electronics introduction to ac voltage controls Module 3.pptx

Ac voltage controllers

1. Module 3 AC Voltage Controllers AC Voltage Controllers: Introduction. Principle of ON-OFF and phase control. Single-phase bidirectional controllers with resistive and inductive loads. Controlled Rectifiers: Introduction. Principle of phase controlled converter operation. Single phase semi-converters. Full converters. Three phase half-wave converters. Three-phase full-wave converters.

2. Introduction Diode rectifiers provide a fixed output voltage only. To obtain controlled output voltages, phase controlled thyristors are used instead of diodes. The output voltage of thyristor rectifiers is varied by controlling the delay or firing angle of thyristors . A phase controlled thyristor is turned on by applying a short pulse to its gate and turned off due to natural or line commutation. If load is highly inductive, it is turned off by firing another thyristor of the rectifier during the negative half cycle of input voltage.

3. The phase control converters can be classified into 2 types, depending on input supply: single phase converters and three phase converters. Each type can be sub divided into semi-converter, full converter and dual converter. A semi-converter is a single quadrant converter and it has one polarity of output voltage and current. A full converter is a two quadrant converter and the polarity of its output voltage can be either positive or negative but output current has only one polarity. A dual converter can operate in four quadrants and both the output voltage and current can be either positive or negative.

4. Principle of phase controlled converter operation

5. During positve half cycle of input voltage, the thyristor anode is positive with respect to its cathose and the thyristor is said to be forward biased. When thyristor T1 is fired at wt = α , T1 conducts and the input voltage appears across the load. When the input voltage starts to be negative at wt = π , thyristor anode is negative with respect to its cathode and T1 is said to be reverse biased, and it is turned off. The time after the input voltage starts to go positive until the thyristor is fired at wt= α is called the delay or firing angle, α Here, output voltage and current have only one polarity. This is not normally used in industrial applications because its output has high ripple content and low ripple frequency.

6. Single phase full converters

7. Consider a single phase full converter with highly inductive load so that the load current is continuous and ripple free. During the positive half cycle, T1 and T2 are forward biased and when these two are fired simulatneously at wt = α , the load is connected to the input supply through T1 and T2. Due to inductive load, T1 and T2 continue to conduct beyond wt = π , even though the input voltage is already negative. During the negative half cycle of the input voltage, T3 and T4 are forward biased and firing of T3 and T4 applies the supply voltage across T1 and T2 as reverse blocking voltage. T1 and T2 are turned off due to line or natural commutation and the load current is transferred from T1 and T2 to T3 and T4.

8. During the period from α to π , the input voltage and current are positive and the power flows from the supply to the load. The converter is said to be operating in rectification mode. During the period π to π + α , the input voltage is negative and input current is positive, and reverse power flows from the load to the supply. The converter is said to be operating in inversion mode. This is extensively used in industrial applications up to 15kW. Depending upon the values of α , the average output voltage could be either positive or negative and it provides two quadrant operation.

9. Single phase dual converters Single phase full converters with inductive loads allow only a two quadrant operation. If two of above mentioned converters are connected back to back, both the output voltage and load current flow can be reversed. The system provides a four quadrant operation called dual converter. These are used in high power variable speed drives. The delay angles are controlled such that one converter operates as a rectifier and the other converter operates as an inverter; but both converters produce the same average output voltage.

10. Because the instantaneous output voltages of the two converters are out of phase, there can be instantaneous voltage difference resulting in circulating current reactor Lr . If v01 and vo2 are the instantaneous output voltages of converters 1 and 2, circulating current can be found by integrating the instantaneous voltage difference starting from wt = π + α 1. Because the two average output voltages during the interval wt = π + α 1 to 2 π - α 1 are equal and opposite, their contribution to the instantaneous circulating current i is zero.

11. For α 1=0, only converter 1 operates, for α 1 = π , only converter 2 operates. For 0 ≤ α 1 < π /2, converter 1 supplies a positive load current = io and thus the circulating current can only be positive. For π /2 < α 1 ≤ π , the converter 2 supplies a negative load current – io and thus only a negative circulating current can flow. At α 1 = π /2 , the converter 1 supplies positive circulating during the first half cycle, and the converter 2 supplies negative circulating during the second half cycle. The instantaneous circulating current depends on the delay angle. For α 1=0, its magnitude becomes minimum when wt = n π , n = 0,2,4…. and maximum when wt = n π , n = 1,3,5….

12. The dual converters can be operated with or without circulating current, only one converter operates at a time and carries the load current; and the other converter is completely blocked by inhibiting gate pulses. Operation with circulating current has the following advantages: 1. Circulating current maintains continuous conduction of both converters over the whole control range, independent of the load. 2. Because one converter always operates as a rectifier and the other converter operates as an inverter, power flow in either direction at any time is possible. 3. Because both converters are in continuous conduction, the time response for changing from one quadrant operation to another is faster.

13. Single phase Semi-converters

14. The load current is assumed to be continuous and ripple free. During the positive half cycle, T1 is forward biased. When T1 is fired at wt = α , the load is connected to input supply through T1 and D2 during the period α ≤ wt ≤ π . During the period from π ≤ wt ≤ π + α , the input voltage is negative and the free wheeling diode Dm is forward biased. Dm conducts to provide the continuity of current in the inductive load. The load current is transferred from T1 and D2 to Dm, T1 and diode D2 are turned off. During the negative half cycle of input voltage, T2 is forward biased, and the firing of T2 at wt = π + α reverses bias Dm. The diode Dm is turned off and the load is connected to the supply through T2 and D1.

15. AC Voltage Controllers-Introduction If a thyristor is connected between ac supply and load, the power flow can be controlled by varying the rms value of ac voltage applied to the load and this type of power circuit is known as an ac voltage controller. The most common applications are: industrial heating, on load transformer connection changing, light controls, speed control of polyphase induction motors and ac magnet controls. There are two types of control: On Off control and Phase angle control In On-Off control, thyristor switches connect the load to the ac source for a few cycles of input voltage and then disconnect it for another few cycles. In phase control, thyristor switches connect the load to the ac source for a portion of each cycle of input voltage.

16. The ac voltage controllers can be classified into two types : 1. Single phase controllers and 2 . Three phase controllers which is subdivided into Unidirectional or half wave control Bidirectional or full wave control. Because the input voltage is ac, thyristors are line commutated and phase control thyristors are relatively inexpensive and slower than fast switching thyristors , they are normally used. Due to line or natural commutation, there is no need of extra commutation circuitry and the circuits for ac voltage controllers are very simple.

17. Principle of On-Off control

18. The thyristor switch connects the ac supply to load for a time t n , the switch is turned off by a gate pulse inhibiting for time t . The on time t n usually consists of an integral number of cycles. The thyristors are turned on at the zero voltage crossings of ac input voltage. This type of control is applied in applications that have a high mechanical inertia and high thermal time constant. Due to zero voltage and zero current switching of thyristors , the harmonics generated by switching actions are reduced. For a sinusoidal input voltage, v s = V m sin 𝟂t = √2 V s sin 𝟂t. If the input voltage is connected to load for n cycles and is disconnected for m cycles, the rms output voltage can be found by Vo =[ ] = Vs √(n/( m+n )) = Vs √k where k = n/( m+n ) and k is called the duty cycle. Vs is the rms phase voltage.

19. Principle of Phase Control The power flow to the load is controlled by delaying the firirng angle of thyristor T 1 . Due to the presence of diode D 1 , the control range is limited and the effective rms output voltage can only be varied between 70.7 and 100%. The output voltage and input current are symmetric and contain a dc component. If there is an input transformer, it may cause a saturation problem. This circuit is a single phase half wave controller and is suitable only for low power resistive loads, such as heating and lighting. Because the power flow is controlled during the positive half cycle of input voltage, this type of controller is known as unidirectional controller.

20. Single phase bidirectional controllers with resistive loads The problem of dc input current can be prevented by using bidirectional control, and a single phase full wave controller with a resistive load. During the positive half cycle of input voltage, the power flow is controlled by varying the delay angle of thyristor T1; and thyristor T2 controls the power flow during the negative half cycle of input voltage. The firing pulses of T1 and T2 are kept 180 o apart.

21. By varying α from 0 to π, Vo can be varied from Vs to 0. The gating circuits for T1 and T2 must be isolated. It is possible to have a common cathode for T1 and T2 by adding two diodes . T1 and d1 conduct together during positive half cycle and T2 and D2 conduct during negative half cycle. Since this circuit can have a common terminal for gating signals of T1 and T1, only one isolation circuit is required, but at the expense of two diodes. Due to two power devices conducting at same time, the conduction losses of devices would increase and efficiency would be reduced.

22. A single phase full wave controller can also be implemented with one thyristor and four diodes. The four diodes act as a bridge rectifier. The voltage across thyristor T1 and its current are always unidirectional. With a resistive load, the thyristor current would fall to zero due to natural commutation in every half cycle.

23. If there is a large inductance in the circuit, T1 may not be turned off in every half cycle of input voltage, and this may result in a loss of control. It would require detecting the zero crossing of the load current to gurrantee turn off of the conducting thyristor before firing the next one. Three power devices conduct at the same time and the efficiency is also reduced. The bridge rectifier and thyristor act as a bidirectional switch, which is commercially available as a single device with a relatively low on state conduction loss. Gating sequence: Generate a pulse signal at the positive zero crossing of the supply voltage vs. Delay the pulse by the desired angle α for gating T1 through a gate isolating circuit. Generate another pulse of delay angle α + π for gating T2.

24. The gating signals of thyristors could be short pulses for a controller with resistive loads but not suitable for inductive loads. When T2 is fired at 𝟂t = π + α, T1 is still conducting due to load inductance. By the time the current of T1 falls to zero and T1 is turned off at 𝟂t = β = π + δ, the gate pulse of T2 has already ceased and T2 cannot be turned on. As a result, only T1 opertes , causing asymmetric waveforms of output voltage and current. This can be resolved by using continuous gate signals with a duration of (π – α). As soon as the current of T1 falls to zero, T2 would be turned on. A continuous gate pulse increases the switching loss of thyristors and requires a larger isolating transformer for the gating circuit.

25. A train of pulses with short durations are normally used. Load voltage and current can be sinusoidal if the delay angle α is less than the load angle ϴ. If α is greater than ϴ, the load current would be discontinuous and nonsinusoidal . Notes: If α = ϴ, sin (β - ϴ) = sin (β - α) = 0 and β – α = δ = π Because the conduction angle δ cannot exceed π and the load current must pass through zero, the delay angle α may not be less than ϴ and the control range of delay angle is ϴ ≤ α ≤ π. If α ≤ ϴ and the gate pulses of thyristors are of long duration, the load current would not change with α, but both thyristors would conduct for π. T1 would turn on at 𝟂t = ϴ and T2 would turn on at 𝟂t = π + ϴ.  

26. G ating sequence: Generate a train of pulse signal at the positive zero crossing of the supply voltage vs. Delay this pulse by the desired angle α for gating T1 through a gate isolating circuit. Generate another continuous pulse of delay angle α + π for gating.