Power Electronics- AC -AC Converters.pptx

This PPT is about AC-AC Converters, including AC Voltage controllers and Cycloconverters, their types, and control.

1. 20EE014 POWER ELECTRONICS UNIT 5 - AC to AC Converters D.Poornima , Assistant Professor ( Sr.Gr ), Department of EEE, Sri Ramakrishna Institute of Technology, Coimbatore

2. INTRODUCTION Power flow through ac system can be controlled by adding series impedance This will cause unwanted voltage drop across the impedance and some power is dissipated Auto-transformers or variacs are used to control the voltage across load, but is inefficient. Nowadays silicon controlled rectifiers (SCRs) are used as ac voltage controller, equivalent to auto transformer. This method is very efficient Ac voltage controller blocks the unwanted power flow rather than dissipate at the control device 2

3. 3 SCRs have capability to flow current in one direction only. When two SCRs are connected back to back, it is possible to flow current in bidirectional and is used in ac circuits Ac-to-ac converters are used to vary the RMS output voltage at load at constant frequency and these converters are called as ac voltage controllers or ac voltage regulators. When ac-to-ac converters converts a fixed frequency and voltage system into another ac system at different frequency with variable voltage, these converters are known as cycloconverters . In cycloconverter, there is no intermediate converter stage.

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5. 5 Semiconductor switches such as thyristors or triacs are used in ac voltage controllers. Thyristors or triacs are natural or line commutated - no requirement of commutation circuit Disadvantage - introduction of harmonics in the ac supply voltage and output voltage Applications of Cycloconverters area of high-power low speed induction motor drives applications specially used in cement mills, rolling mills Applications of AC Voltage Controllers Lighting control system Temperature and heating control system Speed control of induction motors On line tap changing of transformers, etc. .

6. CONTROL STRATEGIES IN AC VOLTAGE CONTROLLERS 6

7. Integral cycle control (On-Off Control) Load power can be controlled by connecting the source to the load for a few complete cycles and then disconnecting the source from the load for another number of cycles This controller is also know as burst-firing or cycle selection or cycle syncopation . Harmonics and radio frequency interference are very low in this controller. 7

8. Integral cycle control (On-Off Control) Suitable for systems with large time constants Average power to the load can be varied from 0% to 100% Used for heating loads and motor speed control 8

9. AC Phase Control Load power can be controlled by delaying the instant of firing of the thyristors By this, only a part of the voltage waveform appears across the load, providing voltage control Suitable for loads with short time constants 9

10. Single Phase Voltage Controller with R Load 10

11. Single Phase Voltage Controller with R Load RMS output voltage across load at firing angle α 11 RMS output voltage can be controlled from V to 0 by changing firing angle from 0° to 180°. The rms value of load current is I = V /R The average value of output voltage over any complete cycle is zero The average value of output voltage for half cycle duration is given by

12. Single Phase Voltage Controller with RL Load 12

13. Three Phase AC Voltage Controllers

14. S ingle-phase ac voltage controllers in high power load applications, the distorted waveform of line current creates the unbalance in three phase line current. T his unbalance line current flows through neutral which must be always avoided. Therefore, three phase ac voltage controllers should be used in high power applications in place of single phase ac voltage controllers. There are different circuit configurations of three phase Ac voltage controllers 14

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16. Three Phase Voltage Controller with Y Connected R Load 16

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18. In three phase ac voltage controller, two or three thyristors of different phases must conduct at a time. Single pulse gate signal cannot be used, a train of gate trigger signal is used for the whole conduction period. In R load, thyristor conduction starts at ωt = 30° and stops at ωt =180°. Range of firing angle is 150° only i.e 0° ≤ α ≤ 150° To get a balanced line current, the thyristors must be triggered symmetrically. Triggering sequence will be α , α + 120°, α + 240° for thyristor T1, T3 and T5 respectively. Thyristors T2, T4 and T6 are triggered at α + 180°, α + 300°, α + 420° respectively The operation of converter can be divided into 6 modes of operation 18

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26. Performance Control of AC Voltage Controllers

27. Power Factor Control in AC Voltage Controllers Power factor control, is the process of reducing the amount of reactive power. The device used in this case is called a power factor controller PFC From the power triangle, the reactive power is at right angle (90°) to the true power and is used to energize the magnetic field. Although reactive power does not have a real value in electronic equipment, the bill for electricity comprises real and reactive power costs. 27

28. Power Factor Control in AC Voltage Controllers Power factor - ratio of the real power in kW to the reactive power in kVAr Value ranges from 0 to 1. If power factor is 0.8 and above, device is said to be using power efficiently. Incorporating a PFC ensures the power factor ranges from 0.95 to 0.99. Power factor controllers (PFC) are mainly used in industrial equipment to minimize reactive power generated by fluorescent lighting and electric motors. To ensure power factor is improved without causing harmonic distortion, the conventional capacitors should not be used. Instead, filters combination of capacitors and reactors for harmonic suppression are used. 28

29. Power Factor Control in AC Voltage Controllers This type of harmonic filter is referred to as a single tuned filter. A quality factor Q of this filter is defined as quality factor of its reactance (XL) at Q tuning frequency where Q is given by ( nXL /R). 29

30. Sequence Control of AC Voltage Controllers Sequence control is employed for the improvement of system performance and for the reduction of harmonics in the input current and output voltage. Sequence control means the use of two or more stages of voltage controllers in parallel for the regulation of output voltage. The term ‘sequence control’ means that the stages of voltage controllers in parallel are triggered in a proper sequence one after the other so as to obtain a variable output with low harmonic content. There are two stage and multistage sequence controllers 30

31. Multistage Sequence Control Multistage sequence control is employed when it is desired to have harmonic content lower than that in a two-stage sequence control. The transformer has n secondary windings. Each secondary is rated for v s /n where v s is the source voltage. Voltage of terminal a with respect to 0 is v s Voltage of terminal b is (n — 1)v s /n and so on. 31

32. Multistage Sequence Control If voltage control from v do = (n — 3) v s /n to v co = (n — 2) v s /n is required, then thyristor pair 4 is fired at α = 0° and the firing angle of thyristor pair 3 is controlled from α = 0° to 180° whereas all other thyristor pairs are kept off. . 32

33. Multistage Sequence Control For controlling the voltage from v bo = (n.— 1) v s /n to v ao = v s thyristor pair 2 is triggered at α = 0° whereas for pair 1, α is varied from 0° to 180° by keeping the remaining (n — 2) SCR pairs off. Thus, the load voltage can be controlled from v s /n to v s by control of triggering the adjacent thyristor pairs. 33

34. Multistage Sequence Control The presence of harmonics in the output voltage depends upon the magnitude of voltage variation. If the voltage variation is a small fraction of the total output voltage, the harmonic content in the output voltage is small. 34

35. Cycloconverter

36. Introduction Cycloconverter is a direct frequency converter which converts ac power at fixed frequency to ac power at variable frequency. It is also called as one-stage frequency changer. In cycloconverter frequency changes in steps. Usually it is a SCR based converter with natural or line commutation. To reduce harmonic current and to improve efficiency and power factor, cycloconverter based on forced-commutation devices are used. 36

37. 37 Applications of Cyclo-converters HVDC transmission systems. Static Var Generation. Aircraft or shipboard power supplies. Speed control of high power AC drives. Grinding mills. Cement mill drives. Mine winders. high power low-speed induction motor drive, low-frequency three phase/single phase induction or traction motor drives

38. Types of Cycloconverters In general, cycloconverter are classified as 1. Step-down cycloconverter 2. Step-up cycloconverter Depending upon the phases, there are three types of cycloconverters such as 1 . Single-phase to single-phase cycloconverters 2. Three-phase to single-phase cycloconverters 3. Three-phase to three-phase cycloconverters The single phase to single phase cycloconverters are two types namely 1. Mid-point-type cycloconverters 2. Bridge -type cycloconverters 38

39. 1. Step-down cycloconverter Output frequency f o is less than the supply (input) frequency f s , i.e., f o < f s. Naturally commutated and the output frequency is limited to a value that is a fraction of input frequency. Commonly used in low speed ac motor drives up to 15 MW with frequencies from 0 to 20 Hz. 2. Step-up cycloconverter Output frequency f o is greater than the supply (input) frequency f s , i.e., f o > f s . Forced commutated and the output frequency is limited to a value that is a multiple of input frequency. Fast switching devices and microprocessors are used to implement advanced conversion strategies. Also known as forced commutated direct frequency changers.. 39

40. Single-phase To Single-phase Step-up Cycloconverters

41. Bridge-type Cycloconverters 41

42. Working Two single-phase controlled converters are operated as bridge rectifiers. Their delay angles are such that the output voltage of one converter is equal and opposite to that of the other converter. If α p is the delay angle of positive converter, the delay angle of the negative converter is: α n = π – α p First, During + ve half cycle, P1,P2 and N1 and N2 are forward biased At ωt =0°, P1 and P2 are triggered Load voltage follows the input voltage, + ve voltage At ωt = ωt1, P1, P2 are force commutated and N1,N2 are triggered The output voltage reverses direction, load voltage is – ve At ωt = ωt2, after π P3, P4 are triggered and N3,N4 are force commutated High frequency turning and force commutation of thyristor pairs gives frequency modulated wave 42

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44. Single-phase To Single-phase Step-Down Cycloconverters

45. Bridge-type Does not require forced commutation P1,P2 is triggered at ωt = α ,+ ve output voltage At ωt =π, voltage and current is zero P1, P2 is reversed biased when – ve voltage appears P3,P4 is triggered at ωt =π + α , and output voltage is again positive After a certain number of pulses, the same is done with N1, N2 and N3, N4 45

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47. Three-phase Cycloconverters

48. Types of Three Phase Cycloconverters Single-phase to single-phase cycloconverters are commonly used in low power industrial applications These converters are not suitable for medium and high power applications. Three-phase cycloconverters are developed for medium and high power applications Three phase cycloconverter are classified as Three phase to single phase cycloconverters Three phase to three phase cycloconverters 48

49. Three Phase to Single Phase Cycloconverters 49

50. Working The converter circuit that allows the current during positive half cycle of load current is called positive converter group , group permitting flow of current during negative half cycle of load current is know as negative group converter . The basic principle is to vary progressively the firing angle of three thyristor of three phase half-wave circuit. Firing angle at A, 𝛼 is 90°, At B, 𝛼 is somewhat less then 90°, at C it is still further reduced and so on. At G 𝛼 is zero and the mean output voltage , given by 𝑉 𝑜 =𝑉 𝑑𝑜 cos𝛼, is maximum at G. At A, the mean output voltage is zero as 𝛼=90° 50

51. Working After point G, a small delay in firing angle is further introduced progressively at points H,I,J,K,L and M. Change in firing angle = frequency reduction factor × 120° At M, the firing angle is again 90° and the value of mean output voltage is zero. Mean output voltage is obtained by joining point pertaining to average voltage values. In one half cycle of fundamental frequency output voltage, there are eight half cycles of supply frequency voltage. Output frequency 𝑓𝑜= 1/8𝑓𝑠 where fs is the supply frequency. 51

52. 52 three- PhaSe to three- PhaSe CyCloConverter

53. Three Phase to Three Phase Cycloconverters 53

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55. Working Out of several configuration of 3 phase to 3 phase cycloconverter, this is the most important scheme, used for large industrial drive. This scheme employs 36 thyristor and is called 6 pulse, 3 phase to 3 phase cycloconverter. Each phase group consist of 3 phase dual-converter with two IGR The load phase, must not be interconnected The magnitude of output voltage in 3 phase bridge circuit is doubled of that in the 18 thyristor circuit. Three phase bridge circuit gives a smooth variations of output voltage, but its control circuit is complex and expensive. 55

56. Matrix Converters

57. Working Matrix converters (MCs) are AC-AC power conversion topologies widely explored and applied in the industry for their attractive features of sinusoidal input and output currents, considerable size reduction, and reliable operation due to the omission of bulky passive components. A matrix converter consists of a matrix of 9 switches connecting the three input phases to the three output phases directly. Any input phase can be connected to any output phase at any time depending on the control. No two switches from the same phase should be on at the same time, otherwise this will cause a short circuit of the input phases. Usually controlled by PWM to produce three-phase variable voltages at variable frequency. 57

58. 58 The input terminals of the converter are connected to the grid, the output terminal are connected to a three phase current-fed system, like an induction motor. The capacitive filter on the voltage-fed side and the inductive filter on the current-fed side are necessary. Their size is inversely proportional to the switching frequency

59. Working There are three methods of matrix converter control − Space vector modulation Pulse width modulation Venturi - analysis of function transfer. With nine bi-directional switches the matrix converter can theoretically assume 512 (2 9 ) different switching states combinations. But not all of them can be usefully employed. Regardless to the control method used, the choice of the matrix converter switching state combinations must comply with two basic rules. The input phases should never be short-circuited and The output currents should not be interrupted. These rules imply that one and only one bi-directional switch per output phase must be switched on at any instant. By this constraint, there are 27 permitted switching combinations. 59

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