Advanced Power Electronics

Biyani's Think Tank Concept based notes

Advanced Power Electronics (B.Tech)

Disha Mehtani Lecture

Deptt. of Electrical and Electronics Biyani Institute of Engineering and Technology, Jaipur

2

For free study notes log on: www.gurukpo.com

Published by :

Think Tanks Biyani Group of Colleges Concept & Copyright :

Biyani Shikshan Samiti Sector-3, Vidhyadhar Nagar, Jaipur-302 023 (Rajasthan)

Ph : 0141-2338371, 2338591-95 Fax : 0141-2338007 E-mail : [email protected] Website :www.gurukpo.com; www.biyanicolleges.org Edition : 2013 Price : Leaser Type Setted by : Biyani College Printing Department

While every effort is taken to avoid errors or omissions in this Publication, any

mistake or omission that may have crept in is not intentional. It may be taken note of

that neither the publisher nor the author will be responsible for any damage or loss of

any kind arising to anyone in any manner on account of such errors and omissions.

Advanced Power Electronics 3


Preface

I am glad to present this book, especially designed to serve the needs of the

students. The book has been written keeping in mind the general weakness in understanding the fundamental concepts of the topics. The book is self-explanatory and adopts the “Teach Yourself” style. It is based on question-answer pattern. The language of book is quite easy and understandable based on scientific approach.

Any further improvement in the contents of the book by making corrections, omission and inclusion is keen to be achieved based on suggestions from the readers for which the author shall be obliged.

I acknowledge special thanks to Mr. Rajeev Biyani, Chairman & Dr. Sanjay Biyani, Director (Acad.) Biyani Group of Colleges, who are the backbones and main concept provider and also have been constant source of motivation throughout this Endeavour. They played an active role in coordinating the various stages of this Endeavour and spearheaded the publishing work.

I look forward to receiving valuable suggestions from professors of various educational institutions, other faculty members and students for improvement of the quality of the book. The reader may feel free to send in their comments and suggestions to the under mentioned address.

Note: A feedback form is enclosed along with think tank. Kindly fill the feedback form and submit it at the time of submitting to books of library, else NOC from Library will not be given.

Author

4


Syllabus

ADVANCED POWER ELECTRONICS

Syllabus: 6EX4 ADVANCED POWER ELECTRONICS Unit-1 AC Voltage Controllers: Principle of On-Off Control, Principle of Phase control, Single Phase Bi-directional Controllers with Resistive Loads, Single Phase Controllers with Inductive Loads, Three Phase full wave AC controllers, AC Voltage Controller with PWM Control. Unit-2 Inverters: Principle of Operation, Single-phase bridge inverters, Three phase bridge Inverters: 180 and 120 degree of conduction. Voltage control of Single Phase and Three Phase Inverters, Current Source Inverters, Harmonics and its reduction techniques. Unit-3 Cycloconverters: Basic principle of operation, single phase to single phase, threephase to three-phase and three phase to single phase cycloconverters. Output equation, Control circuit. Unit-4 DC Power Supplies: Switched Mode DC Power Supplies, flyback converter, forward converter, half and full bridge converter, resonant DC power supplies, bidirectional power supplies. Unit-5 AC Power Supplies: Switched mode power supplies, Resonant AC power supplies, bidirectional AC power supplies. Multistage conversions, Control Circuits: Voltage Mode Control, Current Mode Control



Unit 1

Ac voltage controllers

Q1. Give introduction about ac voltage controllers. Explain principle of on- off

control. Ans:

If a thyristor switch is connected between ac supply and load, the power can be controlled by varying the rms value of ac voltage applied to the load and this type of power circuit is known as ac voltage controller. The most application of ac voltage controllers are:

I. Industrial heating II. On-load transformer tap changing

III. Light controls IV. Speed control of induction motors V. AC magnet controls

For power transfer, two types of control are normally used:

1. On-off control

2. Phase angle control

In on-off control, thyristor switches connect the load to the ac source for a few cycles of the input voltage and then disconnected for a few cycles. In phase control, thyristor switches connect the load to the ac source for a portion of each cycle.

Principle of on -off control:

The thyristor switch connects the ac supply to load for a particular time,the switch is turned off by a gate pulse. The on time consists of an integral number of cycles. The thyristors are turned off at the zero voltage crossisngs of input voltage.

6


The output voltage is found from:

Note that k is called the duty cycle, and the power factor and output voltage vary with the square root of k.

Q2. Explain principle of phase control. Ans. The principle of phase control can be explained with the following circuit.

kVnm

nVV

tdtVmn

nV

ssrmso

srmso

2/12

0

22 )(sin2)(2



Due to the presence of diode D1, the control range is limited. The rms output voltage can only be varied between 70.7 to 100%. The output voltage and input current are asymmetrical and contain a dc component. This circuit is a single-phase half-wave controller and is suitable only for low power resistive loads, such as heating and lighting. Since the power flow is controlled during the positive half-cycle of input voltage, this type of controller is also known as unidirectional controller. The rms value of the output voltage is found from:

The average value of the output voltage is: Q3. Describe Single Phase Bi-directional ac voltage Controllers with Resistive

Loads. Ans:

The problem of dc input current can be prevented by using bidirectional or full-wave control. During the positive half cycles of input voltage, the power flow is controlled by varying the delay angle of thyristor T1 and thyristor T2 controls the power floe during the negative half cycles of input voltage. The firing pulse of T1 and T2 are kept 180 degree apart. Single Phase Bi-directional ac voltage Controllers with Resistive Loads are shown in this figure.

2/1

2/1222

22

)]2

2sin2(

2

1[

)]}(sin2)(sin2[2

1{

so

sso

VV

tdtVtdtVV

)1(cos2

2

)](sin2)(sin2[2

1 2

so

ssdc

VV

tdtVtdtVV

8


The rms value of the output voltage is: Q4. Describe Single Phase Bi-directional ac voltage Controllers with inductive

load. Ans:

In practice, most loads are inductive to a certain extent. A full-wave controller with an inductive load is shown next. The gating signals of thyristors could be short pulses for a controller with a resistive load. However, they are not suitable for inductive loads. When thyristor T2 is fired, thyristor T1 is still conducting due to the inductive load. By the time the current of T1 falls to zero and T1 is turned off, the gate current of T2 has already ceased. Consequently, T2 will not be turned on. This difficulty can be resolved by using a continuous gate signal with a duration of π - α.

However a continuous gate pulse increases the switching loss of thyristors. In practice a train of pulses with short duration are used to overcome the loss problem.

The rms value of the output load voltage is found from:

2/1

2/1

22

2

2sin(

1

)(sin22

2

so

so

VV

tdtVV

2/1

2/1

22

2

2sin

2

2sin(

1

)(sin22

2

so

so

VV

tdtVV



Q5. Describe three phase full wave controllers. Ans:

The unidirectional controllers, which contain dc input current and higher harmonic content due to the asymmetrical nature of the output voltage waveform, are not normally used in ac motor drives. A three-phase bidirectional control is commonly used. The circuit diagram of a three-phase full-wave controller is shown next. For 0 < α < 60o:

For 60o < α < 90o: For 90o < α < 150o:

2/1

)8

2sin

46(

16 so VV

2/1

)16

2cos3

16

2sin3

12(

16 so VV

2/1

)16

2cos3

16

2sin

424

5(

16 so VV

10


Q6. Explain ac voltage control with PWM. Ans:

The input power factor of controlled rectifiers can be improved by PWM type of control. The naturally commutated thyristor controllers introduce lower-order harmonics in load and the supply side. The performance of ac voltage controllers can be improved by PWM control.

The following circuit shows a single-phase ac voltage controller with PWM.

The switches S1 and S2 are turned off several times during the positive and negative half-cycles. S'1 and S'2 provide the freewheeling paths for the load current.



Unit 2

Inverters

Q1. Define inverter and explain principle of operation. Ans1. INVERTER:

A device which converts DC power-to-AC power at desires output voltage and frequency is known as inverter. The function of an inverter is to change a dc input to a symmetrical ac output voltage of desired magnitude and frequency. The output voltage can be fixed or variable at a fixed or variable frequency. A variable output voltage can be obtained by varying the input dc voltage and maintaining the gain of the inverter constant. If the dc voltage is fixed, a variable output voltage is obtained by varying the inverter gain (PWM).The inverter gain is defined as the ratio of the ac output voltage to dc input voltage. Inverters can be broadly classified into single-phase and three-phase inverters.

Principle of operation: Single phase bridge inverters: These are of 2 types:

i. Single phase half bridge inverter ii. Single phase full bridge inverter

Single phase half bridge inverter consists of two choppers and two diodes. Transistors switched on and off alternately. Need to isolate the gate signal for Q1 (upper device).Each provides opposite polarity of Vs/2 across the load.

When Q1 on, Q2 off, vo = Vs/2 When Q1 off, Q2 on, vo = -Vs/2 The rms output voltage is: V0=VS/2.

The principle of single-phase inverter operation can be explained with the following figure.

12


Q2 . Explain steady-state analysis of single phase inverter. Ans: The instantaneous output voltage is given by:

rms value of the fundamental component

Load Current for a highly inductive load

Transistors are only switched on for a quarter-cycle, or 90 .Fourier Series of the output current for an RL load:



Q3. Explain working of single phase full bridge inverter. Ans3:

It consists of four choppers four diodes and 3-wire DC source.Q1-Q2 and Q3-Q4 switched on and off alternately. Need to isolate the gate signal for Q1 and Q3 (upper).

Each pair provide opposite polarity of Vs across the load.

When Q1-Q2 on, Q3-Q4 off, vo = Vs

When Q3-Q4 on, Q1-Q2 off, vo = -Vs

The rms output voltage is Vo = Vs.

Q4. Describe three phase bridge inverters. Ans:

Three-phase inverters are used for high power applications. Three-single phase inverters can be connected in parallel to form a three-phase inverter. This arrangement will require 12 transistors, 12 diodes, and three single-phase transformers.

14


The gating signals of the single phase inverters should be 120 degrees with respect to each other. The transformer primaries are isolated from each other, while the secondary’s may be connected in wye or delta.

A three-phase output can be obtained from a configuration of six transistors and six diodes.Two types of control signals can be applied to the transistors: 180o conduction or 120o conduction.



180 degree Conduction:

For this mode of operation, each device conducts 180 degrees. The sequence of firing is: 123, 234, 345, 456, 561, 612.The gating signals are shifted from each other by 60 degrees.

16


120 degree Conduction: In this mode, each transistor conducts for 120 degrees. The sequence of firing is: 61, 12, 23, 34, 45, 56, 61.

Voltage waveform for 120 degree mode:

Q5. Give explanation about voltage control In single phase inverters. Ans: Voltage Control of Single-Phase Inverters: These are the commonly-used Techniques:

i. Single-Pulse-Width-Modulation

ii. Multiple-Pulse-Width-Modulation

iii. Sinusoidal-Pulse-Width-Modulation

iv. Modified-Sinusoidal-Pulse-Width-Modulation

Single-Pulse-Width-Modulation



One Pulse per Half-Cycle Pulse Width Controls the Output Voltage

Carrier and Reference Signals

18


Compare the Reference Signal with the Carrier. Frequency of the Reference Signal determines the frequency of the Output Voltage. Modulation Index = M = Ar/Ac

Gate Signals and Output

Rms value of the Output Voltage



Harmonics contents in single pulse modulation:

Multiple-Pulse-Width-Modulation

20


Multiple Pulses per Half-Cycle of Output Voltage

Gate Signal Generation

Compare the Reference Signal with the Carrier. Frequency of the Reference Signal determines the Output Voltage Frequency. Frequency of the Carrier determines the number of pulses per half-cycle. Modulation Index controls the Output Voltage.



Gate Signals and Output Voltage

Number of pulses per half cycle = p = fc/2fo = mf /2 where mf = frequency modulation ratio

. Harmonic contents in multiple pulse modulation:

22


This is found that harmonics contents in multiple pulse modulation is lower than the single pulse modulation. In single pulse modulation harmonics contents are 86%. And in multiple pulse modulation harmonics contents are 63%.



Unit 3

Cycloconverters

Q1. Define cycloconverter and explain principle of working. Ans:

Cycloconverters directly convert ac signals of one frequency (usually line frequency) to ac signals of variable frequency. These variable frequency ac signals can then be used to directly control the speed of ac motors. Thyristor-based cycloconverters are typically used in low speed, high power (multi-MW) applications for driving induction and wound field synchronous motors.

Basically they are of two types: i. Step down cycloconverters

ii. Step up cycloconverters

In step down cycloconverters the output frequency f0 is lower than the supply frequency fs. (f0<fs) In step up cycloconverters the output frequency f0 is greater than the supply frequency fs. (f0>fs) Single-Phase to Single-Phase Cycloconverters: Mid point type: The basic principle of cycloconversion is illustrated by the single phase-to-single phase converter shown below. A positive center-tap thyristor converter is connected in anti-parallel with a negative converter of the same type. This allows current/voltage of either polarity to be controlled in the load.

24


The waveforms are shown below:

An integral half-cycle output wave is created which has a fundamental frequency f0=(1/n) fi where n is the number of input half-cycles per half-cycle of the output. The thyristor firing angle can be set to control the fundamental component of the output signal. Step-up frequency conversion can be achieved by alternately switching high frequency switching devices



(e.g. IGBTs, instead of thyristors) between positive and negative limits at high frequency to generate carrier-frequency modulated output.

Q2. Explain working of single phase Bridge type cycloconverter. Ans:

It consists of 8 thyristors. Four thyristors are forward biased called pconverter and rest four are reversed baised called n convereter. The load is connected between the p converter and n converter.

Wave form of single phase bridge type cycloconverter:

26


Q3. Explain three phase to single phase cycloconverters. Ans:

3 to single phase conversion can be achieved using either of the dual converter circuit topologies shown below:

They can be of the following types:

Q4. Explain three phase to three phase cycloconverters. Ans:



Each phase group functions as a dual converter but the firing angle of each

group is modulated sinusoidally with 2 /3 phase angle shift -> 3 balanced voltage at the motor terminal. An inter-group reactor (IGR) is connected to each phase to restrict circulating current. An output phase wave is achieved by sinusoidal modulation of the thyristor firing angles. A variable voltage, variable frequency motor drive signal can be achieved by adjusting the modulation depth and output frequency of the converter.

A 3 to 3 cycloconverter can be implemented using 18 thyristors as shown below:

The synthesized output voltage wave contains complex harmonics which can be adequately filtered out by the machine’s leakage inductance.

28


A 3 to 3 bridge cycloconverter (widely used in multi-MW applications) can be implemented using 36 thyristors as shown below:

Q5. Write output equation for a three phase cycloconverter. Ans:

The output phase voltage v0 can be written as:

where V0 is the rms output voltage and 0 is the output angular frequency. We can also write:

where the modulation factor, mf is given by:



Unit 4

Dc power supplies

Q1 What is SMPS? Classified it. Ans:

SMPS means Switch Mode Power Supply. This is used for D.C to D.C conversion. This works on the principle of switching regulation. The SMPS system is highly reliable, efficient, noiseless and compact because the switching is done at very high rate in the order of several KHz to MHz. The SMPS regulators are used in B.S.N.L at various locations like CDOT, E10B and Transmission systems etc.

They are classified as: 1. Flyback

2. Forward

3. Push-pull

4. Half Bridge

5. Full-Bridge

Q2. Describe Flyback and forward cycloconverter. Ans:

Flyback cycloconverter: Mode 1 Operation – when Q1 is ON.

Then the Current builds up in the primary winding and Secondary winding has the opposite polarity D1 OFF.C maintains the output voltage, supplies load current. Circuit configuration for flyback converter:

30


Mode 2 Operation – when Q1 is turned OFF

THE Polarity of the windings reverses and Diode D1 conducts, charging C and providing current to the load RL. Secondary current falls to 0 before the next cycle begins. Waveform Summary:

Forward Converter: It Includes a “reset” winding to return energy and secondary “dot” so that D2 forward biased when Q1 is ON – no energy stored in the primary. It Operates in continuous mode.



Circuit configuration of forward converter:

Mode 1 Operation – when Q1 is ON. Then the Current builds up in the primary winding and energy will transferred to the load. Mode 2 Operation – when Q1 is turned OFF. Then the Polarity of transformer voltages reverses and D2 turns OFF, D1 and D3 turn ON. Waveform summary

32


Q3. Explain Push-Pull Converter. Ans: Push-Pull Operation:

When Q1 is ON, then the Vs will across the lower primary winding and when Q2 is ON, the Vs will across the upper primary winding. Circuit configuration of push pull converter

Q4. Explain Half-Bridge and full bridge Converter. Ans:

Cuircuit configuration of half bridge converter:

Mode 1 Operation: When Q1 is ON, D1 is conducting and the energy will transferred to the load.



Mode 2 Operation: When Both the transistors are OFF and D1 continues to conduct due to current in L1 .

Mode 3 Operation: When Q2 ON, D2 is conducting and energy will transferred to the load.

Mode 4 Operation: When Both the transistors are OFF and D2 continues to conduct due to current in L1.

Waveform Summary

Circuit configuration of full bridge converter: Mode 1 Operation: When Q1,Q4 are ON and Q2,Q3 are OFF and D1 is conducting then the energy will transferred to the load. Mode 2 Operation: When all transistors are OFF and D1 continues to conduct due to current in L1.

34


Mode 3 Operation: When Q2,Q3 are ON, Q1,Q4 OFF and D2 is conducting then energy will transferred to the load. Mode 4 Operation: When all transistors are OFF and D2 continues to conduct due to current in L1

. Waveform Summary



Q5. Draw the diagram of resonance dc power supplies. Ans:

If the variation of the dc output voltage is not wide, resonant pulse inverters can be used. The inverters frequency which could be the same as resonant frequency is very high and the inverter output voltage is almost sinusoidal. Resonance dc power cab be of 2 types: half bridge and full bridge Half-Bridge Resonant Inverter:

Full bridge resonant inverter:

36


Q6. Draw a circuit for bidirectional dc power supply. Ans:

The Output inverter and Rectifier are combined into a cycloconverter (converts one frequency ac into another frequency ac) and the power flow in either direction. The direction of the power flow depends on the values of V0, Vs and turn ratio. For power flow from source to load the inverter operates in inversion mode. And for power flow from load to source the inverter operates as rectifier. The bidirectional converters allow the inductive current to flow in either direction and the current floe becomes continous.

Circuit diagram of bidirectional power supply



Unit 5

Ac Power Supplies

Q1. Explain workin of UPS in ac power supplies. Ans:

The switch mode ac power supply can be use as standby source for “critical” loads (computer) and as primary source when normal ac not available. The stand by power supplies also known as UPS(uninterruptible power supply). There are 2 common configurations for it:-

Configuration 1: Load normally supplied by Main Supply.

Operating Modes: • Normal condition

When Main supply applied to the load and rectifier keeps batteries charged • Main supply failure

38


The Inverter supplies power to the load and when interruption is over, main supply is applied to the load.

Configuration 2 Inverter supplies power to the load: Operating Modes: • Normal condition

When Inverter operates continuously, powering the load and rectifier maintains the charge on the batteries. The Inverter conditions the supply to the load. • Inverter failure

The Main supply will powers the load.



Q2. Explain the types of ac power supplies. Ans:

They can be classified as: • Switched-mode

• Resonant

• Bidirectional

Switched-Mode AC Power Supply: It consists of two inverters. One is at Input side and operates at high-frequency. Reduces size of transformer and DC filter components at the input of the output-side inverter. Other is at output side and operates at output frequency.

40


Resonant AC Power Supply: It consists of two inverters. One is at Input side and is a resonant inverter. Other is at output side operates with PWM control at the output frequency.

Bidirectional AC Power Supply: The Output inverter and Rectifier are combined into a cycloconverter (converts one frequency ac into another frequency ac) and the power flow in either direction.



Q3. Explain multistage conversion. Ans:

There are four conversions : AC-DC – DC-AC – AC-DC – DC-AC. Ultimately an AC – AC Conversion. The pair of rectifier and inverter can be replaced by converter. The switching functions this converter can be synthesized to combine the functions the rectifier and the inverter. This converter which converts ac- ac directly called the forced commutated cycloconverter. The ac-dc-ac-dc conversion is shown in figure and can be performed by two forced commutated cycloconverters.

Configuration of multistage conversion:

42


Bibliography

1. Dr. P. S. bhimbhra, Power electronics , Jain book agency

2. Sachin S. Sharma , Power electronics, Jain book agency

3. Muhammad H. Rashid, Power Electronics : Circuit, Devices and Applications,

Pearson Education ,INC

4. Ashfaq Ahmed, Power Electronics For Technology, Prentice Hall PTR, 1999

5. Erickson, Robert, Fundamental of Power Electronics, 2nd ed. 2001

Web site: 6. Google.com

7. Wikipedia.com

8. Power electronics.com

9. http://books.google.co.in/books?id=-

WqvjxMXClAC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=on

epage&q&f=false

10. http://books.google.co.in/books?id=chOFQgAACAAJ&source=gbs_similarboo

ks

http://books.google.co.in/books?id=-WqvjxMXClAC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false



Advanced Power Electronics

Documents

Transcript of Advanced Power Electronics