report

INDEX

PARTICULARS PAGE NO.

CERTIFICATE

ACKNOWLEDGEMENT

LIST OF FIGURES

CHAPTER- 1. INTRODUCTION

CHAPTER- 2. TYPES OF AC-DC CONVERTER

2.1) Uncontrolled Converter

2.1.1) Single Phase Half Wave AC-DC Converter

2.1.2) Single Phase Full Wave Centre-Tap AC-DC Converter

2.1.3) Single Phase Full Wave Bridge AC-DC Converter

2.1.4) Three Phase Half Wave AC-DC Converter

2.1.5) Three Phase Full Wave AC-DC Converter

2.2) Controlled Converter

2.2.1) Single Phase Half Wave AC-DC Converter

2.2.2) Single Phase Half Wave AC-DC Converter with

Freewheeling Diode

2.2.3) Single Phase Full Wave Centre-Tap AC-DC Converter

2.2.4) Single Phase Full Wave Bridge Converter

2.2.5) Three Phase Half Wave AC-DC Converter

2.2.6) Three Phase Full Wave AC-DC Converter

CHAPTER- 3. POWER QUALITY ISSUES

3.1) Power Quality

3.1.1) What is Power Quality?

3.1.2) Disadvantages of Poor Power Quality

3.1.3) Categories of Power Quality

3.1.3.1) Transients

3.1.3.2) Voltage Sags

3.1.3.3) Frequency Variations

3.1.3.4) Waveform Distortions

3.1.3.5) Flicker

3.1.3.6) Voltage Fluctuations

3.1.3.7) Grounding

CHAPTER -4. PROBLEMS ASSOCIATED IN AC-DC CONVERTER

4.1) Introduction

4.2) Poor Power Factor

4.2.1) What is Power Factor?

4.2.2) Causes of Poor Power Factor

4.2.3) Effects of Poor Power Factor

4.2.4) Need Of Power Factor Correction

4.2.5) Advantages of good power factor

4.3) Harmonics

4.3.1) Triplen Harmonics

4.3.2) Nontriplen Harmonics

4.3.3) Harmonic Factor (HFn)

4.3.4) Total Harmonic Factor (THD)

4.3.5) Distortion Factor (DFn)

4.3.6) Lowest Order Harmonics

4.3.7) Sources of Harmonics

4.3.8) Causes of Harmonics

4.3.9) Effects of Harmonics

CHAPTER-5. REMEDIES FOR PROBLEMS ASSOCIATED IN AC-DC

CONVERTER

5.1) Power Factor Correction

5.2) Types of Power Factor Correction Techniques

5.2.1) Passive Power Factor Correction

5.2.2) Active Power Factor Correction

5.2.2.1) Extinction Angle Control

5.2.2.2) Symmetrical Angle Control

5.2.2.3) Pulse Width Modulation (PWM) Control

5.2.2.4) Sinusoidal Pulse Width Modulation (SPWM) Control

5.2.2.5) Three Phase PWM Rectifier

5.3) Harmonic Reduction Technique

5.3.1) Low Pass (L-C) Filter Circuit on AC side

5.3.2) Active Shaping of Input (Line) Current

5.3.3) Using Multipulse Rectifiers

CHAPTER-6. CONCLUSION

CHAPTER 1

INTRODUCTION

Power electronics is the application of solid-state electronics for the control and

conversion of electric power. It also refers to a subject of research in electrical

engineering which deals with design, control, computation and integration of nonlinear,

time varying energy processing electronic systems with fast dynamics.

In AC/DC conversion, transformation of AC current into DC current occurs

for use in common applications. Alternating current (AC) periodically reverses direction,

which cannot be used to power certain systems. Direct current (DC) results in a one-way

flow of electrons, and does not alternate like AC. AC to DC conversion is also known as

rectification. Rectification was first used in the early 1900’s, as the mercury-arc

rectifier was invented by Peter Cooper Hewitt in 1902 to allow for the conversion of

large AC power sources to DC.As technology advanced, different applications and types

of rectifiers allowed for more efficient power conversion.

Rectification is used throughout the world in order to power everyday devices.

AC current is used for transmission because of its qualities allowing it to be manipulated

for easier distribution throughout the world. A transformer can step the voltage up in

order to reduce line loss and step the voltage back down to distribute the voltage to

useable levels. In order to supply direct current, for instance, to charge a battery or to

supply power to a circuit, the current most undergo rectification. Power is delivered from

the supplier in the form of alternating current, and in order to harness this power, a

rectifier must be used to convert the current to a useable form: direct current. Most

electronics in a household are meant to run off of DC, such as solid-state lighting, so the

rectification of AC power is very important in modern times.

CHAPTER 2

TYPES OF AC-DC CONVERTER

There are two types of ac-dc converter-

1. Uncontrolled Converter

2. Controlled Converter

2.1. Uncontrolled Converter

2.1.1. Single Phase Half Wave AC-DC Converter

This is the simplest and probably the most widely used rectifier circuit at relatively small

power levels. The output voltage and current of this rectifier are strongly influenced by

the type of the load.

Fig(2.1) Single phase half wave rectifier

(a) circuit diagram (b) waveforms

The ripple factor of output current can be reduced by connecting an inductor in series

with the load resistance because of such high ripple content in the output voltage and

current this rectifier is seldom used with a pure resistive load. As in the previous case,

the diode D is forward biased when the switch S is turned on at ωt = 0. However, due to

the load inductance i0 increases more slowly. Eventually at ωt = π, v0 becomes zero

again. However, i0 is still positive at this point. Therefore, D continues to conduct beyond

ωt = π while the negative supply voltage is supported by the inductor till its current

becomes zero at ωt = β. The diode remains in forward biased longer than π radians

(although the source is negative during that duration) the point when current reaches zero

is when diode turns off. This point is known as the extinction angle, β, beyond this point,

D becomes reverse biased. Both v0 and i0 remains zero till the beginning of the next cycle

where upon the same process repeats.

Output average voltage,

V 0 AV=1

2 π∫0

β

√2 V i sinωt dωt = √2 V i

π (1−cos β2 )

RMS output voltage,

V

0 RMS=¿√ 12π∫0

β

2V i2 sin2 ωt dωt ¿ = V i

√ 2 √ 2 β−sin 2 β2π

=

2.1.2. Single Phase Full Wave Center-Tap AC-DC Converter

Fig 2.2 shows the circuit diagram and waveforms of single phase centre tap uncontrolled

full wave rectifier supplying an RL load. The split power supply can be thought of to

have been obtained from the secondary of a center tapped ideal transformer.

Fig(2.2) Single phase full wave center-tap ac-dc converter


When the switch is closed at the positive going zero crossing of v1 the diode D1 is

forward biased and the load is connected to v1. The currents i0 and ii1 start rising through

D1. When v1 reaches its negative going zero crossing both i0 and ii1 are positive which

keeps D1 in conduction. Therefore, the voltage across D2 is vCB=v2-v1.Beyond the

negative going zero crossing of D2 becomes forward biased and the current i0

commutates to D2 from D1. The load voltage v0 becomes equal to v2 and D1 starts

blocking the voltage vAB =v1-v2 .The current i0 however continues to increase through D2

till it reaches the steady state level after several cycles. Steady state waveforms of the

variables are shown in Fig 2.2(b) from ωt = 0 onwards. It should be noted that the

current, i0 once started, always remains positive. This mode of operation of the rectifier

is called the

“Continuous conduction mode” of operation .Where i0 remains zero for some duration

of the input supply waveform. This mode is called the “discontinuous conduction

mode” of operation.


V

0 AV =1π∫0

π

v0 dωt=¿

2√ 2V i

π¿

RMS output voltage,

V 0 RMS=√ 1π∫0

π

2V i2 sin2 ωt dωt=V i

2.1.3. Single Phase Full Wave Bridge AC-DC Converter

Another type of circuit that produces the same output waveform as the full wave rectifier

circuit above, is that of the Full Wave Bridge Rectifier. This type of single phase rectifier

uses four individual rectifying diodes connected in a closed loop "bridge" configuration

to produce the desired output. The main advantage of this bridge circuit is that it does not

require a special centre tapped transformer, thereby reducing its size and cost. The single

secondary winding is connected to one side of the diode bridge network and the load to

the other side as shown below.

During the negative half cycle of the supply, diodes D3 and D4 conduct in series,

but diodes D1 and D2switch "OFF" as they are now reverse biased. The current flowing

through the load is the same direction as before. As the current flowing through the load

is unidirectional, so the voltage developed across the load is also unidirectional the same

as for the previous two diode full-wave rectifier, therefore the average DC voltage across

the load is 0.637Vmax. However in reality, during each half cycle the current flows

through two diodes instead of just one so the amplitude of the output voltage is two

voltage drops ( 2 x 0.7 = 1.4V ) less than the input VMAX amplitude. The ripple frequency

is now twice the supply frequency (e.g. 100Hz for a 50Hz supply).

Fig(2.3) Single phase bridge ac-dc converter



V 0 AV=√2 V i

π∫0

π

sinωt dωt=2 √ 2 V i

π

RMS output voltage,

V 0 RMS=√ 1π∫0

π

2V i2 sin2 ωt dωt = V i

2.1.4. Three Phase Half Wave AC-DC Converter

The half wave uncontrolled converter is the simplest of all three phase rectifier

topologies. Although not much used in practice it does provide useful insight into the

operation of three phase converters. Fig. 2.4 shows the circuit diagram, conduction table

and wave forms of a three phase half wave uncontrolled converter supplying a resistive

inductive load. For simplicity the load current (i0) has been assumed to be ripple free. As

shown in Fig. 2.4(a), in a three phase half wave uncontrolled converter the anode of a

diode is connected to each phase voltage source. The cathodes of all three diodes are

connected together to form the positive load terminal. The negative terminal of the load

is connected to the supply neutral. Fig. 2.4(b) shows the conduction table of the

converter. It should be noted that for the type of load chosen the converter always

operates in the continuous conduction mode. The conduction diagram for the diodes (as

shown in Fig. 2.4 (c) second waveform) can be drawn easily from the conduction

diagram. Since the diodes can block only negative voltage it follows from the conduction

table that a phase diode conducts only when that phase voltage is maximum of the three.

(In signal electronics the circuit of Fig. 2.4(a) is also known as the “maximum value”

circuit).

Fig(2.4) Three phase half wave ac-dc converter

(a ) circuit diagram (b) conduction table (c) waveforms

Once the conduction diagram is drawn other waveforms of Fig. 2.4(c) are easily obtained

from the supply voltage waveforms in conjunction with the conduction table. The phase

current waveforms of Fig. 2.4(c) deserve special mention. All of them have a dc

component which flows through the ac source. This may cause “dc saturation” in the ac

side transformer. This is one reason for which the converter configuration is not

preferred very much in practice.

2.1.5. Three Phase Full Wave AC-DC Converter

Fig. 2.5 shows the circuit diagram, conduction table and wave forms of a three phase full

wave uncontrolled converter supplying a supplying resistive load.

(a)

(b)

Fig(2.5) Three phase full wave ac-dc converter


Fig. 2.5(b) are easily obtained from the supply voltage waveforms.

Top group: Diode with its anode at the highest potential will conduct. The other two

will be reversed.

Bottom group: Diode with the its cathode at the lowest potential will conduct. The other

two will be reversed.

For example, if D1 (of the top group) conducts, vp is connected to van. If D6 (of the bottom

group) conducts, vn connects to vbn. All other diodes are off. The resulting output

waveform is given as: v0 = vp-vn .For peak of the output voltage is equal to the peak of the

line to line voltage vab. All of them have a dc component which flows through the ac

source. This may cause “dc saturation” in the ac side transformer. This is one reason for

which the converter configuration is not preferred very much in practice.

2.2. Controlled Converter

2.2.1. Single Phase Half Wave AC-DC Converter

Fig 2.6 (a) and (b) shows the circuit diagram and the waveforms of a single phase fully

controlled half wave rectifier supplying a resistive inductive load.

Fig(2.6) Single phase half wave ac-dc converter


As in the case of a resistive load, the thyristor T becomes forward biased when the

supply voltage becomes positive at ωt = 0. However, it does not start conduction until a

gate pulse is applied at ωt = α. As the thyristor turns on at ωt = α the input voltage

appears across the load and the load current starts building up. However, unlike a

resistive inductive load, the load current does not become zero at ωt = π, instead it

continues to flow through the thyristor and the negative supply voltage appears across

the load forcing the load current to decrease. Finally, at ωt = β (β > π) the load current

becomes zero and the thyristor undergoes reverse recovery. From this point onwards the

thyristor starts blocking the supply voltage and the load voltage remains zero until the

thyristor is turned on again in the next cycle. It is to be noted that the value of β depends

on the load parameters. Since the thyristors does not conduct over the entire input supply

cycle this mode of operation is called the “discontinuous conduction mode”.


V 0 AV=1

2 π∫α

β

√2 V i sinωt dωt=V i

√2 π(cosα−cos β )

RMS output voltage,

V0 RMS=√ 1

2 π∫αβ

2 v i sin2 ω t d ω t=V i

√2 ( β−απ

+sin 2α−sin2 β

2 π )12

2.2.2. Single Phase Half Wave AC-DC Converter with Freewheeling Diode

For single-phase, half wave rectifier with R-L load, the load (output) current is not

continuous. A FWD (sometimes known as commutation diode) can be placed as shown

below to make it continuous.

(a)

(b)

Fig(2.7) Single phase half wave ac-dc converter with Freewheeling diode


Note that both D1 and D2 cannot be turned on at the same time. For a positive cycle

voltage source, D1 is on, D2 is off. The equivalent circuit is shown in Figure (b). The

voltage across the R-L load is the same as the source voltage. For a negative cycle

voltage source, D1 is off, D2 is on. The voltage across the R-L load is zero. However,

the inductor contains energy from positive cycle. The load current still circulates through

the R-L path. But in contrast with the normal half wave rectifier, Hence the “negative

part” of vo as shown in the normal half-wave disappear.

2.2.3. Single Phase Full Wave Center-Tap AC-DC Converter

The circuit diagram of single-phase full wave converter using a center –tapped

transformer as shown in fig.2.8(a).

(a)

(b)

Fig(2.8) Single phase full wave center-tap ac-dc converter


When terminal a is positive with respect to midpoint, and midpoint is positive with

respect to b, it is assumed here that load ,or output ,current is continuous and turns ratio

from primary to each secondary is unity. Thyristors SCR1 and SCR2 are forward biased

during positive and negative half cycles respectively. Suppose second SCR are is already

conducting.SCR1 is therefore forward biased and when triggered at delay angle α, SCR1

gets turned on. At this firing angle α, supply voltage applies 2Vmsinα reverse biases

SCR2,this SCR is therefore turned off. Here SCR1 is called the incoming thyristor and

SCR2 is called the outgoing thyristor.

2.2.4. Single Phase Full Wave Bridge AC-DC Converter

The centre-tap full wave single phase rectifier offers as good performance as possible

from a single phase rectifier in terms of the output voltage form factor and ripple factor.

They have a few disadvantages however. These are

1. They require a split power supply which is not always available.

2. Each half of the split power supply carries current for only one half cycle.

Hence they are underutilized.

3. The ratio of the required diode PIV to the average output voltage is rather

high..

These problems can be removed by using a single phase full bridge rectifier as shown in

Fig 2.9 (a). This is one of the most popular rectifier configuration and are used widely

for applications requiring dc power output from a few hundred watts to several kilo

watts. Fig 2.9(a) shows the rectifier supplying an R-L-E type load which may represent a

dc. motor or a storage battery.

(c)

Fig(2.9) Single phase full wave bridge ac-dc converter

(a) circuit diagram (b) conduction table (c) waveforms

Indeed, the R–L–E load shown in this figure may represent the electrical equivalent

circuit of a separately excited dc motor. It is one of the most popular converter circuits

and is widely used in the speed control of separately excited dc machines. The single

phase fully controlled bridge converter is obtained by replacing all the diode of the

corresponding uncontrolled converter by thyristors. Thyristors T1 and T2 are fired

together while T3 and T4 are fired 180º after T1 and T2. From the circuit diagram of Fig

2.9(a) it is clear that for any load current to flow at least one thyristor from the top group

(T1, T3) and one thyristor from the bottom group (T2, T4) must conduct. It can also be

argued that neither T1T3 nor T2T4 can conduct simultaneously. For example whenever

T3 and T4 are in the forward blocking state and a gate pulse is applied to them, they turn

on and at the same time a negative voltage is applied across T1 and T2 commutating

them immediately. Similar argument holds for T1 and T2. For the same reason T1T4 or

T2T3 can not conduct simultaneously. Therefore, the only possible conduction modes

when the current i0 can flow are T1T2 and T3T4. Of course it is possible that at a given

moment none of the thyristors conduct. This situation will typically occur when the load

current becomes zero in between the firings of T1T2 and T3T4. Once the load current

becomes zero all thyristors remain off. In this mode the load current remains zero.

Consequently the converter is said to be operating in the discontinuous conduction mode.

Fig 2.9(b) shows the voltage across different devices and the dc output voltage

during each of these conduction modes. It is to be noted that whenever T1 and T2

conducts, the voltage across T3 and T4 becomes –vi. Therefore T3 and T4 can be fired

only when vi is negative i.e., over the negative half cycle of the input supply voltage.

Similarly T1 and T2 can be fired only over the positive half cycle of the input supply.

The voltage across the devices when none of the thyristors conduct depends on the off

state impedance of each device. The values listed in Fig 2.9(b) assume identical devices.

Under normal operating condition of the converter the load current may or may not

remain zero over some interval of the input voltage cycle. If i0 is always greater than zero

then the converter is said to be operating in the continuous conduction mode. In this

mode of operation of the converter T1T2 and T3T4 conducts for alternate half cycle of

the input supply.

2.2.5. Three Phase Half Wave AC-DC Converter

The figure 2.10(a) shows the three-phase half-wave rectifier topology. To control the

load voltage, the half wave rectifier uses three common-cathode thyristor arrangement.

In this figure, the power supply and the transformer are assumed ideal. The thyristor will

conduct (on state), when the anode-to-cathode voltage vAK is positive, and a firing current

pulse iG is applied to the gate terminal. Delaying the firing pulse by an angle a does the

control of the load voltage. The firing angle α is measured from the crossing point

between the phase supply voltages, as shown in figure 2.10(b). At that point, the anode-

to-cathode thyristor voltage vAK begins to be positive. The figure 2.10(b)shows that the

possible range for gating delay is between α=0° and α=180°, but in real situations,

because of commutation problems, the maximum firing angle is limited to around 160°.

In figure 2.10(b), when the load is resistive, the current id has the same waveform of the

load voltage. As the load becomes more and more inductive, the current flattens and

finally becomes constant. The thyristor goes to the non-conducting condition (off state)

when the following thyristor is switched on, or the current, tries to reach a negative

value. With the help of figure 2.10(b), the load average voltage can be evaluated, and is

given by:

V D=V MAX

23

π∫

−π3

+α

π3+α

cosωt dωt=V MAX

sinπ3

π3

. cos α

Where VMAX is the secondary phase-to-neutral peak voltage, It can be seen from

equation that changing the firing angle α, the load average voltage VD is modified. When

α is smaller than 90°, VD is positive, and when α becomes larger than 90°, the average dc

voltage becomes negative. In such a case, the rectifier begins to work as an inverter, and

the load needs to have the capability to generate power reversal by reversing its dc

voltage.

(a)

(b)

Fig(2.10)Three phase half wave ac-dc converter


2.2.6. Three Phase Full Wave AC-DC Converter

Parallel connection via interphase transformers permits the implementation of rectifiers

for high current applications. Series connection for high voltage is also possible, as

shown in the full wave rectifier of figure 2.11(a). With this arrangement, it can be seen

that the three common cathode valves generate a positive voltage respect to the neutral,

and the three common anode valves produce a negative voltage. The result is a dc

voltage twice the value of the half wave rectifier. Each half of the bridge is a three-pulse

converter group. This bridge connection is a two-way connection, and alternating

currents flow in the valve-side transformer windings during both half periods, avoiding

dc components into the windings, and saturation in the transformer magnetic core. These

characteristics made the also called Graetz Bridge the most widely used line commutated

thyristor rectifier. The configuration does not need any special transformer, and works as

a six-pulse rectifier. The series characteristic of this rectifier produces a dc voltage twice

the value of the half-wave rectifier. The load average voltage is given by:

V D=2.V MAX

23

π∫

−π3

+α

π3+α

cosωt dωt=2.V MAX

sinπ3

π3

cosα

Where VMAX is the peak phase-to-neutral voltage at the secondary transformer terminals,

(a)

(b)

Fig(2.11) Three phase full wave ac-dc converter


The figure 2.11(b) shows the voltages of each half wave bridge of this topology, vDpos and

vDneg, the total instantaneous dc voltage vD, and the anode to-cathode voltage vAK in one

of the bridge thyristors. The maximum value of vAK is √ 3.VMAX, which is the same as of

the half-wave converter, and the interphase transformer rectifier. The double star rectifier

presents a maximum anode-to-cathode voltage of 2 times VMAX.

CHAPTER 3

Power Quality Issues

3.1. Power Quality

3.1.1. What is Power Quality?The Power Quality of a system expresses to which degree a practical supply system

resembles the ideal supply system. If the Power Quality of the network is good, then any

loads connected to it will run satisfactory and efficiently. Installation running costs and

carbon footprint will be minimal. If the Power Quality of the network is bad, then loads

connected to it will fail or will have a reduced lifetime, and the efficiency of the

electrical installation will reduce. Installation running costs and carbon footprint will be

high and operation may not be possible at all.

3.1.2. Disadvantages of Poor Power Quality

Poor Power Quality can be described as any event related to the electrical network that

ultimately results in a financial loss. Possible consequences of poor Power Quality

include

1. Unexpected power supply failures (breakers tripping, fuses blowing) or equipment

failure

2. Equipment overheating (transformers, motors,…) leading to their lifetime reduction.

3. Damage to sensitive equipment (PC‟s, production line control systems,…).

4. Electronic communication interferences.

5. Increase of system losses.

6. Penalties imposed by utilities because the site pollutes the supply network too much.

7. Health issues with and reduced efficiency of personnel, ...

8. Need to oversize installations to cope with additional electrical stress with

consequential increase of installation and running costs and associated higher carbon

footprint.

9. Connection refusal of new sites because the site would pollute the supply network too

much.

10. Impression of unsteadiness of visual sensation induced by a light stimulus whose

luminance or spectral distribution fluctuates with time (flicker).

3.1.3. Categories of Power Quality

3.1.3.1. Transients

Potentially the most damaging type of power disturbance, transients fall into two

subcategories:

1. Impulsive 2. Oscillatory

Impulsive

Impulsive transients are sudden high peak events that raise the voltage and current levels

in either a positive or a negative direction. These types of events can be categorized

further by the speed at which they occur (fast, medium, and slow). Impulsive transients

can be very fast events (5 nanoseconds [ns] rise time from steady state to the peak of the

impulse) of short-term duration (less than 50 ns). The impulsive transient is what most

people are referring to when they say they have experienced a surge or a spike. Many

different terms, such as bump, glitch, power surge, and spike have been used to describe

impulsive transients. One example of a positive impulsive transient caused by

electrostatic discharge (ESD) event.

Fig(3.1) Positive Impulse Transient

Oscillatory

An oscillatory transient is a sudden change in the steady-state condition of a signal's

voltage, current, or both, at both the positive and negative signal limits, oscillating at the

natural system frequency. In simple terms, the transient causes the power signal to

alternately swell and then shrink, very rapidly. Oscillatory transients usually decay to

zero within a cycle (a decaying oscillation). These transients occur when you turn off an

inductive or capacitive load, such as a motor or capacitor bank. An oscillatory transient

results because the load resists the change. This is similar to what happens when you

suddenly turn off a rapidly flowing faucet and hear a hammering noise in the pipes. The

flowing water resists the change, and the fluid equivalent of an oscillatory transient

occurs.

Fig(3.2) Oscillatory transient

3.1.3.2. Voltage Sags

A sag is a reduction of AC voltage at a given frequency for the duration of 0.5 cycles to

1 minute’s time. Sags are usually caused by system faults, and are also often the result of

switching on loads with heavy startup currents. Voltage sags and momentary power

interruptions are probably the most important PQ problem affecting industrial and large

commercial customers. These events are usually associated with a fault at some location

in the supplying power system. Interruptions occur when the fault is on the circuit

supplying the customer. But voltage sags occur even if the faults happen to be far away

from the customer's site. Voltage sags lasting only 4-5 cycles can cause a wide range of

sensitive customer equipment to drop out.

Fig(3.3) Voltage Sags

3.1.3.3. Frequency Variations

Frequency variation is extremely rare in stable utility power systems, especially systems

interconnected via a power grid. Where sites have dedicated standby generators or poor

power infrastructure, frequency variation is more common especially if the generator is

heavily loaded. What would be affected would be any motor device or sensitive device

that relies on steady regular cycling of power over time. Frequency variations may cause

a motor to run faster or slower to match the frequency of the input power. This would

cause the motor to run inefficiently and lead to added heat and degradation of the motor

through increased motor speed and additional current draw.

Fig(3.4) Frequency variations

3.1.3.4. Waveform Distortion

There are five primary types of waveform distortion:

1. DC offset

2. Harmonics

3. Interharmonics

4. Notching

5. Noise

DC offset

Direct current (dc) can be induced into an ac distribution system, often due to failure of

rectifiers within the many ac to dc conversion technologies that have proliferated modern

equipment. DC can traverse the ac power system and add unwanted current to devices

already operating at their rated level. Overheating and saturation of transformers can be

the result of circulating dc currents. When a transformer saturates, it not only gets hot,

but also is unable to deliver full power to the load, and the subsequent waveform

distortion can create further instability in electronic load equipment. A dc offset is

illustrate in fig.3.5 . The solution to dc offset problems is to replace the faulty equipment

that is the source of the problem. Having very modular, user replaceable, equipment can

greatly increase the ease to resolve dc offset problems caused by faulty equipment, with

less costs than may usually be needed for specialized repair labor.

.

Fig(3.5) DC Offset

Harmonics

Harmonic distortion is the corruption of the fundamental sine wave at frequencies that

are multiples of the fundamental. (e.g., 180Hz is the third harmonic of a 60Hz

fundamental frequency; 3 X 60 = 180). Symptoms of harmonic problems include

overheated transformers, neutral conductors, and other electrical distribution equipment,

as well as the tripping of circuit breakers and loss of synchronization on timing circuits

that are dependent upon a clean sine wave trigger at the zero crossover point. Harmonic

distortion has been a significant problem with IT equipment in the past, due to the nature

of switch-mode power supplies (SMPS). These non-linear loads, and many other

capacitive designs, instead of drawing current over each full half cycle, “sip” power at

each positive and negative peak of the voltage wave. The return current, because it is

only short-term, (approximately 1/3 of a cycle) combines on the neutral with all other

returns from SMPS using each of the three phases in the typical distribution system.

Instead of subtracting, the pulsed neutral currents add together, creating very high neutral

currents, at a theoretical maximum of 1.73 times the maximum phase current. An

overloaded neutral can lead to extremely high voltages on the legs of the distribution

power, leading to heavy damage to attached equipment. At the same time, the load for

these multiple SMPS is drawn at the very peaks of each voltage half-cycle, which has

often led to transformer saturation and consequent overheating. Other loads contributing

to this problem are variable speed motor drives, lighting ballasts and large legacy UPS

systems. Methods used to remove this problem have included over-sizing the neutral

conductors, installing K-rated transformers, and harmonic filters.

Fig(3.6) Typical harmonic waveform distortion

Interharmonics

Interharmonics are a type of waveform distortion that are usually the result of a signal

imposed on the supply voltage by electrical equipment such as static frequency

converters, induction motors and arcing devices. Cycloconverters (which control large

linear motors used in rolling mill, cement, and mining equipment), create some of the

most significant interharmonic supply power problems. These devices transform the

supply voltage into an AC voltage of a frequency lower or higher than that of the supply

frequency. The most noticeable effect of interharmonics is visual flickering of displays

and incandescent lights, as well as causing possible heat and communication

interference. Solutions to interharmonics include filters, UPS systems, and line

conditioners.

Fig(3.7) Interharmonic waveform distortion

Notching

Notching is a periodic voltage disturbance caused by electronic devices, such as variable

speed drives, light dimmers and arc welders under normal operation. This problem could

be described as a transient impulse problem, but because the notches are periodic over

each ½ cycle, notching is considered a waveform distortion problem. The usual

consequences of notching are system halts, data loss, and data transmission problems.

One solution to notching is to move the load away from the equipment causing the

problem (if possible). UPSs and filter equipment are also viable solutions to notching if

equipment cannot be relocated.

Fig(3.8) Notching

Noise

Noise is unwanted voltage or current superimposed on the power system voltage or

current waveform. Noise can be generated by power electronic devices, control circuits,

arc welders, switching power supplies, radio transmitters and so on. Poorly grounded

sites make the system more susceptible to noise. Noise can cause technical equipment

problems such as data errors, equipment malfunction, long term component failure, hard

disk failure, and distorted video displays. There are many different approaches to

controlling noise and sometimes it is necessary to use several different techniques

together to achieve the required result. Some methods are:

• Isolate the load via a UPS

• Install a grounded, shielded isolation transformer

• Relocate the load away from the interference source

• Install noise filters

• Cable shielding

Fig(3.9) Noise

3.1.3.5. Flicker

Flicker is defined as 'Impression of unsteadiness of visual sensation induced by a light

stimulus whose luminance or spectral distribution fluctuates with time’. From a more

practical point of view one can say that voltage fluctuations on the supply network cause

change of the luminance of lamps, which in turn can create the visual phenomenon called

flicker. While a small flicker level may be acceptable, above a certain threshold it

becomes annoying to people present in a room where the flicker exists. The degree of

annoyance grows very rapidly with the amplitude of the fluctuation. Further on, at

certain repetition rates of the voltage fluctuation, even small fluctuation amplitudes can

be annoying. The influence of the flicker phenomenon on people is complex to analyse

given that it depends not only on technical aspects like the lamp characteristics to which

the fluctuating voltage is applied but also on the appreciation of the phenomenon by the

eye/brain of each individual.

3.1.3.6. Voltage Fluctuations

Since voltage fluctuations are fundamentally different from the rest of the waveform

anomalies, they are placed in there own category. A Voltage fluctuation is a systematic

variation of the voltage waveform or a series of random voltage changes, of small

dimensions, namely 95 to 105% of nominal at a low frequency, generally below 25 Hz.

Any load exhibiting significant current variations can cause voltage fluctuations. Arc

furnaces are the most common cause of voltage fluctuation on the transmission and

distribution system. One symptom of this problem is flickering of incandescent lamps.

Removing the offending load, relocating the sensitive equipment, or installing power line

conditioning or UPS devices, are methods to resolve this

Fig(3.10)Voltage fluctuations

3.1.3.7. Grounding

Grounding of equipment was originally conceived as a personnel safety issue. From a

power quality perspective, improper grounding can be considered in three broad

categories:

1. Ground loops,

2. Improper neutral-to-ground connections, and

3. Excessive neutral-to-ground voltage.

The ground loop problem is a significant issue when power, communications,

and control signals all originate in different locations, but come together at a common

electrical point.Transients induced in one location can travel through the created ground

loop, damaging equipment along the way.

Improper neutral-to-ground connections will create a ‘‘noisy’’ ground reference

that may interfere with low-voltage communications and control devices. Excessive

neutral-to ground voltage may damage equipment that is not properly insulated or that

has an inexpensive power supply Figure 3.11 shows an example of an improper neutral-

to ground connection, and how this connection can create power quality problems. Load

current returning in the neutral conductor will, at the point of improper connection to

ground, divide between neutral and ground. This current flow in the ground conductor

will produce a voltage at the load equipment, which can easily disrupt equipment

operation.

Fig(3.11) Improper Neutral-to-Ground

Figure 3.12 shows an example of the possibility for excessive neutral-to-ground voltage

and how this can lead to power quality problems For load equipment that produces

significant voltage drop in the neutral, such as laser printers and copying machines when

the thermal heating elements are on, the voltage from the neutral to the ground reference

inside the equipment can exceed several volts. In many cases, this voltage is sufficient to

damage printed circuit boards, disrupt control logic, and fail components.

Fig(3.12) Excessive Neutral-to-Ground

CHAPTER 4

PROBLEMS IN AC-DC CONVERTER

4.1. INTRODUCTION

Most of the power conversion applications consist of an AC-to-DC conversion stage

immediately following the AC source. The DC output obtained after rectification is

subsequently used for further stages. Current pulses with high peak amplitude are drawn

from a rectified voltage source with sine wave input and capacitive filtering. The current

drawn is discontinuous and of short duration irrespective of the load connected to the

system. Since many applications demand a DC voltage source, a rectifier with a

capacitive filter is necessary. However, this results in discontinuous and short duration

current spikes. When this type of current is drawn from the mains supply, the resulting

network losses,the total harmonic content, and the radiated emissions become

significantly higher. At power levels of more than 500 watts, these problems become

more pronounced. Two factors that provide a quantitative measure of the power quality

in an electrical system are Power Factor (PF) and Total Harmonic Distortion (THD).

The amount of useful power being consumed by an electrical system is predominantly

decided by the PF of the system.

4.2. Poor Power Factor

4.2.1. What is Power Factor?

In simple terms, power factor can be defined as the ratio of real power to apparent

power.

PF = P

(Vrms∗Irms ) or PF = WattsV . A .

where P is the real input power and Vrms and Irms are the root mean square (RMS)

voltage and current of the load. Correlating to the thesis work these can be considered as

inputs given to the power factor corrector. The power factor is a number between 1 and

0. When the power factor is not equal to 1, it is an indication that the current waveform

does not follow the voltage waveform. The closer the power factor is to 1 the closer the

current waveform follows the voltage waveform.

Real power (watts) produces real work and is known as the energy transfer component.

Reactive power is the power required to produce the magnetic fields (lost power) to

enable the real work to be done. Reactive power comes into action when there is a

mismatch between the demand and supply of power. Apparent power is the total power

that is derived from the power company in order to supply the required power to the

consumer. Although the active power is responsible for doing work, it is from apparent

power only that the current flowing into the load can be determined.

In case the load is a pure resistance, only then the real power and the product of the RMS

voltage and current will be the same i.e power factor will be 1. In any other case, the

power factor will be below 1.

Fig(4.1)current waveforms with and without PFC

These waveforms illustrate that PFC can improve the input current drawn from the mains

supply and reduce the DC bus voltage ripple. The objective of PFC is to make the input

to a power supply look like a simple resistor. This allows the power distribution system

to operate more efficiently, reducing energy consumption.

4.2.2. Cause Of Poor Power Factor

The power factor gets lowered as the real power decreases in comparison to the

apparent power. This becomes the case when more power drawn. This may result from

increase in the amount of inductive loads (which are sources of Reactive Power) which

include – Transformers, Induction motors, Induction generators (wind mill generators),

High intensity discharge (HID) lighting etc. However in such a case the displacement

power factor is affected and that in turn affects the power factor. The other cause is the

harmonic distortion which is due to presence of the non linear loads in the power

systems. Due to the drawing of non sinusoidal current there is further reduction in the

power factor.

4.2.3. Effects of Poor Power Factor

It is sometimes considered that the wattless component of a current at low

power factor is circulated without an increase of mechanical input over that necessary for

actual power requirements. This is inaccurate because internal work or losses due to

extra current are produced and must be supplied by the prime mover. Since these extra

losses manifest themselves in heat, the capacity of the machine is reduced. Moreover,

wattless components of current heat the line conductors, just as do energy components,

and causes losses in them. The loss in any conductor is always W= I2(R) where W = the

loss in watts, I = the current in amperes in the conductor, and R = the resistance in ohms.

It requires much larger equipment and conductors to deliver a certain amount of power at

a low power factor than a power factor close to 1.

4.2.4. Advantages of good power factor

1. For the same active power taken by the load, the line current drawn from the network

reduces.

2. The lower total current will translate to a less heat losses in the circuit wiring, meaning

greater system efficiency (less power wasted), therefore reduced energy costs.

3. Life time of these devices increase.

4. Penalties for bad power factor are canceled.

5. Electrical bill is reduced.

4.3. HARMONICS

Harmonics are sinusoidal waves that are integral multiples of the fundamental wave.

They appear as continuous, steady-state disturbances on the electrical network.

harmonics are altogether different from line disturbances, which occur as transient

distortions due to power surges. Harmonic frequencies are integral multiples of the

fundamental supply frequency, i.e. for a fundamental of 50 Hz, the third harmonic would

be 150 Hz and the fifth harmonic would be 250 Hz. Figure 4.2 shows a fundamental

sinewave with third and fifth harmonics.

Fig(4.2) Fundamental with third and fifth harmonics

4.3.1. Related Terms

4.3.1.1. Triplen harmonics:-

In commercial buildings, most non-linear (harmonic generating) loads are

single phase caused by electronic lighting ballast, copying machines, uninterruptible

power supplies and personal computers. Triplen harmonics are those which are the 3rd,

9th, 15th harmonic. These are the most damaging to an electrical system because these

triplen harmonic on the A-phase, B-phase and C-phase are in sequence with each other.

Meaning, the triplen harmonics present on each of the three phases add together on the

neutral rather than cancel each other out. The result can be overheating and failure of

electrical components.

4.3.1.2. Non triplen harmonics:-

Non-Triplen harmonics are either positive sequence or negative sequence.

Positive sequence harmonics rotate same direction as the electrical current, typically A-

B-C. Conversely, negative sequence harmonics rotate in the opposite direction of the

electrical current. Non-triplen harmonics are generated by three-phase loads.The voltage

waveform is very sinusoidal whereas the current waveform looks more like a series of

pulses. Thus the current waveform has a high harmonic content. The extent of the change

in shape (distortion) is quantified by measuring the harmonic content of the waveform.

Waveforms of any shape can be analyzed mathematically ,any distorted waveform can

be broken down into a series of waveforms of single frequencies, at different amplitudes

and phase positions. The different frequencies are the harmonics. (A harmonic is an

integer multiple of the base or fundamental frequency, so the 3rd harmonic of 60Hz

would be 3 x 60 z = 180 Hz.) All Power Sight models incorporate harmonics

measurement to the 50th or 63rd harmonic.

4.3.1.3. Harmonic Factor (HFn)

The harmonic factor (of the nth harmonic) , which is a measure of individual

harmonic contribution, is defined as

HFn = Von/Vo1 for n > 1 …(1)

Where V1 is the rms value of the fundamental component and Von is the rms value of the

nth harmonic component.

4.3.1.4. Total Harmonic Distortion (THD)

This term has come into common usage to define either voltage or current

“distortion factor”.The total harmonic distortion, which is a measure of closeness in

shape between a waveform and its fundamental component, is defined as

%THD = (√(U22+U3

2+…….)/U1)*100

4.3.1.5. Distortion Factor (DFn)

THD gives the total harmonic content, but it does not indicate the level of each harmonic

component. If a filter is used at the output of inverter, the higher order harmonics would

be attenuated more effectively. Therefore knowledge of both the frequency and

magnitude of each harmonic is important. The DF indicate the amount of HD that

remains in a particular waveform after the harmonics of that waveform have been

subjected to a second order attenuation (i.e., divided by n2). Thus, DFn is a measure of

effectiveness in reducing unwanted harmonics without having to specify the values of

the second-order load filter and is defined as

DFn = (1/Vo1)[ (Vo2/22) + (Vo3/32) + (Vo4/42) + …………………]

The DF of an individual (or nth) harmonic component is defined as

DFn = Von/Vo1n2 for n>1

4.3.1.6. Lowest order harmonic (LOH)

The LOH is that harmonic component whose frequency is closest to the

fundamental one, and its amplitude is greater than or equal to 3% of the fundamental

component.

4.3.2. Sources of The Harmonic

Non-linear generally do not cause reactive power to flow at the fundamental line

frequency. They can, however, draw higher RMS currents and hence add to distribution

system losses for a given load. The non-linear nature of these loads then daws non-pure

sine wave currents thus causing harmonics of the fundamental current to be present.

Since harmonic distortion is caused by non-linear elements connected to the power

system, any device that has non-linear characteristics will cause harmonic distortion.

Some of which never cause serious problems, are:

1) Transformer saturation and inrush

2) Transformer neutral connections

3) MMF distribution in AC rotating machines

4) Electric arc furnances

5) Fluorescent lighting

6) Computer switch mode power supplies

7) Battery charges

8) Imperfect ac sources

9) Variable frequency motor drives(VFD)

10) Inverters

11) Television power supplies

Switch mode power supplies, Uninterruptable Power Supplies (UPS) and electronic

lighting ballasts may have low power factors and generate harmonic distortions. This is

not because they are high frequency switching converters but rather because the input

stage is usually a low cost rectifier/capacitor filter.

4.3.3. Causes of Harmonics

The root cause of harmonics is “Non-linear loads” such as various semiconductor

devices through which current is not proportional to the applied voltage. Non-linear

loads create harmonics by drawing current in abrupt short pulses, rather than in a smooth

sinusoidal manner.

Fig(4.4) Differences between Linear and Non-Linear Loads

The terms “linear” and “non-linear” define the relationship of current to the voltage

waveform. A linear relationship exists between the voltage and current, which is typical

of an across-the-line load. A non-linear load has a discontinuous current relationship that

does not correspond to the applied voltage waveform.

4.3.4. Effects of Harmonics

The effects of harmonics on circuits are similar to the effects of stress and high

blood pressure on the human body. High levels of stress or harmonic distortion can lead

to problems for the utility's distribution system, plant distribution system and any other

equipment serviced by that distribution system. Effects can range from spurious

operation of equipment to a shutdown of important plant equipment, such as machines or

assembly lines. Harmonics can lead to power system inefficiency. Some of the negative

ways that harmonics may affect plant equipment are listed below:

Conductor Overheating: a function of the square rms current per unit volume

of the conductor. Harmonic currents on undersized conductors or cables can

cause a “skin effect”, which increases with frequency and is similar to a

centrifugal force.

Capacitors: can be affected by heat rise increases due to power loss and reduced

life on the capacitors. If a capacitor is tuned to one of the characteristic

harmonics such as the 5th or 7th, overvoltage and resonance can cause dielectric

failure or rupture the capacitor.

Fuses and Circuit Breakers: harmonics can cause false or spurious operations

and trips, damaging or blowing components for no apparent reason.

Transformers: have increased iron and copper losses or eddy currents due to

stray flux losses. This causes excessive overheating in the transformer windings.

Typically, the use of appropriate “K factor” rated units are recommended for non-

linear loads.

Generators: have similar problems to transformers. Sizing and coordination is

critical to the operation of the voltage regulator and controls. Excessive harmonic

voltage distortion will cause multiple zero crossings of the current waveform.

Multiple zero crossings affect the timing of the voltage regulator, causing

interference and operation instability.

Utility Meters: may record measurements incorrectly,resulting in higher billings

to consumers.

Drives/Power Supplies: can be affected by misoperation due to multiple zero

crossings. Harmonics can cause failure of the commutation circuits, found in DC

drives and AC drives with silicon controlled rectifiers (SCRs).

Computers/Telephones: may experience interference or failures.

CHAPTER 5

REMEDIES OF PROBLEMS IN AC-DC CONVERTER

5.1. POWER FACTOR CORRECTION

5.1.1. What is Power Factor Correction (PFC)?

Power factor correction is a modern concept which deals with increasing the

degraded power factor of a power system by use of external equipments. The objective

of this described in plain words is to make the input to a power supply appear as a simple

resistor. As long as the ratio between the voltage and current is a constant the input will

be resistive and the power factor will be 1.0. When the ratio deviates from a constant the

input will contain phase displacement, harmonic distortion or both and either one will

degrade the power factor.

In simple words, Power factor correction (PFC) is a technique of counteracting the

undesirable effects of electric loads that create a power factor ( PF ) that is less than 1.

5.1.2. What is the need of PFC ?

Constant increasing demand of consumer electronics has resulted in that the

average home has a huge variety of mains driven electronic devices. These electronic

devices have mains rectification circuits, which is the dominant reason of mains

harmonic distortion. A lot of modern electrical and electronic apparatus require to

convert ac to dc power supply within their architecture by some process. This causes

current pulses to be drawn from the ac network during each half cycle of the supply

waveform. Though a single apparatus (a domestic television for example) may not draw

a lot of reactive power or it with improvement in semiconductor devices field, the size

and weight of control circuits are on a constant decrease. This has also positively

affected their performance and functionality and thus power electronic converters have

become increasingly popular in industrial, commercial and residential applications.

However this mismatch between power supplied and power put to use cannot be detected

by any kind of meter used for charging the domestic consumers. It results in direct loss

of revenues.

Furthermore 3-phase unbalance can also be created within a housing scheme since

different streets are supplied on different phases. The unbalance current flows in the

neutral line of a star configuration causing heating and in extreme cases cause burn out

of the conductor.

The harmonic content of this pulsating current causes additional losses and

dielectric stresses in capacitors and cables, increasing currents in windings of rotating

machinery and transformers and noise emissions in many products, and bringing about

early failure of fuses and other safety components. The major contributor to this problem

in electronic apparatus is the mains rectifier. In recent years, the number of rectifiers

connected to utilities has increased rapidly, mainly due to the growing use of computers.

Hence it has become very necessary to somehow decrease the effect of this distortion.

Power factor correction is an extra loop added to the input of household applications to

increase the efficiency of power usage and decrease the degree of waste.

5.1.3. Types of Power Factor Correction (PFC) Techniques

Power Factor Correction (PFC) can be classified as two types :

1. Passive Power Factor Correction

2. Active Power Factor Correction

5.1.3.1. Passive PFC

In Passive PFC, only passive elements are used in addition to the diode

bridge rectifier, to improve the shape of the line current. By use of this category of power

factor correction, power factor can be increased to a value of 0.7 to 0.8 approximately.

With increase in the voltage of power supply, the sizes of PFC components increase in

size. The concept behind passive PFC is to filter out the harmonic currents by use of a

low pass filter and only leave the 50 Hz basic wave in order to increase the power factor.

Passive PFC power supply can only decrease the current wave within the standard and

the power factor cannot never be corrected to 1. And obviously the output voltage cannot

be controlled in this case.

Advantages of Passive PFC

It has a simple structure.

It is reliable and rugged.

In this equipments used don’t generate high-frequency EMI.

Only the construction of a filter is required which can be done easily. Hence the

cost is very low.

The high frequency switching losses are absent and it is insensitive to noises and

surges.

Disadvantages of Passive PFC

For achieving better power factor the dimension of the filter increases.

Due to the time lag associated with the passive elements it has a poor dynamic

response.

The voltage cannot be regulated and the efficiency is somewhat lower.

Due to presence of inductors and capacitors interaction may take place between

the passive elements or they may interact with the system and resonance may

occur at different frequencies.

Although by filtering the harmonics can be filtered out, the fundamental

component may get phase shifted excessively thus reducing the power factor.

The shape of input current is dependent upon the fact that what kind of load is

connected.

5.1.3.2. Active PFC

An active PFC is a power electronic system that is designed to have control

over the amount of power drawn by a load and in return it obtains a power factor as close

as possible to unity. Commonly any active PFC design functions by controlling the input

current of the load in order to make the current waveform follow the mains voltage

waveform closely (i.e. a sine wave). A combination of the reactive elements and some

active switches are in order to increase the effectiveness of the line current shaping and

to obtain controllable output voltage.

The switching frequency further differentiates the active PFC solutions into

two classes.

a) Low frequency active PFC

Switching takes place at low-order harmonics of the line-frequency and it is

synchronized with the line voltage.

b) High frequency active PFC

The switching frequency is much higher than the line frequency.

The power factor value obtained through Active PFC technique can be more than 0.9.

With a suitable design even a power factor of 0.99 can be reached easily. Active PFC

power supply can detect the input voltage automatically, supports 110V to 240V

alternative current, its dimension and weight is smaller than passive PFC power supply

which goes against the traditional view that heavier power supply is better.

Advantages of Active PFC

The weight of such a system is very less.

The dimension is also smaller and a power factor value of over 0.95 can be

obtained through this method.

Diminishes the harmonics to remarkably low values.

By this method automatic correction can be obtained for the AC input voltage.

It is capable of operating in a full range of voltage.

Disadvantages of Active PFC

The layout design is bit more complex.

Since it needs PFC control IC, high voltage MOSFET, high voltage U-fast, choke

and other circuits; it is highly expensive.

5.1.4. Methods used for Power Factor Improvement

The six schemes used for power factor improvement are:

1. Extinction angle control

2. Symmetrical angle control

3. Pulse width modulation (PWM) control

4. Sinusoidal Pulse Width Modulation (SPWM) Control

5. Three phase PWM Rectifier

6. Delta Modulation Technique

5.1.4.1. Extinction angle control

The circuit diagram of a single phase full wave half-controlled (semi) force-

commutated bridge converter is shown in Fig. 5.1(a). The thyristors, T1 & T2, are

replaced by the switches, self-commutated devices, such as power transistor or

equivalent. The power transistor is turned on by applying a signal at the base, and turned

off by withdrawing the signal at the base. A gate turn-off thyristor (GTO) also may be

used, in which case, it may be turned off by applying a short negative pulse to its gate,

but is turned on by a short positive pulse, like a thyristor. In extinction angle control,

switch, S1 is turned on at ωt = 0 , and then turned off by forced commutation at (ωt =

π−β) . The switch, S2 is turned on at ωt = π, and then turned off at (ωt = 2π−β) . The

output voltage is controlled by varying the extinction angle, β. Fig. 5.1(b) shows the

waveforms for input voltage, output voltage, input current, and the current through

thyristor switches. The fundamental component of input current leads the input voltage,

and the displacement factor (and power factor) is leading. This feature may be desirable

to simulate a capacitive load, thus compensating the line voltage drops.

(a)

(b)

Fig(5.1) Single-phase forced-commutated semi-converter

(a) circuit diagram (b) Waveforms for extinction angle control

The average output voltage is

V dc=2

2 π∫0

π−β

√2 V sin ωt d (ωt )

¿ √2π

V (1+cos β )

The value of Vdc is varied from (2√2/π)V to 0, as β varies from 0 to π.

The rms value of output voltage is

V 0=√ 22 π

∫0

π−β

2V 2 sin2 ωtd (ωt )

¿V √ 1π ((π−β )+1

2sin 2β )

Here also, Vo varies from V to 0.

This scheme of extinction angle control can also be used for single phase full wave full

controlled bridge converter with four switches, instead of two needed in the earlier case.

5.1.4.2. Symmetrical Angle Control

This control can be applied for the same half-controlled force commutated

bridge converter with two switches, S1 and S2 as shown in Fig. 5.2(a). The switch,

S1 is turned on at ωt=(π−β)/2 and then turned off at ωt=(π+β)/2 . The other switch,

S2 is turned on at ωt=(3π−β)/2 and then turned off at ωt=(3π+β)/2. The output

voltage is varied by varying conduction angle, β. The gate signals are generated by

comparing half-sine waves with a dc signal as shown in Fig. 5.2(b). The half-sine

waves can be obtained using a full wave diode (uncontrolled) bridge converter. The

gate signals can also be generated by comparing triangular waves with a dc signal as

shown in Fig. 5.2(c). In the second case, the conduction angle varies linearly with the

dc signals, but in inverse ratio, i.e., when the dc signal is zero, full conduction (β=π)

takes place, and the dc signal being same as the peak of the triangular reference

signal, no conduction (β=0) takes place. Fig. 5.2(a) shows the waveforms for input

voltage, output voltage, input current and the current through the switches. The

fundamental component of input current is in phase with input voltage, and the

displacement factor is unity (1.0). Therefore, the power factor is improved.

(a)

(b)

Fig. (5.2) Symmetrical angle control

The average output voltage is

V dc=2π

∫(π−β)

2

( π+β )2

√2V sin ωtd (ωt )=[ 2 √ 2π

V sin( β2 )]

The value of Vdc varies from 2√2V/π to 0 as β varies from π to 0.

The rms value of output voltage is

V 0=√ 22 π

∫(π−β)

2

( π+β )2

2 V 2 sin2 ( ωt )d (ωt )=V √ 1π

( β+sin β )

5.1.4.3. Pulse Width Modulation (PWM) Control

If the output voltage of single phase half-controlled converter is controlled by

delay angle, extinction angle or symmetrical, there is only one pulse per half cycle in the

input current of the converter, and as a result, the lowest order harmonic is third. It is

difficult to filter out the lower order harmonic current. In Pulse Width Modulation

(PWM) control, the converter switches are turned on and off several times during a half

cycle, and the output voltage is controlled by varying the width of pulses. The gate

signals are generated by comparing a triangular wave with a dc signal as shown in Fig.

5.3c. In this case, all the pulse widths obtained are equal. Fig. 5.3a shows the input

voltage, output voltage, and input current. The lowest order harmonic can be eliminated

or reduced by selecting the number of pulses per half cycle. However, increasing the

number of pulses would also increase the magnitude of higher order harmonics, which

could easily be filtered out. The earlier case of symmetrical angle control can be

considered as single pulse PWM.

(a)

(b)

Fig. 5.3 Pulse-width-modulation control

The details of output voltage and current waveforms of the converter are given. The

output voltage (i.e., performance parameters) can be obtained in two steps: (i) by

considering only one pair of pulses such that, if one pulse starts at ωt=α1, and ends at

ωt=α1+δ1, the other pulse starts at ωt=π+α1, and ends at ωt=(π+α1+δ1), and (2) then by

combining the effects of all pairs of pulse.

If mth pulse starts at and its width is ωt=αm its width is δm, the average output voltage due

to p number of pulses is found as

V dc=∑m=1

p [ 2π

∫α m

α m+δm

√2V sin ωtd (ωt )]¿ √2 V

π∑m=1

p

[ cosα m−cos (α m+δm ) ]

If the load current with an average value of Ia is continuous and has negligible ripple, the

instantaneous input current is expressed in a Fourier series as

is ( t )=I dc+ ∑n=1,3,5…

α

(ancos nωt+bn sin nωt )

Due to symmetry of the input current waveform, even harmonics are absent, and Idc is

zero. The Fourier coefficients are obtained as

an=1π∫0

2 π

is ( t ) cosnωt d ( ωt )

¿∑m=1

p [ 1π

∫αm

α m+δm

I a cosnωt d (ωt )− 1π

∫π+α m

π+α m+δm

I a cos nωt d (ωt )]=0

bn=1π∫0

2 π

i s ( t ) sin nωt d (ωt )

¿∑m=1

p [ 1π

∫αm

α m+δm

I a sin nωt d (ωt )−1π

∫π+αm

π+α m+ δm

I a sin nωt d (ωt )]¿

2 I a

nπ∑m=1

p

[ cosnα m−cos n ( αm+δm ) ]

So, the equation for is(t) is written as

is ( t )= ∑n=1,3,5…

α

√2 I n sin ( nωt+φn )

Whereφn=tan−1 ( an/bn )=0, andI n=√(an

2+bn2 )

√2=

bn

√2

5.1.4.4. Sinusoidal Pulse Width Modulation (SPWM) Control

It is well known that the power control of a DC load feeding by the grid is

achieved by the use of an AC-DC converter structure operating through a sPWM

technique. In figure(5.4) one can see such a converter structure consisting of a MOSFET

single phase rectifier bridge in series connected with a switching MOSFET5. In the case

of an ohmic – inductive load a parallel freewheeling diode is necessary. The rectification

becomes by the parasitic bridge MOSFET diodes, while the MOSFETs 1-4 enable the

power inversion, if an active load is considered.

Fig.(5.4) An AC-DC converter structure for supplying a DC load

The sPWM operation can be succeeded by comparison of a sinusoidal voltage waveform

(Uc) in phase to the grid voltage (Ug) with a high frequency triangular waveform in

order to obtain a switching pulse waveform. The pulse duration inside of a half

sinusoidal period is not constant and the pulse of the maximum duration is located exact

at the middle of the half period, while the pulse of the minimum duration appears at the

beginning of that, as it appears in figure 2a. Figure 3 shows the waveforms of the grid

voltage (50Hz) and the corresponding current pulse waveforms (switching frequency 5

kHz). In case of an ohmic DC load the basic harmonic of the grid current pulse

waveform (fig.3a) is in phase with the grid sinusoidal voltage waveform. If the DC load

is ohmic inductive one, then the basic current harmonic is shifted in relation to the

voltage waveform U g (fig.3b). In the case that a sinusoidal waveform Uc is leading

upon the grid voltage Ug by an angle ‘a’ via comparison to the triangular waveform

(fig.2b), a grid current pulse waveform is obtained of which the basic harmonic is shifted

to the grid voltage. In this way the grid current basic harmonic can becomes in phase

with the grid sinusoidal voltage, if we have an ohmic-inductive DC load. It means that

the power factor can be corrected. In this paper an extensive investigation of the

influence of the leading or lagging angle ‘a’ to the power factor via simulation as well as

experimentally has been carried out.

Fig.(5.5) Pulse waveforms obtained by sPWM when ‘a’=00 (2a)

and ‘a’≠00 (2b)

Fig.

(5.6). Grid voltage and current in the case of ohmic load (3a) and

ohmic inductive load (3b)

5.1.4.5. Three phase PWM Rectifier

The configuration of the conventional 3-phase AC input power supply for the

telecommunication system is illustrated in Fig.5.7. The system consists of a 3-phase

PWM rectifier where current is inputted with the power factor of 1 to output the

intermediate voltage and the DC-DC converter which isolates the intermediate voltage to

convert it into DC 48V. And six active switches are required for the former and 4

switches for the latter, and in addition, a gate driving circuit is required respectively.

Also, a DC capacitor smoothing the intermediate voltage together with a voltage detector

to control this voltage is required. In the meantime, Fig5.8 shows the basic configuration

of the proposed 3-phase rectifier. The secondary side of the isolating transformer has a

same configuration as that of the conventional system shown in Fig.1. The 3-phase AC

input side consists of a DC series circuit comprising the transformer connected between

each wire and a active switch. Primary windings of the transformer are all in magnetic

connection and the terminal voltage of each winding is equal. The terminal voltage of the

transformer and the voltage flowing through the transformer are controlled in a manner

that input phase current iR .iS and iT could he the power factor of 1. The proposed system

has less switches than the conventional system. Since the intermediate voltage required

in the conventional system is no more used in the proposed system, the DC smoothing

capacitor and the voltage detector relating to the intermediate voltage can be eliminated

from the proposed system.

Fig(5.7)Conventional circuit

Fig(5.8)Proposed circuit

Circuit Operation

The operation principle of the proposed rectifier is described by using the equivalent

circuit in Fig. 5.7. In Fig.5.7 the winding of each converter is connected to the common

core, therefore the voltages vT across three primary windings of the transformer are

equal. When QRS is in ON state, the voltage VT is equal to vRS and while Qm is in ON

state, the voltage is vTR. When all of QRS. QST and QTR are cut off, the voltage is equal to

zero due to the reflow of io through the transformer. The switching states of QRS.QST and

QTR are controlled so that the power factor of 1 for iR. is and iT can be obtained.

The circuit configuration and control method of a novel 3- phase PWM rectifier are

proposed and verified by the point of view based on the basis of experiment results.

Despite its simple configuration, superior performance is obtained including the ability

to meet the requirements of IEC61000-3-2 Class A for harmonics. The main

characteristics of the proposed rectifier are as follows:

(1) Sinusoidal current with high power factor is obtained.

(2) The circuit is simple to the extent that it can be composed of 1 isolated transformer

and 3 active switches only.

(3) The average terminal voltage of the transformer could be

kept at zero within the control period to prevent the excited current of the transformer

from increasing and to make the transformer more compact.

(4) DC power isolated by fewer conversion steps than conventional type is obtained.

(5) The electrical stress of the IGBT is less than in the fly-back system.

Fig(5.9) Efficiency and Power factor

5.1.4.6. Delta Modulation Technique

It is well known that the input voltage and current waveforms of ideal AC-DC

converters are sinusoidal and in phase. However, the current waveforms of practical AC-

DC converters are non-sinusoidal and contain certain harmonics. As a result of that, the

phase shift between the input current fundamental component and the voltage of AC-DC

converter is increased. The power factor (PF) which depends on the delay angle of AC-

DC converter, the phase shift between the input current and voltage and the circuit

component are then reduced. With the aid of modern control technique and the

availability of high speed semiconductor devices, the input current can be made

sinusoidal and in phase with the input voltage, thereby having an input power factor of

approximate unity. Delta modulation, also known as ripple controller control, maintains

the AC DC converter input current within a defined window above and below a

reference sine wave. The greatest benefits of delta control are that it offers fast load

transient response and eliminates the need for feedback-loop compensation .The other

well-known characteristic is the varying operating frequency.

Principle Of Operation Of DMT

Figure5.7 shows a unity PF circuit that combines a full bridge AC-DC converter and a

full bridge voltage

inverter (frequency converter). The control circuits of AC-DC converter have two main

functions:

• Ensuring a unity PF (sinusoidal current which is in phase with the input voltage).

• Ensuring a constant voltage Ud across the capacitor

The first function 1 can be easily realized, as the boundary values of the hysteresis band

Iw2 and Iw1 are generated such that the 1st harmonic component of the current derived

from the trolley is in phase with the voltage, as will be shown later in the paper. The

switching transistors change their state as soon as current Isa reaches the reference

boundary value of the hysteresis band. The second function is obtained by using a

voltage controller Rv of voltage Ud which generates a suitable value for the reference

current Iref that is derived from the trolley (during motoring regime Iref>0, and during

braking regime Iref<0). From the simulation obtained in the next sections it is clearly

evident that there is a certain relationship between the amplitude of current Ia, current Id,

current Ic and consequently with voltage Ud at the output.The voltage controller Rv

regulates the mean value of voltage Ud which is always estimated at the instant of

sampling of Rv. With respect to the required current waveform it is good to have Iw

constant during each halve cycle of the required current. This delta method of control

keeps the input current Isa within the window hysteresis band around the reference

current Iref which leads the sinusoidal value of this current Ia to be in phase with the

sinusoidal voltage Ua and without any dc offset. To obtain a sinusoidal current the

sampling of the controller must be synchronized with the current waveform and the

sampling period must be TT = 0.01. A new value of Iref must be estimated exactly at the

zero-crossing of current Ia.

Fig(5.10) Frequency Converter Circuit Arrangement For Unity PF

This conclusion is based on the assumptions that filter L1, C1 at the output of the AC -

DC converter is not added into the dc circuit and that all components used are ideal.

However, in practical applications, these assumptions have certain types of error. Output

voltage ripple also includes output capacitor C1- caused ripple and L1-caused ripple. And

all components used are not ideal, so there will be delay in the whole control loop. Given

these realities, the current waveform Id then includes a clear harmonic component of

frequency f =100Hz . This component increases the ripple of current Id and voltage Ud.

When the controller sampling period is TT =0.01s , then the output of the controller

under steady state conditions Iref is purely constant. The sampling period TT of Rv may

cause a time delay in the voltage control circuit. Therefore, it is necessary to reduce TT

during transients. This therefore will result in a staircase waveform of the current but the

dynamic properties of the voltage control loop are improved substantially. Figure 5.10

shows clearly how the voltage and current derived from the trolley are measured. It is

therefore evident that voltage Ua must be measured at the primary side of the

transformer. The current on the other hand, may be measured at the secondary side since

current Ia may be obtained as the average value of current Isa.

5.2. Harmonic Reduction Technique

As IEEE-519 standard says “The harmonic contents should not exceeds above

5%”.The important point to be noted is that, recently due to increasing use of power

electronic units, utility or electricity supply agencies (boards), have restricted that the

power is drawn by the consumers, so as to decrease the harmonic content in the input

current, or make it sinusoidal, and at the same time, improved load power factor is

achieved. Some schemes are presented, in brief.

5.2.1. Low pass (L-C) filter circuit on ac side

Before going into the aspect, let us take a rebook at the input current drawn in the

circuit shown in Fig. 5.11a. Assuming that output (load) current is constant (dc) without

any ripple, the ac input (source) current is square wave in nature , as this current changes

sign, when the input voltage changes sign. If a Fourier analysis of the above current is

done, there are harmonic components present in it. Just as filters have been used on the

output (dc) side, a low pass (L-C) filter is used on the input (source) side to reduce the

harmonic components in the input current. The inductors used tend both to improve the

power factor and also reduce harmonics as given earlier. The overall energy efficiency

remains the same, though additional losses occur in the inductors, but conduction losses

in the diodes are reduced.

Fig.5.11(a) Output and input currents

Fig. 5.11 (b) Low pass (L-C) filter on source (AC) side

5.2.2. Active Shaping of Input (line) Current

By using a power electronic converter for current shaping, as shown in Fig. 5.12(a),

it is possible to shape the input current drawn by the single phase bridge converter

(rectifier) to be sinusoidal and also in phase with the input voltage. The choice of the

power electronic converter is based on the following considerations:

• No need for electrical isolation between the input (dc) and output (dc) sides

• the power flow is always unidirectional from the utility side to the equipment

• the cost, power losses and size of the circuit used should be small.

The basic principle of operation is as follows. At the input side, the current, is, is desired

to be sinusoidal, and also in phase with the voltage, vs, as shown in Fig. 5.12(b).

Therefore, at the full wave bridge converter output, iL and sv have the same waveform as

shown in Fig. 5.12(c).

Fig. 5.12 Active harmonic filtering: (a) step-up converter for current shaping; (b) line

waveforms; (c) │vs│ and iL.

5.2.3. Using Multi-Pulse Rectifiers

One of the technical applications where the multi-pulses rectifiers are used is the

area of the adjustable speed drives. Electrical drives, from which AC adjustable speed

drives with induction motors are the most widely used, belong to significant sources of

electromagnetic interference. Harmonic currents generated by them into the supply

network can have a negative impact on the network and devices connected into it. There

are various technical solutions for reducing harmonics of the adjustable speed drives

such as AC and DC reactors, shunt passive filters, broad-band harmonic filters, active

filtering, drives with input active front end rectifier instead of the standard six pulse one,

multi-pulse rectifiers, supplying high-power drives or sensitive loads from separated

transformers, or some combinations of them .Multi-pulse rectifiers provide a good

solution for harmonics suppression, because they are able to eliminate theoretically, or in

practice to reduce by a significant way ,some harmonics of important orders . In the case

of the twelve-pulse rectifiers, it is the elimination mainly of the dominant 5th and 7th

harmonics, at the eighteen-pulse rectifiers, beside both these harmonics, also the

elimination of following significant orders of 11th and 13th harmonics. Input current of

the electric drive with this type of input rectifier has then lower level of distortion. The

number of eliminated harmonics depends on the number of pulses of the used rectifier –

the rectifier with higher pulse number has higher number of eliminated harmonics, which

means lower input current distortion with lower total harmonic distortion

THDi.

1. Harmonic Elimination Using Twelve-pulse Rectifier

AC adjustable speed drives with the standard input six pulse diode rectifier in the

frequency converter draw a distorted current from the network, which can be, with the

reference to its simplified waveform shown in Fig.5.13, described by the formula:

i (ωt )=∑h=1

∞8

hπI m sin

hd2

coshπ6

sinhπ2

sin (hωt )…….(1)where

h – harmonic order

d – rectifier diode operating angle

Im – current magnitude

Fig.(5.13) Simplified input current waveform of the six-pulse diode rectifier.

It can be determined from the equation (1) that only harmonics of these orders are in the

current spectrum:

h = 5, 7, 11, 13, 17, 19, 23, 25, 29, 31...... (2)

By connecting of two six-pulse rectifiers in parallel or in series and connected to the

three-winding transformer, input current spectrum with harmonics of these orders is

theoretically achieved:

h = 11, 13, 23, 25, 35, 37.....(3)

(which means that dominant harmonics of the 5th and 7th orders are eliminated. To

achieve this elimination, phase shift between secondary phases voltages is necessary, e.g.

the designed 300 as it is shown in Fig. 5.14.

Fig.(5.14)Phasor diagram of secondary voltages of the transformer Dd0y1 with the 300

phase shift

The connection of the twelve-pulse rectifier with the Dd0y1 transformer with marked

harmonic currents in phase A of the primary winding and in phase a of both secondary

windings is shown in Fig. 5.15.

Fig.(5.15) Twelve-pulse rectifier with the transformer Dd0y1 with marked

harmonic currents in one phase.

To demonstrate the elimination of the 5th and 7th harmonics, magnetomotive forces are

calculated for both secondary windings from the formulas:

Fmh21 = N21Ihf 21 (S1)……….. (4)

Fmh22 = N22Ih22 (S2)………… (5)

Where

h – harmonic order (the 5th and 7th)

N21 – number of turns of secondary winding S1

N22 – number of turns of secondary winding S2

Ihf21 – phase harmonic currents of secondary winding S1

Ih22 – phase harmonic currents of secondary winding S2

The ratio of turn numbers of the windings S1 and S2 is determined by the formula:

N 21

N 22

=√3……………(6)

and the currents:

I h 21

I h 22

= 1√3

………………….(7)

It is obvious from the formulas (4)–(7) that the harmonics will be eliminated in the

primary winding if the phase shift between phase harmonic currents and thereby

magnetomotive forces of the secondary windings is 180 degrees. It will be achieved if

the phase shift between 1st harmonics of secondary phases voltages or currents is the

designed 30 degrees, which means the phase shift –150 degrees for the 5th harmonic, as

shown in Fig. 5.16(a), and 210 degrees the for 7th harmonic, as shown in Fig. 5.16(b) for

one phase. Harmonics of the 17th, 19th, 29th, 31st… order are also eliminated by the same

principle.

Fig.(5.16)Elimination of the 5th harmonic (a) and of the 7th harmonic (b) for one

phase of secondary windings.

2. Harmonic Elimination Using Eighteen-pulse Rectifier

Eighteen-pulse rectifier is composed of three six-pulse rectifiers connected in parallel or

in series. In spectrum of the input current only harmonics of these orders will be

theoretically:

h = 17, 19, 35, 37.......... (8)

which means that significant harmonics of the 5th, 7th, 11th and 13th orders are

eliminated. To achieve this elimination, phase shift of 20 degrees between secondary

phase voltages is necessary,as it is shown in Fig. 5.17.

Fig.(5.17) Phasor diagram of secondary voltages of the transformer

Dy1z1+200z1-200 with the 200 phase shift

The connection of the eighteen-pulse rectifier with the transformer Dy1z1+200z1-200

with marked harmonic currents in phase A of the primary winding and in phase a of

secondary windings is shown in Fig. 5.18.

Fig.(5.18) Eighteen-pulse rectifier with the transformer Dy1z1+200z1-200 with marked

harmonic currents in one phase.

To demonstrate the elimination of the 5th, 7th, 11th and 13 th harmonics, magnetomotive

forces for the 1st harmonic current are calculated for all secondary windings from the

formulas:

Fm121 = N21I121 (S1)…………… (9)

Fm122a = N22aI122 (S2a)……….. (10)

Fm122b = N22bI122 (S2b)……….. (11)

Fm123a = N23a I123 (S3a)……….. (12)

Fm123b = N23bI123 (S3b)………... (13)

where the number of turns and the currents of secondary windings are marked in Fig.

5.18 .Phasor diagrams of currents and magnetomotive forces are shown in Fig. 5.19.

Fig.(5.19) Currents (a) and magnetomotive forces (b) of secondary windings in one

phase of the transformer Dy1z1+200z1-200 (for the 1st harmonic

current)

The particular phase shifts shown in Fig. 5.19 are between two magnetomotive forces of

the 1st harmonic current. For the 5th harmonic they must be multiplied by five and

turned in the opposite direction. The result is in Fig. 5.20. The sum of the magnetomotive

force phasors is equal to zero, so the 5th harmonic current will not be in the primary

current spectrum. The analogous principle can be applied for the next harmonics of the

7th, 11th and 13th order, it is only necessary to bear in mind the phasor turning in the

correct direction.

Fig.(5.19)Magnetomotive forces of secondary windings in one phase of the

transformer Dy1z1+200z1-200 (for the 5th harmonic current)

Fig.(5.20)The sum of the magnetomotive force phasors from Fig. 5.19

5.3. Applications of AC-DC Converter

1. BP5034D5 for vacuum cleaner

2. BP5034D12 for cordless telephone

3. BP5034D15 for DC motor control

4. BP5034D24 for rice cookers

5. BP5038A for electric carpets

6. BP5041A for invertor lighting

7.BP5046 for washing basket

8. BP5065C for washing machine

9. BP5085-15 for air cleaner

And many more…………………………

• Heating and lighting control• Induction heating• Uninterruptible power supplies (UPS)

• Fluorescent lamp ballasts: Passive; Active• Electric power transmission• Automotive electronics• Electronic ignitions• Motor drives• Battery chargers• Alternators• Energy storage• Electric vehicles• Alternative power sources: Solar; Wind; Fuel Cells

And more……..

CHAPTER 6

CONCLUSION

report

Documents

spectral distribution

visual sensation

total harmonic

delta modulation

poor power

power factor

split power

total harmonic