report
TRANSCRIPT
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
(a) circuit diagram (b) waveforms
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.
Output average voltage,
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
(a) circuit diagram (b) waveforms
Output average voltage,
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
(a) circuit diagram (b) waveforms
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
(a) circuit diagram (b) waveforms
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”.
Output average voltage,
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
(a) circuit diagram (b) waveforms
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
(a) circuit diagram (b) waveforms
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
(a) circuit diagram (b) waveforms
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
(a) circuit diagram (b) waveforms
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