AC-AC Converter Circuit Breaker Testing

A Novel Single-Phase AC-AC Converter for Circuit Breaker Testing Application
A Novel Single-Phase AC-AC Converter for
Circuit Breaker Testing Application
Department of EEE Page 1

ABSTRACT
This Project a novel single-phase ac-ac converter with power factor correction
and output current control for circuit-breaker testing according to the IEC 60898
standard. The important advantages of the proposed circuit are low component count
and fast responses for the standard requirement, especially a current step at the
beginning of the test. The proposed single-phase ac-ac converter can operate in either
buck or boost mode to accommodate the need for a wide range of output current while
satisfying the ramping and step current requirements in the standard. The control
circuits consist of two parts, dc voltage control of dc-link capacitors and ac output
current controls operating simultaneously. The proposed circuit is verified through
both computer simulation and hardware experiment. An example of a 50A circuit
breaker testing according to the IEC 60898 is demonstrated in the project.

CHAPTER -1
INTRODUCTION

INTRODUCTION
A CIRCUIT-BREAKER (CB) is indispensable equipment in residential,
commercial and industrial systems. It is designed to protect an electrical circuit from
damage caused by overload or short circuit. The capability of interrupting the flow of
the current to protect devices enables its utilization in virtually every applications. The
time tripping characteristics
of the CB and the test procedures detailed in IEC 60898 are necessary in the process of
quality control. Commercially available current sources for CB testing are designed
using a motor-driven tap-changing auto-transformer for ac output current regulation.
Recently, several ac-ac converters have been developed and improved in terms of
higher current rating capability and higher efficiency. Also, they have included the
power factor correction (PFC) to regulate the input current to be sinusoidal wave
shaping with nearly unity power factor. In practice, the ac-ac converters are widely
applied to various industrial applications such as UPS, voltage stabilizer, electric
welding, and etc .
Several topologies of single-phase ac-ac converter had been reported. The
single-phase ac-ac two-leg, three-leg and fourleg (two full-bridges) converters have
been presents [. They are widely adopted choices of converters in UPS, motor drive or
grid-connected applications . These topologies consist mainly of two stages; a
controlled rectifier (e.g., boost PFC topology) and a single-phase inverter. They can be
operated in either buck or boost mode for the desired level of ac output voltage. The
output frequency is controllable and can be set to the values different from the input
frequency. The three-leg and two-leg single-phase ac-ac converters are operated in hard
switching scheme, producing the switching loss and electromagnetic interferences . In
addition, the two-leg single-phase converter has high ripple voltage at the dc-link
capacitors. In a buck-type ac-ac converter reported in the ac output voltage is controlled
using the modified sinusoidal pulsed-width modulation (SPWM).
This topology introduces a distortion on the ac output voltage at the zero-
crossings. Also, the output voltage is limited to the values less than the input voltage

due to its buck-type topology. Resonant converter topologies have also been used in the
ac-ac conversion applications . The resonant converter is suitable for applications with
a fixed frequency where the output frequency cannot be significantly differed from this
frequency. For instance, the 60 Hz signal is not achievable with the 50 Hz input
signal.In ac-ac z-source converters, the z-source network is primarily used to store
energy and the output voltage is load dependent . The output voltage is varied through
the duty ratio created from a PWM signal. The key requirements of ac-ac converter for
circuit breaker testing are a wide range of output voltage with low harmonic distortion
and high input power factor. Therefore, it must becapable of operating in buck and
boost modes with the required dynamics of step current and ramp rate specified .
The single-phase ac-ac converter has been presented in the system application
for CB testing according to the CB testing standard (IEC 60898) in Table I. It used four
switches to control input current and output current with the same ground. The first part
integrates rectifier and boost converter to regulate the dc-link voltages and control the
ac input current with sinusoidal waveform in phase with the ac input voltage. The
output current control is based on half-bridge topology to drive the positive and
negative pulses by means of SPWM technique to the output. Recently, a novel single
phase ac-ac converter has been presented and it is also suitable for CB testing
application because it supports all key requirements of CB testing standard. This
converter operates similarly with converter in , but its difference is about the converter
output separated ground instead of shared ground. The ac-ac converter application for
CB testing does not require the ac current output sharing the same ground with ac input
voltage. As a result, the novel ac-ac converter has been selected to implement the
proposed system for CB testing application. In addition, the proposed system is realized
by the digital implementation according to the CB testing standard (IEC 60898). The
novel ac-ac converter topology can be compared with the four-leg converters (two full-
bridges) . They are the same operation both input current control and output
voltagecontrol with separated ground between input and output sides. A number of
components is counted and compared between novel converter and four-leg converter
as shown in Table II. The novel topology has the reduced number of switches from
eight to only four switches. Although the number of diodes is increased but the cost of

switches is much more expensive than one of diodes.The CB testing under the over
current test condition consists of five tests, a, b, c, d, and e where the test currents vary
from 1.13 to 20 times of the nominal rated current of CB (In) as shown in Table I . The
first three tests (a, b, and c) require that the peak of the test current is gradually varied
whereas a step change is required in the other tests. The time current characteristics for
over current CB test provided in Table I are the requirements for various CB types (B,
C and D). Note that the current characteristics of each test are determined by the initial
condition specified in the standard. For instance, the initial condition for tests a, c, d, or
e is cold, meaning that there is no previous loading of CB before testing.
Fig.1. Proposed ac-ac converter with dc-link voltage and ac output current controllers.

Power electronics
Introduction:
The application of solid state electronics in which the electric power is
controlled and converted is called power electronics. As it deals with designing,
computation, control, and integration of electronic systems where energy is processed
with fast dynamics which is non linear time varying ,it is referred in electrical and
electronic engineering as a research subject.

Mercury arc valves are the first electronic devices with high power. The conversion is
performed in modern systems with thyristors, diodes, transistors which are the
semiconductor switching devices, pioneered in the 1950s by R.D.Middle Brook and
others. In power electronics processing of substantial amounts of electrical energy is
done in contrast to electronic systems concerned with transmission and processing of
signals and data. The most typical power electronics device found in many consumer
electronic devices, such as battery chargers, personal computers, television sets, etc is
an AC/DC converter. Its power ranges from tens of watts to several hundred watts. The
variable speed drive which is used to control an induction motor is the common
application in industry. VSDs power ranges from few hundred watts to tens of mega
watts.
HISTORY:
Power electronics had started with the development of the mercury arc rectifier
which was invented by Peter Cooper Hewitt in 1902.It converts alternating current
(AC) to direct current(DC).A research had started and continued on applying of
thyratrons and grid controlled mercury arc valves for power transmission from 1920’s.
A valve with grading electrodes was developed by Uno Lamm which made mercury arc
valves useful for transmission of high voltage direct current.
In 1947 Walter H.Brattain and john bardeen invented the bipolar point contact
transistor at Bell labs under the direction of William Shockley. The bipolar junction
transistor which was invented by shockley, improved the performance and stability of
transistors, and reduced the costs of transistors. In the 1950s, semiconductor power
diodes were invented which replace the vaccum tubes. In 1956 there was great increase
in the range of power electronics applications with the introduction of the silicon
controlled rectifier (SCR) by general electric. The switching speed of bipolar junction
transistors was allowed for high frequency DC/DC converters in1960s. Power
MOSFET became available from 1976 and the Insulated gate bipolar transistor(IGBT)
was introduced in 1982.

TYPES OF SYSTEMS:
The power conversion systems are classified based on the type of input and output
power as follows:
 AC to DC (rectifier)
 DC to AC(inverter)
 DC to DC (DC to DC converter)
 AC to AC(AC to AC converter)
 DC/AC converters (inverters)
An AC output waveform from a DC source is produced in DC to AC converters.
Applications of DC/AC converters are adjustable speed drives (ASD), uninterruptable
power supplies (UPS), active filters, photovoltaic generators, voltage compensators,
and flexible AC transmission systems(FACTS).
Topologies used in these converters are divided into two different types:
 Voltage source inverters .
 Current source inverters.
In Voltage source inverters (VSIs) voltage waveform is a output which is
independently controlled. In current source inverters (CSIs) current waveform is the
controlled AC output. The power switching devices which are fully controllable
semiconductor power switches results the DC to AC power conversion. Fast transitions
than the smooth ones are produced as the output waveforms are made up of discrete
values. By the controlling of modulation technique it is detected when the power
valves are on and off and for how long around the fundamental frequency the ability to
produce near sinusoidal waveforms is dictated. Space-vector technique, and the

selective-harmonic technique, the carrier-based technique, or pulse width modulation,
are included in the common modulation techniques.
We can use voltage source inverters in both single-phase and three-phase
applications. Single-phase VSIs are widely used for power supplies, single-phase UPSs,
and utilize half-bridge and full-bridge configurations, and when used in multicell
configurations, elaborate high-power topologies. In applications where sinusoidal
voltage waveforms are required, such as ASDs, UPSs, and some types of FACTS
devices such as the STATCOM. Three-phase VSIs are used. In the case of active filters
and voltage compensators where arbitrary voltages are required these are used. To
produce an AC output current from a DC current supply Current source inverters are
used. This type of inverter is practical for three-phase applications where high-quality
voltage waveforms are required.
There was a widespread interest on a relatively new class of inverters, called
multilevel inverters. Because of the fact that power switches are connected to either the
positive or to the negative DC bus, operation of CSIs and VSIs can be classified as two-
level inverters. A sine wave could better approximated by the AC output if more than
two voltage levels were available to the inverter output terminals. The multilevel
inverters, although more complex and costly, offer higher performance due this reason.
Whether or not they require freewheeling diodes, each inverter type differs in the DC
links used. Depending on its intended usage it can be made to operate in square wave or
pulse width modulation mode. We can implement PWM in many different ways and
higher quality waveforms are produced where a simplicity is offered in square wave
mode.
The output inverter section is fed from a constant-voltage source in Voltage
Source Inverters (VSI). The modulation technique which is to be selected for a given
application is determined by the wanted quality of the current output waveform. The
VSI output has discrete values. At the selective harmonic frequencies the loads should
be inductive for a smooth current waveform to be obtained. Capacitive load causes a
choppy current waveform with large and frequent current spikes to be received by load
if the load and source has no inductive filtering between them.

AC TO AC CONVERTERS
The control of voltage, phase, and frequency of the waveform allows the
converting of AC power to Ac power applied to a load from a supplied AC system.
Based on the frequency of the wave form there are two main categories that can be used
to separate the type of converters. The converter which does not allow the user to
modify the frequencies are known as AC regulators or AC voltage Controllers. The
converters which allow the user to change the frequency are known as frequency
converters for AC to AC conversion.
There are three types of converters in frequency converters. They are matrix
converter, cyclo converter, DC link converter (aka AC/DC/AC converter).
AC Voltage Controller: The AC voltage Controller is used to vary the RMS
voltage across the load at constant frequency. Pulse Width Modulation AC Chopper
Control (PWM AC Chopper Control), ON/OFF Control, Phase-Angle Control is the
three control methods that are generally accepted. In Three-phase circuits as well as
Single-phase circuits, all the three methods can be implemented.
ON/OFF Control: For speed control of motors or for heating loads this method
id typically used. This control method involves turning the switch off for m integral
cycles and turning the switch on for n integral cycles. Undesirable harmonics are to be
created because of turning the switches off and on. During zero-current conditions
(zero-crossing) and zero-voltage conditions the switches are turned off and on,
effectively reducing the distortion.
Phase-Angle Control: To implement a phase-angle control on different
waveforms there are various circuits exits such as full-wave or half-wave voltage
control. SCRs, Triacs and diodes are typically used power electronic components. The

user can delay the firing angle with the use of these components in a wave which will
only cause part of the wave to be outputted.
PWM AC Chopper Control: The other two control methods often have poor
harmonics, output current quality and input power factor .PWM can be used instead of
other methods to improve these values. Turning the switches off and on several times
within alternate half-cycles of input voltage can be done by this PWM AC Chopper.
Matrix Converters and CycloConverters: For ac to ac conversion in industries
CycloConverters are widely used, because they are able to use in high-power
applications. They are commutated direct frequency converters that are synchronized
by a supply line. The output voltage waveforms of the cycloconverters have complex
harmonics with higher order harmonics, machine inductance can filter the harmonic
which causes the machine current to have fewer harmonics, losses and torque
pulsations can be caused by the remaining harmonics. There are no inductors or
capacitors in a cycloconverter unlike other converters i.e. any storage devices. For this
reason the instantaneous output power and the input power are equal.
Single-Phase to Single-Phase Cycloconverters: Because of the decrease in both
the power and size of the power electronics switches these started drawing more
interest recently. The single-phase high frequency ac voltage can be either sinusoidal or
trapezoidal. For control purpose these might be zero voltage commutation or zero
voltage intervals.
Three-phase to Single-Phase cycloconverters: These three-phase to single-phase
cycloconverters are two kinds: 3φ to 1φ half wave cycloconverters and 3φ to 1φ bridge
cycloconverters. Either at polarity both the negative and positive converters can
generate voltage, resulting positive current supplied by the positive converter and the
negative current supplied by the negative converter.
New forms of cycloconverters are being developed with recent advances such as
matrix converters. The matrix converter utilize bi-directional, bipolar switches is the

first change first noticed in them. Matrix of 9 switches connecting the three input
phases to the three output phases are present in a single phase to a single phase matrix
converter. At any time without connecting any two switches any output phase and input
phase can be connected together from the same phase at the same time, otherwise a
short circuit of the input phase will be caused. Matrix converters are more compact,
lighter and versatile than other converter solutions. As a result to regenerate energy
back to the utility they are able to achieve higher temperature operation, higher levels
of integration, natural bi-directional power flow and broad output frequency.
ELECTRICAL MACHINE:
Conversion of Electrical energy into Mechanical energy can be done by the electric
machine. A machine that converts mechanical energy to electrical energy is a
Generator, A machine which converts electrical energy to mechanical energy is a
Motor, and the one which changes the voltage level of an alternating current is a
Transformer.
1.2.1GENERATOR:

Fig 1.Electrical generator
An electric device which converts mechanical energy to electrical energy is
called a Generator. It forces electrons to flow through an external electrical circuit.it is
similar to a water pump. A water pump does not create the water inside but creates a
flow of water. The prime mover which is the source of mechanical energy may be a
turbine steam engine or reciprocating, an internal combustion engine, water falling
from a water wheel or turbine, a hand crank or any other source of mechanical energy.
Mechanical or Electrical terms are used to describe the two main parts of the
Electrical machine. The rotating part is the rotor; the stationary part is the stator in
mechanical terms. The power producing component is the Armature and the magnetic
field component is the field in Electrical terms. The armature can be on either the stator
or the rotor. The permanent magnets or the electromagnets are mounted on either the
stator or the rotor provides the mechanical energy.
The generators are divided into two types:
AC Generators and DC Generators
AC Generator:
A machine which converts the mechanical energy into alternating current electricity is
the AC generator because the power transferred into the armature current is greater than
the power transferred into the field current. They always have the rotor with field
winding and the stator with armature winding.

Several types of AC Generators are there:
An induction generator in which the currents in the rotor are induced by the
stator magnetic flux. The rotor can be drived above the synchronous speed by the prime
mover which causes the opposing rotor flux. This rotor flux cuts the stator coils by
producing active current in them, thus sending power back to the grids. An induction
generator cannot be an isolated source of power because it draws reactive power from
the connected system.
In a synchronous generator (Alternator) the separate current source provides the current
for the magnetic field.
DC Generator:
It produces direct current electrical energy from mechanical energy. With in
mechanical limits it can operate at any speed and always output a direct
currentwaveform. The direct current generators known as dynamos works on exactly
the same principle as the alternators but have a commutator on the rotating shaft which
converts the armature alternate current into direct current.
1.2.2 MOTOR:
Fig 2 Electric motor
An electric device which converts electrical energy into mechanical energy is electric
motor which is a reverse part of the generator. The electric motor operates through
interacting magnetic fields and current carrying conductors to generate rotational force.

The motors and generators have many similarities and many motors can work as
generators and vice versa. These are used in industrial fans, machine tools, house hold
appliances, disk drives and power tools. They may be powered by Alternate current and
Direct current.
Motors are divided into two types: AC motors and DC motors.
AC Motor:
It converts alternating current into mechanical energy. It consists of two basic
parts an outside stationary stator having coil with an alternating current to produce a
rotating magnetic field and an inside rotor attached to the output shaft that is given a
torque by the rotating field. By the type of rotor used the ac motors are divided into two
types.
Induction (asynchronous)motor: the induced current creates the rotor magnetic
field. To provide the induced current the rotor must turn slightly slower than the stator
magnetic field. Squirrel-cage rotor,solid core rotor and the wound rotor are the three
types of induction motor rotors.
Synchronous motor does not base on induction so it can rotate exactly at the
supply frequency or sub-multiple. The direct current or a permanent magnet generates
the magnetic field of the rotor.
DC Motor:
A brushed dc motor uses rotating electrical magnets, internal commutation, and
stationary permanent magnets to produce torque from dc power supplied to the motor.
The electric current from the commutator to the spinning wire windings of the rotor
inside the motor can be carried by the brushes and springs. Brushless dc motors uses a
rotating permanent magnet in the rotor and stationary electric motors in the motor. A
motor controller converts d to ac. The complication of transferring power from outside
the motor to the spinning rotor eliminates in this brushless motors. A stepper motor is

an example of brushless synchronous dc motor and it can divide a full rotation into
large number of steps.
1.2.3 TRANSFORMER:
A static device which converts alternating current from one voltage level to
another level (lower or higher) or the same level without changing the frequency is
called aTransformer. Through inductively coupled conductors(transformer coils) the
transformer transfers electrical energy from one circuit to another. A varying magnetic
flux in the transformer core is created by a varying electric current in the first or
primary winding and thus a varying magnetic field through the secondary winding. A
varying Electro Motive Force (EMF) or “Voltage” in the secondary winding induces by
the varying magnetic field, This effect is called Mutual Induction.
There are two types of transformers
Step-Up transformer
Step-down transformer

CHAPTER-2
POWER FACTOR CORRECTION

POWER FACTOR CORRECTION:
PFC (power factor correction; also known as power factor controller) is a
feature included in some computer and other power supply boxes that reduces the
amount of reactive power generated by a computer. Reactive power operates at right
angles to true power and energizes the magnetic field. Reactive power has no real value
for an electronic device, but electric companies charge for both true and reactive power
resulting in unnecessary charges. PFC is a required feature for power supplies shipped
to Europe.
In power factor correction, the power factor (represented as "k") is the ratio of
true power (kwatts) divided by reactive power (kvar). The power factor value is
between 0.0 and 1.00. If the power factor is above 0.8, the device is using power
efficiently. A standard power supply has a power factor of 0.70-0.75, and a power
supply with PFC has a power factor of 0.95-0.99.
PFC is not used solely for computer power supplies. In other industries, PFC
equipment is used to reduce the reactive power produced by fluorescent and high bay
lighting, arc furnaces, induction welders, and equipment that uses electrical motors.

Power factor correction of linear loads:
A high power factor is generally desirable in a transmission system to reduce
transmission losses and improve voltage regulation at the load. It is often desirable to
adjust the power factor of a system to near 1.0. When reactive elements supply or
absorb reactive power near the load, the apparent power is reduced. Power factor
correction may be applied by an electric power transmission utility to improve the
stability and efficiency of the transmission network. Individual electrical customers
who are charged by their utility for low power factor may install correction equipment
to reduce those costs.
Power factor correction brings the power factor of an AC power circuit closer to
1 by supplying reactive power of opposite sign, adding capacitors or inductors that act
to cancel the inductive or capacitive effects of the load, respectively. For example, the
inductive effect of motor loads may be offset by locally connected capacitors. If a load
had acapacitive value, inductors (also known as reactors in this context) are connected
to correct the power factor. In the electricity industry, inductors are said
to consume reactive power and capacitors are said to supply it, even though the energy
is just moving back and forth on each AC cycle.
The reactive elements can create voltage fluctuations and harmonic noise when
switched on or off. They will supply or sink reactive power regardless of whether there
is a corresponding load operating nearby, increasing the system's no-load losses. In the
worst case, reactive elements can interact with the system and with each other to create
resonant conditions, resulting in system instability and severe overvoltage fluctuations.
As such, reactive elements cannot simply be applied without engineering analysis.
An automatic power factor correction unit consists of a number
of capacitors that are switched by means of contactors. These contactors are controlled
by a regulator that measures power factor in an electrical network. Depending on the
load and power factor of the network, the power factor controller will switch the
necessary blocks of capacitors in steps to make sure the power factor stays above a
selected value.

Instead of using a set of switched capacitors, an unloaded synchronous
motor can supply reactive power. The reactive power drawn by the synchronous motor
is a function of its field excitation. This is referred to as a synchronous condenser. It is
started and connected to the electrical network. It operates at a leading power factor and
puts vars onto the network as required to support a system's voltage or to maintain the
system power factor at a specified level.
The condenser's installation and operation are identical to large electric motors.
Its principal advantage is the ease with which the amount of correction can be adjusted;
it behaves like an electrically variable capacitor. Unlike capacitors, the amount of
reactive power supplied is proportional to voltage, not the square of voltage; this
improves voltage stability on large networks. Synchronous condensers are often used in
connection with high-voltage direct-current transmission projects or in large industrial
plants such as steel mills.
For power factor correction of high-voltage power systems or large, fluctuating
industrial loads, power electronic devices such as the Static VAR
compensator or STATCOM are increasingly used. These systems are able to
compensate sudden changes of power factor much more rapidly than contactor-
switched capacitor banks, and being solid-state require less maintenance than
synchronous condensers.
Power factor correction (PFC) in non-linear loads:
Passive PFC:
The simplest way to control the harmonic current is to use a filter that passes
current only at line frequency (50 or 60 Hz). The filter consists of capacitors or
inductors, and makes a non-linear device looks more like a linear load. An example of
passive PFC is a valley-fill circuit.
A disadvantage of passive PFC is that it requires larger inductors or capacitors
than an equivalent power active PFC circuit. Also, in practice, passive PFC is often
less effective at improving the power factor.

Active PFC:
Active PFC is the use of power electronics to change the waveform of current drawn by
a load to improve the power factor.[22]
Some types of the active PFC
are boost, buck, buck-boost and synchronous condenser. Active power factor correction
can be single-stage or multi-stage.
In the case of a switched-mode power supply, a boost converter is inserted between the
bridge rectifier and the main input capacitors. The boost converter attempts to maintain
a constant DC bus voltage on its output while drawing a current that is always in phase
with and at the same frequency as the line voltage. Another switched-mode converter
inside the power supply produces the desired output voltage from the DC bus. This
approach requires additional semiconductor switches and control electronics, but
permits cheaper and smaller passive components. It is frequently used in practice. For
example, SMPS with passive PFC can achieve power factor of about 0.7–0.75, SMPS
with active PFC, up to 0.99 power factor, while a SMPS without any power factor
correction has a power factor of only about 0.55–0.65.
Due to their very wide input voltage range, many power supplies with active PFC can
automatically adjust to operate on AC power from about 100 V (Japan) to 230 V
(Europe). That feature is particularly welcome in power supplies for laptops.
Dynamic PFC:
Dynamic power factor correction (DPFC), sometimes referred to as "real-time power
factor correction," is used for electrical stabilization in cases of rapid load changes (e.g.
at large manufacturing sites). DPFC is useful when standard power factor correction
would cause over or under correction. DPFC uses semiconductor switches, typically
thyristors, to quickly connect and disconnect capacitors or inductors from the network
in order to improve power factor.

CHAPTER-3
BUCK BOOST CONVERTER

BUCK BOOST CONVERTER:
The buck–boost converter is a type of DC-to-DC converter that has an output
voltage magnitude that is either greater than or less than the input voltage magnitude. It
is equivalent to a flyback converter using a single inductor instead of a transformer.
Two different topologies are called buck–boost converter. Both of them can
produce a range of output voltages, from an output voltage much larger (in absolute
magnitude) than the input voltage, down to almost zero.
The inverting topology:
The output voltage is of the opposite polarity than the input. This is a switched-
mode power supply with a similar circuit topology to theboost converter and the buck
converter. The output voltage is adjustable based on the duty cycle of the switching
transistor. One possible drawback of this converter is that the switch does not have a

terminal at ground; this complicates the driving circuitry. Neither drawback is of any
consequence if the power supply is isolated from the load circuit (if, for example, the
supply is a battery) because the supply and diode polarity can simply be reversed. The
switch can be on either the ground side or the supply side.
A buck (step-down) converter combined with a boost (step-up)
converter:
The output voltage is typically of the same polarity of the input, and can be
lower or higher than the input. Such a non-inverting buck-boost converter may use a
single inductor which is used for both the buck inductor and the boost inductor,[2][3][4]
it
may use multiple inductors but only a single switch as in
the SEPIC and Ćuk topologies.
Principle of operation
Fig. 1: Schematic of a buck–boost converter.

Fig. 2: The two operating states of a buck–boost converter: When the switch is turned on, the input
voltage source supplies current to the inductor, and the capacitor supplies current to the resistor
(output load). When the switch is opened, the inductor supplies current to the load via the diode D.
The basic principle of the buck–boost converter is fairly simple (see figure 2):
 While in the On-state, the input voltage source is directly connected to the inductor (L).
This results in accumulating energy in L. In this stage, the capacitor supplies energy to
the output load.
 While in the Off-state, the inductor is connected to the output load and capacitor, so
energy is transferred from L to C and R.
Compared to the buck and boost converters, the characteristics of the buck–boost
converter are mainly:
 Polarity of the output voltage is opposite to that of the input;
 The output voltage can vary continuously from 0 to (for an ideal converter). The
output voltage ranges for a buck and a boost converter are respectively to 0 and
to .
Conceptual overview :
Like the buck and boost converters, the operation of the buck-boost is best
understood in terms of the inductor's "reluctance" to allow rapid change in current.
From the initial state in which nothing is charged and the switch is open, the current

through the inductor is zero. When the switch is first closed, the blocking diode
prevents current from flowing into the right hand side of the circuit, so it must all flow
through the inductor. However, since the inductor doesn't like rapid current change, it
will initially keep the current low by dropping most of the voltage provided by the
source. Over time, the inductor will allow the current to slowly increase by decreasing
its voltage drop. Also during this time, the inductor will store energy in the form of a
magnetic field.
Continuous mode:
Fig 3: Waveforms of current and voltage in a buck–boost converter operating in
continuous mode.
If the current through the inductor L never falls to zero during a commutation cycle, the
converter is said to operate in continuous mode. The current and voltage waveforms in
an ideal converter can be seen in Figure 3.
From to , the converter is in On-State, so the switch S is closed. The rate of
change in the inductor current (IL) is therefore given by

At the end of the On-state, the increase of IL is therefore:
D is the duty cycle. It represents the fraction of the commutation period T during which
the switch is On. Therefore Dranges between 0 (S is never on) and 1 (S is always on).
During the Off-state, the switch S is open, so the inductor current flows through the
load. If we assume zero voltage drop in the diode, and a capacitor large enough for its
voltage to remain constant, the evolution of IL is:
Therefore, the variation of IL during the Off-period is:
As we consider that the converter operates in steady-state conditions, the amount of
energy stored in each of its components has to be the same at the beginning and at the
end of a commutation cycle. As the energy in an inductor is given by:
it is obvious that the value of IL at the end of the Off state must be the same with the
value of IL at the beginning of the On-state, i.e. the sum of the variations of IL during
the on and the off states must be zero:
Substituting and by their expressions yields:

This can be written as:
This in return yields that:
From the above expression it can be seen that the polarity of the output voltage is
always negative (because the duty cycle goes from 0 to 1), and that its absolute value
increases with D, theoretically up to minus infinity when D approaches 1. Apart from
the polarity, this converter is either step-up (a boost converter) or step-down (a buck
converter). Thus it is named a buck–boost converter.
Discontinuous mode[
Fig 4: Waveforms of current and voltage in a buck–boost converter operating in
discontinuous mode.
In some cases, the amount of energy required by the load is small enough to be
transferred in a time smaller than the whole commutation period. In this case, the
current through the inductor falls to zero during part of the period. The only difference
in the principle described above is that the inductor is completely discharged at the end

of the commutation cycle (see waveforms in figure 4). Although slight, the difference
has a strong effect on the output voltage equation. It can be calculated as follows:
Because the inductor current at the beginning of the cycle is zero, its maximum
value (at ) is
During the off-period, IL falls to zero after δ.T:
Using the two previous equations, δ is:
The load current is equal to the averagdiode current ( ). As can be seen on figure
4, the diode current is equal to the inductor current during the off-state. Therefore, the
output current can be written as:
Replacing and δ by their respective expressions yields:
Therefore, the output voltage gain can be written as:
Compared to the expression of the output voltage gain for the continuous mode, this
expression is much more complicated. Furthermore, in discontinuous operation, the
output voltage not only depends on the duty cycle, but also on the inductor value, the
input voltage and the output current.

Chapter-4
CIRCUIT BREAKER

CIRCUIT BREAKER:
A circuit breaker is an automatically operated electrical switch designed to
protect an electrical circuit from damage caused byoverload or short circuit. Its basic
function is to detect a fault condition and interrupt current flow. Unlike a fuse, which
operates once and then must be replaced, a circuit breaker can be reset (either manually
or automatically) to resume normal operation. Circuit breakers are made in varying sizes,
from small devices that protect an individual household appliance up to
large switchgeardesigned to protect high voltage circuits feeding an entire city.
OPERATION:
All circuit breaker systems have common features in their operation, although
details vary substantially depending on the voltage class, current rating and type of the
circuit breaker.
The circuit breaker must detect a fault condition; in low voltage circuit breakers
this is usually done within the breaker enclosure. Circuit breakers for large currents or
high voltages are usually arranged with protective relay pilot devices to sense a fault
condition and to operate the trip opening mechanism. The trip solenoid that releases the
latch is usually energized by a separate battery, although some high-voltage circuit
breakers are self-contained with current transformers, protective relays and an internal
control power source.
Once a fault is detected, contacts within the circuit breaker must open to interrupt
the circuit; some mechanically-stored energy (using something such as springs or
compressed air) contained within the breaker is used to separate the contacts, although
some of the energy required may be obtained from the fault current itself. Small circuit
breakers may be manually operated, larger units have solenoids to trip the mechanism,
and electric motors to restore energy to the springs.
The circuit breaker contacts must carry the load current without excessive heating,
and must also withstand the heat of the arc produced when interrupting (opening) the

circuit. Contacts are made of copper or copper alloys, silver alloys and other highly
conductive materials. Service life of the contacts is limited by the erosion of contact
material due to arcing while interrupting the current. Miniature and molded-case circuit
breakers are usually discarded when the contacts have worn, but power circuit breakers
and high-voltage circuit breakers have replaceable contacts.
When a current is interrupted, an arc is generated. This arc must be contained,
cooled and extinguished in a controlled way, so that the gap between the contacts can
again withstand the voltage in the circuit. Different circuit breakers use vacuum,
air, insulating gas or oil as the medium the arc forms in. Different techniques are used to
extinguish the arc including:
 Lengthening / deflection of the arc
 Intensive cooling (in jet chambers)
 Division into partial arcs
 Zero point quenching (Contacts open at the zero current time crossing of
the AC waveform, effectively breaking no load current at the time of opening. The zero
crossing occurs at twice the line frequency, i.e. 100 times per second for 50 Hz and 120
times per second for 60 Hz AC)
 Connecting capacitors in parallel with contacts in DC circuits.
Finally, once the fault condition has been cleared, the contacts must again be closed to
restore power to the interrupted circuit.
Low-voltage MCB (Miniature Circuit Breaker) uses air alone to extinguish the arc.
Larger ratings have metal plates or non-metallic arc chutes to divide and cool the
arc. Magnetic blowout coils or permanent magnets deflect the arc into the arc chute.
In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a
jet of oil through the arc. Gas (usually sulfur hexafluoride) circuit breakers sometimes

stretch the arc using a magnetic field, and then rely upon the dielectric strength of the
sulfur hexafluoride (SF6) to quench the stretched arc.
Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other
than the contact material), so the arc quenches when it is stretched a very small amount
(less than 2–3 mm (0.079–0.118 in)). Vacuum circuit breakers are frequently used in
modern medium-voltage switchgear to 38,000 volts.
Air circuit breakers may use compressed air to blow out the arc, or alternatively,
the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced
air thus blowing out the arc.
Circuit breakers are usually able to terminate all current very quickly: typically
the arc is extinguished between 30 ms and 150 ms after the mechanism has been tripped,
depending upon age and construction of the device.
SHORT-CIRCUIT CURRENT:
Circuit breakers are rated both by the normal current that they are expected to
carry, and the maximum short-circuit current that they can safely interrupt.
Under short-circuit conditions, the calculated maximum prospective short circuit
current may be many times the normal, rated current of the circuit. When electrical
contacts open to interrupt a large current, there is a tendency for an arc to form between
the opened contacts, which would allow the current to continue. This condition can create
conductive ionized gases and molten or vaporized metal, which can cause further
continuation of the arc, or creation of additional short circuits, potentially resulting in the
explosion of the circuit breaker and the equipment that it is installed in. Therefore, circuit
breakers must incorporate various features to divide and extinguish the arc.
In air-insulated and miniature breakers an arc chute structure consisting (often) of
metal plates or ceramic ridges cools the arc, and magnetic blowout coils deflect the arc
into the arc chute. Larger circuit breakers such as those used in electrical power

distribution may use vacuum, an inert gas such as sulfur hexafluoride or have contacts
immersed in oil to suppress the arc.
The maximum short-circuit current that a breaker can interrupt is determined by
testing. Application of a breaker in a circuit with a prospective short-circuit current
higher than the breaker's interrupting capacity rating may result in failure of the breaker
to safely interrupt a fault. In a worst-case scenario the breaker may successfully interrupt
the fault, only to explode when reset.
MCB used to protect control circuits or small appliances may not have sufficient
interrupting capacity to use at a panel board; these circuit breakers are called
"supplemental circuit protectors" to distinguish them from distribution-type circuit
breakers.
Standard current rating:
Circuit breakers are manufactured in standard sizes, using a system of preferred
numbers to cover a range of ratings. Miniature circuit breakers have a fixed trip setting;
changing the operating current value requires changing the whole circuit breaker. Larger
circuit breakers can have adjustable trip settings, allowing standardized elements to be
applied but with a setting intended to improve protection. For example, a circuit breaker
with a 400 ampere "frame size" might have its overcurrent detection set to operate at only
300 amperes, to protect a feeder cable.
International Standard--- IEC 60898-1 and European Standard EN 60898-1 define
the rated current In of a circuit breaker for low voltage distribution applications as the
maximum current that the breaker is designed to carry continuously (at an ambient air
temperature of 30 °C). The commonly-available preferred values for the rated current are
6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A, 100 A,[5]
and 125 A
(Renard series, slightly modified to include current limit of British BS 1363 sockets). The
circuit breaker is labeled with the rated current in amperes, but without the unit symbol
"A". Instead, the ampere figure is preceded by a letter "B", "C" or "D", which indicates
the instantaneous tripping current — that is, the minimum value of current that causes

the circuit breaker to trip without intentional time delay (i.e., in less than 100 ms),
expressed in terms of In:
Circuit breakers are also rated by the maximum fault current that they can interrupt; this
allows use of more economical devices on systems unlikely to develop the high short-
circuit current found on, for example, a large commercial building distribution system.
In the United States, Underwriters Laboratories (UL) certifies equipment ratings, called
Series Ratings (or “integrated equipment ratings”) for circuit breaker equipment used for
buildings. Power circuit breakers and medium- and high-voltage circuit breakers used for
industrial or electric power systems are designed and tested to ANSI/IEEE standards in
the C37 series.
Types of circuit breakers
Front panel of a 1250 A air circuit breaker manufactured by ABB. This low voltage power
circuit breaker can be withdrawn from its housing for servicing. Trip characteristics are
configurable viaDIP switches on the front panel.
Many different classifications of circuit breakers can be made, based on their features
such as voltage class, construction type, interrupting type, and structural features.

Low-voltage circuit breakers
Low-voltage (less than 1,000 VAC) types are common in domestic, commercial and
industrial application, and include:
MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip
characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers
illustrated above are in this category.
There are three main types of MCBs: 1. Type B - trips between 3 and 5 times full load
current; 2. Type C - trips between 5 and 10 times full load current; 3. Type D - trips
between 10 and 20 times full load current. In the UK all MCBs must be selected in
accordance with BS 7671.
MCCB (Molded Case Circuit Breaker)—rated current up to 2,500 A. Thermal or
thermal-magnetic operation. Trip current may be adjustable in larger ratings.
Low-voltage power circuit breakers can be mounted in multi-tiers in low-voltage
switchboards or switchgear cabinets.
The characteristics of low-voltage circuit breakers are given by international
standards such as IEC 947. These circuit breakers are often installed in draw-out
enclosures that allow removal and interchange without dismantling the switchgear.
Large low-voltage molded case and power circuit breakers may have electric
motor operators so they can open and close under remote control. These may form part of
an automatic transfer switch system for standby power.
Low-voltage circuit breakers are also made for direct-current (DC) applications,
such as DC for subway lines. Direct current requires special breakers because the arc is
continuous—unlike an AC arc, which tends to go out on each half cycle. A direct current
circuit breaker has blow-out coils that generate a magnetic field that rapidly stretches the
arc. Small circuit breakers are either installed directly in equipment, or are arranged in
a breaker panel.

Inside of a circuit breaker
The DIN rail-mounted thermal-magnetic miniature circuit breaker is the most common
style in modern domestic consumer units and commercial electrical distribution
boards throughout Europe. The design includes the following components:
1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the
status of the circuit breaker (On or Off/tripped). Most breakers are designed so they can
still trip even if the lever is held or locked in the "on" position. This is sometimes referred
to as "free trip" or "positive trip" operation.
2. Actuator mechanism - forces the contacts together or apart.
3. Contacts - Allow current when touching and break the current when moved apart.
4. Terminals
5. Bimetallic strip - separates contacts in response to smaller, longer-term over currents
6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the
device after assembly.

7. Solenoid - separates contacts rapidly in response to high over currents
8. Arc divider/extinguisher
Magnetic circuit breakers:
Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force
increases with the current. Certain designs utilize electromagnetic forces in addition to
those of the solenoid. The circuit breaker contacts are held closed by a latch. As the
current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's
pull releases the latch, which lets the contacts open by spring action. Some magnetic
breakers incorporate a hydraulic time delay feature using a viscous fluid. A spring
restrains the core until the current exceeds the breaker rating. During an overload, the
speed of the solenoid motion is restricted by the fluid. The delay permits brief current
surges beyond normal running current for motor starting, energizing equipment, etc.
Short circuit currents provide sufficient solenoid force to release the latch regardless of
core position thus bypassing the delay feature. Ambient temperature affects the time
delay but does not affect the current rating of a magnetic breaker
Thermal magnetic circuit breakers:
Thermal magnetic circuit breakers, which are the type found in most distribution
boards, incorporate both techniques with the electromagnet responding instantaneously to
large surges in current (short circuits) and the bimetallic strip responding to less extreme
but longer-term over-current conditions. The thermal portion of the circuit breaker
provides an "inverse time" response feature, which trips the circuit breaker sooner for
larger overcurrents but allows smaller overloads to persist for a longer time. On very
large over-currents during a short-circuit, the magnetic element trips the circuit breaker
with no intentional additional delay.

Common trip breakers
Three-pole common trip breaker for supplying a
three-phase device. This breaker has a 2 A rating
When supplying a branch circuit with more than one live conductor, each live
conductor must be protected by a breaker pole. To ensure that all live conductors are
interrupted when any pole trips, a "common trip" breaker must be used. These may either
contain two or three tripping mechanisms within one case, or for small breakers, may
externally tie the poles together via their operating handles. Two-pole common trip
breakers are common on 120/240-volt systems where 240 volt loads (including major
appliances or further distribution boards) span the two live wires. Three-pole common
trip breakers are typically used to supply three-phase electric power to large motors or
further distribution boards.
Two- and four-pole breakers are used when there is a need to disconnect multiple
phase AC, or to disconnect the neutral wire to ensure that no current flows through the
neutral wire from other loads connected to the same network when workers may touch
the wires during maintenance. Separate circuit breakers must never be used for live and
neutral, because if the neutral is disconnected while the live conductor stays connected, a
dangerous condition arises: the circuit appears de-energized (appliances don't work), but
wires remain live and some RCDs may not trip if someone touches the live wire (because

some RCDs need power to trip). This is why only common trip breakers must be used
when neutral wire switching is needed.
Medium-voltage circuit breakers
Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into
metal-enclosed switchgear line ups for indoor use, or may be individual components
installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for
indoor applications, but are now themselves being replaced by vacuum circuit breakers
(up to about 40.5 kV). Like the high voltage circuit breakers described below, these are
also operated by current sensing protective relays operated through current transformers.
The characteristics of MV breakers are given by international standards such as IEC
62271. Medium-voltage circuit breakers nearly always use separate current sensors
and protective relays, instead of relying on built-in thermal or magnetic overcurrent
sensors.
Medium-voltage circuit breakers can be classified by the medium used to extinguish the
arc:
 Vacuum circuit breakers—With rated current up to 6,300 A, and higher for generator
circuit breakers. These breakers interrupt the current by creating and extinguishing the
arc in a vacuum container - aka "bottle". Long life bellows are designed to travel the 6-10
mm the contacts must part. These are generally applied for voltages up to about 40,500
V, which corresponds roughly to the medium-voltage range of power systems. Vacuum
circuit breakers tend to have longer life expectancies between overhaul than do air circuit
breakers.
 Air circuit breakers—Rated current up to 6,300 A and higher for generator circuit
breakers. Trip characteristics are often fully adjustable including configurable trip
thresholds and delays. Usually electronically controlled, though some models
are microprocessor controlled via an integral electronic trip unit. Often used for main
power distribution in large industrial plant, where the breakers are arranged in draw-out
enclosures for ease of maintenance.

 SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas.
 Medium-voltage circuit breakers may be connected into the circuit by bolted connections
to bus bars or wires, especially in outdoor switchyards. Medium-voltage circuit breakers
in switchgear line-ups are often built with draw-out construction, allowing breaker
removal without disturbing power circuit connections, using a motor-operated or hand-
cranked mechanism to separate the breaker from its enclosure. Some important
manufacturer of VCB from 3.3 kV to 38 kV are ABB, Eaton, Siemens, HHI(Hyundai
Heavy Industry), S&C Electric Company, Jyoti and BHEL.

CHAPTER 5
SOFTWARE SPECIFICATION

INTRODUCTION TO MATLAB:
MATLAB is high performance language for technical computation, visualization, data
analysis are integrated, also provides an easy to use environment for programming where
problems and solutions are expressed in familiar mathematical notation.
USES OF MATLAB:
Computation and Math
Development of Algorithm
Prototyping, Modeling, and simulation
Visualization, Data analysis, and exploration
Engineering and Scientific graphics
Application development in that including Graphical User Interface building
MATLAB is an interactive system whose basic data element is an array that does
not require any dimensioning. It allows us to solve many technical computing problems,
especially those with matrix and vector formulations. The name MATLAB stands for
matrix laboratory. MATLAB provides easy access to matrix software developed by the
LINPACK and EISPACK projects, which together represent the art in software for matrix
computation.
MATLAB has evolved over a period of years. It is the standard instructional tool
for introductory and advanced courses in mathematics, engineering, and science in
university environment. MATLAB is the tool for high-productivity research,
development, and analysis in industries.
MATLAB features a family of application-specific solutions called toolboxes.
Very important to most users of MATLAB, toolboxes allow you to learn and apply
specialized technology. Comprehensive collections of MATLAB functions (M-files) are
called toolbox. Toolboxes extend the MATLAB environment to solve particular type of
problems. signal processing, control systems, neural networks, fuzzy logic, wavelets,
simulation, and many other type of toolboxes are present.

In the MATLAB system we have five main parts:
The MATLAB language :
This is a high-level matrix/array language with control flow statements, functions, data
structures, input/output, and object-oriented programming features. It allows both
"programming in the small" to rapidly create quick and dirty throw-away programs, and
"programming in the large" to create complete large and complex application programs.
The MATLAB Working Environment:
This is the set of tools and facilities that you work with as the MATLAB user or
programmer. It includes facilities to manage the variables in your workspace and
importing and exporting data. It also has tools for developing, managing, debugging, and
profiling M-files, MATLAB's applications.
Handle Graphics.
This is the MATLAB graphics system. It includes high-level commands for two-
dimensional and three-dimensional data visualization, image processing, animation, and
presentation graphics. It also includes low-level commands that allow you to fully
customize the appearance of graphics as well as to build complete Graphical User
Interfaces on your MATLAB applications.
The MATLAB mathematical function library
This is a vast collection of computational algorithms ranging from elementary functions
like sum, sine, cosine, and complex arithmetic and sophisticated functions like matrix
inverse, matrix eigenvalues, Bessel functions, and fast Fourier transforms.

The MATLAB Application Program Interface (API).
This is a library that allows you to write C and Fortran programs that interact with
MATLAB. It include facilities for calling routines from MATLAB (dynamic linking),
calling MATLAB as a computational engine, and for reading and writing MAT-files.
3.3 COMPONENTS OF MATLAB
 Command Window
 Command History
 Workspace
 Editor
 Current Directory
MATLAB Command Window
It allows a user to enter simple commands. The Command Window is the main window in
which you communicate with MATLAB.
MATLAB Command History
The Command History window displays a log of statements that we ran in the current and
previous MATLAB sessions. It lists the time and date of each session in your operating
system in short date format, followed by the statements in that session.
MATLAB Workspace
MATLAB Workspace consists of the variables you create and store in memory during a
MATLAB session.
EDITOR
The Editor/Debugger provides basic text editing operations as well as access to M-file
debugging tools.In editor we can write the program or code which can be edited and
executed.

CURRENT DIRECTORY
MATLAB maintains directory called current directory for working with M-files and MAT-
files.
Use cd with no arguments to display the current directory.
Use cd with a path to change the current directory.
It contains programs which are present as M-files in MATLAB
SIMULINK:
In this project we use Simulink which is present in MATLAB.Simulink is a
software add-on to matlab which is a mathematical tool developed by The Math works, a
company based in Natick. Matlab is powered by extensive numerical analysis capability.
Simulink is a tool used to visually program a dynamic system (those governed by
Differential equations) and to view results. Any logic circuit, or control system for a
dynamic system can be built by using standard building blocks which are available in
Simulink Library. Various toolboxes for different techniques, such as Fuzzy Logic,
Neural Networks, dsp, Statistics etc. are present in Simulink, which enhance the
processing power . The availability of templates / building blocks, avoid the necessity of
typing code for small mathematical processes is the main advantage.
Simulink is a data flow graphical programming language tool for modeling,
simulating and analyzing multi domain dynamic systems.graphical block diagramming
tool and a customizable set of block libraries is its primary interface. It offers tight
integration with the rest of the MATLAB environment and can either drive MATLAB or
be scripted from it. Simulink is widely used in control theory and digital signal
processing for multidomain simulation and Model-Based Design.

Process of opening and using MATLAB
Step 1: Click on MATLAB icon then the MATLAB window as shown in figure below is
opened.
Step 2:Now click on the Simulink which is present at the top of MATLAB window

Step3: After clicking on Simulink, the following window is opened which is called
Simulink library in which we have different types of tool boxes.
Step 4: Click on the new model icon which is present at the top of Simulink library
window then the following window opens in which the program is written.

3.4 APPLICATIONS OF MATLAB
Calculus
Linear algebra
Probability and statistics
MATLAB is not only for pure math or engineering purposes, but it also has many
applications in finances.
MATLAB®
language supports the vector and matrix operations that are fundamental to
engineering and scientific problems.
It enables fast development and execution With the MATLAB language, you can program
and develop algorithms faster than with traditional languages because you do not need to
perform low-level administrative tasks, such as declaring variables, specifying data types,
and allocating memory.
In many cases, MATLAB eliminates the need for ‘for’ loops. As a result, one line of
MATLAB code can often replace several lines of C or C++ code.

MATLAB provides all the features of a traditional programming language, including
arithmetic operators, flow control, data structures, data types, object-oriented programming
(OOP), and debugging features.

Chapter-5
Project Implementation
HVDC transmission, power conditioning:
A high-voltage, direct current (HVDC) electric power transmission system uses direct
current for the bulk transmission of electrical power, in contrast with the more
common alternating current (AC) systems.[1]
For long-distance transmission, HVDC
systems may be less expensive and suffer lower electrical losses. For underwater power
cables, HVDC avoids the heavy currents required to charge and discharge the cable
capacitance each cycle. For shorter distances, the higher cost of DC conversion equipment
compared to an AC system may still be warranted, due to other benefits of direct current
links.
HVDC allows power transmission between unsynchronized AC transmission systems.
Since the power flow through an HVDC link can be controlled independently of the phase
angle between source and load, it can stabilize a network against disturbances due to rapid
changes in power. HVDC also allows transfer of power between grid systems running at
different frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of
each grid, by allowing exchange of power between incompatible networks.
The modern form of HVDC transmission uses technology developed extensively in the
1930s in Sweden (ASEA) and inGermany. Early commercial installations included one in
the Soviet Union in 1951 between Moscow and Kashira, and a 100 kV, 20 MW system
between Gotland and mainland Sweden in 1954.[2]
The longest HVDC link in the world is
currently the Xiangjiaba–Shanghai 2,071 km (1,287 mi), ±800 kV, 6400 MW link
connecting the Xiangjiaba Dam to Shanghai, in thePeople's Republic of China.[3]
Early in
2013, the longest HVDC link will be the Rio Madeira link in Brazil, which consists of two
bipoles of ±600 kV, 3150 MW each, connecting Porto Velho in the state of Rondônia to
the São Paulo area, where the length of the DC line is 2,375 km
High voltage transmission
High voltage is used for electric power transmission to reduce the energy lost in
the resistance of the wires. For a given quantity of power transmitted, doubling the voltage

will deliver the same power at only half the current. Since the power lost as heat in the
wires is proportional to the square of the current for a given conductor size, but does not
depend on the voltage, doubling the voltage reduces the line losses per unit of electrical
power delivered by a factor of 4. While power lost in transmission can also be reduced by
increasing the conductor size, larger conductors are heavier and more expensive.
High voltage cannot readily be used for lighting or motors, so transmission-level voltages
must be reduced for end-use equipment. Transformers are used to change the voltage levels
in alternating current (AC) transmission circuits. Because transformers made voltage
changes practical, and AC generators were more efficient than those using DC, AC became
dominant after the introduction of practical systems of distribution in Europe in 1891[5]
and
the conclusion of the War of Currents competition at the same time in the US between
the direct current (DC) system of Thomas Edison and the AC system of George
Westinghouse.
Practical conversion of power between AC and DC became possible with the development
of power electronics devices such as mercury-arc valves and, starting in the 1970s,
semiconductor devices as thyristors, integrated gate-commutated thyristors (IGCTs), MOS-
controlled thyristors (MCTs) and insulated-gate bipolar transistors (IGBT).
Harmonics (electrical power)
Harmonic voltages and currents in an electric power system are a result of non-linear
electric loads. Harmonic frequencies in the power grid are a frequent cause of power
quality problems. Harmonics in power systems result in increased heating in the equipment
and conductors, misfiring in variable speed drives, and torque pulsations in motors.
Reduction of harmonics is considered desirable.
Current harmonics
In a normal alternating current power system, the current varies sinusoidally at a specific
frequency, usually 50 or 60 hertz. When a linear electrical load is connected to the system,

it draws a sinusoidal current at the same frequency as the voltage (though usually not
in phase with the voltage).
Current harmonics are caused by non-linear loads. When a non-linear load, such as
a rectifier, is connected to the system, it draws a current that is not necessarily sinusoidal.
The current waveform can become quite complex, depending on the type of load and its
interaction with other components of the system. Regardless of how complex the current
waveform becomes, as described through Fourier series analysis, it is possible to
decompose it into a series of simple sinusoids, which start at the power
system fundamental frequency and occur at integer multiples of the fundamental
frequency.
Further examples of non-linear loads include common office equipment such as computers
and printers, Fluorescent lighting, battery chargers and also variable-speed drives.
A compact fluorescent lamp is one example of an electrical load with a non-linear
characteristic, due to therectifier circuit it uses. The current waveform, blue, is highly
distorted. In this example the voltage waveform is also distorted from a sine wave, due to
many such non-linear loads on this power system.
Voltage harmonics
Voltage harmonics are mostly caused by current harmonics. The voltage provided by the
voltage source will be distorted by current harmonics due to source impedance. If the
source impedance of the voltage source is small, current harmonics will cause only small
voltage harmonics.
Harmonics fundamentals
Harmonics provides a mathematical analysis of distortions to a current or voltage
waveform. Based on Fourier series, harmonics can describe any periodic wave as

summation of simple sinusoidal waves which are integer multiples of thefundamental
frequency.
Harmonics are steady-state distortions to current and voltage waves and repeat every cycle.
They are different from transient distortions to power systems such as spikes, dips and
impulses.[1]
Total harmonic distortion
Total harmonic distortion, or THD is a common measurement of the level of harmonic
distortion present in power systems. THD is defined as the ratio of total harmonics to the
value at fundamental frequency.
where Vn is the RMS voltage of nth harmonic and n = 1 is the fundamental frequency.
Effects
One of the major effects of power system harmonics is to increase the current in the
system. This is particularly the case for the third harmonic, which causes a sharp increase
in the zero sequence current, and therefore increases the current in theneutral conductor.
This effect can require special consideration in the design of an electric system to serve
non-linear loads.[2]
In addition to the increased line current, different pieces of electrical equipment can suffer
effects from harmonics on the power system.
Motors
Electric motors experience losses due to hysteresis and losses due to eddy currents set up in
the iron core of the motor. These are proportional to the frequency of the current. Since the
harmonics are at higher frequencies, they produce higher core losses in a motor than the
power frequency would. This results in increased heating of the motor core, which (if
excessive) can shorten the life of the motor. The 5th harmonic causes a CEMF (counter
electromotive force) in large motors which acts in the opposite direction of rotation. The

CEMF is not large enough to counteract the rotation, however it does play a small role in
the resulting rotating speed of the motor.
Telephones
In the United States, common telephone lines are designed to transmit frequencies between
300 and 3400 Hz. Since electric power in the United States is distributed at 60 Hz, it
normally does not interfere with telephone communications because its frequency is too
low.
Sources
A pure sinusoidal voltage is a conceptual quantity produced by an ideal AC
generator built with finely distributed stator and field windings that operate in a uniform
magnetic field. Since neither the winding distribution nor the magnetic field are uniform
in a working AC machine, voltage waveform distortions are created, and the voltage-time
relationship deviates from the pure sine function. The distortion at the point of generation
is very small (about 1% to 2%), but nonetheless it exists. Because this is a deviation from
a pure sine wave, the deviation is in the form of a periodic function, and by definition, the
voltage distortion contains harmonics.
When a sinusoidal voltage is applied to a certain type of load, the current drawn
by the load is determined by the voltage and impedance and follows the voltage
waveform. These loads are referred to as linear loads; examples of linear loads are
resistive heaters, incandescent lamps, and constant speed induction and synchronous
motors.
In contrast, some loads cause the current to vary disproportionately with the
voltage during each cyclic period. These are classified as nonlinear loads, and the current
taken by them has a nonsinusoidal waveform.
When there is significant impedance in the path from the power source to a
nonlinear load, these current distortions will also produce distortions in the voltage

waveform at the load. However, in most cases where the power delivery system is
functioning correctly under normal conditions, the voltage distortions will be quite small
and can usually be ignored.
Waveform distortion can be mathematically analysed to show that it is equivalent
to superimposing additional frequency components onto a pure sinewave. These
frequencies are harmonics (integer multiples) of the fundamental frequency, and can
sometimes propagate outwards from nonlinear loads, causing problems elsewhere on the
power system.
The classic example of a non-linear load is a rectifier with a capacitor input filter,
where the rectifier diode only allows current to pass to the load during the time that the
applied voltage exceeds the voltage stored in the capacitor, which might be a relatively
small portion of the incoming voltage cycle.
Other examples of nonlinear loads are battery chargers, electronic ballasts, variable
frequency drives, and switching mode power supplies.
OPERATION ARRANGEMENT OF THE PROPOSED COMPENSATOR
The schematic of the setup based on the CIGRÉ benchmark is shown in Fig.1.
The grid impedance comprised of , and in is such that the short-circuit ratio
(SCR) is 2.5, indicative of a very weak grid. The total capacitance in the shunt passive
filter is 273 F. In addition, the total inductance is 450 mH. The
goal of reactive compensation is to reduce or eliminate the reactive power , and to
improve the harmonic content in currents. The subscript represents the concerned
branch ( for the branch with the -connected transformer and for the branch
with the -connected transformer); whereas rep-resents the concerned phase ( , or ).
Unless otherwise men-tioned, small letters refer to the instantaneous quantity. The capital
letters represent average values of active and reactive power on the ac side, and dc -link
power, voltages, and currents. The rms values of currents and voltages on the ac side are
also rep-resented by capital letters.
The structure of the SPVC in each phase is shown in Fig. 2. The value of is 10 F. The
reference polarity of is chosen such that the reactive power of the compensator is

posi-tive when it is supplying reactive power and vice-versa.
Fig. 2. SPVC in one phase
SWITCHING STRATEGIES FOR SPVC
Three modes of operation are discussed as follows.
A. Discharge Mode
In this mode, should have a polarity in order to assist commutation. With reference to Fig.
2, if the current flow direction is from the grid to the converter, switching and on would
connect the positive dc bus to the converter and the negative dc bus to the grid side and
would be positive.
This would discharge the SPVC dc link and would go down according to
where is the residual voltage on the SPVC dc link before the start of discharge activity, is
the instantaneous current flowing from the grid to the converter, refers to the instant at
which the concerned switches are turned on, and when they are turned off. The situation is
depicted in Fig. 3(a).Assuming to be large, and, thus, would fall to zero. If reaches zero
and the switch

combination is not changed, the dc link would not charge in the opposite direction.
This is so because two conduction paths via - and - would be established.
Similar reasoning can be used to switch on the complemen-tary set of switches in the
negative half cycle of to discharge the SPVC. The situation is presented in Fig. 3(b) and
is governed by

C. Bypass Mode
turned on alone, would select the negative dc bus or , switched on alone, would select the
positive dc bus of the SPVC as the bypass path during the positive half cycle. Similarly, ,
turned on alone, would select the negative dc bus and , switched on alone, would select the
positive dc bus as the bypass path during the negative half cycle of . Fig. 3(e).
CONTROL OF SPVC:
The SPVC can only inject as much voltage in a phase as is available on its dc-link
capacitor. Second, the larger the series injected voltage, the earlier the commutating-
voltage crossover will be. The strategy adopted is to charge the SPVC dc link when the
concerned phase is carrying the peak current. The discharge would occur during

commutation of current into the concerned phase. Current commutates into a phase
during the positive and negative half cycles. Therefore, the sequence should be to charge
during the positive half cycle, discharge during the commutation in the negative half
cycle, charge again during the negative half cycle, discharge during the commutation in
the Positive half cycle, and, finally, charge again during the posiThe starting point of
control is depicted in Fig. 4. And are compared to half of the reference reactive power .
This is done because the two branches are identical in their reactive power consumption
and, thus, the SPVCs in the two have to take an equal burden. Since the current into the
LCC rectifier terminal is trapezoidal, we can assume that the current is constant (equal
to ) during the interval in which the SPVC dc link has to be charged. The peak of this
voltage can be calculated using (3) or (4) assuming the initial voltage on the SPVC dc
link is to be zero and
verse instantly. It should be noted that is in radians. Assuming (the line voltage at the
primary side of the converter transformers) as the reference, the load-side line voltage on
the YY transformer ( will be in phase with it At the positive-going zero crossing of , phase
has to take negative current from phase after a delay equal to the firing delay angle . An
assisting polarity voltage has to be injected prior to this zero crossing to force early
crossover and commutation. This can be calculated using (5) and (6)

SIMULATION SETUP AND RESULTS:
Simulink / Sim Power Systems was used to simulate two cases. Case I
demonstrates reactive power control with a changing active power reference and case II
shows the capability of SPVC to follow a changing reactive power reference with fixed .

In addition to the control loop in Fig. 4, two more control loops have been added. The
first (depicted in Fig. 5) is for active power control which compares the reference dc-link
current on the HVDC line with to generate the required firing delay angle. The second
loop (shown in Fig. 6) is for keeping at a fixed value around 15 , a compromise between
minimum reactive power consumption and fast power control above rated power for
limited duration. is kept constant through the use of relatively slow on load tap changers
(OLTCs) on the converter transformers, which change the voltage magnitude upon
sensing the change in to bring it back to the reference value. Sim PowerSystems does not
contain OLTC transformer blocks for time-domain simulations. We have, therefore,
created an alternative solution by creating the grid voltages behind its impedance using
controlled-voltage source blocks.
The loop in Fig. 6 controls these voltages based on the comparison of The
simulations start with the SPVC bypassed in both cases. The SPVC is connected at 3 s
and demonstrates its capability of following . At 6 s, the reference ( or ) is changed,and
the performance of the SPVC is demonstrated for the next 3s. The base values are 1000
MVA, 500 kV, and 2 kA for plotting
Case1:
The active and reactive powers are plotted in Fig. 7. As the SPVC is connected, drops from
a value of 0.55 to 0 p.u., as

CONCLUSION
In this Project, the novel single-phase ac-ac converter with ac output current controls is
proposed and digitally implemented according to the circuit-breaker (CB) testing standard
(IEC 60898). Both simulation and experimental results show the improved transient step
current control performance over the traditional ac current source based on the motor
driven tap changing of auto-transformer. The proposed system is simple and low cost with
minimum number of switches employed. The boost PFC topology with dual dc-link
voltage controls is also incorporated in the proposed system. A laboratory prototype rated
600A(rms) output was constructed to verify the proposed system. According to results, the
proposed topology accomplishes the sinusoidal input line current with unity power factor
and low THDi of output current. The satisfactory transient step responses of output current
are obtained.

REFERENCES
[1] Circuit-breaker for over current protection for household and similar installations, IEC
60898-1, 2003-2007.
[2] S. Kitcharoenwat, , M. Konghirun, A. Sangswang, “A Controlled Current AC-AC
Converter for Circuit Breaker Testing,” in Proc. Electrical Machines and Systems Conf.
(ICEMS), 2012, pp. 1-6
[3] S. Kitcharoenwat, , M. Konghirun, A. Sangswang, “A Digital Implementation of Novel
Single Phase AC-AC Converter with Power Factor Control,” in Proc. Electrical Machines
and Systems Conf. (ICEMS), 2012, pp. 1-6
[4] S. B. Bekiarov and A. Emadi, “A New On-Line Single-Phase to Three- Phase UPS
Topology with Reduced Number of Switches,” in Proc, IEEE PESC, 2003, pp.451-456.
[5] J.-H. Choi, J.-M. B. kwon, j.-H. jung, and B.-H. Kwon, “High- Performance Online
UPS Using Three-Leg-Type Converter,” IEEE Trans. Ind. Electron., vol. 52, no.3, pp.
889-897, Jun. 2005.
[6] C. B. Jacobina, T. M. Oliveira and E. R. C. da Silva, “Control of the Single-Phase
Three-Leg AC/AC Converter,” IEEE Trans. Ind. Electron., vol.53, no. 2, pp.467-476, Apr
2006.
[7] I. S. de Freitas, C. B. Jacobina, E. C. dos Santos, “Single-Phase to Single-Phase Full-
Bridge Converter Operating With Reduced AC Power in the DC-Link Capacitor,” IEEE
Trans. Power Electron., vol.25, no.2, pp. 272-279. Feb.2010.

[8] K. Georgakas and A. Safacas, “Modified sinusoidal pulse-width modulation operation
technique of an AC-AC single-phase converter to optimise the power factor,” IET Power
Electronics, vol.3, lss.3, pp. 454- 464.
[9] L. Garcia de Vicuña, M. Castilla, J. Miret, J. Matas and J. M. Guerrero, “Sliding-Mode
Control for a Single-Phase AC/AC Quantum Resonant Converter,” IEEE Trans. Ind.
Electron., vol. 56, no. 9, pp. 3496-3504, Sep. 2009.
[10] H, Sugimura , Sang-Pil Mun, Soon-Kurl Kwon, T. Mishima, M. Nakaoka, “Direct
AC-AC resonant converter using one-chip reverse blocking IGBT-based bidirectional
switches for HF induction heaters,” in Proc. Industrial Electronics Conf. (ISIE), 2008, pp.
406-412.
[11] M.-K. Nguyen, Y.-G. Jung and Y.-C. Lim, “Single-Phase AC–AC Converter Based
on Quasi-Z-Source Topology,” IEEE Trans. Power Electron., vol. 25, no. 8, pp. 2200-
2209, Aug. 2010.
[12] X. P. Fang, Z. M. Qian, and F. Z. Peng, “Single-Phase Z-Source PWM AC-AC
Converters,” IEEE Power Electron. Letters, vol. 3, no. 4, pp 121-124.
[13] M. Chen, A. Mathew, Jian Sun, “Nonlinear Current Control of Single- Phase PFC
Converters,” IEEE Trans. Power Electron., vol. 22, no. 6, pp. 2187-2194, Nov. 2007.
[14] M. Chen, A. Mathew, and J. Sun, “Mixed-signal control of single-phase PFC based a
nonlinear current control method,” in Proc. IEEE Power Electron. Spec. Conf., 2006, pp.
1–7.

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