cycloconverter

CYCLOCONVERTERS Document By SANTOSH BHARADWAJ REDDY Email: [email protected]

Engineeringpapers.blogspot.com More Papers and Presentations available on above site INTRODUCTION: It is well known that the cycloconverter, which means that a cycloconverter is a type of power controller in which an alternating voltage at supply frequency is converted directly to an alternating voltage at load frequency without any intermediate DC stage. In a line commutated cycloconverter, the supply frequency is greater than the load frequency. The operating principles were developed in the 1930 when the grid control mercury arc rectifier became available. The techniques were applied in Germany, where the three phase 50Hz supply was converted to single phase AC supply at 16 Hz for railway traction. Single phase to three phase cycloconverter driven induction motors are ideal for use in a single phase traction system. Several attempts have been made to develop micro processor based control strategies for controlling a cycloconverter. It has been observed that for highly inductive loads as in case of induction motors, a simple and straight forward firing sequence at constant angles results in a direct short circuit of supply voltage due to simultaneous triggering of semi conductor controlled rectifier (SCR) in the opposite groups, thus

leading to undesirable output waveforms and high short circuits circulating. The various methods that are normally employed to overcome this problem include: 1 Open loop banking. 2 Closed loop banking. 3 In corporation of a circulating current inductor (CCI) to limit the short circuit currents and 4 Forced commutation methods. In this project here the different types of various load conditions has been demonstrated that for low and medium

loads

simply a CCI is connected in series with each SCR, but in the case of high

load a resistor is connected in series to SCR along with the

CCI. This is because of high short circuit currents caused due to high value of inductance.

Harmonics: The power electronics equipments, such as rectifier, inverter, cycloconverters and choppers have switching devices and their operation produces current and voltage harmonics into the system from which they are working. These harmonics affect the operation of other equipments connected to the same system through conduction (or) radio interface. The cycloconverter output voltage waveforms have complex harmonics. Higher order harmonics are usually filtered by the machine

inductance,

therefore

the

machine

current

has

less

harmonics. The remaining harmonics causes harmonic losses and torque

pulsations. Note that in a cycloconverter,

unlike other

converters, there are no inductors are conductors, i.e. no storage devices. For this reason, the instantaneous input power and the output power are equal. There are several factors affecting the harmonic content of the waveforms. In addition to this, the pulse number effects the harmonic content of the waveforms. A greater number of pulses have less harmonic content. Therefore, a 6-pulse (bridge) cycloconverter produces fewer harmonics than a 3-pulse (half-wave) cycloconverter. Moreover, if the output frequency gets closer to the input frequency, the harmonics increases. Finally, low power factor and discontinuous conduction, both contribute to harmonics. For a typical p-pulse converter, the order of the input harmonics is “pn+1” and that of the output harmonics is “pn”. Where p is the pulse number and n is the integer. The firing angle (α), in cycloconverter operation is sinusoidally modulated. The modulated frequency is the same as the output frequency and sideband harmonics are induced at the output. Therefore, the output waveform is expected to have harmonics at frequency related to both the input and output frequencies.

Scope of the project: The objective of this project is reducing the short circuit occurrences and at the same time helps to reduce the harmonics in the load voltage waveform. Here different types of load conditions, it can be demonstrated that a simple and a straight forward firing sequence at constant firing angles results in direct short circuit of supply voltage due to simultaneously triggering of SCR’s. Thus leads to undesirable

output waveforms. Those undesirable waveforms are reduced in high loads by placing a resistance in conjunction with the SCR.

Organisation of thesis: The thesis is organized in to 8 chapters; Chapter 1 describes about the introduction of the single phase to three phase cycloconverter, the modeling, analysis and simulations for an existing single phase to three phase cycloconverter wit 50Hz line frequency and also deals with an elimination of harmonics. Chapter 2 deals with an introduction to thyristors, principle of operation, and some of the static characteristics of SCR. Chapter 3 deals with an explanation of single phase cycloconverters, Principle of operation, and analysis of single phase Cycloconverters and also to get the output waveforms and at the same time which can also deals with an explanation of three phase cycloconverters, blocking mode cycloconverter, circulating current cycloconverters and also to get an output waveforms of these two modes. Chapter 4 deals with implementation of conversion of single phase to three phase to three phase cycloconverter was studied in this chapter and also circuit diagram for single phase to three phase cycloconverter was drawn along with the expected output waveforms. In chapter 5, the discussion about an analysis of single phase to three phase cycloconverter, simulation of single phase to three phase cycloconverter, and also discuss with an analysis of low loads, medium

loads and high loads.

In chapter 6, MATLAB/SIMULINK, the most widely used software in industrial applications for modeling of the dynamic system is explained. Analyzing the circuit using the MATLAB software was explained. In chapter 7, simulation results are presented for various loads i.e., for

along with

their harmonic waveforms. The conclusion, future scope of the project and applications of cycloconverters are presented in chapter 8.

CHAPTER 2 2.1 HISTORY OF SCR Power electronics originated at the beginning of the 19th century. Many technical articles and several books on the subject were published during the period from 1930-1947. These dealt primarily with the application of grid-controlled gas filled tubes. Because of the limitations of the mercury-arc rectifier and gas-filled thyratrons, only a relatively number of equipments was manufactured. It was in 1930 where the field-effect principle was first disclosed in U.S. patent by Julius Lilifienfeld, a former professor of physics at the University of Leipzig who has recently immigrated to the United States. In 1947, the point contact transistor was demonstrated by Walter H. Brattain and John Bardeen (Shockley 1972 & 1976), with William Shockley as an intensely interested observer. After an additional week of further experimentation and polishing of the demonstration, it was repeated for several key Bell Laboratories managers on December 23, 1947, the date that has come to be taken as the “official” date of reduction to practice. Walter H. Brattain, John Bardeen and William Shockley shared a Nobel Prize for the transistor in 1956. The invention of the bipolar junction transistor in 1948 was the

beginning

semiconductor

of

semiconductor

diodes

spawned

electronics. revolution

This

device

electronics.

and

Drastic

reduction in size, cost and power consumption were achieved simultaneously with greatly increased equipment complexity and capability.

At the same time, in 1948 Shockley and Person tried fabricating rudimentary field effect transistor (FET) using evaporated layers of germanium on dielectric. However, it was not until Bardeen theorized on the surface state phenomenon and Shockley published his theoretical analysis of the unipolar field-effect transistor. Semiconductor power diodes become available shortly after 1950. However, it was not until late 1957, when the most popular member f the thyristor family the SCR was announced by General Electric (1957), that semiconductor power electronics really began, starting from a single 16-Adevice, the thyristor family has grown tremendously. Hundreds of thyristors are available from numerous manufacturers through out the world.

2.2 TERMINAL CHARECTERISTICS OF SCR:

SCR is a four layer, three junction, p-n-p-n semiconductor switching device. It has three terminals; anode, cathode and gate. Figure 2.1(a) shows the structure of SCR. Basically a SCR consists of four layers and three junctions. The layers are alternatively p and n

type, J1; J2 & J3 are the junctions which formed between adjacent layers. The terminal connected to outer p region is called anode (A), the terminal connected to outer n region is called cathode and that connected to to inner p region is called gate (G). SCRs are available at high ratings but they are switched on by a low voltage supply of about 1 A and 10 W and this gives immense power amplification capability of this device. As SCRs are solid state devices, they are compact, posse’s high reliability and have low loss. Because of these useful features, SCR is almost universally employed these days for all power controlled devices. Unlike the diode, a SCR also blocks the current flow from anode to cathode until it is triggered into conduction by a proper signal between gate and cathode terminals.

2.3 Static V-I characteristics of SCR

CHAPTER 3 3.1 SINGLE PHASE CYCLOCONVERTER: In industrial applications, two forms of electrical energy are used: Direct current (Dc) and Alternating current (Ac). Usually constant voltage constant frequency single-phase (or) three phase Ac is readily available.

However,

for

different

application,

different

forms,

magnitudes (or) frequencies are required. There are four different conversions between Dc and Ac power sources. These conversions are done by circuits called power converters. The Converters are classified as: 1.

Rectifiers: From single phase or three phase Ac to variable voltage

Dc.

These

Rectifiers

use

line

voltages

for

their

commutation as such these are also called line-commutated (or) naturally- commutated Ac to Dc converters. These are used in Dc Drives, metallurgical and chemical industries, and excitation systems for synchronous machines. 2.

Choppers: A Dc chopper converts fixed Dc input voltage to a controllable Dc output

voltage. The chopper circuits require

force, (or) load, commutation transistors. Choppers find wide application in Dc drivers, subway cars, trolley trucks, buttery – driven vehicles. 3.

Inverters:

From

Dc

to

variable

magnitude

and

frequency, single phase or three phase Ac. These

variable

converters

use line, load ( or ) forced commutation for turning – off the thyristors, inverters find wide use in

induction

synchronous-motor

heating,

transmission.

drives,

induction

motor and UPS,

HVDC

4.

Cycloconverter: from single phase are three phase Ac to variable magnitude and variable frequency, single phase are three phase Ac. Line commutation is more common in these converters,

through

forced

and

load

commutated

Cycloconverters are also employed. These are primarily used for slow speed large Ac drives like rotary kiln. . This article explains what Cycloconverters and their types, how they operate and their applications. Traditionally, Ac-Ac conversion using semiconductor switches is done in two different ways: 1. Two stages ( Ac-Dc and then Dc-Ac ) as in Dc link converters 2. One stage ( Ac-Ac ) Cycloconverters as shown in Figure

Cycloconverters are used in high power applications driving induction and Synchronous motors. They are usually phasecontrolled and they traditionally

use thyristor due to their case

of phase commutation.

Fig 3.1: Block diagram of cycloconverter There are other newer forms of cyclo conversion such as Ac-Ac matrix Converters and high frequency Ac- Ac converters and these uses self-controlled switches. These converters, however, are not popular yet Some applications of Cycloconverters are:

 Cement mill drives.  Ship propulsion drives.  Rolling mill drives.  Scherbius drives.  Ore grinding mills.  Mine winders.

3.1.1 PRINCIPLE OF OPERATION: To understand the operation principles of cycloconverter, the single phase to Single phase cycloconverter Figure 3.2 should be studied first. This converter consists of back-to back connection of two full-wave rectifier circuits. Figure 3.3 shows the operating waveforms for this converter with a resistive load. The input voltage V is an Ac voltage at a frequency f , as shown in Figure 3.3(a). For easy understanding assume that all the thyristor are fired at α = 00 firing angle, i.e., thyristor act like diodes. Note that the firing angles are named as α and α

n

p

for the positive converter

for the negative converter.

Consider the operation of the cycloconverter to get one-fourth of the input frequency at the output. For the first two cycles of input voltage Vs the positive Converter operates supplying current to the load. It rectifies the input voltage therefore, the load sees 4 positive half cycles as seen in Figure. In the next two cycles, the negative converter operates supplying current to the load in the reverse direction. The current waveforms are not shown in the figures because the resistive load current will have the same waveform as the voltage but only scaled by the resistance. Note that when one of the converters

operates the other one is disabled, so that there is no current circulating between the two rectifiers

Fig 3.2: single phase to single phase cycloconverter

3.1.2

ANALYSIS

OF

SINGLE

PHASE

CYCLOCONVERTER: The frequency of the output voltage Vo in Figure 3.3 (b) is 4 times less than that of Vs, the input voltage, i.e. fo/ fi =1/4. Thus, this is a step-down cycloconverter. On the other hand, cycloconverters that have fo / fi ≥ 1 frequency relation are called Step-up cycloconverters. Note that step-down cycloconverters are more widely used than the step-up ones. The frequency of Vo can be changed by varying the number of cycles the positive and the negative converters work. It can only change as integer multiples of f, in 1ϕ - 1ϕ cycloconverters.

With the above operation, the 1ϕ -1ϕ cycloconverter can only supply a certain voltage at a certain firing angleα . The Dc output of each rectifier is: Vd = (2 √2 /∏) V cos α

(3.1)

Where V is the input rms voltage The Dc value per half cycle is shown as dotted in Figure 3.3(d). Then the peak of the fundamental output voltage is Vo1 = (4 / ∏)(2 √2/∏) V cos α (3.2) Equation (3.2) implies that the fundamental output voltage depends on α .For α = 00.

Where

V 01 = V d o * 1 = V d o

(3.3)

V d o = 4 / ∏ (2 √2 )/∏ V

(3.4)

If α is increased to π /3 as in the Figure 3.3 (d), then V01 = Vdo * 0.5

(3.5)

Thus varying α , the fundamental output voltage can be controlled. Constant α

operation gives a crude output waveform with rich harmonic

content. The dotted lines in Figure 3.3(b) and Figure 3.3(c) show a square wave. If the square wave can be modified to look more like a sine wave, the harmonics would be reduced. For this reason α is modulated as shown in figure 3.3 (d). Now,, the sixstepped dotted line is more like a sine wave with fewer harmonics. The more pulses there are with different firing angles (α ), the less are the harmonics.

Fig 3.3 single phase to single phase cycloconverter waveforms of (a)

Input voltage

(b)

Output voltage for zero firing angle

(c)Output voltage with firing angle ∏/3 rad (d) Output voltage with varying firing angle

3.2 THREE PHASE CYCLOCONVERTERS: Many alternative arrangements of cycloconverter circuit having varying degrees of complexity, and providing singular or multi phase outputs are feasible. As in the case of rectifier or phase controlled converter circuit, from the view point of reducing the external harmonics voltages and currents to a minimum, the pulse number of the cycloconverter circuit should be as high as possible. Of course, this

necessarily implies that a relatively large number of the thyristors be employed in the circuit and therefore, this requirement generally cannot be met economically, unless the applications is such that the large number of thyristors are required in any case, purely from the view point of realizing the necessary output power. In practical applications, the cycloconverter is commonly required to deliver a three phase from a three input.

3.2.1THREE

PHASE

TO

THREE

PHASE

CYCLOCONVERTER: If the outputs of three 3 ϕ

- 1ϕ converters of the same kind are

connected in wye (or) delta and if the output voltages are

radians

phase shifted from each other, the resulting converter is a three phase to three phase (3 ϕ

- 3 ϕ

)

cycloconverter. The resulting

Cycloconverters are shown in Figure and Figure with wye connections. If the three converters connected are half-wave converters, then the new converter is called a 3 ϕ

- 3 ϕ half-wave cycloconverter. If

instead, bridge converters are used, then the result is a 3ϕ bridge cycloconverter. 3ϕ

- 3 ϕ

- 3 ϕ

half-wave cycloconverter is also

called a 3- pulse cycloconverter or an 18- thyristor cycloconverter. On the other hand, the 3 ϕ

- 3 ϕ bridge cycloconverter is also called a 6-

pulse cycloconverter or a 36-thyristor cycloconverter. The operation of each phase is explained in the previous section.

The three phase Cycloconverters are mainly used in Ac machine drive systems running

three phase synchronous and induction machines.

They are more advantages when used with a synchronous machine due to their output power factor Characteristics. A cycloconverter can supply lagging. Leading or unity power factor loads while its input is always lagging. A synchronous machine can draw any power Factor current from the converter. This characteristic operation matches the cycloconverter to the synchronous machine. on the other hand, induction

machines

can

only

draw

cycloconverter does not have an edge converters

in this

aspect

for

lagging

current,

compared

running

so

the

to the other

an Induction machine.

However, Cycloconverters are used in Scherbius drives for speed control purposes driving wound rotor induction motors. Cycloconverters produce harmonic rich output voltages, which

will

be

Cycloconverters

discussed

in

the

following

sections

when

are used to run an ac machine the leakage

inductance of the machine filters most of the higher frequency harmonics are reduces the magnitudes of the lower order harmonics.

3.2.2 BLOCKING MODE CYCLOCONVERTER: The operation of these Cycloconverters was explained briefly before, they do Not let circulating current flow, and therefore they do not need a bulky Inter Group Reactor (IGR).when the current goes to zero, both positive and negative converters are blocked. The converters stay off for a short delay time to assure that the load current ceases. Then, depending on the polarity, one of the converters is enabled. With each zero crossing of the current, the converter, which was disabled before the zero, can be used for this purpose. The operation waveforms for a three-pulse blocking mode cycloconverter are given in figure 3.6

The blocking mode operation has some advantages and disadvantages over the circulating mode operation during the delay time, the current stays at zero distorting the voltage and current waveforms.

This

distortion

means

complex

harmonics

patterns

compared to the circulating mode Cycloconverters. In addition to this, the current reversal problem brings more control complexity. However, no bulky IGRs are used. So the size and cost is less than of the circulating current case. Another advantage is that only one converter is in conduction at all times rather than two this means less losses and higher efficiency.

Fig 3.6 Blocking mode operation waveforms

3.2.3 CIRCULATING CURRENT CYCLOCONVERTERS:

In this case, both of the converters operate at all times producing the same fundamental output voltage. The firing angles of the converters satisfy the firing angle condition

α

r

+α

n

=π

Thus when one converter is in rectification mode the other one is in inversion mode and vice versa. If both of the converts are producing pure sine waves, then there would not be any circulating current because the instantaneous potential difference between the outputs of the converters would be zero. In reality, an inter group reactor is connected between the outputs of two phase-controlled converters. The voltage waveform across the inter group reactor can be seen in Figure. This is the difference of the instantaneous output voltages produced by the two converters. The center tap voltage of inter group reactor is the voltage applied to the load and it is the mean of the voltages applied to the ends of inter group reactor, thus the load voltage ripple is reduced. The circulating current cycloconverter applies a smoother load voltage with fewer harmonic compared to the blocking mode case. Moreover, the control is simple because there is no current reversal problem. However, the bulky inter group reactor is a big disadvantage for this converter. In addition to this, the number of devices conducting at any time is twice that of the blocking mode converter. Due to these disadvantages, this cycloconverter is not attractive. The blocked mode cycloconverter and the circulating current cycloconverter can be combined to give a hybrid system, which has the advantages of both. The resulting cycloconverter looks like a circulating mode cycloconverter circuit, but depending on the polarity of the output current only one converter is enabled and the other one is disabled as with the blocking mode Cycloconverters. When the load

current decreases below a threshold, both of the converters are enabled. Thus the current has a smooth reversal. When the current increases above a threshold in the other direction, the outgoing converter is disabled. This hybrid cycloconverter operates in the blocking mode most of the time so a smaller inter group reactor (IGR) can be used. The efficiency is slightly higher than that of the circulating current cycloconverter but much less than the blocking mode cycloconverter. Moreover, the distortion caused by the blocking mode operation disappears due to the circulating current operation around zero current. Moreover, the control of the converter is still less complex than that of the blocking mode cycloconverter.

Fig 3.7 circulating mode operation waveforms

CHAPTER 4 4.1

SINGLE

PHASE

TO

THREE

PHASE

CYLOCONVERTER Recently, with the decrease in the size and price of power electronic switches, single phase to three phase cyclo converters started drawing more research interest. Usually, an bridge inverter produces a high frequency single phase voltage waveform, which is fed to the cycloconverter either through high frequency transformer or not. If

a

transformer

is

used,

it

isolates

the

inverter

from

the

cycloconverter. In addition to this, additional taps from the transformer can be used to power other converters producing high frequency AC link. The single phase high frequency AC voltage can be either sinusoidal or trapezoidal. There might be zero voltage intervals for control purposes or zero voltage commutation. Figure 4.1 shows the circuit diagram of a typical high frequency AC link converter. These converters are not commercially available yet.

Fig 4.1: High frequency AC link converter

4.2 PRINCIPAL OF OPERATION: Figure 4.2 shows the power circuit of a cycloconverter for a three phase output with a single phase input. Each phase consists of two converters, a positive converter and a negative converter connected in anti parallel. For example SCR 1 and SCR 2 from the positive converter and SCR 3 and SCR 4 from the negative converter, essentially, three sets of dual converters are required for the generation of a three phase output from a single phase source. A variable frequency can be generated by alternatively switching the positive and negative converter of one of the dual converters. The other two converters to be triggered in such a manner that a predefined phase

sequence as well as phase difference of 120

degrees is maintained between output A,B and C. In figure 4.2 Lx represents the leakage inductance of the transformer. Lc corresponds to the circulating current inductor fed for limiting the short circuit current. The load components are R and L where R includes the resistance of the load, transformer and circulating current inductor (CCI). Figure 4.3 shows the idealized waveforms with a straight forward firing sequence of 121,343,121 at zero firing angle for purely resistive load (L,Lx,, Lc→0). The SCR 1 turns off when the input voltage Vi goes through zero in the negative direction at point P,Q and R are called cross over points when the change over from a positive group to negative one takes over and vice versa. This cross over point is most susceptible to problems in highly inductive (High L/R loads).

4.3 CIRCUIT DIAGRAM OF SINGLE PHASE TO THREE PHASE CYCLO CONVERTERS:

Figure 4.2: power circuit of single phase to three phase center tapped configuration cycloconverter

4.4 WAVEFORMS

Figure 4.3: idealized waveforms for input and output voltages of each phase

CHAPTER 5 5. 1. ANALYSIS OF 1-ф TO 3-ф CYCLO CONVERTER: The cycloconverter of Figure 4.2 is redrawn in Figure 5.1 for purpose of simulation. The parameter values indicated in the diagram corresponds to the single phase to three phase cycloconverter experimental setup. In subsequent discussions, the simulated waveforms will be shown for phase A only. The waveforms for phase B and C are identical to that of phase A. Voltages V1 and V2 are taken to be 112V (rms) at 50Hz. The gate pulses to the various SCR’s are given at a firing angle (α) where 0< α< .the gate pulses remain high during α to π - α. In the actual system these pulses are chopped at 10 KHz to prevent the isolating transformer from being saturated. The firing sequence, 121,343,121.... and so on corresponding to N=3 is reported in this project. The load parameters R1 and VL1 are selected for simulating for Low , Medium , and High

loads.

The following are the different types of loads are given below: 1. Low

Load.

2. Medium 3. High

Load.

Load.

5.2.1. Low

Load:

Figure 7.2(a) shows the various waveforms for phase A with α=0. Ll=0.001H, and Rl= 45Ω. The load voltage and load current are almost in phase. Consequently, each SCR conducts until the end of the corresponding input half cycle. The waveforms are very close to the idealized waveforms of Figure 4.3. The harmonic spectra of the load voltage and the load current are shown in the Figure 7.2(b). It may be observed that there are two dominant harmonics. They are located at 2fi f0 with fi=50Hz and f0= . The magnitude of the (2N-1)th harmonic is larger than the (2N+1)th component, there values being 34% and 25% of the fundamental, respectively. In addition, the magnitudes of the other harmonics are also substantial.

5.2.2 Medium

Load:

Figure 7.4(a) shows the waveform for phase A for Rl=10Ω and Ll=0.1 H. It may be observed that SCR 1 , SCR 2, SCR 3, and SCR 4 conducts simultaneously for a varied period of time. Due to this overlap conduction of SCR’s the load voltage drops down to almost zero values for overlap period. This causes discontinuity in the load voltage, where as the load current remains continuous. The load current continues to build up during the successive conduction of SCR of a given group thus the magnitude of the load current at the end of the output half cycle is non-zero.

Consequently at the crossover point, SCR 2 continuous to conduct and prevents conduction of SCR 4 for an appreciable time. A similar problem occurs at the next cross over point. Where over conduction of SCR 3 does not allow SCR 1 to be triggered. The conduction of SCR 1 is again delayed until SCR 3 is turned off. From the harmonic spectra of the load voltage and the load current shown in Figure 7.4(b), it may be observed that the (2N1)th harmonic in the load current is very small as compared to the case of low

load. This can be attributed to the reduction of ripple in the

load current due to large load inductance. However, the harmonic content in the load voltage waveform has increased. Specifically, (2N1)th components

are

now

71%

and 50%

of

the

fundamental,

respectively. The higher order harmonics such as (4N+1) in the load voltage have also increased. This can be attributed to simultaneous conduction of SCR in a given group, during which the load voltage drops down to a very low value.

5.2.3. HIGH

LOAD:

If the load is more inductive, the conducting SCR of a group out of conduction for the entire input half cycle. This is illustrated in Figure 7.6(a) for Rl=5Ω and Ll= 0.1H. it can be observed from the Figure 7.6(a) that SCR 2 does not turn off at the crossover point and SCR 4 is not allowed to be turned on at all. Also, when SCR 3 is triggered during the next negative half of the input, SCR 2 also gets a favorable voltage to stay on. Consequently, SCR 2 and SCR 3 are simultaneously on for slightly less than 20ms.

6.1 INTRODUCTION TO MATLAB MATLAB, developed by Math works inc., is a software package

for

high

performance

numerical

computation

and

visualization. The combination of analysis capabilities, flexibility, reliability, and powerful graphics makes MATLAB the premier software package for electrical engineers. MATLAB provides an interactive environment with hundreds of reliable and accurate built in mathematical functions. These functions provide solutions to a broad range of mathematical problems including matrix algebra, complex arithmetic linear systems, differential

equations,

signal

processing,

optimization,

nonlinear

systems, and many other types of scientific computations. The most important feature of MATLAB is its programming capability, which is very easy to learn and to use, and which allows user-developed functions. It also allows access to FORTRAN algorithms and C codes by means of external interfaces. MATLAB has been enhanced by the very powerful SIMULINK program. SIMULINK is a graphical mouse-driven program for the simulation of dynamic systems. SIMULINK enables students to simulate linear, as well as nonlinear, systems easily and efficiently.

6.2 INTRDUCTION TO SIMULINK: SIMULINK is an interactive environment for modeling, analyzing, and simulating a wide variety of dynamic systems. SIMULINK provides a graphical user interface for constructing block diagram models using “drag and drop” operations. A system is configured in terms of block diagram representation from a library of standard components. SIMULINK is very easy to learn. A system in block diagram representation is built easily and the simulation results are display quickly. SIMULINK is particularly useful for studying the effects of nonlinearities on the behavior if the system, and as such, it is also an ideal research tool. The key features of SIMULINK are:  Interactive simulation with live display.  A comprehensive block library for creating liner, nonlinear, discrete or hybrid multi input/output systems.  Seven integration methods for fixed-step, variable-step, and stiff systems.  Unlimited hierarchical model structure.  Scalar and vector connections.  Mask facility for creating custom blocks and block libraries.

6.2.1

SIMULATION

SOLVER:

PARAMETERS

AND

We set the simulation parameters and select the solver by choosing Parameters from the simulation menu. SIMULINK displays the simulation parameters dialog box, which uses three “pages” to manage

simulation

parameters,

Solver,

Workspace

I/O,

and

Diagnostics.

SOLVER PAGE: The

solver

page

appears

when

you

first

choose

Parameters from the Simulation menu or when you select the Solver tab. The Solver page allows you to:

• Set the start and stop times- you can change the start time and stop time for the simulation by entering new values in the Start time and Stop time fields. The default start time is 0.0 seconds and the default stop time is 10.0 seconds.

• Choose the solver and specify solver parameters – The default solver provides accurate and efficient results for most problems. Variable–step solvers can modify their step sizes during the simulation.

These

are

ode45, ode23, ode113, ode15s,

ode23s, and discrete. The default is od45.

For fixed–step

solvers, you can choose ode5, ode3, ode2, ode1, and discrete.

• Output options – The output options area of the dialog box enables you to control how much output the simulation generates. You can choose from three popup options. These are: Refine output, produce additional output, and produce specified output only.

WORKSPACE I/O PAGE: The Workspace I/O page manages the input from and the output to the MATLAB workspace, and allows: •

Loading input from the workspace – input can be specified either as MATLAB command or as a matrix for the Import blocks.

•

Saving the output to the workspace – you can specify return variables by selecting the Time, State, and /or Output check boxes in the Save to workspace area.

DIAGNOSTICS PAGE: The Diagnostic page allows you to select the level of warning messages displayed during a simulation.

6.2.2 THE SIMULATION PARAMETERS DIALOG BOX: Table below summarizes the actions performed by the dialog box buttons, which appear on the button of each dialog box page.

Button Apply

Action Applies the current parameter values and keeps the dialog box open. During a simulation, the parameter values are applied immediately.

Revert

Changes the parameter values back to the values they had when the Dialog box was most recently opened and applies the parameter.

Help Close

Displays help text for the dialog box page Applies the parameter values and closes the dialog box. During a simulation, the parameter values are applied immediately.

6.2.3 BLOCK DIAGRAM CONSTRUCTION: At the MATLAB prompt, type SIMULINK. The SIMULINK BLOCK LIBRARY, containing seven icons, and five pull-down menu heads, appears. Each icon contains various components in the titled category. To see the content of each category, double click on its icon. The easy-to-use pull-down menus allows you to create a SIMULINK block diagram, or open an existing file, perform the simulation, and make any modifications. Basically, one has to specify the model of the system (state space, discrete, transfer functions, nonlinear ode’s, etc), the input (source) to the system, and where the output (sink) of the simulation of the system will go. Generally when building a model, design it first on the paper, then build it using the computer. When you start putting blocks together into a model, add the blocks to the model window before adding the lines that connect them. This way, you can reduce how often you need to open block libraries

Fig 1: source and load voltage of each phase with α=0° for LOW L/R load

Fig 2: source and load currents of each phase with α=0° for LOW L/R load

Fig 3: Harmonic spectra of output voltage for phase A

Fig 4: Harmonic spectra of output voltage for phase B

Fig 5: Harmonic spectra of output voltage for phase C

Fig 6: Harmonic spectra of output current for phase A

Fig 7: Harmonic spectra of output current for phase B

Fig 8: Harmonic spectra of output current for phase C

GATE PULSES OF EACH THYRISTOR:

Fig: source and load voltage of each phase with α=0°

Fig: source and load currents of each phase with α=0°

Fig: Harmonic spectra of load voltage for phase A

Fig: Harmonic spectra of load voltage for phase B

Fig: Harmonic spectra of load voltage of for phase C

Fig: harmonic spectra of load current for phase A

Fig: harmonic spectra of load current for phase B

Fig: Harmonic spectra of load current for phase C

Fig: harmonic spectra of load voltage for phase A

Fig: harmonic spectra of load voltage for phase B

CHAPTER-8

8. 1. CONCLUSIONS: Detailed Simulation for an existing single-phase to three-phase cycloconverter with 50 Hz line frequency and output frequency of 16.6666 Hz (N=3) have been carried out. The reactive loading constraints on the cycloconverter operating under the group trigger mode have been studied. It has been found that for low

loads

both load voltage and the load current contain large harmonic components have been found to be present in the load voltage wave form for all kinds of loads. Delayed conduction of the incoming SCR at the crossover-point has been clearly demonstrated for a medium

load.

The load current waveform is relatively free of harmonic distribution, but the load voltage waveform contains relatively higher harmonic components. The short circuit free operation for high

loads with a

constant firing angle method is not possible because of the short circuit occurrences due to high ratio of . It is recommended that a resistor is connected in series with circulating current inductor (CCI) for limiting the short circuit currents in the practical system.

8.2. APPLICATIONS: At present, the applications of cycloconverter are:  Speed control of high-power AC drives.  Induction heating.  Static VAR generation.  For converting variable speed alternator voltage to constant frequency output voltage for use as power supply in Aircraft or Shipboards.  Slow and Speed large AC drives like Rotary Kilns and Traction vehicles.  Aircrafts VSCF systems.

8. 3. FUTURE SCOPE OF PROJECT: Cycloconverters are used in the industry for the control of frequency and voltage. The single phase to three phase cycloconverter induction motors drives can be used for a single phase traction system. In this project we used constant firing angle method. This technique uses less CPU time and memory. By using this we can reduce these two efforts. We can implement this on 8- bit popular Micro processor for single phase to three phase cyclo converter fed induction motor.

8. 4. FUTURE DEVELOPMENT ON POWER ELECTRONICS: Lower voltages, higher current and better regulation are needed for the coming generation of Very Large Scale Industries (VLSI). Highly regulated voltages on the order of three volts, or less, currents in tens and hundreds of amperes and load dynamics of 100 A/sec will be required. Narain G. Hingorani, vice president of the Electrical System Division for EPRI (Electrical Power Research Institute, Palo Alta, CA, USA), says power electronics should be national priority. He points out (1987) that over the next over 20 years the development of semi conductor devices for high voltage, high current applications will bring about a major transformation in industrial system. He says this “second electronics revolution” has always begun and is being used to make AC/DC converters for High-voltage DC (HVDC) transmission, static VAR compensators, UPS to protect sensitive equipment and drives for adjustable speed motors (Miller 1987).

High frequency resonant converters will be found wide applications in the switching power supplies for personal computers in the near future. Built in Un-interrupted Power Supply (UPS) for the personal computer will possible in the future.

Microprocessors will play a more important role in the power converting system in the future. Dedicated micro controller and power control IC (integrated control) will be developed. Smaller size, lower cost, higher power handling capability and more reliable power converting system will be on the market.

BIBLIOGRAPHY 1. Muhammad H. Rashid, “power electronic circuits, Devices, and Applications”, published by Pearson Education (Singapore) Pte. Ltd., Indian Branch, Delhi. 2. “Microprocessor based single phase to three phase cycloconverter,” IEEE Trans. Ind. Electron., Vol. 37, no. 4, Aug. 1990. 3. G. N. Achary, U. N. Rao, S. S. Shekhawat, “A Single-Phase To Three-Phase Static Converter,” in proc. IEE, Power Electronics, Power Semiconductors and their Applications, 1977. 4. K.Kant, “Analysis and Design of Cycloconverter Fed Induction

Motor Drive”, Ph.D. dissertation, department Engineering, I.I.T., Delhi, India, 1982.

of

Electrical

5. Vineeta and K.Kant, “An Efficient Algorithm for the Control of

Microprocessor Based Single Phase to Three Phase Cycloconverter,” IEEE Trans. In. Electron. Vol. 37. No. 1, Feb. 1990. S. K. Tso. M. E. Spooner, and J. Consgrove, “Efficient Microprocessor Based Cycloconverter Control,” in Proc. IEE, pt. B, Vol. 127, no. 3, May 1980. 6. L. Gyugi and B. R. Pelly, Static Power Frequency Changer, New York: Wiley Interscience, 1976.

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