UP GRADATION OF EXISTING EHVAC LINE BY

COMPOSITE AC-DC TRANSMISSION

A Project Report submitted in partial fulfillment of the

Requirements for the award of the Degree

BACHELOR OF TECHNOLOGY

IN

ELECTRICAL & ELECTRONICS ENGINEERING

Submitted By

K.KUMARA SWAMY K.BAGHYA LAKSHMI (07541A0266) (07541A0261)

K.V.R.SRIKANTH D.DIVYA (07541A0259) (07541A0250)

B.RAJASEKHAR K.VARASRINIVAS (07541A0210) (06541A0243)

Under the Esteemed Guidance of

Mr.T.VIJAY MUNI M.Tech

Assistant professor

Department of Electrical & Electronics Engineering

SRI SARATHI INSTITUTE OF ENGINEERING & TECHNOLOGY

(Approved by A.I.CT.E & Affiliated to JNTU, Kakinada)

Nuzvid, Krishna Dist – 521 201, Andhra Pradesh

2011

SRI SARATHI INSTITUTE OF ENGINEERING & TECHNOLOGY

(Approved by A.I.CT.E & Affiliated to JNTU, Kakinada)

Nuzvid, Krishna Dist – 521 201, Andhra Pradesh

Department of Electrical and Electronics Engineering

CERTIFICATE

This is to certify that the project report entitled “COMPARISION OF 84-PULSE

VSC AND CASCADED MULTILEVEL CONVERTER BASED STATCOM” that is

a bonafide record of work done by K.KUMARA SWAMY (07541A0266),

K.BHAGYA LAKSHMI (07541A0261), K.V.R.SRIKANTH (07541A0259),

D.DIVYA (07541A0250), B.RAJASEKHAR (07541A0210), K.VARASRINIVAS

(06541A0243) submitted in partial fulfillment of the requirements for the award of

degree of Bacholer of Technology in Electrical & Electronics Engineering to the

JNTUK during the academic year (2010-2011). The results embodied in this project

report have not been submitted to any other University or Institute for the award of any

degree or diploma.

Date:

Signature of Guide Signature of HOD

T.VIJAY MUNI D.R.K.PRAVEEN

Assistant Professor Associate Professor

External Examiner

ACKNOWLEDGEMENT

All the success in the every step of our project involves great efforts of the masters

who guided us all through the way, forbidding many obstacles and making us to achieve this

project a grand success.

With the sense of gratitude we wish to express our profound regards to our project

guide Mr.T.VIJAY MUNI, M.Tech for his supervision in framing our project in an

outstanding manner and for his remarkable guidance and encouragement throughout the

project.

We convey our heartfelt thanks to Mr.D.R.K.PRAVEEN, Head of the Electrical

and Electronics Engineering department for his infallible co-operation in the evolution of

our project.

We would like to sincerely thank to the Principal of Sri Sarathi Institute of

Engineering & Technology Prof.P.BHUPAL REDDY for providing necessary facility to

carry out project work successfully.

A special note of thanks to all the faculty members of Electrical and Electronics

Engineering department, for sharing their years of experience and adding momentum to

our project.

Project Associates - - -

K.Kumara Swamy

K.Bhagya Lakshmi

K.V.R.Srikanth

D.Divya

B.Rajasekher

K.Vara Srinivas

ABSTRACT

Recently proposed concept of simultaneous ac-dc power transmission enables the

long EHV ac lines to be loaded close to their thermal limits. The conductors are allowed

to carry certain amount of dc current superimposed on usual ac. This paper presents the

power up gradation of existing EHV ac line corridor by converting it into composite ac-

dc power transmission line. This paper presents the feasibility of converting a double

circuit ac line into composite ac–dc power transmission line to get the advantages of

parallel ac–dc transmission to improve stability and damping out oscillations. No

alterations of conductors, insulator strings or towers of the original line are needed.

Substantial gain in the load ability of the line is obtained. Master current controller senses

ac current and regulates the dc current orders for converters online such that conductor

current never exceeds its thermal limit. Simulation study is carried out on MAT Lab

SIMULINK software package.

TABLES OF CONTENTS

CHAPTER NO TITLE PAGE NO

1 INTRODUCTION 1

2 SIMULTANEOUS AC-DC POWER TRANSMISSION 2

3 DESCRIPTION OF THE SYSTEM MODEL 8

4 HVDC 10

4.1. Natural commutated converters 10

4.2. Capacitor Commutated Converters 11

4.3 Forced Commutated Converters 11

5 TYPES OF HVDC SYSTEMS 12

5.1 Types of HVDC System 12

5.1.1 Mono-polar HVDC system 12

5.1.2 Bipolar HVDC system 12

5.1.3 Homo-polar HVDC system 13

5.2 Voltage Source Converter 14

5.2.1 Voltage Source Converter based on IGBT technology 15

5.3 GTO/IGBT (Thyristor based HVDC) 15

6 12 PULSE CONVERTER 18

6.1 HVDC Converter Station 19

6.2 Basic Control Principles 20

7 COMPARISON OF DIFFERENT HVAC-HVDC 26

8 SCHEMATIC DIAGRAM OF POWER UP GRADING 31

BY COMBINING AC-DC TRANSMISSION

9 SIMULINK 38

10 SIMULATION RESULTS 65

CONCLUSION 68

BIBLIOGRAPHY 69

List of Figures

S.No Name of the Figure Page No.

1 Basic scheme for composite ac–dc transmission 2

2 IEEE Type AC4A Excitation System 8

3 Mono-polar HVDC system 12

4 Bipolar HVDC system: 12

5 Homo-polar HVDC system: 13

6 Multi-terminal HVDC system: 14

7 12-pulse converter. 18

8 Main elements of a HVDC converter station with one bipole

consisting of two 12-pulse converter unit.

19

9 Schematic diagram of power up grading by combining AC-

DC transmission

31

10 Sending end voltage and receiving end voltage

11 Sending end current and receiving end current 65

12 Active and Reactive Power 66

13 Combined AC-DC Currents 67

CHAPTER 1

INTRODUCTION

In recent years, environmental, right-of-way, and cost concerns have delayed the

construction of a new transmission line, while demand of electric power has shown

steady but geographically uneven growth. The power is often available at locations not

close to the growing load centers but at remote locations. These locations are largely

determined by regulatory policies, environmental acceptability, and the cost of available

energy. The wheeling of this available energy through existing long ac lines to load

centers has a certain upper limit due to stability considerations. Thus, these lines are not

loaded to their thermal limit to keep sufficient margin against transient instability.

The present situation demands the review of traditional power transmission theory

and practice, on the basis of new concepts that allow full utilization of existing

transmission facilities without decreasing system availability and security. The flexible ac

transmission system (FACTS) concepts, based on applying state-of-the-art power

electronic technology to existing ac transmission system, improve stability to achieve

power transmission close to its thermal limit.

The basic proof justifying the simultaneous ac–dc power transmission is

explained in reference. In the above references, simultaneous ac–dc power transmission

was first proposed through a single circuit ac transmission line. In these proposals Mono-

polar dc transmission with ground as return path was used. There were certain limitations

due to use of ground as return path. Moreover, the instantaneous value of each conductor

voltage with respect to ground becomes higher by the amount of the dc voltage, and more

discs are to be added in each insulator string to withstand this increased voltage.

However, there was no change in the conductor separation distance, as the line-to-line

voltage remains unchanged. In this paper, the feasibility study of conversion of a double

circuit ac line to composite ac–dc line without altering the original line conductors, tower

structures, and insulator strings has been presented.

CHAPTER 2

SIMULTANEOUS AC-DC POWER TRANSMISSION

Fig. 1 depicts the basic scheme for simultaneous ac–dc power flow through a

double circuit ac transmission line. The dc power is obtained through line commutated

12-pulse rectifier bridge used in conventional HVDC and injected to the neutral point of

the zigzag connected secondary of sending end transformer and is reconverted to ac again

by the conventional line commutated 12-pulse bridge inverter at the receiving end. The

inverter bridge is again connected to the neutral of zig-zag connected winding of the

receiving end transformer.

The double circuit ac transmission line carriers both three-phase ac and dc power.

Each conductor of each line carries one third of the total dc current along with ac current.

Resistance being equal in all the three phases of secondary winding of zig-zag

transformer as well as the three conductors of the line, the dc current is equally divided

among all the three phases.

Fig. 1. Basic scheme for composite ac–dc transmission

The three conductors of the second line provide return path for the dc current.

Zig-zag connected winding is used at both ends to avoid saturation of transformer due to

dc current. Two fluxes produced by the dc current flowing through each of a

winding in each limb of the core of a zig-zag transformer are equal in magnitude and

opposite in direction. So the net dc flux at any instant of time becomes zero in each limb

of the core. Thus, the dc saturation of the core is avoided. A high value of reactor is

used to reduce harmonics in dc current. In the absence of zero sequence and third

harmonics or its multiple harmonic voltages, under normal operating conditions, the ac

current flow through each transmission line will be restricted between the zigzag

connected windings and the three conductors of the transmission line. Even the presence

of these components of voltages may only be able to produce negligible current through

the ground due to high value of . Assuming the usual constant current control of

rectifier and constant extinction angle control of inverter [4], [8]–[10], the equivalent

circuit of the scheme under normal steady-state operating condition is given in Fig. 2.

The dotted lines in the figure show the path of ac return current only. The second

transmission line carries the return dc current, and each conductor of the line carries

along with the ac current per phase. And are the maximum values of rectifier and

inverter side dc voltages and are equal to times converter ac input line-to-line

voltage. R, L, and C are the line parameters per phase of each line. , are

commutating resistances, and, are firing and extinction angles of rectifier and

inverter, respectively. Neglecting the resistive drops in the line conductors and

transformer windings due to dc current, expressions for ac voltage and current, and for

active and reactive powers in terms of A, B, C, and D parameters of each line may be

written as

Neglecting ac resistive drop in the line and transformer, the dc power and of

each rectifier and inverter may be expressed as

The net current in any conductor is offseted from zero.In case of a fault in the

transmission system, gate signals to all the SCRs are blocked and that to the bypass SCRs

are released to protect rectifier and inverter bridges. The current in any conductor is no

more offseted. Circuit breakers (CBs) are then tripped at both ends to isolate the faulty

line. CBs connected at the two ends of transmission line interrupt current at natural

current zeroes, and no special dc CB is required. Now, allowing the net current through

the conductor equal to its thermal limit

Let be per-phase rms voltage of original ac line. Let also be the per-phase

voltage of ac component of composite ac–dc line with dc voltage superimposed on

it. As insulators remain unchanged, the peak voltage in both cases should be equal

Electric field produced by any conductor possesses a dc component superimpose on it a

sinusoidally varying ac component. However, the instantaneous electric field polarity

changes its sign twice in a cycle if is insured. Therefore, higher creep

age distance requirement for insulator discs used for HVDC lines are not required. Each

conductor is to be insulated for, but the line-to-line voltage has no dc component

and. Therefore, conductor-to-conductor separation distance of each

line is determined only by rated ac voltage of the line. Allowing maximum permissible

voltage offset such that the composite voltage wave just touches zero in each every cycle;

The total power transfer through the double circuit line before conversion is as follows:

Where the transfer reactance per phase of the double is circuit line, and is the

power angle between the voltages at the two ends. To keep sufficient stability margin,

is generally kept low for long lines and seldom exceeds 30. With the increasing

length of line, the load ability of the line is decreased [4]. An approximate value of

may be computed from the load ability curve by knowing the values of surge impedance

loading (SIL) and transfer reactance of the line

Where M is the multiplying factor and its magnitude decreases with the length of line.

The value of M can be obtained from the loadability curve. The total power transfer

through the composite line

The power angle between the ac voltages at the two ends of the composite

line may be increased to a high value due to fast controllability of dc component of

power. For a constant value of total power, may be modulated by fast control of the

current controller of dc power converters. Approximate value of ac current per phase per

circuit of the double circuit line may be computed as

The rectifier dc current order is adjusted online as

Preliminary qualitative analysis suggests that commonly used techniques in

HVDC/AC system may be adopted for the purpose of the design of protective scheme,

filter, and instrumentation network to be used with the composite line for simultaneous

ac–dc power flow. In case of a fault in the transmission system, gate signals to all the

SCRs are blocked and that to the bypass SCRs are released to protect rectifier and

inverter bridges. CBs are then tripped at both ends to isolate the complete system. A

surge diverter connected between the zig-zag neutral and the ground protects the

converter bridge against any over voltage.

CHAPTER 3

DESCRIPTION OF THE SYSTEM MODEL

A synchronous machine is feeding power to infinite bus via a double circuit,

three-phase, 400-KV, 50-Hz, 450-Km ac transmission line. The 2750-MVA (5 * 550),

24.0-KV synchronous machine is dynamically modeled, a field coil on d-axis and a

damper coil on q-axis, by Park’s equations with the frame of reference based in rotor [4].

It is equipped with an IEEE type

AC4A excitation system of which block diagram is shown in Fig. 3. Transmission

lines are represented as the Bergeron model. It is based on a distributed LC parameter

travelling wave line model, with lumped resistance. It represents the L and C elements of

a PI section in a distributed manner (i.e., it does not use lumped parameters).

It is roughly equivalent to using an infinite number of PI sections, except that the

resistance is lumped (1/2 in the middle of the line, 1/4 at each end). Like PI sections,

the Bergeron model accurately represents the fundamental frequency only. It also

represents impedances at other frequencies, except that the losses do not change. This

model is suitable for studies where the fundamental frequency load flow is most

important. The converters on each end of dc link are modeled as line commutated two

six- pulse bridge (12-pulse), Their control system consist of constant current (CC) and

constant extinction angle (CEA) and voltage dependent current order limiters (VDCOL)

control. The converters are connected to ac buses via Y-Y and Y- converter transformers.

Each bridge is a compact power system computer-aided design (SIMULINK)

representation of a dc converter, which includes a built in six-pulse Graetz converter

bridge (can be inverter or rectifier), an internal phase locked oscillator (PLO), firing and

valve blocking controls, and firing angle /extinction angle measurements. It also

includes built in RC snubber circuits for each thyristor. The controls used in dc system

are those of CIGRE Benchmark , modified to suit at desired dc voltage. Ac filters at each

end on ac sides of converter transformers are connected to filter out 11th and 13th

harmonics. These filters and shunt capacitor supply reactive power requirements of

converters.

A master current controller (MCC), shown in Fig. 4, is used to control the current

order for converters. . It measures the conductor ac current, computes the permissible dc

current, and produces dc current order for inverters and rectifiers.

CHAPTER 4

HVDC

Over long distances bulk power transfer can be carried out by a high voltage

direct current (HVDC) connection cheaper than by a long distance AC transmission line.

HVDC transmission can also be used where an AC transmission scheme could not (e.g.

through very long cables or across borders where the two AC systems are not

synchronized or operating at the same frequency). However, in order to achieve these

long distance transmission links, power convertor equipment is required, which is a

possible point of failure and any interruption in delivered power can be costly. It is

therefore of critical importance to design a HVDC scheme for a given availability.

The HVDC technology is a high power electronics technology used in electric

power systems. It is an efficient and flexible method to transmit large amounts of electric

power over long distances by overhead transmission lines or underground/submarine

cables. It can also be used to interconnect asynchronous power systems

The fundamental process that occurs in an HVDC system is the conversion of

electrical current from AC to DC (rectifier) at the transmitting end and from DC to AC

(inverter) at the receiving end.

There are three ways of achieving conversion

1. Natural commutated converters

2. Capacitor Commutated Converters

3. Forced Commutated Converters

4.1 Natural commutated converters: (NCC)

NCC are most used in the HVDC systems as of today. The component that

enables this conversion process is the thyristor, which is a controllable semiconductor

that can carry very high currents (4000 A) and is able to block very high voltages (up to

10 kV). By means of connecting the thyristors in series it is possible to build up a

thyristor valve, which is able to operate at very high voltages (several hundred of

kV).The thyristor valve is operated at net frequency (50 Hz or 60 Hz) and by means of a

control angle it is possible to change the DC voltage level of the bridge..

4.2 Capacitor Commutated Converters (CCC).

An improvement in the thyristor-based Commutation, the CCC concept is characterized

by the use of commutation capacitors inserted in series between the converter

transformers and the thyristor valves. The commutation capacitors improve the

commutation failure performance of the converters when connected to weak networks.

4.3 Forced Commutated Converters.

This type of converters introduces a spectrum of advantages, e.g. feed of passive

networks (without generation), independent control of active and reactive power, power

quality. The valves of these converters are built up with semiconductors with the ability

not only to turn-on but also to turn-off. They are known as VSC (Voltage Source

Converters).a new type of HVDC has become available. It makes use of the more

advanced semiconductor technology instead of thyristors for power conversion between

AC and DC. The semiconductors used are insulated gate bipolar transistors (IGBTs), and

the converters are voltage source converters (VSCs) which operate with high switching

frequencies (1-2 kHz) utilizing pulse width modulation (PWM).

CHAPTER 5

TYPES OF HVDC SYSTEMS

There are different types of HVDC systems which are

5.1 TYPES OF HVDC SYSTEM

5.1.1Mono-polar HVDC system:

In the mono-polar configuration, two converters are connected by a single pole

line and a positive or a negative DC voltage is used. In Fig. There is only one Insulated

transmission conductor installed and the ground or sea provides the path for the return

current.

5.1.2 Bipolar HVDC system:

This is the most commonly used configuration of HVDC transmission systems.

The bipolar configuration, shown in Fig. Uses two insulated conductors as Positive and

negative poles. The two poles can be operated independently if both Neutrals are

grounded. The bipolar configuration increases the power transfer capacity. Under normal

operation, the currents flowing in both poles are identical and there is no ground current.

In case of failure of one pole power transmission can continue in the other pole which

increases the reliability. Most overhead line HVDC transmission systems use the bipolar

configuration.

5.1.3 Homo-polar HVDC system:

In the homo polar configuration, shown in Fig. Two or more conductors have the

negative polarity and can be operated with ground or a metallic return. With two Poles

operated in parallel, the homopolar configuration reduces the insulation costs. However,

the large earth return current is the major disadvantage.

5.1.4 Multi-terminal HVDC system:

In the multi terminal configuration, three or more HVDC converter stations are

geographically separated and interconnected through transmission lines or cables. The

System can be either parallel, where all converter stations are connected to the same

voltage as shown in Fig(b). or series multiterminal system, where one or more converter

stations are connected in series in one or both poles as shown in Fig. (c). A hybrid

multiterminal system contains a combination of parallel and series connections of

converter stations

5.2 VOLTAGE-SOURCE CONVERTER:

A voltage-source converter is connected on its ac-voltage side to a three-phase

electric power network via a transformer and on its dc-voltage side to capacitor

equipment. The transformer has on its secondary side a first, a second, and a third phase

winding, each one with a first and a second winding terminal. Resistor equipment is

arranged at the transformer for limiting the current through the converter when

connecting the transformer to the power network. The resistor equipment includes a first

resistor, connected to the first winding terminal of the second phase winding, and

switching equipment is adapted, in an initial position, to block current through the phase

windings, in a transition position to form a current path which includes at least the first

and the second phase windings and, in series therewith, the first resistor, which current

path, when the converter is connected to the transformer, closes through the converter

and the capacitor equipment, and, in an operating position, to interconnect all the first

winding terminals for forming the common neutral point.

In VSC HVDC, Pulse Width Modulation (PWM) is used for generation of the

fundamental voltage. Using PWM, the magnitude and phase of the voltage can be

controlled freely and almost instantaneously within certain limits. This allows

independent and very fast control of active and reactive power flows. PWM VSC is

therefore a close to ideal component in the transmission network. From a system point of

view, it acts as a zero inertia motor or generator that can control active and reactive

power almost instantaneously. Furthermore, it does not contribute to the shortcircuit

power, as the AC current can be controlled.

5.2.1Voltage Source Converter based on IGBT technology

The modular low voltage power electronic platform is called PowerPak. It is a

power electronics building block (PEBB) with three integrated Insulated Gate Bipolar

Transistor (IGBT) modules. Each IGBT module consists of six switches forming three

phase legs. Various configurations are possible. For example three individual three-phase

bridges on one PEBB, one three phase bridge plus chopper(s) etc. The PowerPak is easily

adaptable for different applications.

The IGBT modules used are one Power Pak as it is used for the SVR. It consists

of one three-phase bridge (the three terminals at the right hand side), which provides the

input to the DC link (one IGBT module is used for it) and one output in form of one

single phase H-bridge (the two terminals to the left) acting as the booster converter. For

the latter two IGBT modules are used with three paralleled phase legs per output

terminal. By paralleling such PEBBs adaptation to various ratings is possible.

5.3 GTO/IGBT (Thyristor based HVDC):

Normal thyristors (silicon controlled rectifiers) are not fully controllable switches

(a "fully controllable switch" can be turned on and off at will.) Thyristors can only be

turned ON and cannot be turned OFF. Thyristors are switched ON by a gate signal, but

even after the gate signal is de-asserted (removed), the thyristor remains in the ON-state

until any turn-off condition occurs (which can be the application of a reverse voltage to

the terminals, or when the current flowing through (forward current) falls below a certain

threshold value known as the holding current.) Thus, a thyristor behaves like a normal

semiconductor diode after it is turned on or "fired".

The GTO can be turned-on by a gate signal, and can also be turned-off by a gate

signal of negative polarity.

Turn on is accomplished by a positive current pulse between the gate and

cathode terminals. As the gate-cathode behaves like PN junction, there will be some

relatively small voltage between the terminals.The turn on phenomenon in GTO is

however, not as relieable as an SCR(thyristor) and small positive gate current must be

maintained even after turn on to improve relieabilty.

Turn off is accomplished by a negative voltage pulse between the gate and

cathode terminals. Some of the forward current (about one third to one fifth) is "stolen"

and used to induce a cathode-gate voltage which in turn induces the forward current to

fall and the GTO will switch off (transitioning to the 'blocking' state.)

GTO thyristors suffer from long switch off times, whereby after the forward

current falls, there is a long tail time where residual current continues to flow until all

remaining charge from the device is taken away. This restricts the maximum switching

frequency to approx 1kHz.

It may however be noted that the turn off time of a comparable SCR is ten times

that of a GTO.Thus switching frequency of GTO is much better than SCR.

Gate turn-off (GTO) thyristors are able to not only turn on the main current but

also turn it off, provided with a gate drive circuit. Unlike conventional thyristors, they

have no commutation circuit, downsizing application systems while improving

efficiency. They are the most suitable for high-current, high speed switching applications,

such as inverters and chopper circuits.

Bipolar devices made with SiC offer 20-50X lower switching losses as compared

to conventional semiconductors. A rough estimate of the switching power losses as a

function of switching frequency is shown in Figure 4. Another very significant property

of SiC bipolar devices is their lower differential on-state voltage drop than similarly rated

Si bipolar device, even with order of magnitude smaller carrier lifetimes in the

drift region.

This property allows high voltage (>20 kV) to be far more reliable and thermally

stable as compared to those made with Silicon. The switching losses and the temperature

stability of bipolar power devices depends on the physics of operation of the device.

The two major categories of bipolar power devices are: (a) single injecting

junction devices (for example BJT and IGBT); and (b) double injecting junction devices

(like Thyristor-based GTO/MTO/JCT/FCT and PIN diodes). In a power BJT, most of the

minority carrier charge resides in the low doped collector layer, and hence its operation

has been approximated as an IGBT. The limited gain of a BJT will make the following

analysis less relevant for lower voltage devices.

Silicon carbide has been projected to have tremendous potential for high voltage

solid-state power devices with very high voltage and current ratings because of its

electrical and physical properties. The rapid development of the technology for producing

high quality single crystal SiC wafers and thin films presents the opportunity to fabricate

solid- state devices with power-temperature capability far greater than devices currently

available. This capability is ideally suited to the applications of power conditioning in

new more- electric or all-electric military and commercial vehicles.

These applications require switches and amplifiers capable of large currents with

relatively low voltage drops. One of the most pervasive power devices in silicon is the

Insulated Gate Bipolar Transistor (IGBT). However, these devices are limited in their

operating temperature and their achievable power ratings compared to that possible with

SiC. Because of the nearly ideal combination of characteristics of these devices, we

propose to demonstrate the first 4H-SiC Insulated Gate Bipolar Transistor in this Phase I

effort. Both n-channel and p-channel SiC IGBT devices will be investigated. The targeted

current and voltage rating for the Phase I IGBT will be a >200 Volt, 200 mA device, that

can operate at 350 C.

CHAPTER 6

12-PULSE CONVERTERS

The basic design for practically all HVDC converters is the 12-pulse double

bridge converter which is shown in Figure below. The converter consists of two 6-pulse

bridge converters connected in series on the DC side. One of them is connected to the AC

side by a YY-transformer, the other by a YD transformer. The AC currents from each 6-

pulse converter will then be phase shifted 30°. This will reduce the harmonic content in

the total current drawn from the grid, and leave only the characteristic harmonics of order

12 m±1, m=1,2,3..., or the 11th, 13th, 23rd, 25th etc. harmonic. The non-characteristic

harmonics will still be present, but considerably reduced. Thus the need for filtering is

substantially reduced, compared to 6-pulse converters. The 12-pulse converter is usually

built up of 12 thyristor valves. Each valve consists of the necessary number of thyristors

in series to withstand the required blocking voltage with sufficient margin. Normally

there is only one string of thyristors in each valve, no parallel connection. Four valves are

built together in series to form a quadruple valve and three quadruple valves,

Figure: 12-pulse converter.

Main elements of a HVDC converter station with one bipole consisting of two 12-

pulse converter unit.

together with converter transformer, controls and protection equipment, constitute a

converter. The converter transformers are usually three winding transformers with the

Windings in Yy d N-connection. There can be one three-phase or three single phase

transformers, according to local circumstances. In order to optimize the relationship

between AC- and DC voltage the converter transformers are equipped with tap changers.

6.1 HVDC converter stations

An HVDC converter station is normally built up of one or two 12-pulse

converters as described above, depending on the system being mono- or bipolar. In some

cases each pole of a bipolar system consists of two converters in series to increase the

voltage and power rating of the transmission. It is not common to connect converters

directly in parallel in one pole. The poles are normally as independent as possible to

improve the reliability of the system, and each pole is equipped with a DC reactor and

DC filters. Additionally the converter station consists of some jointly used equipment.

This can be the connection to the earth electrode, which normally is situated

Mono-polar HVDC transmission Voltage in station B according to reversed polarity

convention.

some distance away from the converter station area, AC filters and equipment

for supply of the necessary reactive power.

6.2 BASIC CONTROL PRINCIPLES

DC transmission control

The current flowing in the DC transmission line shown in Figure below is

determined by the DC voltage difference between station A and station B. Using the

notation shown in the figure, where rd represents the total resistance of the line, we get

for the DC current

and the power transmitted into station B is

In rectifier operation the firing angle α should not be decreased below a certain

minimum value αmin, normally 3°-5° in order to make sure that there really is a

positive voltage across the valve at the firing instant. In inverter operation the

extinction angle should never decrease below a certain minimum value γmin,

normally17°-19° otherwise the risk of commutation failures becomes too high. On the

other hand, both α and γ should be as low as possible to keep the necessary nominal

rating of the equipment to a minimum. Low values of α and γ also decrease the

consumption of reactive power and the harmonic

distortion in the AC networks.

To achieve this, most HVDC systems are controlled to maintain γ = γmin in

normal operation. The DC voltage level is controlled by the transformer tap changer in

inverter station B. The DC current is controlled by varying the DC voltage in rectifier

station A, and thereby the voltage difference between A and B. Due to the small DC

resistances in such a system, only a small voltage difference is required, and small

variations in rectifier voltage gives large variations in current and transmitted power. The

DC current through a converter cannot change the direction of flow. So the only way to

change the direction of power flow through a DC transmission line is to reverse the

voltage of the line. But the sign of the voltage difference has to be kept constantly

positive to keep the current flowing. To keep the firing angle α as low as possible, the

transformer tap changer in rectifier station A is operated to keep α on an operating value

which gives only the necessary margin to αmin to be able to control the current.

Converter current/voltage characteristics

The resistive voltage drop in converter and transformer, as well as the non current

voltage drop in the thyristor valves are often disregarded in practical analysis, as they are

normally in the magnitude of 0.5 % of the normal operating voltage. The commutation

voltage drop, however, has to be taken into account as this is in the magnitude of 5 to 10

% of the normal operating voltage. The direct voltage Ud from a 6-pulse bridge converter

can then be expressed by

where αis the firing angle,

If the converter is operating as inverter it is more convenient to operate with

extinction angle γ instead of firing angle α. The extinction angle is defined as the angle

between the end of commutation to the next zero crossing of the commutation voltage.

Firing angle α, commutation angle μ and extinction

angle γ are related by

In inverter mode, the direct voltage from the inverter can be written as

The current/voltage characteristics expressed in above are shown for normal values of

id and dxN. In order to create a characteristic diagram for the complete transmission, it is

usual to define positive voltage in inverter operation in the opposite direction compared

to rectifier operation.

It is clear that to operate both converters on a constant firing/extinction angle

principle is like leaving them without control. This will not give a stable point of

operation, as both characteristics have approximately the same slope. Small differences

appear due to variations in transformer data and voltage drop along the line. To gain the

best possible control the characteristics should cross at as close to a right angle as

possible. This means that one of the characteristics should preferably be constant current.

This can only be achieved by a current controller.

If the current/voltage diagram of the rectifier is combined with a constant current

controller characteristic we get the steady state diagram in Figure below for converter

station A. A similar diagram can be drawn for converter station B. If we apply the

reversed polarity convention for the inverter and combine the diagrams for station A and

station B we get the diagram in Figure below In normal operation, the rectifier will be

operating in current control mode with the firing angle

Steady state ud/id diagram for converter station A Steady state ud/id diagram for

converter station A.&B

The inverter has a slightly lower current command than the rectifier and tries to

decrease the current by increasing the counter voltage, but cannot decrease γ beyond

γmin. Thus we get the operating point A. We assume that the characteristic for station B

is referred to station A that is it is corrected for the voltage drop along the transmission

line. This voltage drop is in the magnitude of 1-5 % of the rated DC voltage. If the AC

voltage at the rectifier station drops, due to some external disturbance, the voltage

difference is reduced and the DC current starts to sink. The current controller in the

rectifier station starts to reduce the firing angle α, but soon meets the limit αmin, so the

current cannot be upheld. When the current sinks below the current command of the

inverter, the inverter control reduces the counter voltage to keep the current at the

inverter current command, until a new stable operating point B is reached. If the current

command at station A is decreased below that of station B, station A will see a current

that is to high and start to increase the firing angle α, to reduce the voltage. Station B will

see a diminishing current and try to keep it up by increasing the extinction angle γ to

reduce the counter voltage. Finally station A meets the γmin limit and cannot reduce the

voltage any further and the new operating point will be at point C. Here the voltage has

been reversed to negative while the current is still positive, that is the power flow has

been reversed. Station A is operating as inverter and station B as rectifier. The difference

between the current commands of the rectifier and the inverter is called the current

margin. It is possible to change the power flow in the transmission simply by changing

the sign of the current margin, but in practice it is desirable to do this in more controllable

ways. Therefore the inverter is normally equipped with a αmin limitation in the range of

95-105°. To avoid current fluctuations between operating points A and B at small voltage

variations the corner of the inverter characteristic is often cut off. Finally, it is not

desirable to operate the transmission with high currents at low voltages, and most HVDC

controls are equipped with voltage dependent current command limitation.

Master control system

The controls described above are basic and fairly standardized and similar for all

HVDC converter stations. The master control, however, is usually system specific and

individually designed. Depending on the requirements of the transmission, the control

can be designed for constant current or constant power transmitted, or it can be designed

to help stabilizing the frequency in one of the AC networks by varying the amount of

active power transmitted. The control systems are normally identical in both converter

systems in a transmission, but the master control is only active in the station selected to

act as the master station, which controls the current command. The calculated current

command is transmitted by a communication system to the slave converter station, where

the pre-designed current margin is added if the slave is to act as rectifier, subtracted if it

is to act as inverter. In order to synchronize the two converters and assure that they

operate with same current command (apart from the current margin), a tele-

communications channel is required.

Should the telecommunications system fail for any reason, the current commands

to both converters are frozen, thus allowing the transmission to stay in operation. Special

fail-safe techniques are applied to ensure that the telecommunications system is fault-

free. The requirements for the telecommunications system are especially high if the

transmission is required to have a fast control of the transmitted power, and the time

delay in processing and transmitting these signals will influence the dynamics of the total

control system.

CHAPTER 7

COMPARISON OF DIFFERENT HVAC-HVDC

In order to examine the behavior of the losses in combined transmission and

not in order to provide the best economical solutions for real case projects. Thus, most of

the configurations are overrated, increasing the initial investment cost and consequently

the energy transmission cost. The small number of different configurations analyzed

provides a limited set of results, from which specific conclusions can be drawn regarding

the energy transmission cost. Nevertheless, the same approach, as for the individual

HVACHVDC systems, is followed in order to evaluate the energy availability and the

energy transmission cost.

Presentation of Selected Configurations and Calculation of the Energy

Transmission Cost

For the combined HVAC-HVDC transmission systems only 500 MW and 1000 MW

wind farm were considered.

The choice for the transmission distance was limited to 50, 100 and 200 km. The three

following, general combinations were compared:

1. HVAC + HVDC VSC

2. HVAC + HVDC LCC

3. HVDC LCC + HVDC VSC

The specific configurations for each solution, based on the transmission distance and the

size of the wind farm, are presented in Tables .

Table: Configurations for the study of combined transmission systems. Wind farm

rated at 500 MW

Configurations for the study of combined transmission systems. Wind farm rated

at 1000

Only the rated power of each transmission technology changes every time while the

distance to shore and the condition of the onshore grid remain the same.

1. The HVAC system has a voltage level of 220 kV and it connected to a weak

grid 50 km from the offshore substation.

2. The HVDC VSC system is connected to a grid of medium strength at a

distance of 100 km from the offshore substation.

3. The HVDC LCC system is connected to a strong grid 200 km from the offshore

substation.The average losses for the cases described above were calculated by

Barberis table-1 and Todorovic table-2. The losses and the results concerning

the energy unavailability and the energy transmission cost are presented in

Table -3.

1000 MW Windfarm with Multiple Connection Points to Shore

Besides the combinations of the transmission technologies presented above, three cases

of transmission solutions from a 1000 MW windfarm are analysed. In these cases the

windfarm is connected to three different onshore grids, utilizing all three transmission

technologies studied so far.

Average power losses, energy unavailability and energy transmission cost for

transmission solutions from a 1000 MW windfarm with multiple connection points

to shore.

CHAPTER 8

SCHEMATIC DIAGRAM OF POWER UP GRADING BY COMBINING AC-DC

TRANSMISSION

phi = 80 deg. 3rd harm.

power upgrading by combining ac dc transmission

500kV, 60 Hz

5000 MVA equivalent

345kV, 50 Hz,

10,000 MVA equivalent

phi = 80 deg. 3rd harm.

Discrete,

Ts = s.

A+B+C+A-B-C-

a3

b3

c3

Zigzag

Phase-Shifting Transformer3

A+B+C+A-B-C-

a3

b3

c3

Zigzag

Phase-Shifting Transformer2

A+B+C+A-B-C-

a3

b3

c3

Zigzag

Phase-Shifting Transformer1

A+B+C+A-B-C-

a3

b3

c3

Zigzag

Phase-Shifting Transformer

Scope

Rectifier

Control and Protection

A

B

C

+

-

Rectifier

Master Control

Master ControlInverter

Control and Protection

A

B

C

+

-

Inverter

A B C

AC filters

50 Hz

600 Mvar

A B C

AC filters

60 Hz

600 Mvar

Distributed Parameters Line1

Distributed Parameters Line

Open this block

to visualize

recorded signals

Data Acquisition

i+

-

Current Measurement

A

B

C

a

b

c

Brect

A

B

C

a

b

c

Binv

A

B

C

A

B

C

A

B

C

A

B

C

A

B

C

A

B

C

Blocks functionalities

Three-Phase Source

The Three-Phase Source block implements a balanced three-phase voltage source

with an internal R-L impedance. The three voltage sources are connected in Y with a

neutral connection that can be internally grounded or made accessible. You can specify

the source internal resistance and inductance either directly by entering R and L values or

indirectly by specifying the source inductive short-circuit level and X/R ratio.

Three-Phase Parallel RLC Branch

The Three-Phase Parallel RLC Branch block implements three balanced branches

consisting each of a resistor, an inductor, a capacitor, or a parallel combination of these.

To eliminate either the resistance, inductance, or capacitance of each branch, the R, L,

and C values must be set respectively to infinity (inf), infinity (inf), and 0. Only existing

elements are displayed in the block icon. Negative values are allowed for resistance,

inductance, and capacitance

Three-Phase Transformer (Three Windings)

This block implements a three-phase transformer by using three single-phase

transformers with three windings. You can simulate the saturable core or not simply by

setting the appropriate check box in the parameter menu of the block. See the Linear

Transformer and Saturable Transformer block sections for a detailed description of the

electrical model of a single-phase transformer.

The three windings of the transformer can be connected in the following manner:

Y Y with accessible neutral (for windings 1 and 3 only) Grounded Y Delta (D1), delta

lagging Y by 30 degrees Delta (D11), delta leading Y by 30 degrees

Universal Bridge

The Universal Bridge block implements a universal three-phase power converter

that consists of up to six power switches connected in a bridge configuration. The type of

power switch and converter configuration are selectable from the dialog box.

The Universal Bridge block allows simulation of converters using both naturally

commutated (or line-commutated) power electronic devices (diodes or thyristors) and

forced-commutated devices (GTO, IGBT, MOSFET).

The Universal Bridge block is the basic block for building two-level voltage

sourced converters (VSC).

Connection Port

The Connection Port block, placed inside a subsystem composed of

SimPowerSystems blocks, creates a Physical Modeling open round connector port on the

boundary of the subsystem. Once connected to a connection line, the port becomes solid .

Once you begin the simulation, the solid port becomes an electrical terminal port, an

open square .

You connect individual SimPowerSystems blocks and subsystems made of sim

Power Systems blocks to one another with Sim Power Systems connection lines, instead

of normal Simulink signal lines. These are anchored at the open, round connector ports .

Subsystems constructed of SimPowerSystems blocks automatically have such open round

connector ports. You can add additional connector ports by adding Connection Port

blocks to your subsystem

Breaker

The Breaker block implements a circuit breaker where the opening and closing

times can be controlled either from an external Simulink signal (external control mode),

or from an internal control timer (internal control mode).

The arc extinction process is simulated by opening the breaker device when the

current passes through 0 (first current zero crossing following the transition of the

Simulink control input from 1 to 0).

When the breaker is closed it behaves as a resistive circuit. It is represented by a

resistance Ron. The Ron value can be set as small as necessary in order to be negligible

compared with external components (typical value is 10 m). When the breaker is open it

has an infinite resistance.

If the Breaker block is set in external control mode, a Simulink input appears on

the block icon. The control signal connected to the Simulink input must be either 0 or 1: 0

to open the breaker, 1 to close it. If the Breaker block is set in internal control mode, the

switching times are specified in the dialog box of the block.

If the breaker initial state is set to 1 (closed), SimPowerSystems automatically

initializes all the states of the linear circuit and the Breaker block initial current so that

the simulation starts in steady state.

A series Rs-Cs snubber circuit is included in the model. It can be connected to the

circuit breaker. If the Breaker block happens to be in series with an inductive circuit, an

open circuit or a current source, you must use a snubber.

Distributed Parameter Line

Implement an N-phase distributed parameter transmission line model with lumped

losses

The Distributed Parameter Line block implements an N-phase distributed

parameter line model with lumped losses. The model is based on the Bergeron's traveling

wave method used by the Electromagnetic Transient Program (EMTP).In this model, the

lossless distributed LC line is characterized by two values (for a single-phase line)

For multiphase line models, modal transformation is used to convert line quantities from

phase values (line currents and voltages) into modal values independent of each other.

The previous calculations are made in the modal domain before being converted back to

phase values. In comparison to the PI section line model, the distributed line represents

wave propagation phenomena and line end reflections with much better accuracy.

Description of the Control and Protection Systems

The control systems of the rectifier and of the inverter use the same Discrete

HVDC Controller block from the Discrete Control Blocks library of the

SimPowerSystems Extras library. The block can operate in either rectifier or inverter

mode. At the inverter, the Gamma Measurement block is used and it is found in the same

library. The Master Control system generates the current reference for both converters

and initiates the starting and stopping of the DC power transmission.

The protection systems can be switched on and off. At the rectifier, the DC fault

protection detects a fault on the line and takes the necessary action to clear the fault. The

Low AC Voltage Detection subsystem at the rectifier and inverter serves to discriminate

between an AC fault and a DC fault. At the inverter, the Commutation Failure Prevention

Control subsystem [2] mitigates commutation failures due to AC voltage dips. A more

detailed description is given in each of these protection blocks.

HVDC Controller Block Inputs and Outputs

Inputs 1and 2 are the DC line voltage (VdL) and current (Id). Note that the

measured DC currents (Id_R and Id_I in A) and DC voltages (VdL_R and VdL_I in V)

are scaled to p.u. (1 p.u. current = 2 kA; 1 p.u. voltage = 500 kV) before they are used in

the controllers. The VdL and Id inputs are filtered before being processed by the

regulators. A first-order filter is used on the Id input and a second-order filter is used on

the VdL input.

Inputs 3 and 4 (Id_ref and Vd_ref) are the Vd and Id reference values in p.u.

Input 5 (Block) accepts a logical signal (0 or 1) used to block the converter when Block =

1.

Input 6 (Forced-alpha) is also a logical signal that can be used for protection purposes. If

this signal is high (1), the firing angle is forced at the value defined in the block dialog

box.

Input 7 (gamma_meas) is the measured minimum extinction angle of the converter 12

valves. It is obtained by combining the outputs of two 6-pulse Gamma Measurement

blocks. Input 8 (gamma_ref) is the extinction angle reference in degrees. To minimize

the reactive power absorption, the reference is set to a minimum acceptable angle (e.g.,

18 deg).

Finally, input 9 (D_alpha) is a value that is subtracted from the delay angle maximum

limit to increase the commutation margin during transients.

The first output (alpha_ord) is the firing delay angle in degrees ordered by the regulator.

The second output (Id_ref_lim) is the actual reference current value (value of Id_ref

limited by the VDCOL function as explained below). The third output (Mode) is an

indication of the actual state of the converter control mode. The state is given by a

number (from 0 to 6) as follows:

0 Blocked pulses

1 Current control

2 Voltage control

3 Alpha minimum limitation

4 Alpha maximum limitation

5 Forced or constant alpha

6 Gamma control

Synchronization and Firing System

The synchronization and generation of the twelve firing pulses is performed in the

12-Pulse Firing Control system. Use Look under mask to see how this block is built. This

block uses the primary voltages (input 2) to synchronize and generate the pulses

according to the alpha firing angle computed by converter controller (input 1). The

synchronizing voltages are measured at the primary side of the converter transformer

because the waveforms are less distorted. A Phase Locked Loop (PLL) is used to

generate three voltages synchronized on the fundamental component of the positive-

sequence voltages. The firing pulse generator is synchronized to the three voltages

generated by the PLL. At the zero crossings of the commutating voltages (AB, BC, CA),

a ramp is reset. A firing pulse is generated whenever the ramp value becomes equal to the

desired delay angle provided by the controller.

Table I shows the simulation results for power transfer of composite ac-dc power

transmission through different lengths of lines at transmission angle of 60o. It can be

observed that there is a substantial up gradation of power transfer capacity of the line by

simultaneous ac - dc power transmission as compared to ac transmission alone. The fact

is illustrated in Fig. 4.

TABLE-I. SIMULATION RESULTS AT 60O TRANSMISSION ANGLE

FOR DIFFERENT LINE LENGTHS

Lenth(KM) 400 500 600 700 800

Active

power(MW)

290 270 260 250 230

Reactive

power(MVAR)

2500 3000 3500 3700 4400

Sendingend

current(KA)

2.1 2.2 1.9 1.7 1.4

Receivingend

current(KA)

1.8 1.6 1.5 1.7 1.6

Sendingend

voltage(KV)

200 250 280 300 320

Receiveingend

voltage(KV)

170 200 230 210 200

CHAPTER 9

SIMULINK

Simulink is a graphical extension to MATLAB for modeling and simulation of

systems. In Simulink, systems are drawn on screen as block diagrams. Many elements of

block diagrams are available, such as transfer functions, summing junctions, etc., as well

as virtual input and output devices such as function generators and oscilloscopes.

Simulink is integrated with MATLAB and data can be easily transferred between the

programs. In these tutorials, we will apply Simulink to the examples from the MATLAB

tutorials to model the systems, build controllers, and simulate the systems. Simulink is

supported on UNIX, Macintosh, and Windows environments; and is included in the

student version of MATLAB for personal computers.

Simulink is started from the MATLAB command prompt by entering the following

command: Simulink

Alternatively, you can hit the New Simulink Model button at the top of the

MATLAB command window as shown below:

When it starts, Simulink brings up two windows. The first is the main Simulink

window, which appears as:

The second window is a blank, untitled, model window. This is the window into

which a new model can be drawn.

Basic Elements

There are two major classes of items in Simulink: blocks and lines. Blocks are

used to generate, modify, combine, output, and display signals. Lines are used to transfer

signals from one block to another.

Blocks: There are several general classes of blocks:

Sources: Used to generate various signals

Sinks: Used to output or display signals

Discrete: Linear, discrete-time system elements (transfer functions, state-space

models, etc.)

Linear: Linear, continuous-time system elements and connections (summing

junctions, gains, etc.)

Nonlinear: Nonlinear operators (arbitrary functions, saturation, delay, etc.)

Connections: Multiplex, Demultiplex, System Macros, etc.

Blocks have zero to several input terminals and zero to several output terminals.

Unused input terminals are indicated by a small open triangle. Unused output terminals

are indicated by a small triangular point. The block shown below has an unused input

terminal on the left and an unused output terminal on the right.

Lines

Lines transmit signals in the direction indicated by the arrow. Lines must always

transmit signals from the output terminal of one block to the input terminal of another

block. On exception to this is a line can tap off of another line, splitting the signal to each

of two destination blocks, as shown below (click the figure to download the model file

called split.mdl).

Lines can never inject a signal into another line; lines must be combined through

the use of a block such as a summing junction.

A signal can be either a scalar signal or a vector signal. For Single-Input, Single-

Output systems, scalar signals are generally used. For Multi-Input, Multi-Output systems,

vector signals are often used, consisting of two or more scalar signals. The lines used to

transmit scalar and vector signals are identical. The type of signal carried by a line is

determined by the blocks on either end of the line.

Simple Example

The simple model (from the model file section) consists of three blocks: Step,

Transfer Fcn, and Scope. The Step is a source block from which a step input signal

originates. This signal is transfered through the line in the direction indicated by the

arrow to the Transfer Function linear block. The Transfer Function modifies its input

signal and outputs a new signal on a line to the Scope. The Scope is a sink block used to

display a signal much like an oscilloscope.

There are many more types of blocks available in Simulink, some of which will

be discussed later. Right now, we will examine just the three we have used in the simple

model.

Modifying Blocks

A block can be modified by double-clicking on it. For example, if you double-click on

the "Transfer Fcn" block in the simple model, you will see the following dialog box.

This dialog box contains fields for the numerator and the denominator of the

block's transfer function. By entering a vector containing the coefficients of the desired

numerator or denominator polynomial, the desired transfer function can be entered. For

example, to change the denominator to s^2+2s+1, enter the following into the

denominator field:

[1 2 1]

and hit the close button, the model window will change to the following,

which reflects the change in the denominator of the transfer function.

The "step" block can also be double-clicked, bringing up the following dialog box.

The default parameters in this dialog box generate a step function occurring at

time=1 sec, from an initial level of zero to a level of 1. (in other words, a unit step at t=1).

Each of these parameters can be changed. Close this dialog before continuing.

The most complicated of these three blocks is the "Scope" block. Double clicking on

this brings up a blank oscilloscope screen.

When a simulation is performed, the signal which feeds into the scope will be displayed

in this window. Detailed operation of the scope will not be covered in this tutorial. The

only function we will use is the autoscale button, which appears as a pair of binoculars in

the upper portion of the window.

Running Simulations

To run a simulation, we will work with the following model file:

Before running a simulation of this system, first open the scope window by

double-clicking on the scope block. Then, to start the simulation, either select Start from

the Simulation menu (as shown below) or hit Ctrl-T in the model window.

The simulation should run very quickly and the scope window will appear as shown

below.

Note that the simulation output (shown in yellow) is at a very low level relative to the

axes of the scope. To fix this, hit the autoscale button (binoculars), which will rescale the

axes as shown below.

Note that the step response does not begin until t=1. This can be changed by

double-clicking on the "step" block. Now, we will change the parameters of the system

and simulate the system again. Double-click on the "Transfer Fcn" block in the model

window and change the denominator to

[1 20 400]

Re-run the simulation (hit Ctrl-T) and you should see what appears as a flat line in

the scope window. Hit the autoscale button, and you should see the following in the

scope window.

Notice that the auto scale button only changes the vertical axis. Since the new

transfer function has a very fast response, it it compressed into a very narrow part of the

scope window. This is not really a problem with the scope, but with the simulation itself.

Simulink simulated the system for a full ten seconds even though the system had reached

steady state shortly after one second. To correct this, you need to change the parameters

of the simulation itself. In the model window, select Parameters from the Simulation

menu. You will see the following dialog box.

There are many simulation parameter options; we will only be concerned with the

start and stop times, which tell Simulink over what time period to perform the simulation.

Change Start time from 0.0 to 0.8 (since the step doesn't occur until t=1.0. Change Stop

time from 10.0 to 2.0, which should be only shortly after the system settles. Close the

dialog box and rerun the simulation. After hitting the autoscale button, the scope window

should provide a much better display of the step response as shown below.

Building Systems

In this section, you will learn how to build systems in Simulink using the building

blocks in Simulink's Block Libraries. You will build the following system.

First you will gather all the necessary blocks from the block libraries. Then you

will modify the blocks so they correspond to the blocks in the desired model. Finally, you

will connect the blocks with lines to form the complete system. After this, you will

simulate the complete system to verify that it works.

Gathering Blocks

Follow the steps below to collect the necessary blocks:

Create a new model (New from the File menu or Ctrl-N). You will get a blank

model window.

Double-click on the Sources icon in the main Simulink window.

This opens the Sources window which contains the Sources Block Library. Sources are

used to generate signals.

Drag the Step block from the sources window into the left side of your model

window.

Double-click on the Linear icon in the main Simulink window to open the Linear

Block Library window.

Drag the Sum, Gain, and two instances of the Transfer Fcn (drag it two times)

into your model window arranged approximately as shown below. The exact alignment

is not important since it can be changed later. Just try to get the correct relative

positions. Notice that the second Transfer Function block has a 1 after its name. Since

no two blocks may have the same name, Simulink automatically appends numbers

following the names of blocks to differentiate between them.

Double-click on the Sinks icon in the main Simulink window to open the Sinks

window.

Drag the Scope block into the right side of your model window.

Modify Blocks

Follow these steps to properly modify the blocks in your model.

Double-click your Sum block. Since you will want the second input to be

subtracted, enter +- into the list of signs field. Close the dialog box.

Double-click your Gain block. Change the gain to 2.5 and close the dialog box.

Double-click the leftmost Transfer Function block. Change the numerator to [1 2]

and the denominator to [1 0]. Close the dialog box.

Double-click the rightmost Transfer Function block. Leave the numerator [1], but

change the denominator to [1 2 4]. Close the dialog box. Your model should appear as:

Change the name of the first Transfer Function block by clicking on the words

"Transfer Fcn". A box and an editing cursor will appear on the block's name as shown

below. Use the keyboard (the mouse is also useful) to delete the existing name and type

in the new name, "PI Controller". Click anywhere outside the name box to finish

editing.

Similarly, change the name of the second Transfer Function block from "Transfer

Fcn1" to "Plant". Now, all the blocks are entered properly. Your model should appear

as:

Connecting Blocks with Lines

Now that the blocks are properly laid out, you will now connect them together.

Follow these steps.

Drag the mouse from the output terminal of the Step block to the upper (positive)

input of the Sum block. Let go of the mouse button only when the mouse is right on the

input terminal. Do not worry about the path you follow while dragging, the line will

route itself. You should see the following.

The resulting line should have a filled arrowhead. If the arrowhead is open, as

shown below, it means it is not connected to anything.

You can continue the partial line you just drew by treating the open arrowhead as an

output terminal and drawing just as before. Alternatively, if you want to redraw the line, or

if the line connected to the wrong terminal, you should delete the line and redraw it. To

delete a line (or any other object), simply click on it to select it, and hit the delete key.

Draw a line connecting the Sum block output to the Gain input. Also draw a line

from the Gain to the PI Controller, a line from the PI Controller to the Plant, and a line

from the Plant to the Scope. You should now have the following.

The line remaining to be drawn is the feedback signal connecting the output of the

Plant to the negative input of the Sum block. This line is different in two ways. First,

since this line loops around and does not simply follow the shortest (right-angled) route

so it needs to be drawn in several stages. Second, there is no output terminal to start

from, so the line has to tap off of an existing line.

To tap off the output line, hold the Ctrl key while dragging the mouse from the point on

the existing line where you want to tap off. In this case, start just to the right of the

Plant. Drag until you get to the lower left corner of the desired feedback signal line as

shown below.

Now, the open arrowhead of this partial line can be treated as an output terminal. Draw

a line from it to the negative terminal of the Sum block in the usual manner.

Now, you will align the blocks with each other for a neater appearance. Once

connected, the actual position of the blocks does not matter, but it is easier to read if

they are aligned. To move each block, drag it with the mouse. The lines will stay

connected and re-route themselves. The middles and corners of lines can also be

dragged to different locations. Starting at the left, drag each blocks so that the lines

connecting them are purely horizontal. Also, adjust the spacing between blocks to leave

room for signal labels. You should have something like:

Finally, you will place labels in your model to identify the signals. To place a

label anywhere in your model, double click at the point you want the label to be. Start

by double clicking above the line leading from the Step block. You will get a blank text

box with an editing cursor as shown below

Type

an r in this box, labeling the reference signal and click outside it to end editing.

Label the error (e) signal, the control (u) signal, and the output (y) signal in the

same manner. Your final model should appear as:

To save your model, select Save As in the File menu and type in any desired

model name. The completed model can be found here.

Simulation

Now that the model is complete, you can simulate the model. Select Start from the

Simulation menu to run the simulation. Double-click on the Scope block to view its

output. Hit the auto scale button (binoculars) and you should see the following.

Simulink Basics Tutorial - Interaction with MATLAB

We will examine three of the ways in which Simulink can interact with MATLAB.

Block parameters can be defined from MATLAB variable.

Signals can be exchanged between Simulink and MATLAB.

Entire systems can be extracted from Simulink into MATLAB.

Taking Variables from MATLAB

In some cases, parameters, such as gain, may be calculated in MATLAB to be used in a

Simulink model. If this is the case, it is not necessary to enter the result of the MATLAB

calculation directly into Simulink. For example, suppose we calculated the gain in

MATLAB in the variable K. Emulate this by entering the following command at the

MATLAB command prompt.

K=2.5

This variable can now be used in the Simulink Gain block. In your Simulink model,

double-click on the Gain block and enter the following in the Gain field.

Close this dialog box. Notice now that the Gain block in the Simulink model shows the

variable K rather than a number.

Now, you can re-run the simulation and view the output on the Scope. The result should

be the same as before.

Now, if any calculations are done in MATLAB to change any of the variab used in the

Simulink model, the simulation will use the new values the next time it is run. To try this,

in MATLAB, change the gain, K, by entering the following at the command prompt.

K=5

Start the Simulink simulation again, bring up the Scope window, and hit the

auto scale button. You will see the following output which reflects the new, higher gain.

Besides variab, signals, and even entire systems can be exchanged between MATLAB

and Simulink.

Simulink is a platform for multinomial simulation and Model-Based

Design for dynamic systems. It provides an interactive graphical environment and a

customizable set of block libraries, and can be extended for specialized applications.

TOOL BOXES of MATLAB

SIGNAL PROCESSING

The Signal Processing Block set extends Simulink with efficient frame-based

processing and blocks for designing, implementing, and verifying signal processing

systems. The block set enables you to model streaming data and multirate systems in

communications, audio/video, digital control, radar/sonar, consumer and medical

electronics, and other numerically intensive application areas.

Embedded Target for Motorola® MPC555

The Embedded Target for Motorola® MPC555 lets you deploy production code

generated from Real-Time Workshop Embedded Coder directly onto MPC5xx

microcontrollers. You can use the Embedded Target for Motorola MPC555 to execute

code in real time on the Motorola MPC5xx for on-target rapid prototyping, production

deployment of embedded applications, or validation and performance analysis.

Real-Time Windows Target

Real-Time Windows Target enables you to run Simulink and State flow models in

real time on your desktop or laptop PC. You can create and control a real-time execution

entirely through Simulink. Using Real-Time Workshop, you generate C code, compile it,

and start real-time execution on Microsoft Windows while interfacing to real hardware

using PC I/O boards. Other Windows applications continue to run during operation and

can use all CPU cycles not needed by the real-time task.

Real-Time Workshop

Real-Time Workshop generates and executes stand-alone C code for developing

and testing algorithms modeled in Simulink. The resulting code can be used for many

real-time and non-real-time applications, including simulation acceleration, rapid

prototyping, and hardware-in-the-loop testing. You can interactively tune and monitor the

generated code using Simulink blocks and built-in analysis capabilities, or run and

interact with the code outside the MATLAB and Simulink environment.

Real-Time Workshop Embedded

Real-Time Workshop Embedded Coder generates C code from Simulink and

State flow models that has the clarity and efficiency of professional handwritten code.

The generated code is exceptionally compact and fast—essential requirements for

embedded systems, on-target rapid prototyping boards, microprocessors used in mass

production, and real-time simulators. You can use Real-Time Workshop Embedded

Coder to specify, deploy, and verify production-quality software.

To let you make a side-by-side comparison between the capabilities and characteristics of

the code generated by Real-Time Workshop and Real-Time Workshop Embedded Coder,

the demos for both products have been placed together on the Real-Time Workshop.

Sim Driveline

SimDriveline extends Simulink with tools for modeling and simulating the

mechanics of driveline (drive train) systems. These tools include components such as

gears, rotating shafts, and clutches; standard transmission templates; and engine and tire

models. SimDriveline is optimized for ease of use and speed of calculation for driveline

mechanics. It is integrated with Math Works control design and code generation products,

enabling you to design controllers and test them in real time with the model of the

mechanical system.

Sim Events

Sim Events extends Simulink with tools for modeling and simulating discrete-

event systems using queues and servers. With Sim Events you can create a discrete-event

simulation model in Simulink to model the passing of entities through a network of

queues, servers, gates, and switches based on events. You can configure entities with

user-defined attributes to model networks in packet-based communications,

manufacturing, logistics, mission planning, supervisory control, service scheduling, and

other applications. Sim Events lets you model systems that are not time-driven but are

based on discrete events, such as the creation or movement of an entity, the opening of a

gate, or the change in value of a signal.

Sim Mechanics

Sim Mechanics extends Simulink with tools for modeling and simulating

mechanical systems. It is integrated with Math Works control design and code generation

products, enabling you to design controllers and test them in real time with the model of

the mechanical system.

Sim Power Systems

SimPowerSystems extends Simulink with tools for modeling and simulating

basic electrical circuits and detailed electrical power systems. These tools let you model

the generation, transmission, distribution, and consumption of electrical power, as well as

its conversion into mechanical power. SimPowerSystems is well suited to the

development of complex, self-contained power systems, such as those in automobiles,

aircraft, manufacturing plants, and power utility applications.

Simulink Accelerator

The Simulink Accelerator increases the simulation speed of your model by

accelerating model execution and using model profiling to help you identify performance

bottlenecks.

Simulink Control Design

Simulink Control Design provides advanced functionality for performing linear

analysis of nonlinear models. You can extract linear approximations of a model to

analyze characteristics such as time and frequency responses and pole-zero dynamics. A

graphical user interface (GUI) and programming capabilities reduce the complexity and

time required to develop the linear zed models.

Simulink Fixed Point

] Simulink Fixed Point enables the intrinsic fixed-point capabilities of the

Simulink product family, letting you design control and signal processing systems that

will be implemented using fixed-point arithmetic.

Simulink Parameter Estimation

Simulink Parameter Estimation is a tool that helps you calibrates the

response of your Simulink model to the outputs of a physical system, eliminating the

need to tune model parameters by trial and error or develop your own optimization

routines. You can use time-domain test data and optimization methods to estimate model

parameters and initial conditions and generate adaptive lookup tables in Simulink.

Simulink Report Generator

The Simulink Report Generator automatically creates documentation from

Simulink and State flow models. You can document software requirements and design

specifications and produce reports from your models, all in a standard format. You can

use the pre built templates or create a template that incorporates your own styles and

standards.

Simulink Response Optimization

Simulink Response Optimization is a tool that helps you tune design parameters

in Simulink models by optimizing time-based signals to meet user-defined constraints. It

optimizes scalar, vector, and matrix-type variables and constrains multiple signals at any

level in the model. Simulink Response Optimization supports continuous, discrete, and

multi rate models and enables you to account for model uncertainty by conducting Monte

Carlo simulations.

Simulink Verification and Validation

Simulink Verification and Validation enables you to develop requirements-

based designs and test cases in Simulink and State flow and measure test coverage. By

linking requirements to your design and test cases and performing coverage analysis at

the model level, you can trace requirements, validate your design, identify inadequate

requirements, and expose unnecessary constructs and design flaws.

State flow

State flow is an interactive design and simulation tool for event-driven systems.

State flow provides the language elements required to describe complex logic in a

natural, readable, and understandable form. It is tightly integrated with MATLAB and

Simulink, providing an efficient environment for designing embedded systems that

contain control, supervisory, and mode logic.

CHAPTER 10

SIMULATION RESULTS

Sending end voltage and receiving end voltage

Sending end current and receiving end current

Active and Reactive Power

Combined AC-DC Currents

CONCLUSION

The feasibility to convert ac transmission line to a composite ac–dc line has been

demonstrated. For the particular system studied, there is substantial increase (about

83.45%) in the load ability of the line. The line is loaded to its thermal limit with the

superimposed dc current. The dc power flow does not impose any stability problem. The

advantage of parallel ac–dc transmission is obtained.

Dc current regulator may modulate ac power flow. There is no need for any

modification in the size of conductors, insulator strings, and towers structure of the

original line. The optimum values of ac and dc voltage components of the converted

composite line are 1/2 and times the ac voltage before conversion, respectively.

BIBLIOGRAPHY

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