1 Power Taping

Possibility of Power Tapping From Composite AC–DC Power Transmission

Lines

ABSTRACT

A recently proposed concept of simultaneous ac–dc power transmission enables the long extra high-voltage ac lines to be loaded close to their thermal limits. The conductors are allowed to carry a certain amount of dc current superimposed on usual ac. This paper presents the feasibility of small power tapping from composite ac–dc power transmission lines which would pass over relatively small communities/rural areas having no access to a major power transmission network. The proposed scheme is digitally simulated with the help of a PSCAD/EMTDC software package. Simulation results clearly indicate that the tapping of a small amount of ac power from the composite ac–dc transmission line has a negligible impact on the normal functioning of the composite ac–dc power transmission system.

This paper presents the power up gradation of existing EHV ac line corridor by converting it into composite ac-dc power transmission line. No alterations of conductors, insulator strings or towers of the original line are needed.

Index Terms—PSCAD simulation, simultaneous ac–dc power transmission, small

power tapping

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CONTENTS

(1) Introduction

(2) Simultaneous ac-dc power transmission

(3) Description of the system model

(4) HVDC

(5) Types of HVDC systems

(6) Comparison of different HVAC-HVDC

(7) 12 Pulse converter

(8) Simulink diagram of power tapping from combining AC-DC

transmission

(9) Simulink

(10) Results

(11) Conclusion

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INTRODUCTION

PRESENTLY, about half of the world’s population, especially those in

developing countries, live without electricity.These days, the supply of electricity is

considered essential to avail normal facilities of daily life. Its availability is fundamental

for economic development and social upliftment. Large power (steam, hydro, nuclear)

stations are usually located far from load centers. The wheeling of this available electric

energy from these remotely located stations to load centers is achieved either with extra

high-voltage (EHV) ac or HVDC transmission lines. These EHV ac /HVDC transmission

lines often pass over relatively small communities/rural areas that do not have access to a

major power transmission network. It is most desirable to find methods for connecting

these communities to the main transmission system to supply cheap and abundant

electrical energy.

However, the HVDC transmission system does suffer a significant disadvantage

compared to EHV ac transmission, in regards to the tapping of power from a transmission

system. Techno-economical reasons prevent the tapping of a small amount of power from

HVDC transmission lines. This is considered a major drawback due to the fact that in

many instances, HVDC transmission lines pass over many rural communities that have

little or no access to electricity. In the past, a number of schemes have been proposed for

small power tapping from HVDC lines. Most of these schemesuse forced commutated or

line commutated inverters to tap off the power from the HVDC system. These schemes

inherently required additional commutation circuitry or local generation which, in turn,

leads to the high cost of installations and operational complications.

Ekstrom and Lamell [3] have proposed a scheme for small power tapping from an

HVDC line based on a current source line-commutated single-phase thyristor bridge,

connected in series with the HVDC line. To start the tap operation, a local dc voltage

source is required in this scheme. The available bulk electric energy can also be wheeled

by simultaneous ac-dc power transmission recently proposed by the authors [2]. In this

scheme, the conductors are allowed to carry superimposed dc current along with ac

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current. The feasibility of simultaneous ac-dc power transmission has been proved by

laboratory modeled experimental verification as well as digital simulation. Substantial

power upgrading has already been demonstrated by converting the EHV ac line into a

composite ac-dc transmission line without any alteration. The transmission angle can be

increased up to 80 in a composite ac-dc line without losing transient stability, which is

impossible in a pure EHV ac line.

From this composite ac–dc line, small power tapping is also possible despite the

presence of a dc component in it. This paper proposes a simple scheme of small power

tapping from the composite ac–dc power transmission line along its route. In this study,

the tapping stations are assumed to draw power up to 10% of the total power transfer

capability of the composite line. However, more power tapping is also possible subject to

the condition that it is always less than the ac power component.

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.

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

SYSTEM UNDER STUDY:The network depicted in Fig. has been taken up for the feasibility of a small

power tap for remote communities from the composite ac–dc power transmission system. The details of power tap substations are shown in Fig. The modeling details of the network components are described in . A synchronous machine is delivering power to an infinite bus via a double-circuit three-phase, 400-kV, 50-Hz, 450-km ac transmission line. The minimum value of ac phase voltage and maximum value of dc voltage with respect to ground of the converted composite ac–dc line, respectively, are 1/2 and

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times that of per phase voltage before conversion of the conventional pure EHV ac line . The line considered is converted to a composite ac–dc transmission line with an ac rated voltage of 220 kV and a dc voltage of 320 kV. In a composite ac–dc transmission line, the dc component is obtained by converting a part of the ac through a line-commutated 12-pulse rectifier bridge similar to that used in a conventional HVDC.

The dc current thus obtained is injected into the neutral point of the zig-zag-connected secondary windings of sending end transformer. The injected current is distributed equally among the three windings of the transformer. The same is reconverted to ac by the conventional line commutated inverter at the receiving end. The inverter bridge is connected to the neutral of zig-zag-connected winding of the receiving end transformer. The transmission line is connected between the terminals of the zig-zag windings at both ends. The double-circuit transmission line carries both three-phase ac as well as dc power after conversion to a composite ac–dc line. The zig-zag connection of secondary windings of the transformer is used at both ends to avoid saturation of the core due to the flow of the dc component of current. The replacement of a Y-connected transformer from a conventional EHV ac line with a zig-zag transformer in composite ac–dc power transmission is accomplished along with the reduction of ac voltage in such a way that the insulation-level requirements remain unaltered. However, the neutral point of this transformer needs insulation to withstand the dc voltage. Moreover, the zig-zag transformer transfers only 25% of the total power by transformer action.

Fig. 2(a) shows the line-to-ground voltage (i.e., phase voltage) characteristic of the composite ac–dc line which is offset from zero as it possesses a dc voltage component superimposed on a sinusoidally varying ac voltage component. Since the line-to-line voltage does not carry any dc componentit has a pure ac waveshape as demonstrated in Fig. 2(b). The composite-line conductor current characteristic illustrated in Fig. 2(c) indicates the presence of a dc component injected along with an ac component whereas Fig. 2(d) shows the current characteristic at the input of the tap substation. The voltage and current waveforms at the tap substations as depicted in Fig. 2(b), (d), and (e), indicate neither the presence of the dc component nor distortion in waveforms though these are derived from the composite line. To tap ac power from the line, the transformer can be directly connected to the conductors of the line without breaking them.

In this study of a composite ac-dc transmission line, the ac-line voltage component has

been selected as 220 kV. Each tapping station transformer (rated as 120 MVA, 220/66

kV, is connected to the local ac load via a circuit breaker (CB) as depicted in

Fig. 1(b). These CBs are provided for local protection, to clear the fault within the local

ac network. The nature of the local load considered here is that of a summer time

residential class with the following characteristics.

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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

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

each rectifier and inverter may be expressed as

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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

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

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

Each conductor is to be insulated for , but the line-toline 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 is the transfer reactance per phase of the double 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 loadability 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

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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.

SMALL POWER TAPPING STATION REQUIREMENTS:The main requirements of a small power tapping stations are as follows.

• The per unit cost of the tap must be strongly constrained (i.e., the fixed cost must be kept as low as possible).

• The tap must have a negligible impact on the reliability of the ac–dc system. This implies that any fault in the tap must not be able to shutdown the whole system.

• The tap controls should not interfere with the main system (i.e., the tap control system has to be strictly local). Failure to achieve this leads to a complex control system requirement and, thus, higher cost of hardware.

• Small tap stations having a total rating less than 10% of the main terminal rating have potential applications where small, remote communities or industries require economic electric power.

The tapping stations considered in this study are of fairly small power rating, up to 10% of the total transfer capacity of the composite ac-dc power transmission line. Short interruption of the power supplies should be tolerable at the occurrence oftemporary earth faults on the main simultaneous ac–dc power transmission system. Further, any fault occurring within tapping station and its local ac network is to be cleared by local CBs. These tapping stations will not depend upon the telecommunicationlinks with the main composite ac–dc transmission system

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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,

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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.

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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

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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..

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.

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 (1-2kHz) utilizing

pulse width modulation (PWM).

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Configurations of HVDC

There are different types of HVDC systems which are

Mono-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.

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.

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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.

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

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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.

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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.

Voltage 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.

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.)

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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

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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.

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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, 23th, 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.

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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.

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

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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.

BASIC CONTROL PRINCIPLES

Control System Model

The control model mainly consists of measurements and generation of firing

signals for both the rectifier and inverter. The PLO is used to build the firing signals. The

output signal of the PLO is a ramp, synchronized to the phase-A commutating bus line-

to-ground voltage, which is used to generate the firing signal for Valve 1. The ramps for

other valves are generated by adding 60 to the Valve 1 ramp. As a result, an equidistant

pulse is realized. The actual firing time is calculated by comparing the order to the value

of the ramp and using interpolation technique. At the same time, if the valve is pulsed

but its voltage is still less than the forward voltage drop, this model has a logic to delay

firing until the voltage is exactly equal to the forward voltage drop. The firing pulse is

maintained across each valve for 120 .

The and measurement circuits use zero-crossing information from commutating bus

voltages and valve switching times and then convert this time difference to an angle

(using measured PLO frequency). Firing angle (in seconds) is the time when valve turns

on minus the zero crossing time for valve.

Extinction angle (in seconds) for valve is the time at which the commutation bus voltage

for valve crosses zero (negative to positive) minus the time valve turns off. The control

schemes for both rectifier and inverter of the CIGRÉ HVDC system are available in the

example file in PSCAD/EMTDC Version 4.0.1.

24

Following are the controllers used in the control schemes:

• Extinction Angle Controller;

• dc Current Controller;

• Voltage Dependent Current Limiter (VDCOL).

1) Rectifier Control: The rectifier control system uses Constant Current Control

(CCC) technique. The reference for current limit is obtained from the inverter side.

This is done to ensure the protection of the converter in situations when inverter side does

not have sufficient dc voltage support (due to a fault) or does not have sufficient load

requirement (load rejection). The reference current used in rectifier control depends on

the dc voltage available at the inverter side. Dc current on the rectifier side is measured

using proper transducers and passed through necessary filters before they are compared to

produce the error signal. The error signal is then passed through a PI controller, which

produces the necessary firing angle order. The firing circuit uses this information to

generate the equidistant pulses for the valves using the technique described earlier.

2) Inverter Control: The Extinction Angle Control or control and current control

have been implemented on the inverter side. The CCC with Voltage Dependent Current

Order Limiter (VDCOL) have been used here through PI controllers. The reference limit

for the current control is obtained through a comparison of the external reference

(selected by the operator or load requirement) and VDCOL (implemented through lookup

table) output. The measured current is then subtracted from the reference limit to produce

an error signal that is sent to the PI controller to produce the required angle order.

The control uses another PI controller to produce gamma angle order for the

inverter. The two angle orders are compared, and the minimum of the two is used to

calculate the firing instant.

Control System Model

The control blocks available in SIMULINK have been used to emulate the control

algorithm described above Section, and enough care has been taken. Some control

parameters required conversion to their proper values due to differences in units. The

rectifier side uses current control with a reference obtained from the inverter VDCOL

output (implemented through a lookup table), and the inverter control has both current

control and control operating in parallel, and the lower output of the two is used to

25

generate the firing pulses. The angle is not provided directly from the converter valve

data. It needed to be implemented through measurements taken from valve data. The

control block diagrams are shown in Fig.

26

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

2In 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, normally

17°-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

27

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

28

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

29

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

30

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.

31

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.

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 windfarm were considered.

The choices 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 .

32

Table:Configurations for the study of combined transmission systems.

Windfarm rated at 500 MW

33

Configurations for the study of combined transmission systems. Windfarm 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.

34

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.

35

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

transmissionm solutions from a 1000 MW windfarm with multiple connection

points to shore.

36

Simulink circuit

37

Zig zag transformer

Controlling circuit

38

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

39

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

40

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

41

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.

42

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.

43

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:

44

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.

45

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

46

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:

47

[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.

48

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.

49

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.

50

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.

51

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

52

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.

53

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.

54

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.

55

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.

56

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

57

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.

58

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.

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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 positions 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 block so that the lines

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

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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

Typ

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

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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 autoscale 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.

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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.

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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 autoscale

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

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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 Blockset extends Simulink with efficient

frame-based processing and blocks for designing, implementing, and verifying signal

processing systems. The blockset 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.

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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 Stateflow 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.

SimDriveline

SimDriveline extends Simulink with tools for modeling and

simulating the mechanics of driveline (drivetrain) systems. These tools include

components such as gears, rotating shafts, and clutches; standard transmission

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templates; and engine and tire models. SimDriveline is optimized for ease of use and

speed of calculation for driveline mechanics. It is integrated with MathWorks control

design and code generation products, enabling you to design controllers and test them

in real time with the model of the mechanical system.

SimEvents

SimEvents extends Simulink with tools for modeling

and simulating discrete-event systems using queues and servers. With SimEvents 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. SimEvents 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.

SimMechanics

SimMechanics extends Simulink with tools for modeling and

simulating mechanical systems. It is integrated with MathWorks 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

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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 linearized 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

calibrate 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

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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 multirate 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

Stateflow is an interactive design and simulation tool

for event-driven systems. Stateflow 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.

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RESULTS:

70

71

72

73

74

75

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CONCLUSION:

The feasibility of tapping a small amount of power to feed remotely located

communities in the same simple way as tapping in the case of an EHV ac line is

demonstrated for the composite ac–dc transmission system. It is also economical

compared to complicated methods of tapping from the HVDC line. The results clearly

demonstrate that the tapping of a small amount of ac component of power from the

composite ac–dc transmission line has a negligible impact on the dc power transfer.

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REFERENCES:

[1] L. Chetty, N. M. Ijumba, and A. C. Britten, “Parallel-cascaded tapping station,” in Proc. IEEE Int. Conf. Power System Technology, 2004, pp. 1674–1878.

[2] H. Rahman and B. H. Khan, “Power upgrading of transmission line by combining ac-dc transmission,” IEEE Trans. Power Syst., vol. 22, no.1, pp. 459–466, Feb. 2007.

[3] A. Ekstrom and P. Lamell, “HVDC tapping station: Power tapping from a dc transmission line to a local ac network,” in Proc. AC-DC Conf., London, U.K., 1991, pp. 126–131.

[4] Task force on SmallHVDCTaps,Working Group, “Integration of small taps into (existing) HVDC links,” IEEE Trans. Power Del., vol. 10, no. 3, pp. 1699–1706, Jul. 1995.

[5] M. R. Aghaebrahimi and R. W. Menzies, “Small power tapping from HVDC transmission system: A novel approach,” IEEE Trans. Power Del., vol. 12, no. 4, pp. 1698–1703, Oct. 1997.

[6] PSCAD/EMTDC, User’s Guide Manitoba-HVDC Research Centre. Winnipeg, MB, Canada, Jan. 2003.

[7] P. S. Kundur, Power System Stability and Control. New York: Mc-Graw-Hill, 1994.

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