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PARUL INSTITUTE OF ENGINEERING & TECHNOLOGY

Department of Electrical Engineering Advanced Power System-I

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INDEX

Sr.

No.Aim of the Experiment

1 MATLAB Simulation Based Automatic Generation Control (AGC).

2 MATLAB Simulation Based Automatic Voltage Regulator (AVR).

3 The HVDC Projects presently in service in INDIA.

4Study of reactive power control using shunt compensation and series

compensation.

5 Study of Thyristor Controlled Rectifier Using MATLAB Simulink.

6 Study of Thyristor Switched Capacitor Using MATLAB Simulink.

7 Study of HVDC Transmission.

8 Design of SIX Pulse Converter Using MATLAB Simulink.

9 Design of Twelve Pulse Converter Using MATLAB Simulink.

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PRACTICAL NO. 1

Aim: MATLAB Simulation Based Automatic Generation Control (AGC)

Automatic Generation Control (AGC):

The megawatt (MW) output of a generator is regulated by controlling the driving torque, Tm,provided by a prime-mover turbine. In a conventional electromechanical system, it could be a

steam or a hydraulic turbine. The needed change in the turbine-output torque is achieved by

controlling the steam/ water input into the turbine. Therefore, in situations where the output

exceeds or falls below the input, a speed-governing system senses the deviation in the generatorspeed because of the load-generation mismatch, adjusts the mechanical driving torque to restore

the power balance, and returns the operating speed to its rated value. The speed-governor output

is invariably taken through several stages of mechanical amplification for controlling the inlet(steam/ water) valve/ gate of the driving turbine. Figure 1.1 shows the basic speed-governingsystem of a generator supplying an isolated load. The operation of this basic feedback-control

system is enhanced by adding further control inputs to help control the frequency of a large

interconnection. In that role, the control system becomes an automatic generation control (AGC)with supplementary signals.

Fig:1.1 speed-governor system.

To avoid competing control actions, in a multi generator unit station each speed-governor system

is provided with droop (R) characteristics through a proportional feedback loop (RFigure shows an AGC on the principal generating unit with supplementary control. In contrast,

the second, third, and remaining generating units in a multiunit station operate with their basic

AGCs. In a complex interconnected system, the supplementary control signal may be determined

by a load-dispatch center.

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Fig: 1.2 AGC with supplementary control on the principal generating unit.

MATLAB Simulink Circuit Diagram and Output Waveforms

Fig: 1.3 MATLAB Circuit diagram for AGC

Fig: 1.4 MATLAB output waveform for AGC

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Fig: 1.5 MATLAB Circuit diagram for AGC with feed back input

Fig: 1.6 MATLAB output waveform for AGC with feed back input

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PRACTICAL NO. 2

Aim: MATLAB Simulation Based Automatic Voltage Regulator (AVR)

Automatic Voltage Regulator

Main componentexciterdelivers dc power to generator field. Basic role provide constancy of generator terminal voltage during normal small and slow

changes in load.

Common practice to have exciter with enough margins to give powerful boost in excitationlevel during emergency situations as well.

Modern exciterseither brushless or static designExciter Types Brushless designconsists of inverted three phase synchronous generator. The latter has its armature on the rotor and field on stator. AC armature voltage rectified in diodes mounted on rotating shaft and then fed directly into

main generator field, Design eliminates need for slip rings and brushes.

Static designexcitation power is obtained directly from generator terminals or from stationservice bus.

AC power is rectified in thyristor bridges and fed into main generator field via slip rings. Static excitersvery fast, contribute to improved transient stability.

Fig: 2.1 Brushless AVR loop

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Exciter Modeling

Suppose |V| decreasesresult in an increase in the error voltage e Results in increase in vR, ie, vf and if Which in turn increases the d-axis flux, the internal generator emf E and terminal voltage V Mathematical modeling:

|V|ref- |V| = e (1)

And vR= KA e (2)

Where KA is the amplifier gain

Taking Laplace Transform of these two equations|V|ref(s) - |V|(s) = e(s)

And GA = vR(s)/e(s) = KA

Last equation implies instantaneous amplifier responsein reality amplifier will have a delayrepresented by a time constant TA

GA = vR(s)/e(s) = KA/ (1 + sTA) (3)

If Re and Le represent the resistance and inductance of the exciter fieldvR= Re ie + Led (ie)/dt (4)

Taking Laplace TransformvR(s) = Re ie(s) + Les ie(s) (5)

Measured across the main field, the exciter produces K1armature per ampere of field currenti.e.,

vf= K1 ie (6)

Taking Laplace Transform of the two equations and eliminating ie, we obtain the transferfunction of the exciter

Ge = vf(s)/v R(s) = Ke/ (1 + sTe) (7)

Where Ke=K1/Re and Te=Le/Re

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Generator Modeling

Need to close the loop by establishing the missing dynamics between field voltage vfandthe generators terminal voltage |V|

Relationship between vf and |V| will depend on generator loading, with simplest possiblerelationship existing at low or zero load when V = E

Applying KVL to field windingvf= Rfif+ Lffd(if)/dt (8)

The field flux linking the armaturefa = facost =- fasin(t-/2)

Therefore the induced emf E=-dfa/dtor E = facos(t-/2)

The RMS value bein|E| = fa/2 = Lfaif/2

Therefore if = 2|E|/Lfa

and if= 2|E|/Lfa (9)

Substituting the value of ifin equation (8)vf= 2Rf|E|/Lfa + (2/Lfa) (Lffd(E)/dt) (10)

Taking Laplace Transform|E|(s)/vf(s)|V|(s)/vf(s) = KF/ (1 + sTd0) (11)

Where KF = Lfa/2Rfand Tdo = Lff/Rf is the O. C. d-axis time constant

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Fig: 2.2Block Diagram of AVR Loop

The open-loop transfer function G(s) equalsG(s) = K/ (1 + sTA) (1 + sTe) (1 + sTd0)

And the open loop gain K is defined by

K = KA Ke KF

Typical values of the time constants are TA = 0.02 to 0.1 sec, Te = 0.5 to 1.0 sec and Td0 =5 to 10 sec

Static Performance of the AVR loop

The AVR loop must1. Regulate the terminal voltage |V| to within required static accuracy limit

2. Have sufficient speed of response

3. be stable Thus, the static accuracy requirement is:

For a constant reference input |V|ref0, error e0must be less than some specified percentagep of the reference. i.e. e0 = |V|ref0 - |V|0 < (p/100). |V|ref0

For a constant input, transfer function is obtained by setting s = 0 in|V| = (G(s)/ (1 + G(s)) |V|ref, giving

e0 = |V|ref0 - |V|0

e = |V|ref0 - (G(0)/ (1 + G(0))|V|ref0

= (1/ (1 + G(0)) |V|ref0

= (1/ (1 + K)) |V|ref0

=>1/ (1 + K) < p/100 or K > 100/p-1

e.g. for static error p to be less than 1%, K must exceed 99.

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Dynamic Response of the AVR loop

The time response of the loop is given by|V|(t) = L-1{|V|ref(s) G(s)/ [1 + G(s)]}

Mathematically, the response depends on the eigenvalues or closed-loop poles which can beobtained from the characteristic equation

1 + G(s) = 0

The open-loop transfer function given by

G(s) = K/ (1 + sTA) (1 + sTe) (1 + sTd0) is of third order

Therefore will have 3 eigenvalues s1, s2, and s3. If s1, s2, s3are distinct and real, transient response components are of the form A1es1t,

A2es2t, A3es3t.

If two eigenvalues e.g. s2, s3are complex conjugate, j, transient component will haveoscillatory term of the form A1et sin (t+).

For AVR loop to be stable, transient components must vanish with time requires all threeeigenvalues to be located in left hand s-plane.

For good tracking ability, transients must vanish fast. Real part of eigenvalues determines the rapidity of exponential decayhigh speed loop must

posses eigenvalues located well to the left in s-plane.

Amplitude factors A1, A2, A3 express relative size of transient terms.

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MATLAB Simulink Circuit Diagram and Output Waveforms

Fig: 2.3 MATLAB Simulink Circuit Diagram and Output Waveforms with KA=10

Fig: 2.4 MATLAB Simulink Circuit Diagram and Output Waveforms with KA=12.16

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Fig: 2.5 MATLAB Simulink Circuit Diagram and Output Waveforms with KA=0

Fig: 2.6 MATLAB Simulink Circuit Diagram and Output Waveforms with KA=10 and

Feed Back through Stabilizer

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PRACTICAL NO. - 3

Aim: The HVDC Projects presently in service in INDIA

High Voltage Direct Current transmission HVDC

One of the most exciting new technical developments in electric power system in the last

three decades has been High Voltage Direct Current transmission. From the first of HVDClinks to the recent, the voltage has increased from 100 KV to 800 KV, the rated power from 20

MW to 6300 MW and the distance from 96 km to 1370 km.

Preceding and accompanying this rapid growth of Direct Current Transmission were

developments in High Voltage, High power valves, in control and protection system, in DC

cables and in insulation for overhead DC lines.

In India three HVDC projects are in operation.

i. The Rihand-Delhi HVDC transmission project having 1500 MW capacity and 500 KVDC voltages is the first commercial long distance DC transmission project in India.

ii. Vindhyachal 2x250 MW Back to back DC converter station which asynchronouslyconnected the Northern and Western regions for exchange of power.

iii. The Nation HVDC experimental line project, which links Lower Sileru in A.P. to Barsoorin M.P. Phase 1 of this project is capable of transmitting 100 MW at 100 KV DC.

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PRACTICAL NO.- 4

Aim: Study of reactive power control using shunt compensation and series compensation.

Shunt Compensation:

Passive reactive-power compensators include series capacitors and shunt-connected inductorsand capacitors. Shunt devices may be connected permanently or through a switch. Shunt reactors

compensate for the line capacitance, and because they control over voltages at no loads and light

loads, they are often connected permanently to the line, not to the bus. Figure 4.1 shows the

arrangements of shunt reactors on a long-distance, high-voltage ac line. Many power utilitiesconnect shunt reactors via breakers, thereby acquiring the flexibility to turn them off under

heavier load conditions. Shunt reactors are generally gapped-core reactors and, sometimes, air-

cored. Shunt capacitors are used to increase the power-transfer capacity and to compensate forthe reactive-voltage drop in the line. The application of shunt capacitors requires careful systemdesign. The circuit breakers connecting shunt capacitors should withstand high-charging in-rush

currents and also, upon disconnection, should withstand more than 2-pu voltages, because the

capacitors are then left charged for a significant period until they are discharged through a largetime-constant discharge circuit. Also, the addition of shunt capacitors creates higher-frequencyresonant circuits and can therefore lead to harmonic over voltages on some system buses.

Fig: 4.1 Basic diagram of Shunt Compensation

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Series Compensation:

Series capacitors are used to partially offset the effects of the series inductances of lines. Seriescompensation results in the improvement of the maximum power-transmission capacity of theline. The net effect is a lower load angle for a given power-transmission level and, therefore, a

higher-stability margin. The reactive-power absorption of a line depends on the transmission

current, so when series capacitors are employed, automatically the resulting reactive-powercompensation is adjusted proportionately. Also, because the series compensation effectively

reduces the overall line reactance, it is expected that the net line-voltage drop would become less

susceptible to the loading conditions.

In an interconnected network of power lines that provides several parallel paths, for power flowbetween two locations, it is the series compensation of a selected line that makes it the principal

power carrier. Series compensation is defined by the degree of compensation; for example, a 1-

pu compensation means that the effective series reactance of a line will be zero. A practicalupper limit of series compensation, on the other hand, may be as high as 0.75 pu. One impact ofthe passive compensation of lines is that whereas the shunt-inductive compensation makes the

line electrically resonant at a supersynchronous frequency, the series compensation makes the

line resonant at a subsynchronous frequency. The subsynchronous resonance (SSR) can lead toproblematic situations for steam turbinedriven generators connected to a series-compensatedtransmission line. These generators employ multiple turbines connected on a common shaft with

the generator. This arrangement constitutes an elastically coupled multimass mechanical systemthat exhibits several modes of low-frequency torsional resonances, none of which should be

excited as a result of the subsynchronous-resonant electrical transmission system. The

application of series compensation requires several other careful considerations. The application

of series capacitors in a long line constitutes placing lumped impedance at a point. Therefore, thefollowing factors need careful evaluation:

1. The voltage magnitude across the capacitor banks (insulation);

2. The fault currents at the terminals of a capacitor bank;3. The placement of shunt reactors in relation to the series capacitors (resonant over voltages);

4. The number of capacitor banks and their location on a long line (voltage profile).

Effect on Power-Transfer CapacityA simple system analysis can be performed to develop a basic understanding of the effect

of shunt and series compensation on power-transmission capacity. Consider a short, symmetrical

electrical line as shown in Fig. 2(a). For an uncompensated line, and assuming Vs = Vr =V, the

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power equation becomes

Fig: 4.2 The series compensation of a short, symmetrical transmission line.

From the voltage-phasor equations and the phasor diagram in Fig. 2(a),

Series Compensation:

If the effective reactance of a line is controlled by inserting a series capacitor, and if the line

terminals voltages are held unchanged, then a X lchange in the line reactance will result in a Ilchange in the current, where

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Therefore, from Eq. (1), the corresponding change in the power transfer will be

Using Eqs. (2) and (3), Eq. (4) may be written as

As Xlis the reactance added by series capacitors, XlI2

l= Qse represents the incremental varrating of the series capacitor. Therefore

Shunt Compensation:

Reconsider the short, symmetrical line described in Fig. 4.2 (a). Apply a shunt capacitor at the

midpoint of the line so that a shunt susceptance is incrementally added (Bc), as shown in Fig.4.3. For the system in this figure, the power transfer in terms of the midpoint voltage on the line

is

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The differential change in power, P, as a result of a differential change, Vm, is given as

Fig: 4.3 The midpoint-capacitor compensation of a short, symmetrical line.

Also as shown in Fig. 4.3

The current Ic in the midline shunt capacitor modifies the line currents in the sending andreceiving ends of the line to the following:

Substituting the results of Eq. (8) in Eq. (7), we get

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If the midpoint voltage of the line is approximately equal to V cos / 2, then the incrementalrating of the shunt-capacitor compensation will be Qsh = V2m Bc,

By comparing Eqs., we deduce that for an equivalent power transfer on a short electrical line,

Assuming an operating load angle = 30, we get the ratio of the ratings of series (Qse) toshunt (Qsh) compensators to be 0.072, or 7.2%.

From the foregoing discussion, it is clear that the var net rating of the series compensator

is only 7.2% of that required of a shunt compensator for the same change in power transfer.

Therefore, one concludes that the series-capacitive compensation is not only achieved with a

smaller MVAR rating, but also that it is automatically adjusted for the entire range of the lineloading. However, the cost of the compensator is not directly related only to the MVAR-rating

series capacitor costs increase because they carry full line current and also both their ends must

be insulated for the line voltage.

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PRACTICAL NO. - 5

Aim: Study of Thyristor Controlled Rectifier Using MATLAB Simulink

A TCR is one of the most important building blocks of thyristor-based SVCs. Although it can be

used alone, it is more often employed in conjunction with fixed or thyristor-switched capacitors

to provide rapid, continuous control of reactive power over the entire selected lagging-to-leadingrange.

The Single-Phase TCR

A basic single-phase TCR comprises an anti-parallelconnected pair of thyristor valves, T1 andT2, in series with a linear air-core reactor, as illustrated in Fig. The anti-parallelconnected

thyristor pair acts like a bidirectional switch, with thyristor valve T1 conducting in positive half-cycles and thyristor valve T2 conducting in negative half-cycles of the supply voltage. The firingangle of the thyristors is measured from the zero crossing of the voltage appearing across its

terminals.

The controllable range of the TCR firing angle, a, extends from 900

to 1800. A firing angle of 90

0

results in full thyristor conduction with a continuous sinusoidal current flow in the TCR. As the

firing angle is varied from 900

to close to 1800, the current flows in the form of discontinuous

pulses symmetrically located in the positive and negative half-cycles, as displayed in Fig. Once

the thyristor valves are fired, the cessation of current occurs at its natural zero crossing, a process

known as the line commutation. The current reduces to zero for a firing angle of 1800. Thyristor

firing at angles below 90

0

introduces dc components in the current, disturbing the symmetricaloperation of the two antiparallel valve branches. A characteristic of the line-commutation

process with which the TCR operates is that once the valve conduction has commenced, any

change in the firing angle can only be implemented in the next half-cycle, leading to the so-called thyristor dead time. Let the source voltage be expressed as

Fig: 5.1 A TCR

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Fig: 5.2 Current and voltages for different a in a TCR.

The 3-Phase TCR

A 3-phase, 6-pulse TCR comprises three single-phase TCRs connected in delta, as shown in Fig.5.3. The inductor in each phase is split into two halves, as shown in Fig. 5.4, one on each side of

the anti-parallelconnected thyristor pair, to prevent the full ac voltage appearing across thethyristor valves and damaging them if a short-circuit fault occurs across the reactors two endterminals. The phase- and line-current waveforms are also displayed in Fig. 5.3.

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If the 3-phase supply voltages are balanced, if the three reactor units are identical, and also if allthe thyristors are fired symmetricallywith equal firing angles in each phasethen the

symmetric current pulses result in both positive and negative half-cycles and the generating ofonly odd harmonics. The percentage values of harmonic currents with respect to fundamentalboth in the phases and in the linesare the same.

The delta connection of the three single-phase TCRs prevents the triplen (i.e., multiples of third)harmonics from percolating into the transmission lines. The cancellation of its 3rd and multiple

harmonics can be explained as follows: Let iABn, iBCn, and iCAn be the nth-order harmonic-

phase currents in the respective delta branches, and let iAn, iBn, and iCnbe the currents in the

respective lines connected to the delta-configured TCR. Then, the 3rd harmonic currents areexpressed as

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Fig: 5.3 delta-connected TCR and its phase and line currents for different .

The TCRs do not possess high overload capability because the air-core designs of their reactors.

If the TCRs are expected to transiently withstand high over voltages, a short-term overloadcapacity must be built into the TCR by design, or additional thyristor-switched overload reactorsmay need to be installed.The TCR responds rapidly, typically in duration of one-and-a-half to three cycles. The actual

response time is a function of measurement delays, TCR controller parameters, and systemstrength..

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Fig: 5.4 LC or LCR configurations

These filters are tuned to the dominant 5th and 7th

harmonic frequencies. Sometimes, specific

filters for 11th and 13th harmonics or a simple high-pass filter are also installed. If an individual

phase control of the TCR is envisaged, or if the network resonance conditions so necessitate a3rd harmonic filter needs to be installed in parallel with the TCR.

The schematic diagram of a 6-pulse TCR with filters is depicted in Fig. As it is desirable inpower-system applications to have controllable capacitive reactive power, a capacitor is

connected in shunt with the TCR. This capacitor may be fixed, or it may be switchable by meansof mechanical or thyristor switches. The main advantages of the TCR are flexibility of control

and ease in uprating. Different control strategies can be easily implemented, especially those

involving external supplementary signals to achieve significant improvements in systemperformance. The voltage reference and current slope can be controlled in a simple manner.

Modular in nature, a TCR SVC can have its rating extended by the addition of more TCR banks,

as long as the coupling transformer rating is not exceeded.

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Fig: 5.5 MATLAB TCR circuit

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Fig: 5.6 MATLAB Simulink Current & Voltage Graph

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PRACTICAL NO.- 6

Aim: Study of Thyristor Switched Capacitor Using MATLAB Simulink

THE THYRISTOR-SWITCHED CAPACITOR (TSC)

Before describing the configuration and operating characteristics of a TSC, certain concepts areexplained.

Switching a Capacitor to a Voltage Source

The circuit shown in Fig. 6.1 consists of a capacitor in series with a bidirectional thyristor

switch. It is supplied from an ideal ac voltage source with neither resistance nor reactance

present in the circuit. The analysis of the current transients after closing the switch brings forthtwo cases:

1. The capacitor voltage is not equal to the supply voltage when the thyristors are fired.

Immediately after closing the switch, a current of infinite magnitude flows and charges thecapacitor to the supply voltage in an infinitely short time. The switch realized by thyristors

cannot withstand this stress and would fail.

2. The capacitor voltage is equalto the supply voltage when the thyristors are fired. The analysis

shows that the current will jump immediately to the value of the steady-state current. The steady

state condition is reached in an infinitely short time. Although the magnitude of the current does

not exceed the steady-state values, the thyristors have an upper limit ofdi/ dtvalues that they canwithstand during the firing process. Here, di/ dtis infinite, and the thyristor switch will again fail.

It can therefore be concluded that this simple circuit of a TSC branch is not suitable.

For LC circuits tuned to resonance frequencies of three times the supply frequency and higher,

the magnification factor is close to 1.0; for tuning below 3q0, the magnification factor increases

very rapidly. For practical schemes, therefore, n should be chosen higher than 3 (typically,

between the 4th and 5th harmonic).

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Fig: 6.1 TSC Circuit and Waveforms

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Fig: 6.2 MATLAB TSC circuit

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Fig: 6.3 MATLAB Simulink Current & Voltage Graph

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PRACTICAL NO.- 7

Aim: Study of HVDC Transmission

Electric power transmission was originally developed with direct current. The availability of

transformers and the development and improvement of induction motors at the beginning of the

20th Century, led to greater appeal and use of a.c. transmission d.c. transmission now becamepractical when long distances were to be covered or where cables were required.

WHY USE DC TRANSMISSION?

The question is often asked, Why use d.c. transmission? One response is that losses are lower,

but this is not correct. The level of losses is designed into a transmission system and is regulatedby the size of conductor selected. d.c. and a.c. conductors, either as overhead transmission lines

or submarine cables can have lower losses but at higher expense since the larger cross-sectional

area will generally result in lower losses but cost more.When converters are used for d.c. transmission in preference to a.c. transmission, it is generally

by economic choice driven by one of the following reasons:

1. An overhead d.c. transmission line with its towers can be designed to be less costly per unit of

length than an equivalent a.c. line designed to transmit the same level of electric power. However

the d.c. converter stations at each end are more costly than the terminating stations of an a.c. line

and so there is a breakeven distance above which the total cost of d.c. transmission is less than itsa.c. transmission alternative. The d.c. transmission line can have a lower visual profile than an

equivalent a.c. line and so contributes to a lower environmental impact. There are other

environmental advantages to a d.c. transmission line through the electric and magnetic fields

being d.c. instead of ac.

2. If transmission is by submarine or underground cable, the breakeven distance is much less

than overhead transmission. It is not practical to consider a.c. cable systems exceeding 50 km butd.c. cable transmission systems are in service whose length is in the hundreds of kilometers and

even distances of 600 km or greater have been considered feasible.

3. Some a.c. electric power systems are not synchronized to neighboring networks even thoughtheir physical distances between them is quite small. This occurs in Japan where half the country

is a 60 hz network and the other is a 50 Hz system. It is physically impossible to connect the two

together by direct a.c. methods in order to exchange electric power between them. However, if ad.c. converter station is located in each system with an interconnecting d.c. link between them, it

is possible to transfer the required power flow even though the a.c. systems so connected remain

asynchronous.

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CONFIGURATIONS:

The integral part of an HVDC power converter is the valve or valve arm. It may be noncontrollable if constructed from one or more power diodes in series or controllable if constructedfrom one or more thyristors in series. Figure 7.1 depicts the International Electro technical

Commission (IEC) graphical symbols for valves and bridges (1). The standard bridge or

converter connection is defined as a double-way connection comprising six valves or valve armswhich are connected as illustrated in Figure 2. Electric power flowing between the HVDC valve

group and the a.c. system is three phase. When electric power flows into the d.c. valve group

from the a.c. system then it is considered a rectifier. If power flows from the d.c. valve group

into the a.c. system, it is an inverter. Each valve consists of many series connected thyristors inthyristor modules. Figure 2 represents the electric circuit network depiction for the six pulse

valve group configuration. The six pulse valve group was usual when the valves were mercury

arc.

Fig: 7.1 SIX Pulse HVDC Configuration

Advantages of HVDC Systems:

The classical application of HVDC systems is the transmission of bulk power over long distancesbecause the overall cost for the transmission system is less and the losses are lower than AC

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transmission. A significant advantage of the DC interconnection is that there is no stability limitrelated to the amount of power or the transmission distance. Long Distance Bulk Power

Transmission. When large amounts of power are to be delivered over long distances, DCtransmission is always an alternative to be considered. AC transmission becomes limited by:

Acceptable variation of voltage over the transmission distance and expected loadinglevels.

Need to maintain stability, that is, synchronous operation across the transmission, after adisturbance, both transiently and dynamically.

Economic effects of additions necessary to correct the above limitations.The DC line, requiring as few as two conductors (one only for submarine with earthreturn) compared to the AC lines use of three, requires a smaller right of way and a lessobtrusive tower.

A qualitative comparison between AC and DC lines with regard to impact on theenvironment is as follows:

Visual impact constitutes an environmental advantage for a DC line, since the tower size for thesame power is lower when compared to the tower size of an AC line. Right-of-way width of a

DC line compared to an AC line is considerably reduced. This facilitates suitable routes in

densely populated areas and in regions with difficult terrain.

Corona phenomenon has a substantially different nature in DC than in AC transmission.

Generally, for a bipolar DC transmission line and an AC transmission line with almost the same

rms conductor voltage to earth and equal transmitting capacity, annual mean Corona Losses (CL)

are more favorable for the DC than the AC case, particularly in poor weather Conditions.

Radio interference (RI) results from Corona discharges, which generate high frequency currentsin the conductors producing electromagnetic radiation, in the vicinity of the lines. RI

measurements have shown that radio noise from a DC line is considerably lower than from AC

lines of similar capacity.

Audible noise (AN) values resulting from comparable DC and AC lines during fair weather are

quite similar. However, during rain, the better performance and the lower interference levels

generated by DC compared to AC lines are considered an advantage.

With regard to magnetic fields, conditions for DC lines are quite different than AC lines. Since aDC line has an unchanging electric field, it exerts effectively no magnetic field on thesurroundings. The DC field of a monopolar line is comparable to the strength of the earthsmagnetic field.

Regarding generation and emission by DC lines of positively charged ions, O3, N2 and freeelectrons, research studies and investigations of possible consequences have shown, up to now,

no evidence of hazard from any operating DC line.

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HVDC transmission refers to that the AC power generated at a power plant is transformed intoDC power before its transmission. At the inverter (receiving side), it is then transformed back

into its original AC power and then supplied to each household. Such power transmissionmethod makes it possible to transmit electric power in an economic way through up-conversionof voltage, which is an advantage in existing AC transmission technology and to overcome many

disadvantages associated with AC power transmission as well. The overall structure of an HVDC

system is as shown in Figure 1.4 and its basic components are described below.

AC Breaker. This is used to isolate the HVDC system from the AC system when the HVDC

system is malfunctioning. This breaker must be rated to carry full load current, interrupt fault

current, and energize the usually large converter transformers. The purposes of this breaker arefor the interface between AC switch yards or between AC busbar and HVDC system (Figure).

AC Filters and Capacitor Bank. The converter generates voltage and current harmonics at boththe AC and DC sides. Such harmonics overheat the generator and disturb the communicationsystem. On the AC side, a double tuned AC filter is used to remove these two types of

harmonics. In addition, the reactive power sources such as a capacitor bank or synchronous

compensator are installed to provide the reactive power necessary for power conversion.

Twelve Pulse Valve Group:

Nearly all HVDC power converters with thyristor valves are assembled in a converter bridge of

twelve pulse configuration. Figure 7.2 demonstrates the use of two three phase converter

transformers with one d.c. side winding as an ungrounded star connection and the other a delta

configuration. Consequently the a.c. voltages applied to each six pulse valve group which makeup the twelve pulse valve group have a phase difference of 30 degrees which is utilized to cancel

the a.c. side 5th and 7th harmonic currents and d.c. side 6th harmonic voltage, thus resulting in a

significant saving in harmonic filters. Figure 3 also shows the outline around each of the threegroups of four valves in a single vertical stack. These are known as quadrivalves and areassembled as one valve structure by stacking four valves in series. Since the voltage rating of

thyristors is several kV, a 500 kV quadrivalves may have hundreds of individual thyristors

connected in series groups of valve or thyristor modules. A quadrivalve for a high voltageconverter is mechanically quite tall and may be suspended from the ceiling of the valve hall,

especially in locations susceptible to earthquakes.

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Fig: 7.2 TWELVE Pulse HVDC Configuration

APPLICATIONS OF HVDC CONVERTERS:

The first application for HVDC converters was to provide point to point electrical power

interconnections between asynchronous a.c. power networks. There are other applications whichcan be met by HVDC converter transmission which include:

1. Interconnections between asynchronous systems. Some continental electric power systems

consist of asynchronous networks such as the East, West, Texas and Quebec networks in NorthAmerica and island loads such as the Island of Gotland in the Baltic Sea make good use of

HVDC interconnections.

2. Deliver energy from remote energy sources. Where generation has been developed at remotesites of available energy, HVDC transmission has been an economical means to bring theelectricity to load centers. Gas fired thermal generation can be located close to load centers and

may delay development of isolated energy sources in the near term.

3. Import electric energy into congested load areas. In areas where new generation is impossibleto bring into service to meet load growth or replace inefficient or decommissioned plant,

underground d.c. cable transmission is a viable means to import electricity.

4. Increasing the capacity of existing a.c. transmission by conversion to d.c. transmission. New

transmission rights-of-way may be impossible to obtain. Existing overhead a.c. transmissionlines if upgraded to or overbuilt with d.c. transmission can substantially increase the power

transfer capability on the existing right-of-way.

5. Power flow control. A.c. networks do not easily accommodate desired power flow control.Power marketers and system operators may require the power flow control capability provided

by HVDC transmission.

6. Stabilization of electric power networks. Some wide spread a.c. power system networksoperate at stability limits well below the thermal capacity of their transmission conductors.

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HVDC transmission is an option to consider increasing utilization of network conductors alongwith the various power electronic controllers which can be applied on a.c. transmission.

The synchronous compensator has been the preferred means of a.c. voltage control as it increasesthe short circuit ratio and serves as a variable reactive power source. Its disadvantages includehigh losses and maintenance which add to its overall cost.

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PRACTICAL NO.- 8

Aim: Design of SIX Pulse Converter Using MATLAB Simulink

Fig: 8.1 MATLAB SIX-Pulse converter circuit

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Fig: 8.2 MATLAB Simulink Pulses Graph

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Fig: 8.3 MATLAB Simulink Output Voltage Graph

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PRACTICAL NO.- 9

Aim: Design of Twelve Pulse Converter Using MATLAB Simulink

Fig: 9.1 MATLAB TWELVE pulse converter circuit

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Fig: 9.2 MATLAB Simulink Output Voltage Graph

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Fig: 9.3 MATLAB Simulink Output Pulses Y graph

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Fig: 9.4 MATLAB Simulink Output Pulses d Graph

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