Static Exciation Power Section

7/28/2019 Static Exciation Power Section

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The Power converter is a thyristor six-legs, unidirectional and totalcontrolled type(GRAETZ BRIDGE): tyristors are adopted (SCR Silicon Controlled Rectifiers).

.

R

S

T

1

4

3

6

5

2

+

-

The thyristors parameters are calculated on the basis of the transformersecundary voltage Vac and of converter rated current.Safety coefficients are usually foreseen

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DIODES AND THYRISTORS GENERAL INFORMATIONS

The diode is a semiconductor component realized by a junction PN. TheCURRENT can flow only in ONE direction, from the anode, connected to the zoneP, to the cathode, K connected to the zone N.The diode conducts when the voltage between anode and cathode is positive,UAK>0 (direct polarization); it doesn't conduct when it is negative (inversepolarization). In a real diode UAK is very low (less than 1 V) and the current intensity

assumes high values; the inverse voltage can be high and the diode is crossed by asmall current.The relationship between current and volatge illustrated in fig.

The P-N Junction Diode

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THYRISTORS

The thyristor is in the conduction status when it is subject to a direct polarizationvoltage between anode and cathode (VAK>0) and when the current flowing through it,drived from a positive pulse applied to the gate electrode, reaches at least thelachting value; the conduction automatically stops when the VAKvoltage is < 0.The pulses that command the thyristors conduction are generated from excitationcontrol.

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http://upload.wikimedia.org/wikipedia/en/6/62/Thyristor_char.JPG


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IDEAL three phase diode converter characterist ics

Input Voltage Vc =380 Volt (rms value),max value Vmax =2*380= 537 VoltOutput voltage ( average value) =Vm= 1.35 *Vc = 513 Volt

Ideal rectified voltage ( comparison with phase to phase input vol tages)

0 0.0017 0.0033 0.005 0.0067 0.0083 0.01 0.01170.0133 0.015 0.0167 0.0183 0.020

108

217

325

433

542

650

Ideal rectified voltage

VmVmaxVu t( )

vab t( )

vbc t( )

vca t( )

vab t( )

vbc t( )

vca t( )

t

.

.

.

.

.

.

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. Fully controlled thyristor converter : firing characteristic

phases sequencei: .. R S T R .. fired thyristor : .. 1+6 1+2 3+2 3+4 5+4 5+6 1+6 ..

Converter output voltage with 3 alfa values: 30,90,150

Each thyristor is in the conduction state for 1/3 of period (= 120 )(= 6.66 ms )The commutation from a thyristor to the other every 3.3 ms

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Thyristors f ired sequence

ramo 1

leg

ramo 2

leg

ramo 3

leg

ramo 4

leg

ramo 5leg

ramo 6

leg

T/6 T/6 T/6 T/6 T/6

T

T/6

VRSVRS VRT VST VSR VTR VTS

The functional unity that produces the correct impulses sequence is the PHASESHIFTER.

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Fully controlled thyristor converter: Output voltage as a function of firingangle alfa

0 40 80 120 160 2001.2 10

3

600

0

600

1.2 103

Exc. output voltage average value

0Ed 1( )

5 150

1

Al fa= 5is the maximum forcing excitat ion vol tage (= 386 Vdc)The minimum ALFA value is settable ( es: 8 - 10 )

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Converter Redundancy

With the purpose to increase the degree of reliability of the system,the section of power is realized redundant; the use of two identicalconvermters in parallel it allows to dominate the presence of ananomaly of operation of a converter, guaranteeing so, thecontinuity of operation of the same system.

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The static fi led breaker ( Crow bar)

A crow bar circuit is a fast de excitation method

Two thyristors connected in anti parallel The discharge resistence Double function: protection and de excitation

+

mmmmmmmmm

DischargeResistence

Converter Field

The device consists of two thyristors in antiparallel and connected inseries to a discharge resistor

The positive crow-bar CBP guarantees a fast de-energization of

generator field ( shutdown or trip) and operates as a protection forcontinuous side direct overvoltages.

The negative CBN operates as a protection for transient voltage inducedinto generatot field , during line or machine failures (ex. Pole slip).

In any case the crow bar circuit senses the polarity of the overvoltageand causes the appropriate power thyristor to turn on, quickly dissipatingthe field energy through the discharge resistor to minimize the potential

damage.The Crow bar thyristors firing circuits are redundant

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Th field flashing circuit

It is necessary ONLY if the excitation is fed by generator terminal

crow

bar G~converter

3

pre-excitation

power supplycircuit

31

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THE POWER SECTION

Two converters are mounted in the same cubicle of the board .We see the heatsinks ( black color) the thyristors inside ( white color)and the protection fuses outside ( white color).Cooling converter type: natural convention

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Static Exciation Power Section