Power Generation

THYRIPARTCompound-Excitation Systemfor Synchronous Generators

Excitation Systems

Operating Characteristics Standard circuitry

THYRIPART

Load dependentShort circuit supportingLow voltage gradient dv/dtBlack start capabilityDigital open and closed loop controlHigh reliability and availabilityExtraordinary flexibilityEasy maintenanceBest control capabilities

The THYRIPART excitation system is a static excitation systemsupplied by voltage and current from the generator andincorporating a transistorized voltage regulator. It supplies theenergy required to excite the generator directly via slip rings.

The outstanding flexibility of the standard system and the modularconstruction make the THYRIPART system suitable for applicationin industrial-, steam- and hydro power plants and particularlysuitable for the modernization of existing excitation systems.

This basic circuit with its excellent dynamic properties also featuresa transistorized controller which provides a parallel circuit to theexcitation winding.

The transistorized controller is a DC chopper regulator, a standardproduct of DC drive technology. It operates in parallel with therectifier, i.e. it is connected in parallel to the excitation winding.The transistorized chopper can both add and subtract current tothe field current supplied by the basic THYRIPART circuit, thusincreasing or reducing the field current. This control featurepermits a relative voltage stability of 0.5%.

In island mode it is even possible to operate the excitation systemwithout closed loop control. The hardware components trans-former, Harz circuit and bridge converter are capable to controlthe generator voltage to an accuracy of (depending on generatorcharacteristics) approximately 10 %.

The modular construction the graphically supported flexiblesoftware of the digital open and closed loop control as well asthe little space requirement makes the system ideal for the mo-dernization of existing excitation systems.

For new systems adaptation to specific requests is possible.

Mechanical Design

The THYRIPART excitation system is installed in a standard cubiclespecially designed for equipment comprising open loop andclosed-loop controls and power electronics devices. Modulardesign and easy accessibility of all components facilitate all settingand optimizing operations. The standard cubicle group is 1500to 2400 mm wide and 600 mm deep at a height of 2200 mm.The adaptability to all kinds of local conditions and existing plantmay necessitate deviating cubicle dimensions, especially in thecase of modernization projects.

As the generator lead or neutral point has to go through thecurrent transformer and the voltage transformer will be connectedto the generator lead the transformers should be assembled closeto the generator. Therefore the transformers are supplied bulkyfor erection into a separate room of MV compartment.

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THYRIPART-compartments(examples)

The functional principle is based on a Harz circuit which uses anoscillating circuit to convert a voltage source into a current source.The no load component of the excitation is supplied from theexcitation voltage transformer and converted into a current sourceby the Harz circuit The load component is supplied by theexcitation current transformer. The generator current flows throughthe primary winding. The no load component from the Harzcircuit flows through an additional winding and is added by thisto the load component. A third winding supplies the resultingexcitation current which is rectified in a bridge converter and fedto the generator field.

The current-dependent component also helps to boost or maintainexcitation in the event of voltage dips or short circuits.

The oscillating circuit is supplied by the voltage generated fromresidual magnetism during run-up to speed or also at rated speed.This enables the generator to operate as a self-excited machinewithout an external power source.

THYRIPART Standard Circuit

THYRIPART

The voltage transformer (1) supplies the no load part of theexcitation.The oscillating circuit consisting of the reactor (3) and the capacitor(4) transforms the no load part into a current source.The current transformer (2) supplies the load part of the excitationand adds it to the no load part.The B6U converter bridge (7) rectifies the excitation current. The components (1)-(7) alone control the generator voltage withan accuracy of max. 10 % The digital open and closed loop control (8) receives the actualvalues from the measurement transformers (5) and controls thetransistor chopper (9). This chopper adds or subtracts a part ofthe excitation current. This results in an accuracy of 0,5 % forthe generator voltage.

Main components are:

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In manual mode, the actual value excitation current is detectedvia a shunt resistor (10) and controlled by the closed loop control.

The over voltage protection (11) prevents from over voltages inthe field circuit. It is self restoring so that excitation can go onafter over voltages caused by load steps or short circuits.

To de-excite the generator, the de-excitation contactor (6) shortsthe feeding current source. Additionally the shorted current sourceprevents the generator from self-excitation out of the residualmagnetism if the generator is at speed or being turned.

Optionally it is possible to speed the de-excitation up by the fastde-excitation circuit (12) consisting of the de-excitation switchand the de-excitation resistor.

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5 8 9

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711

1012

Mode of Operation

THYRIPART

Base ExcitationThe excitation current supplied by the THYRIPART consists of thevoltage-dependent no-load component and the current-dependentload component.The no-load component is generated from an oscillating circuitaccording to the Harz circuit principle. The oscillating circuitconsisting of reactor L and capacitor C is connected to the generatorlead and the voltage transformer respectively.At a frequency near resonance, the oscillating circuit produces acurrent at the leads between the reactor and the capacitor. Thiscurrent is proportional to the voltage applied (Harz principle). Byadjusting the reactor inductance (by varying the air gap) orchanging the tap on the current transformer this current can beset to correspond to the no-load field current when the generatoris at nominal voltage.Based on the Harz principle this current is independent of ambientconditions as e.g. the rotor temperature. At rated speed the ratedvoltage is reached automatically.As this current flows through the reactor its phase is offset by90el from that of the current.During load operation a current component decoupled directlyfrom the generator current is added to this no-load component.Decoupling and addition are performed in the current transformer(T).The generator current flows through the primary winding of theCT, which only has a few turns, and the current from the oscillatingcircuit flows through the first secondary winding.The sum of the two currents (adapted according to the phaseangle and the transformation ratio) is picked off the secondsecondary winding, rectified, and fed to the field winding.This base excitation controls the generator current to an accuracyof min. 10 %.

Voltage built upThe oscillating circuit operates near resonance frequency. Theresult is a voltage rise which supplies a little excitation currenteven if the supply voltage generated from residual magnetism islow. By this the generator voltage rises and therefore also theexcitation current. This results in a voltage built up to ratedgenerator voltage.

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U(G)(400 V)

-L -C

-T

I(G)

If

Principle diagram of the basic excitation

Current- and Voltage transformer (example)

Comanche Peak nuclear power plant, USA: THYRIPART systemsqualified to American standard 1E

Closed loop controlThe closed loop control controls a transistor chopper, whichadapts the excitation current from the Harz circuit by currentaddition or subtraction. This results in a control accuracy of 0,5 % for the generator voltage.

Automatic Voltage Regulator (Automatic control system)The actual value of the generator voltage is compared with anadjustable generator voltage setpoint. The resulting signal iscompared with the output of the excitation limiter and taken tothe input of the PI voltage controller. The PI voltage controllerwith adjustable gain and time response provides an output signalwhich is applied to the setpoint input Iref of the secondary fieldcurrent controller. The output of this controller governs thegeneration of the frequency-modulated driving pulses for thepower transistors of the associated output stage, which is operatedwith a clock frequency of about 2,5 kHz. The DC current flowsthrough the phase U and W of the transistor power circuit.

Automatic cos or reactive power regulator at the generator leads (Automatic control system)The cos regulator compares the actual value with an adjustablecos reference value. In case of a deviation the reference valueof the voltage regulator is adjusted until the deviation is reducedto zero. In isolated or no-load operation of the generator theoperation mode of the automatic control system is switched overfrom cos regulation to voltage regulation.

Manual control system (Excitation current regulator)The P-action control amplifier of this regulator receives a filteredsetpoint signal that is compared with the actual value of the fieldcurrent. The output signal controls the frequency-modulationdrive circuit for the power transistors of the associated outputstage.

Of the three control systems the automatic one is normally inoperation, even during starting and sto