Power Generation

THYRIPART®Compound-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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2

3 4

5 8 9

6

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 by90°el 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 ϕ or reactive power regulator at the generator leads (Automatic control system)The cos ϕ regulator compares the actual value with an adjustablecos ϕ 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 ϕ 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 stopping of the electric set.The automatic control system includes the measuring and setpointdevices and the control and monitoring circuits for the followingfunctions:

Generator voltage regulationFast secondary control and limitation of the output current ofthe field current chopper regulator and/or field-forcing limiter.Limiting controller for the under-excited range (under excitationlimitation)Delayed high limiting control for the over-excited range(over excitation limitation)Delayed generator current limiter (stator current limitation)

THYRIPART®4

OptionsV/Hz-limiter (over fluxing limiter)Cos - ϕ - or reactive power regulator at the supply pointVAR joint control of several generators

Commissioning modeThe manual control system is designed as excitation currentregulator which permits generator characteristics to be recordedduring commissioning and inspections, and also short-circuitoperation of the generator to be carried out for setting theprotective relays. When the automatic voltage regulator is faulted,it can also be used for the operational adjustment of the generatorexcitation.

Follow-up controlThe setpoint value of the field current controller is continuouslyupdated during operation on the automatic control system, thusensuring rapid and nearly bump less changeover to manual controlin the event of a fault. Automatic switchover takes place whencertain fuses or protective circuit breakers operate or in case offailure of the automatic control system.

Control accuracyThe control accuracy of all control systems is ± 0.5 %

Control board inside the transistor power part

THYRIPART®

When operating a synchronous generator, it is necessary toobserve the permissible combinations of active and reactivepower, which can be seen from the capability diagram.

LMO Limit characteristic of the underexcited rangeOP Limit set by the stator temperature risePQ Limit set by the rotor temperature rise

Similar characteristics with reversed active-power flow apply tomotor operation of the generator.

The underexcitation limiter corrects the reactive power by raisingthe machine voltage as necessary to ensure that, in case of anexcursion beyond the limit characteristic L-M-O, the operatingpoint is returned to that characteristic before the machine istripped by the underexcitation protection.

Limiting controllers

Limit characteristics of a synchronous machine in generator operation

Minimum excitation limiting

Overexcitation limiting and secondary field current limiting

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The overexcitation limiter ensures that, in the overexcited range,the operating point always keeps within capability curve sectionP-Q of the generator. In response to system voltage drops causedby high reactive power requirements, switching manipulationsor faults, the voltage regulator raises the excitation level so as tokeep the generator voltage constant. The overexcitation limitingdevice acts as a safeguard against thermal overloading of therotor.

The overexcitation limiter admits excitation current values betweenthe maximum continuous current and the maximum excitationcurrent (field forcing) for a limited period of time so that thegenerator can back up the system in response to short-time systemvoltage dips.

The secondary excitation current limiter (field-forcing limiter),in contrast, has the task of limiting the excitation current to themaximum permissible value as quickly as possible.

The stator current limiter ensures the delayed limitation ontoworking points, within the N-P range of the generator powerdiagram. The main task of the stator current limiter is to preventthe generator stator from thermal overload, which can be causedby a high reactive power at increased active power. The statorcurrent limiter also permits increased excitation values for alimited period so that the generator can back up the system.

Max. turbine-output

Overexcited

Active Power

Underexcited

Reactive Power

O

PN

M

L

Q

Active Power

Max. turbine-output

Reactive Power

Underexcited

M

L

O

N

Excitation current If(Rated Value)

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30

20

10

1 1.1 1.5

Subordinate excitationcurrent limitation

Overexcitationlimitation

t(s)

The four quadrant chopper allows positive as well as negative outpot voltageswith positive or negative output currents. If the mean value of the output voltageis bigger than the excitation voltage of the base excitation, an additional currentis fed into the field circuit and the excitation current rises. If the mean value ofthe voltage is lower, a part of the base excitation current is fed back into the DClink. The result is a lower excitation current.

Open loop control

The open loop control system contains interlocksfor switching the excitatioin on and off. The pro-tective OFF signal on the other hand is applieddirectly to the coil of the excitation contactor. Theopen loop control also affects the changeoverbetween the automatic and manual closed loopcontrol systems.

Every operating mode of the excitation system ismonitored and displayed; slow control processesare carried out and evaluated.

In addition to detailed internal fault indication, theinternal monitoring routine makes the followingalarms available at the cabinet terminals:

Fault with Protective Off commandFault in automatic control system and switchoverto manual control systemGroup alarm triggered by various internal fault signals.Group alarm triggered by various internal fault signals causing starting lockout.

Process alarms are also made available for externalindication:

Excitation is onExcitation is offAutomatic Voltage Regulator is onExcitation Current Regulator is onCos ϕ – or VAR Regulator is onLimiters are active

Further more detailed signals are optionally possible.

THYRIPART®6

The power circuit uses transistor chopper regulators, which provides the necessaryexcitation power via a DC link.

The field voltage is adjusted by varying the pulse/pause ratio and the field circuitresistance causes the field circuit to vary accordingly. The field current is measuredin the output stage and the signal is converted for the field current controller.

Power circuit

Principle connection of the transistor power part

Principle of the pulse wide modulation

+ + + + + + + + +

positive zero negative

-220 V

0

+220 V

- - -------

Rectifier Smoothing Adjuster

Output

Control

P

N

L1

L2

L3

parameters can be changed easily by selecting the correspondingparameter in the parameter list. It is possible to choose betweena complete parameter list, pre-defined parameter lists withselection of parameter for a special application (e.g. input/ out-put) or a user defined parameter list by entering the interestingparameter numbers. A complete upread of parameters allowseasy documentation.

THYRIPART®

PC Tools

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The operator friendly software tool SIMOVIS/DriveMonitorguarantees simply commissioning of the THYRIPART®. Througha serial interface the voltage regulator can be connected with thePC for easy configuration.

Customer friendly ConfiguringThe SIMOVIS/DirveMonitor software for Microsoft Windows9x/2x/NT allows the complete parameterization of the powercircuit. Actual values can be monitored in the parameter list and

The trace recording function contains followingfeatures:

Monitoring of up to 8 analog signalsMonitoring of 16 binary signals per unused analog signal (e.g. 32 binary and 6 analog signals)Maximum recording time of 280s at a maximum sampling rateof 280ms or 1.4s at minimum sampling rate of 1.4ms Freely adjustable sampling rate between 1.4 and 280ms in steps of 1.4msFault recording is automatically triggered by programmable fault signals (triggered by binary signals e.g. faults or by comparing an analog value (condition: </>/=/<>) with a predefined value)Adjustable pre-trigger between 0% (no pre-trigger, only future) and 100% (only past, no future)

THYRIPART®

Trace

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Trace is an add-on for SIMOVIS/Drive Monitor that permitsvisualization of recorded data. You can also store the data readout of the device and open it again later. It is also possible toimport such data into text processing programs, such as MicrosoftWord, or into spreadsheet programs, such as Microsoft Excel.

You can perform simple measurements of amplitudes and instantsusing two moveable cursors.

THYRIPART®

Configuring with D7-ES

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The complex voltage regulator with its limiters and calculationsas well as control functions such as interlocking is integratedinside the programmable T400 circuit board. The T400 can beconfigured with the graphic D7-ES configuring tools, based onWindows 95/NT.

Thus, it is very easy to implement even complex customizedsupplementary functions inside the THYRIPART®. Completestandard software packages are available for functions andapplications which are frequently required.

The picture below shows an example of a software structure anda selection of available functions out of the function block library.

Technical Data

THYRIPART®10

Atucha nuclear power plant, Argentinia:four THYRIPART® systems 7.2 MVA foremergency generators of Atucha 2.

THYRIPART®

The range of application for THYRIPART® excitation systems are low and middle voltage generators with small and mediumpower when load dependence and short circuit support is required or the condition of the rotor insulation does not allowthe high voltage gradient caused by thyristor converters. This mainly applies for older generators. Other reasons are theblack start capability and the high availability. The standard application range is (extended application range on demand):

Rated generator voltage U(GN) max. 13,2 kV

Rated generator current I(GN) max. 2300 A

Rated excitation current I(fN) max. 960 A

Auxiliary supply from station battery 250 V: For power circuit control

Power consumption: < 0,1 kW continuously< 0,2 kW short-time

For signal and controller power supply (24V DC)

Power consumption: < 0,2 kW continuously

Instrument transformers: Potential transformers:connected to 3 phases of the generator voltage.Power consumption < 5 VA per phase;Secondary voltage rating 100 V to 120 V.

Current transformers:Two-phase (L1, L3), for measuring the generator current.Secondary current rating 5 A or 1 A,< 5 VA (plus CT cable losses).

The transformers are not included in the scopeof supply of the voltage regulator .

Standards: The THYRIPART® excitation system is rated and designed according to IEC-, EN-, DIN-, VDE-, IEEE-421- standards. Service and maintenance of the excitation system THYRIPART® can be executed according the VGB4- instructions.

Published by and copyright 2004:Siemens AGPower GenerationFreyeslebenstrasse 191058 Erlangen, Germanyfax: 0049-9131-18-4369www.siemens.com/powergeneration

Subject to change without prior notice

The information in this document contains generaldescriptions of the technical options available whichdo not always have to be present in individual cases.The required features should therefore be specified ineach individual case at the time of closing the contract.

Excitation Systems