REPLACING AGED STATIC EXCITER SYSTEMS

8/17/2019 REPLACING AGED STATIC EXCITER SYSTEMS

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ALTERNATE SOLUTIONS TO REPLACING AGED

STATIC EXCITER SYSTEMS

Richard C. Schaefer, Basler Electric CompanyThomas W. Eberly, Consultant

Abstrac t: Over the years, many rotatingexciters have been replaced for staticexciters on synchronous machines. Asthese replacement static exciters age,much of the analog hardware becomesobsolete and can no longer be supported bythe manufacturer, again forcing thereplacement of the entire excitationsystem. For many systems, the powerrectifier bridge has proven to be reliable with

replacement components for power SCRsand power diodes utilized in the bridge stillbeing accessible. Today, rather thanreplacing the entire excitation system,another approach is to replace only theanalog portion and keep the power rectifierbridge; hence, saving on the overall cost ofa replacement rather than the investment ofthe entire excitation system.

This paper will discuss the results of thisalternative solution and will demonstrate theimproved performance gained byimplementing a digital front-end controllerinto the existing power rectifier bridge/s thatcan result in substantial cost saving to aproject.

INTRODUCTIONIn the field today, there are many staticexciters that are coming into their 15 to 20year life cycle of replacement due toobsolescence of the analog control boards.Now multiple solutions are available forretrofitting older static exciter technology.

Approaches may include:

1. Replacing the complete static excitersystem, including the power potentialtransformer.

2. Keeping the power potential transformerand replacing the voltage regulatorcontrol and power rectifier bridge(s).

3. Retrofitting only the analog controls andkeeping the power rectifier bridge(s),breaker, contactors and power potentialtransformer (PPT).

The reuse of the rectifier bridges hasbecome increasingly popular as rectifier

bridge technology utilized years ago hasexhibited high reliability and proven toremain in excellent operating condition.Hence, replacing only the analog controlsdue to obsolescence and keeping the powerSCR rectifier bridge/s may be a favorableeconomic solution for an excitation systemupgrade.

In addition, there are other factors that canprompt the need to re-evaluate the existingexcitation system controls’ ability to complywith new rules and guidelines in the powerindustry. With the passage of the 2005Energy Act, an emphasis on generatorexcitation model validation, performancetesting, and limiter coordination withprotective relaying has become mandatory.These issues, along with the increasingneed to add a power system stabilizer to theexcitation system have emphasized theneed to modernize existing equipment andminimize the time required for testing.

With new digital excitation systems,

integrated features reduce the packagespace requirements of an overhaul, whileWindows “friendly” operating software withbuilt-in testing tools have madecommissioning and data gathering easier.

This paper addresses a retrofit solutionwhere a GE Bus Fed static exciter,manufactured in the 1980s, was upgraded


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with a new digital controller, including a newdigital firing circuit and gate amplifier boardto interface with the existing half-wave 3SCR and 3 Power Diode Rectifier Bridge,

AC breaker/field flash contactor, and powerpotential transformer.

THE PROBLEMIn Corona, California, a GE Frame V, 64MVA, 13.8 kV, 3600 RPM cogenerationplant required an upgrade in two of itscontrol systems that would improve theperformance and efficiency of thepower/electrical blocks. The excitationsystem and the turbine control system werechosen as the major elements involved inthis upgrade. The power plant’s turbinecontrols were modernized with a new digital

controller communication system. Theexcitation system was replaced due to theproblems of obsolete printed boards,reactive power control problems, andgeneral lack of equipment support. Bothsystems use Modbus communication, whichis beneficial in streamlining the plant’soperation. The existing static exciter waslocated in a compact, restricted area thatcontained two parallel convention-cooledrectifier bridges with load sharing reactorson each bridge, an ac field breaker, a field

flash contactor, and field flashing scheme. A1970s rack-mounted analog voltageregulator assembly was mounted on thefront door, along with a power supply, avar/Power Factor controller, and othermiscellaneous hardware filling in thecabinet’s interior. (See Figure 1.)

Figure 1: GE Bus Fed Analog Voltage Regulator

RETROFIT WITH A DECS-400

DIGITAL EXCITATION CONTROLThe original equipment represented a 1970stechnology design that incorporated manytransformers and huge heat dissipatingpower supplies to provide dc to the Bus Fed

card rack. The demolition eliminated all ofthe analog controls except the two SCRbridges, ac field molded case breaker, dcfield flash contactor, the field flash resistor,and dc control interface.

The new equipment included a DigitalController, DECS-400, mounted on the frontdoor to replace the old AVR rack, a newprogrammable IFM150 digital firing moduleprogrammed for 3 SCR application, and agate amplifier board that ensured sufficient

energy pulses to fire the SCRs on the twoexisting convection-cooled rectifier bridges.The balance reactors used for the SCRbridges remained as they were installed.

Digital technology offers substantiallyimproved performance and featureintegration over its analog predecessor. Avoltage regulation of ¼ of 1%, under andover excitation (off-line/on-line) limiters, fieldcurrent regulation for manual control, autotracking for bumpless transfer between any

control modes and a communicationsModbus protocol via a RS 485 serial port toaid control and streamline plant operations.Modbus communications provided a meansto obtain metering, control, andannunciation from the excitation system.

Generator voltage matching wasimplemented that automatically caused thegenerator voltage to match the utility busvoltage to prevent bumps insynchronization. Voltage matching also

reduced the time for synchronization, sinceoperator intervention was no longerrequired.

Operating software is the key tocommissioning the excitation system quicklyand efficiently. Here, a customer-friendlyWindows-based BESTCOMS operatingsoftware communication program utilized a


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laptop computer for setup andcommissioning, while testing tools includedin the BESTCOMS aided startup andeliminated otherwise externally connectedtest equipment such as chart recorders.Features included are identified in the table

on the last page.

The demolition included eliminating all theanalog controls that made up the originalexcitation system except the rectifierbridges. See Figures 2 and 3.

Figure 2: Existing 3 SCR, 3 Diode Half-WaveBridge

Figure 3: Original equipment parts removedduring demolition

Figure 4: DECS-400 Installed in Cabinet Door

The Digital Controller was installed on thefront door of the cabinet. See Figure 4. Thespecial sequence panel included interposingrelays, a programmable firing module,autotransformer for matching the existingpower potential transformer secondaryvoltage to the firing circuit chassis, controltransformer for the power supply, and aField Isolation Module for monitoringgenerator field current and field voltage. A

subpanel chassis was specially designed tofit on the back door in between the doorchannels. See Figure 5.

Figure 5: Back door of DECS-400 panel showingIFM Firing Circuit and related hardware


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A gate amplifier board chassis was mountedin the cabinet interior back wall to amplifyfiring pulses and to ensure adequate energyfrom the firing circuit necessary to fire thegates of the power SCRs. Output contactsfrom the DECS-400 digital controller

interfaced to the existing dc field flashcontactor used for starting the machine.Figure 6 shows the cabinet lineup with theDigital Controller mounted. Figure 7 showsthe partial schematic of the newinterconnected system, highlighting theinterface of the old and new hardware.

Figure 6: Exciter cabinet lineup with

DECS-400 installed

Figure 7: Partial schematic of the new interconnected system


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A special interface transformer was used tomatch the power potential transformersecondary voltage to the required voltage ofthe new digital firing circuit. The digital firingmodule offered the flexibility of beingprogrammed for various types of bridges,

half wave or full wave SCR bridgeoperation. And for half wave bridges,programmable for SCRs located on eitherthe positive or negative rail. See Figure 8.

Figure 8: Firing Circuit setup for Half-WaveBridge

After the installation, the wiring interfaceand checkout was completed and the

excitation system and generator was readyto build voltage. The two-channel chartrecorder in the BESTCOMS operatingsoftware was utilized to monitor voltagebuildup and perform the voltage step testsneeded to determine theexcitation/generator performance. Figures 9through 11 illustrate the screenshotsresulting from data collected duringcommissioning. Unlike the older analogexcitation systems that required voltagebuildup in manual mode, the new excitation

system safely built voltage in AVR modewithout concerns of voltage overshoot dueto windup.

Using the two (2) channel real time chartrecorder, Figure 9 demonstrates the voltagebuildup in voltage regulation mode. Note thesmooth, stable operation as generatorvoltage builds up to nominal.

Figure 9: Voltage Buildup in Voltage RegulatorMode

Voltage step tests were performed to verifyperformance based on the selected gainsfor the digital controller. Here, the generatorvoltage responded within .3 seconds for a5% voltage step change in the positivedirection. See Figure 10. The analysisscreen in BESTCOMS allows up to 10%voltage step tests to be performed, as wellas time duration for the length of voltagestep change. Comparing the performancewith that of the old analog system, the new

digital system exhibited approximately 5times faster voltage response compared toits analog voltage regulator predecessorusing the same power SCR bridge/s. Fasterresponse translates into improved transientstability and generator relay coordinationafter a system disturbance.


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Figure 10: 5% Voltage Step Change in VoltageRegulator Mode

Data is saved in the form of either ascreenshot pasted into a word processingdocument or a data file for future record.

Also available is oscillography that savesthe information into an IEEE COMTRADE orlog file.

Figure 11 demonstrates the operation of theUnder Excitation Limiter (UEL) at a lowercalibrated value to verify dynamicperformance and stable operation.

Note that the performance for the UELresponds in less than 1 second with only asingle, small underdamp swing. Uponconclusion of the commissioning, theexcitation system was placed in Var Modefor normal operation.

Figure 11: Under Excitation Limit Dynamic StepTest

CONCLUSIONThe successful replacement of the analogcontrols with state of the art digitaltechnology met the expectations of theproject and enabled the generator andexcitation system to be commissionedquickly and efficiently with new features to

enhance the system’s reliability for manymore years of successful operation. Figures12 and 13 illustrate a Basler SSE staticexciter of the early 1980s being replacedwith a digital front end while keeping thepower bridge. Figure 14 illustrates aSiemens Allis Static Exciter before and aftera digital front end replacement.


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Figure 12: Using the analog technology;many lights, switches and meters

on front of doors of a Basler SSE Static Exciter

Figure 13: New front door provided with newcutouts for DECS-400, a serial cable is used for

a laptop connection.

Figure 14: The Lineup of the Siemens AllisOriginal Equipment; The DECS-400 digitalcontroller installed on front of Siemens Allis

Cabinet. Metering and obsolete switches wereremoved.

DIGITAL CONTROLLER INTEGRATED

FEATURES, DECS-400• Voltage Regulation: Better than 0.20%

Accuracy- Other Operating Modes- Field Current Regulation- Var or Power Factor Control

• Automatic Nulling: Nulling betweenoperating modes and redundant DECS

• Selectable Underfrequency or Volts/HertzRatio Limiter

• Minimum Excitation Limiter: Flexible 5 pointmap on real/reactive power axis or Internalgenerated UEL curve

• Maximum Excitation Limiter• Dual PID Setting Groups: Allows for

programmed changes in PID gain settingsfor use with Power System Stabilizer oralternate transmission systems


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• Autovoltage Matching: Automaticallymatches generator voltage to bus voltage

• (2) Preposition Set points: Programmable for AVR, Manual, Var/PF Controller

• Reactive Droop or Line Drop Compensation• Loss of Voltage Sensing: Transfers to

manual control automatically due to loss ofvoltage sensing at the voltage regulator

• Oscillography: 600 points, 6 programmableparameters, holds up to 6 records

• Sequence of events: stores 127 records• Real Time Monitoring (Chart Recorder for

test analysis)• Built-in Dynamic Analyzer for measuring

frequency response of generator andexcitation system

• Protection- Field Over Voltage- Generator Over/Under Voltage- Field Overcurrent

- Loss of Voltage Sensing- Loss of Field- Volts/Hertz Protection

• Field over voltage, generator over/undervoltage, field over current, and loss of fieldprotections have dual set points selectablevia programmable logic

• HMI providing Metering, Set point control, Alarm annunciation

• Irig B Time Synchronization stamp• Generator Field Temperature Monitoring

(Static Exciter)• (2) Analog Transducers Outputs

• Optional Built-in Power System Stabilizer

REFERENCES[1] Front End Analog Conversion to Digital

Saves Cost for Upgrades of ExcitationSystem, Waterpower XV Conference,July 2007. J. Wilson, EWEB; R.Schaefer and Kiyong Kim, BaslerElectric Company.

[2] R. C. Schaefer, Excitation Upgrade

Keeps Existing Power SCR Bridges andPPT, Application Note EX-BRDG1,Basler Electric, 2006.

[3] R. C. Schaefer, DECS-400 ProvidesFront End Digital Control to GE Bus FedPower Bridges, Application NoteEX-BUSFED, Basler Electric, 2007.

[4] R. C. Schaefer, Basler’s SSE AnalogConversion to Digital Provides FeatureEnhancements and Saves Cost of

Power Bridge Replacement, ApplicationNote EX-SSE1, Basler Electric, 2006.


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If you have any questions or need

additional information, please contact

Basler Electric CompanyRoute 143, Box 269, Highland, Illinois U.S.A. 62249

Tel +1 618.654.2341 Fax +1 618.654.2351

e-mail: [email protected]

No. 59 Heshun Road Loufeng District (N),Suzhou Industrial Park, 215122, Suzhou, P.R.China

Tel +86(0)512 8227 2888 Fax+86(0)512 8227 2887e-mail: [email protected]

P.A.E. Les Pins, 67319 Wasselonne Cedex FRANCE Tel +33 3.88.87.1010 Fax+33 3.88.87.0808


55 Ubi Avenue 1 #03-05 Singapore 408935 Tel +65 68.44.6445 Fax+65 65.68.44.8902

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REPLACING AGED STATIC EXCITER SYSTEMS