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PDA-IVTMPeriodic On-line Partial Discharge Monitoring Using a Portable Instrument for Hydro Generators
IRIS POWER ON-LINE PRODUCTS
A USERS PERSPECTIVE:
By combining on-line monitoring and a diminished schedule of off-line testing, the utility hasswitched from time-based to a condition-based procedure for allocating resources for testing andmaintenance, the maintenance staff can discern more thoroughly the condition of the insulationand what, if any, repairs or changes in operating procedures need to be done to increase thelifespan of the unit.
IRIS POWER, THE WORLDS LARGEST PROVIDER OF ON-LINE PERIODIC AND CONTINUOUS PARTIAL DISCHARGE MEASUREMENT SYSTEMS
The test enables predictivemaintenance on hydro generatorstator windings, resulting in theincreased availability andextension of operating life. Thetechnique was introduced morethan three decades ago and hassince been used on over 5000hydro generators to identifydeteriorated stator windings.
The method is non-destructiveand based on proven scientificand empirical principles and isrecommended by manufacturersand industry standards such asthe IEEE Std. 1434-2000.
The monitoring technique isbased on the application of 80pFcapacitive couplers, resulting in ahigh frequency measurement
range and a favorable signal-to-noise ratio. This enables theautomatic separation andrecording of both partialdischarges and noise, so that testresults can be easily interpretedby users. The most commonmethod of monitoring hydrogenerators involves using a PDA-IV portable instrument withmultiple sets of permanentlyinstalled capacitive couplers. Theinstrument is controlled by meansof a computer and includesWindowsTMbased control anddata display software.
Alternatively, continuousmonitoring systems are availablefrom Iris Power. They can beintegrated with plant SCADA andfacilitate remote monitoring.
IRIS POWER ON-LINE PRODUCTS
PERIODIC ON-LINE PARTIAL DISCHARGE MONITORING OFHYDRO GENERATORS
C1 C2 Ring Bus
Coupler Termination Box
HOW THE PDA-IVINSTRUMENTMEASURES ANDANALYZES PARTIALDISCHARGES
The PDA-IV test has beendesigned to monitor partialdischarges during normalgenerator operation, i.e.under normal electrical,mechanical, thermal andenvironmental operatingstresses. The readings arenot affected by externalinterference (or noise) suchas power system corona,output bus arcing or othercommon noise sources. Thetests are typically performedsemi-annually and take about30 minutes per generator.
Differential Coupler Configurationfor Hydro Generators
HYDRO GENERATOR TESTING
The 80pF capacitors block the 50/60Hz power frequency voltage allowing high frequency fast rise-time voltagepulses (which are caused by partial discharges in the winding) to pass through. Couplers are normally installedin a differential configuration. For example, in a winding with two parallel circuits per phase, two couplersare installed, one on each parallel, as shown in the diagram below. The lengths of the coaxial cables betweenthe couplers and the termination box are trimmed so that noise pulses originating outside the machine arriveat the two instrument inputs simultaneously. Partial discharge pulses originating in the winding arrive at thetwo instrument inputs at different times. Consequently, the system can digitally distinguish between noise andwinding partial discharge.
IRIS POWER ON-LINE PRODUCTS
PDA-IV TEST PROCEDURE
With the generator operating under normal service conditions, theoperator connects the PDA-IV instrument to a coupler terminationbox and to a portable computer running the Iris Power software. Themagnitude, phase position and number of partial discharge pulsesdetected at each coupler are then recorded and can be viewedimmediately, or stored for a subsequent analysis. The resultspresented to the user include:
graphs depicting the nature and severity of particular insulationaging mechanisms
trend curves highlighting the progression of these mechanismsover time
statistical values which can be compared with historical databasesfor similar generators.
In most cases, the assessment of the condition of the stator windinginsulation, based on the on-line partial discharge measurement, canbe performed independently by the user after a short training course.Iris Power can also assist the user with result interpretation using itsunique database of over 140,000 results, collected over 20 years ongenerators of all makes and sizes.
Stator winding insulationproblems occur in 40% of allhydro generators and are one ofthe principal causes of forcedoutages, which result in lostrevenues. On-line partialdischarge (PD) monitoring, aproven technique developed byOntario Hydro and the CanadianElectrical Association in the late1970s, helps to minimize the risksof unexpected stator windingfailure. Using permanentlyinstalled couplers and a portableinstrument (the PDA-IV), utilityplant personnel worldwide havefound this method reliable andeasy to use.
In its 30-year history, the on-linepartial discharge test hasproduced results on thousands ofhydro generators. Published casestudies by dozens of PDA testusers have confirmed thefollowing benefits:
the root causes of statorproblems can be identified andoften repaired at an earlierstage.
the monitoring can increasethe availability of generators,and can contribute to theextension of the useful life ofthe unit stator winding
intervals between generatorinspection outages can beincreased, if the PDA resultsare good
most stator winding in-servicefailures can be avoided
there is an exceptionally lowrate of false indications
clear levels at which machinesneed maintenance have beenestablished
can be applied to existingand new hydro generators.
PLANNING PREDICTIVE MAINTENANCE USING ON-LINE PARTIAL DISCHARGE MONITORING
The PDA-IV instrument is usedin conjunction with 80pF, highvoltage capacitors,permanently installed atspecific locations on the statorwinding. Iris Power suppliesthe compact capacitivecouplers made of high qualitymica splittings, encapsulatedin a specifically formulatedepoxy compound. These arecertified to be discharge freeat twice the operating voltageand are resistant to surfaceelectrical tracking.
Installed PDA couplers within a hydro generator
IRIS POWER LP HAS BEEN THE WORLD LEADER IN MOTOR AND GENERATOR WINDING DIAGNOSTICS SINCE 1990,PROVIDING A FULL LINE OF ON-LINE AND OFF-LINE TOOLS, AS WELL AS COMISSIONING AND CONSULTING SERVICES.
PDA-IV is a trademark of Iris Power LP
Windows is a registered trademark of MicrosoftCorporation in the United States and other countries.
HEAD OFFICEIRIS Power LP3110 American DriveMississauga, ON L4V 1T2CanadaPhone: 1-905-677-4824Fax: 1-905-677-8498
Ver 6 11/09
DETECTION OF PARTIAL DISCHARGES
Partial discharges in degrading high voltage stator windings give rise tosmall voltage pulses which travel through the stator winding. Themagnitude and number of these pulses depends on the degree ofinsulation deterioration. That is, as the magnitude and number of partialdischarge voltage pulses increase, the rate of the electrical insulationdeterioration must also be increasing. The partial discharge signals areoften mixed with electrical noise from sources such as corona andoutput bus arcing, but are separated for analysis by the PDA-IVinstrument.
WORLDWIDE INDUSTRYACCEPTANCE OF ON-LINE PDMONITORING
The PDA partial discharge test has won worldwideacceptance with all majorutilities. This generalacceptance has beenachieved because:
the test has been proveneffective in thousands ofinstallations, by identifyinghydro generators whichrequire maintenance
the test equipment is cost-effective
the test is performed on-line and requires nogenerator shut-down
generator station staff canperform the test andinterpret the results withminimal training.
PD in Parallel C1
PD in Parallel C2
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