M5100 SFRA

Doble Engineering Company 85 Walnut St. Watertown, MA USA 02472-4037 Tel +1 617.926.4900 Fax +1 617.926.0528 http.www.doble.com

SWEEP FREQUENCY

RESPONSE ANALYZER

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M5100 SWEEP FREQUENCY RESPONSE ANALYZER

BACKGROUND

Power transformers are purchased with the projectedservice life of forty years, yet evidence from constructionprograms since the 1960's, and international studies showthat, on average, fifteen years is a more realistic estimate.

Power transformers are specified to withstand themechanical forces of shipping and subsequent in-serviceshort circuits across the terminals. The major causes oftransformer failure are related to bushings and load tapchangers. However, the main tank is susceptible to failuresthat stem from mechanical changes caused by shortcircuits, system faults, and the aging process. Furtherdegradation can be triggered by moisture levels, insulationquality of paper and oil, and gassing.

Minor stresses, compounded by further in-service faults,can signal the start of major transformer failures. Thesemechanical degradations are difficult to detect throughvisual inspection. Such inspections are often limited byin-service demand and off-line availability. The best wayto avoid these failures and assure a long life is to trackchanges in performance of the main tank, bushings andload tap changers.

Constant change in the utility industry has changedmaintenance practices forever. Economic realities,reductions in personnel, loss of expertise, complextechnologies, and the demand for continuous operation ofequipment all mandate new test instrumentation solutions.To be effective, use of these “tools” must provide acomprehensive, efficient, and reliable means of acquiringtest data and diagnosing transformer conditions withminimal human intervention.

Doble Engineering is dedicated to achieving this goal bydelivering the products and services you need to develop acomplete protocol of dynamic tests, specifically to assessthe condition of transformers.

SUMMARY

The M5100 SFRA (Sweep Frequency Response Analyzer)is the latest addition to the family of M-Series products thatprovides test data for complete transformer conditionassessment.

The M5100 SFRA Instrument is a compact, rugged,field-portable system, which consists of a laptop computer,application-specific hardware and a Windows98ΤΜ

operating system. When applied to power and distributiontransformers, it detects mechanical failure ormovement of windings caused by system events. Inaddition, transporting of mobile units, or new/rewoundtransformers can result in the same types of failures.

There is a direct correlation between the electrical RLC(Resistance Inductance Capacitance) networkrepresentation of a transformer and its physicalconstruction: the tank, core, and windings. Series/parallelcombinations of resistance, inductance, and capacitancecan accurately represent the layout of windings, the core,and the tank. Any mechanical movement of these compo-nents can effect the RLC network.

Identifying the RLC network and its associated frequency-dependent transfer function can be accomplished by usingthe SFRA method. Changes in the impedance networkand transfer function can be useful in detecting andidentifying a wide range of problems.

The M5100 SFRA Instrument can detect the following:

· Core grounds

· Core movement

· Shorted turns or open windings

· Axial deformation of windings

· Hoop buckling of windings

· Partial winding collapse

· Broken or loose clamping structures for

winding/core

M5100 SFRA Instrument

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TEST SETUP AND CONNECTIONS

The M5100 SFRA Instrument performs off-line testingonly. Therefore, its test connections are made to atransformer that has been safely grounded and isolatedfrom the power system.

The above figure shows the test connections for a typicaltransformer under test. Three cables are connected fromthe instrument to the bushings of the winding under test, H1and H3. The output of the instrument, a variable frequencyexcitation source, is connected to the H1 bushing, alongwith the reference cable, by a single termination clamp.A measurement cable, the third connection, is clamped onthe return side of the winding's bushing (H3) to measureany attenuation or phase shift, as referenced to H1.

The M5100 SFRA Instrument outputs a 10V peak to peakwaveform that will vary in frequency, at a controlled rate,from a range of 10 Hz to 10 MHz in five selectable bands.The transfer function models the distributive RLC networkof the transformer and determines its frequency-dependentresponse. The M5100 SFRA Instrument compares thesignal into the specimen to the signal output, and calculatesattenuation and phase shift for all frequencies.

A test for each of the windings can be performed andcompared, from winding to winding or from simulartransformers, to provide a reference for analyzing the testresults.

FULLY AUTOMATED CONTROL

The M5100 SFRA Instrument performs fully automatedtesting. The Instrument prompts the operator to fill ininformation about the transformer under test. The operatorinitiates the test by selecting the START TEST button andidentifying the test by name. The operator can observe thetest in progress by such indications as:

· Plotting of the test data points in attenuation (dB) and frequency

· Test in progress bar graph

· "STATUS" that alerts the operator to an abnormal occurrence: for example, when no measurementinput is detected and caused by a faulty connection

· "CURRENT FREQUENCY" and "RESPONSE" at anypoint in the test.

When the test data is complete, the Frequency Responsedata, is plotted graphically. Tools for zooming, dualcursors, simultaneous band viewing, linear/log plotting anddata viewing allow you to investigate and analyze theresults consisting of magnitude and phase.

M5100 SFRA Instrument Test Connections

Zoomed Graph with a Cursor Enabled

Typical Sub Band Response Graph

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CONTACT US

Learn more about how the Doble M5100 SFRA Instrument can help optimize your operations. Contact your ClientService Engineer or local representative for details about purchasing or adding the M5100 SFRA Instrument to yourEquipment and Service Contract, or discuss the Transformer Diagnostic Test Data Set approach to power transformercondition assessment and our other M-Series diagnostic instruments.

VISIT OUR WEB SITE OR CALL US

Doble Engineering Company85 Walnut StreetWatertown, MA 02472-9107 USA, +1 617.926.4900, Fax: +1 617.926.0528Email: M5100@doble.com Web site - www.doble.com

DEMONSTRATED VALUE

Doble Engineering has focused on condition assessment of substation apparatus through assessment strategies,instruments and techniques that have helped electrical power companies succeed since the 1920s.

M5100 SFRA Instrument’s Control Screen Showing Test Results