Source 1Source 1Source 1Source 1Source 1NLTC leads orcore related

Source 2Source 2Source 2Source 2Source 2seriesautotransformer

POWER EQUIPMENT DIVISION

Power transformers represent thelargest portion of capital investmentin transmission and distributionsubstations. The financial conse-quence of losing a single unit canhave a multimillion-dollar impact.Acoustic emission (AE) is a well-known technique that is used todetect and locate acoustic sourcesin power transformers.

Do you have a gassingtransformer?

Would you like to know the location ofthe fault?Without taking the transformer out ofservice?Can�t afford taking the unit(s) out ofservice for inspection and need to knowthe condition of your equipment?Do you want to monitor your trans-former during special operating conditions (overload, solarstorms, commissioning)?You can use AE!!!

Advantages of this technique include:! Applied on-line! Non-invasive! More sensitive than electric methods for on-site tests! Locate the source(s) in a three-dimensional plot! Can be used on manufacturer facilities or repair/

refurbishment shops to locate a defect whendetected by electric methods

! The performance of this technique is enhanced whenused in conjunction with Dissolved Gas Analysis(DGA).

AE detects:! Partial discharge ! Arcing! Hot spots ! Loose connections! Static electrification in ! Core clamping problems

GSU transformers

Other parameters (load current, pump current, tempera-ture, load tap changer motor current) are acquired alongwith acoustic emission data in order to correlate thisinformation with the operating conditions on the trans-former during the test.

Case Histories:1. GSU TRANSFORMER, 362.5

MVA, 500/24 kV, SHELL FORM.This unit was tested during thefirst 36 hours after commissioningto detect PD/Arcing due to staticelectrification. A sister unit failedcatastrophically 2 years before,just minutes after commissioningdue to this phenomena. Dis-charges resulting from staticelectrification were detected andlocated in the lower and upperpart of the high voltage coil.

2. TRANSFORMER, 85 MVA, 230/13.8 kV, CORE FORM. This trans-former was tested during aninduced voltage test in a refur-bishment shop. RIV and electricPD detection techniques indicatedpartial discharges in Phases A and

C. However, acoustic emission test results detectedand located an acoustic source in the lower part ofPhase B coil. Internal inspection revealeddiscolored insulation in low voltage (LV) and loadtap changer (LTC) leads of Phase B. The samediscoloration was found in LV and LTC leads of PhaseA, but not in the same degree as Phase B. Phase Cinsulation did not show any degradation.

Testing Power Transformerswith Acoustic Emission

Power transformer

Three-dimensional plot showing the location of two different sources.

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3. AUTOTRANSFORMER, 243 MVA, 500/230/13.8 kV,SHELL FORM. This unit was tested twice. The firsttest revealed an acoustic source of low amplitudelocated in the middle of the high voltage wall. Thesecond test (one year later) indicated the same sourcein the same location, but with higher-amplitude andhigher-energy characteristics. An internal inspectionperformed in that area indicated that the acousticactivity was generated by degradation on the no-loadtap changer (NLTC) leads.

4. GSU-TRANSFORMER, 784 MVA, 25/500 kV, SHELLFORM. An AE test was performed because this unitwas gassing heavily. The detection and location ofone low-amplitude source was obtained close to thetop of the core and a low voltage bar. Acousticactivity was particularly intense just before thesecond group of pumps operated and diminished afew minutes after both cooling groups were running.This behavior indicated the existence of a thermalproblem that corresponded with the diagnosisobtained by Dissolved Gas Analysis (DGA).

5. GSU-TRANSFORMER, 784 MVA, 25/500 kV, SHELLFORM. Sister unit of the previous case. This unit wasnot gassing and no acoustic activity was detected inthe area where the acoustic source was detected forits sister unit. Find out more about testing transformers with

acoustic emission, call (630) 260-1650 today!

6. TRANSFORMER, 400 MVA, 500/161/13.8 kV, SHELLFORM. Several areas of acoustic activity weredetected in this transformer. After filtering extrane-ous noise and performing the data analysis, oneacoustic source was located at the bottom of the unit,in the core. Internal inspection results located theorigin of this activity: overheating in the connectionbetween ground and core laminations, several inchesaway from the calculated location.

Location of one source in a two-dimensional view.

Load Current, Current of Pumps # 1 and # 2 , and Temperature.

Quality Services Laboratories (QSL-Plus) and its prede-cessor, Physical Acoustics Quality Services (PAQS) havemore than 17 years of experience using acoustic emis-sion for the condition assessment of power transformers.Through the years, a large database has been developedthat allows data comparison between similar designs.QSL-Plus is also an active participant on the ongoingEPRI TC project �Development of a new acoustic emissiontechnique for the detection and location of gassing sources inpower transformers,� intended to improve upon this tech-nique.