Specifications subject to change without notice. Copyright © 2013 MISTRAS Group, Inc. All Rights Reserved. www.mistrasgroup.com

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#300A-13109-02

Services Division Application Brief

Power transformers account for the largest portion of capital investment in transmission and distribution substations. Failure of a single power transformer carries a multimillion dollar impact with a build or replacement timeline of 18-24 months. Yet 53 percent of transformer failures are preventable. But what if there was a way to protect that substantial investment while safeguarding against catastrophic failure?

MISTRAS offers that answer with diagnostic and monitoring of power transformers with Acoustic Emission (AE) technology.

AE monitoring is a well-known, accepted tech-nique to detect and locate AE signals in power transformers – it’s the Silver Bullet for protecting critical capital investments by helping prevent millions of dollars in potential losses. The value of protection for system critical hardware like power transformers cannot be overstated.

Do you have a gassing transformer?Using AE diagnostic and monitoring techniques on power transformers has a host of useful and versatile functions. These functions can be utilized while keeping power transformers on-line, therefore preventing a costly outage and saving customers lost revenue. Also, AE can not only provide an accurate assessment of your equipment’s condition during on-line testing, but also can monitor your transformer during special operating conditions like overload, solar storms, commissioning, etc. It can even determine under which operating conditions a fault is active.

you Can use aCoustiC emission for aLL these funCtions anD more!!!

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 two and three- dimensional plot • Can be used on manufacturer facilities or repair/refurbishment shops to locate a defect when detected by electric methods• The performance of this technique is enhanced when used in conjunction with Dissolved Gas Analysis (DGA).

ae DeteCts:• Partial discharge• Arcing• Hot spots• Loose connections• Static electrification in GSU Transformers• Core clamping problems• Mechanical defects

Other parameters (load current, vibration, gas monitoring data, pump current, temperature, and load tap changer motor current) are acquired along with Acoustic Emission data in order to correlate this information with the operating conditions on the transformer during the test/monitoring. A grading system provides information on the intensity of the fault. All results are compared to a database of over 1000 tests.

Acoustic Emission Diagnostic and Monitoring of Power Transformers

Benefits of ae monitoring of PowEr TrAnsforMErs GEnErATor sTEP uP (Gsu) GEnErATion siDE• Prevent plant or mill shutdown• Eliminate need for on-site spare unit• Avoid capital expenses in the millions• Prevent loss of revenue• Avoid negative profit impact• Prevent EPA fugitive emissions due to leaking oil

transmission• Eliminate need for on-site spare unit• Avoid capital expenses in the millions• Avoid negative profit impact• Prevent EPA fugitive emissions due to leaking oil• Increase in customer satisfaction• Avoid operating capacity reduction on remaining transformers• Maintain dispatch flexibility• Offers vandalism protection

DistriBution Increased customer satisfaction by preventing:• Unplanned outages• Negative effects on commercial and residential customers• Negative public relations• Loss of revenue and profit• Legal action• Fines

Specifications subject to change without notice. Copyright © 2013 MISTRAS Group, Inc. All Rights Reserved. www.mistrasgroup.com

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www.mistrasgroup.com/substationsWORLDWIDE HEADQUARTERS:195 Clarksville Rd • Princeton Jct, NJ 08550 • USAT: +1.609.716.4000 • F: +1.609.716.0706E-MAIL: sales@mistrasgroup.com

#300A-13109-02

Acoustic Emission Diagnostic & Monitoring of Power Transformers

RIV and electric PD indicated partial discharges in Phases A and C during an induced voltage test. However, AE test results detected and located signals in Phase B coil. Internal inspection revealed discolored insulation in low voltage (LV) and load tap changer (LTC) leads of Phase B. The same discoloration was found in LV and LTC leads of Phase A, but not in the same degree as Phase B.

“Locating and assessing faults in power transformers,” T&D World, June 2004.CoMMErCiAl iMPACT:Accurate location of the fault allowed the repair facility a timely repair, returning the unit to service immediately.

transformer, 85 mva, 230/13.8 kv, Core form.

Testing done in first 36 hours after commissioning to detect PD/Arcing due to static electrification. A sister unit failed catastrophically two years before, minutes after commissioning due to this phenomena. Discharges resulting from static electrification were detected and located. “Reduction of failure risk in power transformers through the detection and location of incipient faults

using AE”, TECHCON 2003.CoMMErCiAl iMPACT:Although static electrification was detected and located, it was determined to be minor, which allowed the transformer to remain in operation. The data collected is now a baseline for this transformer design.

One of three sister units gassed. DGA indicated partial discharge. AE test showed two areas of strong activity, indicative of severe fault. Investigation and urgent follow-up was recommended. “IEEE C57-127 Std. IEEE Guide for detection and location of Acoustic Emissions from partial discharges in oil immersed power transformer and reactors, case study 6” August 2007.

CoMMErCiAl iMPACT:Unit remained operative since it was still under warranty. Subsequently, transformer failed two months later leading to outage. Emergency spare unit was installed — with all the costs of a major replacement and installation.

Sister unit of the previous case. This unit was not gassing and no acoustic activity was detected in the area where the acoustic source was detected for its sister unit. “Detection, location, assessment and classification of faults in energized power transformers using acoustic emission.” CIGRE Transformer Colloquium, 2003.

CoMMErCiAl iMPACT:Baseline data from a non-gassing transformer was obtained, facilitating the identification of active sources when the same unit (or a transformer of the same design) is tested in the future.

AE test was performed on heavy gassing unit. Detection and location of one high-amplitude source was obtained close to the top of the core and a low-voltage bar. Acoustic activity was particularly intense just before second group of pumps operated and diminished a few minutes after both cooling groups were running. This behavior indicated the existence of a thermal problem

corresponding with diagnosis obtained by DGA. Internal inspection revealed overheated bus bar that was repaired on-site.CoMMErCiAl iMPACT:The plant estimated $3 million savings since AE monitoring reduced necessary outage to one week rather than the month necessary for a manufacturer repair.

A high-temperature fault was diagnosed after an increase in combustible gases. Continuous monitoring installed April 2008. Removal from service planned in July 2008 after combustible gas increase, but AE test confirmed increase in activity during gassing, but also indicated a significant decrease in activity afterward, transformer continued in operation. “Visher Ferry T1 34.5/2.4 kV

6 MVA GSU In-Service Condition Assessment and Tear Down Findings 2010”CoMMErCiAl iMPACT:Real-time information from acoustic monitoring system indicated the fault was only active under certain operating conditions, which were modified. The unit stayed in operation for eight months, avoiding $1.3 million of loss of revenue.

6 MVA, 3Φ, 34.5/2.4 kV

Gsu-TrAnsforMEr, 784 MVA, 25/500 kV, sHEll forM.

Gsu-TrAnsforMEr, 784 MVA, 25/500 kV, sHEll forM.

243 MVA, 1Φ, 500/230/13.8 kV

Gsu TrAnsforMEr, 362.5 MVA, 500/24 kV, sHEll forM.

Acoustic Energy Pump Current load Current(A) oil Temperature(Co)

3D Location plot 2D Location plot Correlation with operating parameters