ULTRASONIC OR TEV? - hvds.co.nz

1 104 PT&D February/March ORTEV?By Greg Linton, HV Diagnostic Services LtdFigure 1 ULTRASONIC CASE STUDYA routine substation survey in August2004 located a discharge of 20% FSDfrom the breather of a CT chamber. Thisactivity, originally measured at 12% in2002, was definitely on the increaseand the deterioration is typicallyexponential-like as the discharge gainsmomentum. The TEV readings quiterightly appeared normal and wereunchanged from previous surveys. Aninspection under a scheduled outagediscovered discharge surrounding anearthing shim between layers of gasketmaterial of the CT (see figure 1). Thefaulty component was replaced and asubsequent re-test has shown theinstallation free of any CASE STUDYThe annual visit to a Waikato Industrial site towards the end of 2004revealed an unexpected increase in levels to one substation over theprevious 12 months.

2 The highest reading was recorded on the unusedbusbar end cap of an oil filled fuse switch (see figure 2). Although thebackground level was unchanged,indicating it was not external noise, thedischarge was affecting all readings onthe switchboard (see figure 3). Thedecision was made to replace theoffending switch in a controlled mannerprior to the beginning of their busyseason. An investigation by the switchmanufacturer confirmed that theyfound the likely source of the dischargearound the teflon washers installed onthe end of the buss connectors underthe red cover identified. The substationwas again surveyed in Feb 2005 withthe readings returning to the levelspreviously Discharge Testing is a well established systemfor measuring the insulation performance insidehigh voltage Switchgear. However, for it to be ofmost benefit, the testing must be performed usingcomplementary techniques, otherwise you are not beingpresented with all the facts.

3 The term Partial Discharge is somewhat generic andits most commonly recognised form is Corona is often heard from overhead structures, particularlywhen atmospheric conditions suit, with the resultingdamage over time to components well known DISCHARGE This Corona can also be described as Ultrasonicdischarge because of the frequency range it occupies andcan be identified early on, using a suitably tuned detector,well before any audible signal allows human ears to hear Switchgear these surface discharges generallyoccur due to differences or interruptions in the electricalstress surrounding an insulating component and canresult from inadequate clearances, sharp metal cornersand edges, or simply dirt and debris. As an airbornesignal, a clear path is required between the discharge andtransducer to allow transmission and detection of thesignal. This limits its use to air insulated components only,the typical areas being dry termination boxes and theswitchtank bushings, shutters and spouts.

4 It is also wellsuited for the online testing of MSU s and other modularepoxy insulated ring main EARTH VOLTAGEBy contrast, the Transient Earth Voltage (TEV) methoddeveloped by EA Technology of the UK, allows activity tobe located through any insulating medium provided thereis a break in the metal skin between cabinets and formation of a fast moving electromagneticwaveform at the insulation breakdown and its associatedcapacitively transferred charge can be detected on theswitchgears outer surface. TEV will therefore find defectswithin oil and pitch filled components and would seem tobe the only method , very rarely is the same defect identified byboth methods. This is due to the fundamental differencesbetween the types of discharge. ULTRASONIC is almostalways between phases or off into the air, whichincidentally can become ionized providing the perfectenvironment for discharge to continue.

5 TEV indicates aleakage to earth through voids or damage to theinsulation and it is this breakdown that is revealed by therange of instruments developed by EA is the difference and why do weuse both ULTRASONIC and TEV whenlocating Partial Discharge? This articlewill serve to explain the differencesbetween the two methods in widestuse round the world Early detection of developing faults and timely repair does improve network reliability and is a cost effective way to manage maintenance. Both TEV and ULTRASONIC measurements need to be made to truly assess the condition of HV Switchgear. PD testing is not just for aged gear. Benchmark surveys soon after commissioning provide confidence now and allow meaningful comparison in the future. The importance of an annual test program is highlighted above, not only to identify significant changes but to allow deferment of works onminor problems to when they are actually 2 Figure 32004 Levels2003 ReadingsAvailable services include:Non-intrusive Switchgear Surveys & Monitoring utilising TEV and ULTRASONIC detection & termination inspections and Cable Mapping & VLF PILC Cable condition assessment and/or new XLPE circuit acceptance and support from EA Technology, UK with access to innovative products and services for HV asset owners and the Electrical Supply details on website Diagnostic Services Ltd50 Disraeli St (PO Box 33078) Christchurch, New ZealandPhone 64 3 962 0225 Fax 64 3 366 0680 Mobile 021 663 491 Email Website DISCHARGEAll you really need to the proof!

6 Dedicated to the prevention of unplannedoutages through Online Partial Discharge Location and Greg Linton for further Partial Discharge since 1998,no-one s been doing it longer!