Apparatus Testing in Poor Weathter Conditions 20-25
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APPARATUS TESTING IN POOR WEATHER CONDITIONS APPARATUS TESTING IN POOR WEATHER CONDITIONS SAFETY In general, any outdoor activity in bad weather constitutes a risk and should not be perormed without a thorough risk assessment and appropriate authorization. A severe risk is posed by lightning. Doble Engineering does not recommend testing in any situation where lightning is a present or an imminent threat. equipment status and motivation for testing Beore we start testing, are there reasons to suspect the equipment may be in poor condition? I not – can the testing be deerred? I there are, then the motivation or testing is based on suspicions in regard to the equipment condition, and we must use that knowledge to guide us toward the tests to be perormed and subsequent interpretation o test r esults. Poor weather will affect results through temperature and through contamination o the test object with water, ice, snow, etc. Lower temperatures will affect the inductance, capacitance, and resistance directly, but will also lead to water crystallization in dielectric material. Te ice crystals will affect results and may lead to erroneous conclusions. In situations where the equipment, say a power transormer, has recently been switched out o service, it is likely still warm; thereore, testing should be planned to take place beore it has significantly cooled down. As long as appropriate saety measures are taken, valid test results should be obtained. individual test expectations Different tests will be affected by poor weather in different ways. Power Factor – Surace contamination and low temperatures may affect both the current and watts losses, giving either overly pessimistic or overly optimistic power-actor results. Care should be taken to include test guards and in interpretation o results in low temperatures. Capacitance and DC Resistance – Tese tests should not be significantly affected by temperature or surace contamination, but this will depend on the level o contamination and the temperature change. DC resistance should decrease with lower temperatures – but the effect may be masked through connection and contact resistances and appropriate correction curves should be used. Sweep Frequency Response Analysis – Tis test relies on inductance-capacitance combinations to produce resonances to a low-voltage ac signal. I mpulse tests used to pr oduce a requency response may be very susceptible to temperature and surace contamination issues while sweep systems are relatively immune, unless the surace contamination is extreme. Leakage Reactance – Tis is a high current test which may be significantly affected by surace contamination but much less affected by temperature variations. case 1: power transformer test in freezing conditions Tree high voltage General Electric type U bushings were tested in a large, oil-filled, power transormer during a winter storm that had snow, ice, and reezing rain as part o the weather conditions. Te transormer was tested with the oil temperature well belo w reezing. Te three bushing temperatures were also below reezing when tested. APPARA TUS TESTING IN POOR WEATHER CONDITIONS COVER STORY 20 • FALL 2011 BY TONY MCGRAIL, Doble Engineering NETA WORLD • 21 COVER STORY Apparatus testing in po or and/or extreme weather conditions presents several challenges – access to equipment, safety, and setting up the equipment under test so as to provide useful results. Tis paper looks at some individual case studies which demonstrate the precautions needed and the “Do’s and Don’ts” of such testing.
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APPARATUS TESTING IN
POOR WEATHERCONDITIONS
aPParatUs testinG in Poor Weather conditions
SAfety
In enera, an otoor actiit in ba weather constittes a risk an sho not be perorme withot a thoroh risk assessment an appropriate athorization. A seere risk is pose b ihtnin.dobe Enineerin oes not recommen testin in an sitation where ihtnin is a present or animminent threat.
equipment status and motivation for testing
Beore we start testin, are there reasons to sspect the eqipment ma be in poor conition? I not– can the testin be eerre? I there are, then the motiation or testin is base on sspicions in
rear to the eqipment conition, an we mst se that knowee to ie s towar the tests to be perorme an sbseqent interpretation o test rests.
Poor weather wi aect rests throh temperatre an throh contamination o the test object with water, ice, snow, etc. lower temperatres wi aect the inctance, capacitance, an resistanceirect, bt wi aso ea to water crstaization in ieectric materia. Te ice crstas wi aectrests an ma ea to erroneos concsions.
In sitations where the eqipment, sa a power transormer, has recent been switche ot o serice,it is ike sti warm; thereore, testin sho be panne to take pace beore it has sinicant cooe own. As on as appropriate saet measres are taken, ai test rests sho be obtaine.
individual test expectations
dierent tests wi be aecte b poor weather in ierent was.
Power Factor – Srace contamination an ow temperatres ma aect both the crrent an wattsosses, iin either oer pessimistic or oer optimistic power-actor rests. Care sho be taken
to ince test ars an in interpretation o rests in ow temperatres.
Capacitance and DC Resistance – Tese tests sho not be sinicant aecte b temperatreor srace contamination, bt this wi epen on the ee o contamination an the temperatrechane. dC resistance sho ecrease with ower temperatres – bt the eect ma be maskethroh connection an contact resistances an appropriate correction cres sho be se.
Sweep Frequency Response Analysis – Tis test reies on inctance-capacitance combinations to proce resonances to a ow-otae ac sina. Impse tests se to proce a reqenc response ma be er ssceptibe to temperatre an srace contamination isses whie sweep sstems are reatie immne, ness the srace contamination is extreme.
Leakage Reactance – Tis is a hih crrent test which ma be sinicant aecte b sracecontamination bt mch ess aecte b temperatre ariations.
case 1: power transformer test in freezing conditions
Tree hih otae genera Eectric tpe u bshins were teste in a are, oi-e, power transormerrin a winter storm that ha snow, ice, an reezin rain as part o the weather conitions.Te transormer was teste with the oi temperatre we beow reezin. Te three bshin temperatres were aso beow reezin when teste.
aPParatUs testinG in Poor Weather conditions
COVER STORY
20 • FALL 2011
By tony MCGraIl, Doble Engineering
NETA WO
COVER
Apparatus testing in poor and/or extreme weather conditions presents several challenges –
access to equipment, safety, and setting up theequipment under test so as to provide useful
results. Tis paper looks at some individual casestudies which demonstrate the precautions needed
and the “Do’s and Don’ts” of such testing.
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Both hih-otae an ow-otae winin power actors (CH an Cl) are hiher than norma, whie the interwinin power actor (CHl) is acceptabe. Tis tpe o ariation is an inication o possibe srace contamination eects as the interwinin insation test oes not ince ron path osses as part o the measrement.
Te same transormer was teste ater on the same a. Wins were now oshore, bshins wereceane an rie, an the sn ha warme the porceain which hepe to remoe an remnantmoistre accmate beore retestin the bshins. est rests are ien in abe 4.
uner the same test reime, the mA rests hae not chane reat, bt the watts oss rests haerece sbstantia, refectin the srace contamination impact. CHl has not chane sinicant,bt the aes o CH an CHl hae come within expecte ranes or a new transormer anare acceptabe.
Te rin o bshins has proce acceptabe CH an Cl aes, as expecte.
case 3: power transformer le akage reactance and
sfra in freezing conditions
In this case a 230/72 kv 50/67/83 MvA power transormer ha come ot o serice aer a ow- otae bshin aire. Te bshin was repace an the transormer ceane interna. A rane o tests was perorme to ensre the interit o the transormer ha not be en compromise prior to re-
enerization. leakae reactance rests ha not been taken or the nit since actor tests, an sweepreqenc rests were taken or the rst time. Te transormer is pictre in Fire 1.
COVER
aPParatUs testinG in Poor Weather conditions
Te test rests or the transormer’s power-actor oera winin tests were not acceptabe orserice, with power-actor aes reater than 1.0 percent or ow-otae an hih-otae winins(Cl & CH). Bshin power-actor aes, the C1 test as ien in abe 1, were aso hiher thanacceptabe or in-serice bshins o this tpe.
Inerte C1 test aes or the bshins were comparabe to the stanar C1 test aes which inicatethe hih C1 power actor-aes were attribte to the bshin’s interna main core insation annot bein infence neatie b exterior conitions. Sch testin is er se to rther eterminethe impact o srace contaminants on rests.
Te transormer was warme p an a bshin sraces were rie. Te power-actor tests were perorme a secon time an the transormer oera power actor improe sinicant.Te bshin C1 test aes remaine at the hiher than acceptabe test ae that was obtainein the preios test. Te bshins were tente, an heat was appie to warm the bshins aboereezin. Aer seera hors o heatin, the bshins were aain teste an a o the C1 aninerte C1 power-actor aes were within 0. 1 percent o the namepate ae, as shown in abe 2.
Te rests aboe reezin are sinicant improe an emonstrate the ssceptibiit o sch testrests to temperatre. Interpretation o sch rests reqires an nerstanin o the impact o srace contaminants an o temperatre on the test process an the rests themsees.
Bshin temperatre sties perorme b dobe hae shown that gE, tpe u bshin C1 power-actor ae is infence b temperatre. When this tpe o bshin is teste beow reezin, the power actor aes increase an ee o to an ee ate ae aboe the recore namepate ae.Tis chane in power actor with temperatre was seen in both a known oo bshin an a knowneectie bshin.
dobe Enineerin proies temperatre correction tabes which bein at 0°C.
case 2: power transformer tests in high humiditY
In this case, a power transormer was teste in hih hmiit, oer 94 percent, with onshore oceanbreezes simtaneos brinin aeroso moistre to the nit. Oera test rests are ien in abe 3.
COVER STORY
aPParatUs testinG in Poor Weather conditions
table 1:
Power Factors Measured in Freezing Conditions
table 3:
Test Results inHigh Humidity,
Damp Conditions
tabl
Test R
and D
table 2:
Power Factors Measured in Normal Conditions
22 • FALL 2011 NETA WO
figu
Transfin Free
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NETA WOaPParatUs testinG in Poor Weather conditions 24 • FALL 2011
recommended practices
When power-actor testin in sbzero conitions, water or moistre in the apparats co conensean reeze. Tis wi ater the oera ieectric conition, aectin the test rests, tpica recin the watts oss, an sbseqent iein ower power-actor aes. Tis ma ie the ase impressiono an improement in the oera conition o the insation sstem! Care sho be taken whentestin in reezin conitions to make sre the context an interpretation o rests are cearan nerstoo.
Can power-actor tests be perorme on apparats which is at a temperatre aboe reezin bt whenthe ambient is beow reezin ? yes! Aain, care mst be taken an there are correction cres aaiabeto reerence test rests to stanar temperatres.
Tere are cear risks when testin in rain or snow that are associate with saet. Te inceeectroction an sip an trip hazars. Te ariation in srace eakae impeances wi mean that power-actor an reate measrements wi be compromise as watts oss wi rise as the ron pathcapacit increases. Hih hmiit ma hae the same eect. Howeer, sweep reqenc responseanasis measrements an c resistance measrements sho be reatie naecte an ma be perorme ness the srace contamination ies a short circit across the srace. Aition o acoar to the testin reime ma ie rther inormation abot the state o the insation an thsai in ianoses.
coNcluSioNS
It is important when testin in poor or naorabe weather to nerstan both the motiation ortest an the ike impact o the aerse weather on acta test rests. In terms o motiation, we mstconsier what we expect to n – are there reasons to sspect the interit o the apparats nertest? In terms o interpretation o rests, we mst consier the iniia test mae an the ike impact o aerse conitions on those rests an ths the interpretation o those rests.
ac kn ow le dg em en tsTe athor wo ike to thank his coeaes at dobe Enineerin, incin lon Pon, ManeSameron an Marsha re, or their thoht comments an contribtions..
COVER
With appropriate precations, the oowin tests were perorme:
•BushingC1andC2andpower-factormeasurements •Windingpowerfactorandcapacitance •HVsingle-phaseexcitation •DCwindingresistance •Turnsratio •Sweepfrequencyresponseanalysis •Leakagereactance
Eerthin was acceptabe except or anomaos eakae reactance measrements which inicate a ariation that was not seen in the sweep reqenc response anasis measrements. leakae reactanceresponses which ince the tertiar winin showe an anoma, inicatin ariation on one phase.
B ensrin that the test object ha been sitab ceane an prepare, there was scient spportor the interit o the measrements or a case o anoma to be on within the transormer.Tis was eenta trace to two C shortin bocks which ha both, in act, not been shorte an which ths showe as extra bren on the eakae reactance rests.
In this case oo test practices aowe or reiabe rests an sbseqent anasis which ientie thecase o the anoma within the transormer.
aPParatUs testinG in Poor Weather conditions
Tony McGrail is presently the Doble Engineering Solutions Manager or On-Linediagnostics, providing condition, criticality and risk analysis or utility companies. PreviouslyTony has spent over 10 years with National Grid in the UK and the US; he has been both
a substation equipment specialist, with a ocus on power transormers, circuit breakers and integrated condition monitoring, and has also taken on the role o substation asset manager
and distribution asset manager, identiying risks and opportunities or investment in an ageing infastructure. Tony is a Fellow o the IET, a member o the IEEE and the IAM, iscurrently chair o the Doble Client Committee on Asset and Maintenance Management and acontributor to SFRA and other standards at IEEE, IEC and CIGRE. His initial degree was in
Physics, supplemented by an MS and a PhD in EE and an MBA. Tony is an Adjunct Proessor at Worcester Polytechnic Institute, MA, leading courses in power systems analysis.
COVER STORY
FPOPhoto from Magna Feature
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