ML12338A545

Official Exhibit - ENT000245-00-BD01 - Excerpt of Chapter 3: Design and Manufacture of Extruded Solid-Dielectric Power Distribution Cables

ML12338A545
Person / Time
Site:	Indian Point
Issue date:	03/29/2012
From:	- No Known Affiliation
To:	Atomic Safety and Licensing Board Panel
SECY RAS
References
RAS 22115, 50-247-LR, 50-286-LR, ASLBP 07-858-03-LR-BD01
Download: ML12338A545 (2)

Text

United States Nuclear Regulatory Commission Official Hearing Exhibit Entergy Nuclear Operations, Inc.

In the Matter of:

(Indian Point Nuclear Generating Units 2 and 3)

ASLBP #: 07-858-03-LR-BD01 Docket #: 05000247 l 05000286 Exhibit #: ENT000245-00-BD01 Identified: 10/15/2012 Admitted: 10/15/2012 Withdrawn:

Rejected: Stricken:

Other:

ENT000245 Submitted: March 29, 2012 194 Chapter 3 Design and Manufacture of Extruded Solid-Dielectric Power Distribution Cables lead to dielectric failure . Reports of cable failures suspected to be caused by water trees about the time of the original stud y by Vahlstrom [39] also originated in Japan [42].

These unexpected discoveries of possible unsatisfactory service life of polyethylene and crosslinked polyethylene power cables were disturbing to the power utilities and the cable industry and led to urgent studies of the problem in many countries including Canada [43- 46]. Most of the work has been carried out on HMWPE and unfilled XLPE. The quantities of EPR and filled XLPE cables in underground distribution are relatively small; furthermore the opaque nature of these materials makes the detec-tion of trees difficult. Trees can be observed easily in XLPE and HMWPE cables because these insulation s are translucent. Water treeing of dielectrics is discussed and illustrated in Chapter 2. Water trees are apparent by their characteristic dark appear-ance when contrasted to the translucent, thin wafers cut from the cable insulation.

Because of the lack of field evidence, it is not possible to conclude that EPR cables have failure rates comparable to HMWPE and XLPE. It has been reported that the treeing susceptibility of EPR cables is about equal to that in XLPE and less th an that in HMWPE [47].

Water trees , sometimes called electrochemical trees , have basic characteristics dif-ferent than electrical trees. Electrical trees are characterized by the occurrence of partial discharge, require high electric stress to initiate and rapidl y lead to catastrophic dielec-tric failure. Water trees can be initiated at much lower dielectric stress, grow very slowly, are associated with no measurable partial discharge, and may completely bridge the insulation from conductor to shield without dielectric breakdown, altho ugh the dielectric strength is much reduced , in particlar the direct current (dc) breakdown value [48]. Electric treeing, a well -known phenomenon since the early days of electrical engineering, occurs in poorly designed or overstressed insulation systems. The mechan-ism of failure is kno wn and understood. Water treeing is a process studied intensively since circa 1970, but the incepti on and growth of the treelike structure has no univer-sally agreed theoretica I basis. Th e Ii(era t ure on wa tel' treeing is large because the inves-tigations, although only encompassing a short time period, are intensive. Bernstein [49]

in his review of watn treeing theor y. gi\es the major requirements and factors influen-cing the growth but suggests that the mechanism of inception is not known. It is accepted that two fundamental co ndition s are required: (i) a polar liquid , usually water, must be present and (ii) vol tage stress. Electrical trees require only voltage stress.

Other factors , listed in no pa It icula I' order of importance, have been enumerated by Bernstein [49]. These are aging time, material nature, contaminants/impurities, tem-perature, temperature gradien L eLI ble design, magni tude of operating voltage stress , test frequency , antioxidant, voltage stab ili ze rs, water nature , and semiconducting layer type.

It has been shown that water in the interstices of the stranded conductor greatly enhances the tree growth even when the cable is immersed in water, particularly when a temperature gradient exists in the insulation [50]. Badher et al. proposed a physical model of aging in polymeric cables [51] and later proposed short - and long-term elec-trical tests based on the model [52]. Lyle and Kirkland reported on an accelerated aging test procedure for the growth of water trees and determination of cable life when subjected to various test conditions [53] . The importance of water in the strand and increased temperatures were again demonstrated.

Although the problem of water treeing has not been elimina ted, manufacturers are improving processing technology to improve the service life. Cables manufactured

uti on Cables Section 3.5 Solid-Dielectric Insulation Techniques 195 wa ter trees today are improved over those that exhibited early failures circa 1970. Tape strand Japan [42]. shielding is now unacceptable. The number of voids, protrusions , and contaminants hylene and have been significantl y reduced. Tree-retardant XLPE insulation compounds are now les and the widely used. Some experts believe that voltage stress should be reduced even though this s including action would increase the first cost of the cable. Hermetically sealed sheaths have been l1d unfilled developed [54], although these designs might be econom ically attractive only for trans-istribution mission voltages. Experience indicates that such sheaths provide good service lives for s the detec- cables rated at 138 kV and higher. The great importance of keeping water out of the VPE cables strand is now accepted , and several manufacturers now offer a strand filler to prevent cussed and water ingress. Modern cable design has improved markedly over the last decade, but a uk appear- final so lution to water treeing and premature failure in wet environments has not yet insulation. been assured. Figure 3.12 illustrates the rising trend of fai lure rates. Recent field inves-EPR cables tigations [5 5] led to the conclusions that failures in XLPE begin to intensify after la- IS ed that the years of service and that water treeing and internal defects (inclusions) we re the most han that in often iden tified problems . Unfortunately, there is an unavoidable time lag between possible effective corrective measures and corroborative evidence from service failure eristics dif- rates.

e of partial phic dielec-3.5 SOLID-DIELECTRIC INSULATION TECHNIQUES grow very

tely bridge Since the 1950s there ha ve been great advances in the techniques of insulating medium-though the voltage solid-d ielect ri c cab les. Formerly the insulations were the thermosetting* com-breakdown pounds based on butyl rubber or the thermoplastic compound , polyethylene. The butyl

)f electrical rubber insulant wa s applied by an extruder, called a tuber. The vu lcan ization of the le mechan-insulation was a separate operation in an open steam vessel or autoclave.

in tensi vel y Semiconducting fabr ic tapes were utilized for conductor and insulation shielding. In no UI1lver-

the in ves-

~nstein [49] ~ 20.0

)rs influen - '"

0)

10.0 1 ~/ 1

'~3 own. It is 0.

~ 5.0 id, usually tage stress . o E

' '1:'

7-1/~ 17' o

lerated by

• ities, tem- ~ 1.0

%~;-:

'J>

stress, test ~ 0.5

-/-

ting layer ~

o - t>-~r-;t

-"+-0 a ,~_

L/- f -

~/~1 ,-+-=-\~!

~

or greatly E" 0.1 Figure 3.12 Failul"C r" tcs or po lymeric power r1 y when a cable rated 5- 35 kV: ' Irter Thue [4 6]: (a) and Z 0.05 a physical (b). Average failure r;t te I*o r ,til c"blcs o perat- 1964 1966 1968 1970 1972 1974 1976 1978 term elec- ing in a particular ye' lr (eI) XLPE 'Illd (b) PE. Curves a and b (c) Failure I*ate vel*s u, number o i" years in ser- 2 4 6 8 10 12 14 16 ated aging vice: XLPE + PE. Year in service (curve c) life when trand and

• A thcrlll osC I i, d ef ined as a m a teri a l, cured under hea t. that docs no t soften when rehea ted. Thi s definition is not stric*tl l" truc fo r cro sslin ked pol ye th ylene because it docs soften and ha s lo wer ph ysica l

turers are strength tl1<ln Ill o, t \ ulclI1i/ed rubbers a t high temperatures . H owever. for practical cable application s. th e tUfactured definiti o n is , uit ahk .