JUL 1 4 .1988

NBSIR 88-3774

A Review of Candidate Methods

for Detecting Incipient Defects

Due to Aging of Installed Cables

in Nuclear Power Plants

Frederick I. Mopsik, Edward F. Kelley and Francoise D. Martzloff

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Center for Electronics and Electrical Engineering

Gaithersburg, MD 20899

May 1988

Contract No. NRC-FIN-D1715

Jit Vora, Contract Officer

Prepared for: Nuclear Regulatory Commission

5650 Nicholson Lane South

Rockville, MD 20852

75 Year* Stimulating Amenca s Progress

1913-1988

NBSIR 88-3774

A REVIEW OF CANDIDATE METHODS

FOR DETECTING INCIPIENT DEFECTS

DUE TO AGING OF INSTALLED CABLES

IN NUCLEAR POWER PLANTS

Frederick I. Mopsik, Edward F. Kelley and Francoise D. Martzloff

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Center for Electronics and Electrical Engineering

Gaithersburg, MD 20899

May 1988

Contract No. NRC-FIN-D1715

Jit Vora, Contract Officer

Prepared for: Nuclear Regulatory Commission

5650 Nicholson Lane South

Rockville, MD 20852

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARD, Ernest Ambler, Director
-
A REVIEW OF CANDIDATE METHODS FOR DETECTING INCIPIENT DEFECTS

DUE TO AGING OF INSTALLED CABLES IN NUCLEAR POWER PLANTS

Frederick I. Mopsik, Edward F. Kelley and Frangois D. Martzloff

National Bureau of Standards

Gaithersburg MD 20899

ABSTRACT

Several types of test methods have been proposed for detecting incipient defects

due to aging in cable insulation systems, none offering certainty of detecting all

possible types of defects. Some methods apply direct detection of a defect in the

cable; other methods detect changes in electrical or non-electrical parameters from

which inferences can be drawn on the integrity of the cable. The paper summarizes

the first year of a program conducted at the National Bureau of Standards to assess

the potential of success for in situ detection of incipient defects by the most

promising of these methods.

INTRODUCTION

Among the many components being considered for aging assessment and residual life

extension of nuclear power plants, cables merit consideration because of their

unique role in plant safety systems and support systems. Stresses imposed on the

cables include mechanical stresses at the time of construction and installation,

followed by many years of operational environment conditions. Aging takes place

under the combined effects of humidity, heat, oxidation, and radiation environment

associated with each cable system. And yet, under accident situations, the aged

cables are expected to withstand the mechanical and environmental stresses associated with such an event and to perform their intended function. This situation,

however, has not escaped the attention of system designers, plant operators and

regulators. The detection of incipient defects has become a recognized subject of

study. This paper presents a progress report on some of the work performed at the

National Bureau of Standards (NBS) to assess applicability of known insulation test

methods for in situ detection of incipient defects.*

A review of existing practices and of the literature on insulation evaluation tests

shows that existing facilities depend mainly on the use of cables that have been

qualified to appropriate national codes and standards prior to installation and

operation. There are ongoing programs, however, for assessing aging of installed

cables, but no simple method has been found for performing nondestructive in situ

tests. There are many recognized tests for assessing the condition of a cable

insulation but most of them are destructive and are typically performed in the

laboratory on specimens that include new cables, artificially aged cables, and

naturally aged cables. Tests on artificially aged cables, where the aging is

accelerated by one or more environmental overstresses, are unavoidably subject to

debate on the validity, conservatism, or ambiguity of the acceleration process.

* Performed under the sponsorship of USNRC Nuclear Plant Aging Research Program.
A very important concept for in situ insulation evaluation by nondestructive test

methods is that trends in test results yield much more information than absolute

values obtained from single tests. Therefore, any program of insulation evaluation

is likely to require establishing and updating a data base of the test results.

Thus, in addition to the technical challenge of the test method itself, a significant part of the total challenge will be the logistics of managing the data base.

A further challenge will be the credibility of conclusions based on small changes

observed over a long period of time by tests methods that have yet to be generally

accepted. One aspect of acceptability criteria is that the test methods should be

simple enough to be performed by moderately qualified personnel, with a minimum of

subjective interpretation of the results.

Figure 1 shows a simplified diagram illustrating the range of severity in cable

defects and the types of detection methods that could be applied for detecting such

defects by nondestructive in situ tests. There are two major categories of tests:

(1) tests providing direct detection of single or multiple defects, and (2) tests

revealing a change in the overall dielectric or mechanical characteristics of the

insulation, from which inferences can be derived on the adequacy of the insulation.

The methods reviewed during 1987 by the NBS multidisciplinary team fall in these

two categories and will be briefly described. The plans for further assessment of

promising candidate methods will also be presented, soliciting cooperation of all

parties in beginning the task of correlating test results. From such a cooperative

effort, tests methods could gain acceptability for demonstrating the behavior of

naturally aged cables and an assessment of the residual life of cable systems.

SEVERITY LOCALIZED

OF DEFECTS DEFECTS

(DIRECT)

GENERAL

CONDITION

(INFERENCES)

SUBTLE CLOSED CRACKS

OPEN CRACKS

POLLUTED CRACKS

SHORT CIRCUITS

OBVIOUS OPEN CIRCUITS

t I TDR PD

t OHMMETER

& MEGGER

I

TDS &

MECHANICAL

Figure 1 . Range of defects and test methods

DIRECT DETECTION METHODS

Methods for the direct detection of incipient defects in the insulation are inherently based on tests assessing the electrical characteristics of the insulation and

conductor system. In contrast, methods based on inferences can apply some other

type of test. For instance, a mechanical test can be performed, from which

inferences are drawn on the basis of prior knowledge of correlation between the

observed mechanical behavior and the ultimate electrical quality of the insulation.

The direct test methods reviewed by the NBS team include two basic approaches, each

including two specific implementations. The first approach is time-domain reflectometry; the second is based on the detectable occurrence of partial discharges,

with a possibility of combining the two approaches in a single method.

Time-Domain Ref lectometry (TDR)

The basic concept of TDR consists of launching a fast- transition waveform into one

end of a transmission line while the instantaneous voltage, at the same end of the

line, is displayed as a function of time. Reflections caused by discontinuities in

2
the impedance of the transmission path cause a voltage change which is displayed at

a time corresponding to the round-trip time between the origin and the site of the

discontinuity. For a given velocity of propagation, the round-trip time may be

converted into the distance from the sending end of the line to the impedance

discontinuity

There are several advantages, as well as limitations, to the use of TDR techniques

to determine the presence of damage or morphological changes in the insulation of a cable. The primary advantage of TDR techniques is the ability to determine the

location of the damage without the removal of the cable from its installation in

raceways or conduits. Additionally, since TDR techniques use low-voltage pulses,

the method does not stress the cable being tested and thus avoids inducing damage

resulting in subsequent cable failure. The primary disadvantage appears to be the

lack of discrimination between impedance discontinuities caused by artifacts (such

as splices or connectors in the cable) and significant damage to the cable caused

by abrasion or cracking of the insulation.

In order to understand the capabilities and limitations of TDR techniques to detect

and locate damage in the cables, two approaches were used. The first approach is

experimental, using commercial TDR equipment to identify the practical limitations

of reflectometry on cables containing known defects. The second approach is theoretical, modeling the cable system by computer-aided electronic circuit analysis

methods. From the combination of these approaches, insight was gained into the

following questions:

• What is the smallest impedance change that may be resolved by TDR

methods using equipment with a given noise level and dynamic range?

• What are the limits of the electrical characteristics of the cable,

such as impedance mismatch of a cable to the pulse generator, which

cause attenuation and consequently will affect the defect detection

capability?

• What is the resolution capability of a TDR system when a small

impedance change, representing a defect, is located near a large

impedance change such as is encountered in an electrical connector?

® How far can the fast- transition pulse travel in a cable before its

fast transition is altered, losing its capability for fine defect

resolution?

One of the fundamental limitations of the TDR method for the detection of defects

in cables may be the long-term stability of a TDR system. For TDR to be useful, it

will be necessary to perform differential measurements on cables, that is, measure

the TDR signature of cables over a long period of time to determine the changes

attributable to damage. Since the system measurment accuracy may be a major consideration for the detection and location of defects in a cable, uncertainties

associated with the TDR instrumentation may limit the sensitivity of the method.

Such uncertainties include noise within the instrumentation as well as noise

introduced by the cable. Furthermore, major impedance changes associated with

connectors can mask adjacent changes associated with incipient defects which,

ironically, are most likely to be near cable connections.

For a specific cable, it is possible to use analytical techniques to predict the

characteristics of a defect detectable by TDR techniques. Conversely, modeling a hypothetical defect and comparing the result of TDR simulation with an actual TDR

signature can provide insights for explaining a change in TDR signature over a period of time.

3
A system has been developed by EG&G Idaho, called the Electrical Circuit Characterization And Diagnostic (ECCAD) system. This system includes TDR as well as the

measurement of several other parameters under computer control. The measurements,

however, cannot reveal small changes which are highly localized along the length of

the circuit (such as a single crack or small abrasion in the insulation) . It must

be pointed out that these limitations do not mean that an ECCAD system is of little

value to industry. Most of the advantages have been documented in the literature

[1] , [2] , [3]

the limitations, however, should also be recognized.

Reliable detection of incipient defects will depend upon the sensitivity of the

system and on the physical length and severity of the change of dielectric constant

or loss factor of the insulation. Previous DOE- sponsored tests [4] performed at

NBS demonstrated the limitations of TDR on the detection of isolated defects along

shielded cables. In that study, it was shown that TDR could not be used to find a

breakdown channel in a concentric-cylinder transmission line.

In considering this limitation, it must be remembered that the ECCAD system records

the TDR signature in its entirety, that is, it records the signature for the entire

length of the circuit tested, and at low resolution so that the entire range of the

reflection coefficient can be observed (from -1 to 1). If we assume that the TDR

signature is temporally digitized in 512 points with a 10-bit resolution (0.1%),

then a 15-cm section of cable would represent a 1-ns duration in the TDR signature.

A 100 -m cable would display a TDR signature of about 600 ns in duration. A small

defect of 1 centimeter or less in length, unless inducing greater losses, would

hardly be visible even if it changed the TDR signature by 1%.

To illustrate these concerns, a simple experiment was conducted to compare the TDR

signature for a fully insulated test wire against the signature for the same test

wire after a piece of insulation has been removed. Small changes in the wire position produced dramatic (a few percent) changes in the TDR signature in the vicinity

of the test region. Such a result alerts us to potential difficulties in taking

TDR signatures on unshielded cables. If the unshielded cables were moved, the TDR

signatures would be changed. Thus, if historical TDR data were taken for later

reference, whenever an unshielded cable is moved in a tray of cables, it would be

necessary to make a new record of all the TDR signatures affected by the move.

The conclusion of this review was that there are defects which TDR techniques at

low voltage cannot successfully detect: those which cause small changes in the

insulation dielectric over a small length of the circuit. One possible technique

was then considered: TDR-type signatures obtained with short (less than 100 ns),

high-voltage pulses generating partial discharges and thus making the defect more

detectable. The acronym LEPT (Low Energy Pulse Testing) was coined to emphasize

the low energy aspect of the approach.

Low-Energy Pulse Testing (LEPT)

For the initial tests, it vas expected that pulses with peaks of 2 kV to 10 kV and

widths of less than 100 ns could accomplish the purpose. For reference, it should

be noted that a discharge from a 1-nF capacitor charged at 5 kV is at the threshold

of detection by human fingers — it is a less severe pulse than one experiences by

touching a metal object after walking on a carpet on a dry day. This electrostatic

discharge involves the human body capacitance (0.15 nF) charged as high as 15 kV.

Most non- electronic equipment can survive such a discharge without difficulty.

Exploratory tests with a simple bare wire insulated from a ground plane by a thin

sheet of insulating material produced detectable partial discharges for pulses as

narrow as 30 ns, at amplitudes of 2 kV. However, because the scope of this project

4
does not include the development of new techniques, experiments were not conducted

beyond the exploratory stage. Further exploratory development work could produce

more conclusive results.

Partial Discharges Detection

Detection of partial discharges is a tool that has been widely used in the electrical industry, but the classical techniques have limitations that would make

difficult their application to in situ cable tests. One of the major problems is

the poor signal/noise ratio in environments other than a well-shielded high voltage

laboratory. Another limitation in nondestructive tests is the permissible amount

of discharge, because the discharge phenomenon could be described as consuming part

of the insulation life. Consequently, the voltage at which the tests are conducted

could become an issue.

Two detection methods using signal processing techniques have been described in the

literature [5], [6]. The advantage of the method developed by Weeks and Steiner is

that signal correlation and averaging techniques will allow detection of very small

discharges, even in the presence of substantial background noise, and at relatively

low voltages. We intend to explore these methods to evaluate their capability for

detecting discharges originating from multiple sites in complex cable systems, and

at the lowest possible voltage.

METHODS BASED ON INFERENCES

Mechanical Properties

Changes in the mechanical properties of insulating materials have been associated

with changes in the electrical insulation integrity of cables, forming a useful

body of knowledge which is generally accepted as a valid assessment of a particular

cable [7], [8]. Testing to determine these changes includes the measurement of

parameters such as tensile strength and elongation. In this form, these tests can

only be destructive, and are performed in the laboratory on specimens obtained from

cables removed from service.

A new method that could be applicable for in situ tests has been proposed by Toman

and Gardner [9]. In this method, a test fixture is used that could be installed at

accessible points of installed cables. A fixed anvil and a moving wedge produce a controlled deformation of the insulation for plotting of the stress/strain characteristic of the insulation. According to reported laboratory tests of this method,

a significant correlation has been established between the results of the stress/

strain measurement and the electrical integrity of the insulation. To the extent

that a test limited to the accessible points can be used to assess the overall condition of the cable, this method appears promising.

Dielectric Tests

Measuring changes of insulation resistance is a well - accepted method for assessing

the quality of insulating systems. It has been used in the electrical industry for

many decades, not just limited to cables. Note that it is the change, rather than

the absolute value that is generally considered significant. In some instances,

empirically determined thresholds are also used to signal a process of degradation

beyond which reliability decreases sharply [10]. In addition to the resistance,

generally determined by application of a dc test voltage of 500 or 1000 V, or some

5
other value related to the rated voltage of the equipment, the 'polarization index'

is also determined. This index is the ratio of two insulation resistance values

measured, for instance, after one minute and ten minutes of test voltage application. While highly empirical, this method has been found useful as a quick check

to assess the insulation quality (not to detect single point defects) . Because it

involves the time -varying change of the dielectric material, it might have some of

the elements of the much more sophisticated method of time-domain spectroscopy,

discussed later in this paper.

Another well-known method, sometimes cited disparagingly, is the application of an

ac test voltage at some arbitrary value above the service voltage of the system.

Historically, this method has been enshrined as a 'twice rated plus one thousand'

criterion, but its roots are more a demonstration of adequate clearances than

adequate insulation . The inference is that if the equipment can withstand that

level of overstress, sufficient margin is available to ensure reliable service.

The objection is, of course, that the test can be destructive or, worse yet, pass

the insulation tut leave it in a weakened state compared to its state before the

test. Attempts were made to limit this situation by applying the overvoltage for

brief periods, from which the nickname of 'thumper' was derived. Selecting an

appropriate test voltage is an exercise in brinkmanship between demonstrating

sufficient margin and creating defects in the insulation. Going beyond such

methods was one of the objectives of the NBS assessment program, and it is cited

here only to provide a historical perspective.

Time-Domain Spectroscopy (TDS)

Time-domain spectroscopy is a relatively new technique for determination of the

frequency spectrum of the dielectric loss of a material from its measured time

behavior in response to a step-voltage excitation. The state-of-the-art instrument, developed before the NBS program was initiated, is described in Ref [11].

With this instrument, a precisely defined voltage step is applied to a guarded,

three- terminal insulation specimen and the charge response is measured for time

durations ranging from 10 fis to up to 3000 s. By conversion of the time-domain

signals to frequency domain, the response of the material can be expressed over the

equivalent frequency range. The resolution on loss measurement (tan 5 ) is in the

order of one part per million. This measurement is clearly a lumped circuit method

in which the only geometric parameters involved are those that define a geometric

capacitance. The remarkable sensitivity of the TDS method is illustrated by the

measurements described in the next two sections.

The instrument is capable of detecting almost any relaxation in any polymer that

falls within the instrumental frequency range, including low- loss and high-quality

insulation. Its frequency range is well adapted to relaxations with time scales

that are measured mechanically. The technique is not concerned with particular

features along a cable but rather with the nature of the total cable insulation.

The anticipation is that environmentally- induced changes in the cable insulation

will either lead to relaxations, change them or shift their time scales. Furthermore, we assume that these changes could be used to anticipate future problems.

Specifically, shifting of the loss peak corresponding to the glass - transition of a

polymer would indicate possible embrittlement of the material and potential failure

on flexure. These changes, however, are very material-dependent and their occurrence and effect would have to be established on a case-by-case basis. Thus, it is

important to establish a catalog of materials (cables) to assess the generic applicability of the methods.

Another potential application of TDS is the measurement of the true dc conductivity

of the insulation. Normally, for many polymers, this measurement is very difficult

as dipolar relaxation can easily occur on the time scale of the measurement. This
limitation is a problem with the standard ASTM method of measuring current 1 minute

after application of the voltage. However, a frequency- domain analysis of the loss

spectrum can determine a true conductance, even in the presence of losses.

For installed cables, we recognize several limitations. First, the cable insulation sections are effectively in parallel so that only an average quantity is

measured, not a particular defective section. Second, the cables need a ground

reference definition (i.e., a shield) so that the environment is not significantly

included. Third, the measurement can be conducted only if the cable is disconnected from any parallel load. (The test could, however, be made on a three-phase

cable with a load left connected, applying the test voltage on all three conductors

together, with respect to ground.) Finally, cable length will limit the maximum

frequency obtainable; however, it seems that the most interesting changes occur at

the low-frequency side of the spectrum. Nevertheless, the advantages of the method

appeared attractive enough to motivate several experiments, using the existing NBS

instrument on specimens obtained from other parties involved in the aging assessment program. Two such measurements are reported here.

TDS measurements on naturally aged cables

Three pieces of coaxial cable were obtained from the Shippingport nuclear power

plant. Two samples were obtained from EG&G- Idaho, expected to be a pair of the

same type of cable, one having been irradiated in the harsh environment, the other

as having been removed from the control room and thus not irradiated. Actually,

the samples were of the same type, but from different manufacturers. This difference of manufacturer may leave a question on an observed difference between the two

samples. To avoid this ambiguity, another sample was obtained after a visit to the

Shippingport site. This third sample was taken from the control room, where a more

recent addition had been installed, and was of the same manufacturer as the sample

removed from the containment.

The cable that was exposed to radiation environment could be readily identified

from the others. The outer PVC jacket on the exposed cable had a few cracks

perpendicular to the cable axis and was much stiffer and harder to cut. The mold

marks from the braid were not as prominent, indicating that shrinkage had occurred.

The polyethylene dielectric was yellow, as contrasted to the white color it

normally has in new cable and in the samples that were not exposed to radiation.

The three samples were tested at 30 and 50°C with frequency coverage from 10"^ to

6x10^ Hz. Because of the small sample capacitance and low loss levels, the data

were taken as a four-shot average to minimize noise. At 30°C, the recordings from

all samples were similar, but at 50°C, the irradiated sample clearly showed a difference. The recorded curves for e", the loss index, are shown in Figure 2.

Figure 2. TDS Measurements on naturally aged cable samples

7
For the irradiated cable, there is a clear, rapid increase in e" at frequencies

below 1 Hz. The increase is so large that it is ten times larger at 0.01 Hz than

the range used to display the three results. This increase is due mainly to the

presence of a dc conductivity that is not present in the unirradiated cables. In

contrast, the two different, unirradiated cables show a low, relatively constant e"

with no evidence of a dc conductivity. While there are small, differences between

the two samples, these are clearly distinguished from the larger effects of

radiation.

TDS measurements on artificially aged specimens

Six samples of cross-linked polyolefin insulation were supplied by Sandia National

Laboratories, where accelerated aging tests are under investigation. They were of

the type used for low-voltage power applications and were supplied as tubes 10 cm

long with the 12 AWG center conductor missing. The inner diameter was 0.23 cm and

the outer diameter was 0.39 cm. The tubes came in three colors: red, white and

black. One set was in the original, unaged state while the other set had been aged

at Sandia for 16 days at 120°C followed by 16 days exposure at 65 krad/h, for a

total dose of 25 Mrad. The samples were prepared for measurement by reinserting a

center conductor and applying a conductive layer to the outside surface so that a

cylindrical capacitor was obtained. The capacitance and loss were measured between

the central conductor and the guarded, outer electrode.

The results for the three different color samples are given in Figure 3 which shows

the loss component as a function of frequency measured at 50°C. At 10 kHz, the

measured capacitance was 15 pF, corresponding to a dielectric constant of 2.4. This

result should be contrasted to polyethylene which has a dielectric constant of 2.1.

If one considers the unaged samples, the high dielectric constant indicates that

these samples contain not only colorant, left over cross-linking agent residues,

and the normal additives, but also an inorganic filler. The high loss at low

frequencies also suggest the presence of a filler as the loss seems to be related

to an ionic conduction mechanism. It should also be noted that the colorant did

seem to create small but systematic differences between the samples.

Upon aging with radiation, a great reduction is apparent in the low-frequency loss.

It should be noted that the loss at the lowest frequency for the unaged cables is

several times off-scale. This behavior is a complete reversal from the

Shippingport polyethylene coaxial cables where the effect of radiation was an

increase in the low-frequency loss. However, the loss in these samples after the

decrease is still much greater than that of the coaxial cable. This change in

conductive loss is a very strong diagnostic feature even though, as noted below,

the samples are still at an early stage in the possible service life of this

insulation. A less prominent change is the appearance of new relaxations in the

irradiated material, especially one centered near 1 Hz, which is apparent only in

detailed comparisons of the data, not reported here. While this relaxation is not

strong, it is definitely not present in the original material; it could possibly

grow with further aging and irradiation and may become important near the end of

the service life of the insulation. Only further measurements on more samples can

confirm this conjecture.

The changes in dielectric loss spectra do change quite strongly with aging. While

the correlations with aging are hard to make from these limited measurements, these

changes are much more pronounced than the changes in mechanical properties reported

in the Sandia measurements, indicating great potential for the method. However,

more samples need to be examined, not only to track further aging in this material,

but also to determine the behavior of other insulation systems. At the present

stage of the work, no speculation has been made to explain the difference.
0.75 T

M

0 . 60 -

0.45 -

0.30

0.15 _

CONTROL-WHITE. RED

CONTROL-BLACK

10“3 io-2 0.1 1.0 10

Freq (Hz)

Figure 3. Effect of aging for three samples with different colorants

Developing a data base for TDS correlation

A commercial version of the Time-Domain Dielectric Spectrometer has just become

available, with capabilities fully compatible with the studies reported here. The

package consists of a desk- top computer and an instrumentation system contained in

a standard instrument rack enclosure. The instrument uses a low-voltage output

that is capable of driving loads up to 1000 pF.

Since TDS is a nondestructive technique, it can be used on existing cable systems

during outage and test times at a plant. Another interesting possibility for

application of the TDS measurements is the following: if a specimen of the same

material as the cable of interest were placed in the same environment as the cable

within the containment, it could be connected to the outside by a pair of wellshielded or impervious cables, and characterized by TDS at any time, without

disturbing the power plant operation. Since the TDS test is non- destructive , there

would be no limit on the number of tests made on that specimen, . from which

conclusions could then be drawn on the advancing aging of the adjacent cables. Of

course, appropriate temperature balancing would be necessary. From the experience

of other aging test plans, there might be a question about introducing additional

leads in the containment to provide voltage/current stress on the specimen.

Nevertheless, the benefit from such a test plan might outweigh the objections.

The major expectation from a TDS measurement would be to obtain an indication of

embrittlement, or other form of degradation leading to reduced cable life. This

expectation to quantify aging, to determine margins, and to determine residual

life, in turn, will require establishing a correlation between the embrittlement

and operational quality of the material. This correlation has not yet been

developed, but programs conducted by others are aimed at acquiring end-of-life data

which could then be correlated with measurements performed with time -domain

spectroscopy. Limited initial attempts to interest U.S. utilities in such a cooperative program have not been successful. The EPRI workshop offers an

opportunity to propose this program informally to a greater number of utilities.

The concept has also been discussed with Electricite de France (EdF) and with

Siemens-KWU. We have received some specimens of artificially aged cables from EdF,

where the cables received a complete evaluation using the EdF procedures (which do

not include TDS at this time) . A similar arrangement is under consideration with

Siemens-KWU

9
CONCLUSIONS AND RECOMMENDATIONS

The review of candidate methods, predictably, has not identified a universally

acceptable technique for detecting incipient defects (Figure 4). However, at least

three interesting possibilities exist: time-domain ref lectometry , partial discharge

signal processing, and time-domain spectroscopy. The last two will be further

investigated at NBS . Other methods have not been actively pursued, pending further

development. In particular, the idea of Low Energy Pulse Testing offers some

interesting possibility but would require a major development program. The method

of stress/strain measurements is currently under development by its inventors, and

we are looking forward to further results.

SEVERITY

OF

DEFECTS

SUBTLE

OBVIOUS

CLOSED CRACKS

OPEN CRACKS

MILD AGING

SEVERE AGING TDR &

IMPEDANCE

SHORTS MERS.

OPEN CIRCUIT OHM METER

Figure 4. Summary of types of defects and candidate methods

The correlation of partial discharge signals offers an opportunity to extend a

technology which has been demonstrated in the laboratory to the more difficult

environment of in situ tests at a power plant. A state-of-the-art system will be

assembled at NBS, to continue the evaluation toward that type of environment.

Time-domain spectroscopy promises a very powerful detection method for the overall

aging of cable insulation. The benefits from the sensitivity of this method,

however, cannot be demonstrated until a convincing corre-lation is established

between TDS measurements and insulation integrity. Waiting for the demonstration

to be accomplished before joining a cooperative program is a vicious circle. Thus,

the goal of this paper, in addition to reporting the NBS work, is to present a strong recommendation for establishing, perhaps through EPRI , a cooperative program

for a data base on TDS measurements correlation.

ACKNOWLEDGMENTS

The evaluation and experiments reported in this paper were conducted by a

multidisciplinary team including T.F. Leedy, the authors and with the program

guidance and encouragement of J.P. Vora. Review of the program was also provided

by B . A . Bell, R.E. Hebner, and K.N. Mathes . The program was supported by funding

from the Nuclear Regulatory Commission, Office of Research, under the Nuclear Plant

Aging Research (NPAR) Program.

10
REFERENCES

1. Meyer, L.C. "Nuclear Plant-Aging Research on Reactor Protection Systems,"

NUREG/CR-4740 , EGG-2467, October 1986.

2. Dinsel, M.R. , Donaldson M.R. , and Soberano, F.T. "In Situ Testing of the

Shippingport Atomic Power Station Electrical Circuits," NUREG/CR-2956

,

EGG-2443, April 1987.

3. Meininger, R.D., Soberano, F.T. and Dinsel, M.R. "Three Mile Island

Techniques to Detect Degradation in Electrical measurement and Control

Circuits," Proceedings International Conference on Nuclear Power Plant

Aging, Availability Factor, and Reliability Analysis

San Diego, CA,

July 8-12, 1985.

4. Anderson, W.E., Ramboz , J.D., and Ondrejka, A.R. "Final Report: Technical

Contributions to the Development of Incipient Fault Detection/Location

Instrumentation," NBSIR 86-3392, National Bureau of Standards, April 1986.

5. Weeks, W.L. and Steiner, J.P. "Instrumentation for the Detection and

Location of Incipient Faults on Power Cables," IEEE Transactions PAS-104,

No. 7, July 1982.

6. Jenni, A. "Partial Discharge Measurements on Power Cables," Wire Industry,

September 1982.

7. Al-Hussaini, T.J. and Stoner, J.E. "On-going Qualification of Cables in a Pressurized Water Reactor Environment," Proceedings , Nuclear Science

Symposium, Orlando, FL, November 1984.

8. Michel, W.O., Siemens-KWU, technical discussion with NBS , December 1987.

9. Toman, G.J. and Gardner, J.B. "Development of a Nondestructive Cable

Insulation Test," Proceedings of ANS Meeting on Operability of Nuclear

Power Plant Systems in Normal and Adverse Environments

,

October 1986.

10. Pays, M. and Rey , C., Electricite de France, technical discussion with NBS,

December 1987.

11. Mopsik, F.I., "Precision Time-domain Dielectric Spectrometer," Review

Scientific Instruments (1984).

11
NBS-114A (REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publ ication Date

SHEET (See instructions) NBSIR 88-3774 MAY 1988

4. TITLE AND SUBTITLE

A Review of Candidate Methods for Detecting Incipient Defects Due to Aging of

Installed Cables in Nuclear Power Plants

5. AUTHOR(S)

F.I. Mopsik, E.F. Kelley, F.D. Martzloff

6. PERFORMING ORGANIZATION (If joint or other than NBS. see in struction s)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8 . Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State , ZIP)

10.

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185. FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most si gn i fi cant information. If document includes a significant

bi bl iography or literature survey, mention it here)

Several types of test methods have been proposed for detecting incipient defects

due to aging in cable insulation systems, none offering certainty of detecting all

possible types of defects. Some methods apply direct detection of a defect in

the cable; other methods detect changes in electrical or non-electrical parameters

from which inferences can be drawn on the integrity of the cable. The paper

summarizes the first year of a program conducted at the National Bureau of Standards

to assess the potential of success for in situ detection of incipient defects by

the most promising of these methods.

12.

KEY WORDS (Six to twelve entries ; alphabetical order; capitalize only proper names; and separate key words by semicolon s) cable aging; insulation aging; partial discharge detection; test methods;

time-domain refl ectometry ; time-domain spectroscopy

13. availability

Unlimited

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, | Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

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PRINTED PAGES

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