Electrical condition monitoring techniques for low-voltage cables used in nuclear power plants

L. Verardi, D. Fabiani, G.C. Montanari

Department of Electrical, Electronic and Information Engineering

Università di Bologna, Viale Risorgimento 2 40136 Bologna, Italy

Pavel Žák

ÚJV Řež, a. s. Hlavní 130, Řež 25068 Husinec Czech Republic

Abstract - In this paper, dielectric spectroscopy is adapted to

investigate the dielectric response, in the frequency range 10-2-106 Hz, of a low-voltage cable for nuclear power plants. Cable samples were aged under accelerated stress conditions up to an equivalent aging time of 60 years. The aging procedure consisted of two phases: thermal aging by means of high temperature followed by gamma-irradiation at room temperature. Dielectric spectroscopy has been adapted to measure the dissipation factor Tan(δ) of inner insulation and outer sheath. A significant reduction of Tan(δ) can be observed in the frequency range 101-104 Hz, indicating that accelerated aging process mainly affects the dipolar polarization. Furthermore, the shape of the dielectric response presents a global minimum which shifts to lower frequency as the aging time increases. Experimental results show that the frequency in correspondence of this minimum value, as well as Tan(δ) at the reference frequency of 100 Hz, can be used as aging indicators.

I. INTRODUCTION

Nuclear power plants’ (NPPs) safety systems rely on low-voltage power, instrumentation and control (I&C) cables, which operate in areas characterized by relatively high temperature and gamma-irradiation. Several stresses can lead to insulation aging [1-3]: the degradation causes polymers to become more and more brittle, thus no more useful as cable electrical insulation which requires good thermal, mechanical and electrical properties. As these cables are related to fundamental safety systems, they must be able to withstand unexpected accident conditions and therefore, their condition assessment is extremely important, particularly if a plant lifetime extension is being envisaged [4,5].

Nowadays, the integrity and functionality of these cables are monitored through destructive testing (thermal analysis, such as OIT, TGA and DSC, or mechanical testing, e.g. elongation at break) [4-6]. The insulating materials are usually characterized through accelerated aging, which consists in applying different aging stress levels (e.g. temperature, dose rate) to material samples and extrapolate the measurement outcomes to the lower stress levels typically found in operation [7-9].

The investigation of electrical aging markers which can provide information about the state of the cable by non-

destructive testing methods would improve significantly the present diagnostic techniques [10-13].

Ethylene Propylene Rubber (EPR, EPDM) and Ethylene Vinyl Acetate (EVA) are polymeric materials widely used in nuclear environments [14,15,18]. It should be borne in mind that the insulating polymers produced by cable providers involve many inorganic compounds playing the role of flame retardants, anti-oxidants, stabilizers, lubricants, plasticizers and dyes [16-18].

Additives can improve the mechanical or chemical properties of the insulation and may exceed the 60% of the material composition. Their presence obviously alters the effects of aging: the application of different aging stress levels, e.g. performing accelerated aging tests, can change the balance between the reactions (such as chain scissions, cross-linking and oxidation), with different consequences on the macroscopic properties of the investigated materials. Therefore, it is not possible to identify general rules concerning the chemistry of the aging processes and the performance of the insulating material, even if the degradation mechanisms of the pure polymer are already known [18]. Any experimental evidence obtained from materials actually used as insulations can help to understand the consequences of the aging processes on their properties.

The aim of this paper, is to evaluate thermal and radiation aging of NPP cable insulating materials through dielectric response measurements [19,20].

This work has been made within the 7th framework EU project “Ageing Diagnostics and Prognostics of low-voltage I&C cables” ADVANCE.

II. EXPERIMENTAL SETUP A. Specimens

The cross section of the samples here investigated is shown in Fig. 1. In addition to the primary insulation, the structure of LV cables for NPPs usually includes shielding components and outer sheath, whose composition often differs from that relevant to primary insulation. In this case, the structure of the samples consists of :

• multiple strand conductor: Cu.

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978-978-1-4673-4744-0/13/$31.00 ©2013 IEEE

Total diameter = 1.35 mm • Insulation: 2 layers. Inner: EVA-b

EPDM-based. Average thickness: 1.15 • Screen: copper polyester tape. • Jacket: EVA-based. Thickness: 1.6÷3.4

The radiochemical tests carried out to chpolymeric materials revealed that the insulatcomposed of:

• inner layer: Aluminium Trihydrate (AT34%, Carbon Black 1%. CrosslinkDicumyl Peroxide (DCP).

• outer layer: EPDM 41%, Chalk 29%Carbon Black 1%. Crosslinking activat

The outer sheath consists of: • ATH 59,5%, EVA 33%, Chalk 6%,

1.5%. Crosslinking activator: DCP. This type of cable is currently installed at th

(Spain). It is designed for the following conditions:

• Temperature: 40÷50°C. • Integrated gamma dose after 60 years: 6

Fig. 1. Cable cross section. It is possible to recognize thtwo layers forming the insulation.

B. Aging Procedure

The accelerated aging was carried out by mtemperature and gamma-radiation, applied seque

The thermal aging was performed at Tecnato(San Sebastián de los Reyes, Madrid, Spain)were left in the oven at the temperature of 12hours. The total aging time was split in six chours each. Each aging cycle corresponds toequivalent thermal aging.

The radiation aging was carried out at Úfacilities (Řež, Czech Republic). The gammaprovided by a 60Co gamma-ray source placed inthe irradiation chamber. The dose rate depend

based; Outer: mm

4 mm haracterize the tion is mainly

TH) 65%, EVA king activator:

%, ATH 29%, tor: DCP.

Carbon Black

he Garoña NPP environmental

675 kGy.

he shield and the

means of high entially. om laboratories . The samples 20ºC for 1221 ycles of 203.5 o 10 years of

ÚJV irradiation a-radiation was n the center of

ds generally on

the distance from the cobalt sowere fastened on a perforate(Ø=80cm). Dosimetry was basedThe samples were irradiated temperature to reach a total acorresponding to the integrated The average dose rate was 0.45 kwas split in six cycles of 113 kGy C. Electrical Tests

Electrical properties of the agthrough dielectric spectroscopy, Frequency ranged between 10-2-1of 3 Vrms. Test temperature was k

The insulation has been testedto both conductors and measuelectromagnetic shield. The ousupplying the voltage to the shiefrom a copper wire mesh surround

III. EXPERIMENTAL RESU

Figure 2 shows the behavior

cable sheath over the whole freqdielectric analyzer. In the plot arfrom the unaged specimen and 1017.5 h of thermal treatment, c10 years, 30 years and 50 years omost significant differences are c104 Hz, the following figures wfrequency range. Fig. 2 shows cdissipation factor decreases grincreases. Furthermore, its lowfrequency as aging time increases

Figure 2 Dissipation factor vs. freqfrequency range of the dielectric anal

brevity, only the results obtained from plotted

ource, so that the specimens ed stainless steel cylinder d on the system alanine/ESR.

for 2202 hours at room absorbed dose of 676 kGy, gamma dose after 60 years. kGy/h. Also this aging phase y each (average).

ged cables were investigated using a Dielectric Analyzer.

106 Hz with an input voltage ept at 25°C.

d supplying the input voltage uring the signal from the

uter sheath has been tested eld and measuring the signal ding the cable.

ULTS AND DISCUSSION

of the dissipation factor of quency range covered by the re shown the results obtained

after 203.5 h, 610.5 h and corresponding respectively to of equivalent aging. Since the concentrated in the range 101-will be focused only on this clearly that the value of the radually as the aging time west value shifts to lower s.

quency, plotted over the whole yzer: 10-2-106 Hz. For the sake of the sheath after thermal aging are d.

505

Figure 3 shows the results of dielectric spectroscopy performed on the inner insulation, in the frequency range 101-104 Hz.

Figures 3a and 3b show the results obtained during accelerated thermal aging and the subsequent radiation aging, respectively. In these graphs, it is possible to notice a progressive decrease of the dissipation factor over the whole frequency range. The thermal aging (Fig. 3a) causes a gradual reduction of Tan(δ), which stabilizes only after 30 years of equivalent aging. Indeed, the three plots corresponding to 40 years, 50 years and 60 years of equivalent thermal aging are almost perfectly overlapped. The application of gamma-radiation, then, leads to a further reduction of the dissipation factor, particularly noticeable for the first specimens (10 and 20 years of equivalent aging, see fig. 3b). Increasing the aging time, the effect of the radiation on the insulation seems to become negligible, and the results are almost equal to those obtained after thermal aging. Only at frequencies between 10 Hz and 100 Hz it is possible to observe a gradual reduction of

Figure 3 Dielectric response of the cable insulation vs. frequency in the frequency range 101-104 Hz, during thermal aging (a) and during the

subsequent application of radiation aging (b). Different symbols indicate the aging times.

the dissipation factor with the aging time. Figure 4 shows the effects of the accelerated aging

treatment on the dissipation factor of the outer sheath. A comparison between the dielectric response of insulation and sheath could help to disclose the effect of the material composition on its electrical properties.

Figure 4a shows the effects of thermal aging on Tan(δ) of cable sheath, which decreases progressively with aging time. The variation is gradual and continues during the whole aging process. In addition, a gradual displacement of the minimum value of Tan(δ) to lower frequency is clearly visible (see fig. 4a). The gamma-radiation aging affects the dielectric response, causing a further decrease of the dissipation factor (fig. 4b). As observed from the results obtained testing the inner insulation, the shape of the curve approaches the one obtained after 1221 hours of thermal aging. After 60 years of equivalent thermal aging the effects of the irradiation are almost negligible.

In order to highlight previous considerations, it is possible

Figure 4 Dielectric response of the cable sheath vs. frequency in the

frequency range 101-104 Hz, during thermal aging (a) and during the subsequent application of radiation aging (b). Different symbols indicate

the aging times.

(a)

(b)

(a)

(b)

506

to extract from the measurement some indicators correlated with the aging time. For example, it is possible to choose a reference frequency and analyze the variation of the dissipation factor.

Figure 5 shows the trend of the dissipation factor at the reference frequency of 100 Hz. It can be clearly seen that cable insulation is not significantly influenced by the application of radiation aging (Fig. 5a). On the contrary, the sheath (Fig. 5b) is affected remarkably also by radiation aging. The variation of Tan(δ) due to thermal aging is lower than that relevant to insulation at the same aging time. In addition, radiation application leads to a further decrease of the measured loss factor value. It is noteworthy that the lowest value reached by the dissipation factor is about the same found for the insulation. After 30-40 years of equivalent aging, in fact, Tan(δ) reaches a sort of end-point, indicating the conclusion or the stabilization of the phenomena leading to the variation of the property.

Figure 5 Dissipation factor vs. equivalent aging time at the reference

frequency of 100 Hz. White symbols indicate the results after thermal aging, black symbols indicate the outcomes at the end of the entire aging

process.

Similar considerations can be done observing the plots of Fig. 6, which show the frequency at which the minimum value of Tan(δ) is reached vs. the aging time. Also in this case, fig. 6a corresponds to the insulation and fig. 6b to the outer sheath. The analysis of this indicator confirms the results obtained from the measurement of Tan(δ) at 100 Hz.

Several causes can be responsible of the results previously shown. First of all, the fact that radiation aging affects only the cable sheath and not the inner insulation can be explained by diffusion-limited oxidation (DLO). The value of dose rate used to accelerate the aging (carried out at room temperature), in fact, can lead to a non-uniform polymer aging. To have a uniform oxidation with high dose rates, oxygen molecules have to diffuse quickly inside the insulation, which cannot occur at atmospheric pressure. Therefore, at high dose rate only the surface of the outer sheath is affected. This could be revealed by infrared spectroscopy, comparing the oxidation peaks of the inner and outer part [3].

Figure 6 Frequency corresponding to the minimum value of Tan(δ) vs. equivalent aging time. White symbols indicate the results after thermal

aging, black symbols indicate the outcomes at the end of the entire aging process.

(a)

(b)

(a)

(b)

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Moreover, it should be noted that the sheath, due to its protective function, can be more resistant to thermal aging and in order to reach the same level of degradation of the inner insulation, the application of gamma-radiation is required. Thermal analysis (OIT, DSC and TGA), under investigation, will provide useful information about the role played by the additives and will detect changes in the polymer morphology (e.g. degree of cristallinity). Finally, mechanical tests are required to correlate electrical properties with the degradation of polymer mechanical properties.

IV. CONCLUSIONS

Dielectric spectroscopy results show a remarkable decrease

of the dissipation factor Tan(δ) with the aging time, in the frequency range 101-104 Hz. These results would suggest that morphological changes, associated with dipolar polarization phenomena, occur in the insulation due to aging.

The accelerated aging, carried out applying thermal and radiation stresses sequentially, affects the dielectric response in the frequency domain reducing the value of Tan(δ). Moreover, the minimum value assumed by Tan(δ) shifts to lower frequency as the aging time increases.

The effects of the accelerated aging on outer sheath and insulation are different. The insulation presents a larger reduction of Tan(δ) with thermal aging: after 30 years of equivalent thermal aging the property does not undergo further reductions, suggesting that an end-point of insulation degradation is reached. On the contrary, the dissipation factor of the outer sheath reaches the same values of the insulation only after gamma-irradiation aging. Also in this case, it is possible to observe that the property, after 40 years of equivalent aging (thermal + radiation), reaches the same end-point of the insulation.

The aging indicators extracted from the measurement, i.e. the Tan(δ) at the reference frequency of 100 Hz together with the frequency at which the global minimum of the dissipation factor is located, are able to quantify previous considerations.

Further measurements of sample chemical and mechanical properties could clarify the phenomena that cause the variation of the electrical properties shown in this paper.

ACKNOWLEDGMENTS

The authors thank Dr. Juan Carlos Cano for the thermal

aging carried out at Tecnatom A.S. (San Sebastián de los Reyes, Madrid, Spain). The authors want also to thank Dr. Vit Plaček for the radiation aging test carried out at ÚJV (Řež, Czech Republic). Finally, the authors want to thank Dr. Grazyna Przybytniak (Institute of Nuclear Chemistry and Technology, Warsaw, Poland) and Matthias Meyer (Nexans, Nuremberg, Germany) for the preliminary characterization of

the insulating materials which revealed their composition.

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