Masaki Kanegami

Mechanical aging trend in ethylene propylene rubber-insulated safety cables sampled from bwr nuclear power containment

Statistical analysis has been performed on variously aged ethylene propylene rubber insulation on cables removed from boiling water reactor-type containment. Temperature and dose rate conditions of such cables are not identical. Therefore, elongation at break data observed for such insulation are converted to each equivalent period under 60 °C, 10 mGy/h condition. The aging trend is found to be approximately slower by half than that expected from accelerated aging test results. Furthermore, this aging trend is used as a reference to fit to each elongation value, and the fitted curve is extrapolated to find the end of service life, which is defined as 70 % of absolute elongation. Lifetimes estimated from this method and from predictions by the accelerated aging test are compared on an Arrhenius plot. It is suggested that the latter lifetime estimation is shorter than actual. The temperature dependence also suggests a difference in a basic chemical reaction under the two aging conditions, which results in apparent differences in activation energies and pre-exponential factors.

Remaining antioxidant and thermal-radiation degradation in ethylene-propylene-diene rubber

We have evaluated the thermal and radiation degradation of flame-retardant ethylene-propylene rubber (EPR) insulators used for safety cables in nuclear power plants. Accelerated degradation of EPR sheets was conducted under conditions of (1) simultaneous thermal-radiation aging (100 °C and 1.0 kGy/h), (2) radiation aging (room temperature and 1.0 kGy/h), and (3) thermal aging (100 °C or 200 °C, and 0 kGy/h). The degraded samples were analyzed by infrared spectroscopy, elongation at break, and oxidation induction time. The infrared measurements were not well suited for estimating the oxidation of EPR samples. The mechanical properties and induction time measurements show that the samples exposed to conditions (1) and (2) showed similar degradation behavior and the cross-linking reaction might be the main cause of the degradation.

Effect of mechanical vibration on the behavior of partial discharges in generator windings

As mechanical vibration usually exists in HV motors and generators, its influence on the partial discharge (PD) needs to be studied so as to improve the reliability of on-line PD monitoring method. In this report, the mechanical vibration frequency was synchronized to the power frequency and the effects of vibration phase and frequency were investigated. Because the length of the discharge gap changed with the vibration, obvious variation in the slot-PD patterns was observed at different vibration lead phase and frequency. Moreover, when the vibration amplitude was large, it was also found that the PD behavior at low voltage just above the PD inception voltage was different from that at higher voltage. This was explained in terms of the effects of the field variation in the gap due to the vibration and the charges left on the insulating surface due to PD.

Relationship between remaining antioxidant content and radiation-thermal degradation in crosslinked polyethylene

To clarify the deterioration mechanism of cables installed in nuclear power plants, three kinds of XLPE sheets with various antioxidant contents were aged by 100 Gy/h γ-ray irradiation at room temperature, 60, and 100°C. The remaining antioxidant content estimated from an oxidation induction time of a differential calorimetric thermogram as well as the elongation at break decreases faster as the initial antioxidant content decreases. On the other hand, an evaluation parameter called oxidation degree, which is the absorbance ratio of the carbonyl group to the methylene group obtained by infrared spectroscopy, increases with aging time. The oxidation degree increases with an increase in the elongation at break, and the relationship of these two parameters can be plotted around a sigmoid curve. Furthermore, the elongation at break and oxidation degree show reasonably stable transitions until the antioxidant content decreases to a certain value. After such a period, the elongation at break starts to change drastically and becomes sufficiently low value. It is clarified from these experimental results that there exists a certain antioxidant content that effectively inhibits oxidation.

ESCA analysis of semiconducting layer materials in XLPE power cables

This paper describes an application of ESCA analysis to semiconducting materials in XLPE power cables in terms of the oxygen ratio (integrated intensity of oxygen spectrum/integrated intensity of carbon spectrum). Analyzed materials were carbon black, an additive mixed into semiconducting materials, and several semiconducting materials treated in various cross-linking conditions. It was found that the additive increases the oxygen ratio of semiconducting materials as well as does the cross-linking treatment. The higher the oxygen ratio of the semiconducting materials, the smaller is the average lamellar angle in the XLPE insulation at the semiconducting interface. The angle is thought to be a parameter which expresses the hyperstructure of polymer insulation and smaller angles correspond to a better state of the semiconducting interface in XLPE power cables.

Waveform analysis of partial discharge current in liquid nitrogen immersed insulation

This paper deals with the partial discharge characteristic in liquid nitrogen immersed insulation using a high frequency detection system. AC voltages of 3-6 kV were applied on a needle-plane electrode system in which a polyethylene terephthalate film was inserted. Partial discharge detection was carried out using a wide band current detection system that has a frequency range up to 250 MHz. Frequency analysis was performed in both the low and high frequency bands. It was found that the signal component within the 150-250 MHz range correlated with the discharge mode, whereas that within the 1-1O MHz range correlated with PD charge magnitude. The results suggest that partial discharge measurement in the high frequency range is an effective method for monitoring discharge mode.

Statistical analysis of aging trends in EPR-insulated safety cables sampled from BWR containments

Statistical analysis was performed on aging trends of ethylene-propylene rubbers used for safety cables within boiling water reactor (BWR) containments of nuclear power plants. In order to obtain a regressed aging trend curve, observed mechanical elongation at break data was converted for equivalent period in an environment of 60°C and 10 mGy/h. Dropping speed is found to be approximately two times slower in service-used cables than that predicted by an accelerated aging test. This newly found aging trend was used as a reference to fit to each elongation value, and the fitted curve is extrapolated to find available service durations. An Arrhenius plot shows that the duration predicted from the acceleration test is shorter than the actual duration. Activation energy is lower and the pre-exponential factor in the Arrhenius plot is higher in newly estimated service durations. Low-oxygen-concentration environment in BWR containments could be a contributor to such behavior.

Influence of mechanical vibration on the behavior of partial discharges in generator stator windings

As mechanical vibration usually exists in HV motors and generators, its influence on the partial discharge (PD) needs to be studied so as to improve the reliability of on-line PD monitoring method. In this report, the mechanical vibration frequency was synchronized to the power frequency and the effects of vibration phase were investigated. Because the length of the discharge gap changed with the vibration, obvious variation in the slot-PD patterns was observed at different vibration phase. Moreover, when the vibration amplitude was large, it was also found that the PD behavior at low voltage just above the PD inception voltage was different from that at higher voltage. This was explained in terms of the effects of the field variation in the gap due to the vibration and the charges left on the insulating surface due to PD.

Partial discharge monitoring system with use of resistance temperature detector laid in stator coil slot of hydro power generator

It is necessary to estimate precisely the breakdown strength of coil insulation if utilities intend to continue using stator coils which have been used for a long time. For this purpose, monitoring of the maximum discharge magnitude (qm) of partial discharges (PDs) in coil insulation has been found to be useful. A monitoring method has been developed using the resistance temperature detector (RTD) laid in the stator coil slot of a hydro power generator as a PD current pulse detector. For the purpose of deciding diagnosis criteria based on the PD monitoring, long term PD characteristics should be clarified. We set monitoring systems in 4 hydro power stations. Over a period of one year, we have monitored several combinations of PD, line voltage, coil temperature and output current every hour. We analyzed these data and found strong correlation between qm and coil temperature.

Investigation of the voltage life of mica-alumina composite insulation at high temperatures

Experiments and investigations were carried out on the voltage life of a bar-coil model with a nickel-plated copper conductor covered with mica-alumina composite insulation at a temperature range of 550–850 °C. Diffusion of copper into the insulation layer was observed on the specimens aged for a long period. Insulation with mica paper gave faster diffusion speed and shorter voltage life than insulation with mica flake. The diffusion speed became higher with an increase in temperature. The ac current gradually increased with aging time and increased rapidly just before breakdown. There was the relationship I = aEn (a, n: constants) between ac current I and applied stress E, and n was approximately equal to 1 in the region where thermal unbalance did not occur. Arrheniuss law held in the relationship between ac current and aging time. Therefore, the breakdown mechanism might be that the effective insulation thickness decreased due to copper diffusion into the insulation layer and the ac current increased gradually until thermal breakdown in the last stage. If copper diffuses into the insulation layer, even with no voltage application, the aging time required to decrease the breakdown voltage to a certain level (the voltage life) obeys Arrheniuss law. If the voltage life is dominated by diffusion into the insulation layer, the activation energy for voltage life in the aging test becomes twice that for diffusion, both with and without voltage application.