STUDY OF PAPER-IMPREGNATED INSULATION OF POWER CABLES

V. M. ANIKEENKO, V. F. GRAFOV, M. N. TRESKINA

(Presented by the scientific seminar of the Department of EIKT)

During the operation of cable lines, the insulation is subjected to thermal and electric fields, which cause irreversible changes in the electrical and physico-mechanical characteristics; the degree of change in these characteristics indicates the state of the insulation \[1, 2, 3\].

The purpose of the present work was to study the state of the insulation of an SB 3×95 power cable rated at 10 kV and to predict its further operation after 30 years of service under conditions of partial depletion of the insulation due to leakage of the impregnating compound through through-cracks in the sheath. In cables, especially those with sector-shaped conductors and a nonradial electric field, depletion leads to the appearance and development of ionization processes and to cable failure.

In view of the above, we studied \(\operatorname{tg}\delta\), \(R_{\text{ins}}\), the ionization-inception voltage, the electric strength under short-term and prolonged exposure to a voltage of frequency 50 Hz, and the mechanical strength of the paper insulation. For comparison of the degree of aging of the insulation of the SB cable, the insulation of a “new cable” (manufactured in February 1971), grade ASB 3×70 rated at 10 kV, was studied in parallel in the initial state and after 50 and 200 hours of depletion under laboratory conditions. The study was carried out on cable samples and models whose design provided for the exclusion of surface leakage current, edge effect, etc.

During operation of the cable line, partial depletion and aging of the paper insulation occurred. These processes had the greatest effect on the layers of tape located at the conductor surface: thus, \(E_{\text{br}}\) of the first layers is 25 kV/mm, and that of the outer layers of the phase and belt insulation is, respectively, 36.7 kV/mm and 45 kV/mm. At the same time, the tensile strength of the tapes of the phase insulation is 7.01 kg/mm² for the SB cable and 8.17 kg/mm² for the ASB cable. It was established that the ionization-inception field strength in the insulation of the SB cable is at the level of 0.855 kV/mm, and in the ASB cable 0.94 kV/mm. The results of the study of \(\operatorname{tg}\delta\) and \(R_{\text{ins}}\) as functions of temperature and time of artificial depletion are presented in Table 1.

As follows from Table 1, for the connection scheme of cable samples “one conductor against the other two, connected to the lead sheath,” \(\operatorname{tg}\delta\), at a test voltage lower than the ionization-inception voltage, is 1.5–2 times greater in the SB cable at elevated temperatures than in the insulation of the ASB cable.

Table 1

The characteristics given above make it possible only to assess qualitatively the condition of cable insulation, but do not make it possible to estimate quantitatively its service life. In order to predict the reliability of the insulation of SB and ASB cables, tests were carried out on four batches of model specimens of each cable under prolonged exposure to an alternating voltage of power frequency. The test results, processed—

Fig. 1. Dependence of \(\ln[-\ln P(\tau)] - \ln Z\) on \(\dfrac{1}{Z}\) for SB cable at various \(E_{\max}\)

Fig. 2. Dependence of \(\ln[-\ln P(\tau)] - \ln Z\) on \(\dfrac{1}{Z}\) for phase specimens of ASB cable at various \(E_{\max}\)

... obtained with the aid of the Weibull distribution, and are broken curves having 2–4 rectilinear segments.

At the Department of Electrical Insulation and Cable Engineering of TPI, a method has been developed for estimating the reliability of cable products [4]. The reliability equation in the absence of external and internal mechanical stresses has the form

\[ P(\tau)=\exp\left[ -\frac{d\cdot S_{\text{м}}}{0.482\cdot v_0\cdot f}\cdot Z\cdot \exp\left( -\frac{0.482\cdot \ln \dfrac{0.65}{p^{5/3}}}{Z} \right) \right], \tag{1} \]

\[ Z=\left[ \ln \frac{ 1-\dfrac{1}{\gamma} }{ A\cdot e^{-bT}\cdot f\cdot E\cdot \ln \dfrac{2D}{A\cdot e^{-bT}\cdot \beta\cdot f\cdot L} } \cdot \frac{ 1 }{ D-A\cdot e^{-bT}\cdot \beta\cdot f\cdot L-2k_i\cdot \ln \tau/\tau_0 } \right]^{0.82}. \tag{2} \]

The procedure for determining the parameters of equation (1) is described in [4]. The results of the calculations are given in Table 2 and Figs. 1–2.

Table 2

As follows from Figs. 1–2, all dependences

\[ \{\ln[-\ln P(\tau)]-\ln Z\}=f\left(\frac{1}{Z}\right), \]

corresponding to different field strengths and temperatures for each cable model, fall on one straight line. Therefore, the statement of the author of [6] regarding changes in the mechanism of insulation failure at different probabilities of its breakdown should be considered erroneous.

Table 3

On the basis of the parameters determined from equation (1), calculations were performed for the reliability of the insulation of a cable line 2700 m long at a voltage of 10 kV, made of ASB 3×70 and SB 3×95 cable at temperatures of 333, 343, and 353°K. The results of the calculations of probabili-

the probability of failure-free operation of the insulation of a cable line are presented in Table 3.

Conclusions

1. During the operation of cables with viscous impregnation, the properties of the insulation deteriorate: the short-time electrical strength of both individual tapes and insulated conductors decreases, and dielectric losses increase in the elevated-temperature range (60–80°C). A substantial deterioration of the insulation located near the lead sheath was observed.

2. The method for calculating the service life and reliability of electrical insulation, developed at TPI, made it possible, on the basis of experimental data, to calculate the reliability of cable operation in a state of significant depletion of the insulation, without taking couplings into account, for various service lives.

3. It has been established that at a line operating temperature of 60°C the cable can operate without failures for 7–8 years, at 70°C for 2 years, and at 80°C for 1 year.

References

1. N. A. Voskresenskii, A. K. Mann. Study of the ionization characteristics of cable insulation with viscous impregnation. Elektricheskie stantsii, 1959, No. 7.

2. M. A. Greisukh, G. S. Kuchinskii et al. Paper-oil insulation in high-voltage structures. GEI, 1963.

3. S. N. Koikov, A. N. Tsikin. Electrical aging of solid dielectrics. Energiya, 1968.

4. V. S. Dmitrevskii. Calculation of electrical insulation. TPI, 1972.

5. V. S. Dmitrevskii, V. F. Grafov et al. Aging of solid dielectrics and reliability of electrical insulation. Abstracts of a report at the Scientific Council on theoretical and electrophysical problems of electric power engineering. Novosibirsk, 1970.

6. I. M. Oudin, I. Rerollect, H. Thevenon. Rev. gen. electr., 1968, 77, No. 7, 430–434.