Ionization Characteristics of Polyethylene Insulation of Power Cables with Sector Conductors*

M. N. Treskina

(Presented by the scientific seminar of the Department of Electrical Insulation and Cable Engineering)

At present, polyethylene is widely used as insulation for power cables. The electric strength of sheet polyethylene is 600–800 kV/mm, whereas the maximum breakdown voltage of polyethylene cable insulation is 20–90 kV/mm. One of the factors leading to such a substantial decrease in the electric strength of polyethylene applied as insulation on worm presses is the presence of gas inclusions in the insulation layer.

Polyethylene is characterized by a high coefficient of thermal expansion, exceeding the coefficients of thermal expansion of aluminum and copper by factors of 9 and 13, respectively. Cooling of polyethylene insulation applied on a worm press takes place from the outer surface of the layer. The solidification front moves from the outer surface of the insulation toward the inner layers, where pores are formed during cooling as a result of shrinkage. Moreover, the main number of inclusions is located at the boundary between the current-carrying conductor and the insulation. The sizes of the gas pores vary and may range from \(10^{-3}\) mm to 1 mm. Some time after extrusion, the pressure in the pores equalizes with atmospheric pressure [1].

During cable operation, the electric-field strengths arising in gas inclusions may be sufficient for ionization of the gas in the inclusions. At the same time, irreversible changes occur on the surface of the polyethylene—the destruction of the polymer, accompanied by the formation of gaseous and solid substances. The solid substances contain an increased amount of carbon, and the surface of the gas pore becomes electrically conductive. Further development of ionization may lead to the “short-circuiting” of some pores and to the appearance of deep pits, as a result of which the electric strength of the cable insulation decreases [2].

Therefore, in view of the low corona resistance and considerable porosity of polyethylene insulation, knowledge of their ionization characteristics is necessary for assessing cable reliability.

Three-core cables with polyethylene insulation for voltages of 1–6 kV are manufactured with sector conductors without a screen over the conductor. In recent years there has been a transition from stranded (M) to single-

* Students of TPI D. S. Omelchenko and Z. A. Shandra took part in the work.

single-wire (O) sector conductors, which makes it possible to reduce the need for equipment by a factor of 4–5, the need for production floor space by a factor of 9, and to increase labor productivity in cable manufacture by a factor of 10 [3].

The purpose of the present work was to investigate and compare the ionization characteristics of polyethylene insulation in multistrand and single-wire unshielded sector conductors.

For practically used designs of cables with polyethylene insulation for voltages of 6, 10, and 35 kV, calculations were carried out to determine the sizes of the minimum gas inclusions in which ionization will occur when line voltage is applied to the insulation (Fig. 1). In this work, flat pores were adopted as the most dangerous of the possible pore shapes. A layer of polyethylene insulation containing a gas inclusion may be regarded as an inhomogeneous two-layer dielectric if the pore is located at the conductor or at the sheath, and as a three-layer dielectric if the pore is located within the thickness of the insulation.

Fig. 1. Dependence of the size of the minimum inclusion on the cross section of current-carrying conductors for cables with polyethylene insulation: 1—6 kV, cable with sector conductors; 2—10 kV and 3—35 kV cables with round conductors

In a radial electric field, the field strength in a gas inclusion of thickness \(\Delta\), located at radius \(r\), is determined by the formula

\[ E_{\mathrm{g}} = \frac{U}{ r \varepsilon_{\mathrm{g}} \left[ \frac{1}{\varepsilon_{\mathrm{p}}}\ln\frac{r}{r_1} + \frac{1}{\varepsilon_{\mathrm{g}}}\ln\frac{r+\Delta}{r} + \frac{1}{\varepsilon_{\mathrm{p}}}\ln\frac{r_2}{r+\Delta} \right] } \ \frac{\mathrm{kV}}{\mathrm{mm}}, \]

where \(r_1\) and \(r_2\) are the radii over the conductor and over the insulation,

\(\varepsilon_{\mathrm{g}}\) and \(\varepsilon_{\mathrm{p}}\) are the dielectric constants of gas and polyethylene.

Comparison of the \(E_{\mathrm{g}}\) arising in pores at various \(\Delta\) with the breakdown strength of air makes it possible to determine the size of the minimum inclusion [4]. Taking into account that the maximum field strengths in the insulation occur at the surface of the conductor, where the probability of formation of gas inclusions is greatest, it is necessary to analyze dangerous pores located at the current-carrying conductor. As can be seen from Fig. 1, for cables with the smallest permissible conductor cross sections, pores of thickness \(0.05\) mm and greater are dangerous from the standpoint of the occurrence of ionization in them. As the conductor cross section increases, the size \(\Delta_{\min}\) increases. Because of the sector conductors, pores in the insulation of a 6 kV cable are more dangerous than in a 10 kV cable.

To study the ionization characteristics, aluminum conductors insulated with polyethylene 2 mm thick, with a cross section of 70 mm², manufactured at the Kamkabel plant, were used. From the insulated stranded and solid conductors, 20 specimens of each were cut. On specimens 1.8 m long, a layer of semiconducting rubber of identical resistance was applied as an electrode.

To eliminate edge effects, a winding of unvulcanized insulating rubber in the form of a sleeve was made at the edges of the electrodes.

Fig. 2. Integral distribution curve of the values of $\operatorname{tg}\delta$ of the polyethylene insulation of stranded (M) and solid (o) sector conductors

To compare the homogeneity of the polyethylene insulation of the specimens, $\operatorname{tg}\delta$ was measured. As can be seen from Fig. 2, in this batch of specimens there is a considerable scatter of $\operatorname{tg}\delta$. Specimens with a small scatter of $\operatorname{tg}\delta$ values were selected for study $(2 \cdot 10^{-3} \div 6 \cdot 10^{-3})$.

Measurements of $\operatorname{tg}\delta$, $\operatorname{tg}\delta = f(U, t^\circ C)$, were carried out on an R—525 bridge. The ionization inception voltage $U_i$ and the ionization intensity $J_i$ were determined by a bridge circuit using the differential method [5]. The error in determining the ionization inception voltage averaged 10%.

The ionization intensity $J_i$ is expressed in relative units.

To study the temperature dependence of the ionization characteristics, taking into account the insignificant thickness of the insulation, the specimens, bent into rings 40 cm in diameter, were placed in a thermostat. The ionization characteristics of the specimens before and after bending did not change. The holding time of the specimens at a specified temperature before measurement was 0.5 h. Figure 3 gives the dependences of $\operatorname{tg}\delta$, $E_i$, and $J_i$ on temperature. For the entire batch of specimens, the ionization inception field strength $E_i$ at 20°C for stranded conductors is 30—60% lower than $E_i$ for solid conductors. Calculations (Table 1) showed that, when line voltage arises on the insulation of a 6-kV cable (emergency condition), all pores 0.1 mm thick and larger will be ionized. For these cables such pore sizes cannot be permitted.

Table 1

Calculated values of the maximum field strengths in the insulation ($E_{\max}$) in the pore ($E_{\Gamma}$), and the size of the dangerous pore ($\Delta_{\min}$) in the insulation of a cable for a voltage of 6 kV with a sector-conductor cross section of 70 mm²

According to the data obtained, the ionization-inception field strength in the insulation of multiwire conductors is 1.5–2.4 kV/mm, which corresponds to ionizable pore sizes of 0.14–0.09 mm according to [2], while for single-wire conductors \(E_u = 3.5\text{–}4.5\) kV/mm, which corresponds to pore sizes of 0.06–0.045 mm. Consequently, the wire-like character of the conductor surface leads to the appearance of larger pores in the polyethylene insulation of conductors M as compared with O. The greater ionization intensity in the insulation of conductors M as compared with O is possibly determined by the larger number of pores in comparison with conductors O.

With increasing temperature, a decrease in \(E_u\) and an increase in \(J_u\) were observed (Fig. 3). In works [4, 6] an increase was shown in the ionization-inception voltage upon heating the polyethylene insulation of cables having screens over the conductors; this was explained by a decrease in the size of gaseous inclusions and by an increase in the pressure in them during thermal expansion of the polyethylene. The decrease in \(E_u\) upon heating insulated sector conductors without a screen over the conductor can be explained by the more significant thermal expansion of polyethylene compared with aluminum and by an increase in the size and number of pores at the conductor–insulation boundary. This agrees with the increase in ionization intensity with increasing temperature, which is more pronounced for conductors M (Fig. 3).

For the purpose of studying the influence of prolonged exposure to an electric field on the ionization characteristics, the insulated M and O conductors were subjected to ionization aging. The specimens were kept for 500 hours under a voltage of industrial frequency exceeding the ionization-inception voltage by 15%. After specified intervals of time, measurements were made of \(\operatorname{tg}\delta = f(U)\), \(U_u\), \(J_u\) (Fig. 4).

Fig. 3. Temperature dependence of \(\operatorname{tg}\delta\), \(E_u\), \(J_u\) of the polyethylene insulation of multiwire (M) and single-wire (O) sector conductors

With increasing duration of ionization aging, \(\operatorname{tg}\delta\) of the insulation increases, more significantly for M. Ionization of gaseous inclusions in polyethylene can cause oxidative processes, destruction of polyethylene chains, and formation of carbonyl groups and radicals. These processes were the cause of an increase in the capacitance of the specimens during ionization aging: the capacitance of conductors M increased by 10–16%, and that of conductors O by 5–8%. Consequently, during ionization aging there occurs an increase in the dielectri-

permeability and \(\tg \delta\) of polyethylene insulation, more significant for cores M than for cores O.

Over 125 hours of ionization aging, an increase in \(E_u\) and a decrease in \(J_u\) were observed (Fig. 4), which may be connected with an increase in the pressure in the pores at the initial stage of ionization. Some deterioration of the ionization characteristics of cores O is observed during 60 hours of aging. Cores O are characterized by high values of \(E_u\), small pore sizes, and, consequently, considerable ionization intensities in these pores [2]. The latter is possibly the cause of the rapid “closure” of small pores. Therefore the decrease in \(E_u\) and increase in \(I_u\) of cores O in the first 60 hours of aging can be explained by the “closure” of small pores.

During prolonged ionization, polyethylene is destroyed (erosion), and pore sizes increase. On the other hand, the “closure” of some pores leads to a local increase in field strength, which facilitates ionization of other pores. These causes are possibly associated with the decrease in \(E_u\) and increase in \(J_u\) during prolonged ionization aging of polyethylene insulation (Fig. 4).

Fig. 4. Dependence of \(\tg \delta\), \(E_u\), and \(I_u\) of polyethylene insulation of multistrand (M) and single-wire (O) sector cores on the duration of ionization aging

The insulated cores investigated were a model of cable cores with a voltage of 6 kV. Taking into account the application of insulation 3.5 mm thick, it may be assumed that, under equivalent conditions, ionization aging in the cable will proceed more intensively.

To assess the change in the electric strength of polyethylene insulation in the process of ionization aging, the study was carried out on flat samples of high-pressure polyethylene 0.6 mm thick, of the same grade as the core insulation. The surface of the sample was subjected to ionization occurring in an artificial pore at a voltage 15% higher than the ionization inception voltage. Breakdown of the samples was carried out at industrial-frequency voltage with plane–plane electrodes in transformer oil.

The obtained average values of the breakdown strength of polyethylene are given in Table 2.

In the process of ionization aging, the electric strength of polyethylene decreases: over 200 hours, the reduction in \(E_{\text{br}}\) amounted to 25%.

On the basis of the results obtained, the following conclusions may be drawn:

1. With a decrease in the cross section of current-conducting cores, the size of the minimum pore at which ionization is possible decreases. For na-

cessful (from the standpoint of absence of ionization in gas pores) operation of the insulation of cables rated at 1.6 and 35 kV, the dimensions of pores located at the conductor–insulation boundary must not exceed certain values. Owing to the small radii of curvature of sector-shaped conductors and the absence of screens over the conductors, the danger of ionization arising in the insulation of 6 kV cables is greater than in the insulation of 10 kV cables, and the values of the minimum pores in the insulation of a 6 kV cable are smaller than in a 10 kV cable.

Table 2

Change in the electric strength of HV polyethylene with ionization-aging time

1. \(\tg\delta\) of the polyethylene insulation of a cable of one length is not a constant quantity; fluctuations of the values of \(\tg\delta\) reach one order of magnitude.

2. The ionization-inception field strength in the insulation of single-wire conductors is 30–60% higher than \(E_i\) of stranded conductors, while the ionization intensity is lower than \(J_i\) in the insulation of conductors of type M.

3. With an increase in temperature from 20°C to 80°C, there occurs a 60% decrease in the ionization-inception field strength and a 50–170% increase in the ionization intensity in the polyethylene insulation of sector-shaped conductors without a conductor screen. Moreover, the larger percentage increase of \(J_i\) pertains to the insulation of stranded conductors.

4. With prolonged ionization aging, \(\tg\delta\) of the polyethylene insulation increases. The increase in \(\tg\delta\) of the insulation of conductors of type M is 3 times greater than that of conductors of type O.

5. In the course of ionization aging during the first period, the ionization-inception field strength increases and the ionization intensity decreases. With prolonged ionization aging, a decrease in \(E_i\) and an increase in \(J_i\) are observed. In the course of ionization aging, the electric strength of the polyethylene insulation decreases.

REFERENCES

1. A. G. Mirzoev. Some questions of the technology of manufacturing power cables with polyethylene insulation. “Application of Plastics in the Cable Industry,” VNII EM, 1964.

2. A. N. Perfil’ev. Ionization of air inclusions in the polyethylene insulation of high-voltage cables. Vestnik elektropromyshlennosti, 12, 1961.

3. S. A. Parkhomovich. Some questions of the economics of cable production. Vestnik elektropromyshlennosti, 6, 1962.

4. R. V. Kulakova. Power cables with plastic insulation. GEI, 1963.

5. F. H. Kreüger. Detection of internal discharges, in particular in high-voltage cables. High-Voltage Cable Lines. Collection of translations, issue 2, GEI, 1959.

6. Development of a 10 kV power cable with polyethylene insulation. NIIKP Report, No. 635—57.