열산화에 따른 XLPE의 전기트리 열화현상

조영신, 이홍기．, 심미자．．, 김상욱

서울시립대학교 화학공학과, ．．생명과학과

．우석대학교 화학공학과

Effects of Thermal Oxidation on Electrical Treeing Deterioration Phenomena in XLPE

Y. S. Cho, H. K. Lee．, M. J. Shim．．, S. W. Kim

Dept. of Chem. Eng., ．．Dept. of Life Sci., The Univ. of Seoul

．Dept. of Chem. Eng., Woosuk Univ.

Introduction

Polyethylene is the most widely used polymer in the electric power cable industry because of its well combination of electrical properties and processibility. Cross-linked version of polyethylene(XLPE) possesses excellent electrical strength and low dielectric loss over the wide frequency range which make it an ideal material for uses as an insulation for medium voltage(MV) and high voltage(HV) power cables[1]. Under the high thermal and electrical stresses, the electrical system can be deteriorated during its service life and these processes can be accelerated under the presence of oxygen resources diffused from surroundings or by-produced during processing step or long-term operation[2,3]. From these view points, it is very important to observe the effects of thermal oxidation on the deterioration of electrical insulation by electrical treeing of pre-breakdown phenomena. This study shows the electrical treeing deterioration of oxidized XLPE. Electrical treeing phenomena, DSC test results and optical intensity of oxidized XLPE were studied and the dielectric breakdown mechanism was discussed.

Experiment

Specimen used in this study was a block of needle-plane electrode geometry of XLPE insulation in power distribution 22.9 kV CN/CV cable(Hanil Power Cable Co.). Needle electrode which had well controlled curvature of radius of 3 □ was inserted into the sample at 100℃ with inserting zig[4]. It was annealed for 12 hrs and then cooled down slowly to room temperature to minimize the mechanical defects by thermal stress. The test specimen was set in insulating silicon oil at room temperature and a commercial AC voltage was applied to the needle electrode until dielectric breakdown. The electrical tree was observed by microscope and analyzed by using image analyzer(Image Pro Plus).

Results and Discussion

XLPE insulated power distribution cables have circular conductor of compacted stranded copper wires(①) as shown in Fig. 1. Extruded semi-conducting conductor screens(②) fill the interstices between the wires and provide a smooth circular envelope around the conductor and reduce the concentration of electrical stress around

Fig. 1. Photograph of power distribution 22.9 kV CN/CV cable.

the conductor. The XLPE insulation part(③) takes the largest portion of cables and more than half of the electrical accident has been reported in it. So it is very important to develop a good electrical insulator for the reliable and stable supply of electric power. XLPE insulated power distribution and transmission cables can be used at 90℃. Furthermore, XLPE permits short-circuit peak temperature of the conductor up to 250℃ and, in MV applications withstands overloads lasting several hours each year at 130℃, which is quite impossible with thermoplastic PE(LDPE, low density polyethylene). So the oxidation resistance of insulation under high temperature is very important[2,3].

Fig. 2 DSC thermograms of oxidized XLPE.

The thermal oxidation characteristics of XLPE are shown in Fig. 2. Virgin and air-oxidized samples were oxygen-oxidized at 153.4℃, respectively. About 1 mg of sample was heated at the heating rate of 10℃/min from room temperature to oxidizing temperature in N2 gas flow rate of 50 ml/min and the gas was replaced by oxygen at the same flow rate and oxidizing temperature. The oxidation initiation temperature(OIT) and most rapidly oxidizing temperature(MOT) were obtained. All the pre-oxidized sample showed lower values as shown in Fig. 2. When the oxidation

Fig. 3. Discolored XLPE at 180℃ of air for 30 hrs.

deterioration is started, the oxidation process is accelerated by the metallic impurities or contaminants and produces carbonyl compounds so that the system is discolored as  shown in Fig. 3. When the polymeric insulator contacts with oxygen diffused from outside or presented inside the system at high temperature by Joule's heating of conductor under high electric field, it is thermally oxidized, though deep inside the insulation, and the electrical insulating capability is reduced. The color intensity of oxidized region increases toward outside where the sample contacted with oxygen directly at high temperature. So that the dielectric breakdown phenomena of treeing may be affected by thermal-oxidation reaction.

Fig. 4. Electrical tree from the tip of HV needle electrode inserted into the oxidized XLPE to simulate the defects or impurities.

Fig. 4 shows the electrical tree from the tip of high voltage needle electrode inserted into the oxidized XLPE in the air convection oven of 150℃ for 30 days to simulate the defects or impurities. The electrode separation was 2 mm and the supplied electrical stress was 7.5 kV for 30 sec. at room temperature. It is generally agreed that the long-term performance of XLPE insulated cables depends on the number and size of voids, contaminants, defects and impurities around which electrical stress is magnified and electrical tree is initiated. When these are associated with water, then can sometimes give rise to water-treeing deterioration. The initial tree was started from the needle electrode and propagated toward the counter plane electrode, little by little. At the initial stage, low density tree was observed but as the voltage applying time increased the branching density of tree increased. Treeing phenomena have been reported as the main cause of dielectric failure in the electric power cable and the initiation and growth mechanism have been investigated worldwide to minimize the occurrence of electrical tree and to suppress the propagation toward the outer electrode[5,6]. But there has not been enough study about the spatial random pattern because of its complexity and variety. Also it is difficult to correlate the phenomenological information to the insulating capability. Many researchers have reported that the electrical tree has fractal characteristics or self-similarity[7,8]. Recently the fractal concept was introduced by Bennoit Mandelbrot and used to characterize the complex natural phenomena. The fractal phenomena have been characterized by the fractal parameter of dimension showing the spatial complexity. Several ways of obtaining the fractal dimension were designated and the scale covering method was most frequently used. More detail things about the fractal characteristics of electrical tree in the oxidized XLPE samples will be discussed next.

References

1. N. Yoshifuji, T. Niwa, T. Takahasi, and H. Miyata, "Development of the New Polymer Insulating Materials for HVDC Cable", IEEE Trans. on PD, 7, 1053(1992).

2. N. Shimizu, K. Uchida, and K. Horii, "Effect of Absorbed Oxygen on Tree Initiation in Polyethylene", T. IEE Japan, 108-A, 551(1988).

3. K. Uchida and N. Shimizu, "Effects of Oxygen on AC electrical Tree Initiation in Polyethylene at High Temperature", ibid., 112-A, 121(1992).

4. Y.S. Cho, J.H. Park, M.J. Shim, and S.W. Kim, "Effects of Defects on Dielectric Breakdown Phenomena and Life Time of Polymeric Insulation", Proc. of the 5th ICPADM in Seoul, 447(1997).

5. J.V. Champion, S.J. Dodd, and G.C. Stevens, "Analysis and Modeling of Electrical Tree Growth in Synthetic Resins over a Wide Range of Stressing Voltage", J. Phys. D: App. Phys., 27, 1020(1994).

6. J.C. Fothergill, L.A. Dissado, and P.J.J. Sweeney, "A Discharge-Avalanche Theory for the Propagation of Electrical Trees: A Physical Basis for their Voltage Dependence", IEEE Trans. on DEI, 1, 474(1994).

7. Y.S. Cho, M.J. Shim, and S.W. Kim, "Fractal Characteristics of Electrical Treeing Phenomena in Electric Power Distributing Cable Insulation", Proc. of the 5th ICPADM in Seoul, 443(1997).

8. S. Maruyama, S. Kobayashi, and K. Kudo, "Fractal Characteristics of AC Electrical Trees in Polymeric Insulating Materials", T. IEE Japan, 113-A, 480(1993).