PEA에 의한 DGEBA/MDA/SN 계의 공간전하 특성

조영신, 이홍기*, 심미자**, 김상욱

서울시립대학교 화학공학과, **생명과학과

*우석대학교 화학공학과

Space Charge in DGEBA/MDA/SN System by PEA

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

Electric components are operated under complex conditions of high temperature, high electric field, mechanical and chemical stresses, etc. To increase the reliability and stability of electric power supply, synthetic polymers are widely employed in the field of electrical insulation replacing such materials as porcelain, impregnated paper, mineral oil etc. [1].

It is known that most of the causes of electric failure in polymeric insulator are induced from the dielectric breakdown of electrical insulators by treeing deteriorations [2,3]. The electrical tree is initiated from the stress concentration of thick insulation. Accumulation of space charge in the polymeric insulation increases or decreases the electric field strength at some points of insulation by distorting the electric field [4,5]. When the reinforced electric field exceeds the dielectric strength of the polymeric insulation, then the electric failure is induced by treeing deterioration.

It has been reported that the space charge plays a leading role in aging by tree initiation and propagation under ac stress [6,7]. So the measurement of space charge distribution is important for understanding the electric breakdown initiated from the electrical insulation [8,9]. This work is focused on the space charge behaviour in the modified new epoxy resin system. In this study, the characteristics of space charge in the DGEBA/MDA/SN epoxy resin system were observed by PEA(pulsed- electro-acoustic) method.

Experiment

The typical thickness of DGEBA/MDA/SN system was 1.0 mm. Measurements of charge distributions were made within 2 min after short-circuiting at the end of the application of voltage for 30 min. The spatial resolution of the PEA system was 45 □, so that small peaks smaller than 45 □ were not significant. Pulsed electric field applied between the two sample electrodes induces a perturbation force based on the coulomb force law by the movement of charges located in the dielectric and launches corresponding acoustic wave. The piezoelectric transducer(PVDF, polyvinylidene fluoride) detects the acoustic signal and the signal is transformed to a voltage signal. The space charge profile information is extracted and calibrated using digital signal processing. The electric pulse has an amplitude of 2 kV and a width of 10 ns.

Results and Discussion

Fig. 1 shows the effects of SN contents on the charge distributions of DGEBA/

Fig. 1. Effects of SN contents on the space charge distribution by PEA method.

MDA epoxy resin system. Homocharge develops in pure DGEBA/MDA sample while heterocharge develops in the modified systems with SN. The ions or    dipoles are polarized to form heterocharge right after the voltage application. This heterocharge increases the local electric field at the electrode/sample interface. The heterocharge near electrode stimulates the injection of homocharge from the electrode.

Fig. 2 shows the homocharge dependence on SN contents near the cathode in DGEBA/MDA system. Under dc voltage application, a heterocharge near the electrode reduce the breakdown voltage, whereas a homocharge will increase it. However, after polarity reversal under ac conditions the homocharge is converted to hetero space charge and reduce the breakdown voltage. The negative charge near the cathode are plotted as a function of SN contents. The amount of charge near the cathode decreases with the increase of SN content. The unreacted SN can provide a bridge for charge transport so that the heterocharge was developed near the counter electrode and decrease the dielectric breakdown strength of the epoxy resin system.

Fig. 3 shows the effects of post-curing on the space charge formation in DGEBA/MDA/SN(10) system. As the post-curing time increased, the hetero-charge near the electrode increased. So the formation of hetero space-charge can reduce the dielectric properties of this system. By the formation of hetero space-charge, the electric field at the vicinity of needle apex is reinforced and distorted. The magnified electric field can cause the internal discharge. By the discharge, the polymeric system

Fig. 2. Effects of SN contents on the space charge formation.

Fig. 3. Effects of post-cure time on the space charge distribution

by PEA method.

is eroded and several pits are formed at the apex of needle. The pits grow and reach the counter electrode. This is the breakdown mechanism by space charge formation.

Conclusions

From the study on the effects of space charge on dielectric  deterioration of the modified DGEBA/MDA system by adding reactive additive nitrile, the following conclusions were obtained. Space charge accumulated around the defects and impurities changes the electric field and initiates the electrical treeings. Homo charge develops in pure DGEBA/MDA sample, while heterocharge develops in the modified systems with SN. The unreacted SN can provide a bridge for charge transport so that the heterocharge is developed.

Acknowledgement

The authors wish to acknowledge the financial support of the Korean Research Foundation made in the Program Year of 1996-1999.

References

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