Electrical Breakdown and Space Charge Formation at High-Temperature Region in Long-Time Heating Treatment PVC

• Published in: Electrical engineering in Japan 2014-07, Vol.188 (2), p.18-28
• Main Authors: Miura, Masakazu, Fukuma, Masumi, Kishida, Satoru
• Format: Article
• Language: English
• Subjects: Breakdown; Conduction heating; Degradation; Deterioration; Electric fields; electrical breakdown; Heat treatment; high temperature region; polyvinyl chloride; Polyvinyl chlorides; Space charge; thermal degradation
• ISSN: 0424-7760; 1520-6416
• DOI: 10.1002/eej.22528

SUMMARY Polyvinyl chloride (PVC) is widely used as an insulating material in various electrical products. It is reported that an exothermic reaction reaching temperatures above 150 °C can be caused by overload currents or inferior electrical wire connections before the ignition of electrical products. The exothermic phenomenon may cause deterioration of insulating properties in PVC due to its chemical decomposition. It is necessary to clarify the degradation of insulating properties in PVC under thermal stress exceeding 150 °C for the safe use of electric products. In this investigation the space charge distribution and conduction current in the heat‐treated PVC sheet were measured in the range from room temperature to 200 °C in the presence of a dc electric field, using a high‐temperature PEA system. Positive charge injection and increasing conduction currents were observed before breakdown above 100 °C in 100 °C 300‐h heat‐treated samples and in non–heat‐treated samples. The results indicate the thermal breakdown process from the analysis of conduction currents and electric fields. In samples exposed to higher temperatures (150 °C 100 h), the breakdown strength deteriorated strongly in the range from room temperature to 90 °C. Increases in conduction current were observed in the entire temperature range before breakdown of the 150 °C 100‐h heat‐treated PVC. This indicates that heat treatment above 150 °C degrades the breakdown properties in the range from room temperature to 90 °C due to thermal decomposition accompanied by dehydrochlorination in PVC. The electric field is intensified near the cathode due to positive charge accumulation, and the breakdown strength begins to deteriorate only above 90 °C. This shows that thermal stress exceeding 150 °C causes deterioration of insulating properties and that the breakdown process is affected by space charge formation in PVC.