| Composite insualtors have been applied enormously since its first operating in transmission lines thanks to its many advantages,such as small size,light weight and excellent anti-contaminated characteristic.The fact that huge amounts of composite insulators are utilized in operation requires that they need to have good performance to guarantee the safe of transmission lines.Interface between core and housing is an important structure in a composite insulator.Poor interface quality may lead to degradation of both electrical performance and mechanical performance and finally result in abnormal fracture.So as to esure the interface quality,efficient and reliable methods are needed to test the interface properity of composite insulators.Sixty-eight composite insulators in Guangdong and Hainan were sampled from transmission lines.Several experiments including water boiling test,DC(direct current)leakage current test,power frequency voltage infrared thermography test and step front impulse voltage test were conducted.Test results were analyzed and compared for verification.Then interface property test methods were proposed.The current-voltage curves were analyzed during DC leakage current test.Besides,rates of current change as well as insulation resistance change were computed.Results show that leakage currents grow obviously after boiling.Differences of current rise degree can be found among test samples.For most 110 k V samples,their leakage current changing degrees(ΔI%)are below 500% when voltage gradient is set as 1k V/cm,with their currents increasing from 30μA to about 150μA.The ΔI%s of most 220 k V smaples are less than 25 times at 0.5 k V/cm.Insulation resistances of each sample decrease after boiling.Differences of resistance decrease degree can be found among test samples,too.Most 110 k V smaples show their insulation resitstance changing degrees(ΔR%)between-50%～-80%,with their insulation resistance declining from 4000MΩ to 1000MΩ at 1k V/cm.Many 220 k V smaples show their insulation resitstance changing degrees(ΔR%)between-50%～-80% at 0.5 k V/cm.The temperature rise process was observed during power frequency voltage infrared thermography test.Additionally,the infrared image axial temperature and characteristcs of heating sections were analyzed.Results indicate that the temperatures of samples rise quickly in the first 5min,then they become smooth and remain stable around 30 min.Most smaples have their axial temperature uniformly distributed,with their sheath temperature around 40℃.Several samples show local temperatures of some parts much higher than average.The highest temperature reaches 133.1℃.The temperatures of 20 K heating sections are between 23℃and 133.1.0℃,while their lengths are between 1.1cm and 22.9cm.There are 5 samples having 80 K heating sections with 1.3cm in length.Their temperatures are all above 84.8℃.The number of fitting ends generating heating sections are 3 times as many as that of no-fitting ends.The connection zones are prone to form interface defects.Finally,based on the test results metioned above,two kinds of interface property test methods are proposed,namely DC leakage characteristic method and power frequency voltage infrared thermography method.For the four problem smaples,the success rete of infrared thermography method is 100% while DC leakage method is able to identify three of them.DC leakage method is sutable for long interface defects.In order to detect interface defects in composite insulators reliably,these two methods are supposed to be combined with each other.Research in this paper improves interface property test methods for composite insulators,which are able to detect interface defects effectively.This is meaningful for the safe operation of transmission lines. |
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