Porcelain insulators and porcelain shells are important equipments in the operation of power plants and transformer substations which insulation and supporting wire. Internal deficiencies were easily generated during manufacturing process. Also, local stress of porcelain insulators and porcelain shells increased in formidable natural conditions. Even a tiny deficiency can also result in the destruction of porcelain insulators and porcelain shells and induce heavy accidents. Destruction of in service porcelain insulator mainly depends on three factors: material performance, stress level and hazard degree of deficiencies. Nondestructive examination of in service porcelain insulator is established in the analysis of material performance and stress, identifies whether deficiency existed, and the deficiency equivalent, deficiency property and the hazard degree to the porcelain insulator. So the authenticity of nondestructive examination of porcelain insulator is a critical constituent of the safe reliability.

The conventional examination is mainly checking the internal and surface deficiencies of porcelain insulators and porcelain shells. The common internal nondestructive examination is ultrasonic and radio examination, penetrant detection is only used in detecting surface deficiencies. But radial graphic inspection is not applicable in internal examination of porcelain insulator. The other detection methods such as magnetic powder detection and eddy current detection is not suitable. So the nondestructive examination of porcelain insulator is only ultrasonic detection.

Ultrasonic examination can detect the cracks buried inside 2~3mm of flange mouth of porcelain insulators and porcelain shells and the other methods can not do this. Through field investigation we can find that 95% fracture position located in flanges face(as shown in Fig.1.).So this place is the major in our ultrasonic examination work.

Ultrasonic examination is sensitive for plane deficiencies in material. When detect sound beam normal to deficiencies during detection progress then can get higher deficiencies reflect echo (as shown in Fig.2 and Fig.3.). Such as the detectable rate of porcelain insulators crack deficiency is very high. But due to the limitation of diffusion and sound of ultrasonic beam, we can get enough deficiencies reflect echo wave if the deficiencies big enough or reflect angle suitable for strip and sphericity deficiencies of porcelain insulators and porcelain shells.

Detection equipment is the critical factor of detection results, also it is the important facility to increase the authenticity of the nondestructive examination of porcelain insulators and porcelain shells. Now the models of ultrasonic detection equipments are various, from the angle of removable and utility, economic, and easy to study, it is better to choose the home equipments. We can build well foundation of ultrasonic detection if we can choose good equipments, detectors, test blocks and correct detection parameters.

We should use every detection chance of inservice units to detect the porcelain insulators and porcelain shells. The surface of the porcelain insulators and porcelain shell should be cleaned before detection. Before the ultrasonic examination, we should do penetrant test on the doubtful place to determine whether micro-cracks existed in the exteriority. To indeterminate place, we should do detection registration and the reflect echo of deficiencies from the internal surface should be especially pay attention to analyze, judge and make correct evaluation.

The new porcelain insulators and porcelain shells should be overall detected before bringing into service and the detection results should be recorded. Then we can do comparison between the next detection and the original deficiencies to determine whether the deficiencies extended after operated several days. The porcelain insulators and porcelain shells which have rather large deficiency should be replaced immediately.

The State Grid Co. defines the ultrasonic detection of porcelain insulators and porcelain shells definitely in the Technical Supervision Specification of PillarInsulators Used in 72.5KV and upward Voltage Class:

1. External appearance and ultrasonic detection of high voltage pillar insulators should be proceeded after conveyed to the loading bay.

2. Ultrasonic detection of high voltage pillar insulators should be proceeded again after installment and debugging.

3. Ultrasonic detection periodicity of pillar insulators operating in the transformer substations is one year for new equipments and 3 years for 72.5KV and upward from the commission day and the detection periodicity changing into one year after 3 periodicities, the detection rate should be 100%.

Now it is impossible to detect the deficiencies in the material completely with single nondestructive examination method. We should increase the safe operation reliability of equipments with various facilities to find the deficiencies and erase them. We should choose correct detection methods and dimension to increase the reliability of nondestructive examination results. First of all, we should know the material and ordinance of porcelain insulators and porcelain shells, potential position and properties of deficiencies. The best detection method and equipments should be selected after evaluating the shape of deficiencies, determination of orientation harmfulness and calculating the maximal permitted size of deficiencies under related standards. We should use several detection methods to get more information if possible. Of course we can increase the reliability of detection results of porcelain insulators and porcelain shells by the other facilities except from nondestructive examination and make comprehensive judgments found in multi-disciplinary knowledge. Finally, it is important to know the purpose of nondestructive examination and the material properties of porcelain insulators and porcelain shells. We can warrant the safe operation of porcelain insulators and porcelain shells by nondestructive examination.