## 3 Results and Discussion  
### Criteria of Insulator String Condition Level Based on Laboratory Test  
The samples of frames when the UV emission elongated from the first to fourth cup-and-pin insulator are shown in Figure 2. The UV count value that was displayed directly on the camera screen was ignored since the observation made was qualitative. The UV was clearly seen at the junction between the first and second cup-and-pin insulator, the second and third cup-and-pin insulator, and the third and fourth cup-and-pin insulator. This condition was represented by testing with a voltage of 130 kV.  
The condition with a voltage stress of 200 kV is displayed in Figure 3. The UV emission extended from the first to the sixth cup-and-pin insulator. The UV was found at the connection (at each cap-and-pin element) between the first and second cup-and-pin insulator, second and third cup-and-pin insulator, third and fourth cup-and-pin insulator, and fourth and fifth cup-and-pin insulator.  
The flashover was generated by testing with a voltage of 215 kV. The UV elongation had reached the 10 th cup-and-pin insulator as seen in Figure 4. The UV appeared in all connections, namely between the first and second cup-and-pin insulator, the second and third cup-and-pin insulator, the third and fourth cup-and-pin insulator, the fourth and fifth cup-and-pin insulator, the fifth and sixth cup-and-pin insulator, the sixth and seventh cup-and-pin insulator, the seventh and eighth cup-and-pin insulator, the eighth and ninth cup-and-pin insulator, and the ninth and tenth cup-and-pin insulator. Although the UV had fully extended from the energized terminal to the grounded terminal, the protection system did not operate immediately. The protection system switched off the AC voltage supply after a voltage of 215 kV was held for about 1 min.  
Figure 4: UV connection between the cup-and-pin insulators that had reached 100% of string length.  
Figure 3: UV connection between the cup-and-pin insulators that had reached 60% of string length.  
Figure 2: UV connection between the cup-and-pin insulators that had reached 40% of string length. The first cup-and-pin insulator was closest to the energized terminal.  
If the UV pattern was observed deeply, it could be generalized in two types. The first one was that the UV bridged the two adjacent cup-and-pin insulators. The second one was that the UV did not bridge the two adjacent cup-and-pin insulators. On that basis, the insulator string condition level was then assessed based on the UV existence and pattern. If the UV was not found, the insulator string condition level was categorized as one (1). If the UV was seen, but it did not bridge the two adjacent cup-and-pin insulators, the insulator string condition level was two (2). If the UV appeared and bridged the two adjacent cup-and-pin insulators, the insulator string condition level was three (3). In other words, level three was the worst state of insulator string condition, while level one was the safest state.  
The categorization of insulator string condition which was created in the laboratory test was in accordance with the contamination flashover mechanism as defined in [3]. There were seven phases before flashover condition. They were: (1) heating of the surface layer; (2) drying of the surface layer; (3) dry zone formation; (4) onset of dry zone arcing; (5) glow and streamer discharges; (6) slow arc growth; (7) fast arc growth. These seven phases could be simplified into three severity levels based on the discharge form. The first level included the phases 1-3 that had no discharge form. The second level comprised phases 4-5 that related to partial discharge development. The third level consisted of phases 6-7 relating to partial arc emergence.