Comparison of Methodologies to Detect Damaged Composite Insulators

As application of composite insulators on overhead lines has spiraled, methods to monitor them for critical damage (especially internal defects) have taken on prime importance. Apart from the obvious impact on reliability of networks, trustworthy condition assessment is also important to assure the safety of maintenance personnel prior to live-line work. Unfortunately, no single diagnostic technique has emerged that can identify all the possible types of damage that could exist in a composite insulator. As a result, a variety of complementary tools and procedures are typically employed. Those methods that can be applied in-service are of greatest interest to power system operators since inspections can then be conducted with lines energized.

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In recent years, CIGRE WG B2.03 (now disbanded) and IEEE 15.09.04.01 both worked toward the goal of issuing a state-of-the art summary of the different diagnostic techniques for composite insulators. An edited 2012 contribution to INMR by insulator expert Igor Gutman in Sweden, which appeared in INMR WEEKLY TECHNICAL REVIEW of Aug 17, 2020, reviewed and compared alternative techniques.

INMR recognizes that there are varied opinions on the comparative effectiveness of different assessment methodologies and tools. This addendum to that original contribution contains remarks recently proposed by Charles Jean, technical expert at Positron, to complement what was originally presented.


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A.Corona Ring

Gutman Contribution from 2012:                                                  

Experiments conducted at STRI showed that the E-field probe had a detection rate similar to that of the UV camera and identified most defects located at the fitting. This was also the only diagnostic tool that could clearly detect a ‘floating potential’ defect, i.e. moisture in the middle of an insulator. The E-field probe, however, cannot detect low severity defects near the fitting due to its sled construction that is obstructed by the corona ring. It therefore does not allow measurements closer than about 15 cm (i.e. the first four sheds from both ends) from a fitting equipped with corona rings.

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Jean Remarks from 2020:                                                                              

As indicated in the original article, a low severity defect cannot be detected near the corona ring due to mechanical obstruction. Near the corona ring center, the E-field is low. To detect a conductive defect, presence of an E-field is required. The corona ring is acting as a shield to protect the insulator by practically eliminating the voltage differential on that section of the insulator and therefore also the E-field under the ring and near the ring along its axis. Any conductive defects in this zone are not dangerous and, even if there was a conductive path in this area, it would have practically no influence on the overall electrical insulation properties of the entire composite insulator. The number of sheds in this protected area may vary depending on the mechanical design of the end fitting and the shed spacing. In the Fig. below, two sheds are located inside the protected area near the corona ring. Measurements are not necessary near the corona ring because defects near the corona ring do not compromise the electrical safety of the insulator.

Comparison of Methodologies to Detect Damaged Composite Insulators two sheds are located inside the protected area near the corona ring
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B. IR Thermal Detection

Gutman Contribution from 2012:                                                                 

For the specific types of defects investigated, IR thermal detection is superior to visual inspection or other techniques.

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Jean Remarks from 2020:

Low severity defects will not produce IR radiation. The IR method is not as sensitive as the E-field method. It cannot detect floating potential defects and defects that are not visible from the view of the camera. Connected defects may generate E-field intensity 10 to 100 times higher than floating potential defect. The presence of a connected path for the current will produce some heat that can be detectable using the IR method. The fact that the E-field method detects floating defects demonstrates that the electric field method is indeed very sensitive to small defects (connected or floating). It is far more sensitive in detecting small defects and does not require the presence of a current in the defect. For maintenance purposes, it is preferable to detect low severity defects as early as possible.

Comparison of Methodologies to Detect Damaged Composite Insulators Floating potential defects not detectable by the IR method
Floating potential defects (not detectable by the IR method).
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The sensitivity of the IR method is too low to allow the detection of low severity defects such as this floating potential defect that was detected by the E-field method. The above graphic was produced by the Composite Insulator Tester.

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C. Table 1

Gutman Contribution from 2012 with remark by Jean:                                                                  

Actually, the E-field diagnostic is now often performed from a Bucket Truck.

Comparison of Methodologies to Detect Damaged Composite Insulators E field diagnostic
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