One of the most important parameters for composite insulators is hydrophobicity, allowing superior insulation performance compared to ceramic cap & pin strings. However, these initial hydrophobic properties can deteriorate in service if there is continuous corona activity either directly on the housing or from nearby metal fittings. In fact, this can prove to be a specific mechanism leading to premature ageing and therefore must always be taken into account, even at the design stage. Electric field must always be controlled in the most vulnerable areas of these types of insulators.
Composite insulators have seen growing application in relatively clean environments due to their comparative ease of handling and attractive acquisition costs. More recently, voltage upgrading as well as compact design of new AC lines have become additional niche areas where composite insulators are being applied in clean environments. In the case of the latter applications, insulator arrangements are often designed relatively short to fit into the reduced space window of towers. Therefore, limiting maximum E-field becomes even more critical. Another growing area of application are composite station post insulators, especially those having a solid core since these do not differ much in flange design from composite line insulators. Three criteria have to be taken into account to assure optimal dimensioning of composite insulators equipped with grading rings:
1. Limiting electric field on grading ring & end fitting;
2. Limiting electric field along surface of insulator housing;
3. Limiting electric field at ‘triple point’ (where air & housing meet metal fitting).
All three are normally verified by E-field calculations, the first by the standard RIV test described in IEC 60437 2nd Edition (1997-09). The third criterion cannot be verified by a test while the second is as yet not verifiable by any test. Power companies, however, are now increasingly interested in having such verification.
Establishment of Criteria for Maximum E-Field
According to CIGRE Brochure 284, maximum E-field on the surface of a composite insulator (i.e. at the tip of the first shed from the end fitting) is estimated between 0.6 and 1.0 kV/mm. But this range may be optimistic. For example, research by U.S.-based EPRI indicated that a maximum limit on E-field of 0.45 kV/mm is preferable while past STRI research proposed 0.4 kV/mm. Others have estimated critical E-field level at only about 0.38 kV/mm. For maximum E-field on the metal fitting, CIGRE recommended a limit of 2.2 kV/mm. According to an earlier paper from EPRI, the value indicated for surface E-field on metal fittings and grading rings should be 2.1 kV/mm and this value is often being used as a reference for design purposes. According to internal CIGRE discussions, some utilities are now specifying values as low as 1.6 kV/mm – probably to account for possible manufacturing defects, surfaces that have become slightly damaged from improper handling or ageing of the grading rings in service. In an earlier paper, STRI had recommended 1.8 kV/mm.
Newer research summarizes work carried out to determine a practical limit for permissible E-field on insulator surfaces for design purposes. Initial work by EPRI to determine E-field threshold levels for water-induced corona (first published in 1999) has been expanded based on small as well as full-scale tests to refine these thresholds. For example, results from both natural ageing tests and artificial ageing tests show a clear tendency for reduced hydrophobicity on sheath sections where E-field exceeds about 0.3 to 0.4 kV/mm. Further fine-tuning the threshold has been based on both small-scale and full-scale laboratory tests as well as data from service experience. This has led to the following final criterion: average E-field on the insulator sheath should not be permitted to exceed 0.42 kV/mm for more than 10 mm along the surface. Such an averaging approach was introduced to avoid small yet significant geometry issues, which do not properly reflect insulator performance (i.e. there will be a sharp increase in E-field at such points). As for the end fitting seal (i.e. the triple point), E-field must not be allowed to exceed 0.35 kV/mm. Calculations should be modeled using 3-D E-field simulations and laboratory testing can also be considered.
Finally, the following criteria, presently adopted at STRI, have been used for many practical applications:
* Limit of E-field on grading ring & end fitting: 1.8 kV/mm
* Limit of average E-field along housing surface: 0.42 kV/mm
* Limit of E-field at triple point: 0.35 kV/mm
Practical E-Field Calculations
E-field calculations were performed for several power companies, totalling over 20 design arrangements using composite insulators. These calculations, involving both line and station post insulators, were intended to check the limits of E-field and in some cases to make recommendations on optimal design and positioning of grading rings as well as arcing horns.
Development of New Water Drop Corona Induced (WDCI) Test Methodology
Pre-Test on Short Insulators
As example, the Swedish Grid wanted to verify the results of E-field calculations on the surface of composite insulators. The first challenge was to create a wetting method that simulates water drop corona but still allows the test to be performed without disturbance from other parts of the insulator. Standard IEC rain could be one option but this created a lot of corona from the metal parts, thereby disturbing the focus of main interest, i.e. the surface close to the HV fitting. After some experimentation, the following points were concluded based on a pre-test using short insulators of about 52 kV voltage class:
• The most promising pattern of water drops for simulation and detection is a saturated surface with both large and small droplets. This was produced by hand spraying, according to IEC TS 62073, with water of about 100 μS/cm standard conductivity.
• Water drop induced corona can be simulated, detected and documented at maximum operating voltage and even below this level.
Testing performed on short insulators provided a better understanding of the mechanism of water drop corona and formed the basis to proceed with full-scale tests.
Full-Scale Testing on Different Insulator Designs
Full-scale (420 kV voltage class) testing was initially performed on one design. The goal was to find the most promising detection technique to document results, whether: daylight UV-camera; high-speed camera with light amplification; RIV-measurement; PD-measurement; acoustic measurement; or still photography with long shutter time. After comprehensive testing, it was decided that still image documentation was the best option as far as ease of use (i.e. allowing repeatability and reproducibility) as well as analyzing results and defining acceptance criteria. It was also determined that it is important to document results with photos taken from above as well as below the test object since corona could be hidden if viewed from only one direction. It is also desirable to promote greater efforts to create standardized techniques for corona detection. The preliminary test method developed and fine-tuned on a single composite insulator design was then verified on 3 much different designs of end fittings. The intention was to also refine the test criterion, taking into account possible ageing of the housing surface due to corona. Another issue was to fine-tune the desired range of humidity for the test. To evaluate ageing, corona inception voltage on the insulators was investigated after different periods of exposure. Effective (i.e. actual) corona exposure time differs between the tests for different insulators. It is known that, when exposure time is longer or recovery time for hydrophobicity is shorter, hydrophobicity and corona inception voltage decrease more rapidly. This might help explain different gradients of decrease in inception levels over time for different insulators. When E-field is sufficiently high to initiate water-induced corona, the surface starts to deteriorate and becomes more hydrophilic, as was observed during the tests. It was also shown that when contact angle of droplets is reduced, level of E-field needed to initiate corona also goes down. Thus, it was decided to establish the criterion for new insulators only, namely that no corona should occur at maximum operating voltage.
While investigating influence of relative humidity (RH), it was discovered that corona inception voltage at different humidity levels deviated by more than 10%. The results of electric field calculations at different voltage and humidity levels seem to correspond better with laboratory observations at higher humidity levels. In fact, since air is humid when water droplets are applied to insulators in service (e.g. from rain, dew, etc.), it is reasonable to use a more humid range while performing the test. Thus, the appropriate RH range proposed for the standard RIV tests seemed to be 45 to 75%.
The first proposed STRI approach for the criterion for visible corona was 75% of maximum operating voltage. Further development tests were performed to fine-tune the procedure of voltage application and, during these tests, observations of visible corona were compared to the results of E-field calculations. The procedure was as follows: the voltage was increased until water induced corona could be detected on the surface of insulators and then the voltage was decreased to 110% of maximum operating voltage (267 kV). This voltage application procedure was applied to 9 new insulators (3 identical insulators of same design). Resulting data is complemented by the E-field calculations at the same voltage levels. Predictions based on E-field calculations are supported by appearance/ absence of corona during the test. After fine-tuning, as discussed, this criterion was changed to 100% of maximum operating voltage (and it was also stated that the test is valid only for new insulators).
Testing Complicated Insulation Structures
One of the recent practical applications of the WDCI-test was for complicated structures using composite insulators. For example, Elia, the TSO in Belgium, has upgraded their double- circuit 150 kV transmission network into double-circuit 380 kV lines. The traverses are removed on affected towers and insulators replaced with complex composite cross-arms. One of these designs has gone through a full-scale WDCI-test, as described above. The acceptance criterion was that there should be no visible corona at 100% of maximum operating voltage. Absence of corona was predicted by electric field calculations at both 243 kV and 292 kV and verified by test results (blue marked areas confirm that no corona should be seen based on calculations and yellow marked areas indicate that corona might be observed). Similar results were found for the 420 kV full-scale, three-phase insulation structure used by National Grid in the U.K., i.e. corona was not observed at maximum operating voltage.
Final Proposal for WDCI Test
The final Water Drop Corona Induced (WDCI) test procedure should be performed as follows:
Set-up & Environment: As per RIV-test (a tower simulation could be useful to make the test more representative of service conditions) using a brand new insulator. The environmental conditions should be according to IEC 60437.
Test Objects: Three insulators of same design. If all pass, the test is passed. If one fails, the test shall be repeated with 6 new insulators and if none fail, the test is passed. If more than one failed during the first test, the test fails.
Wetting Method: The wetting should be made by the spray bottle recommended for the wettability test according to IEC 62073 from a distance of circa 25 cm and for approximately 5s. All metallic parts should be properly dried with paper/cloth to make sure no water could initiate corona that masks the main test. Conductivity of the water should be as standard IEC rain, 100 μS/cm.
Voltage Application: The voltage should be applied similar to the RIV test, i.e. first should be increased to corona inception voltage (or, if reached first, 120% of maximum operating voltage) and then reduced to a target voltage of 100% of maximum operating voltage. The observation of visible corona should be done at these two voltage levels and the voltage should be kept for 60 seconds at each level in order to allow proper documentation using still cameras.
Detection Techniques: The observation of corona should be made by two standard photo cameras, installed in parallel, with 30s shutter time. It is important to document results with photos taken both from above and below the test object since the corona could be hidden from one of the directions.
Acceptance Criterion: The proposed criterion is the absence of visible corona at 100% of maximum operating voltage.
Criteria for limitation of E-field in sensitive parts of a composite insulator have been established after comprehensive research performed separately by STRI and EPRI, but leading to basically the same numerical results. It is possible to verify the results of E-field calculations using the newly developed Water Drop Corona Induced (WDCI) test procedure. This test almost fulfils all typical IEC requirements with final reproducibility verifications conducted in another test laboratory. The test method is cost-effective if performed in conjunction with a standard RIV and/or corona test and can also be considered an additional type test in any user specification for composite insulators. A combination of E-field calculations and their verification by laboratory testing creates a solid basis for optimal dimensioning and positioning of grading rings and arcing horns on composite insulators.