In ex e tional circ mstan es, a tec nical commit e may pro ose the publ cation of a tec nical sp cif i ation when • the req ired s p ort can ot b o tained for the publ cation of an Inter
General
Clauses 5 and 6 focus on verifying the specified withstand degree of pollution at designated test voltages Additionally, the insulator's characteristics can be assessed across various voltage levels, corresponding to specific creepage distances This involves measuring the maximum withstand degree of pollution at different voltage levels or determining the maximum withstand voltage at varying pollution degrees Procedures for these evaluations are outlined in Clauses B.2 and B.3.
Directions for checking the laboratory equipment for artificial pollution tests, when requested, are given in Annex E.
Determination of the maximum withstand degree of pollution at a given test
Insulators must undergo various tests at specified test voltages and different pollution levels as outlined in sections 5.2 and 6.3 These tests should follow the procedures detailed in sections 5.6 or 6.6 and can be conducted in any order However, if two tests result in flashover at any pollution level, no additional tests should be performed at that or a higher pollution level Similarly, if three tests demonstrate withstand at any pollution level, no further tests should be conducted at that or a lower pollution level.
If three individual tests at a specific pollution level result in successful withstand, that pollution level can be considered the maximum withstand degree at the test voltage This is valid only if the next higher pollution level, as specified in sections 5.2 or 6.3, results in two tests that conclude with flashover.
I n the case of salt fog tests, the preconditioning process (see 5 5) shall be performed on the insulator before the determination of the maximum withstand salinity
If four withstand tests are conducted at a salinity of 224 kg/m³ and all tests succeed, the maximum withstand salinity is determined to be at least 224 kg/m³ Conversely, if one flashover occurs alongside three successful withstand tests at the same salinity, then 224 kg/m³ is regarded as the maximum withstand salinity.
Determination of the maximum withstand voltage at a given degree of
Individual tests will be conducted on insulators with a specified pollution level, using a series of voltage levels that increase by approximately 1.05 times each step The testing will adhere to the procedures outlined in sections 5.6 or 6.6 Tests may be performed in any order, but specific conditions apply: if two tests result in flashover at any voltage level, no additional tests will be conducted at that or higher levels; similarly, if three tests demonstrate withstand at any voltage level, no further tests will be performed at that or lower levels.
If three individual tests at any voltage level demonstrate withstand capability, that voltage can be considered the maximum withstand voltage for that pollution degree However, this is contingent upon the next higher voltage level resulting in two tests that conclude with flashover.
I n case of salt-fog tests, the preconditioning process (see 5 5) shall be performed on the insulator before the determination of the maximum withstand voltage.
Determination of the 50 % withstand voltage at a given degree of pollution
The insulator must undergo a minimum of ten "useful" tests under specified pollution conditions Testing should adhere to sections 5.6 or 6.6, with the applied voltage adjusted using the up-and-down method Each voltage step should not exceed 10% of the anticipated 50% withstand voltage.
The initial "useful" test is identified as the first test that produces a result differing from the previous one This test, along with at least nine subsequent tests, will be deemed useful for determining the 50% withstand voltage.
The calculation of the 50 % withstand voltage shall be made using the following formula:
Ui is an applied voltage level ni is the number of groups of tests carried out at the same applied voltage level Ui
N is the number of useful tests
More details on the up-and-down method and processing of the relevant results are found in
Alternatively, the method of maximum likelihood can be used to obtain U50 % More information about this method will be found in the relevant literature
Additional recommendations concerning the solid layer method procedures
General
The following recommendations delve into solid layer methods, offering criteria for auxiliary controls during testing They are designed to assist users who may lack expertise in this area, helping to prevent inaccurate results.
Contamination practice
When using the spraying or "flow-on" method, the insulator can be treated while positioned in the test chamber In contrast, the dipping method requires the insulator to be contaminated prior to its installation in the chamber For insulators made up of multiple units in series, each unit must be dipped individually and maintained in a vertical position until the contaminant has fully dripped off and dried.
After a contaminating operation, if a blotched layer appears on the insulator, it must be washed and cleaned again as per section 4.3.2 Subsequent trial contaminations should be conducted, each followed by appropriate washing, until a continuous layer is achieved on the insulator Testing can then commence Generally, experience indicates that repeating the operation several times is sufficient to prepare the insulator surface for satisfactory contamination without the need for any preconditioning process.
Drying of the pollution layer
Natural drying of the pollution layer on insulators can be effective if maintained for 6 to 8 hours with relative humidity below 70% Lower humidity levels can reduce drying times When using hot air to expedite drying, it is crucial that the method does not deposit materials that could impact the insulator's wetting or pollution level Additionally, controlling the speed of the hot air flow is essential to avoid disturbing the pollution layer on the insulator.
Checking the wetting action of the fog
The test insulator, contaminated with the same suspension as that which will be used for the actual test, shall be placed in the location where it will be tested
A dummy insulator, ideally of the same type and similarly polluted, can be utilized in place of the test insulator It should be positioned as close as possible to the location of the insulator being tested and maintained at an equivalent height above the floor.
During the operation of fog generators in a real test scenario, the conductance of the wet layer on the test insulator must be measured periodically following a specific procedure.
Each measurement of the layer conductance consists of applying to the insulator a voltage
For frequencies ranging from 0 to 100 Hz, the overall creepage distance must not be lower than 1,000 V per meter while measuring the current The voltage should be applied long enough to accurately assess the voltage and current values, yet brief enough to prevent significant errors caused by heating or drying of the pollution layer Additionally, no discharge activity should take place during this process.
The increase of the layer conductance with time shall be monitored The rise time Tc shall be
The temperature control (Tc) must be maintained between 5 minutes and 70 minutes, as illustrated in Figure C.1 If Tc falls outside this range, repeated humidification with varying steam input rates should be performed until it meets the specified limits The determined steam input rate will then be utilized in the actual tests.
N OTE 1 The li mi ts for the ri se ti me are based on experience from a c tests and h ave been confirmed by some d c tests
It is recommended to install a monitor insulator in the test chamber to gather valuable information for future revisions of the testing procedures.
Checking fog uniformity for large or complex test objects
For large or complex test objects, such as those exceeding 3 meters in length or featuring multiple insulated components under voltage, it is crucial to ensure uniform fog density and wetting rates across all parts Uneven wetting, particularly when the live end is wet while other areas remain dry, can result in inaccurate test outcomes.
Measurements are conducted in proximity to the test object or at designated positions that correspond to its location when it is absent The test laboratory determines the appropriate measurements, with common examples including various testing methodologies.
• Conductance measurement on polluted dummy insulators (measured as described in Clause C.4)
The dummy insulators shall be placed at positions that fall within the following guidelines: – Within 1 to 1 5 m lateral distance from the test object
For test objects up to 3 meters tall, a single measurement site located near the middle is adequate However, for objects taller than 3 meters, additional measuring sites are necessary to encompass the entire height It is advisable to maintain a maximum distance of 3 meters between adjacent measuring sites.
Conductance measurements on polluted test objects are carried out using appropriate electrodes, such as copper adhesive tape, as outlined in Clause C.4 It is essential to position the electrodes to ensure a uniform wetting distribution across the test object In scenarios involving multiple parallel insulator configurations, individual measurements on each insulator arm are unnecessary.
I t is recommended to carry out the wetting rate check as described in Clause C 4 after any adjustments in order to ensure that the steam input rate is still correct.
Fog input to the test chamber
Fog is created by steam from boiling water in open tanks or by low-velocity steam introduced through large spray nozzles The fog can only enter the test chamber once the steam generation stabilizes Consequently, when using tanks for steam production, they must remain covered until the water reaches its boiling point.
The insulator shall be positioned so that th e visible fog surrounds it as uniformly as possible
The temperature rise in the test ch amber, measured at the heigh t of the test object, shal l not exceed 1 5 K by the end of the test
Duration of the withstand test
Current understanding suggests that the risk of flashover is minimal when a test object endures the test voltage for 100 minutes after fog application This risk can be assessed by measuring the leakage current on the energized test object during the test, which typically peaks and then declines as the layer is washed away If 30 minutes have elapsed since the onset of discharge activity, as indicated by leakage current measurements, the flashover risk is deemed negligible.
When flashover is judged improbable by measurement of the leakage current or other techniques, the test can be stopped and considered as a withstand.
Evaluation of the reference salt deposit density ( SDD )
To ensure accurate sampling, it is essential to completely remove the pollution layer from the selected insulator area This process requires a minimum of three consecutive wipings of the designated area.
To effectively dissolve the collected deposit, use 2 to 4 liters of de-ionized water per square meter of the surface being sampled The success of the removal process can be evaluated by measuring the remaining deposit.
It is recommended to ensure that the ratio of local SDD measurements, whether taken from the upper or lower surface of sheds, to the overall SDD of the entire sampled insulator area falls within the range of 0.7 to 1.3.
Figure C.1 – Determination of layer conductance and evaluation of its rise time T c = t 2 – t 1
La ye r c on du ct an ce (p er u ni t o f t he m ax im um v al ue )
Annex D (informative) Information to check equipment for artificial pollution tests
Table D.1 provides the withstand characteristics of selected reference suspension insulators to aid in the calibration of laboratory equipment for artificial pollution tests These values, derived from a laboratory situated at an altitude of less than 1,000 meters above sea level, adhere to the guidelines outlined in the current technical specification.
N OTE 1 Val ues to complete Tabl e D 1 wi l l be i nserted after d iscu ssion of avail abl e resu lts from d i fferent l aboratories
The reference insulators listed in Table D.1 should not be considered representative of their respective classes Their selection does not indicate any judgment of merit or assumption of superior performance compared to other similar types under testing or operating conditions.
Th e choi ce of these i nsu lators i s based on the recommend ati ons for the comparati ve i nter-l aboratory tests of the
Laboratories starting artificial pollution tests or which are located at altitudes higher than
At altitudes of 1,000 meters above sea level, or when alternative testing methods are employed, variations in results may occur compared to the values presented in Table D.1.
The withstand degree of pollution and the 50% withstand voltage (U50%) for reference insulators were evaluated through artificial pollution tests conducted in a vertical position The results were obtained following the testing procedures outlined in the current technical specification.
I nsu lator type and nu m ber of u ni t per stri n g
Maximum withstand degree of pollution U 50 %
Applied voltage kV unit Salt fog method
Salt fog method Solid layer method a
Salinity Creepage distance U 50 % SDD Creepage distance U 50 % mm/kV kV kg/m 3 mg/cm 2 kg/m 3 mm kV mg/cm 2 mm kV
Anti fog creepage d istance: 570 mm
An ti fog creepage d istance: 390 mm
The values presented are determined from a single laboratory, raising concerns about the reproducibility of the results Additionally, the impact of the steam rate and the absolute amount of steam, which varies with the size of the testing chamber, remains to be clarified The testing chamber has a diameter of 320 mm.
Supplementary information on artificial pollution tests on insulators for voltage systems of ± 600 kV and above (solid layer method procedure B)
General
This annex provides essential supplementary information regarding testing facilities, wetting systems, the assessment of wetting action, and steam input rates, specifically related to the solid layer method—procedure B for conducting artificial pollution tests on insulators designed for voltage systems exceeding ±600 kV.
Test chamber
The recommended minimum distance between the live parts of a test object and any earthed structure should exceed the maximum phase-to-earth test voltage (in kV) divided by 200 This same distance must also be upheld between the live part of the voltage supplier bushing and any part of the test object.
Fog generator
Fog can be produced through various methods, including the use of boilers in a test chamber, horizontal and vertical nozzles supplied by an external steam generator, or a combination of these techniques.
When nozzles are used they shall not be directly pointed at the test object.
Wetting action and uniformity of fog density
Check of the wetting action and fog distribution uniformity according to Clauses C 4 and C 5 is recommended at least at each change of test configuration
To reduce significant scatter in test results for large test objects, especially at UH V test levels, it is essential to maintain precise control over all test parameters.
During high-temperature conditions, such as in summer, it may be essential to increase the initial steam input rate to approximately 0.080 kg/h/m³ This rate should be maintained for the first 10 to 20 minutes of the test before being adjusted downward for the remainder of the testing period.
Test of very large insulators
To assess the pollution performance of insulators, particularly for comparing various types and profiles, conducting tests on relatively short insulator sets (with an arcing distance of at least 1.5 m, provided they represent the full set in terms of radial geometry and profile) may be adequate.
The following problems need further investigation:
The effectiveness of preconditioning with direct current (d.c.) voltage significantly impacts the outcomes of the salt fog procedure Additionally, the presence of non-soluble materials can alter the test results during the solid layer procedure Furthermore, the steam rate and the actual d.c electric field play crucial roles in determining the wetting characteristics of the materials tested.
– application of atmospheric correction factors;
– salt fog test method for voltages greater than ±600 kV
I EC 60507, Artificial pollution tests on high-voltage ceramic and glass insulators to be used on a c systems
[1 ] Outdoor insulation in polluted conditions: Guidelines for selection and dimensioning
Part 2: The DC case CIGRE Technical Brochure No 51 8