Iec ts 61956 1999

SPÉCIFICATION TECHNIQUE CEI IEC TECHNICAL SPECIFICATION TS 61956 Première édition First edition 1999 09 Méthodes d’essai pour l’évaluation de la formation des arborescences d’eau dans les matériaux is[.]

Domaine d'application

This technical specification outlines testing methods for evaluating polyethylene (PE) and cross-linked polyethylene (PRC) compositions in relation to water trees, aiming to estimate their performance under alternating electrical stress in the presence of water Two methods are detailed: Method I, which assesses insulating materials alone, and Method II, which involves "sandwich" structures that include an insulating material in intimate contact with semiconductor screens.

Définitions

The formation of water treeing occurs through a degradation process, particularly observed in low-density polyethylene (LDPE) and cross-linked polyethylene (XLPE) under alternating stress and humid conditions This process leads to the creation of dielectric weak zones that make up the water tree structures.

Water treeing is characterized by its tree-like structure with hydrophilic dendrites, initially appearing as chains of water-filled cavities that later form a network of microscopic channels with hydrophilic surfaces These structures typically develop under electrical conditions in humid environments and can grow to lengths of about a millimeter over several years There are two main types of water trees: a) the butterfly-type tree, which resembles a butterfly knot, consists of straight branches radiating in opposite directions from a central point and is usually oriented in the direction of the electric field; b) the open tree, which resembles a tree with a trunk reaching the surface of the insulation or the insulation/screen interface, with branches generally aligned with the electric field from the surface of the insulation or interface.

LICENSED TO MECON Limited - RANCHI/BANGALORE FOR INTERNAL USE AT THIS LOCATION ONLY, SUPPLIED BY BOOK SUPPLY BUREAU.

METHODS OF TEST FOR THE EVALUATION OF WATER TREEING

This technical specification outlines testing methods for assessing water treeing in polyethylene (PE) and crosslinked polyethylene (XLPE) compounds under alternating electric stress in wet conditions It details two approaches: Method I focuses on evaluating insulating materials independently, while Method II assesses insulating sandwiches that include an insulating material in direct contact with semiconducting screens.

This technical specification references several normative documents, which are integral to its provisions For dated references, any amendments or revisions to these publications are not applicable However, parties involved in agreements based on this specification are encouraged to consider the latest editions of the referenced normative documents In the case of undated references, the most recent edition of the relevant normative document is applicable Additionally, members of IEC and ISO keep registers of currently valid International Standards.

IEC 60243-1:1998, Electrical strength of insulating materials – Test methods – Part 1: Tests at power frequencies

IEC 61072:1991, Methods of test for evaluating the resistance of insulating materials against the initiation of electrical trees

Water treeing is a degradation process that occurs in low density polyethylene (LDPE) and cross-linked polyethylene (XLPE) when subjected to alternating current (a.c.) stress and moist conditions This phenomenon leads to the development of dielectrically weakened regions known as water trees.

Water trees are hydrophilic, dendritic structures that develop under wet and electrical conditions, often reaching lengths of about 1 mm over several years There are two main types of water trees: the bow tie tree, which features straight branches radiating from a central point and aligns with the electric field within the insulation, and the vented tree, which has a trunk that extends to the insulation surface or interface, with branches oriented away from the surface in the direction of the electric field.

2 Méthode d’essai I (essai sur plaques)

Principe

Les essais sur plaques (variante A et variante B) ont pour but d'évaluer le développement d'arborescences d’eau dans les matériaux isolants à base de polyéthylène basse densité

(PEBD) et de polyéthylène réticulé (PRC).

The two test variants utilize plate-shaped specimens and the same testing cell Both tests are preselection trials aimed at differentiating and selecting insulators based on their water tree formation.

The constraint concentration test (variant A) is primarily designed to assess the development of open tree structures originating from the tips of protrusions on the screen or penetrations in the insulation at the insulation/screen interface, simulated by needle-shaped water-filled impressions A plate with several identical impressions is subjected to an electric field in a test cell, as outlined in section 2.3.1, with water present, leading to an electric field concentration at these points.

This essay evaluates the formation of butterfly-type branching structures within the mass of the test tube, specifically by positioning away from areas of high electric field concentration.

The uniform electric field test (variant B) aims to assess the development of butterfly-shaped branching when a plate is exposed to water while simultaneously subjected to a uniform electric field in a testing cell as described in section 2.3.1.

Eprouvettes

Circular test specimens with a diameter of (35 ± 1) mm can be produced by cutting with a punch from a plate of one of the following thicknesses: (4.0 ± 0.1) mm, (3.0 ± 0.1) mm, or (2.0 ± 0.1) mm It is essential that comparisons between materials are made using specimens of identical thickness.

For tests involving open tree structures, plates can be produced by molding pellets under pressure In the case of butterfly-type tree structures, it is advisable to homogenize the composition through extrusion to prevent the concentration of additives or impurities on the surface of the pellets In all scenarios, it is crucial to take extreme precautions to avoid contamination of the materials, which could subsequently affect the plates and strips.

In the case of compositions based on PRC (using dicumyl peroxide), the sheets are preformed in a press at approximately 130 °C They are then heated to 180 °C, maintained at this temperature for 30 minutes, and cooled to 70 °C, all while under pressure The sheets removed from the press are degassed for 72 hours.

(90 ± 2) °C pour l'élimination des résidus volatiles.

In the case of LDPE-based materials (without crosslinking agents), the sheets are preformed at approximately 130 °C in a press, then heated to around 200 °C and cooled, still under pressure, to about 70 °C.

L'expérience a montré que lorsque l'on moule des plaques avec des compositions à base de PE ou de PRC, une pression exercée d'au moins 5 N/mm 2 conduit à des résultats satisfaisants.

The plaque tests (variant A and variant B) are intended to assess the development of water trees in low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) based insulating materials.

Both test variants utilize plaque-shaped specimens and the same test cell, serving as screening tests to differentiate and preselect insulating compounds based on their resistance to water treeing.

The stress concentration test (variant A) aims to evaluate the formation of vented trees originating from the tips of screen protrusions or insulation intrusions at the insulation/screen interface This is simulated using needle-shaped, water-filled indentations A plaque featuring multiple identical indentations is subjected to both water and an electric field simultaneously, creating electric field stress concentrations at these specific points within a designated test cell.

By means of this test, bow tie treeing can be additionally assessed within the bulk of the test specimen, away from the stress concentration areas.

The uniform field stress test (variant B) aims to evaluate the growth of bow tie trees when a plaque is subjected to both water and a consistent electric field within a specified test cell.

Test specimens in the form of disc-shaped slabs with a diameter of (35 ± 1) mm can be achieved by punching them out from plaques of one of the following thicknesses:

(4,0 ± 0,1) mm; (3,0 ± 0,1) mm; (2,0 ± 0,1) mm Comparison between materials should be made at equal thicknesses only.

For vented tree testing, plaques can be produced from pellets through press molding In bow tie tree testing, it is advisable to homogenize the compound via extrusion to prevent the concentration of additives and impurities on the pellet surfaces It is crucial to exercise extreme caution to avoid contamination of both the materials and the resulting plaques and slabs.

XLPE based compounds, utilizing dicumyl-peroxide, undergo a preforming process within the press frame at approximately 130 °C The plaques are then heated to 180 °C for 30 minutes while under pressure, followed by a cooling phase to around 70 °C After removal from the frame, the plaques are annealed for 72 hours at a temperature of (90 ± 2) °C to eliminate volatile by-products.

LDPE-based materials, when not using a crosslinking agent, are initially pre-formed at approximately 130 °C within the press frame They are then heated to around 200 °C and subsequently cooled to about 70 °C, all while maintaining pressure.

Experience shows that when making plaques from XLPE or PE based compounds, a press force of at least 5 N/mm 2 of plaque gives satisfactory results.

Appareillage d'essai

Cellule d'essai

La cellule d'essai [10], représentée à la figure 1, comprend:

– une coupelle en polyéthylène haute densité (PEHD) avec, dans son fond, une ouverture circulaire;

– une électrode de terre, incluant une base support en PEHD.

– six vis en nylon pour comprimer l'éprouvette entre la coupelle et l'électrode de terre;

– un couvercle en plastique transparent à travers lequel l'électrode haute tension est installée Cette électrode doit être constituée d'un métal inoxydable, comme le palladium, le platine ou un métal équivalent.

Figure 2 illustrates the test cell, which can be constructed using various materials, provided that the testing principles are maintained Care should be taken in selecting materials to prevent soluble contaminants from the construction materials from leaching into the test solution.

Electrodes

La coupelle de la cellule d'essai contient une solution de NaCl (voir 2.3.3) devant constituer l'électrode haute tension Il convient que l'électrode d’arrivée à immerger dans la solution de

NaCl soit faite d’un métal noble comme le platine ou le palladium.

L'électrode de terre est une électrode circulaire en laiton ayant une surface supérieure plate, de 20 mm à 25 mm de diamètre, dont les bords sont arrondis selon la figure 1.

Liquide d'essai

La solution de NaCl (1,8 mmol/litre) est obtenue en dissolvant 0,1 g de NaCl dans 1 l d’eau distillée et désionisée.

This water is prepared through distillation in a glass-free apparatus, then passes through a mixed bed deionization column and is stored in a sealed polyethylene container Before use, the water must be checked, ensuring its pH is within the required range.

(7 ± 0,1) et sa conductivitộ telle que (σ ≤ 100 àS/m).

Ensemble support d'aiguilles/piston

The needle/piston support assembly, illustrated in Figure 3, is specifically designed for testing under concentrated electric fields This setup allows for the simultaneous imprinting of eight needle tips, alongside two rows of four needles positioned at the center of the upper face of the specimen Each of the eight needle tips extends by (0.5 ± 0.05) mm from the surface of the needle support and is secured in place by a pair of screws to ensure accurate positioning.

According to IEC 61072, the needles must be made of stainless steel, with a tip curvature radius of (4 ± 1) µm and an angle of 30° The preferred overall diameter ranges from 0.7 mm to 1.0 mm.

Before use, needles must be cleaned and dried carefully to avoid damaging their tips The shape and curvature of each needle's tip should be checked after cleaning, as well as before and after mounting on the holder Great care must be taken to ensure that the needle tips are free from any signs of corrosion.

1) Des aiguilles de ce type peuvent être obtenues auprès de Ogura Juwel Industry Co Ltd 7-12 Omori Kita 5,

Chome, Ota-ku, Tokyo 143, Japon.

The test cell [10], shown in figure 1, comprises:

– a cup, made of high density polyethylene (HDPE), with a circular opening at its bottom;

– a ground electrode assembly including a HDPE base support;

– six nylon screws to compress the test specimen between the cup and the ground electrode;

– a transparent plastic cover through which the high voltage lead is installed This lead shall be made of a noble metal, such as palladium, platinum or other.

Figure 2 illustrates a dimensional drawing of the test cell, which can be constructed using various materials and designs, provided that the fundamental testing principle is maintained It is crucial to select materials carefully to prevent the leaching of soluble contaminants into the test solution.

The test cell's cup is designed to hold a NaCl solution, serving as the high voltage electrode It is essential that the lead submerged in the NaCl solution is composed of a noble metal, such as platinum or palladium.

The ground electrode is a circular brass electrode having a flat upper surface, 20 mm to

25 mm in diameter, with rounded edges according to figure 1.

The NaCl solution (1,8 mmol/litre) is obtained by dissolving 0,1 g of NaCl in 1 l of distilled and deionized water.

This water is produced by passing distilled water through a mixed bed deionizing column using a non-glass apparatus and storing it in an airtight polyethylene container Prior to use, the water must be tested for pH, which should be 7.0 ± 0.1, and conductivity, which should not exceed 100 µS/m.

The needle holder – plunger assembly, illustrated in Figure 3, is designed for stress concentration testing, enabling simultaneous indentation with eight needle tips arranged in two parallel rows of four Each needle tip extends (0.5 ± 0.05) mm from the holder's surface and is securely fixed in place by two set screws to maintain precise positioning on the upper surface of the test specimen.

The needles according to IEC 61072 shall be made of stainless steel and have a tip radius of

(4 ± 1) àm and an included angle of 30°, the preferred overall diameter is 0,7 mm to 1,0 mm 1)

Prior to use, the needles shall be cleaned and dried, taking care not to damage the needle tip.

Each needle must be thoroughly inspected for its tip radius and shape after cleaning and before and after being secured in the holder It is crucial to take extreme care to ensure that the needle tips are free from any corrosion products.

1) Such needles may be obtained by suppliers as Ogura Juwel Industry Co Ltd 7-12 Omori Kita 5, Chome,

The needle holder and pistons are assembled as follows: first, the needle support is installed on the mounting plate using the needle holder mounting screws Next, the needle holder is connected to the piston with a connecting screw After tightening the screw, the piston is raised to align with the upper surface of the needle holder's mounting plate Any gap between the mounting plate and the upper cell guide can be eliminated by adjusting the screw tightness A detailed diagram of the needle support is provided in Figure 4.

Mode opératoire (essai de vieillissement)

Mode opératoire sous champ électrique uniforme (variante B)

L'éprouvette circulaire est fixée dans la cellule d'essai Les six vis en Nylon sont resserrées uniformément en serrant progressivement et sans excès les vis diamétralement opposées.

The test cell cup is filled three-quarters full with the test liquid and sealed with a cover that has a high-voltage electrode The lower part of the test cell, including the sample, is submerged in silicone oil to eliminate surface discharges The high-voltage electrode is connected to an alternating voltage source (48 Hz to 62 Hz), while the ground electrode is connected to the ground It is important that the average value of the resulting electric field is 5 kV effective/mm, for instance, 20 kV effective for a 4 mm thick plate.

Les conditions de vieillissement préférentielles sont 240 h à température ambiante D'autres valeurs de champ électrique, d'autres durées et températures de vieillissement sont possibles.

Mode opératoire avec concentration de champ électrique (variante A)

La coupelle de la cellule d'essai est préparée comme décrit en 2.4.1 Afin d'effectuer les empreintes dans la plaque, la cellule d'essai (sans couvercle), l'ensemble porte-aiguilles et piston,

A 10 ml solution of NaCl and a weight of 1,000 g are placed in a preheated oven at 60 °C After 15 minutes, the NaCl solution is poured into a dish, and the piston is positioned in the cell The piston is carefully slid down until the needle tips rest on the test tube's surface The weight is gently placed on the piston handle, and the oven is closed After one hour, the weight is removed, and the cell with the piston is taken out of the oven Cooling to room temperature takes approximately 30 minutes Once the piston is removed, the cell dish is filled three-quarters full with the NaCl solution The needles are then immediately cleaned with distilled water, dried by blowing, and stored in a dry place to prevent corrosion.

La cellule d'essai est couverte, reliée à la haute tension et à la terre, et l'éprouvette est vieillie comme indiqué en 2.4.1.

Examen après vieillissement

Examen microscopique des arborescences de type nœud papillon (variante B)

After aging under tension, the specimen undergoes an examination to identify any potential branching structures The specimen is sliced using a microtome into several sections perpendicular to the aged surface in the presence of water, measuring 20 mm in length and 0.1 mm to 0.2 mm in thickness; 20 sections are evenly examined All sections are stained following the procedures outlined in [13] The stained sections are then analyzed under a microscope at a magnification of ×100 In each section, the maximum length of butterfly-shaped branches is measured, with density calculated only for those exceeding 50 µm (including both wings) Due to the high number of observed branches in the material, only 1/5 of the total surface area of each section is examined The average density of branches longer than 50 µm for all examined sections is then calculated and recorded in the test report.

To assemble the needle holder and plunger, first, attach the needle holder to the mounting plate using the mounting screws Then, connect the needle holder to the plunger with the connecting screw After tightening the screw, lift the plunger to align the upper surface of the needle holder mounting plate with the bottom surface of the cell-top guide Adjust the screw's tightness to eliminate any gap between the mounting plate and the cell-top guide.

A detailed drawing of the needle holder is shown in figure 4.

2.4.1 Test procedure with uniform field stress (variant B)

The circular test specimen is securely placed in the test cell, and the six nylon screws are tightened evenly and gradually This is achieved by working on diametrically opposite screws to avoid applying excessive torque.

The test cell is filled to three-quarters with the test liquid and capped with a cover that includes a high voltage lead bushing To prevent surface discharges, the lower part of the test cell, along with the test specimen, is immersed in silicone oil The high voltage lead is connected to an a.c voltage source operating between 48 Hz and 62 Hz, while the ground electrode is connected to ground The target average electric field stress is set at 5 kV r.m.s./mm.

20 kV r.m.s in case of a slab 4 mm thick.

The preferred ageing conditions are 240 h at room temperature Other field stresses, ageing times and temperatures are possible.

2.4.2 Test procedure with stress concentration (variant A)

To prepare the test cell, follow the procedure outlined in section 2.4.1 Begin by placing the test cell (without its cover), the needle holder-plunger assembly, 10 ml of NaCl solution, and a 1,000 g weight into a preheated oven at 60 °C for 15 minutes After this time, pour the NaCl solution into the cup and insert the plunger assembly into the recessed area of the cell, ensuring the plunger is fully pulled back Gently slide the plunger down until the needle tips contact the test specimen's surface, then carefully place the weight on the plunger handle and close the oven After one hour, remove the weight and take the cell with the plunger out of the oven, allowing it to cool to room temperature for approximately 30 minutes.

After removing the plunger assembly, the cup of the test cell is filled to three-quarters with

NaCl solution The needles are immediately cleaned in distilled water, blown dry and stored in a dry area to prevent corrosion.

The test cell is covered, connected to high voltage and to ground and the test specimen is aged as described in 2.4.1.

2.5.1 Microscopic inspection regarding bow tie trees (variant B)

After voltage aging, the test specimen undergoes an examination for water trees It is microtomed perpendicular to the water-aged surface into slices measuring 20 mm in length and 0.1 to 0.2 mm in thickness, with a total of 20 evenly distributed slices being analyzed.

All slices will be dyed according to the procedures outlined in [13] The dyed slices are then analyzed using an optical microscope at a magnification of ×100 The maximum length of the observed bow tie trees in each slice is measured, and their density is calculated, considering only those that exceed a certain size.

The study involves examining trees with a length exceeding 50 àm, including both wings, across a significant number of observed trees To streamline the analysis, only 1/5 of the total area of each slice is assessed The average density of these trees is calculated and documented in the test report for all examined slices.

Examen microscopique des arborescences ouvertes et nœud papillon (variante A)

The examination process is similar to that described in section 2.5.1 For the analysis of open branching structures, samples are sliced using a microtome into sections measuring 0.1 mm to 0.2 mm in thickness, ensuring that a row of four tips is included in each slice The sections located between the rows of tips can be utilized for examining butterfly-type branching structures.

The length of open tree structures is measured using the same microscope as in section 2.5.1 at each imprint level The distance is measured from this point to the furthest part of the tree structure, resulting in a distribution of tree lengths (eight open trees per sample) If the end of the imprint cannot be identified, the length of the tree structure, particularly the longest open trees, can be approximately determined by subtracting the needle penetration length from the total distance between the sample surface and the furthest part of the tree structure.

Rapport d'essai

Le rapport d'essai doit comprendre:

– l'identification du matériau, le type et sa dénomination usuelle;

– le mode de préparation des éprouvettes essayées et le préconditionnement auquel elles ont été soumises;

– les valeurs nominale et mesurée de l'épaisseur de l'éprouvette;

– le nombre d'éprouvettes essayées par niveau de tension;

– le mode opératoire appliqué (sous champ électrique uniforme ou sous champ électrique divergent);

– les conditions de vieillissement (tension appliquée, durée et température);

– la longueur maximale des arborescences de type nœud papillon observées;

– la densitộ moyenne des arborescences de type nœud papillon dộpassant 50 àm en longueur (mm –3 );

– la longueur maximale des arborescences ouvertes;

– la longueur moyenne des arborescences ouvertes;

3 Méthode d’essai II (essai sur coupelle)

Objet et principe de l'essai

The purpose of Test Method II is to quantify the tendency of a polymer insulating material to form water treeing when in direct contact with semiconductor screen materials This simulates the actual configuration of the insulation system in extruded power cables.

To compare various insulators, both screens are made from a known semiconductor material that serves as a common reference Other screen materials can also be utilized to investigate how the development of water treeing varies with different combinations of insulating and screening materials.

During cup testing, samples are simultaneously exposed to humidity, a uniform and constant alternating electric field, and a specified temperature cycle Following a predetermined aging process, the samples undergo a steadily increasing alternating voltage until breakdown occurs Using Weibull statistics as a basis, the dielectric strength at 63.2% is compared to that measured on similar samples that were not subjected to aging (reference group).

2.5.2 Microscopic inspection regarding vented and bow tie trees (variant A)

The inspection process for vented tree examination is similar to the method described in section 2.5.1 The test specimen is microtomed into slices ranging from 0.1 mm to 0.2 mm thick, ensuring that a row of four tips is included in each slice Additionally, the slices located between the rows of tips are suitable for bow tie tree observation.

The vented tree length is measured using the same microscope as in section 2.5.1 at each indentation tip, with the distance to the foremost front of the tree recorded A tree length distribution is established based on eight vented trees per test specimen If the indentation tip is not identifiable, the tree length of longer vented trees can be approximated by subtracting the needle penetration length from the total distance between the test specimen's surface and the foremost front of the tree.

The test report shall include:

– identification of the material, type, common name;

– method of preparing the test specimens and preconditioning to which they were subjected;

– the nominal and measured range of thickness of the test specimen;

– the number of test specimens tested at one voltage;

– the applied test method (uniform field stress or with stress concentration);

– the ageing conditions (applied voltage, time and temperature);

– the maximum length of the observed bow tie trees;

– the average density of bow tie tree density exceeding 50 àm in length (mm –3 );

– the maximum length of the vented trees;

– the average length of the vented trees;

3 Test method II (cup test)

3.1 Object and principle of test

Test method II is designed to measure the tendency of polymeric insulating materials to develop water treeing when in direct contact with semiconductive screen materials This approach simulates the conditions experienced by insulation in extruded power cables.

To effectively compare various insulating materials, both screens are typically constructed from a standard semiconductive material for consistency Additionally, utilizing different screen materials allows for the investigation of how water tree growth is influenced by various combinations of insulating and screen materials.

The test involves cup-shaped specimens subjected to simultaneous humidity, a constant uniform alternating current (a.c.) electric field, and temperature cycling Following a specified ageing period, these specimens are then exposed to a steadily increasing a.c voltage until breakdown is achieved.

Weibull statistics, the 63,2 % value of the electric breakdown strength is compared with that measured on similar test specimens that have not been exposed to ageing (reference group).

The reduction in dielectric rigidity serves as a key indicator of material vulnerability to water tree formation To obtain additional characteristics, the samples are sliced into thin sections using a microtome for microscopic examination.

Pour plus de détails voir aussi [15].

Eprouvette d’essai

Homogénéisation

L'homogénéisation du matériau isolant est destinée à prévenir l'accélération du dévelop- pement des arborescences d'eau au niveau des interfaces entre granules.

Homogenization is carried out in a clean laboratory extruder at temperatures that prevent any alteration of the materials, such as avoiding cross-linking An extruder that produces ribbons approximately 60 mm wide and about 6 mm thick is an appropriate choice.

Le matériau homogénéisé encore chaud doit être enveloppé dans une feuille d'aluminium propre immédiatement après l'extrusion Le refroidissement à température ambiante se déroule alors naturellement sans procéder à un refroidissement forcé.

The decrease in breakdown strength serves as a key indicator of a material's susceptibility to water treeing To enhance characterization, the test samples are microtomed into thin slices for detailed microscopic analysis.

See also [15] for further details.

The basic configuration of the test specimens is shown in figure 5.

The residual breakdown strength after ageing is a key metric for assessing the impact of ageing on materials It is essential that the cup-shaped test specimens exhibit a flashover voltage that surpasses that of the plane central area of the specimen, as outlined in section 3.3.4.

The insulation material and its screens create the base of a cup filled with water during the aging process To maintain high humidity in the stressed area of the test specimens, the lower screen is equipped with an aluminum backing This aluminum foil effectively prevents water from diffusing through the lower screen, ensuring a consistent humidity level across each layer of the test specimen in a steady state.

The lower screen is flat and earthed, while the upper screen, which operates at high voltage, features a Rogowski profile at the edges This design ensures that the uniform electric field in the flat section of the test specimen is maintained without exceeding limits along the circumference.

The test specimens are produced in three stages: a) homogenizing of the materials (extrusion); b) preshaping of insulation cup and screen material plaques; c) assembling and press-moulding.

To prevent contamination, it is crucial to handle both the materials and the preshaped parts of the test specimens with extreme care.

NOTE Fingerprints on any of the electrically stressed surfaces of the test specimen will spoil the test results.

To ensure optimal quality, all material and part handling should take place in a controlled environment with filtered air, with the use of a flow bench being highly recommended for this purpose, and this requirement applies uniformly across all stages of the production process.

Once the assembly is complete, the electrically stressed components of the test specimens are fully covered with screening materials As a result, the handling of these test specimens before and during the aging tests is not considered highly critical.

The homogenizing of the insulation material is done in order to prevent accelerated water tree growth at granular interfaces.

Homogenizing is conducted in a clean laboratory extruder at controlled temperatures to prevent any alterations in the materials, such as crosslinking A tape extruder capable of producing tapes with a width of around 60 mm and a thickness of about 6 mm is an ideal option for this process.

The hot homogenized material shall be wrapped in clean aluminium foil immediately after extrusion Cooling to room temperature is done in the aluminium foil without any forced cooling.

La forme du matériau homogénéisé n'a pas grande importance Cependant, une forme appropriée correspond à une plaque ronde de 60 mm de diamètre et de 6 mm d'épaisseur.

Elle a alors approximativement les mêmes dimensions que la coupelle isolante préfabriquée.

Des granules peuvent être ajoutés si nécessaire dans la partie haute de la coupelle.

Fabrication

All components, including insulating and shielding materials, are produced using tools depicted in Figures 7 to 11, which must be made of hardened stainless steel It is essential that all surfaces of the tools in contact with the insulating or shielding materials are exceptionally smooth Prior to use, the tools should be sprayed with a silicone-free PTFE product, heated to 180 °C to eliminate any solvents The tools are then wiped with a soft cloth to remove most PTFE particles, leaving only those in the microscopic depressions on the tool surfaces These tools can be utilized to manufacture multiple batches without needing to repeat the PTFE treatment If the same tools have been used for assembling and crosslinking the samples, a thorough cleaning and polishing must be performed before applying the PTFE treatment.

NOTE Il faut éviter les traces de doigts Les manipulations des parties préfabriquées survenant entre la fabrication et le montage s'avèrent particulièrement critiques.

Une presse hydraulique avec chauffage et refroidissement doit être utilisée.

The production of insulating cups and shielding materials occurs at temperatures below the melting point of the material The melting process must be conducted without applied hydraulic pressure, although the press plates should remain in contact with the molds This method allows for the simultaneous manufacturing of multiple components.

L'expérience montre qu'une pression de 30 kN par élément est suffisante Après 1 min à 2 min, les éprouvettes sont refroidies avec maintien de la pression puis sont retirées du moule.

Using the tools shown in Figures 7 to 9 and a 2 mm thick shim placed between the main mold piston and the lower plate of the hydraulic press, the insulating cup is produced with an approximate thickness of 0.9 mm.

Screen materials are produced with a thickness of (0.5 ± 0.05) mm using the tool shown in Figure 10 An aluminum sheet, typically 0.2 mm thick, is placed between the upper side of the mold and the press plate For the lower screen, the sheet is trimmed with a steel brush to ensure good contact with the screen material After molding, the aluminum sheet is removed from the upper screen, and the screen is cut or punched to achieve a diameter of 50 mm.

Les éléments fabriqués doivent toujours être stockés dans des récipients fermés et propres.

Tous les travaux doivent être réalisés dans un environnement propre à air filtré (voir 3.2).

Montage des éléments préfabriqués

The assembly process utilizes the same type of molds as those used in manufacturing (see figures 7 to 9) The lower part of the mold (the base) and the prefabricated elements are inserted into the main mold cylinder To facilitate demolding, a thin sheet of aluminum (0.06 mm to 0.08 mm thick) is wrapped around the outer wall of the prefabricated cup before placing it in the mold Additionally, it is important to spray PTFE on the piston and the bottom part of the mold.

Place the mold on the lower plate of the hydraulic press with the piston facing upward Insert a spacer (60 mm in diameter and 0.9 mm thick) between the piston and the upper plate of the press, applying a pressure of approximately 20 kN per assembled specimen Gradually increase the temperature to reach the curing temperature recommended by material manufacturers, such as 180 °C for polyethylene with dicumyl peroxide.

The shape of the homogenized material is not critical, but an ideal form is a round tablet measuring 60 mm in diameter and 6 mm in thickness, which has a mass comparable to a preshaped insulation cup If needed, additional granules can be incorporated into the top section of the cup.

All components, including insulation and screen materials, are preshaped using hardened stainless steel tools that are well-polished on all surfaces in contact with the materials Prior to use, these tools must be sprayed with a silicone-free PTFE spray and heated to 180 °C to eliminate solvents After heating, the tools are polished with a soft material to retain PTFE only in microscopic depressions This process allows for multiple batches to be preshaped without reapplying the PTFE treatment If the tools have been used for assembling and crosslinking test specimens, they must be thoroughly cleaned and polished before undergoing the PTFE treatment again.

NOTE Fingerprints must be avoided Handling of the preshaped parts between preshaping and assembling is very critical.

A hydraulic press with heating and cooling shall be applied.

The insulation cup and screen materials are preshaped at a temperature close to their melting point, without applying hydraulic pressure, while ensuring the hydraulic press plates are in contact with the molds This process allows for the simultaneous preshaping of multiple test specimen parts.

A press force of 30 kN per test specimen part is adequate, as demonstrated by experience After maintaining the pressure for 1 to 2 minutes, the test specimen parts are cooled and subsequently removed from the mould.

The insulation cup is formed to a thickness of about 0.9 mm, utilizing the tools illustrated in figures 7 to 9, along with a 2 mm thick spacer plate positioned between the piston of the main mold and the lower press plate of the hydraulic press.

The screen materials are pre-shaped to a thickness of $0.5 \pm 0.05$ mm using a specific tool An aluminum foil, typically 0.2 mm thick, is positioned between the mold's upper side and the press plate To enhance adhesion, the aluminum foil for the lower screen is roughened with a steel brush After molding, the aluminum foil for the upper screen is removed, and the screen is then cut or punched to a diameter of 50 mm.

Preshaped parts shall always be stored in closed, clean containers All work shall be done in clean, filtered air surroundings (see 3.2).

Assembly utilizes the same moulds as those used for preshaping The bottom part of the mould, along with the preshaped components, is inserted into the cylinder of the main mould To facilitate easy release from the mould, the outer side-wall of the preshaped cup is covered with a thin layer of aluminium foil, measuring between 0.06 mm and 0.08 mm in thickness.

The piston and the bottom part of the mould should be sprayed with a PTFE spray.

The mould is then placed on the bottom plate of the hydraulic press with the piston up.

A 60 mm diameter and 0.9 mm thick spacer plate is positioned between the piston and the upper plate of the press, where a force of about 20 kN is applied to each assembled test specimen The temperature is then gradually raised to the crosslinking temperature specified by the material manufacturer, such as 180 °C for polyethylene with dicumyl peroxide.

The applied pressure is adjusted to approximately 30 kN per specimen The temperature is then maintained for the recommended duration, such as 30 minutes for polyethylene with dicumyl peroxide The specimens are cooled while maintaining the pressure before being removed from the mold.

Toutes les éprouvettes réalisées doivent être conditionnées dans une étuve à air ventilé à

(90 ± 2) °C pendant 72 h, cela pour évacuer les contraintes mécaniques et enlever tout sous- produit volatile résultant de la procédure de fabrication.

The individual capacity of test specimens must be measured after they have cooled to room temperature To assess the minimum insulation thickness, one of the tested specimens is sliced, allowing for the measurement of its minimum insulation thickness with an accuracy of ±0.01 mm The conversion factor $ k $ between the capacity $ C_x $ and the minimum insulation thickness $ t_{min} $ is defined as follows:

Les objets avec une épaisseur d’isolant minimale calculée située entre 0,65 mm et 0,75 mm sont acceptés pour essai.

Pour la présélection toutes les éprouvettes d’essai doivent être soumises préalablement à un essai de tenue sous tension alternative utilisant les appareillages décrits en 3.3.4 L'essai se déroule comme suit:

– augmentation du champ électrique jusqu'à 45 kV eff./mm en 30 s;

– maintien de la contrainte pendant 1 min;

– augmentation du champ électrique jusqu'à 50 kV eff./mm;

– maintien de cette contrainte de 50 kV eff./mm pendant 1 min;

– diminution du champ électrique jusqu'à zéro en 10 s.

Seules les éprouvettes qui réussissent cet essai de tenue sous tension alternative sont prêtes à être soumises à d’autres essais.

Appareillage d'essai

Installation électrique

Test specimens must be aged under a constant average electric field of 15 kV rms/mm High-voltage stainless steel electrodes with a diameter of 6 mm should be inserted into the PE cover of each tested specimen.

All electrodes must be interconnected, with voltage supplied by a transformer connected to a regulation unit equipped with a rapid disconnection device (within three cycles of industrial frequency) and a timer to measure the aging duration in case of a breakdown during the aging period To minimize damage from a breakdown, it is essential to limit the short-circuit current to a few amperes.

The applied force is set to around 30 kN for each test specimen The temperature is held steady for the suggested duration, such as 30 minutes for polyethylene with dicumyl peroxide Afterward, the test specimens are cooled while maintaining the pressure before being removed from the mould.

All finished test specimens shall be conditioned in an air ventilated oven at (90 ± 2) °C for 72 h.

This is done in order to relieve some of the built-in mechanical stress and to remove all volatile by-products from the manufacturing process.

After the test specimens have cooled to room temperature, the capacitance of each specimen must be measured To determine the minimum insulation thickness, one specimen needs to be cut and sliced for precise measurement, with an accuracy of ±0.01 mm The conversion factor $ k $ relates the capacitance $ C_x $ to the minimum insulation thickness $ t_{\text{min}} $.

Objects with minimum insulation thickness calculated to be between 0,65 and 0,75 mm are allowed for testing.

For preselection all test specimens shall be subjected to an a.c withstand test using the arrangements described in 3.3.4 The test is carried out as follows:

– increase gradually the electric stress to 45 kV r.m.s./mm within 30 s;

– maintain this electric stress for 1 min;

– increase the electric stress to 50 kV r.m.s./mm;

– maintain 50 kV r.m.s./mm for 1 min;

– decrease gradually the electric stress to zero afterwards within 10 s.

Only test specimens that pass this a.c withstand test are ready for further testing.

This section outlines the test equipment used for evaluating test specimens, including the ageing setup and breakdown tests A minimum of eight test specimens from each material will undergo the ageing test.

The test specimens shall be aged at a continuous average electric stress of 15 kV r.m.s./mm.

Stainless steel high voltage electrodes with a diameter of 6 mm shall be inserted in the PE cover of each test specimen.

All high-voltage electrodes must be interconnected and powered by a transformer connected to a regulating unit equipped with a rapid disconnection circuit that operates within three cycles of the power frequency Additionally, a time measuring device is necessary to track the ageing time in the event of a breakdown during the ageing period To minimize damage from a breakdown, it is essential to limit the short circuit current to a few amperes.

Installation thermique

During the aging test, the samples are placed in an oven on a copper plate A copper tube with a diameter of 10 mm is either crimped or welded to the plate, ensuring that the distance between all points on the plate and the tube does not exceed 50 mm Cooling water circulates through the tube during the cooling period.

Pendant la période de chauffage, la circulation d’eau est interrompue et l’élément de chauffage de l'étuve est mis en service jusqu’à ce que la plaque de cuivre atteigne 90 °C.

The temperature must be maintained at (90 ± 2) °C At the end of the heating period, the heating element is turned off, and water circulation is resumed The duration of each heating and cooling period should be automatically controlled using a timing device.

Liquide d’essai

The NaCl solution (1.8 mmol/litre) is prepared by dissolving 0.1 g of NaCl in 1 liter of distilled and deionized water This water is produced through distillation in a non-glass apparatus, followed by passage through a mixed bed deionization column, and is stored in a hermetically sealed polyethylene container Before use, the water must be checked to ensure its pH is (7.0 ± 0.1) and its conductivity (σ ≤ 100 µS/m) The water is then poured into cuvette-shaped test tubes, ensuring that the liquid level is maintained so that the distance between its surface and the lid does not exceed a specified limit.

To compensate for potential water evaporation, deionized water can be added through a 2 mm diameter hole in the cover, as shown in Figure 11 It is important to seal the hole during the aging process.

Equipement pour l'essai jusqu'au claquage

High voltage breakdown tests using alternating current require a low current high voltage transformer equipped with a regulation unit that ensures a continuous increase in voltage, reaching a maximum of at least 100 kV effective Additionally, the regulation unit must include a rapid disconnection device that activates in less than three cycles for industrial frequency in the event of a breakdown.

Dielectric breakdown tests should be conducted in accordance with IEC 60243-1, utilizing a rise rate of 20 kV effective/min Unlike the requirements outlined in IEC 60243-1, the electrical test is performed directly on the cup-shaped specimen.

The arrangement of electrodes during the breakdown test must be designed to minimize the risk of external arcing Similarly, the specimen should be immersed in an insulating liquid, such as silicone oil, that does not cause swelling of the materials involved, or in a gas with a high breakdown voltage, like SF6 It is important to consider potential unforeseen environmental factors that may arise.

La figure 12 représente un exemple de dispositif approprié.

Mode opératoire (essai de vieillissement)

A minimum of 16 test specimens, previously tested according to the procedures outlined in section 3.3, is required for this experiment The specimens must be divided into two groups, each containing at least eight specimens: a reference group with at least eight specimens to determine the initial breakdown voltage, and an aging group with at least eight specimens.

In the ageing test, specimens are positioned on a copper plate within an oven, where a 10 mm diameter copper pipe is welded or soldered to the plate's underside This setup ensures that every point on the plate is within 50 mm of the pipe.

Cooling water is circulated through the tube during the cooling period.

During the heating phase, the cooling water circulation is halted while the oven's heating element is activated until the copper plate reaches 90 °C, which must be maintained within a range of (90 ± 2) °C Once the heating period concludes, the heating element is turned off, and the cooling water circulation resumes The durations of both heating and cooling periods are automatically regulated by a timer device.

A NaCl solution with a concentration of 1.8 mmol/litre is prepared by dissolving 0.1 g of NaCl in 1 liter of distilled and deionized water This water is produced by passing distilled water through a mixed bed deionizing column and storing it in an airtight polyethylene container Prior to use, the water must be tested for pH, which should be maintained at 7.0 ± 0.1, and conductivity.

Test specimens with a conductivity of \$\sigma \leq 100 \, \text{S/m}\$ are filled with a test liquid, ensuring that the liquid level remains within 10 mm of the cover's surface To prevent evaporation, deionized water can be added through a 2 mm diameter hole in the cover, which must be sealed during the ageing period.

AC breakdown tests necessitate the use of a high voltage transformer that operates at low current, equipped with a regulating unit capable of continuously increasing the voltage to a minimum of 100 kV r.m.s Additionally, the regulating unit must feature a rapid disconnection device that can activate within three cycles of the power frequency in the event of a breakdown.

Breakdown tests on test specimens will be conducted in accordance with IEC 60243-1, utilizing a rise rate of 20 kV r.m.s./min However, for cup-shaped test specimens, the electric test will be performed directly, deviating from the standard procedure outlined in IEC 60243-1.

To minimize the risk of external breakdown during the breakdown test, the electrode arrangement must be carefully designed Additionally, the test specimen should be immersed in an insulating liquid, such as silicone oil, that does not cause material swelling, or in a gas with high breakdown strength, like SF6, while considering potential environmental hazards.

An example of a suitable arrangement is shown in figure 12.

At least 16 test specimens manufactured and tested according to the procedures described in

For this test, a total of 3.3 specimens are necessary, divided into two groups of at least eight specimens each: the reference group, which includes at least eight specimens to assess the initial breakdown strength, and the ageing group, also comprising at least eight specimens.

Les éprouvettes du groupe de vieillissement doivent être remplies d’un liquide de vieillissement, les couvercles collés sur les coupelles et les électrodes à haute tension inter- connectées.

Fréquence 50 Hz ou 60 Hz (±2 Hz)

The cooling process lasts for 8 hours, followed by a heating phase of 16 hours The cooling water temperature should be maintained at (20 ± 5) °C During the last five hours of cooling, the test specimens must reach a temperature of (20 ± 5) °C After the cooling period, a heating phase of 16 hours is required, reaching the maximum specific temperature for the material in question, which is (90 ± 2) °C for XPLE This target temperature must be achieved within 3 hours of starting the heating phase.

If a test specimen fractures during aging, it should be removed from the testing apparatus during a cold and stable period for examination of water tree formation to determine the likely cause of the fracture Only one such fracture is permitted during the aging process.

Après que la procédure de vieillissement ait été terminée, on doit procéder à la stabilisation des éprouvettes à une température ambiante avant d'en retirer le liquide de vieillissement.

Après avoir retiré le liquide de vieillissement, les éprouvettes d'essais ne doivent pas être exposées à des températures élevées (dépassant 25 °C).

Après avoir retiré le liquide de vieillissement des éprouvettes, les essais de claquage doivent être achevés dans les 24 h.

Examen des éprouvettes vieillies et non vieillies

Essai de claquage

Les essais de claquage sont effectués conformément à 3.3.4.

To determine the maximum stress in the insulator at the moment of breakdown, the specimen must be sliced with a microtome at the point of failure The minimum insulation thickness is measured using an optical microscope with a precision of ±0.01 mm.

Examen des arborescences d'eau

A total volume of at least 250 mm³ must be examined for both butterfly and open tree configurations This volume should consist of the slices cut by a microtome from all tested samples The examination should be restricted to the flat section of the tested samples, specifically the area with a uniform electric field.

The test specimens of the ageing group shall be filled with ageing liquid, the covers glued on to the cups and the high voltage electrodes connected together.

Frequency 50 Hz or 60 Hz (±2 Hz)

Temperature 8 h cooling/16 h heating Temperature of the cooling water: (20 ± 5) °C.

In the final 5 hours of the cooling period, test specimens must achieve a temperature of (20 ± 5) °C Following this, a 16-hour heating period is initiated, during which the maximum rated temperature for the material, specifically (90 ± 2) °C for XLPE, must be reached within 3 hours.

During the stable cold period of ageing, any test specimen that breaks down must be removed from the test setup and examined for water treeing to determine the likely cause of the failure It is important to note that only one breakdown is permitted during the ageing process.

Once the aging process is finished, all test specimens must be stabilized at room temperature before the aging liquid is removed Following the removal of the liquid, it is crucial that the test specimens are not subjected to temperatures above 25 °C.

The breakdown tests shall be finished within 24 h after the ageing liquid has been removed from the test specimens.

3.5 Examinations of unaged and aged test specimens

Breakdown strength both for unaged and aged test specimens has to be established For the aged samples, water tree length and densities shall also be determined.

The breakdown tests shall be performed according to 3.3.4.

To determine the maximum stress of insulation at breakdown, the test specimen must be microtomed at the breakdown location The minimum insulation thickness is accurately measured using a microscope, with a precision of ±0.01 mm.

A total volume of at least 250 mm 3 shall be examined for both bow tie and vented water trees.

This volume will include microtomed slices from all test specimens, focusing specifically on the flat section, which represents the area of the uniform electric field (refer to figure 6).

En commenỗant à couper au microtome à partir du centre dans un plan perpendiculaire aux surfaces des électrodes, il convient que l’examen soit constitué:

– d’au moins 10 lamelles successives d’une ộpaisseur de 150 àm à 250 àm issues de deux éprouvettes essayées choisies au hasard;

– de deux lamelles d’une ộpaisseur de 150 àm à 250 àm prộlevộes sur chaque ộprouvette restante.

Les arborescences d'eau doivent alors être examinées sur la partie plane de l’éprouvette.

Toutes les lamelles doivent être teintes en utilisant les modes opératoires indiqués en [13].

Les valeurs suivantes doivent être enregistrées:

– l’arborescence la plus longue de type nœud papillon;

– l’arborescence ouverte la plus longue générée à partir de l’écran haute tension;

– l’arborescence ouverte la plus longue à partir de l’écran basse tension;

– la densitộ des arborescences de type nœud papillon dộpassant 50 àm de longueur (mm –3 );

– la densitộ des arborescences ouvertes à partir de l’ộcran haute tension dộpassant 50 àm de longueur (mm –2 );

– la densitộ des arborescences ouvertes à partir de l’ộcran basse tension dộpassant 50 àm de longueur (mm –2 ).

Scope

This technical specification outlines testing methods to evaluate water treeing in polyethylene (PE) and crosslinked polyethylene (XLPE) compounds, assessing their performance under alternating electric stress in wet conditions It details two approaches: Method I focuses on the evaluation of insulating materials independently, while Method II addresses the assessment of insulating sandwiches that include an insulating material in direct contact with semiconducting screens.

Normative references

This technical specification references several normative documents, which are integral to its provisions For dated references, any amendments or revisions do not apply, but parties are encouraged to consider the latest editions of these documents For undated references, the most recent edition is applicable Additionally, IEC and ISO members keep registers of currently valid International Standards.

IEC 60243-1:1998, Electrical strength of insulating materials – Test methods – Part 1: Tests at power frequencies

IEC 61072:1991, Methods of test for evaluating the resistance of insulating materials against the initiation of electrical trees

Definitions

Water treeing is a degradation process that occurs in low density polyethylene (LDPE) and cross-linked polyethylene (XLPE) when subjected to alternating current (a.c.) stress and moist conditions This phenomenon leads to the development of dielectrically weakened regions known as water trees.

Water trees are hydrophilic, tree-like structures that develop under wet and electrical conditions, often reaching lengths of about 1 mm over several years There are two main types of water trees: the bow tie tree, which features straight branches radiating from a central point and aligns with the electric field within the insulation, and the vented tree, which has a trunk that extends to the insulation surface or interface, with branches oriented away from the surface in the direction of the electric field.

2 Méthode d’essai I (essai sur plaques)

Les essais sur plaques (variante A et variante B) ont pour but d'évaluer le développement d'arborescences d’eau dans les matériaux isolants à base de polyéthylène basse densité

(PEBD) et de polyéthylène réticulé (PRC).

The two test variants utilize plate-shaped specimens and the same testing cell Both tests are preselection trials aimed at differentiating and selecting insulators based on their water tree formation.

The constraint concentration test (variant A) is primarily designed to assess the development of open tree structures originating from the tips of screen protrusions or penetrations in the insulation at the insulation/screen interface, simulated by needle-shaped water-filled impressions A plate with several identical impressions is subjected to an electric field in a test cell, as outlined in section 2.3.1, with water present, leading to an electric field concentration at these points.

This essay evaluates the formation of butterfly-type branching structures within the mass of the test tube, specifically by positioning away from areas of high electric field concentration.

The uniform electric field test (variant B) aims to assess the development of butterfly-type branching when a plate is exposed to water while simultaneously subjected to a uniform electric field in a testing cell as described in section 2.3.1.

Circular test specimens with a diameter of (35 ± 1) mm can be produced by cutting with a punch from a plate of one of the following thicknesses: (4.0 ± 0.1) mm, (3.0 ± 0.1) mm, or (2.0 ± 0.1) mm It is essential that comparisons between materials are made using specimens of identical thickness.

For tests involving open tree structures, plates can be produced by molding pellets under pressure In the case of butterfly-type tree structures, it is advisable to homogenize the composition through extrusion to prevent the concentration of additives or impurities on the surface of the pellets In all scenarios, extreme precautions should be taken to avoid contamination of the materials, which could subsequently affect the plates and strips.

In the case of compositions based on PRC (using dicumyl peroxide), the sheets are preformed in a press at approximately 130 °C They are then heated to 180 °C, maintained at this temperature for 30 minutes, and cooled to 70 °C, all while under pressure The sheets removed from the press are degassed for 72 hours.

(90 ± 2) °C pour l'élimination des résidus volatiles.

In the case of LDPE-based materials (without crosslinking agents), the sheets are preformed at approximately 130 °C in a press, then heated to around 200 °C and cooled, still under pressure, to about 70 °C.

L'expérience a montré que lorsque l'on moule des plaques avec des compositions à base de PE ou de PRC, une pression exercée d'au moins 5 N/mm 2 conduit à des résultats satisfaisants.

Principle

The plaque tests (variant A and variant B) are intended to assess the development of water trees in low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) based insulating materials.

Both test variants utilize plaque-shaped specimens and the same test cell, serving as screening tests to differentiate and preselect insulating compounds based on their resistance to water treeing.

The stress concentration test (variant A) aims to evaluate the formation of vented trees originating from the tips of screen protrusions or insulation intrusions at the insulation/screen interface This is simulated using needle-shaped, water-filled indentations A plaque featuring multiple identical indentations is subjected to both water and an electric field simultaneously, creating electric field stress concentrations at these specific points within a designated test cell.

By means of this test, bow tie treeing can be additionally assessed within the bulk of the test specimen, away from the stress concentration areas.

The uniform field stress test (variant B) evaluates the growth of bow tie trees by exposing a plaque to both water and a uniform electric field within a specified test cell.

Test apparatus

Test procedure (ageing test)

Examination after ageing

Test specimen

Test apparatus

Examinations of unaged and aged test specimens

Tiêu đề	IEC TS 61956 1999
Chuyên ngành	Electrical Engineering
Thể loại	Technical Specification
Năm xuất bản	1999

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Số trang	70
Dung lượng	498,88 KB