山西冠恒ASTMD149美標標準固體絕緣材料電壓擊穿的實驗方法

Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies 固體電絕緣材料在工業用電頻率下的電壓擊穿和介電強度的實驗方法

1. Scope*

1.1 This test method covers procedures for the determination of dielectric strength of solid insulating materials at commercial power frequencies, under specified conditions.2,3

1.2 Unless otherwise specified, the tests shall be made at 60Hz. However, this test method is suitable for use at anyfrequency from 25 to 800 Hz. At frequencies above 800 Hz,dielectric heating is a potential problem.

1.3 This test method is intended to be used in conjunction with any ASTM standard or other document that refers to this test method. References to this document need to specify the particular options to be used (see 5.5).

1.4 It is suitable for use at various temperatures, and in any suitable gaseous or liquid surrounding medium.

1.5 This test method is not intended for measuring the dielectric strength of materials that are fluid under the conditions of test.

1.6 This test method is not intended for use in determining intrinsic dielectric strength,direct-voltage dielectric strength,or thermal failure under electrical stress (see Test MethodD3151).

1.7 This test method is most commonly used to determine the dielectric breakdown voltage through the thickness of a test specimen (puncture). It is also suitable for use to determine dielectric breakdown voltage along the interface between a solid specimen and a gaseous or liquid surrounding

medium (flashover). With the addition of instructions modifying Section 12, this test method is also suitable for use for proof testing.

1.8 This test method is similar to IEC Publication 243-1. All procedures in this method are included in IEC 243-1. Differences between this method and IEC 243-1 are largely editorial.

1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific hazardstatements are given in Section 7. Also see 6.4.1.

ASTMD149美標標準固體絕緣材料電壓擊穿的實驗方法(三) 3.1 Definitions:

3.1.1dielectric breakdown voltage (electric breakdown voltage), n—the potential difference at which dielectric failureoccurs under prescribed conditions in an electrical insulatingmaterial located between two electrodes. (See also Appendix X1.)

3.1.1.1 Discussion—The term dielectric breakdown voltage is sometimes shortened to “breakdown voltage.”

3.1.2dielectric failure (under test), n—an event that is evidenced by an increase in conductance in the dielectric under test limiting the electric field that can be sustained.

3.1.3dielectric strength, n—the voltage gradient at which dielectric failure of the insulating material occurs under specific conditions of test.

3.1.4 electric strength, n—see dielectric strength.

3.1.

4.1 Discussion—Internationally, “electric strength” is used almost universally.

3.1.5flashover, n—a disruptive electrical discharge at the surface of electrical insulation or in the surrounding medium, which may or may not cause permanent damage to the insulation.

3.1.6 For definitions of other terms relating to solid insulating materials, refer to Terminology D1711.

ASTMD149美標標準固體絕緣材料電壓擊穿的實驗方法(四)

4. Summary of Test Method

4.2 Most commonly, the test voltage is applied using simple test electrodes on opposite faces of specimens. The options for the specimens are that they be molded or cast, or cut from flat sheet or plate. Other electrode and specimen configurations are also suitable for use to accommodate the geometry of the sample material, or to simulate a specific application for which the material is being evaluated.

4.1 Alternating voltage at a commercial power frequency (60 Hz, unless otherwise specified) is applied to a test specimen. The voltage is increased from zero or from a level well below the breakdown voltage, in one of three prescribed methods of voltage application, until dielectric failure of the test specimen occurs.

5. Significance and Use

5.1 The dielectric strength of an electrical insulating material is a property of interest for any application where an electrical field will be present. In many cases the dielectric strength of a material will be

the determining factor in the design of the apparatus in which it is to be used.

5.2 Tests made as specified herein are suitable for use to provide part of the information needed for determining suitability of a material for a given application; and also, for detecting changes or deviations from normal characteristics resulting from processing variables, aging conditions, or other manufacturing or environmental situations. This test method is useful for process control, acceptance or research testing.

5.3 Results obtained by this test method can seldom be used directly to determine the dielectric behavior of a material in an actual application. In most cases it is necessary that these results be evaluated by comparison with results obtained from other functional tests or from tests on other materials, or both, in order to estimate their significance for a particular material.

5.4 Three methods for voltage application are specified in Section 12: Method A, Short-Time Test; Method B, Step-by- Step Test; and Method C, Slow Rate-of-Rise Test. Method A is the most commonly-used test for quality-control tests. However,the longer-time tests, Methods B and C, which usually will give lower test results, will potentially give more meaningful results when different materials are being compared with each other. If a test set with motor-driven voltage control is available, the slow rate-of-rise test is simpler and preferable to the step-by-step test. The results obtained from Methods B and C are comparable to each other.

5.5 Documents specifying the use of this test method shall also specify:

5.5.1 Method of voltage application,

5.5.2 Voltage rate-of-rise, if slow rate-of-rise method is specified, 5.5.3 Specimen selection, preparation, and conditioning,

5.5.4 Surrounding medium and temperature during test,

5.5.5 Electrodes,

5.5.6 Wherever possible, the failure criterion of the currentsensing element, and

5.5.7 Any desired deviations from the recommended procedures as given. 5.6 If any of the requirements listed in 5.5 are missing from the specifying document, then the recommendations for the several variables shall be followed.

5.7 Unless the items listed in 5.5 are specified, tests made with such inadequate reference to this test method are not in conformance with this test method. If the items listed in 5.5 are not closely controlled during the test, it is possible that the precisions stated in 15.2 and 15.3 will not be obtained.

5.8 Variations in the failure criteria (current setting and response time) of the current sensing element significantly affect the test results.

5.9 Appendix X1. contains a more complete discussion of the significance of dielectric strength tests.

ASTMD149美標標準固體絕緣材料電壓擊穿的實驗方法(六) 6. Apparatus

6.1.3 The controls on the variable low-voltage source shall be capable of varying the supply voltage and the resultant test voltage smoothly, uniformly, and without overshoots or transients, in accordance with 12.2. Do not allow the peak voltage to exceed 1.48 times the indicated rms test voltage under any circumstance. Motor-driven controls are preferable for making short-time (see 12.2.1) or slow-rate-of-rise (see 12.2.3) tests.

6.1.4 Equip the voltage source with a circuit-breaking device that will operate within three cycles. The device shall disconnect the voltage-source equipment from the power service and protect it from overload as a result of specimen breakdown causing an overload of the testing apparatus. If prolonged current follows breakdown it will result in unnecessary burning of the test specimens, pitting of the electrodes,and contamination of any liquid surrounding medium.

6.1.5 It is important for the circuit-breaking device to have an adjustable current-sensing element in the step-up transformer secondary, to allow for adjustment consistent with the specimen characteristics and arranged to sense specimen current. Set the sensing element to respond to a current that is indicative of specimen breakdown as defined in 12.3.

6.1.6 The current setting is likely to have a significant effect on the test results. Make the setting high enough that transients, such as partial discharges, will not trip the breaker but not so high that excessive burning of the specimen, with resultant electrode damage, will occur on breakdown. The optimum current setting is not the same for all specimens and depending upon the intended use of the material and the purpose of the test, it is often desirable to make tests on a given sample at more than one current setting. The electrode area is likely to have a significant effect upon the choice of current setting.

6.1.7 It is possible that the specimen current-sensing element will be in the primary of the step-up transformer. Calibrate the current-sensing dial in terms of specimen current.

6.1.8 Exercise care in setting the response of the current control. If the control is set too high, the circuit will not respond when breakdown occurs; if set too low, it is possible that it will respond to leakage currents, capacitive currents, or partial discharge (corona) currents or, when the sensing element is located in the primary, to the step-up transformer magnetizing current.

6.2 Voltage Measurement—A voltmeter must be provided for measuring the rms test voltage. If a peak-reading voltmeter is used, divide the reading by =2 to get rms values. The overall error of the voltage-measuring circuit shall not exceed 5 % of the measured value. In addition, the response time of the voltmeter shall be such that its time lag will not be greater than 1 % of full scale at any rate-of-rise used.

6.2.1 Measure the voltage using a voltmeter or potential transformer connected to the specimen electrodes, or to a separate voltmeter winding, on the test transformer, that is unaffected by the step-up transformer loading.

6.2.2 It is desirable for the reading of the maximum applied test voltage to be retained on the voltmeter after breakdown so that the breakdown voltage can be accurately read and recorded.

ASTMD149美標標準固體絕緣材料電壓擊穿的實驗方法(七)

6.3 Electrodes—For a given specimen configuration, it is possible that the dielectric breakdown voltage will vary considerably,depending upon the geometry and placement of the test electrodes. For this reason it is important that the electrodes to be used be described when specifying this test method, and that they be described in the report.

6.3.1 One of the electrodes listed in Table 1 shall be specified by the document referring to this test method. If no electrodes have been specified, select an applicable one from Table 1, or use other electrodes mutually acceptable to the parties concerned when the standard electrodes cannot be used due to the nature or configuration of the material being tested.See references in Appendix X2 for examples of some special electrodes. In any event the electrodes must be described in the report.

6.3.2 The electrodes of Types 1 through 4 and Type 6 of Table 1 shall be in contact with the test specimen over the entire flat area of the electrodes.

6.3.3 The specimens tested using Type 7 electrodes shall be

of such size that all portions of the specimen will be within and no less than 15 mm from the edges of the electrodes during test. In most cases, tests using Type 7 electrodes are made with the plane of the electrode surfaces in a

vertical position. Tests made with horizontal electrodes shall not be directly compared with tests made with vertical electrodes, particularly when the tests are made in a liquid surrounding medium.

6.3.6 Whenever the electrodes are dissimilar in size or shape, ensure that the one at which the lowest concentration of stress exists, usually the larger in size and with the largest radius, is at ground potential.

6.3.7 In some special cases liquid metal electrodes, foil electrodes, metal shot, water, or conductive coating electrodes are used. It must be recognized that it is possible that these will give results differing widely from those obtained with other types of electrodes.

6.3.8 Because of the effect of the electrodes on the test results, it is frequently possible to obtain additional information as to the dielectric properties of a material (or a group of materials) by running tests with more than one type of electrode. This technique is of particular value for research testing.

ASTMD149美標標準固體絕緣材料電壓擊穿的實驗方法(八)

6.3.3 The specimens tested using Type 7 electrodes shall be of such size that all portions of the specimen will be within and no less than 15 mm from the edges of the electrodes during test. In most cases, tests using Type 7 electrodes are made with the plane of the electrode surfaces in a vertical position. Tests made with horizontal electrodes shall not be directly compared with tests made with vertical electrodes, particularly when the tests are made in a liquid surrounding medium.

6.3.4 Keep the electrode surfaces clean and smooth, and free from projecting irregularities resulting from previous tests.If asperities have developed, they must be removed.

6.3.6 Whenever the electrodes are dissimilar in size or shape, ensure that the one at which the lowest concentration of stress exists, usually the larger in size and with the largest radius, is at ground potential.

6.3.7 In some special cases liquid metal electrodes, foil electrodes, metal shot, water, or conductive coating electrodes are used. It must be recognized that it is possible that these will give results differing widely from those obtained with other types of electrodes.

6.3.5 It is important that the original manufacture and subsequent resurfacing of electrodes be done in such a manner that the specified shape and finish of the electrodes and their edges are maintained. The flatness and surface finish of the electrode faces must be such that the faces are in close contact with the test specimen over the entire area of the electrodes.Surface finish is particularly important when testing very thin materials which are subject to physical damage from improperly finished electrodes. When resurfacing, do not change the transition between the electrode face and any specified edge radius.

6.3.8 Because of the effect of the electrodes on the test results, it is frequently possible to obtain additional information as to the dielectric properties of a material (or a group of materials) by running tests with more than one type of electrode. This technique is of particular value for research testing.

ASTMD149美標標準固體絕緣材料電壓擊穿的實驗方法(九)

6.4 Surrounding Medium—The document calling for this test method needs to specify the surrounding medium and the test temperature. Since flashover must be avoided and the effects of partial discharges prior to breakdown mimimized,even for short time tests, it is often preferable and sometimes necessary to make the tests in insulating liquid (see 6.4.1).Breakdown values obtained in insulating liquid are often not comparable with those obtained in air. The nature of the insulating liquid and the degree of previous use are factors influencing the test values. In some cases, testing in air will require excessively large specimens or cause heavy surface discharges and burning before

breakdown. Some electrode systems for testing in air make use of pressure gaskets around the electrodes to prevent flashover. The material of the gaskets or seals around the electrodes has the potential to influence the breakdown values.

6.4.1 When tests are made in insulating oil, an oil bath of adequate size shall be provided. (Warning—The use of glass containers is not recommended for tests at voltages above about 10 kV, because the energy released at breakdown has the potential to be sufficient to shatter the container. Metal baths must be grounded.)

It is recommended that mineral oil meeting the requirements of Specification D3487, Type I or II, be used. It shall have a dielectric breakdown voltage as determined by Test Method D877 of at least 26 kV. Other dielectric fluids are suitable for use as surrounding mediums if specified. These include, but are not limited to, silicone fluids and other liquids intended for use in transformers, circuit breakers, capacitors, or cables.

6.4.1.1 The quality of the insulating oil has the potential to have an appreciable effect upon the test results. In addition to the dielectric breakdown voltage, mentioned above, particulate contaminants are especially important when very thin specimens(25 μm (1 mil) or less) are being tested. Depending upon the nature of the oil and the properties of the material being tested, other properties, including dissolved gas content, water content, and dissipation factor of the oil also have the potential to affect the results. Frequent replacement of the oil, or the use of filters and other reconditioning equipment is important to minimize the effect of variations of the quality of the oil on the test results.

6.4.1.2 Breakdown values obtained using liquids having different electrical properties are often not comparable. (See X1.4.

7.) If tests are to be made at other than room temperature,the bath must be provided with a means for heating or cooling the liquid, and with a means to ensure uniform temperature. Small baths can in some cases be placed in an oven (see 6.4.2)in order to

provide temperature control. If forced circulation of the fluid is provided, care must be taken to prevent bubbles from being whipped into the fluid. The temperature shall be maintained within 65°C of the specified test temperature at the electrodes, unless otherwise specified. In many cases it is specified that specimens to be tested in insulating oil are to be previously impregnated with the oil and not removed from the oil before testing (see Practice D2413). For such materials, the bath must be of such design that it will not be necessary to expose the specimens to air before testing.

6.4.2 If tests in air are to be made at other than ambient temperature or humidity, an oven or controlled humidity chamber must be provided for the tests. Ovens meeting the requirements of Specification D5423 and provided with means for introducing the test voltage will be suitable for use when only temperature is to be controlled.

6.4.3 Tests in gasses other than air will generally require the use of chambers that can be evacuated and filled with the test gas, usually under some controlled pressure. The design of

D149 – 094such chambers will be determined by the nature of the test program to be undertaken

ASTMD149美標標準固體絕緣材料電壓擊穿的實驗方法(十)

6.5 Test Chamber—The test chamber or area in which the tests are to be made shall be of sufficient size to hold the test equipment, and shall be provided with interlocks to prevent accidental contact with any electrically energized parts. A number of different physical arrangements of voltage source,measuring equipment, baths or ovens, and electrodes are possible, but it is essential that (1) all gates or doors providing access to spaces in which there are electrically energized parts be interlocked to shut off the voltage source when opened; ( 2)clearances are sufficiently large that the field in the area of the electrodes and specimen are not distorted and that flashovers and partial discharges (corona) do not occur except between the test electrodes; and (3) insertion and replacement of specimens between tests be as simple

and convenient as possible. Visual observation of the electrodes and test specimen during the test is frequently desirable.

7. Hazards

7.1 Warning—It is possible that lethal voltages will be present during this test. It is essential that the test apparatus,and all associated equipment electrically connected to it, be properly designed and installed for safe operation. Solidly ground all electrically conductive parts that any person might come into contact with during the test. Provide means for use at the completion of any test to ground any parts which fall into any of the following cases: (a) were at high voltage during the test; (b) have the potential to acquire an induced charge during the test; or (c) have the potential to retain a charge even after disconnection of the voltage source. Thoroughly instruct all operators in the proper way to conduct tests safely. When making high-voltage

tests,particularly in compressed gas or in oil, it is possible that the energy released at breakdown will be sufficient to result in fire, explosion, or rupture of the test chamber. Design test equipment, test chambers, and test specimens so as to minimize the possibility of such occurrences and to eliminate the possibility of personal injury.

7.2 Warning—Ozone is a physiologically hazardous gas at elevated concentrations. The exposure limits are set by governmental agencies and are usually based upon recommendations made by the American Conference of Governmental ndustrial Hygienists.8 Ozone is likely to be present whenever voltages exist which are sufficient to cause partial, or complete,discharges in air or other atmospheres that contain oxygen.Ozone has a distinctive odor which is initially discernible at low concentrations but sustained inhalation of ozone can cause temporary loss of sensitivity to the scent of ozone. Because of this it is important to measure the concentration of ozone in the atmosphere, using commercially available monitoring devices,whenever the odor of ozone is persistently present or when ozone generating conditions continue. Use appropriate means,such as exhaust vents, to reduce ozone concentrations to acceptable levels in working areas.

8. Sampling

8.1 The detailed sampling procedure for the material being tested needs to be defined in the specification for that material.

8.2 Sampling procedures for quality control purposes shall provide for gathering of sufficient samples to estimate both the average quality and the variability of the lot being examined;and for proper protection of the samples from the time they are taken until the preparation of the test specimens in the laboratory or other test area is begun.

8.3 For the purposes of most tests it is desirable to take samples from areas that are not immediately adjacent to obvious defects or discontinuities in the material. Avoid the outer few layers of roll material, the top sheets of a package of sheets, or material immediately next to an edge of a sheet or

roll, unless the presence or proximity of defects or discontinuities is of interest in the investigation of the material.

8.4 The sample shall be large enough to permit making as

many individual tests as required for the particular material

(see 12.4).

9. Test Specimens

9.1 Preparation and Handling:

9.1.1 Prepare specimens from samples collected in accordance with Section 8.

9.1.2 When flat-faced electrodes are to be used, the surfaces of the specimens which will be in contact with the electrodes shall be smooth parallel planes, insofar as possible without actual surface machining.

9.1.3 The specimens shall be of sufficient size to prevent flashover under the conditions of test. For thin materials it will often be convenient to use specimens large enough to permit making more than one test on a single piece.

9.1.4 For thicker materials (usually more than 2 mm thick)it is possible that the breakdown strength will be high enough that flashover or intense surface partial discharges (corona) will occur prior to breakdown. Techniques that are suitable for use to prevent flashover, or to reduce partial discharge (corona) include:

9.1.4.1 Immerse the specimen in insulating oil during the test. See X1.4.7 for the surrounding medium factors influencing breakdown. This is often necessary for specimens that have not been dried and impregnated with oil, as well as for those which have been prepared in accordance with Practice

D2413,for example. (See 6.4.)

9.1.4.2 Machine a recess or drill a flat-bottom hole in one or both surfaces of the specimen to reduce the test thickness. If dissimilar electrodes are used (such as Type 6 of Table 1) and only one surface is to be machined, the larger of the two electrodes shall be in contact with the machined surface. Care must be taken in machining specimens not to contaminate or mechanically damage them.

9.1.4.3 Apply seals or shrouds around the electrodes, in contact with the specimen to reduce the tendency to flashover.

9.1.5 Materials that are not in flat sheet form shall be tested using specimens (and electrodes) appropriate to the material and the geometry of the sample. It is essential that for these materials both the specimen and the electrodes be defined in the specification for the material.

9.1.6 Whatever the form of the material, if tests of other than

surface-to-surface puncture strength are to be made,define the specimens and the electrodes in the specification for the material.

9.2 In nearly all cases the actual thickness of the test specimen is important. Unless otherwise specified, measure the thickness after the test in the immediate vicinity of the area of breakdown. Measurements shall be made at room temperature(25 6 5°C), using the appropriate procedure of Test Methods.