IEC 60243-3 pdf download – Electrical strength of insulating materials — Test methods — Part 3: Additional requirements for 1,2/50 ? ?s impulse tests

IEC 60243-3 pdf download – Electrical strength of insulating materials — Test methods — Part 3: Additional requirements for 1,2/50 ? ?s impulse tests
1 scope
This part of lEC 60243 gives requirements additional to those in IEC 60243-1 for thedetermination of the electric strength of solid insulating materials under 1,2/50 us impulsevoltage stress.
2 Normative references
The following normative documents contain provisions which，through reference in this text,constitute provisions of this part of lEC 60243.For dated references, subsequent amendmentsto,or revisions of，any of these publications do not apply. However parties to agreementsbased on this part of IEc 60243 are encouraged to investigate the possibility of applying themost recent editions of the normative documents indicated below.For undated references, thelatest edition of the normative document referred to applies.Members of the lEC and the lSomaintain registers of currently valid lnternational Standards.
IEC 60243-1:1998,Electrical strength of insulating materials – Test methods – Part 1:Tests atpower frequencies
3Definitions
For the purpose of this part of lEC 60243, the following definitions, together with those given inclause 2 of lEC 60243-1, apply.
3.1
full impulse-voltage wave (and see figure 1)
aperiodic transient voltage that rises rapidly to a maximum value, then falls less rapidly to zero
3.2
peak value (of an impulse-voltage wave), upmaximum value of voltage
3.3
virtual peak value (of an impulse-voltage wave), u1
value derived from a recording of an impulse-voltage wave on which high-frequencyoscillations, or overshoot of a limited magnitude, may be present
3.4
virtual origin (of an impulse-voltage wave)o1
point of intersection O,with the line of zero voltage of a line drawn through the points of 0,3and 0,9 times the virtual peak value on the front of an impulse-voltage wave (see figure 1)
3.5
virtual front time (of an impulse-voltage wave) t
equal to 1,67 times the interval t, between the instants when the voltage is 0,3 and 0,9 timesthe peak value (t, figure 1).
3.6
virtual time to half-value t2z
time interval tg between the virtual origin O，and the instant on the tail when the voltage hasdecreased to half the peak value
4Significance of the test
ln addition to the information of clause 3 of lEC 60243-1, the following points are of importancein connection with impulse-voltage tests:
4.1 High-voltage equipment may be subjected to transient voltage stresses resulting fromsuch causes as nearby lightning strokes. This is particularly true of apparatus such astransformers and switchgears used in electrical power transmission and distribution systems.The ability of insulating materials to withstand these transient voltages is important inestablishing the reliability of apparatus insulated with these materials.
4.2Transient voltages caused by lightning may be of either positive or negative polarity. In asymmetrical field between identical electrodes， the polarity has no effect on the electricstrength. However,with dissimilar electrodes, there may be a pronounced polarity effect. Whenassymetrical electrodes are used for testing materials with which the tester has no previousexperience or knowledge，it is recommended that comparative tests be made with bothdirections of polarity.
4.3 The standard wave shape is a 1,2/50 us wave, reaching peak voltage in approximately1,2 us,and decaying to 50 % of peak value in approximately 50 us after the beginning of thewave. This wave is intended to simulate a lightning stroke that may strike a system withoutbreakdown.
NOTElf the object being tested has appreciable inductive characteristics , it may be difficult or impossible to attainthe specified wave shape with less than 5 % oscilations, as prescribed in 8.2.2.However, the procedures given inthis publication are expected ordinarily to be applied to configurations of test specimens and electrodes which areprimarlly capacitive.Testing of more complex configurations，such as between coils of completed apparatus ormodels of such apparatus, should be performed in accordance with the specifications for that apparatus.
4.4Because of the short time involved，dielectric heating,other thermal effects，and theinfluence of injected space-charges may be reduced during impulse testing of most materials.Thus,impulse tests usually give higher values than the peak voltage of short-term ac tests.From comparisons of the impulse electric strength with the values drawn from longer timetests, inferences may be drawn as to the modes of failure under the various tests for a givenmaterial.
5 Electrodes and test specimensClause 4 of lEC 60243-1 is applicable.
6 Conditioning before tests
Clause 5 of lEC 60243-1 is applicable.
7 Surrounding medium
Clause 6 of lEC 60243-1 is applicable.
8Electrical apparatus
8.1 Voltage source
The test voltage applied to the electrodes shall be provided by an impulse generator having thefollowing characteristics.
8.1.1 A choice of either positive or negative polarity shall be provided, one of the connectionsto the electrodes being earthed.
8.1.2 Controls within the generator shall be capable of adjusting the shape of the waveapplied to the test specimen under test to have a virtual front time ty of 1,2 us ± 0,36 us, andvirtual time to half-value t2 of 50 us ± 10 us (see figure 1).
8.1.3 The voltage capability and energy-storage capacity of the generator shall be sufficient toapply impulse waves of the proper shape to any test specimens to be tested,up to thebreakdown voltage or specified proof voltage of the material.
8.1.4 The peak value of the voltage is taken as the virtual peak value，provided that theconditions of 8.2.2 are satisfied.
8.2Voltage measurement
8.2.1Provisions shall be made for recording the voltage wave as applied to the testspecimen, and for measuring the virtual peak voltage, the virtual front time, and the virtual timeto half-value within +5 % of the true values.
8.2.2 lf the voltage wave has oscillations with a magnitude of no more than 5 % of the peakvalue, and a frequency of at least 0,5 MHz, a mean curve may be drawn，the maximumamplitude of which is the virtual peak value. lf the oscillations are of greater magnitude,or oflower frequency, the voltage wave is not acceptable for a standard test.
Procedure
Clause 8 of lEC 60243-1 is applicable.
10 Application of voltage
10.1 Breakdown test
Breakdown tests shall be in accordance with clause 10 of lEC 60243-1.