Is,Part-3, - Download as PDF File .pdf), Text File .txt) or read online. Specification for Power Transformer,Insulation Levels and Dielectric Tests. Part 3: Insulation levels, dielectric tests and external clearances in air Foreword . The text of document 14//FDIS, future edition 3 of IEC , prepared by IEC/TC 14 "Power I.S. EN - PDF. View Test Prep - _3 from ELECTRICAL at Engineering College. IS: (Part III) - (Reaffirmed ) Edition () Indian Standard.
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Any additional requirement or tests regarding external insulation which are deemed necessary shall be subject to agreement between the purchaser and the supplier. Leave a message. Which method has been selected is part of the information to be supplied with an enquiry and with an order. The apparent charge shall not be higher than a specified value q. Servo-motor operated automatic line voltage correctors, Part 1:
A suitable standard value should preferably be selected from Table 2. This may be connected to earth through a considerable impedance for example. It may also be necessary to design the winding with higher neutral insulation level because of the test connection to be used for the induced power-frequency test of the transformer see It should be checked that the power-frequency withstand voltage is greater than the above mentioned system-fault voltage.
During impulse tests on a line terminal the neutral terminal shall be connected directly to earth. An increase of the value may sometimes easily be arranged and can improve the interchangeability of the transformer in the system.
The rated voltage of the surge arrester which is to be installed for neutral protection shall be selected at least equal to the maximum power-frequency voltage under such conditions of system faults as are considered.
A chopped-wave impulse test on the neutral is not recommended.
Unless impulse testing on a particular tapping has been agreed. They are then designed in accordance with 5. Motors and other apparatus for auxiliary equipment shall fulfil insulation requirements according to the relevant Indian Standards which are generally lower than the 17 PDF created with pdfFactory Pro trial version www.
Testing conditions determine the choice of tapping required for induced power-frequency test and for switching impulse test see 4.
No definite figures are recommended here. The clauses of this standard are applicable when dry type transformers are intended for general power distribution in public or industrial systems. The line terminals therefore receive a modified induced overvoltage test in accordance with At the end of the test. The peak value divided by 2 shall be equal to the test value.
The voltage shall be. After completion of erection on site a 1 V megohm meter test is recommended. There are also other possible methods. In exceptional cases see 4 the voltage on the neutral terminal may be raised by connection to an auxiliary booster transformer. The choice of a suitable test connection is determined by the characteristics of the transformer and of the test plant.
When the test frequency exceeds twice the rated frequency. If the winding has a neutral terminal. If the ratio between the windings is variable by tappings.
Recommended test connections which avoid excessive overvoltage between line terminals are shown in Fig. Other separate windings shall generally be earthed at the neutral if they are star-connected and at one of the terminals if they are delta-connected. Another winding of the transformer under test may also be connected in series with the high voltage winding. Only a1 is possible. This connection may also be used for a three-phase transformer without unwound magnetic return paths having the neutral insulation designed for less than one-third of the voltage U.
Rated voltages of the two auto-connected windings are Un1. If there is a delta-connected winding. Three different generator connections to the low voltage winding are shown. A three-phase transformer shall be tested either phase by phase in a single-phase connection that gives voltages on the line terminals as shown in Fig.
Um kV. The apparent charge shall not be higher than a specified value q. For other separate windings. The voltage shall be switched on at a level not higher than one-third of U2. The choice shall be agreed between the parties at the time of placing the order.
Specified According to Method 2 5. Occasional higher kicks should be disregarded. U1 shall be 3. Further information may be obtained from Appendix A.
If the apparent charge reading rises above the specified limit for a significant time and then returns below this level again the test may continue without interruption until acceptable readings have been obtained for 30 min.
Difficulties concerning the bushings are mentioned in 2. A failure to meet the partial discharge acceptance criterion shall. Part I General definitions and test requirements first revision. Part III Measuring devices first revision. Occasional high kicks should be disregarded. Part II Test procedures first revision. Suggestions for such procedures are given in Appendix A. The amplitude of opposite polarity of an oscillatory impulse should not exceed 50 percent of the first amplitude.
In the case of a three-phase transformer. The tank shall be earthed. If during any of these applications an external flashover in the circuit or across a bushing gap should occur.
Wider tolerances may. The resulting impulse shape is then often oscillatory. The impulse shape problem may also be treated by alternative methods of earthing during the test see If such elements are present during the test. The amplitude of the impulse applied to the line terminal is not prescribed but shall not exceed 75 percent of the rated lightning impulse withstand voltage of the line terminal. It is also possible to simplify the problem by earthing the non-tested terminals of the winding through resistors.
It is then permissible to earth the non-tested line terminals through resistors not exceeding ohms. The same applies to the testing of the line terminals of the common winding if the line terminals of the high-voltage winding are earthed.
Alternatively by agreement at the time of placing the order the transferred surge method in accordance with Exceptions from this main procedure are given under In this case wider tolerances may be applied by agreement between the parties see In this case. The neutral terminal is connected to earth through an impedance. On the other side the voltages appearing on the non-tested line terminals to earth should not exceed 75 percent of their rated lightning impulse withstand voltage.
The resistance value shall not be higher than ohms and shall be chosen so that the voltage appearing on the terminals is limited to not more than 75 percent of their rated withstand voltage. The resistance shall not exceed 5 ohms. The line terminals of the low voltage winding are connected to earth through resistances of such value that the amplitude of transferred impulse voltage between line terminal and earth or between different line terminals or across a phase winding is as high as possible but not exceeding the rated impulse withstand voltage.
The current flowing from tank to earth. The wave at the low voltage winding terminals may have any shape and shall be acceptable. In most cases an oscillogram of the current flowing to earth from the tested winding will present the best sensitivity for fault indication. A guidance for this purpose is provided in Appendix B. The standard lightning impulse shall have a time to chopping between 2 to 6 microseconds. The latter part of the oscillation pattern is then modified. When it has been agreed to carry out this test it shall be combined with the full lightning impulse test in the manner described below test as per clause The recommended order of the different pulse applications is: The neutral current record or any other supplementary recording presents a superposition of transient phenomena due to the front of the original impulse and from the chopping.
Account should therefore be taken of the possible variations. The peak value of the chopped impulse shall be the same as for the full impulse. The chopping circuit shall be so arranged that the amount of overswing to opposite polarity of the recorded impulse will be limited to not more than 30 percent of the amplitude of the chopped impulse.
Its value is usually less than microseconds. During the test considerable flux is developed in the magnetic circuit. The test is a routine test for windings with Um kV specified according to Method 2 see 5.
The specified test voltage shall appear between line and neutral terminals and the neutral shall be earthed. The impulse voltage can be sustained up to the instant when the core reaches saturation and the magnetizing impedance of the possible impulse duration can be increased by introducing remanence of opposite polarity before each full voltage test impulse. This is accomplished by lower voltage impulses of similar shape but opposite polarity or by temporary connection to a dc voltage source.
In a three-phase transformer the voltage developed between line terminals during the test shall be approximately 1. Oscillographic records shall be obtained of at least the impulse wave-shape on the line terminal under test.
If the oscillographic recording should fail. For a single-phase transformer the neutral of the tested winding shall be earthed. Windings not used for the test shall be suitably earthed at one point but not short-circuited.
This is considered to give sufficiently good sensitivity to arbitrary discharge sources irrespective of location. In a transformer such a partial discharge causes a transient change of voltage to earth. The specified. The comparison of simultaneously collected indications at different terminals may give information about the location of the partial discharge source within the transformer see A The preferred measure of the intensity of a partial discharge is the apparent charge.
If a capacitance tap is not provided it is also possible to insulate the bushing flange from the tank and use it as the measuring terminal. The FIG. The circuit shall represent a reasonably constant resistance. The measuring impedance in its simplest form is the matching impedance of the cable which may. A calibration generator in accordance with IS: A partial discharge-free capacitor shall be used.
The measuring impedance with a protective gap shall be connected between the low-tension terminal of the capacitor and earth see Fig. The rise time should be not more than 0. The equivalent capacitances between the central conductor. This is. Co where U is the voltage step usually 2 to 50 V.
A wide-band system utilizes a relatively large ratio between upper and lower limit of the frequency band. This is based on rate of repetition. Capacitor Co shall then be placed at the high-voltage terminal and a coaxial cable with a matching resistor shall be used from the step voltage generator. One method for calibration between earth and another terminal is shown in Fig. Continuous recording by oscillograph or tape recorder is not obligatory.
The generator shall preferably be battery-operated and have small physical dimensions. This is often facilitated by oscillographic monitoring of the test-disturbances may.
Identification of partial discharge sources by comparison of shape and polarity of individual pulses may be possible. Band-stop filters may be used against radio transmitters. The narrowrband instrument should be operated at a frequency not higher than kHz. This should be followed by consultations between the manufacturer and purchaser to agree to further supplementary tests or other action to show either the presence of serious partial discharge.
Low-pass filters on the supply leads to the transformer under test can help in such cases. The steady state partial discharge level. The testing environment should first be investigated to find any obvious sign of irrelevant sources of partial discharges.
The test object shall not be rejected immediately upon such a result but further investigations shall be undertaken. If there has been no voltage collapse. Below are some suggestions which may be useful during the above courses of action: Such a procedure may be the last resort if other means of improving the behaviour of the transformer or identifying the source have failed.
As the test concerns the internal insulation. It is described in A A particular case is the identification of partial discharge in the dielectric of the condenser bushings see A One is based on correlation of readings and calibrations at different pairs of terminals in addition to the obligatory readings between live terminal and earth. Re-processing of the transformer. It is also possible to identify individual pulse shapes during the test with corresponding calibration wave forms.
Relatively limited variation of the partial discharge level with voltage increase. There are several known and published methods. It is also well known that a limited exposure to a relatively high partial discharge may lead to local cracking of oil and temporarily reduced extinction and reinception voltages. Calibrations are made between winding terminals and earth. Calibrations and tests are made with reference to the terminals as indicated in the informal table in Fig. If calibration pulses are fed in at alternative calibration terminal pairs.
The line with results at 1. If there is an evident correlation between the profile of the test readings at different measuring terminal pairs and the profile obtained at the same measuring terminals for pulses fed in at particular pair of calibration terminals. While the calibration generator is connected to a specific pair of a calibration terminals. The procedure is then repeated for other pairs of calibration terminals.
This means that it is possible to draw a conclusion as to the location of the partial discharge source in terms of the electric circuit diagram of the transformer.
The example. The method as described is successful mainly in those cases where one distinct source of partial discharge is dominant. The physical location may be at any place along the connecting leads between the series winding and the common winding.
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