This standard specifies the test equipment, sample preparation, test steps and precautions for partial discharge. This test method is applicable to measuring partial discharge of extruded insulated power cables of different lengths, that is, measuring the discharge of the cable at a specified voltage and given sensitivity or checking whether the discharge exceeds the specified value. GB/T 3048.12-1994 Test methods for electrical properties of wires and cables Partial discharge test GB/T3048.12-1994 Standard download decompression password: www.bzxz.net

National Standard of the People's Republic of China
Test methods for determining electrical properties of electric cables and wires
Partial discharge test
GB/T3048.12-94
Replaces G3 3018.12-h3
Test methods for determining electrical properties of electric cables and wiresPartial discharge test
This standard is equivalent to IEC885-3 (1988) partial discharge test for whole extruded cable. 1 Subject content and scope of application
This standard specifies the test equipment, sample preparation, test procedures and precautions for partial discharge. This test method is applicable to measuring partial discharge of extruded insulated power cables of different lengths, that is, measuring the discharge of the cable at a specified voltage and a given sensitivity or checking whether the discharge exceeds the specified value. The general requirements, definitions and periodic calibration requirements of the electrical performance test of wires and cables are specified in GB/T3018.1. 2 Reference standards
GB/T3048.1 General principles for electrical performance test methods of wires and cables 3 Test equipment
3.1 Test equipment
It consists of a high-voltage power supply with a kVA capacity that meets the length requirements of the tested cable, a high-voltage voltmeter, a measuring line, a discharge voltage calibrator, a double pulse generator, etc. If necessary, it also includes terminal impedance or reflection suppressor. The noise level of all components of the test equipment should be low enough to obtain the required sensitivity.
The frequency of the test power supply is close to the industrial frequency AC 49~61H of a sine wave. The ratio of the peak value to the effective value is equal to /, with an error of +7%. 3.1.1 Test circuit and instruments The test circuit includes the test piece, a coupled electric penetrator and a measuring circuit. The measurement circuit can be measured by measuring impedance (the input impedance of the measuring instrument and the input unit selected to match the cable impedance), connecting wires, and measuring instruments. The measuring instrument or detector includes a suitable amplifier and an oscilloscope. In addition, instruments can be added as needed to indicate the presence of partial discharge. The measured apparent charge 3.1.2 Double pulse generator The characteristics of the partial discharge test circuit need to be calibrated with a double pulse generator. The double pulse should be synchronized with the I frequency. The two paired and equal pulses have a trap time that can be continuously adjusted from 0.2 to 100. The leading edge of the pulse (rise time) should not exceed 20ms, and the time from 10% of the wave head value to 10% of the wave tail value should not exceed 150ms. 3.1.3 Terminal impedance (characteristic impedance) In order to suppress pulse reflection in the case of an open circuit at the far end of the cable (the cable terminal far away from the detector), a terminal impedance can be connected at the far end. Its impedance The impedance value should be equal to the characteristic impedance value of the cable sample. 3.1.4 Reflection inhibitor If there is no terminal impedance during the test, in order to avoid the influence of pulse transmission, a reflection inhibitor can be used, that is, an electronic switch that can block the input of the detector in most cases and block the reflected pulse when the far end of the cable is open. However, when the local discharge site is at the far end or near it, some interruption is difficult to avoid. 3.2 Determine the characteristics of the test circuit
The characteristics of the test circuit should be determined under the conditions of use. The commonly used test circuits for determining the characteristics of the test circuit are shown in Figures 1 to 5. Similar test circuits can also be applied when the two ends of the cable conductor (and the two ends of the shield) are connected together. Approved by the State Administration of Technical Supervision on May 19, 1994 and implemented on January 1, 1995
3.2.1 Superposition performance
GB/T 3048. 12- 94
If the terminating impedance is not used, the performance of the test circuit to the superposition of traveling waves must be determined. Connect the double pulse generator according to Figure 6 and plot the double pulse curve (see Figure 6, Figure 7 to Figure 9). This verification should be carried out at least once a year or when important circuit components have been repaired or replaced.
3.2.2 Terminal impedance
When the terminal impedance (see Figure 4) is used, its suitability for the test cable should be confirmed in accordance with the method specified in Chapter 8. This verification should be carried out at least once a year or when required or when important circuit components have been repaired or replaced. 3.2.3 Reflection inhibitor
Purpose of using reflection inhibitor The purpose is to obtain a type 1 double pulse curve in accordance with Figure 7. According to Figure 10, the effectiveness of the reflection suppressor should be checked at least once a year and when required or when important circuit components have been repaired or replaced. 3.2.4 Electrical calibration
Electrical calibration should be carried out using the "charge conversion calibration method". In this method, the calibrator is directly connected between the conductor and the metal shield at one end of the test cable, and then a predetermined charge is injected into the test product. The injected charge is required to produce a pulse height of at least 10mm on the oscilloscope. In general, the calibrator should be removed before the high-voltage test transformer is powered, and the amplifier multiplier should not be adjusted. There are two exceptions.
. The capacitor of the calibrator can work under the test voltage and forms part of the test circuit. b Use a secondary calibration circuit. At this time, the calibrator is not affected by the high voltage, but the pulse height produced by the secondary calibration circuit should be checked in advance against the pulse height produced by the primary calibration circuit. The calibration charge gcl (pC) is equal to the product of the calibration pulse amplitude △U (V) and the capacitance Cal (pF) of the calibrator. That is:
4f = Cu. AU
When calibrating, the calibration coefficient of the measuring instrument is multiplied by the instrument reading. The discharge value of the input instrument can be obtained. The stability of the table value should meet the requirements of the corresponding standard.
3.2.5 Sensitivity
a. The sensitivity of the test circuit refers to the minimum discharge m (pC) that the instrument can detect under the condition of background interference, which is expressed by the following formula:
Yaia2khn
Where: Sensitivity—calibration coefficient, calculated. pc/mtm, h.-background T-interference deflection value, mm. Read on an oscilloscope or micro-cubic meter. (2) In order to obtain clear test results, the display height of the product on the oscilloscope should be at least times the apparent background interference height, and if an indicating instrument is used, the reading of 9l should also be at least twice the noise reading. However, for individual interference pulses that can be clearly identified, the background interference height is not included, h. The specific value of sensitivity is determined according to the standards of different types of cable products. 4 Sample preparation As a type test, it is carried out on a short cable sample; as a routine test, it is carried out on the manufacturing length of the product (long cable sample). 5 Test steps The selection of the test circuit should be based on the double pulse diagram to see whether the cable sample belongs to the short cable case (Article 5.1) or the long cable case (Articles 5.2 to 5.4). GB/T 3048. 12 94 The test circuit must be free of discharge to achieve the required sensitivity. Calibration does not need to be done when high voltage is applied.
5.1 Short cable length (including type test) a. Requirements
Short cables can be considered similar to concentrated capacitors. The limit on the length of short cables depends on the test circuit used. The actual value can be determined by the double pulse curve specified in Chapter 7 and is defined as. The test circuits of Figures 1, 2 and 3 are generally selected. Note: When the two ends of the cable are connected together, the length up to 2% is also extended to the short cable. b. Sensitivity test
The calibrator should be connected in parallel to the end of the sample away from the test instrument. The calibration coefficient kz (pC/mm) and the sensitivity 4min (pC) can be calculated by injecting the calibration charge 9 and the corresponding measured deflection value. ke = qeu/α2bzxz.net
Amin — 2kzh.
Where h. is the background interference deflection value (mm). c. Test steps
Only one end of the sample needs to be measured, and the measured deflection value A (mm) is used to calculate the discharge g (pC), that is,
The test voltage is specified in the product standard.
5.2 Test of long cables without terminal impedance
a requirement
When the cable length exceeds, it is still possible to test without terminal impedance, provided that the addition and attenuation phenomena are taken into account. At this time, the double pulse curve diagram is either type 1 curve as shown in Figure 7, or type 2 and type 3 curves as shown in Figures 8 and 9. However, the sample length 1 here should be less than 21, or greater than 212. If the sample length range is 21≤≤21+, other test circuits should be used or tested according to the provisions of Articles 5.3 and 5.4. The commonly used test circuits are shown in Figures 1, 2, 3 and 5. b. Sensitivity test
According to Figure 1, 2, 3 or 5, the calibrator should be connected in parallel to each end of the cable, the first step to the far end, the second step to the near end. In the above two cases, the calibration power of the calibrator and the amplification factor of the amplifier should not change. Record the following values ​​respectively. a: The deflection value measured when the calibrator is connected to the near end, mm a: The deflection value measured when the calibrator is connected to the far end, mm. From a,and calibration power 4 to calculate the calibration coefficient (nc/mm) : kr = qal/a,
and u2 to calculate the attenuation correction factor F: when a2ai F=1
when ±g
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