（Eight Test and Detection Methods for High Voltage Cables）

1. Insulation resistance measurement of cable main insulation 1.1 test purpose Initially judge whether the main insulation is damp and aging, and check whether there are defects in the cable main insulation after the withstand voltage test. The decrease of insulation resistance indicates that the insulation is affected with damp or aged or deteriorated, which may lead to cable breakdown and burning. It can only effectively detect the whole damp and penetrating defects, and is insensitive to local defects. 1.2 Measurement method Measured in each phase, the non-tested phase is grounded together with the metal shield (metal sheath) and armor layer. Use a megohmmeter, and recommend a large-capacity digital megohmmeter (e.g. short-circuit current > 3mA). The measuring voltage of 0.6/1kV cable is 1000V. The measured voltage of cables above 0.6/1kV is 2500V. 5,000 V can also be used for cables above 6/6kV. For cables above 110kV, use a 5000V or 10,000 V electric megohmmeter, which is best equipped with self-discharge function. Wear insulating gloves every time you change the wiring, and fully ground and discharge after each phase test. Electric megohmmeter 1.3 test cycle Handover test After a new terminal or connector is created. 1.4 Pay attention to the problem The “L” terminal lead and “E” terminal lead of megger should have reliable insulation. Before and after the measurement, the cable should be fully discharged for about 2-3 minutes. If a hand-operated megger is used, do not stop shaking the handle before disconnecting the high-voltage lead. The other end of the cable that is not connected to the test equipment should be guarded by someone, and no one is allowed to get close to or contact with it. If the leakage current on the surface of the cable joint is large, shielding measures can be adopted, and the shielded wire is connected to the “G” end of the megger. 1.5 Requirements for insulation resistance value of main insulation Handover: Before and after the withstand voltage test, there is no obvious change in insulation resistance. Pre-test: greater than 1000MΩ Reference standard for insulation resistance value of cable main insulation Note: The values listed in the table are all insulation resistance values converted to a length of 1km. Conversion formula r conversion = r measurement /L, l is the length of the cable to be measured. When the cable length is less than 1km, conversion is not required. 2. Cable main insulation withstand voltage test 2.1 Type of withstand voltage test Cable withstand voltage test is divided into DC withstand voltage test and AC withstand voltage test. DC withstand voltage test is suitable for paper insulated cables, and rubber insulated power cables are suitable for AC withstand voltage test. The cable we usually use is an AC polyethylene insulated cable (rubber insulated power cable), so we only introduce the AC withstand voltage test below. 2.2 withstand voltage test wiring diagram Voltage withstand test wiring diagram 2.3 withstand voltage standard For 110kV and above cables, the resonant withstand voltage test with frequency of 20hz～ 300Hz is recommended. The AC withstand voltage standard during handover is as follows: For 110kV and above cables, the resonant withstand voltage test with frequency of 20hz～ 300Hz is recommended. AC withstand voltage standards during pre-test are as follows: 3. Measurement of insulation resistance of cable outer sheath 3.1 test purpose Check whether the outer sheath of the cable is damaged or damp after laying or running. The reasons for the damage of the outer sheath are: excessive tension or excessive bending during laying; During laying or operation, due to direct external forces such as construction and transportation; The terminal/intermediate joint is subjected to internal stress, natural tension and electrodynamic force; Termite swallowing, chemical corrosion, etc. 3.2 Measurement method For 110kV and above cables, use a 500V electric megohmmeter, and it is best to have a self-discharge function. Wear insulating gloves every time you change the wiring, and fully ground and discharge after each phase test. The protective layer overvoltage protector must be disconnected during the test. GB50150-2006 and Q/CSG 1 0007-2004 require that the handover and pre-test of the insulation resistance value of the outer sheath should not be less than 0.5MΩ/km. 3.3 test cycle Handover test 3 years (for those with outgoing wires on the outer sheath) 3.4 Pay attention to the problem The “L” terminal lead and “E” terminal lead of megger should have reliable insulation. Before and after the measurement, the metal sheath of the cable should be fully discharged for about 2-3 minutes. If a hand-operated megger is used, do not stop shaking the handle before disconnecting the high-voltage lead. The other end of the cable that is not connected to the test equipment should be guarded by someone, and no one is allowed to get close to or contact with it. 4. DC withstand voltage test of cable sheath 4.1 Test Purpose Check whether the outer sheath of the cable is damaged or damp after laying or running. 4.2 Test voltage During the test, the overvoltage protector of the protective layer must be disconnected. Handover test-10kV DC for 1min. Preventive test-5kV DC for 1min. 4.3 test cycle Handover test 3 years 4.4 Test judgment No breakdown occurs. 4.5 Detection site Non-metallic sheath and joint outer sheath (if the thickness of the outer sheath is more than 2mm and the surface is coated with a conductive layer, it is basically applied to cables with voltage levels of 110kV and above). For the cross-connected system, DC withstand voltage test is conducted on each section of the cross-connected system. During the test, the cross-connected connection of the cable metal sheath is disconnected, the metal sheath of the tested section is connected with the DC test voltage, the metal sheath of the non-tested section of the interconnection box is grounded, and the outer sheath of the insulation joint, the insulation splint between the sections of the interconnection box, and the lead coaxial cable are tested together with the cable outer sheath. Wiring diagram of DC withstand voltage test principle of phase A outer sheath in cross-connected grounding mode 4.7 Typical Defects and Defect Analysis No. ① The defect is a typical construction problem. After the fault point is located, the construction party indicates that the cable there has been pricked by a shovel and passed the test after treatment. This defect exposes the problems existing in construction management. Serial number ② Four installation errors of similar insulated connectors were found in the two-circuit cable, which reflected that the level of accessory installers was low, and the defects detected by the outer sheath test avoided the occurrence of similar serial number ⑤ operation failures. The reason for the defect of serial number ③ lies in the lax construction management, and the reason for the defect of serial number ④ lies in the poor installation quality of accessories. The serial number ⑤ is an example of a 110kV cable fault in a certain company, which also reveals that there are problems in both the installation and handover test of accessories. First of all, the process requirements of the manufacturer are unreasonable. The outer layer of the copper braided tape of the cable preform only requires one layer of semi-overlapping insulating tape, and the preform is seriously eccentric in the copper shell, resulting in insufficient insulation margin. Secondly, during the DC 10kV/1min withstand voltage test of the cable outer sheath, the only insulating tape was broken down by the test voltage, but the metal sheath of the non-tested section on the other side of the interconnection box was not grounded during the test, resulting in the defect not being discovered in time. After live operation, the inside of the insulation joint is conductive, which causes the cross-connection system of cable sheath to fail, and the sheath produces about tens of amperes of induced current. The induced current flows through the contact between the copper braid of the joint and the copper shell, and the heat generated melts the prefabricated part of the intermediate joint. The melting area destroys the insulation performance of the stress cone of the rubber prefabricated part, and the field strength is seriously distorted, and the joint is instantly broken down, and the conductor discharges the copper shell, resulting in line tripping. 5. Measure the resistance of metal shielding layer and conductor resistance ratio. 5.1 Test Purpose Measuring the resistance of metal shielding layer and conductor can monitor its corrosion change, and measuring the resistance ratio can eliminate the influence of temperature on DC resistance measurement. 5.2 test cycle Handover test 5.3 Test Methods The DC resistance of metal shield and conductor at the same temperature is measured by double-arm bridge. 5.4 Test judgment Compared with the measured data before operation, there should be no big change. When the ratio of the former to the latter is increased compared with that before operation, it shows that the DC resistance of the shielding layer is increased and the copper shielding layer may be corroded. When the ratio is reduced compared with that before operation, it shows that the contact resistance of conductor connection points in accessories may increase. 6. Cross-interconnection system test 6.1 Schematic Diagram of Cross Interconnection System 6.2 Effect and Composition of Cross Interconnection Compared with non-cross interconnection, the current flowing through the metal sheath is greatly reduced. The highest induced voltage on the metal sheath of the ungrounded end is the voltage induced on the metal sheath of the cable with the longest length. The metal sheath must be disconnected for cross-interconnection, and the insulation between the fractures and the ground must be good. Generally, the cross-interconnection of cable metal sheath is carried out by using the interconnection box. The metal sheath of the grounding end is introduced into the direct grounding box through coaxial cable for grounding; The metal sheath at the ungrounded end is introduced into the cross-connected grounding box through coaxial cable, and the box is equipped with a sheath overvoltage protector to limit the possible overvoltage. Protective grounding box Direct grounding box Cross interconnection box 6.3 Cross-Interconnection Performance Test Dc withstand voltage test of cable outer sheath, insulating joint outer sheath and insulating splint During the test, the protective layer overvoltage protector must be disconnected, and the metal sleeves of the three cables on the other side are grounded in the interconnection box, so that the insulation rings of the insulation joints can also be combined for the test. Test of non-linear resistance type protective layer overvoltage protector The following two items are handover test items, and the preventive test is selected as one of them. Voltammetric characteristics or reference voltage shall conform to the provisions of the manufacturer. The insulation resistance of the nonlinear resistor and its lead to the ground shall be measured with a 1000V megohmmeter, and its value shall not be less than 10MΩ. Inspection of contact resistance and connection position of interconnected box knife (or connecting piece) The connection position should be correct. When measured in normal working position, the contact resistance should not be greater than 20μ ω. 7. Check the phase at both ends of the cable line. 7.1 test purpose Before the new line is put into operation and after the connection mode of the line is changed in operation, check the phase and phase sequence at both ends to prevent accidents caused by phase errors. 7.2 test cycle Handover test. 7.3 Test Methods Check that the phases at both ends of the cable line should be consistent and consistent with the phase of the power grid. For cable lines of 110kV and above, it must be completed in the power failure state, and its method is basically the same as that of overhead lines. 8. Cable line parameter measurement 8.1 test purpose The measurement of DC resistance, positive sequence impedance, zero sequence impedance and capacitance of cable lines are the practical basis for relevant calculations (such as system short-circuit current, relay protection setting value, etc.) before the new lines are put into operation and after the connection mode changes in operation. 8.2 test cycle Handover test. 8.3 Test method Same as overhead line parameters. Because the positive sequence capacitance and the zero sequence capacitance of the cable are the same, it is usually only expressed by the capacitance between the conductor and the metal shield.