This paper includes insulation resistance measurement of cable main insulation, cable main insulation withstand voltage test, cable outer sheath insulation resistance measurement, cable outer sheath DC withstand voltage test, measurement of metal shield resistance and resistance ratio, cross-elective system test, inspection The phase and cable line parameters at both ends of the cable line are measured in eight parts.
First, the insulation resistance measurement of the main insulation of the cable
First, the insulation resistance measurement of the main insulation of the cable
1. Purpose of the test
Initially determine whether the main insulation is damp and aging, and check whether the main insulation of the cable is defective after the withstand voltage test. A decrease in insulation resistance indicates that the insulation is wet or deteriorated, which may cause cable breakdown and burnout. It can only effectively detect the overall moisture and penetration defects and is not sensitive to local defects.
2, measurement method
In each phase measurement, the non-test phase and the metal shield (metal sheath) and the armor layer are grounded together. With a megohmmeter, a large-capacity digital megohmmeter is recommended (eg, short-circuit current >3 mA). The 0.6/1kV cable measures 1000V. The cable measuring voltage above 0.6/1kV is 2500V. The cable above 6/6kV can also be used for 5000V. For the cable of 110kV and above, the electric megohmmeter with 5000V or 10000V is used. The electric megohm meter is equipped with self-discharging function. Insulated gloves are used each time the wiring is changed. After each phase of the test, it should be fully grounded and discharged.
3, test cycle
Handover test, after new terminal or joint
4, pay attention to the problem
The "L" end lead and the "E" end lead of the megger should have reliable insulation. The cable should be fully discharged before and after the measurement, and the time is about 2-3 minutes. If you shake the megohmmeter by hand, do not stop shaking the handle before disconnecting the high voltage lead. The other end of the cable test equipment shall be guarded and no one shall be allowed to approach and contact. If the leakage current of the cable joint surface is large, shielding measures can be used, and the shielded wire is connected to the "G" end of the megohmmeter.
5, the main insulation insulation resistance value requirements
Handover: Before and after the withstand voltage test, there is no significant change in insulation resistance.
Pre-test: greater than 1000MΩ
Cable main insulation insulation resistance value reference standard
Note: The values listed in the table are the insulation resistance values when converted to a length of 1km.
Conversion formula R conversion = R measurement / L, L is the length of the cable under test. When the cable length is less than 1km, no conversion is required.
Second, the cable main insulation withstand voltage test
1, pressure test type
The cable withstand voltage test is divided into DC withstand voltage test and AC withstand voltage test. The DC withstand voltage test is applicable to paper insulated cables, and the rubber and plastic insulated power cables are suitable for AC withstand voltage test. Our conventional cable is AC polyethylene insulated cable (rubber-insulated power cable), so we will only introduce the AC withstand voltage test below.
2, pressure test wiring diagram
3, pressure standard
For 110kV and above cables, the recommended frequency is 20hz ~ 300Hz resonant withstand voltage test. The AC withstand voltage standards at the time of handover are as follows:
For 110kV and above cables, the recommended frequency is 20hz ~ 300Hz resonant withstand voltage test. The AC withstand voltage standards during the pre-test are as follows:
Third, cable outer sheath insulation resistance measurement
1. Purpose of the test
Detect if the cable is damaged or damp after laying or during operation. The reasons for the damage of the outer sheath are: excessive tension or excessive bending during laying; direct external force due to construction and transportation during laying or operation; end/intermediate joints subjected to internal stress, natural tension, and electrodynamic force; termites devour , chemical corrosion, etc.
2, measurement method
For 110kV and above cables, the 500V electric megger is used, and the electric megohm meter is equipped with self-discharging function. Insulated gloves are used each time the wiring is changed. After each phase of the test, it should be fully grounded and discharged. The sheath overvoltage protector must be disconnected during the test.
GB50150-2006, Q/CSG 1 0007-2004 requires that the insulation and resistance of the outer sheath insulation resistance value is not less than 0.5MΩ/km.
3, test cycle
Handover test, 3 years (for the outer sheath with lead wire)
4, pay attention to the problem
The "L" end lead and the "E" end lead of the megger should have reliable insulation. The cable metal sheath should be fully discharged before and after the measurement, and the time is about 2-3 minutes. If you shake the megohmmeter by hand, do not stop shaking the handle before disconnecting the high voltage lead. The other end of the cable test equipment shall be guarded and no one shall be allowed to approach and contact.
Fourth, the cable outer sheath DC withstand voltage test
1. Purpose of the test
Detect if the cable is damaged or damp after laying or during operation.
2, test voltage
The overvoltage protector of the sheath must be disconnected during the test, and the test is to be conducted - DC 10kV, duration 1min, preventive test - DC 5kV, duration 1min
3, test cycle
Handover test, 3 years
4, test judgment
No breakdown occurs.
5, the detection site
Non-metallic sheath and joint outer sheath (the thickness of the outer sheath is 2mm or more, and the surface is coated with a conductive layer, basically for 110kV and above voltage grade cables).
For the cross-external system, the DC withstand voltage test is carried out on each section of the cross-elective system. During the test, the cross-connection of the cable metal sheath is broken, and the metal sheath of the test section is connected to the DC test voltage. The metal sheath of the non-test segment cable on one side is grounded, the outer sheath of the insulated joint, the insulating splint between the interconnecting box segments, and the lead coaxial cable are tested together with the outer sheath of the cable.
6, typical defects and defects analysis
The defect of serial number 1 is a typical construction problem. After the fault point is located, the construction party indicates that the cable has been injured by the iron shovel, and the test passes after the treatment. This defect exposes the problems in construction management.
No. 2 Installation errors of the same type of insulated joints Four points were found in the two cables, reflecting the low level of the installer. The outer sheath test detected the defects to avoid the occurrence of the operation failure similar to the serial number 5.
The reason for the defect of serial number 3 is that the construction management is not strict, and the reason for the serial number 4 defect is that the installation quality of the accessory is poor.
The serial number 5 is an example of a 110kV cable failure in a unit, and there are problems in both the attachment installation and the handover test.
First of all, the manufacturer's process requirements are unreasonable. The outer layer of the copper braid of the cable preform requires only one layer of semi-insulated insulation tape, and the preform is severely eccentric in the copper shell, resulting in insufficient insulation margin.
Secondly, in the DC 10kV/1min withstand voltage test of the outer sheath of the cable, the test voltage punctures only one layer of insulation tape, but the metal sheath of the non-test segment on the other side of the interconnection box is not grounded during the test, resulting in defects. Not found in time.
After the live operation, the insulated joint is internally turned on, causing the cable sheath to cross the lian system to fail, and the sheath generates an induced current of about several tens of amps. The induced current flows through the joint of the copper braid and the copper shell of the joint, and the generated heat fuses the intermediate joint preform. The burned area destroys the insulation performance of the stress cone of the rubber preform, the field strength is severely distorted, and the joint is instantaneously broken down. The conductor discharges the copper shell, causing the line to trip.
Fifth, measure the resistance and resistance ratio of the metal shield
1. Purpose of the test
Measuring metal shield resistance and conductor resistance can monitor changes in corrosion, and measuring resistance ratio can eliminate the effect of temperature on DC resistance measurements.
2, test cycle
Handover test
3. Test method
The DC resistance of the metal shield and conductor at the same temperature was measured with a double-arm bridge.
4, test judgment
There should be no major changes compared to the measurement data before commissioning. When the ratio of the current to the latter increases compared with that before the operation, it indicates that the DC resistance of the shield layer increases, and the copper shield layer may be corroded; when the ratio is reduced compared with before the operation, the conductor in the accessory is indicated. The contact resistance of the connection point is increased.
Sixth, cross mutual lian system test
1. Cross-peel system diagram
2, cross-interconnect effect and composition
Compared to the non-interconnected interconnection, the current flowing through the metal sheath is greatly reduced. The high induced voltage on the non-grounded metal sheath is the voltage induced on the metal sheath of the longest length of the cable. The cross-connection must be disconnected from the metal sheath, and the insulation between the fracture and the ground should be well insulated. Generally, the interconnection box is used for the cross-connection of the metal sheath of the cable. The grounding metal sheath is introduced into the direct grounding box through the coaxial cable; the non-grounding metal sheath is introduced into the cross-connected grounding box through the coaxial cable, and the sheathed overvoltage protector is used to limit the possible overvoltage.
3, cross-connect performance test
DC withstand voltage test of cable outer sheath, insulated joint outer sheath and insulating splint
During the test, the over-voltage protector must be disconnected, and the three-section cable metal sleeve on the other side should be grounded in the interconnection box, so that the insulating rings of the insulated joint can also be tested together. Nonlinear resistance type sheath overvoltage protector test. The following two items are handover test items. One of the preventive tests is selected: the volt-ampere characteristic or reference voltage should be in accordance with the manufacturer's specifications; the non-linear resistance sheet and its lead-to-ground insulation resistance are measured with a 1000V megohmmeter. The insulation resistance between the lead and the case should not be less than 10MΩ. Check the contact resistance and connection position of the interconnecting box knife (or connecting piece), and the connection position should be correct. When measuring at the normal working position, the contact resistance should not be greater than 20μΩ.
Seven, check the phase of the two ends of the cable line
1. Purpose of the test
After the new line is put into operation and the line connection mode is changed during operation, the phase and phase sequence of both ends are checked to prevent phase errors and cause accidents.
2, test cycle
Handover test.
3. Test method
Check that the ends of the cable line are in phase and consistent with the grid phase. For cable lines of 110kV and above, all need to be completed in the power outage state, and the method is basically the same as the overhead line.
Eight, cable line parameter measurement
1. Purpose of the test
Cable line DC resistance, positive sequence impedance, zero-sequence impedance measurement, capacitance measurement, as the actual basis of calculations (such as system short-circuit current, relay protection setting value, etc.) after the new line is put into operation and the line connection mode is changed. .
2, test cycle
Handover test.
3. Test method
The same as the overhead line parameters, because the positive sequence capacitance and the zero sequence capacitance of the cable are the same, it is usually expressed only by the capacitance between the conductor and the metal shield.