VLF cable testing (Very Low Frequency) is a technique for testing of medium and high voltage (MV and HV) cables. VLF systems are advantageous in that they can be manufactured to be small and lightweight; making them useful – especially for field testing where transport and space can be issues. Because the inherent capacitance of a power cable needs to be charged when energised, system frequency voltage sources are much larger, heavier and more expensive than their lower-frequency alternatives. Traditionally DC hipot testing was used for field testing of cables, but DC testing has been shown to be ineffective for withstand testing of modern cables with polymer based insulation (XLPE, EPR). DC testing has also been shown to reduce the remaining life of cables with aged polymer insulation.
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| - VLF cable testing (Very Low Frequency) is a technique for testing of medium and high voltage (MV and HV) cables. VLF systems are advantageous in that they can be manufactured to be small and lightweight; making them useful – especially for field testing where transport and space can be issues. Because the inherent capacitance of a power cable needs to be charged when energised, system frequency voltage sources are much larger, heavier and more expensive than their lower-frequency alternatives. Traditionally DC hipot testing was used for field testing of cables, but DC testing has been shown to be ineffective for withstand testing of modern cables with polymer based insulation (XLPE, EPR). DC testing has also been shown to reduce the remaining life of cables with aged polymer insulation. (en)
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| - VLF cable testing (Very Low Frequency) is a technique for testing of medium and high voltage (MV and HV) cables. VLF systems are advantageous in that they can be manufactured to be small and lightweight; making them useful – especially for field testing where transport and space can be issues. Because the inherent capacitance of a power cable needs to be charged when energised, system frequency voltage sources are much larger, heavier and more expensive than their lower-frequency alternatives. Traditionally DC hipot testing was used for field testing of cables, but DC testing has been shown to be ineffective for withstand testing of modern cables with polymer based insulation (XLPE, EPR). DC testing has also been shown to reduce the remaining life of cables with aged polymer insulation. VLF testing of cables is supported in IEC 60502 (up to 35 kV) and in IEEE 400.2 (up to 69 kV). As higher voltage VLF equipment is developed, standards may be adapted to increase the voltage level for application. The VLF test can be used in a number of ways:
* Apply VLF to cables in a simple withstand approach to detect potential failures (faults) in the cable insulation during a planned outage. The tested cable must withstand an AC voltage for a specified testing time without flashover. This method yields a "pass/fail" statement. VLF cable testing uses different wave shapes, typically sine and square and care must be taken when describing the voltage to be used. RMS and peak voltages have different relationships to each other depending on the wave shape and IEEE 400.2 uses the peak voltage level to equate the wave shapes. Frequency ranges used are within the range of 0.01 Hz to 0.1 Hz, where frequency selection depends on the load presented by the cable. Test voltage levels are either calculated using a multiple of the cable's nominal phase-phase voltage or via tables in IEEE 400.2; typically they are in the range of 1.5 U0 to 3 U0. The VLF cable testing time varies from 15 to 60 minutes. IEEE 400.2 establishes some suggested test voltages and times. Subsequent work by the CDFI has shown there to be no significant change in the efficacy of a VLF test conducted over the frequency range 0.1 to 0.01 Hz when the IEEE 400.2 voltages and times are used.
* Apply VLF to cables in a monitored withstand approach where a diagnostic measurement is made before and during the course of the withstand test. Monitoring a diagnostic enables some additional decision making before the final test voltage is reached. Some cables are not good candidates for withstand testing and a diagnostic indication obtained at a lower voltage can negate the need to perform withstand testing. During the test measurement of a diagnostic parameter can be used to optimise test times. Test times can be shortened for cables with good diagnostic indications or lengthened for cables that show deteriorating diagnostic measurements during the test.
* Apply VLF to measure insulation losses (i.e. the insulation dissipation factor or Tan-delta). In this case, the IEEE 400.2 establishes the criteria for assessment. The test is typically performed over a range of test voltages from 0.5 Uo to 2 Uo depending on the standard/guide that is being followed.
* Apply VLF in order to detect and measure partial discharge. In this case, the IEEE 400.3 outlines a procedure for assessment and IEC 60270 provides the background for partial discharge testing of high voltage apparatus. The test is typically performed over a range of test voltages to identify the different defects and their inception and extinction voltages. (en)
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