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VLF cable testing is a technique for testing of medium voltage cables. The VLF test can be used in two ways: * Apply VLF to measure insulation losses (i.e. the insulation dissipation factor or Tan-delta) at different VLF frequencies that are typically in the range of 0.01 to 0.1 Hz. In this case, the IEEE Std. 400.2 establishes the criteria for assessment. Accessories for partial discharge testing is also available from several VLF test set manufacturers. * Apply VLF to XLPE cables in a monitored withstand approach to detect potential failures (faults) in the cable insulation during a planned outage. The tested cable must withstand a VLF (Very low frequency) AC voltage for a specified testing time without flashover. This method yields a "Go/No Go" statement. VLF cable testing uses different wave shapes typically sine and square, voltages expressed for these wave shapes differ as RMS is not always applicable. In these cases the reference is via the peak voltage. Frequency ranges used are within the range of 0.01 Hz to 0.1 Hz, where frequency selection depends on the load of the cable. Test voltage levels are calculated using a multiple of the cable's nominal voltage, they are in the range of 1.5 U0 to 3 U0. The VLF cable testing time varies from 15 to 60 minutes. Care must be used lowering the test frequency rates, lowering the test voltage, or decreasing the test time, as this affects the water tree growth which is the main purpose of the test in XLPE cables. The object is to grow the water trees through the XLPE insulation till the cable fails, then the aged part of the cable can be replaced. VLF testing can be considered a short and economical testing criteria for network operators, the IEEE Std. 400.2 establishes the criteria for assessment. In the past DC voltages were used for cable testing, which sometimes was actually damaging to the cable insulation. == VLF withstand testing == High voltage withstand tests are used within manufacturing plants to ensure the quality of completed cable system components from MV to EHV. Thus, it is quite natural for utilities to also use withstand tests as commissioning and maintenance tests for cable systems in the field. The goal of these tests is the same as in the factory test, namely to have any weak components of the cable system fail in a controlled manner, such that the minimum number of customers are affected. In fact a recent study (Cable Diagnostic Focused Initiative Project by NEETRAC-Georgia Tech) has shown that withstand tests are among the most routinely employed diagnostic tests in the USA; this study has also shown that the most preferred withstand tests use Very Low Frequency (VLF: 0.01 to 0.1 Hz) AC methods. Some observations for the VLF withstand test are (Based on CDFI results): * VLF tests are very practical for a utility to perform and do not require specialized services * The Survivor rates are high for these tests with expected values, based on 1,000 ft (305 m) cable system segment lengths, in the range of 0.2 to 4% for 30 min tests performed at the IEEE Std. 400.2 voltage levels * IEEE Std. 400.2 provides appropriate time and voltage test levels (determining optimal times and voltages was outside the scope of the work reported here) * VLF tests at IEEE Std. 400.2 test levels do not significantly damage cable systems as would be manifested by cascading (or multiple) failures on test or shortened times to failure in service * Data have been collected using both of the commonly used VLF waveforms, there is little evidence of a significant difference in outcomes that can be ascribed to the voltage waveform * A number of areas for further technically useful work have been identified 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「VLF cable testing」の詳細全文を読む スポンサード リンク
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