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VLF cable testing

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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

VLF tan delta testing

Medium voltage distribution cables and their accessories form a critical part of power delivery systems. The systems employ insulation materials that have a low permittivity and loss. The permittivity and the loss are dielectric properties of the insulation material. As the systems age, these dielectric properties change such that they may provide a convenient way to monitor the insulation degradation. Generally, the dielectric loss is monitored because it can increase several orders of magnitude during the service life of the systems. This approach correlates well with the known mechanisms of degradation, namely the ingress of water and the subsequent growth of water trees for polymeric insulations.

During the last decade, VLF testing for extruded distribution cables has gained interest among the worldwide utilities. The increasing interest is evidenced by recent publications and discussions inside the expert community in which standards are being proposed and continuously discussed. In practice, it is convenient to measure the dielectric properties at a VLF of 0.1 Hz.[1] This both reduces the size and power requirements of the energizing source and increases the resolution. While it seems there is a general consensus as to the interpretation of the dielectric properties for diagnosis, many issues regarding the definition of more accurate means of system evaluation still need further study.

Tan delta measurement constitutes a cable diagnostic technique that assesses the general condition of the cable system insulation, which can be represented by an equivalent circuit that consists of two elements; a resistor and a capacitor. When voltage is applied to the system, the total current is the result of the contributions from the capacitor current and the resistor current. The tan delta is defined as the ratio between the resistor current and the capacitor current. The measurements are carried out offline.

Nowadays, two different criteria are applied for diagnosing a cable insulation system using the Tan δ value. One criterion uses the magnitude of the Tan δ value as a tool for diagnostics while the other uses the difference in Tan δ values for particular electrical stresses or voltage levels. The latter is commonly known as the “Tip-Up” of the Tan δ value.[2] The results for both criteria are often interpreted using recommendations given in the standards. The standards provide a hierarchical level that evaluates the cable insulation system.

International standards and guides

  • DIN VDE 0276 (after laying tests on new cables)
  • IEC 60502-2:2014 Cables for rated voltages from 6 kV (Um = 7,2 kV) up to 30 kV (Um = 36 kV) (after laying tests on new cables)
  • IEEE 400-2012 Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems
  • IEEE 400.2-2013 Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF)
  • CENELEC HD620 S1 (after laying tests on new cables)

See also

References

  1. ^ Eager, G.S.; Katz, C.; Fryszczyn, B.; Densley, J.; Bernstein, B.S. (Apr 1997). "High voltage VLF testing of power cables". IEEE Transactions on Power Delivery. 12 (2): 565–570. doi:10.1109/61.584323.
  2. ^ "IEEE Std. 400-2, Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF)". IEEE-SA.
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