VLF cable testing

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 Guide. 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. IEEE Guide 400.2 establishes some suggested test voltages and times.

VLF withstand testing

High voltage in conjunction with partial discharge measurements are used on solid dielectric cable and accessories 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 partial discharge 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 detect any defective components of the cable system before failure. However in some cases a partial discharge tests are not available and withstand test are used. While far less reliable, withstand tests are simple to operate and the equipment is inexpensive. While withstand tests can't fail the vast majority of defects, some in the industry believe the risk of damaging the cable is worth the few percent of defects they can fail in a controlled manner while the minimum number of customers are affected. A recent study (Cable Diagnostic Focused Initiative Project by NEETRAC-Georgia Tech) has shown that withstand tests have been often applied in the past. 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 simple for a utility to perform and do not require specialized services
Cables that pass VLF tests are more likely to fail in the future.
The failure rates can be high on a cable system basis with some studies showing in the range of 0.2 to 4% for 30 min tests performed at the IEEE Guide 400.2 voltage levels
IEEE Guide 400.2 provides suggested time and voltage test levels but exact parameters are not possible since defect growth rates are not known and can vary widely.
VLF tests at IEEE Guide 400.2 test levels do not significantly damage cable systems 'good' insulation but often can degrade insulation defects without a test failure and 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 failure rate outcomes that can be ascribed to the voltage waveform
Many areas for further technically useful work have been identified since the scatter in the data is large and the results are not definitive.
Massive damage can not be detected withstand test. For example a cut 50% through the insulation can out last a withstand test by months.
Sometimes the test process can introduce contamination or damage which will not be detected by a withstand test but can fail in service years later.

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 can change. The dielectric loss can be assessed since it can increase several orders of magnitude during the service life of the systems. This approach correlates well some lossy growths in aged polymeric insulation such as water trees.

During the last three decade, VLF testing for extruded distribution cables has gained interest among the worldwide utilities. The increasing interest is evidenced by publications and discussions inside the industry. 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 of the resistive component (near DC component) of dielectric loss (not the capacitive component). While it seems there is no 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 significant further study.

Tan delta measurement constitutes a cable diagnostic technique that assesses the general condition of the cable system insulation, which can be represented in an overly simplified 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 guide. The guide provide a hierarchical level that evaluates the cable insulation system. The major caveats with this approach are:

losses can't be located.
Since most systems do not provide simple guard circuits to prevent erroneous loss contributions from the terminations or joints, many systems will show unnecessary high levels of tangent delta when actually the insulation system is in good condition.
A hot cable system can produce significantly elevated tangent delta values as compared to a cold cable
The vast majority insulation defects are not associated with losses. For example a knife cut 50% through solid dielectric insulation which produce the same tangent delta levels before and after the test.

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)

References

↑ 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.
↑ "IEEE Guide 400-2, Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF)". IEEE-SA.

External links

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