These numbers result from many years of VLF field use and research. Testing Methodology When VLF Withstand testing cable, the proper test voltage and time duration are critical for the success of the test. Perform the test as instructed to make best use of VLF technology. Once partial discharge is triggered in a defect under the test voltage, enough time must be allowed to permit the PD to create an electrical tree and grow to failure.
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These numbers result from many years of VLF field use and research. Testing Methodology When VLF Withstand testing cable, the proper test voltage and time duration are critical for the success of the test. Perform the test as instructed to make best use of VLF technology.
Once partial discharge is triggered in a defect under the test voltage, enough time must be allowed to permit the PD to create an electrical tree and grow to failure. The test voltage and time duration are both critical to causing failures at defect locations severe enough to be triggered into PD during the test.
Let the technology and physics work. Lesser defects are not affected, as they are not forced into PD under the test voltage. They remain dormant and are not aggravated by the test voltage since it is below the PD inception voltage level of the defect. IEEE Std. These test sets are acceptable to be used. VLF ac test voltages with cosinerectangular and the sinusoidal wave shapes are most commonly used. While other wave shapes are available for testing of cable systems, recommended test voltage levels have not been established.
It should be noted that the recommended test time for a withstand test is 30 min. If the test cable has a defect severe enough at the withstand test voltage, an electrical tree will initiate and grow in the insulation.
Inception of an electrical tree and channel growth time are functions of several factors including test voltage, source frequency and amplitude, and the geometry of the defect. For an electrical tree from the tip of a needle in PE insulation in laboratory conditions to completely penetrate the insulation during the test duration, VLF ac voltage test levels and testing time durations have been established for the two most commonly used test voltage sources, the cosine-rectangular and the sinusoidal wave shapes.
However, the time to failure will vary according to the type of insulation such as PE, paper, and rubber. Thus the electrical tree growth rate is not the same for all materials and defects.
The voltage levels installation and acceptance are based on the most used, worldwide practices of from less than 2 U0 to 3U0, where U0 is the rated rms phase to ground voltage, for cables rated between 5 kV and 69 kV. Table 3 lists voltage levels for VLF withstand testing of shielded power cable systems using cosine-rectangular and sinusoidal waveforms Bach [B2]; Eager, et al. For a sinusoidal waveform the rms is 0. The rms and peak values of the cosine-rectangular waveform are assumed to be equal.
It should be noted that terminations may need to be added to avoid flashover for installation tests on cables rated above 35 kV. Regarding the test times: The recommended minimum testing time for a simple withstand test on aged cable circuits is 30 min at 0. If a circuit is considered as important, e. A test time within the range 15—30 min may be considered if the monitored characteristic remains stable for at least 15 min and no failure occurs.