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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. 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. The purpose of a withstand test is to verify the integrity of the cable under test. 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.
It should be noted that the recommended test time for a withstand test is 30 min. These numbers result from many years of VLF field use and research.
IEEE Std. Note 1. High Voltage, Inc.
VLF cable testing
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. DC testing has also been shown to reduce the remaining life of cables with aged polymer insulation . As higher voltage VLF equipment is developed, standards may be adapted to increase the voltage level for application. High voltage withstand tests are used in conjunction with partial discharge measurements 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 withstand and partial discharge tests as commissioning and maintenance tests for cable systems in the field.