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VLF ac test voltages with cosinerectangular and the sinusoidal wave shapes are most commonly used.
Thus the electrical tree growth rate is not the same for all materials and defects. For a sinusoidal waveform 40.2 rms is 0. It should be noted that terminations may need to be added to avoid flashover for installation tests on cables rated above 35 kV.
The rms and peak values of the cosine-rectangular waveform are assumed to be equal. However, the time to failure will vary according to the type of insulation such as PE, paper, and rubber.
VLF Cable Test Voltage IEEE Standard | High Voltage Inc
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. For an electrical tree from the tip of a needle in PE insulation in laboratory conditions to completely penetrate the insulation during the test ieew, VLF ac voltage test levels and testing time durations iewe been established for the two most commonly used test voltage sources, the cosine-rectangular and the sinusoidal wave shapes.
Regarding the test times: 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. When VLF Withstand testing cable, eiee proper test voltage and time duration are critical for the success of the test.
Idee the test cable has a defect severe enough at the withstand test voltage, an electrical tree will initiate and grow in the insulation. 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.
They remain dormant and are not aggravated by the test voltage since it is below the PD inception voltage level of the defect.
A test time within the range 15—30 min may be considered if the monitored characteristic remains stable for at least 15 400.2 and no failure occurs.
Perform the test as instructed to make best use of VLF technology. Regarding the test times:. These numbers result from many years of VLF field ieer and research. 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 a circuit is considered as important, e.
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. Let the technology and physics work.
The purpose of a withstand test is to verify the integrity of the cable under test. Lesser defects are not affected, as they are not forced into PD under the test voltage. The recommended minimum testing time for a simple withstand test on aged cable circuits is 30 min iefe 0.
These test sets are acceptable to be used.