O P E R A T O R M A N U A L
20
Recommended VLF Test Voltage Levels and Durations
With much of the world abandoning DC testing of solid dielectric cable, VLF AC hipots are becoming widely
used. Since an AC test is a proof (or withstand) test on a go/no-go (or pass/fail) basis, the test voltage is
applied for a set duration. The cable being tested either holds the test voltage or fails. The question arises,
and there is and has been much debate, as to the proper test voltage and the time length of the test. There
are differing recommendations and standards in existence that define voltage level and duration. Here we
will attempt to summarize what exists and offer recommendations.
5.1 General VLF testing
5.1.1 VLF test parameters
During a VLF test an electrical tree at the site of an insulation defect is forced to penetrate the insulation.
Inception of an electrical tree and channel growth time are functions of test signal frequency and amplitude.
For an electrical tree to completely penetrate the insulation during the test duration, VLF test voltage levels
and testing time durations have been established for the two most commonly used test signals, the cosine-
rectangular and the sinusoidal wave shapes.
The voltage levels (installation and acceptance) are based on most-used practices world-wide of between 2
Uo and 3 Uo, where Uo is the rated voltage for cables rated between 5 kV and 35 kV. The maintenance test
level is about 80% of the Acceptance test level. One can reduce the test voltage another 20% if more test
cycles are applied (Bach [B2]; Baur, Mohaupt, and Schlick [B5]; Krefter [B16]).
Table 4 and Table 5 list voltage levels for VLF withstand testing of shielded power cable systems using
cosine-rectangular and sinusoidal waveforms (Bach [B2]; Eager et al. [B7]; Krefter [B16]; Moh [B17]. For
a sinusoidal waveform the rms is 0.707 of the peak value if the distortion is less than 5%.
VLF test voltage for sinusoidal waveform (see Note 1)
Waveform
Sinusoidal
Cable System
Rating
(Phase to Phase)
[kV]
Installation
(Phase to Ground)
Acceptance
(Phase to Ground)
Maintenance
(Phase to Ground)
[kV rms]
[kV peak] [kV rms]
[kV peak]
[kV rms]
[kV peak]
5
9
13
10
14
7
10
8
11
16
13
18
10
14
15
19
27
21
30
16
22
20
24
34
26
37
20
28
25
29
41
32
45
24
34
28
32
45
36
51
27
38
30
34
48
38
54
29
41
35
39
55
44
62
33
47
46
51
72
57
81
43
61
69
75
106
84
119
63
89
