IEEE 400-2:2013 pdf free download – IEEE Guide for Field Testing of Shielded Power Cable SystemsUsing Very Low Frequency (VLF)(less than 1 Hz)
5.Very low frequency (VLF) ac testing
VLF ac testing methods utilize ac signals in the frequency range from 0.01 Hz to 1 Hz. In withstand testing,(Gnerlich [Bil]),the test object must survive a ‘specified voltage applied across the insulation for aspecified period of time without breakdown of the insulation (Hampton, et al.[B19]). The magnitude of thewithstand voltage is usually greater than the operating voltage. If the accessory or cable insulation issufficiently degraded a breakdown can occur. The cable system may be repaired and the insulation retesteduntil it passes the withstand test.Diagnostic testing allows the determination of the relative amount ofdegradation of a cable system section and establishes, by comparison with figures of merit or accumulateddata, whether a cable system section is likely to continue to perform properly in service. It should be notedthat values of the diagnostic quantity measurements obtained during VLF ac voltage tests may not correlatewith those values obtained at other frequencies,for example, the tangent delta is larger at 0.1 Hz than atpower frequency and partial discharge(PD) may differ in terms of magnitude and inception voltage.
There are risks associated with high-voltage testing and diagnostics. Diagnostic tests can be non-destructiveif they are performed at voltages at or below the normal operating voltage.However, there is a trade-offbetween gathering additional information about the cable under test and going to elevated voltage levels,with the associated higher risk that the cable may fail as the voltage is increased. It should be noted that atthe prescribed withstand levels in Table 3, a failure indicates that the cable is already in a highlycompromised condition. In addition,should a failure occur under test,the resultant fault current andcollateral damage to the cable and surrounding assets may be limited. This may not be the case should thecable fail under operating conditions.
5.1.1 VLF ac withstand voltage test parameters
The purpose of a withstand test is to verify the integrity of the cable under test. If the test cable has a defectsevere 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 testvoltage, source frequency and amplitude, and the geometry of the defect. For an electrical tree from the tipof a needle in PE insulation in laboratory conditions to completely penetrate the insulation during the testduration，VLF ac voltage test levels and testing time durations have been established for the two mostcommonly used test voltage sources, the cosine-rectangular and the sinusoidal wave shapes. However, thetime to failure will vary according to the type of insulation such as PE, paper，and rubber. Thus theelectrical 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 fromless than 2 Uo to 3Up, where Uo is the rated rms phase to ground voltage, for cables rated between 5 kV and69 kV.The maintenance test level is about 75% of the acceptance test level. One can reduce the test voltageby another 20% if the voltage is applied for longer times (Bach [B2]; Baur,Mohaupt, and Schlick, [B6];Krefter [B27]).Evidence (Hernandez-Mejia, et al.[B21]) indicates that increasing the voltage above 3Uo tocompensate for reduced test cycles (time) does not replicate performance either on test or in service ascompared to the lower voltage, longer time tests.
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. [B9]; Krefter [B27j; Moh [B28]). For asinusoidal waveform the rms is 0.707 of the peak valuc, assuming the harmonic distortion is less than 5%.The rms and peak values of the cosine-rectangular waveform are assumed to be equal.