IEEE 400-2-2013 pdf free downlaod – IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF).
5. Very low frequency (VLF) testing VLF testing methods utilize AC signals in the frequency range from 0.01Hz to 1 Hz. VLF testing methods can be categorized as withstand or diagnostic (Gnerlich [B8]). In withstand testing, the test object must withstand a specified voltage applied across the insulation for a specified period of time without breakdown of the insulation. The magnitude of the withstand voltage is usually greater than that of the applied voltage. If the cable insulation is sufficiently degraded a breakdown can occur. The cable system may be repaired and the insulation retested until it passes the withstand test. Diagnostic testing allows the determination of the relative amount of degradation of a cable system section and establishes, by comparison with figures of merit or accumulated data, whether a cable system section is likely to continue to perform properly in service. It should be noted that values of the diagnostic quantity measurements obtained during VLF tests may not correlate with those values obtained during power frequency tests, for example, the dissipation factor is much larger at 0.1 Hz than at power frequency and partial discharge (PD) may differ in terms of magnitude and inception voltage. Diagnostic tests may be nondestructive and are usually performed at lower voltages than withstand tests. However, when the cable system insulation is in an advanced condition of degradation, the diagnostic tests can cause breakdown before the test can be terminated.
The VLF withstand test methods of cable systems are:
⎯ VLF testing with cosine-rectangular waveform
⎯ VLF testing with sinusoidal waveform
⎯ VLF testing with bipolar rectangular waveform
⎯ VLF testing with alternating regulated positive and negative DC step voltages
Examples of the various waveforms are shown in Annex A.
The VLF diagnostic test methods of cable systems are:
⎯ VLF dissipation factor (tan delta) measurement (VLF-DF)
⎯ VLF differential dissipation factor measurement (VLF-DTD)
⎯ VLF dielectric spectroscopy (VLF-DS)
⎯ VLF loss current harmonics (VLF-LCH)
⎯ VLF leakage current (VLF-LC)
⎯ VLF partial discharge (PD) measurement (VLF-PD)
Field testing techniques frequently employ a combination of diagnostic and withstand test methods. Test methods should be selected based on their ease of operation, operator training requirements, cost/benefit ratio, and the cable system age and condition.
1— Connect HV power supply and apply voltage of specified magnitude for specified time. Straightforward pass/fail criterion.
2— Connect HV power supply and ancillary equipment, if required, to make diagnostic measurements. Care is needed to eliminate unwanted interference or stray signals. The density and severity of the defects in a cable system influence the effectiveness of any diagnostic method including the VLF test methods (Bach [B1] and [B2]; Baur, Mohaupt, and Schlick, [B5]). As a general rule, the more severe the defects, the lower the AC dielectric strength, and the larger the loss and harmonic loss currents. Severe defects are, for example, large water trees, large contaminants, large voids, or sharp protrusions that can initiate partial discharges and/or electrical trees at test voltages. Less severe defects are small water trees, small contaminants or voids, and less sharp protrusions. These cable conditions can be grouped as shown in Table 2.IEEE 400-2 pdf download.