Frequently Asked Questions (FAQs)

VLF Section                                      Tan Delta Section                                    Partial Discharge Section

VLF Section                         (Top)

Is VLF damaging to my cables ?

There is no evidence that we are aware of that indicates that VLF can damage healthy cable or reduce the life expectancy of the asset. This is not the case with DC where there is substantial evidence that the use of DC can damage cable that could otherwise be returned to service.

Overvoltage tests are designed (with proper choice of test voltage and test time) to accelerate the damage in already damaged cable such that the cable faults during the test. This facilitates the location of the damage which is required in order to replace the damaged section.

This method avoids the spectacular and sometimes collateral damage associated with cable failure under system conditions whilst removing the damaged cables from the network.

What about experiments that "prove" the destructive nature of VLF ?

There is no need to prove this - it is an accepted fact that any over-voltage test (DC, VLF or system frequencies) can potentially case the cable to fail. This is actually the desired outcome of such a test.

If any test were destructive for the healthy portions of the cable then this would be problematic and we would see the following : (1) Increased failure rates of cables that have been tested (2) Cable manufacturers would void warranties on cables if such tests were used. Neither is true, in fact the data indicates that the opposite is true and many cable manufacturers recommend the use of VLF.

We would urge our customers to assess the source of the "proof" and compare that to the sources of evidence to the contrary.

Can VLF testing be compared with system frequency (50/60 Hz) testing ?

There is a lot of evidence to indicate that the behaviour of cables (breakdown voltages, growth rates of electrical trees etc) is similar (although not identical) at VLF to system frequencies.

Tan Delta Section                (Top)

Can Tan Delta detect water trees ?

Yes the measured Tan Delta values have been shown to be sensitive to moisture ingress in cables.

What about Hybrid (mixed XLPE and paper-oil/EPR) ?

The measured value of Tan Delta is not the only indicator of poor condition of the dielectric. This is of course difficult to predict with a hybrid cable. However the consistency of repeated TD readings (defined as the standard deviation) is similarly an indication of deteriorated dielectric condition and the voltage dependence of the TD readings are similarly valid.

Partial Discharge Section    (Top)

Can Partial Discharge equipment detect water trees ?

No - there is no electrical activity associated with water ingress. By the time that the water tree has converted to an electrical tree and is supporting partial discharges, the damage is normally so severe as to quickly cause the cable to fail.

What length of cables can be tested with Partial Discharge systems ?

It is important to differentiate here between (1) the length of cable that can be energised (2) the length over which PD can be detected (3) the length over which PD can be located.

(1) The length of cable that can be energised depends upon the energising technology. In our opinion it is irrelevant what the length of cable is that can be energised as the limiting factor is the detection and indeed the location. If you cant detect or locate the PD then it is not important how long a length  of cable you are enegrgising.

What is also important in how the inception and extinguishing voltages can be compared with their values at 50 Hz. To energize longer lengths of cable, non-sinusoidal (cosine and Oscillating Wave) generators are used or lower-frequency sinusoidal VLF signals may be used. All of these may cause Partial Discharges that are not reflective of the 50 Hz PD and this is something that should be considered.

Typically we believe that 5000-7000 m is a reasonable length of cable to energise in order to be able to detect and locate PD on a cable. This value can be severely impacted by the issues described in (2) and (3) below.

(2) The detection of PD pulses on a cable depends upon (a) The magnitude of the PD pulse (b) the position of the PD pulse (c) the ambient noise. The higher the magnitude of the PD pulse the more likely to detect it, the closer to the detector, the more likely to detect the PD and the lower the ambient noise the more likely that you are to detect the PD. Remember that PD is a stochastic event (non-determinisic) so detection can be a 

(3) PD mapping relies on the detection of an incident PD pulse plus the detection of a reflected pulse and in some systems it requires an additional reflected pulse. The same limitations described in (2) above apply except that they apply to the reflected pulses as well. This makes location a more difficult problem to solve than pure detection.