Electrical properties of insulating materials under VLF voltage

Cigre 751 talks about electrical properties of insulating materials under applied VLF voltage. In this article, this article shows the findings and future course of action proposed b Cigre:

Summary of findings

The following statements summarize the findings:

Advantages of applying VLF testing:

? Low reactive power consumption compared to power frequency. This is especially advantageous when testing high capacitive test objects, i.e. long distance cable line, generator rods, etc.

? In insulating materials, with low electrical conductivity, the electric field distribution during VLF voltage testing is similar to that at power frequency.

? Creates less or no space charge compared to DC voltage testing. Thus, reducing the risk of space charge-induced breakdown during rapid voltage grounding and/or application of the AC service stress.

? Creates less partial discharge degradation compared to testing at power frequency.

? Has a higher sensitivity to changes in resulting dielectric loss compared to measurements at power frequency.

? In case of water tree-degraded cables, the breakdown voltage at VLF testing is typically up to 2 times higher than that at power frequency.

Disadvantages of applying VLF:

? Depending upon the type of inclusions and impurities, the breakdown voltage at VLF testing can be 2-4 times higher than that at power frequency.

? Special attention need to be addressed to the field grading materials of cable accessories because the electric field distribution may become frequency-dependent, resulting in different internal electric field distributions at VLF and power frequency.

Gaps in knowledge:

? There is a limited number of experiences reported on the impact of VLF testing on the insulation properties of insulation systems based upon SIR and EPR/EPDM materials.

? In general, few reports address the VLF properties of unaged insulation systems.

? Partial discharge (PD) measurements at high voltage VLF testing is not yet fully exploited, mainly because of challenges regarding noise interference, interpretation and comparison of results from PD measurements at VLF and power frequency.

Conclusions

This report discusses results from studies regarding the impact of VLF testing on insulation systems with respect to theory and known practical experience from application of VLF test methods. The presented challenges regarding space charge formation and mechanisms of degradation, providing a base for evaluation of possibilities and limitations regarding application of as high voltage acceptance/after laying tests and diagnostic tools.

? In case of VLF (0.1 Hz) testing of high voltage insulation systems, the risk of space charge

accumulation is very low, and the internal electric field distribution becomes similar to that at power frequency, provided the electrical conductivity of all the insulating materials is kept below approximately

1·10-13 Ω-1m-1

? During VLF testing of installed cable systems, it is important to address the electric field distribution within the accessories, as it may become frequency- and voltage dependent due to high and non-linear conductivity of field grading materials.

? Gradual degradation caused by partial discharge activity is reduced at VLF voltages compared to that at power frequency, mainly due to reduced number of discharges per time unit and reduced voltage across voids and electric tree channels. - It is also expected low rate of water tree degradation at low frequencies.

? Measurement of the low frequency dielectric loss factor becomes is a useful diagnostic indicator, as combined changes of permittivity and conductivity are easier revealed at low frequency than at power frequencies.

? In case of VLF breakdown testing of XLPE cables, experience show that breakdown occur at voltage levels in a broad range, up 4 times that at power frequency. It is particularly difficult to detect voids during VLF withstand testing. In case of water tree-degraded cables, the breakdown voltage at VLF voltage is typically about 2 times higher than at power frequencies.

? The knowledge regarding impacts of VLF testing on unaged insulation systems and SIR and EPR/EPDM based insulating materials is limited.

? More work is needed to improve methodology of interpretation and comparison of results from partial discharge measurements at VLF and power frequencies.