PD 101 Series – #07: PD Measurement on Cable Joint and Termination

16/07/2018
16072

There are three main failure mechanisms that exist in XLPE cables: Cavities, water/electrical trees and protrusions.

In case of a cavity, the lower permittivity within the gas-filled cavity leads to a higher electric field strength in this specific area within the insulation. Once the local electric field strength exceeds the dielectric strength of the insulation, the insulation is overstressed, leading to partial discharge.

Water can enter into the insulation system once the increased water protection is damaged. Due to the water vapor pressure, water can diffuse into the XLPE. A water tree will start growing towards the core conductor as the water molecules are polarized. It will grow in the direction of the electrical field. The field increases due to the higher electrical conductance and the decreased insulation distance. A water tree cannot be detected via a PD measurement, however it can lead to an electrical tree once the electric field strength is too high.

A defect in the inner or outer semi conductive layers, like a tip, leads to an inhomogeneous electrical field. The elevated field strength, once exceeding the dielectric strength of the insulation, will cause partial discharge and the evolution of an electrical tree with this protrusion.

PD measurement on power cable can be performed in 3 different ways which are using coupling capacitor, High Frequency Current Transformer (HFCT) on grounding or sheath cable and UHF sensor on cable end termination.

Use of coupling capacitor is mainly done with power source during factory acceptance test (FAT) or site acceptance test (SAT) as shown in Figure 1

Basic connection diagram of a coupling capacitor at a cable end termination

For on-line PD measurement on cable, HFCT is normally used on sheath cable on joint and grounding point on the end termination. The signal propagation can be very complex; therefore it is beneficial to have a straight through connection. The HFCTs should be connected around the cross-bonding links as shown in Figure 2.

 

 

 

 

 

 

 

 

 

 

The UCS1 is a UHF PD sensor designed to measure and monitor cable end terminations. The directional sensor detects PD signals as transient voltage drops across the insulation of HV cable terminations and can be used with or without parallel installed grounding connections. In order to achieve the best response characteristic, the UCS1 should be installed as close as possible to the termination, using short and low inductive braids as shown in Figure 3.

The UHF approach ensures a very sensitive (local) PD measurement in environments with high disturbance levels

For technical enquiry, please contact Seokhoon Hong (Regional Application Specialist – Partial Discharge) via email seokhoon.hong@omicronenergy.com or contact ENTEC A&T’s Email: support@automationandtesting.vn

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