• Tommy Warvik

Non-destructive diagnosis on critical high voltage cables


post@neka.no

A large portion of the transmission system today consists of aged cables and there is an increasing concern about the state of this cables. Many of them are way passed their life expectancy but this do not mean that they have to be replaced immediately. Even if the cable has reached it`s life expectancy it probably have many years left, with the right maintenance. The only way to find out which cables to prioritize, is to uncover the overall state of the cables.

Figure: Pry-cam.com


Neka performs a non-destructive diagnosis which will reveal whether the cable, parts of the cable, joints or terminations need to be replaced in order to extend the lifetime.


The diagnosis consists of three(four) different tests that will provide essential data regarding the state of the cable. This data combined, will give a good indication regarding the health of the system.


Figure: Pry-cam.com


The diagnosis consists of:

  1. Online PD survey

  2. VLF TD

  3. (VLF PD)

  4. LIRA


Online PD survey

Online PD measurement

The PD test is performed on the cable while it`s still in service. The test reveals whether there is insulation faults present that generate partial discharges at operating voltage and frequency.


If the cable is showing signs of critical PD activity at operating voltage, it is only a matter of time before a failure will occur.


The PD system collects all the necessary information to carry out a complete analysis of the PD phenomenon.



VLF TD

VLF TD at operating voltage

The VLF TD test require that the cable is taken out of service. A voltage sours(VLF 0,1 Hz) is connected to the cable phase and the voltage is raised in steps while the the relation between the effective and reactive power (tanδ) is measured. With this information we can evaluate the condition of the insulating system of the entire cable route.


IEEE 400.2-2013 recommend to test from 0.5 and 1.5 U0 but on fragile cables the voltage can be lowered to a maximum of 1*U0, which means no additional strain



Damoed AC PD diagnosis

VLF PD at operating voltage

Partial discharges (PD) are the main contributor to the insulation breakdown in cable systems. With DAC PD diagnosis, small weaknesses in cables, joints and terminations, that otherwise would pass a pressure-test, will be revealed.






- PD measurement according to; IEC 60270, IEC 60885-3, IEEE 400.3

- All types of cables from 6.3 to 72.5 kV

- With a one-sided measurement, a minimum of 10 km of cable can be tested. For extra long cables, double-sided measurement can be used

- PD mapping (Localization <1 meter)

- PDIV (Inception voltage)

- PDEV (Extinction voltage)

- PRPD (PD pattern)

- PD level (Amplitude / Repetition Rate)

- PD pattern and results comparable to normal 50 Hz


Reveals typical installation defects in joints and terminations such as:

Poorly prepared pex surface (knife cuts and impurities)

Poor semiconductor removal

Poor shrinkage

Misalignment of the stress control



Damoed AC TD diagnosis


DAC Tandelta detects dielectric losses that occur in insulation weaknesses. DAC TD is especially useful for detecting aging and degrading the insulation in paper cables.

- Dipole losses (General aging)

- Interface losses

- PD discharges (Partial discharges




LIRA



LIne Resonance Analysis

A LIRA test reveals all types of changes affecting the insulation capacitance.

The LIRA is perfect for revealing dimensional changes as cable joints, areas influenced by extensive heating, radiation, water intrusion, corrosion,

bends, scratches, mechanical impacts, fatigue and global or local insulation material degradation


Contact:


post@neka.no

+47 95232799

www.neka.no


Kontakt​
+47 95232799
post@neka.no
Harbitzalléen 2A
0275 OSLO
NORWAY
  • LinkedIn - Grey Circle

© 2020 NEKA

Images from Unsplash, PryCam, B2 electronic, Wirescan, onsite hv group

neka_ny-neg.png