HT Cable Testing.pptx, Electrical testing of HT cables

The document talks about HT cable testing. High voltage test, partial discharge test and loss angle test

1. HT Cable Testing Dr. Pradip Chanda

2. Why do we need to test cables Cabling is an expensive business. The expenditure of substitution once the routes are all concealed is bigger. Cable testing is done to To ensure long life of the cables used as it isn’t easy to change them To ensure cable quality To ensure high strength of the material used Classification of cable testing mechanical tests like bending test, dripping and drainage test, and fire resistance and corrosion tests, thermal duty tests, dielectric power factor tests, power frequency withstand voltage tests, impulse withstand voltage tests, partial discharge tests, and life expectancy tests.

3. Preparation of Cable Samples for Testing Improper sampling of cables – leads to excessive leakage or end flashovers. Normal length – 50cm to 10m Ends are protected by electrical and mechanical stress shields * Shields avoid surface leakage currents from entering the circuits

4. Dielectric Power Factor Test Uses high voltage Shering-Bridge Power Factor or Dissipation Factor (tan ∂ ) measured at 0.5, 1.0, 1.66 and 2.0 times the rated voltage (phase to ground) Parameters considered : Maximum Value of Power Factor. Difference in Power Factor between rated voltage and 1.66 times the rated voltage. Difference in Power factor between rated voltage and two times the rated voltage.

5. High Voltage Tests Stage 1- HVAC Test Stage 3 -HV Impulse Test Stage 2 - HVDC Test This test is done for the entire length of the cable – to check the continuity in conduction

6. Partial Discharge Test – Discharge Measurement Why is it important? Life of insulation depends on the internal discharges due to a given magnitude of voltage stress Steps: Detector is connected through a coupling capacitor Transient travelling wave from cavity from near end is detected Far end wave superimposes over near end wave Error factor is high although it can be corrected by carefully studying the shape of waveform generated by discharge detector If the detector is connected through a coupling capacitor to one end of the cable, it will receive the, transient travelling wave directly from the cavity towards the nearer end, and after a short time, a second travelling wave pulse reflected from the far end is observed. Thus, the detected response is the combination of the above two transient pulses.

7. Location of Discharge The voltage dip caused by a discharge at a fault or a void is propagated as a travelling wave along the cable. This wave is detected as a voltage pulse across the terminals of the cable ends. By measuring the time duration between the pulses, the distance at which the discharge is taking place from the cable end can be determined. The shapes of the voltage pulses depend on the nature of the discharges.

8. Partial Discharge Test – Life Tests Life tests help to determine the expected life of a cable under normal stress conditions by subjecting it to short term, heavy stress – i.e. Less Time and High Voltage Relation Between Maximum Electrical Stress ( E m ) and Life of the Cable Insulation in Hours ( t ) : E m = Kt -(1/n) k=field condition and material constant n= Life index depending on material By conducting long duration life tests at increased stress (1 hr to about 1000 hr ) the expected life at the rated stress may be determined.

9. THANK YOU