EE2353 - HIGH VOLTAGE ENGINEERING

Seminar by Ramesh Rajesh/III Year/EEE “B”

TESTING OF CABLES

Why do we need to test cables ?

Important for transmission of electrical energy by both underground and over-ground means

To ensure long life of the cables used as it isn’t easy to change them

Transmission of high voltages is necessary in today’s world

To ensure high efficiency and strength of the material used

Classification of Cable Testing

Mechanical Tests – Bending, Dripping and Drainage, Fire Resistance and Corrosion

Thermal Duty Tests

Impulse Withstand

Voltage TestsPartial Discharge

Tests

Dielectric Power Factor Tests

Power Frequency Withstand

Voltage Tests

Life Expectancy Tests

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

IEC Voltage Step Ratings

1. Conductor2. Semiconducting Layer 13. Insulation4. Semiconducting Layer 25. Outer Conductor and Outer Coating

Structure of HVDC Cable

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:1. Maximum Value of Power Factor.2. Difference in Power Factor between

rated voltage and 1.66 times the rated voltage.

3. Difference in Power factor between rated voltage and two times the rated voltage.

Shering Bridge

Note: Protection from over-voltages must be given in case of breakdown in cables.

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

Start• Cable is prepared

HVAC

• AC Voltage – 2.5 times rated voltage for 10 minutes ; No insulation damage must occur

HVDC

• DC Voltage – 1.8 times rated DC Voltage of -ve polarity for 30 minutes

HV Impulse• Impulse Voltage – Prescribed magnitude – 5

times

Stop

• Cable Ready ; Power Frequency Dielectric Power Factor test done to ensure zero error

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:1. Detector is connected through a coupling capacitor2. Transient travelling wave from cavity from near end is detected3. Far end wave superimposes over near end wave4. Error factor is high although it can be corrected by carefully studying

the shape of waveform generated by discharge detector

Partial Discharge Test – Locating Discharges

A dip in voltage value at a fault or a cavity is propagated as a travelling wave

Detected as a pulseWhat is a travelling wave ? Time duration between each pulse, distance of fault

from either ends is found Pulses tend to be distorted upon amplification

Partial Discharge – Scanning Method

HV

GROUND

Grounded cable just

after production

Discharge is detected by the high energy field

produced between the HV rings

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 ( Em ) and Life of the Cable Insulation in Hours ( t ) :

Em = Kt-(1/n)

k=field condition and material constantn= Life index depending on material

*(1 – 1000 ) hours with high stress – determines expected life at rated stress