ee2353 / high voltage engineering - testing of cables
TRANSCRIPT
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