Cable testing is important to ensure the long life and high efficiency of cables used for high voltage transmission. There are several types of tests conducted on cable samples including mechanical, thermal, impulse withstand voltage, partial discharge, and dielectric power factor tests. Partial discharge tests are particularly important for determining the life of cable insulation by detecting internal discharges under voltage stress. These tests are conducted at various stages of voltage levels on cable samples to certify them for transmission use.

1. EE2353 - HIGH VOLTAGE ENGINEERING
Seminar by Ramesh Rajesh/III Year/EEE “B”
TESTING OF CABLES

2. 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

3. Classification of Cable Testing
Mechanical Tests – Bending, Dripping and Drainage,
Fire Resistance and Corrosion
Thermal Duty
Tests
Impulse
Withstand
Voltage Tests
Partial Discharge
Tests
Dielectric Power
Factor Tests
Power Frequency
Withstand
Voltage Tests
Life Expectancy
Tests

4. 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

5. IEC Voltage Step Ratings

6. 1. Conductor
2. Semiconducting Layer 1
3. Insulation
4. Semiconducting Layer 2
5. Outer Conductor and Outer Coating
Structure of HVDC Cable

7. 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.

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

9. 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

10. 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

11. 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 capacitor
2. Transient travelling wave from cavity from near end is detected
3. Far end wave superimposes over near end wave
4. Error factor is high although it can be corrected by carefully studying
the shape of waveform generated by discharge detector

12. 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 pulse
What is a travelling wave ?
 Time duration between each pulse, distance of fault
from either ends is found
 Pulses tend to be distorted upon amplification

13. Partial Discharge – Scanning Method
H
V
GROUND
Grounded
cable just
after
production
Discharge is detected by
the high energy field
produced between the HV
rings

14. 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 constant
n= Life index depending on material
*(1 – 1000 ) hours with high stress – determines
expected life at rated stress