Why Cable testing is needed
The power supply networks are growing continuously so the quality of power cables is an important factor. Cable testing must be done so that we could determine the condition of the power system, so by this, we could detect cable failures and we can improve the system reliability.
Cable components
Power cable testing
High voltage DC withstand test is the most common method of cable testing, this test can only detect defects which are associated with the conduction, so the defects were detected by the leakage currents
The most common fault in dielectric cables is due to ‘partial discharge’ to perform the partial discharge test the equipment used was very expensive so acoustic equipment are used to do the partial discharge test
Types of cables and their characteristics
Cables can be classified into low, medium and high voltage cables, so according to the cable types the testing and maintenance are done, three-conductor cable has been used in the lower voltage range. Nowadays we use XLPE insulated because of the quality of the insulation material and it is well designed
Due to these features, these cables can be used for extra-high voltage level
What are the procedures for cable fault location
These are the following steps to locate the cable faults
- Fault indication
- Disconnecting and Earthing
- Fault analyses and insulation test
- Cable fault Pre location
- Cable route tracing
- Precise cable fault location
- Cable identification
- Fault marking and repair
- Cable testing and diagnosis
Cable testing
Installation acceptance and maintenance test
- Installation test is a test which is done after cable installation but it is done before the application of joints or terminations
- Acceptance test is also a field test which is conducted after cable installation, which is inclusive of terminations and joints before the cable system is commissioned
- Maintenance test this test is done during the operation life of the cable system
Thermal imaging camera is used for maintenance testing and a Megger for DC high pot testing
Withstand test
This type of test involves the application of a voltage at a nominal level or higher than the prescribed period of time. By doing this test we could determine the cable’s ability to withstand voltage without insulation breakdown
Dielectric response
This kind of test is done to determine the cables dielectric response to the applied electric field, it can be done by measuring properties such as recovery voltage DC leakage current, dissipation factor
This can be done by applying AC or DAC voltage for a prescribed period at a frequency of 20 – 300 Hz
Advantages
- An overall assessment of the cable can be done
- Has the ability to detect the degree of water treeing and conductive type defects
Dielectric response – Dissipation factor
This could determine the loss factor of cables insulation, the loss factor will increase with age, so dissipation factor or Tan delta measurement can be used to determine the cable condition
Dielectric response – DC leakage current
During this test, a DC voltage lower than the withstand voltage is applied between the conductor and the insulation shield. So the leakage current through the cable insulation can be measured
While testing extruded cable system HVDC must not be used leakage current measurement will only take place when the applied voltage reached its steady-state value
Dielectric Response – Recovery voltage
DC voltage is used to charge the cable for a specified time in this test, after that the cable will be discharged to ground through a resistor for short time and then the open circuit voltage is recorded and plotted against time
It is used to determine the level of water tree degradation in extruded insulation
Partial discharge - Acoustic measurements
When a partial discharge occurs energy is released which produces a mechanical wave so the place in which partial discharge occurs acts like an acoustic wave source
Time-domain Reflectometry
This method uses changes in impedance of cable to determine cable imperfections such as faults, open connections, poor neutral, lossy connections, and water ingress