Faults in the cable insulation
Online PD testing is becoming the topic of interest for power cable owners to ensure the safe and continuous operation of power cables. Partial discharge is a localised discharge in the insulation medium due to high voltage stress that does not bridge the space between the two conductors. Partial discharge testing of power cables is one of the diagnostic methods to detect and locate incipient faults in the cable insulation.
If PD source is detected timely in the cable system, it could save the huge cost of unplanned outage to Plant owners.
Regular periodic or continuous monitoring of PDs inside cables and their accessories will help condition monitoring team to identify the location of incipient fault and then planning a scheduled outage of the suspected cable circuit in order to perform corrective measures.
Offline and Online PD
PD testing of power cables can be done offline and online. When doing offline testing, cable is disconnected at the terminations and then connected to separate power supply. There are different types of power supplies for this purpose i.e.
- Very Low Frequency (going as low as 0.1Hz to 0.01Hz),
- Resonant Voltage (frequency ranging from 20 Hz to 300Hz depending on the voltage) or
- Normal power frequency (50Hz or 60Hz) supply.
Online PD testing is done while the cable is in service under normal operating condition by installing sensors at cable termination.
These sensors include but not limited to High Frequency Current Transformer (HFCT), Transient Earth Voltage Sensor (TEV), High Voltage Coupling Capacitor or Acoustic Sensor. Some of these sensors can be installed while the system is online while others (/ or in some cases) require outage of cable system in order to install them.
Problem of high noise worsen the situation when PD signals are superimposed onto noise signals. Simple pulse separation and classification techniques are not effective in such situation. Signal needs to be analysed in frequency domain then.
Following are the guidelines when conducting Online Partial Discharge Testing of Power Cables:
Cable earth shield coming out of the cable (whether single core or three core) must not be touching any earthed metal except at terminating point to Substation Earth. Earth shield at cable termination should be insulated using shrink tubes in order to get access to individual cable earths.
This will help in easy installation and reliable measurement of PD in the cables.
At cable terminations where access to the cable end is not permitted due to Health and Safety Regulations, permanent sensors should be installed with termination box near cable end providing access to the sensors connected to individual cable earths.
Before doing online PD measurement, it is important to know the Cable Return Time. Cable Return Time is defined as the time taken by a pulse for completing a round trip along the circuit length. Cable Return Time improves the accuracy of PD source localization after detection.
Online PD measurement should be conducted for longer time length in order to improve the reliability of test. Although, online periodic measurement of PD in power cables will be able to capture continuous discharge sources and if lucky, some intermittent discharge sources.
It is better to perform PD measurement at both ends of the cable in order to confirm test results.
Most of the PD signals flowing along the length of cables have dominant frequency component of less than 5MHz due to low pass nature of power cable and propagation losses. It should be noted that filtering is not desirable in the range of 100 kHz to 5 MHz as applying such filtering may also filter PD signals.
Care must be taken when filtering is done within this range.
PD signals are phase consistent. However, it is also observed that noise can be phase consistent. This might effect PD analysis if analysis is only considering Phase Resolved patterns. There are numerous analysis tools available in the market for PD detection within high noise. One should not only rely on such sophisticated analysis tools.
Basic rules in identifying a PD signal in noisy data are:
- Pulse Shape: PD pulse travelling along the length of cable has bigger peak followed by few oscillations (in some cases).
- PD activity should be phase consistent.
- If such activity is found within the data, there should be reflected pulse(s) after main PD pulse. Such reflections will help in locating incipient defect site. However, it is also possible that there is no reflected pulses because of long cable lengths and attenuation.
When PD is detected inside power cables, it is suggested to locate the defect source by further investigations. Once PD source is confirmed, it is advised to plan a scheduled outage in order to perform corrective measures.
9. After corrective measures, it is also advised to perform online PD measurement regularly or continuously for some days or weeks in order to confirm the workmanship done during repair.
Few words more //
These are the general guidelines that can be followed to perform online PD measurements in power cables.
However, these guidelines do not favour or reject any technology, system or commercially available tools.
Online PD testing of power cables can be difficult or challenging but not impossible. By using proper procedure and correct methodology, reliable online PD measurements can be done even in the presence of noise. With the availability of huge research material, innovative analysis tools and sophisticated software, reliable online PD testing in power cables is possible as compared to the concept of PD testing 10 years before.