Guidelines To Perform Online Partial Discharge Measurements In Underground Power Cable

By Author: Rugged Monitoring
Total Articles: 20
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What is Power Cable?
A power cable consists of different layers of insulation sandwiched between High Voltage conductor and metallic sheath which as ground of power cable. The purpose of these insulation screens is to control electric field stress due to High Voltage applied on conductor. Any defect to these layers will decrease dielectric strength of these layers and can result in insulation failure of power cable. Due to reduced dielectric strength partial discharges can happen in insulation that can keep on continue before actual insulation breakdown.
Why there are Partial Discharges in power cables?
A partial discharge is a localized electrical discharge that only partially bridges the insulation between conductors, and which can or cannot occur adjacent to a conductor
Consider below cross section of power cable. In absence of any defect it electrical representation is equivalent to an ideal capacitor between High Voltage conductor and metallic sheath.
However, in presence of a defect, e.g., a cavity in insulation, the equivalent electrical representation of power cable will change. Due to series ...
... capacitance of cavity, overall capacitance and hence dielectric strength of power cable insulation will decrease.
Due to decreased dielectric strength, when induced voltage on cavity surpasses its dielectric strength, it collapses resulting release of different forms of energies. This release of energy is equivalent to Partial Discharge phenomenon. One of the energies is release of High Frequency Pulses which travel along the length of power cables on both conductor and metallic sheath. Both fields on conductor and metallic sheath are equal in magnitude and opposite in polarity. The High Frequency pulses can be detected by using High Frequency Current Transformers that can be clamped around cable metallic sheath, mostly at the ends/terminations of power cables.
Guidelines to perform Online Partial Discharge Measurement?
Online Partial Discharge measurement refers to the procedure which is used to perform partial discharge monitoring in power cables while the cable is energized under normal operating conditions. For the purpose of test, a temporary de-energization might be required where safe access to power cable metallic sheath is not available. Below are some general guidelines to perform online partial discharge testing.
1. Installation of High Frequency Current Transformer sensors
High Frequency Current Transformer (HFCT) are clamp on inductive sensors that can be clamped around power cable metallic sheath that is connected to substation earth. While installing HFCT sensors, the polarity of these sensors should be pointing towards earth direction. This will later help in analysing partial discharge signals and their direction. A positive pulse travelling in the direction of polarity of High Frequency Current Transformer will give output as positive pulse and vice versa.
It is very important that High Frequency Current Transformer sensor should be clamped around metallic sheath which is coming out of power cable, passing though HFCT and then connected to earth. It is very important for online partial discharge measurement as mistakes do happen. Such mistakes should be avoided.
Simultaneous sensors measurement is done with help of 3x HFCT on each cable ground, it helps to identify the source of partial discharge, whether PD is coming from outside of power cable or from one of phases of power cables.
A Transient Earth Voltage sensor can be connected to inside or outside of medium voltage air insulated metal clad switchgear to detect local termination defects. Application of HFCT together with TEV can help identify internal versus local PD at termination
2. Solving the challenge of High noise during online partial discharge measurement
It is very well known that Online partial discharge measurements are challenging as compared to Offline partial discharge measurements due to presence of huge noise. Noise can be present in power supply which is powering up equipment. It can be power cable system noise or can be coming from environment. Noise can vary from site to site.
A PD measurement equipment must be equipped with different sets of filters as two different sites are never same. There are different techniques available in commercial systems that can help in eliminating or minimizing noise on site.
3. Select the right partial discharge monitor to analyse multiple signals
When doing online partial discharge measurements, there can be multiple signals; multiple PD sources or multiple noise signals. Most of these noise signals can be eliminated if a PD monitor is equipped with string denoising features like HPM601 PD Monitor from Rugged Monitoring.
Phase Resolved Partial Discharge Graphs also known as PRPD graphs are one the best tools to identify type of defect. Different types of defects have different patterns of PRPD. Hence a knowledge of these patterns will help in better analyzing PD activity.
4. Make sure to perform partial discharge measurement during soak test
It is common procedure to perform a 24-hour or 48-hour soak test on new installation of power cable or a cable with new/replaced termination or joint. During 24 hours, cable is energized but not loaded.
In most of these soak tests by cable installation companies or cable owners, a major element is missing, which is PD measurement during initial 24 hours or 48 hours of soak test. Most of these soak tests on power cables are performed without PD measurement. It is recommended to perform partial discharge testing during complete soak test to ensure that power cable and its accessories are partial discharge free.
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