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Online Partial Discharge Measurements in Underground Power Cable

posted by Rugged Monitoring on 2020-02-28

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.

power cable

Basic Structure of Power Cable

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.

power cable insulation

Electrical Representation of Power Cable Insulation in Absence of Defect

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. 

power cable insulation

Electrical Representation of Power Cable Insulation in Presence of Defect

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.

partial discharge phenomenon

Real Reason Behind Partial Discharge Phenomenon

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

info   Do you know Rugged Monitoring Hsens-H (HFCT Sensor) and Hsens-T (TEV sensor) are equipped with transient overvoltage protection within sensors to minimize transients that can damage the electronics connected to them.

hsenh and hsenst applications

Example Application of Hsens-H (one phase is shown) and Hsens-T

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. 

info    Do you know Rugged Monitoring HPM601 Partial Discharge Monitor is equipped with real-time band pass filters. These filters are software configurable with different options of bandwidths and cut-off frequencies. Apart from Band-Pass filters, HPM601 Monitor is equipped with Wavelet as well as Moving Average filters that are performed in real-time on acquired data. These collection of multiple options of filters make on-site experience like a breeze.

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. 

corona discharge graph

An Example of PRPD Graph of Corona Discharge

surface discharge graph

An Example of PRPD Graph of Surface Discharge

Internal discharge graph

An Example of PRPD Graph of Internal Discharge

info   Do you know HPM601 transfers recorded data using its gigabit ethernet with transfer speed of 10,000 waveforms in high resolution mode or 100,000 peak values of waveforms in low resolution mode. This enables the user to visualize good quality PRPD graphs and help in identifying defect type.

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. 

partial discharge monitor - hpm601

HPM601 Rugged Partial Discharge Monitor 

ifo    Do you know HPM601 PD Monitor from Rugged Monitor is equipped with up to 1 TB of industrial SSD drive. This enable the user of system to leave monitor on-site without laptop. Monitor will keep on recording data for 24 or 48 hours continuously and will keep record of data into its internal memory. That recorded data can be downloaded to laptop later. This will ensure to record any PD activity which might be present during soak test. 

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