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Partial Discharge (PD) Testing for Electrical Cables and Switch Gear Panels
What is Partial Discharge?
Partial discharge (PD) is an electrical discharge between the electrodes inside the insulation of electrical assets. This happens when there is unbalanced electrical stress around the insulation, causing the gases to ionise and the voltage to drop. Understanding the causes of these discharges, which occur most frequently in equipment like turbine generators, cables, transformers, and switchgear, is critical to preventing catastrophic failures. PD is one of the vital parameters indicating the condition and soundness of an electrical insulation system, and its presence or absence must be detected for the sound health and functionality of electrical equipment.
Leading Partial Discharge Testing Services
Types of Partial Discharge
The effects of partial discharge breakdown of insulation include rays, warmth, smell, sound, waves, and high-frequency electric current. It can range through all types of insulation, including air, oil, solid, and gas-insulated switch gear, and at all voltage levels from 3kV to 769kV and beyond.
- Internal Discharge: Occurs within the voids, cavities, or inclusions inside the insulation material. Such voids can be post-manufacturing defects caused by the aging process or the mechanical strain to which these parts are subjected.
- Surface Discharge: This happens along the surface of the insulation, often due to contamination or deterioration. These discharges occur when voltages at the surface of insulations are more than their capability to break down. Small discharges occurring on the surface of the insulation may throw a carbon track along the insulation surface, causing continued insulation damage and possible electrical breakdowns.
- Corona Discharge: Corona discharge refers to the ionisation of the air surrounding a conductor and is commonly observed in the gaseous medium under high voltage conditions. It is a chemiluminescent compound, bluish-purple in colour, which forms ozone and highly reactive nitric oxide. Corona discharge is mainly experienced on sharp ends or tips of conductors where the electric field converges.
- Treeing: The progressive degradation of solid insulation forms a branched, tree-like pattern. It commences with a small void, inclusion, or imperfection and propagates through the insulation material under electrical stress. Treeing can occur in two forms: electrical treeing and water treeing.
- Void Discharge: Specific internal discharge within gaseous voids in solid or liquid insulation. These voids are effective at concentrating the electric field to produce partial discharges once the strength of the electric field in the cavities outweighs the dielectric strength of the gas contained therein. Void discharges can be rather devastating, as each discharge event results in localised overheating of insulation and the initiation of various chemical reactions that further erode the insulation layer. Preventing and minimising void discharges is essential to protect electrical insulation systems.
Benefits of Partial Discharge Testing
- Early Fault Detection: Identifies insulation faults before they develop into catastrophic failures. Through early fault detections, prompt repairs are possible, which adds to the overall life of the equipment.
- Predictive Maintenance: Facilitates condition-based maintenance, enhancing reliability and reducing unexpected and costly downtime.
- Extended Equipment Life: Helps maintain and prolong electrical equipment’s lifespan.
- Safety Improvement: Reduces the risk of electrical failures that can lead to safety hazards.
- Cost Savings: Minimizes repair costs and unplanned outages through early detection of potential issues.
Advancements in Non-Invasive Techniques
Recent developments in non-invasive methods for measuring partial discharge (PD) have enhanced the management and protection of electric equipment.
In the ultrasonic detection method, ultrasonic sensors detect the sound waves of discharges produced at high frequencies, making it an effective system to detect PD. Electromagnetic detection utilises antennas and sensors to pick signals of PD from electromagnetic emissions, hence improving the precision of deadly discharge locations.
Infrared thermography is another method that uses thermal imaging cameras to record temperature fluctuations due to PD activity and visually represent potential issues. Detecting PDs using Ultra-High-Frequency (UHF) sensors and optical fibre sensing is also used today. Altogether, these advancements help to improve the reliability and longevity of the electrical system and play a key role in making the detection of PD more accurate and efficient.
Cable testing and partial discharge (PD) testing are closely related, as both are essential techniques for assessing the condition and reliability of electrical insulation in cables, with PD testing specifically identifying early signs of insulation breakdown that can lead to cable failure
Effective Partial Discharge (PD) Testing Methods
Understanding the different methods involved in partial discharge (PD) testing is crucial. This helps ensure the electrical equipment’s reliability and lifespan. PD testing also usedThese methods can be classified into offline and online testing, each with distinct advantages and applications.
Offline Testing
Oscillating Wave Test (OWT): In this method, oscillating voltage waves are used to assess insulation. It helps identify areas of poor insulation when the equipment is not in use.
Very Low Frequency (VLF) Testing: Here, VLF sinusoidal waveforms are used to apply stress to the insulation of cables. This method is most appropriate and adequate for checking the insulation quality of high-capacitance apparatus such as power cables and transformers.
Online Testing
Acoustic Emission Testing: This method detects PD by analysing electric discharge noise waves. The emissions can be recorded through the acoustic sensors installed on the equipment and are non-invasive.
TEV (Transient Earth Voltage) Testing: TEV defines the transient voltages attributed to PD activity. This method entails using sensors on the outer layers of the equipment and does not require extensive and invasive adjustment, thus making it efficient for monitoring PD.
High-Frequency Current Transformer (HFCT) Testing: measures the high-frequency current involved in the propagation of PD. The HFCT sensors are installed around the conductors to intercept the signals inherent to the PD, which enables achieving relevant information regarding the location and intensity of the discharges.
Partial Discharge (PD) Testing from Ocean
Partial discharge testing is a nonintrusive detection method used to measure the PD when the equipment operates. When undetected for a prolonged period, partial discharge can cause total system failure. Ocean is the leading engineering company in Oman and an industry leader in providing online partial discharge monitoring services. Our detection system has advanced technologies to detect the slightest electrical insulation deterioration.
Ocean provides precise and sophisticated testing devices for various industries to prevent unplanned outages. Their PD testing systems are compatible with Medium volt (MV), High volt (HV), and Extra high volt (EHV) assets operating above 3 kV. This includes switchgear, transformers, cable terminations, and land cables. Whether at the project’s inception or on the verge of a system failure, Ocean will be your trusted partner in safeguarding your operations with our advanced detection systems and reliable services, ensuring optimal performance and efficiency.
