Online PD Monitoring

Online Partial Discharge Monitoring On Electrical Systems

Partial discharge is an invisible threat lurking in your electrical systems. It can be life-threatening if undetected, causing unanticipated and costly loss of production, system failure, and, in some cases, significant destruction or shutdown of operations. Due to such severe risks, it becomes coherent that partial discharge cannot be left unchecked and needs to be addressed for the effective and safe functioning of your electrical systems.

Partial discharge occurs when the insulator or a specific spot within it is unable to withstand the electrical field. While everything may seem normal on the surface, partial discharge remains invisible, making fault detection a subtle and challenging process until the damage becomes evident.

Thanks to ever-evolving technologies, multiple partial discharge testing devices, online and offline, are available today to help you identify the weak spots in your electrical systems. This blog will explore the fundamental principles of partial discharge, the benefits of online monitoring, and how Online Partial Discharge Testing is revolutionizing maintenance strategies for electrical systems.

Understanding Partial Discharge

Partial discharge, a term that may sound innocuous, refers to minor electrical discharges between the electrodes inside the insulation in electrical assets. This happens when there is unbalanced electrical stress around the insulation, causing the gases to ionize 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 key to preventing catastrophic failures.

As technology advances, many techniques have been invented to help detect such insulation defects and mitigate possible damage. Online PD testing and Offline PD testing are the two popular detection methods that can be used to detect partial discharge at an early stage. Online PD testing is done when the device is operating, offering real-time insights, while offline PD testing occurs when the equipment is switched off and energized with a test voltage source. Let’s delve into the possibilities of online partial discharge testing in detail.

Online Partial Discharge Testing Methodology

Online partial discharge testing is a nonintrusive testing approach conducted while the device is in operation. In this method, noninvasive sensors are installed in the device, which helps you continuously monitor the insulation under minimal additional electrical stress on the equipment. Modern software systems are used to analyze the signal patterns and thus detect defects, if any. Online partial discharge testing continuously monitors power cables, transformers, and other high-voltage equipment.

Online Partial Discharge Monitoring of Power Cables

Partial discharge happens when there is an insulation defect in power cables. Online Partial Discharge (PD) monitoring of power cables is a practical way to ensure the reliability of electrical power systems. Using this method, you can identify insulation breakdowns in joints and terminations, which, if not repaired immediately, may lead to prolonged power outages. Partial discharge in power cables can happen if there are weak spots in cable insulations, such as particles, voids, or cracks, or due to damage to cables or cable accessories.

By installing suitable sensors at terminations and joints of the cable system, you can monitor partial discharge either temporarily or continuously. Additionally, Implementing Robust noise suppression and partial discharge source separation algorithms will potentially localize dangerous partial discharge in the cable system.

Partial Discharge in Switchgears

Switchgear may encounter partial discharge when the electric field goes beyond the dielectric strength of the insulation, which is generally made of powder-coated epoxy or fiberglass. It may not result in a complete breakdown, but if the partial discharge goes undetected for a prolonged period, it might lead to equipment failure. Partial discharge in switchgear might be due to natural temperature fluctuations or humidity.

 The switchgear might not look damaged on the outside, but it might be compromised underneath. Partial discharges can occur in various forms, such as corona, surface, or internal discharges. If you get close enough to the damaged equipment during a partial discharge event (a.k.a. an arc), you might smell ozone or hear low crackling noises.

Partial discharge testing and switchgear monitoring can be done through ultrasound, transient earth voltage (TEV), and electromagnetic interference (EMI). These techniques will help detect and eliminate partial discharge activities before they lead to failure, thus extending the lifespan and functionality of your electrical systems.

The Role of Online Partial Discharge Testing

Online partial discharge (OPD) testing allows for the detection and analysis of partial discharge events without obstructing the normal functioning of the equipment. OPD testing has multiple advantages, and let us look at some of them below:

  • Minimal Downtime: Since the equipment’s operation is uninterrupted during testing, there are minimal to no downtimes or disruptions. 
  • Continuous Monitoring: Real-time data collection will facilitate continuous insulation condition assessment.
  • Cost-Effective: Online partial discharge testing will identify risk factors early. This will eliminate the need for total equipment replacement and extend its lifespan, eventually leading to cost savings.
  • Enhanced Safety: Promptly identifying partial discharges in equipment will improve the safety of the entire electric system. 
  • Comprehensive Analysis: OPD testing will deliver a precise diagnosis of the defect and the type, severity, and location of the partial discharge, facilitating a comprehensive analysis and mitigation. 

Devices for Online Partial Discharge Detection

Several technologies and devices are available today to detect partial discharge. Let us look at some of the devices used to detect PD at an earlier stage to avoid further damage. 

Ultrasonic Detectors: Ultrasonic emissions often occur during a partial discharge event, and specialized devices like Ultrasonic Detectors can effectively locate these emissions. PD Detector Pro by IPEC is designed to detect the sources of PD within enclosed spaces, aiding in finding and maintaining electrical systems to prevent potential failures or hazards.

Ultrasonic detectors capture ultrasonic waves using sensitive microphones or piezoelectric sensors. The captured ultrasonic signals are analyzed to determine the presence, intensity, and location of a partial discharge. These detectors can identify ultrasonic emissions even when the device is in operation and differentiate between different types of discharges and background noise. They also offer continuous or periodic monitoring, providing real-time data on the insulation condition.

Transient Earth Voltage (TEV) Sensors: TEV sensors detect and measure electromagnetic pulses emitted during a partial discharge event in metal-clad switchgear. It can detect electromagnetic pulses while the device is in operation. In combination with ultrasonic PD detectors, TEV sensors can provide comprehensive analyses. Devices like TEV antennae are high-performing PD sensors available on the market. 

High-frequency current Transformers (HFCT):  HFCT sensors can detect high-frequency electric current pulses emitted in electrical cables and accessories during a partial discharge activity. These sensors will be clamped around the cables without disrupting the power supply. PD detector Pro by IPEC is an efficient HFCT sensor and helps in the early detection of abnormalities. 

Coupling Capacitors: Coupling capacitors detect partial discharge in rotating devices. Each sensor in a coupling capacitor is a high-voltage capacitor with an integrated quadrupole. The sensor is connected to the rotating device via cable lugs, and a measurement device picks up the partial discharge signals while the device is in operation.

Acoustic Emission Sensors: Sound will be emitted during a partial discharge event. This sound, audible or not, is produced when the current streamer is formed and the material surrounding the streamer is vaporized. This vaporization rapidly releases mechanical energy, which propagates as a pressure field. Acoustic detection systems can be categorized into external and internal systems. External acoustic detection systems employ sensors mounted on the equipment’s outer side to detect the acoustic partial discharge signal. On the other hand, internal systems use sensors inside the equipment to measure the pressure wave directly.

Optical PD Detection: Optical sensors detect the light emitted during a partial discharge event. This device is primarily beneficial for detecting corona discharge during PD. The light is produced due to various ionization, excitation, and recombination processes during the discharge. However, the amount of the emitted light and its wavelength depends on the insulation medium and several other factors such as temperature, pressure, etc. 

Digital PD Monitoring Systems: These integrated systems use multiple sensors and advanced signal-processing algorithms to provide comprehensive PD monitoring and diagnostics.

Online Partial Discharge Testing by Ocean

Online 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. Their detection system has advanced technologies to detect even minute 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 help you save the day with their reliable services and advanced devices. 

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