Acoustic imaging for pre-locating partial discharge
Introduction
Partial discharge (PD) is a phenomenon that occurs in many types of electrical insulation systems when localised electrical stress exceeds the dielectric strength of the insulating material. Essentially, it takes the form of small electrical discharges that, if left unchecked, gradually erode the integrity of the insulation. This deterioration can lead to reduced asset lifespan and potentially catastrophic failures, making early detection and location of PD essential to minimise these risks and ensure the reliability of critical electrical assets.
PD produces several distinct types of emissions among which radio frequency (RF) radiation and acoustic waves can be used to identify and locate PD sources. If acoustic waves are sufficiently powerful acoustic imaging, which relies on the acoustic noise associated with PD, offers an attractive alternative for pre-locating the PD source safely and efficiently.
Challenges in locating partial discharge sources
Locating the precise source of PD is a challenging endeavour using traditional spot testing methods due to the complexity of the instruments used and the technician required ability and training. These methods often require technicians to physically scan equipment surfaces or make contact measurements, which is labour-intensive and potentially hazardous due to the high-voltage environments or confined spaces.
Acoustic imaging for PD pre-location
Acoustic imaging technology offers an attractive, initial solution to the challenges of PD pre-location. By using an array of microphones, an acoustic imager picks up the audible and ultrasonic noise generated by PD. The signals from the microphones are then processed to minimise the effect of stray noise and determine the direction and approximate intensity of the PD. Using a technique known as “beam-forming”, this information is used to produce an image where the PD is represented by a coloured "cloud map" overlaid on a standard digital image of the equipment being investigated. Modern acoustic imaging solutions provide all of this functionality in a compact, self-contained handheld instrument.
One of the most significant advantages of acoustic imaging is its ability to detect PD from a considerable distance, up to 120 meters, with Megger’s MPAC cameras. This capability makes it easy to survey large sections of plant quickly when looking for PD, eliminating the need to approach energised equipment closely or struggle to reach elevated assets like overhead lines. The visual nature of acoustic imaging also enables precise localisation of PD sources, even in complex or hard-to-access areas, facilitating targeted repairs and minimising the need for extensive manual inspection.
Advanced acoustic imaging systems can also classify PD fault types. They do this primarily by using phase resolution, which correlates the sound from the PD with the waveform of the applied voltage. Different types of PD have different phase resolution patterns. Megger acoustic cameras distinguish three types of PD: surface discharge, which is often known as tracking; suspension discharge, which typically occurs across a small air gap; and corona discharge, which is often discharge from a sharp point or edge directly into the air. This extra information about the type of PD can be very useful for identifying the source and determining the most appropriate remedial action.
Benefits of early PD detection and remediation
The early detection of PD, including through acoustic imaging, allows for timely interventions, preventing minor issues from escalating into catastrophic failures and unplanned outages. By identifying and addressing insulation defects before they cause irreversible damage, proactive PD management can significantly extend the lifespan of electrical assets, optimise maintenance practices and reduce overall costs.
In addition to preventing failures and prolonging asset life, acoustic imaging enhances safety by minimising the need for personnel to work near high-voltage equipment during PD inspections. The ability to detect and locate PD from a safe distance reduces the risk of electrical accidents and allows for more efficient and targeted maintenance efforts.
By catching and correcting insulation issues early, facilities can avoid costly repairs, replacements and downtime associated with catastrophic failures. Regular PD monitoring also enables more informed maintenance decisions, allowing resources to be allocated based on the actual condition of assets rather than fixed schedules.
Conclusion
As industries increasingly recognise the value of condition-based maintenance and predictive strategies, the use of acoustic imaging for PD pre-location is becoming an essential part of modern maintenance. Acoustic imaging technology provides a non-invasive, long-range method for detecting and pre-locating PD in high-voltage electrical equipment. Providing ‘advance notice’ of developing faults enables proactive management of insulation defects, saves time and money, prevents failures and optimises asset performance.
