In electrical systems, especially those above 1,000V, sufficient and properly maintained equipment insulation helps ensure equipment longevity. However, manufacturing defects, installation practice irregularities, and vibrational wear and tear can affect insulation integrity. As a result, unwanted electrical partial discharges (PD) may be present.
Under practical conditions, thermal imaging with infrared cameras frequently is deployed to identify unusual levels of heating—not visible to the eye—at insulators; sometimes these are caused by partial discharges. However, defects in insulation material do not always cause an increase in temperature, and affected components’ temperature may be distorted by other normally hot components in the vicinity; these may simply get missed when using this traditional test method.
These defects include, but are not limited to:
- Treeing/Tracking – leakage of electric current between two points separated by an insulating material, caused by dirt, carbon particles, moisture, or other contaminants.
- Arcing – visible plasma discharge between two electrodes that is caused by electrical current ionizing gases in the air. Arcing can be caused by intermittent shorts to ground (lightning is a form of arcing), and treeing/tracking often leads to arcing if left unchecked.
- Corona – an electrical discharge brought about by the ionization of a fluid such as air surrounding a conductor that is electrically charged and is affected by conductor geometry as well as contamination.
Not every insulation defect will create a temperature increase. However, they all produce noise in the ultrasound spectrum, and handheld ultrasound test devices can safely and accurately detect insulation damage—even with defects that do not produce a heat signature.
Arcing, tracking, and corona generate unique sound characteristics in the ultrasound spectrum. A trained technician can discern among them based on experience as well by using on-screen information from the handheld ultrasound tester. All of these emissions produce ionization, whereby neutral atoms and molecules gain or lose electrons, becoming an ion. The byproducts of ionization are ozone (O3) and nitric acid (HN03), which is destructive to most dielectric materials and corrosive to certain metals. Radio frequency interference is another common side effect of electrical partial discharge.
Ultrasound detectors work in the range of 20-100KHz and use heterodyning to translate the ultrasound into the audible range for the technician. Most ultrasonic testers use high-quality over-the-ear headphones designed to attenuate audible noise from plant operations. The detectors also show the intensity of the signal in decibels (dB) and may, depending on device sophistication, also show graphical representations of the sound wave in the frequency and time domain. Software is also commonly available to capture and analyze sound files and verify PD problem type and severity.
Ultrasound test devices are very well adapted to electrical inspections. For example, they can perform PD detection through short distances with a basic airborne sound probe. Scanning around panel seams or via ventilation louvers can be an effective means of detecting PD on some equipment types; alternatively, permanently mounted partial discharge sensors can be mounted on equipment. The partial discharge sensor may be a standalone device or part of an infrared window/visual inspection system, and it ensures that an unadulterated signal is received from inside the equipment without any worry about the inspector’s safety. For longer distances and overhead substation applications, the parabolic sensor is also available with a range of up to 45 meters (150 feet).
One final note: Loose insulators, windings, and even vibrating mechanical screw connections transmit ultrasonic waves into the equipment structure. However, handheld ultrasound test devices can detect these noise sources and transform them into audible frequencies. In this manner, even more-subtle electromechanical defects can be pinpointed, and corrective actions can be taken.