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Ultrasonic techniques for the assessment of partial discharges in electrical equipment.

Partial discharges are small electrical sparks that occur in the electrical insulation of the connection, wires, transformers and coils in large electric motors and generators. Partial discharge analysis is a proactive diagnostic approach that uses partial discharge (PD) measurements to assess the integrity of this equipment. This article describes how airborne ultrasonic instruments can be used to detect and assess partial discharges in electrical equipment.

Partial Discharges

Partial discharges are small electrical sparks that occur in the electrical insulation of the connection, cables, transformers and coils in large electric motors and generators. Partial discharge analysis is a proactive diagnostic approach that uses partial discharge (PD) measurements to assess the integrity of this equipment. Each PD is the result of electrical distribution from an air pocket in the insulation. PD measurements can be made continuously or intermittently and are detected online or offline. The results of the electrical discharges are used to reliably predict that the electrical equipment needs maintenance.

Just as each material has a characteristic tensile strength limit, each material also has an electrical (dielectric) breakdown that represents the electrical intensity required for current to flow and for an electrical discharge to occur. Common insulating materials such as epoxy, polyester and polyethylene have a very high dielectric strength. In contrast, air has a relatively low dielectric strength. The electrical interruption in air causes a very brief flow (lasting only fractions of a nanosecond) of electric current through the air pocket. The measurement of partial discharges is, in effect, the measurement of these breakdown currents.

Electrical equipment can suffer from a variety of manufacturing defects or operational problems that undermine its reliability. The electrical insulation of motors and generators is susceptible to:

Thermal variations Chemical attack Abrasion due to excessive coil movement. In all cases, these stresses will weaken the bonding properties of the polyester or epoxy resins that protect and insulate the windings. As a result, an air pocket develops in the windings.

Partial discharge levels not only provide an early warning of impending equipment failure, but also accelerate the wear process.

Ultrasonic / Acoustic Emission

All electrical equipment in operation produces a wide range of sounds. The high frequency ultrasonic components of these sounds are very short waves, and short wave signals tend to be quite directional. It is therefore relatively easy to isolate these signals from background noise and detect their exact location. In addition, as subtle changes begin to occur in electrical and mechanical equipment, ultrasound allows these potential danger signals to be discovered very early, before the very likely breakdown occurs.

Airborne ultrasonic instruments, often referred to as “ultrasonic translators”, will provide information in two ways: Qualitatively, due to the ability to “hear” ultrasound through noise isolation, and Quantitatively, through incremental readings of the measurement. This is achieved in most ultrasonic translators by an electronic process called “heterodyning”, which accurately converts the ultrasound picked up by the instrument into audible range sounds, which users can know and recognise through headphones.

While the ability to measure sound intensity and see patterns is important, it is equally important to be able to “hear” the ultrasound produced by different equipment. This is precisely what makes these useful instruments, which allow analysts to confirm a diagnosis in the field, able to discriminate between various sounds from different equipment.

The reason why users can accurately determine the location of an ultrasonic signal on a machine is due to its high frequency/short wavelength. Most sounds picked up by humans range from 20 Hz to 20 kHz (20 cycles per second to 20,000 cycles per second). Low frequency sounds in the audible range measure approximately 1.9 cm to 17 metres in length, while ultrasounds perceived by translators measure only between 0.3 - 1.6 cm in length. Since the wavelengths of ultrasound are of lower magnitude, the ultrasonic range is the most suitable environment for locating and isolating sources of trouble in high floor environments.

The use of ultrasound / ultrasonic emission in partial discharges

Ultrasonic testing is often used for the assessment of voltages in excess of 1000 V, especially in enclosed locations. This is especially useful in identifying tracking problems. In enclosed locations, the frequency of tracking is much higher than the frequency of serious faults, which can be identified using techniques such as thermography.

When electricity leaks into power lines or jumps through a gap in an electrical connection, it disturbs the air molecules around it and generates ultrasound. Often these sounds are perceived as a frying or crackling sound, and in other situations it is heard as a buzzing sound.

There are basic problems that can be detected through ultrasound:

Corona: When the voltage on an electrical conductor, such as a high voltage antenna or transmission line, exceeds a threshold value, the surrounding air begins to ionise to form a blue or purple glow. Learn more about the corona effect and corona effect cameras Tracing: Often referred to as “baby arcing”, it follows the path of damaged insulation. Arcs: An arc occurs when electricity flows through space. Lightning is a good example. High voltage applications include: insulators, cables, switches, bus bars, relays, contactors and junction boxes. In substations and components such as insulators, transformers and bushings can be detected.

The method of detecting arcing and corona is similar to the procedure used to detect acoustic emissions from mechanical sources. Instead of hearing a chirp, the user hears a crackle or hum. In some cases, such as when trying to locate interference from radio and television sources or substations, the area of disturbance can be located with a radio transistor or broadband interference locator.

Determining whether or not a problem exists is relatively simple. When comparing sound quality and sound levels between similar equipment, the difference in sound tends to be very different. Alternatively, trends in signal amplitudes over an extended period of time may be indicative of defects.