Acoustic Method for Detecting Partial Discharge - Essay Example

?Acoustic method for detecting partial discharge Introduction A partial discharge in electrical technology is defined as a dielectric breakdown of anelectrical insulation system under conditions like high voltage that leads to a breakdown of the gap between two conductors separated by the dielectric space. Unlike corona discharges that are visible in the form of a luminous glow, partial discharges that occur within insulation systems cannot be seen by the naked eye. Such electrical insulation systems are tested for the presence of partial discharges to evaluate their conditions. Common electrical components like transformers and power cables are tested for partial discharge as part of a quality assessment framework. In fact, detection of partial discharge is a common practice adopted by electrical manufacturers across the world since the 1980s. Testing for partial discharge is necessary to improve the quality and reliability of electrical systems, as it has been cited as one of the primary reasons for failure of electrical systems (Bengtsson, 2003). The presence of partial discharge indicates a partial damage to the insulation that can lead to speaking effects along the insulation surface. Thus, detecting partial discharge helps in the identification of flaws like cracks and voids where there is a maximum probability of electrical stresses that can lead to a breach of the insulation. The analysis of partial discharge can be carried out using several predictive tests that identify any degradation of the insulation before such a condition leads to the damage of the system. Such testing can further be carried out in standard operating conditions and does not require any special arrangements. Zhiqiang, (2004) says that typical methods of detection include acoustic, electrical and chemical techniques that do not affect the insulation system. Besides, all these methods for detection can be undertaken at reasonable costs and thus find widespread acceptance in the industry. This paper discusses the acoustic method of partial discharge (PD) detection by describing the advantages and disadvantages of the technique. Further, data has been sourced from 6 different research papers that analyze the acoustic PD method from different perspectives. Discussions and inferences from these individual papers has been presented and compared wherever appropriate. Possible future trends for acoustic PD testing have also been discussed in brief. Acoustic detection of PD Blackburn (1998) says that the acoustic method for PD detection relies on the identification, processing and storage of the signal resulting from a partial discharge. The acoustic signal generated during a PD occurrence is detected and recorded by the acoustic PD testing arrangement. The technique is similar to the generation of thunder during a storm. The acoustic signal is generated when a current streamer develops within the void space. The material surrounding this volume becomes vaporized leading to an explosion that releases a detectable amount of mechanical energy. The released energy then travels along the electrical system’s volume as a pressure field. The acoustic PD detection technique is mostly used in gas-insulated substations as well as in High-voltage transformers. Detections systems that employ this method are also differentiated based on their construction, into internal and external acoustic PD systems. Internal PD systems consist of a sensor that is placed within the transformer tank to detect the magnitude of the pressure wave through the oil medium. In contrast, external PD systems are based on sensors that are attached to the outside wall of the transformer tank to monitor the pressure wave. Amongst these two methods, the external PD systems are more widely used in the industry due to the ease of construction and operation (Darley, 2001). The main benefit of the acoustic PD detection method over other available techniques is that the information on the position of the PD can be retrieved instantly from the PD detection system through measurements from multiple locations in the transformer tank. This real time estimation helps determine the type of the PD, its severity and the precise location of the fault in the insulation. Eleftherion (2005) says that the detection of the fault position is also helpful in repairing the damaged insulation at the specified spot. The acoustic PD method also has the advantage of being resistant to electromagnetic interference, thereby detecting a highly qualitative signal-to-noise ratio. This minimized the number of false alarms and improves the functionality of the electrical system. Kemp (2005) has studied some of the issues and deficiencies associated with acoustic PD detection. His research shows that the immunity to interference does not necessarily imply the absence of any acoustic noise from the PD system. In fact, acoustic noise is largely generated from mechanical vibrations on the electrical system. However, the frequency of this noise is still very low compared to the strength of the acoustic PD signal, thereby making this a very suitable technique for PD detection. The wave propagation associated with acoustic detection is highly complex and relatively difficult to calculate. Given the inefficiencies within the transformer tank, the waves generated from the pressure wave do not propagate in spherical wave fronts, which are further complicated by the reflections caused from the boundary walls. This can diminish the strength of the signal and introduce noise. The strength of the signal is also influenced by absorption and dispersion within the mineral oil, which can significantly dampen the signal in larger containers. Further, the actual signal strength as detected by the sensor is much lower than the generated intensity due to attenuation effects. As such, the sensor must be capable to detect even the smallest acoustic signals. Blackburn (1998) has shown that PD detection and physical location within high voltage transformers is an important criterion for demonstrating the operational status of electrical insulators within these transformers. He further evaluates the time taken for the insulation to wear down due to electrical and mechanical stresses and concludes that if the insulation damage is not rectified, it could lead to a disruption in the normal operation of the system besides damaging equipment in the vicinity. Darley (2001) has shown that insulation breakdown leads to an accumulation of electrical charge within the device which is then released as an electromechanical energy pulse. He further goes on to add that damage to the insulation accelerates the stress from electrical and mechanical factors thereby hastening the development of the internal flaw in the electrical device. Eleftherion (2005) has estimated the time taken to determine the accurate position and intensity of the acoustic PD pulse to minimize the time required for diagnostics and repair. Kemp (2005) has studied the construction of one particular acoustic PD detection system using a series of optical acoustic sensors that operate based on the phenomenon of Fabry-Perot interferometry. Unlike other commercially available solutions Kemp (2005) shows that the inert property of the acoustic sensor allows for a smooth operation without influencing the functionality of the high-voltage transformer in any manner. This technique was found to result in measurements of better quality and signal amplitudes which were also free from many of the errors and noise found in common measurements. In addition, this method also reduced the impact of multipath errors that occur due to different speeds of signals originating from multiple locations within the transformer. Zhiqiang, (2004) studied the application of acoustic PD detection to power cables using a ‘complex discharge analysis’ method. This method studies PD events when the capacitance within the cable is discharged i.e., during the tail times. This method proved to involve lower voltage stress, reduced the time of estimation to five trials and reduced power consumption due to a longer charging duration. Future work The preceding sections have discussed the utility of the acoustic method in detecting partial discharges. Further studies are required to understand the very nature of the PD mechanism itself. (include author) says that studies so far have led to different conclusions over the nature of acoustic PD signals in terms of duration and frequency. Lack of any common agreement over common characteristics has made it difficult to develop a common standard for detecting partial discharges through the acoustic method. Thus, there is a need to investigate this area in much broader detail to eliminate these deficiencies (Zhiqiang, 2004). Kemp (2005) also cites the need to develop better noise rejection procedures in order to differentiate PD pulses from electromagnetic interference. References 1. Bengtsson (2003), Transformer PD Diagnosis using Acoustic Emission Technique. 10th International Symposium on High Voltage Engineering. Montreal, Canada. 2. Blackburn (1998), Acoustic Detection of Partial Discharges Using Non-Intrusive Optical Fibre Sensors. IEEE International Conference on Condution and Breakdown in Solid Dielectrics, Vasteras, Sweden. 3. Darley (2001), Partial Discharges within Power Transformers and the Use of Ultrasonic Techniques in Their Location. IEE Colloquium on Assessment of Degradation Within Transformer Insulation Systems, London, U.K. 4. Eleftherion (2005), Partial Discharge XXI: Acoustic Emission-Based PD Source Location in Transformers. IEEE Electrical Insulation Magazine. vol. 11, pp. 22-26. 5. Kemp (2005), Partial discharge plant-monitoring technology: Present and future developments. IEE Proc.-Sci. Meas. Technology. vol. 142, pp. 4-10. 6. Zhiqiang, (2004), The Directionality of an Optical Fiber High-Frequency Acoustic Sensor for Partial Discharge Detection and Location. Journal of Lightwave Technology. vol. 18, pp. 795-806. Read More