Partial Discharges In Power Transformers Research Articles

Acoustic measuring of partial discharge in power transformers

Power transformers' reliability can be seriously affected by partial discharges. For this reason, it is necessary to implement technical methods to identify endangered equipment before catastrophic failures occur. A well-known method that can be applied either in the laboratory or in the field is the detection and localization of partial discharges, by means of the analysis of the acoustic signals they produced. An innovative partial discharge detector was developed based on the analysis of an acoustic or electrical PD signal envelope. This paper describes the architecture of the developed acoustic detector, which is composed of a set of ultrasonic sensors, signal conditioning and control modules, a graphical interface and the required software for the location of the affected area within the transformer. The conditioning and control modules perform analog to digital conversion, arrival time measurement, communication and control operations. Finally, some power transformer diagnostic testing is presented and discussed.

Detection and location of partial discharges in power transformers using acoustic and electromagnetic signals

The measurement of partial discharges (PD) is a non-destructive and sensitive diagnostic tool for the condition assessment of insulating systems. Two major tasks of PD measurements may be distinguished, (i) PD detection, hence providing evidence and the type of the PD and (ii) the location of the PD. The question "Where is the PD source?" is amongst others tremendously important for scheduling and starting maintenance/repair actions cost and time efficiently or to perform a risk analysis. Here the possibility to geometrically localize the flaw, by means of arrival times of acoustic PD signals, gets an extremely interesting option. Precise acoustic arrival times are consequently essential to accurately locate PD in a power transformer. The averaging of acoustic PD signals helps to enhance the acoustic sensitivity. The acoustic detection limit is lowered significantly and the determination of the arrival times is made possible for weaker PD. Supplementary steps, like automatic objective arrival time determination or additional wavelet-based de-noising further improve the overall location accuracy. A new location approach works with pseudo-times and allows for the use of robust direct solvers instead of the previously used iterative algorithms.

Optimisation of a sensor for onsite detection of partial discharges in power transformers by the UHF method

Among the different solutions which allow onsite partial discharge measurement in energized power transformers, the UHF technique is gaining general interest. In order to apply this method in existing transformers, it is considered advantageous to design a UHF sensor to be fitted inside the transformer tank. This paper describes the constraints affecting the development of such sensor and the process followed to optimize its design. During this process, different solutions for broadband UHF antennas were analysed. The selection was based on computer simulation and experimental results. Computer simulation was used to evaluate differences in radiation pattern, antenna impedance, gain and effective area. Measurements of the antenna response to different partial discharge sources in oil were performed using an oil dielectric breakdown test set in an electromagnetic shielded laboratory. A group of selected antennas were then tested in a power transformer simulating the broadband signals generated by partial discharges by injecting controlled voltage pulses in the transformer. Finally, a sensor based on a conical monopole antenna structure was built and attached to a specially designed housing to be fitted inside the transformer tank through the drain valve. This final prototype was benchmarked during a high voltage factory test of a power transformer which showed a significant partial discharge activity.

Sulfur Hexafluoride-Filled Extrinsic Fabry–PÉrot Interferometric Fiber-Optic Sensors for Partial Discharge Detection in Transformers

This letter presents a side-hole fiber-based extrinsic Fabry-Perot interferometric (EFPI) sensor whose cavity was filled with sulfur hexafluoride (SF6) for in situ power transformer partial discharge detection. The Fabry-Perot cavity was formed by a polished thin diaphragm and a cleaved side-hole fiber that were thermally fused onto a fused silica ferrule. The cavity was first vacuumed and then inflated with 300-kPa SF6 through the fiber side holes to increase its resistance to electric stress. Experimental results show that the sensor filled with SF6 was able to withstand electric stress of at least 10 kV/mm (ac root-mean-square value) in transformer oil, three times larger than that of a traditional air-filled EFPI sensor.

Knowledge-based diagnosis of partial discharges in power transformers

The abstraction of meaningful diagnostic information from raw condition monitoring data in domains where diagnostic expertise and knowledge is limited and constantly evolving presents a significant research challenge. Expert diagnosis and location of partial discharges in high voltage electrical plant is one such domain. This paper describes the functionality of a knowledge-based decision support system capable of providing engineers with a comprehensive diagnosis of the defects responsible for partial discharge activity detected in oil-filled power transformers. Plant data captured from partial discharge (PD) sensors can be processed to generate phase-resolved partial discharge (PRPD) patterns. This paper proposes a means of abstracting the salient features characterizing the observed PRPD patterns. Captured knowledge describing the visual interpretation of these patterns can be applied for defect diagnosis and location. The knowledge-based PRPD pattern interpretation system can support on-line plant condition assessment and defect diagnosis by presenting a comprehensive diagnosis of PD activity detected and classification of the defect source. The paper also discusses how the system justifies its diagnosis of the PD activity to offer the expert greater confidence in the result, a feature generally absent in 'black-box' pattern recognition techniques. The incremental approach exhibited by the system reflects that of a PD expert's visual interpretation of the PRPD pattern. The paper describes how this functional system design has evolved from the approach taken by PD experts to the visual interpretation of PRPD patterns.

A strategy to locate partial discharges in power transformers using acoustic emission

This paper presents a solution for the problem related to Partial Discharges (PD) source localization in oil- isolated transformers, using Genetic Algorithms. As known in literature, the simple PD detection is not enough to take a decision about intervening, so the localization is necessary to assess the risk and to plan corrective actions. The parameters extracted from the acoustic signals collected by piezoelectric sensors installed outside of the transformer, provide the correct position of PD. From parameters extracted of acoustic signals collected from piezoelectric sensors installed outside the transformer, the proposed algorithm provides the position where the PD is occurring. This work compares the performance of the method using Genetic Algorithms and those using iterative method, which is normally used to solve this problem. The conclusion describes the advantages of the proposed solution.

On-Line Partial Discharge Monitoring and Diagnostic System for Power Transformer

This paper introduces a computerized on-line partial discharge (PD) monitoring and diagnostic system for transformers. The system, which is already in use in a power station, uses wide-band active transducers and a data acquisition unit with modularized and exchangeable components. The system software is a power equipment monitoring and diagnostic system, which is based on the component object model, and was developed for monitoring multiple parameters in multiple power supply systems. The statistical characteristics of PDs in power transformers were studied using 7 experimental models for simulating PDs in transformers and 3 models for simulating interfering discharges in air. The discharge features were analyzed using a 3-D pattern chart with a three-layer back-propagation artificial neural network used to recognize the patterns. The results show that PDs in air and oil can be distinguished. The model can be used for interference rejection on-line monitoring of partial discharge in transformers.

Investigation of propagation of partial discharges in power transformers and techniques for locating the discharge

The location of partial discharges in a power transformer can be determined based on the characteristics of the transfer function from the discharge source to the measuring terminal. Previous studies partially validated the technique using computer simulation and practical experiments based on a PD calibrator to represent a discharge signal. 'Real' discharges produced by insulation defect models were used to study how discharges propagate in an 11 kV plain-disc-type transformer winding: A corona model and a 'floating objects in oil' model produced discharge signals with different durations at various locations along the winding. Measurements were taken at the tap of the bushing capacitance through a conventional discharge detector. The signals were filtered, amplified and fed into a digital storage oscilloscope. The frequency spectra of the measured signals showed significant similarities irrespective of the type of discharge source. The characteristic of the transfer functions, i.e. the crests and troughs in the spectra, could be used for locating the source of the discharge. Energising the transformer increased the level of electric noise, which did affect the low frequency end of the spectra, but did not have any impact on the characteristics used for location.

Fiber Fabry-Perot sensors for detection of partial discharges in power transformers.

A diaphragm-based interferometric fiberoptic sensor that uses a low-coherence light source was designed and tested for on-line detection of the acoustic waves generated by partial discharges inside high-voltage power transformers. The sensor uses a fused-silica diaphragm and a single-mode optical fiber encapsulated in a fused-silica glass tube to form an extrinsic Fabry-Perot interferometer, which is interrogated by low-coherence light. Test results indicate that these fiber optic acoustic sensors are capable of faithfully detecting acoustic signals propagating inside transformer oil with high sensitivity and wide bandwidth.

Optical fiber sensor-based detection of partial discharges in power transformers

In this paper, a fiber optic acoustic sensor system is designed and tested for on-line detection of the partial discharges inside high voltage power transformers. The fiber optic sensor uses a silica diaphragm and a single mode optical fiber encapsulated in a silica glass tube to form an extrinsic Fabry–Perot interferometer. Test results indicate that the developed fiber optic sensors are capable of detecting the acoustic signals propagating inside the transformer oil with high resolution and high frequency.

Location of partial discharges in power transformers using computer-aided acoustic techniques

In this paper, the recent development of a computer-aided partial discharge (PD) acoustic location system is briefly introduced. The paper presents the characteristics of this system with emphasis on the acoustic locating technique and interference countermeasures. Signal recognition, ascertainment of time delay and the location algorithms are discussed. A novel method for identifying multiple PD sources is proposed. By using fuzzy sets and fuzzy subsets as well as their relation equation, one can classify and identify the multiple PD sources. On-site testing results are also included in the paper. The results confirm that the acoustic emission techniques for partial discharge location are feasible and effective.

Piezoelectric reflecting transducer for partial discharge acoustic signals

The paper presents a new design for a piezoelectric transducer for acoustic signals emitted during partial discharges in power transformers. Testing results of its metrological parameters are presented. Examples of partial discharge oscillograms recorded using the new transducer in the measuring path are shown.

Location of Partial Discharges in Power Transformers by Computation and Measurement of Capacitively Transmitted Voltage Pulses

The voltage distribution in a power transformer after a partial discharge has occurred is calculated. The results are compared with voltages, measured at the terminals of the transformer. A partial discharge prediction curve, based on the calculated capacitive distribution is constructed, by which the location of partial discharges is improved. The location method is based on peak values or capacitive pulses measured at the terminals. Calculation of the complete winding resonances after a partial discharge is found not to be accurate enough for internal corona location purposes.