Partial discharge (PD) measurements provide valuable information for assessing the condition of high voltage (HV) insulation systems, contributing to their quality assurance. Different PD measuring techniques have been developed in the last years specially designed to perform on-line measurements. Non-conventional PD methods operating in high frequency bands are usually used when this type of tests are carried out. In PD measurements the signal acquisition, the subsequent signal processing and the capability to obtain an accurate diagnosis are conditioned by the selection of a suitable detection technique and by the implementation of effective signal processing tools. This paper proposes an optimized electromagnetic detection method based on the combined use of wideband PD sensors for measurements performed in the HF and UHF frequency ranges, together with the implementation of powerful processing tools. The effectiveness of the measuring techniques proposed is demonstrated through an example, where several PD sources are measured simultaneously in a HV installation consisting of a cable system connected by a plug-in terminal to a gas insulated substation (GIS) compartment.

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Application of HFCT and UHF Sensors in On-Line Partial Discharge Measurements for Insulation Diagnosis of High Voltage Equipment

Partial discharge (PD) detection in the ultra-high frequency (UHF) range is state of the art. One of the most critical points in this detection method is adopting proper and suitable antenna design. In this paper, a new design of UHF antennas is introduced which is suitable to be used on the transformers in the HV laboratories of the manufacturers. To ensure the applicability of the new antenna, experimental UHF PD measurements are carried out on a transformer tank model as well as on the real power transformers. Installing several UHF antennas on the transformer tank to perform defect localization is also an important issue. There are rigorous limitations and lack of the places to install the antennas. This problem is almost solved by using the mentioned new antenna design. The proper positioning of the antennas is the other critical point in the UHF method. Placing the antennas in improper positions will damage the localization accuracy. In this paper, an automatic algorithm is introduced to determine the best installation positions for the antennas to enhance the localization accuracy. The efficacy of this algorithm is investigated by using the results of experimental measurements on power transformers.

Advancing new techniques for UHF PDdetection and localization in the power transformers in the factory tests

Partial discharge (PD) detection is a widely extended technique for electrical insulation diagnosis. Ultrahigh-frequency detection techniques appear as a feasible alternative to traditional methods owing to their inherent advantages such as the capability to detect PDs online and to locate the piece of equipment with insulation problems in substations and cables. In this paper, four antennas are thoroughly studied by means of their theoretical and experimental behavior when measuring electromagnetic pulses radiated by PD activity. The theoretic study of the band of frequencies in which the pulse emits and the measurement of the parameters $S_{11}$ are complemented with the frequency response and wavelet transform of a set of 500 time signals acquired by the antennas, and the results are analyzed in detail.

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Antenna Parametrization for the Detection of Partial Discharges

Electromagnetic waves generated due to partial discharge (PD) can be captured by ultra-high-frequency (UHF) antennas. The size and frequency band of an UHF antenna for PD online monitoring are critical factors for practical installation inside a transformer. This paper presents a compact multi-band UHF Hilbert fractal antenna with wide frequency band and suitable size for easy installation. Design criteria of a PD antenna were proposed and the Hilbert fractal antenna was selected as an appropriate candidate for PD detection. Principle of Hilbert fractal antenna was discussed and a fourth order Hilbert fractal antenna was optimized for detecting PD signals. Actual PD experiments have been carried out for two typically artificial insulation defect models while the antenna was used for the PD measurement. More simulation studies and actual PD experiments have been done to further examine the effects of electromagnetic wave refraction and reflection by transformer components. The results show that the proposed UHF antenna is qualified for PD online monitoring.

Hilbert fractal antenna for UHF detection of partial discharges in transformers

Nowadays, the localization of partial discharge (PD) in power transformers by measurement of ultra-high-frequency (UHF) electromagnetic (EM) waves from a PD source is receiving more attention. This method is based on comparing the differences in the arrival times, to antennas in the transformer, of EM waves emitted from the PD source. From the measured UHF signal, and factoring in the influence of the transformer active part on the EM wave propagation, the correct determination of an EM wave arrival time in the localization algorithm is a critical task that influences the accuracy of the UHF PD localization method. Nearly all weak points in the dielectric system of power transformers generate PD. These discharges gradually degrade the insulation, leading to transformer failure. It is possible to detect these defects in the early stages of PD generation by measuring and analyzing the PD data. Suitable maintenance and repair strategies can then be implemented.

A novel method for ultra-high-frequency partial discharge localization in power transformers using the particle swarm optimization algorithm

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.

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

Fifteen years since the first application of the UHF method for partial discharge in gas insulated switchgear, this has become widely used and the preferred method for PD diagnostics in GIS. Same technique is currently starting to be applied for onsite PD testing of Power Transformers. Significant advantages of this UHF method include the reduction of background noise and the possibility of on-line testing. It does provide a relief of background noise by selecting a working frequency band where interference is minimal or absent . In order to apply this technique to power transformers in service, a specific UHF sensor which can be fitted in a transformer without the need of a power outage has been developed. The process followed to design several suitable sensors and the experimental tests chosen to select and validate the best of these designs are presented in the paper.

Design and testing of UHF sensors for partial discharge detection in transformers

Partial discharge detection by the UHF method has become a powerful tool to guarantee GIS reliability. This paper presents new experimental results regarding the study of the most common discharge source in a GIS, which is provoked by free metal particles. Particles of different size and material were introduced in an experimental GIS bus and AC voltage applied. The movement of the particles was observed by a video camera and simultaneously the partial discharges were registered by a UHF monitoring system. Several discharge patterns were identified depending on the particle movement and location (ground metal chamber or spacer). These patterns were then used by the monitoring system for defect location and risk assessment for the GIS. Finally, based on more than 10 years experience with UHF monitoring, an overview of HV commissioning procedures and online monitoring systems is presented.

http://ieeexplore.ieee.org/iel5/6841/18401/00847683.pdf

Partial discharge detection of free moving particles in GIS by the UHF method: recognition pattern depending on the particle movement and location

The application of the UHF method to PD measurements in MTS substations presents technical challenges due to the much higher external noise in outdoor substations. To adapt the UHF technique to handle these challenges, a series of tests was performed in the laboratory and at selected substations. Different PD sources were inserted in a test rig, and the proposed UHF detection system was evaluated. On-site measurements using this system showed that the spectra of external and internal PD sources differed considerably, suggesting that the system is capable of yielding reliable indications of the presence of PD in MTS.

J. Lopez-Roldan

Papers

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

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

Design and testing of UHF sensors for partial discharge detection in transformers

Partial discharge detection of free moving particles in GIS by the UHF method: recognition pattern depending on the particle movement and location

Partial discharge diagnostics for mixed-technology switchgear (MTS) in outdoor substations