This paper presents a theoretical study relating to partial discharge (PD) source localization in high voltage power transformers using UHF technique. It also includes a brief review of typical PD sources, continuous UHF-based monitoring system, and some suitable UHF sensor types. Using MATLAB calculations and some basic assumptions, the differences in potential accuracy of PD source location are shown for different numbers of sensors, including the influence of sensor position. In addition, in the case of four UHF sensors, other effects were taken into accounts such as background noise sources, signal attenuation, multivariate wavelet denoising and the first peak detection method. By using the programs, at first, it is shown how to find the corresponding loci on which a PD source can be expected to lie by means of two and three UHF sensors in power transformers. Furthermore, using four sensors, it is presented on two different examples how simulated UHF signals, which travel from the partial discharge source to individual UHF sensors, can be used to find a partial discharge source position with sufficient accuracy. Multivariate wavelet transform was applied to the simulated signals in order to remove noise from the signals, and thus to reduce a localization error. The first peak detection method was applied to two various presumed shapes of UHF signal, in order to find time differences of arrival between related signals at the sensors. The corresponding results of partial discharge source location were analyzed and compared.

Application of UHF method for partial discharge source location in power transformers

Power transformer is one of the most important high voltage apparatus in any power system network. The life span of such apparatus commonly relies upon its insulation condition. The primary reason for an insulation breakdown is partial discharge. Partial discharges (PDs) are electrical discharges that happen in the insulation system of the transformers. Occurrence of PDs results in deterioration of the transformer insulation system. Hence, measuring and detecting the PD at an initial stage is necessary to avoid complete breakdown of the transformer. Currently, various PD detection and measurement methods, such as, electrical, chemical, acoustic, UHF and optical methods are being used for PD monitoring in power transformers. The conventional PD monitoring techniques like electrical or chemical have some limitations. The optical Fiber Bragg Grating (FBG) Sensor has been used as a PD sensor for PD monitoring in the power transformer to overcome these issues. This paper presents a review of FBG sensors’ application as a sensor for PD monitoring in an oil-filled power transformer. The challenges and future prospects in this area are also discussed.

Partial Discharge Detection in an Oil-Filled Power Transformer Using Fiber Bragg Grating Sensors: A Review

More and more transmission power cable circuits with lengths up to several tens of kilometers will be installed and put into operation. This contribution discusses basic aspects of selecting an on-site test voltage and the partial discharge measurement under on-site testing conditions of power cable lines with long length. On the one side, the generation of suitable test voltages for high capacitive loads, especially long cables, is evaluated and experiences of on-site measurements are described. On the other hand, the challenges for sensitive partial discharge measurement on long length power cables are discussed. Under this scope, partial discharge measurement by dual-sided measurement technique is described and examples are shown.

Basic aspects of partial discharge on-site testing of long length transmission power cables

Reliable operation of high-voltage equipment depends on defect-free HV insulation systems. However, defects in the insulation can lead to partial discharge (PD) activity. PD measurement has been shown to be a useful tool for evaluating the condition of high voltage components including gas-insulated switchgear (GIS). This work focuses on characterizing the motion and PD signal behavior of free-moving particles in an experimental GIS arrangement under high dc field stress. For this purpose, the particle geometry, material composition, and amplitude of the applied negative dc voltage are varied. PD signals are measured using a broadband ultra-high-frequency (UHF) sensor and recorded in the time domain. The UHF signal is used to trigger a synchronized high-speed camera in order to observe the correlation between particle motion and UHF PD pulse activity. Certain unique combinations of UHF pulse repetition rates and signal amplitudes in the time domain are observed that depended on and were correlated with particles’ motion. These are evaluated by means of pulse sequence analysis, whose significant clusters enable estimation of particle motion. In particular, particle motion with small hopping amplitudes on the negative electrode—known as “firefly”—is differentiated with high accuracy.

Combined Characterization of Free-Moving Particles in HVDC-GIS Using UHF PD, High-Speed Imaging, and Pulse-Sequence Analysis

Prolonged partial discharge (PD) activities can deteriorate the insulation of a power transformer leading to its complete failure. Therefore, PD monitoring is essential to ensure that PD sources are identified early. Ultra-high Frequency (UHF) PD measurement has gained attention because of some advantages compared to the conventional electrical method, such as low sensitivity to external disturbances. CIGRE recommends providing dielectric windows in newly manufactured transformer tanks for installing UHF sensors. However, there is no general recommendation on the placement of UHF sensors on a transformer tank, yet. This contribution provides knowledge about this discussion based on experimental data obtained from a 300 MVA, 420-kV grid-coupling transformer. UHF signals were measured using antennas at 17 different positions on the transformer tank and were used to provide a general evaluation of signal attenuation depending on distance and different types of propagation paths. Two approaches of signal evaluation, namely, signal power and peak-to-peak amplitude, were compared and found to have a high qualitative correlation, which implies that either method can be used to draw similar conclusions. Additionally, certain sensor positions were analyzed for any inherent benefits and finally, prospective sensor positions were suggested based on their performance and relative distances.

T. Ariaans

Papers

Guidelines for partial discharge detection using conventional (IEC 60270) and unconventional methods