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

To calculate the overvoltages in transformer windings under very fast transient overvoltages (VFTOs) generated by switching operations in gas-insulated substation (GIS), a single-input and multiple-output circuit model, which is based on a multiconductor transmission line (MTL) model, is presented in this paper. The procedure to determine the transfer function is deduced. Vector fitting and recursive convolution are used to calculate the time domain response. The resonant frequencies and the location of the resonance are determined by an analysis of the amplitude-frequency response characteristics. For validation, the calculated results are compared with the measurements on a transformer winding model, and the results are found to be satisfactory

Modeling of Transformer Windings Under Very Fast Transient Overvoltages

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

The paper presents a hybrid model of disc-type power transformer winding for frequency response analysis (FRA) based on traveling wave and multiconductor transmission line (MTL) theories. Each disc of a winding is described by traveling wave equations, which are connected to each other in a form of MTL matrix model. This significantly reduces the order of the model with respect to previously established MTL models of transformer winding. The model is applied to frequency response simulation of two single-phase transformers. The simulations are compared with the experimental data and calculated results using lumped parameter and MTL models reported in other publications. It is shown that the model can be used for FRA result interpretation in an extended range of frequencies up to several mega Hertz and resonance analysis under very fast transient overvoltages (VFTOs).

A Hybrid Winding Model of Disc-Type Power Transformers for Frequency Response Analysis

This work studies the sensitivity of frequency response analysis (FRA) method in detecting different types of deformations on a 6.6 kV plain disc type transformer winding. Winding deformations such as axial displacements and inter-turn faults are experimentally simulated and the measured FRA results are presented. A high frequency transformer-winding model is developed to investigate the sensitivity of the FRA method for these winding deformations. A comparison is carried out and a good agreement is shown between measured and calculated FRA results.

Investigations on sensitivity of FRA technique in diagnosis of transformer winding deformations

A simulation model of a continuous disc type 6.6 kV transformer winding was used to study the propagation behaviour of partial discharge (PD) pulses. The model based on multi-conductor transmission line theory uses a single turn as a circuit element with the capacitance, inductance, and losses calculated as distributed parameters. Transfer functions that describe how the location of the PD source affects the current signals measured at the terminals of the winding were calculated. The paper shows how the position of the zeros in the frequency response of the measured current signals can be used to locate the source of the discharge. Sensitivity studies on the parameters of the model were used to investigate the effect of inaccuracies in the model on the position of the zeros and hence the location of the discharge.

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Simulation of a transformer winding for partial discharge propagation studies

An experimental investigation into the propagation behaviour of partial discharge (PD) pulses in a continuous disc type 6.6kV transformer winding is described in this paper. PD pulses were injected into the winding using a calibrator and the resulting current signals at the line and neutral end terminals measured using wide band current transformers. The location of the troughs (or zeros) in the frequency spectra of the measured signals change in accordance with the position of the injected pulse. The crests (or poles) in the spectra convey information about the resonance frequencies of the winding and are not affected by the position of the injected pulse. The measured spectra are compared with the spectra generated by a simulation model and although differences exist the overall shape and location of the poles and zeros are similar.

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Experimental investigation into the propagation of partial discharge pulses in transformers

Experimental investigation into the behaviour of partial discharges (PD) in different types of defect models are described in this paper. The models were used to study the waveshape and frequency spectra of the signals resulting from discharges. This information is then used to investigate the effect on the signals of the type of discharge. The circuit configuration and data acquisition system are also described. Typical insulation defect models were designed and used to investigate the relationship between PD activity and insulation type. Descriptions of the test results obtained for three types of PDs are described. They include corona, surface discharge along the insulation pressboard in oil and discharge along the outer surface of a bushing. The signals were collected by a wide-band PD measurement system. Analyses of the signals indicate that the type of PD source can be determined using the high frequency components of the spectra. This conclusion is very important in the high-frequency range and importance in the diagnosis process used for PD identification in power transformers.

Detection and characterisation of partial discharges in transformer defect models

A 66 kV continuous disc type winding of a distribution transformer is used to investigate the propagation of partial discharges (PD) with the aim of location The winding was modelled, as multiconductor transmission lines with each turn represented by a transmission line This approach results in the model being valid up to a few MHz in frequency The validity of the model was confirmed by impedance measurements on the winding The transfer functions calculated between probable PD source locations to winding terminals showed that the troughs (or zeros) change in frequency with the location of PD source and hence can be used for the location of PD Transfer functions obtained experimentally using a discharge calibrator as the PD source, showed very good agreement with the calculations

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G. Edwards

Papers

Simulation of a transformer winding for partial discharge propagation studies

Experimental investigation into the propagation of partial discharge pulses in transformers

Detection and characterisation of partial discharges in transformer defect models

An electrical PD location method applied to a continuous disc type transformer winding