Partial discharges (PDs) are among the various indicators that are utilized for monitoring of power transformers. PDs emit electromagnetic waves that can be received by inserting ultrahigh frequency probes into the transformer tank, thus an advantage of this method is higher robustness against external noises. In this contribution, an algorithm is proposed for the localization of PDs in power transformers by capturing and analyzing multiple signal sets. Detecting exact arrival times (ATs) of a signal set is a challenging task in the PD localization. Different AT detection methods are applied to several signal sets to compare their effectiveness. Furthermore, modifications are introduced to enhance precision of the AT detection. A fraction of the analyzed signal sets provide acceptable coordinates for the PD location. The localization algorithm consists of two approaches including wavelet denoising and AT sequence determination to eliminate PD coordinates that lie significantly far from the real location of the PD. The rest of the PD coordinates are then divided into two sections using a clustering method, and the center of the favorable cluster yields the definite PD coordinates. Precision of the PD localization algorithm is validated by three measurements inside a specially designed transformer tank.

The detection and measurement of partial discharge (PD) is a well-known method for evaluating the insulation performance of power transformers. According to the IEC 60270 standard, the apparent charge is measured in a frequency range below 1 MHz using an electrical circuit which quantifies only a portion of the discharge energy conducted to the PD detector through the conductors. However, it is also well known that the PD energy is not only conducted but also transmitted via electromagnetic (EM) and optic waves, acoustic waves, and thermal energy. This article describes the significant challenges and the problem of calibrating the ultrahigh frequency partial discharge measurement method for power transformers and clarifies the complexities theoretically and experimentally.

This paper provides a comparative study on the impulse breakdown performance of SF6 and 20% C3F7CN / 80% CO2 gas mixture under weak non-uniform electric fields. Two geometrically distinct electrode configurations, with similar field utilization factor, were subjected to standard lightning impulse waveform (1.2/50 mu s) and the effects of pressure, gap distance and impulse polarity on breakdown characteristics were studied. The results show that SF6 has a higher breakdown voltage than the C3F7CN/CO2 mixture and the positive 1.2/50 mu s breakdown voltages were found to be higher than the negative counterparts. These results show that the effect of polarity on breakdown voltage does not exclusively depend on electrode geometry but also on pressure and gas type. The results of this paper identify the weaker polarity for 20% C3F7CN / 80% CO2 tested with hemispherical rod-plane and coaxial electrode configurations and the results contribute to the design of SF6-free gas insulated equipment.

Recently, partial discharge (PD) measurement in power transformers using ultra high frequency (UHF) techniques attracts a lot of attentions. But, most investigations on the subject are focused on detection of electromagnetic (EM) waves e.g. design of proper antennas and localization of PD source where just deals with starting point of the UHF signal. Thus, analysis of UHF data including waveforms and patterns is less performed. In this paper, an investigation on influence of PD type on UHF signals in phase domain is presented to find if there is any difference between UHF and IEC patterns. Moreover, UHF PD waveforms in time and frequency domains is studied to realize effect of transformer tank, antenna characteristics and type of PD on them.

In this contribution, the alternative approach of applying the PRPD technique to the captured UHF signals in order to obtain an UHF PRPD pattern as a complementary tool for PD root cause analysis is investigated. For this purpose, different fault scenarios were created inside a specially designed transformer tank. The UHF signals were captured via different antennas using several measurement setups to determine how parameters such as frequency, bandwidth, PD location, etc. affect the sensitivity of the UHF measurement technique for different types of PD. Furthermore, the correlation between the captured PRPD patterns using the electrical measurement according to IEC 60270 and the UHF measurement technique were analyzed and compared. Finally, the results are presented and the advantages and flaws of the PD cause identification via the UHF PRPD technique are discussed.

Location of a partial discharge (PD) fault inside a transformer can be determined using the Ultrahigh Frequency (UHF) PD measurement technique. Hereby, the electromagnetic (EM) waves that propagate due to the PD are captured using specially designed probes that are inserted into the transformer tank through its oil drain valves. The difference in the arrival time (AT) of these signals is the base of an equation system, where the PD location and the PD inception time are unknown. In this contribution, the influence of erroneous determined ATs on the localization error is investigated using a two-dimensional analogy model. Furthermore, a novel method based on detection of abrupt changes in a certain statistical property of the signal for the AT determination is proposed. A test setup is utilized to evaluate the localization algorithm. Finally, the results are presented and the precision of the proposed method for PD localization using the UHF measurement technique is discussed.