Determining whether power transformers are suffering from internal arcing or dangerous levels of partial discharge (PD) is important because failure without warning can result in damage to neighboring equipment, customer dissatisfaction, disruption to economic activity, and the imposition of regulatory fines. This paper provides an overview of the excitation of UHF signals by PD inside transformers. The use of externally mounted sensors has been outlined, and the structure of a dielectric window that can be constructed on an inspection hatch has been outlined. Attenuation of UHF signals propagating inside a transformer tank has been shown to be relatively low. A new approach to locating PD sources in three dimensions has been presented, based on using a numerical model of the transformer materials, which defines electromagnetic propagation velocities on a mesh of 5-cm sub-cells. The concepts of the propagation-velocity matrix (PVM) and the propagation-time matrix (PTM) were thereby introduced. Once the PD source has been located to a specific region of the transformer, the PVM might be used to index a database of physical descriptions of the transformer that could provide additional information to assist with diagnosis.

Partial discharge monitoring of power transformers using UHF sensors. Part I: sensors and signal interpretation

Partial discharges (PD) generate wideband radio frequency interference and, consequently, can be detected using radio receiving equipment. Due to the advances in ultra-high-speed sampling equipment, it is possible to accurately measure the propagation of the PD wavefront as it passes through a 4 element antenna array. From these measurements, the three-dimensional position of the PD source can be calculated using an iterative algorithm. The locating equipment is suitable for use within the vicinity of energized high-voltage plant and can locate sources up to 15 m from the array. Results are presented showing the location ability of the equipment under laboratory and field conditions. A significant advantage is the ability to detect PD sources in energized plant without the need for outages or electrical connections.

Radiometric location of partial discharge sources on energized high-Voltage plant

Because of the large capital value of power transformers and their critical role in the electricity network, there is an increasing need for non-intrusive diagnostic and monitoring tools to assess their internal condition. The UHF technique is being applied increasingly to monitor partial discharges (PD) in power transformers and has been shown to be much more sensitive than acoustic techniques when the signal path passes through solid insulation. Field trials were performed on five power transformers ranging in size from 18 to 1000 MVA and in voltage from 25 to 400 kV. The results of PD tests on power transformers provide sufficient evidence to justify making provision for UHF sensors on new transformers to facilitate their monitoring when required during the service lifetime.

Partial discharge monitoring for power transformer using UHF sensors. Part 2: field experience

The location of a partial discharge (PD) source inside a transformer can be determined from the time differences of arrival (TDOA) between signals that are captured by an array of UHF sensors. The TDOA can be acquired from the received PD waveforms. In this paper, three different methods of acquiring the TDOA from the PD waveforms are discussed. The time difference can be calculated either by taking the first peak of the signal as the arrival instant, or from the cross-correlation of the PD waveforms, or by applying the similarity function to the plots of the PD signals cumulative energy. Computation algorithms for determining the TDOA automatically are introduced so that possible bias from human interpretation is avoided. The presence of noise and its effect on the accuracy of the PD localization will also be presented. Experimental results show the first-peak method has higher accuracy than the two other methods. The application of signal denoising further improves the localization accuracy.

Partial discharge localization in transformers using UHF detection method

Investigations are reported into a range of defects causing partial discharge (PD) activity in high-voltage oil-insulated equipment. The aim is to explore the relationship between radiated ultra-high frequency (UHF) signals and electric charge in a PD current pulse for PD sources in oil insulation. Experiments are performed under high-voltage AC conditions using electrode configurations that included a sharp protrusion, a bad contact, and a free metallic particle. A broadband current transformer is used to measure the PD current pulses with the UHF signals being detected using a pair of sensors mounted inside a metal tank surrounding the PD sources. Current pulses and UHF signals were recorded simultaneously using an oscilloscope with a bandwidth of 3 GHz. The level of correlation between the energy of the radiated UHF signals and the charge content of PD pulses is shown to vary depending on the defect type. Polarity-dependent UHF spectral content and ratio of UHF signal energies measured at the two sensors are shown to exhibit characteristics dependent on the discharge behaviour. These results and the analysis procedures introduced will contribute to the development of effective UHF monitoring systems for oil-insulated high-voltage plant.

UHF and current pulse measurements of partial discharge activity in mineral oil

A new technique for analyzing partial discharge (PD) signals based on ultra-high frequency (UHF) measurements is presented, using results from site tests. Signals from simultaneously active discharge sources can be mapped according to their point of origin in a manner that will simplify the interpretation of phase-resolved discharge patterns. The scheme has the potential to allow automated location and tracking of multiple discharge sources in power transformers and other metal-clad high-voltage plants.

Applying UHF partial discharge detection to power transformers

Sensors operating in the ultra-high frequency (UHF) band have been used to detect internal discharges during site testing of a 1000 MVA oil-filled transformer. The transformer was operated at voltages exceeding normal levels by up to 15%. Two discharge sources were detected at both sensors using UHF test equipment. Discharge signals are analyzed in phase-resolved form and time-of-flight measurements are used to provide further information regarding their origin. The results indicate that the UHF method could prove very effective for transformer testing and online PD monitoring.

Power transformer monitoring using UHF sensors: site trials

Testing UHF partial discharge detection on a laboratory based power transformer

In this paper, partial discharge (PD) is generated using a range of electrode configurations in an oil insulated system. High voltage AC at 50 Hz is applied to each test configuration and the PD activity is recorded using an ultra high frequency (UHF) monitoring system. The PD signals are analysed in a phase resolved form, from which the corresponding statistical distributions are determined. The results indicate that different defect types can be distinguished on the basis of their phase-resolved patterns measured at UHF. Possible physical causes for the discharge patterns are discussed.

G.P. Cleary

Papers

UHF and current pulse measurements of partial discharge activity in mineral oil

Applying UHF partial discharge detection to power transformers

Power transformer monitoring using UHF sensors: site trials

Testing UHF partial discharge detection on a laboratory based power transformer

An investigation of discharges in oil insulation using UHF PD detection