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Science and Technology article citation info: IntroductionLarge power transformers are the most critical component in electric power systems, as they are essential in maintaining a reliable supply of electric energy. There are many factors which cause a transformer malfunction, but those, which can potentially lead to catastrophic failure are winding damages (due to short-circuit, lightning, and other over-voltages) and insulation system failure (moisture, thermal aging, partial discharges). The damage from a catastrophic transformer failure may run into tens of millions of dollars [16]. To avoid such a scenario, power utilities are moving towards continuous transformer condition monitoring, based on dissolved gas analysis and acoustic emission (AE) or electromagnetic (HF/VHF/UHF) partial discharge detection.According to the newest research results and analyses presented by the experts of the CIGRE Working Group A2.37, in the technical brochure 642: Transformer Reliability Survey, the main reason of breakdowns of high voltage power transformers is damage to the windings and the main insulation system [3]. Mechanical defects in the form of winding deformations (axial displacements and radial deformations) and deterioration of insulation properties associated with thermal aging processes [6], can lead to the initiation of the partial discharge (PD) phenomena occurrence.In recent years, in the electric power industry and research centres, a trend consisting in developing and implementing advanced

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: The current power cables IEC standards do not provide adequate recommendations for after-laying testing and diagnosis of offshore export and inter-array power cables. However the standards IEEE 400 and IEEE 400.4 recommend partial discharge monitored testing, e.g., by continuous or damped AC voltages (DAC). Based on the international experiences, as collected in more than 20 years at different power grids, this contribution focuses on the use of DAC for after-laying testing and diagnosis of submarine power cables both the export and inter-array cables. Higher risk of failure, long unavailability, higher repair costs, and maintenance costs imply that advanced quality control is becoming more important. The current state of the existing and drafting international standards are based on onshore experiences and not related to the actual serious problems experienced with failures on export up to 230 kV and inter-array cables up to 66 kV. The application of damped AC as a testing solution in this concern is specially discussed. The advantages of this testing technique, in combination with actual testing examples, show the findings on export and inter-array cables at offshore wind farms.

In this paper an electrical partial discharge (PD) localization technique for a dry type power transformer is presented. The method has been tested in the laboratory on real transformer coil as well as on a transformer in a test bay.Calibration signal is injected through a plate sensor placed along the winding on the surface of the transformer coil in different positions, thus PD signals can be measured at both coil terminal sites using digitizer. Measured PD signal during test can be then evaluate and compared with calculated transfer function. The described technique enables not only localization of the PD source also determination of the apparent charge. This evaluation can be more accurate than conventional PD measurements by use of quadruple and coupling capacitor calibration and can be used during manufacturing process as well as diagnostic in field.