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.

Wax-formation in electrical equipment is usually associated with oil impregnated cables and capacitors. Its formation is generally attributed to high field strength and partial discharges. In context with power transformers x-wax only appeared in the bushings. In recent years, however, there was an increasing number of failures in hermetically sealed power transformers with wax formation in the high voltage windings. Wax formation in transformers may block the cooling ducts and lead to overheating. Especially compact power transformers with synthetic ester and silicone fluid seem to be affected. This investigation sought the necessary conditions under which x-wax is formed in power transformers and how these conditions differ for different types of insulation liquids. A laboratory model of the high voltage insulation of a distribution transformer is used to investigate the influence of temperature, partial discharges (PD) and pressure on the x-wax formation. In hermetically sealed transformers the internal pressure can change depending on the load and ambient conditions. Previous investigations have shown that small reduction of the internal pressure can significantly reduce PD inception voltage and increase the apparent charge of the PDs. This is a transient situation that may happen in wind turbine transformers when the transformer cools down during a calm phase. For this study, different insulation fluids, such as synthetic and natural esters, silicone fluid and mineral oil are stressed with partial discharges for 200 hours at various temperatures both at ambient pressure and at a reduced pressure of approx. 800 mbar. The partial discharges are monitored to allow a comparison between the PD energy and the amount of wax formed during the experiment. The overall behavior of the PD was quite different with the different liquids, requiring an increasing voltage to keep igniting in the case of natural ester. Dissipation factor, permittivity, DC resistance and breakdown voltage of the fluids are measured before and after the PD stress to see if these can be used as indicators for wax generation. The investigation shows, that, when stressed with partial discharges over extended periods of time, all insulation fluids form some sort of solid ageing product, which are being investigated and may or may not be classified as x-wax. The amount of solid and the conditions under which they are formed differs greatly according to type of the fluid.