Steep voltage transients from power converters and vacuum circuit breakers result in increased electric stress and risk of insulation degradation in transformers. This paper presents a time-to-breakdown study for impregnated pressboard using a plane-to-plane electrode set with circular razor-sharp upper electrode on a thin pressboard sheet. The partial discharge repetition rate was measured for each sample by stepwise increasing the voltage at 60 second intervals with sinusoidal or switched voltages, respectively. The PD repetition rate for switched voltage was much higher compared to sinusoidal voltage. Voltage endurance tests were then performed at several voltage amplitudes for both sinusoidal and switched bipolar voltage. The result shows that the time-to-breakdown for switched voltage compared sinusoidal voltage was reduced by a factor of more than 105 with equal peak voltage and frequency. This difference is explained by the creation of a significant amount of space charges around the sharp electrode that act as homocharge and reducing the field for sinusoidal voltage, while it acts as heterocharge and strongly enhance the field for polarity reversal at switched voltages. These experiments show that care should be taken when introducing fast voltage changes to transformer insulation systems such as impregnated pressboard.

Voltage-time (V-t) curves for breakdown are important in insulation coordination for design of transformers. The time to breakdown for a liquid insulated gap is determined by the velocity of a breakdown streamer and the distance it must propagate to short circuit the insulation. Streamer velocities vary with applied voltage, and for commercial insulation liquids the voltage at which the streamer accelerate to very high velocities vary a lot. The processes behind this acceleration are not yet determined. It seems clear that streamer propagation is governed by the high field within the liquid phase in front of the propagating plasma channel. In the context of new results on measured space charge limited currents for several liquids, this paper will discuss how the electric field in front of a streamer may be limited by space charges, suppressing the possibility of the field to increase to a level where propagation will accelerate. Lack of space-charge limiting the field in front of the streamer may explain the early acceleration voltage that esters show compared to mineral oils.

Negative streamers with an applied step voltage in an 80-mm point-to-plane gaps in transformer oil have been studied with a fast 16-frame camera. The negative streamers propagated for around 200 µs – 300 µs before breakdown occurred or the streamer channel disappeared. The streamer velocity was approx. 0.4 mm/µs. For some of the captured images, we have seen that faster positive streamers have propagated in the opposite direction from the propagating negative streamer head. This clearly shows there is little or no contact back to the electrode behind the head of the propagating negative streamer, and a positive field towards the electrode must exist, likely due to charge separation in the streamer head.

Paper explaining measured conduction currents iin a needle to plane gap in a mineral oil and an ester liquid when applying a high voltage. There are large differences between mineral oil and ester.

Preprint of submitted article to TDEI. Compare partial discharge activity at different voltage shapes on aluminium nitride substrates embedded in silicone liquid and gel.

Electrification of society, increasing renewable energy sources and mobility charging lead to new loading patterns for power transformers. Dynamic load conditions induce enhanced mechanical stress on the transformers’ windings, potentially causing degradation of the solid insulation over time and compromising the transformer's short-circuit withstand capability. Thermal expansion of the windings, caused by losses in the copper conductors, occurring as the transformer is load-ed, increases the stress. Conversely, magnetic losses in the core and tie plate expansion contribute to a reduction in stress. This paper presents the effect of step changes in core losses and copper losses by on-line measurements of the clamping pressure, to better understand the mechanical stresses acting upon the solid insulation cellulose materials.

During manufacturing of a power transformer, a fixed clamping pressure is exerted on the winding structure to prevent the winding from buckling due to a short circuit fault. Over the long service life, the transformer is subjected to different loads. The load current influences the clamping pressure in many ways: essentially by repetitive mechanical stresses of the cellulose materials (e.g., pressboard, paper etc.) due to thermal expansion of copper winding and by continuous moisture exchange between the cellulose materials and the insulation oil due to their temperature variations. Although the presence of moisture in cellulose may temporarily increase the clamping pressure, it exacerbates the ageing process. As such, the mechanical properties of pressboard may degrade over time, resulting in a loose clamping structure where the mechanical integrity of the transformer may jeopardize. This paper reports the experimental setup of a test rig to investigate the effect of load variation on the development of clamping pressure. To do so, a stack of dry pressboard samples and heatable brass plates are clamped together and is placed in transformer oil. The heat-flow in the brass plates is controlled, thus emulating different winding losses. Effect of three different loads, i.e., the losses in the winding is controlled to investigate its effect on clamping pressure. The thermal expansion of the brass plates is observed to strongly influence the clamping pressure in a short time scale. Although long term evolution of the clamping pressure is not investigated yet, nonetheless, no significant change in clamping pressure is observed once the temperature of the rig comes back to its initial room temperature.

The electrification of society, increasing renewable energy sources and mobility charging lead to new loading patterns for power transformers. Dynamic load conditions induce enhanced mechanical stress on the transformers’ windings, potentially causing degradation of the solid insulation over time and compromising the transformer’s short-circuit withstand capability. Thermal expansion of the windings, caused by losses in the copper conductors, occurring as the transformer is loaded, increases the stress. Conversely, magnetic losses in the core and tie plate expansion contribute to a reduction in stress. This paper presents the effect of step changes in core losses and copper losses by on-line measurements of the clamping pressure, to better understand the mechanical stresses acting upon the solid insulation cellulose materials. Energisation is found to decrease the clamping pressure following warming up of the transformer, and loading the transformer increased the pressure as the windings increased in temperature. The converse effect was found when unloading and de-energising. The on-line monitoring system provides a new and important step towards ensuring the short-circuit performance of power transformers.

Partial discharges (PDs) on liquid impregnated pressboard insulation have been investigated for sinusoidal and switched voltages for two different liquids – one mineral oil and one synthetic ester. The test arrangements used were a rounded electrode resting on a pressboard plate on a plane electrode representing semi-uniform field, and a sharp-edged conical electrode representing divergent field. The main result shows that the PD inception voltage (PDIV) is considerably lower at switched than at sinusoidal voltage. Moreover, the mineral oil showed a lower inception voltage than the synthetic ester. Finally, a lower PDIV for the sharp electrode compared to the spherical electrode was seen. The differences can be explained by formation and transport of space charges. The space charges will shield the high-field regions when having the same polarity as the voltage and then enhance the electric field right after a voltage polarity reversal. There is minimal time available for charge drift at polarity reversal of switched voltage. Contrary, for sinusoidal slowly varying voltage there is plenty of time for the charges to drift before the opposite voltage peak occurs. Thus, less field enhancement from space charges under sinusoidal voltage. The field enhancement due to space charges of opposite polarity is enhanced for liquids with higher injection current, such as mineral oil.

This article reviews the functional requirements, dielectric performance, and test standards of insulating liquids from a transformer design perspective.

The objective of this article is to characterize partial discharge (PD) inception voltages and patterns at sinusoidal and square voltages in insulating ceramic substrates with generic manufacturing defects. PD tests were conducted on active metal brazed (AMB) aluminum nitride (AlN) substrates either with cavities in the polymer coating or sharp protrusions of the metal brazing. The substrates were tested in silicone liquid instead of silicone gel. Light sensitive imaging was used to localize the PD sites on the surface. PDs at square voltage with rise time of $1.6 ~\mu \text{s}$ were measured with a high-frequency current transformer and a photomultiplier. In both substrate types, the peak–peak value of the PD inception voltage (PDIV) was higher at sinusoidal voltage compared to peak-peak value of the square wave. This difference was more pronounced in uncoated substrates, where discharges incepted at sharp edges surrounded by silicone liquid. For both types of defects, comparable PDIVs were measured at bipolar and unipolar square waves. Results for unipolar pulses are discussed considering the role of charge memory effect.

Partial discharge (PD) characteristics in aluminum nitride (AlN) insulating substrates at sinusoidal and switched voltages have been investigated. The substrates were tested in silicone liquid. PDs we observed both at sharp edges of the copper conductor and inside the ceramic. Phase-resolved PD patterns at sinusoidal voltage were analyzed for both defects. Optical measurement with a photomultiplier tube (PMT) was used to characterize discharges occurring at the surface of the substrates at switched voltages. The average PD inception voltage (PDIV) at sinusoidal voltage counted 8.5 $\mathrm{kV}_{\mathrm{p}}$ in contrast to 5 $\mathrm{kV}_{\mathrm{p}}$ and 6 $\mathrm{kV}_{\mathrm{p}}$ at negative and positive unipolar square pulses respectively. As shown with bipolar pulse tests the PD inception was determined by the peak-to-peak voltage.

Conduction currents have been measured in neat n-Tridecane under stepped voltage with a 50 ns rise time in a point-plane geometry with a 3.5 μm point radius. Currents are observed under both polarities with similar onset threshold. Larger currents are recorded under negative polarity. The shape and polarity dependence of the charge recordings indicate that the charge is generated in the bulk, but charge injection from the needle electrode cannot be ruled out.

This paper presents a novel system for measuring the clamping pressure of transformer windings. The clamping pressure is essential for the integrity of a transformer during short circuit events. The introduction of new loading patterns, increased dynamic loading, may accelerate cellulose degradation and reduced clamping pressure. Direct measured clamping pressure and temperatures together with physical models will facilitate condition-based maintenance and digital twins. The paper explains how the sensors work and presents data from a heat run test as well as in-service conditions. It is demonstrated how the clamping pressure changes with the transformer loading and temperature.

Photomultiplier tubes (PMT) in combination with flexible liquid light guides and lenses have been tested for partial discharge (PD) detection. The coincidence technique was used to eliminate the random noise of PMTs. Results of PD measurements in two different insulation systems, namely a needle-plane arrangement in air and a ceramic substrate in silicone liquid, are presented and discussed.

Detecting partial discharge (PD) sources in transformer bushings under service conditions is challenging as it requires separation of different PD sources from each other and from external noise. In this study, a new method for PD detection in transformer bushings was tested both in a laboratory setup and on a 132/49 kV transformer in service. The bushings were of resin-bonded paper (RBP) type. Both conventional (IEC 60270) and non-conventional PD integration settings were used. The PD measurement impedances were connected both directly to the bushing measurement taps and differentially between the measurement taps of each of the three bushing phases. The differential measurements allowed location of the PD sources to specific bushings. The results show that the differential PD measurement method can be a useful technique for condition assessment of transformers and other multi-phase high voltage apparatus energized from the grid.

Paper insulation on transformer windings may contain moisture, from aging or moisture ingress through breathers. A sudden overload will cause the insulation to heat much faster than the oil, and the moisture forced out of the insulation may not dissolve in the much colder oil, with bubbling as a result. Several studies have addressed bubbling temperature depending on winding moisture, aging state or pre-overload temperature, but some results are conflicting between studies. A setup has therefore been built to study bubbling. One heating system circulates oil with “pre-overload” temperature of up to $120^{\circ}\mathrm{C}$ . Moisture level is controlled by circulating through a cellulose reservoir with cellulose of known moisture. Another system heats one paper covered electrode controlled from $20^{\circ}\mathrm{C}$ to $200^{\circ}\mathrm{C}$ in about 15 minutes. Shock heating with a temperature increase of $80^{\circ}\mathrm{C}$ in 30 seconds is possible. Bubbling can be detected optically through viewports, or by resulting discharges if applying 20 $-\boldsymbol{30\text{kV}}$ between the covered electrode and a high voltage electrode, with an adjustable gap of normally 2mm. The covered electrode is insulated from ground and has a connection for current or discharge measurements. The setup has been tested with low density pressboard with 6% moisture, resulting in a bubbling temperature of $163^{\circ}\mathrm{C}$ , which other studies with paper only have found for about 1% moisture. Presently, the setup does not have adjustable pressurization, but it is made for easy expansion with pressure control, both higher than and lower than normal atmosphere.

This study addresses the dielectric performance of nonpolar hydrocarbon liquids and mineral oils under negative polarity stress. Stopping length for non-breakdown streamers, breakdown voltages and velocities for various pre-breakdown streamer modes have been studied for a selection of model liquids (cyclohexane and white oils), for a gas to liquid oil, and a refined naphthenic transformer oil. Studies of propagation modes were done using an 80 mm point to plane gap and a step voltage with 0.5 mu s rise time. Light emission and pre-breakdown currents have been recorded and instantaneous velocities have been derived from images of propagating streamers. Compared to positive polarity, there are less differences in streamer behaviour in the oils examined under negative polarity. Breakdown voltages and acceleration voltages are higher for negative streamers than for positive ones, while their propagation velocities are lower. While propagation modes for positive voltages are quite distinct, the mode changes for negative ones are more gradual. The behaviour of both positive and negative streamers is in line with the hypothesis that the propagation is governed by electron avalanches and quantum chemical properties of liquid components.