Partial Discharges in Motor Wires at PWM Voltages of Different Smoothness

Author’s recent work has been focused on exploring possibilities for measuring partial discharges (PDs) under the action of pulse width modulated waveforms (PWM), which resulted in a development of an electrical measurement method adopted for such voltages. This solution allows a more flexible PD analysis since voltage shapes appearing in service can be utilized. In this paper, results are presented where several waveforms are applied to quantify the PD properties for motor wire test objects, starting from a non-filtered PWM waveform and continuing with gradually smoothened ones towards an AC voltage shape. The results suggest that non-smoothed PWM voltage introduces considerably more PDs and with larger magnitudes, which most probably influences the lifetime of the insulation system in all tested cases. Additionally, above a certain level of filtering, the use of lower carrier frequencies implies higher PD exposure, which suggests that the filters used should be evaluated together with the carrier frequency to ensure a longer lifetime of the insulation. It was also observed that the changes in duty cycle reduce the PD density at higher frequencies. This suggests that only applying square voltage waveforms with 50 % duty cycle for PD testing may fail to capture the actual stress inflicted

PD Properties when Varying the Smoothness of Synthesized Waveforms

The increased use of power electronic components in power systems makes it important to understand how rapidly rising voltages affect insulation systems. One vital aspect of this challenge is the measurement of partial discharges, PDs, which are considered as being a sign of weakness and can affect the life of insulation considerably. In this paper an approach is presented to measure PDs in a dielectrically insulated cavity when exposed to pulse width modulated (PWM) voltage shapes with different degree of smoothness. This is a continuation of our earlier investigations on the different behavior of PDs where voltages characterized by different rise times were applied. The present investigation shows that the PD amplitude decreases significantly already at a moderate level of PWM voltage smoothness to a magnitude that is about the same as for sinusoidal voltage shape. For the phase resolved PD (PRPD) pattern to become similar to the normal AC pattern it is required that the remains from PWM steps are lower than the extinction voltage. This work elucidates how PDs are affected by synthesized waveforms and limits for a sufficient smoothing level are found, which is of importance when designing insulation systems exposed to fast transients

Core taxa underpin soil microbial community turnover during secondary succession

Understanding the processes that underpin the community assembly of bacteria is a key challenge in microbial ecology. We studied soil bacterial communities across a large-scale successional gradient of managed and abandoned grasslands paired with mature forest sites to disentangle drivers of community turnover and assembly. Diversity partitioning and phylogenetic null-modelling showed that bacterial communities in grasslands remain compositionally stable following abandonment and secondary succession but they differ markedly from fully afforested sites. Zeta diversity analyses revealed the persistence of core microbial taxa that both reflected and differed from whole-scale community turnover patterns. Differences in soil pH and C:N were the main drivers of community turnover between paired grassland and forest sites and the variability of pH within successional stages was a key factor related to the relative dominance of deterministic assembly processes. Our results indicate that grassland microbiomes could be compositionally resilient to abandonment and secondary succession and that the major changes in microbial communities between grasslands and forests occur fairly late in the succession when trees have established as the dominant vegetation. We also show that core taxa may show contrasting responses to management and abandonment in grasslands.Soil microbial communities are compositionally stable across grasslands under secondary succession but differ from paired forest reference sites. Core taxa underpin whole-community turnover patterns, with the underlying assembly processes driven by site-variation in soil pH.imag

Mapping and assessment of forest ecosystems and their services - Applications and guidance for decision making in the framework of MAES

The aim of this report is to illustrate by means of a series of case studies the implementation of mapping and assessment of forest ecosystem services in different contexts and geographical levels. Methodological aspects, data issues, approaches, limitations, gaps and further steps for improvement are analysed for providing good practices and decision making guidance. The EU initiative on Mapping and Assessment of the state of Ecosystems and their Services (MAES), with the support of all Member States, contributes to improve the knowledge on ecosystem services. MAES is one of the building-block initiatives supporting the EU Biodiversity Strategy to 2020.JRC.H.3-Forest Resources and Climat

Stochastic Detection of Partial Discharges

The traditional method to detect partial discharges relies heavily on the use of high-pass filters. Whereas this is a very effective approach for normal power voltage shapes, severe problems arise when partial discharges are to be detected under waveforms generated by modern power electronics. Therefore, a technique that utilizes the random occurrence of partial discharges for detection has been developed and used for semi-square voltages. This technique has evolved since the first publications and new applications have been found. The present paper introduces the technique and discusses the necessary analysis elements, requirements imposed as well as some examples intended to show possible application areas

Partial discharge characteristics in motor insulations under exposure to multi-level inverters

The use of variable speed drives fed by pulse width modulated (PWM) inverters allows a more efficient use of electric energy. The drawback is however that the stress imposed on motor winding insulation increases and partial discharges (PDs) appear under such conditions, being considered the major contributor to the reduction in the insulation life time. We present in this paper changes observed in the PD characteristics of motor insulation systems aimed for different voltage levels. These were a twisted pair test object as well as a higher voltage level motor insulation wire, fed from either two- or three-level inverters. Each of the objects was subjected to PWM waveforms of different rise-times. The twisted pair test objects were insulated by a chromium oxide filled enamel, whereas the motor insulation objects were mica epoxy based. To compare the performances, measurements of extinction voltages and PD characteristics were conducted. Specifically, the total number of PDs and their amplitude per cycle were obtained at 2.5 and 4.5 kV(pp), dependent on the system tested. The experimental results showed that the total summed PD magnitude drops considerably when applying a three level inverter. However the maximum PD magnitude remain at the same level for the same rise time in both cases. Also the extinction voltage remains the same for a specific rise time. The PD magnitude decreases for increased rise time, similarly for both the cases. Thus the presented observations suggest that a change to a multilevel inverter alone may not be sufficient for increasing the motor insulation system life