Thomas Hammarström

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.

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.

Evidence for changing PD properties at short voltage rise times

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 to measure partial discharges, PDs, which are considered as being a sign of weakness and can affect the life of insulation considerably. In this paper we report on continuation of our earlier investigations on the different behavior of PDs when voltages characterized by different rise times are applied. Significant differences in PD characteristics are found, which indicate that the effect on the insulation system is dependent on the voltage wave shape. Applying square-like voltages to a cavity with dielectric insulated electrodes significantly affects the discharge amplitude, its rise time, the inception voltage and the distribution shape. The investigation shows that PD amplitude increases while PD rise time decreases for shorter voltage rise times, being indications of a possible change in the discharge mechanism. This in turn can lead to faster deterioration and reduction of service life and may therefore need to be considered when designing insulation systems exposed to fast transients.

Loss current studies of partial discharge activity

Partial discharge measurements have important applications in evaluating insulation systems as well as for dielectric materials studies. This work investigates a measurement technique to study partial discharge activity by measuring the PD loss current. The method is similar to dielectric response measurement and measures the current through a test object with and without PD. By comparing the dielectric current and the discharge current, all conduction processes, both fast and slow, can be explored. This is in contrast to traditional pulse PD detections, where only the fast PD transients have been measured. The method is demonstrated through measurements of PDs appearing in twisted pair and needle-plate test objects under sinusoidal and square wave voltage excitations. Comparisons are made to results of simultaneously employed traditional pulse PD detection technique.

Partial Discharge of Gel Insulated High Voltage Power Modules subjected to Unconventional Voltage Waveforms

Performances and duration of the new generation of high voltage power electronic components are dependent on dielectric materials aim to insulating their internal terminals. The presence of defects, some due to faults generated during the manufacturing process, but also due to the internal design of layers and connections, can cause local enhancements of electric field and consequently possible activity of partial discharges phenomena or other effects (aging, tracking) that may result in reduction of device reliability. Furthermore, the usage of unconventional voltage waveforms, like square waves or pulse width modulated waves, additionally increases the electrical aging of the insulation system as compared to conventional sinusoidal waves.

Partial Discharges in Motor Wires at PWM Voltages of Different Smoothness

Authors 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.

Partial discharge behavior of a newly developed enamel insulation at various voltage rise times

Investigations of partial discharge (PD) characteristics in twisted pairs are presented. The test objects were made of conventional and chromium oxide filled enameled wires and were exposed to pulse width modulated (PWM) voltages of different rise times. To compare the performance of each insulation system, measurements of both PD inception and extinction voltages were conducted. Additionally, the number of PDs per cycle and their amplitude at different voltage levels were determined. Experimental results showed that the total PD exposure was considerably reduced for the chromium oxide filled enamel insulation as compared to that of the conventional material at the shortest voltage rise time. However, for filtered PWM waveforms with relatively longer rise times this difference became less prominent. The analysis of the results indicated that the observed effects are determined by the presence of chromium oxide filler and its electrical properties.

Partial discharge characteristics within motor insulatioi exposed to multi-level PWM waveforms

The frequent use of variable speed drives fed by pulse width modulated (PWM) inverters allow for a more efficient utilization of electric energy. The drawback is however that the stress imposed on motor winding insulation increases and partial discharges (PDs) may appear under such conditions, being considered the major contributor to the reduction of the insulation life time. This paper presents what influence the choice of PWM level and rise time has on the PD characteristics for two different motor insulations as well as introducing suitable high voltage generation test-set ups for these kinds of measurements. The two different test objects were twisted pair and a pig tail motor insulation test objects, the latter aimed for higher voltage levels. These test objects were fed from either two-, three-, four- or five level inverters of similar type, as often used in actual applications. Additionally a six level inverter was used for verification purposes. The twisted pair test objects were insulated by a polyamide-imide enamel, whereas the motor pig tail insulation objects were mica-epoxy based. To compare the performances, measurements of the PD characteristics are reported and observations concentrate on the importance of the voltage rise time, on the size of the voltage step in relation to the extinction voltages (PDEV). Specifically, the total number of PDs and their amplitude per cycle is obtained. The experimental results show that the total summed PD magnitude (exposure) drops considerably when applying the three- or higher level inverters. However the maximum PD magnitude is less dependent, which indicates that a change to a high level inverter alone may still not be sufficient condition for increasing motor insulation system life. In conclusion a combination of the PDEV level, rise time and PWM level must all together be considered in this complex design process.

Partial discharge characteristics of electrical treeing in XLPE insulation exposed to voltages of different rise times

The use of pulse width modulated (PWM) waveforms allows a more flexible energy applications and management. The downside is that under such operating conditions the voltage stress imposed on the insulation systems increases, particularly as the frequency content is considerably higher than at the conventional 50 Hz sinusoidal waveform. Among various degradation mechanisms, electrical treeing is one important processes and it can be linked with a presence of partial discharge (PD) activity. In this paper, the changes in PD characteristics at different stages of degradation by electrical treeing were investigated for XLPE based insulation exposed to rapidly changing voltages of varying magnitude. The tested material samples were subjected to two square voltage shapes, each characterized by their rise-time. Here the shorter was 1 ps and the longer 0.5 ms, all tests run at 414 Hz. To compare the voltage endurance, the voltage level was gradually increased until detectable PD activity and tree initiation could be observed, both electrically and optically. Typically continuous PD appearance was observed between 23 and 32 kVpp. The total number of PDs and their characteristics were monitored as the trees were gradually growing. Here the experimental results shows that the PDs appears at lower voltage magnitudes for the shorter rise times and that the trees had a significantly less branched appearance as compared to the longer voltage rise time. These results resemble the observations that recently been presented with tests on DC voltage pre-stressed material samples exposed to impulses of reversed polarity. Additionally the measured PD characteristics were non-symmetric, having a clear polarity dependence. This finding suggests a possibility to use the presented approach for a more detailed investigating of degradation processes in solid insulation systems suitable for both HVAC and HVDC applications.

Study of partial discharge activity by excess current

Measurements of partial discharges (PD) play an important role in study of dielectric materials. This work applies a measurement system based on dielectric response technique to study an excess current associated with PD activity, which appears during and after PD activity and shows some prominent properties. The value of such a measurement is demonstrated through studies of two types of test objects, twisted pairs and cavity specimens. A simultaneous use of pulse PD detection in the study shows a correlation between changes in PD amplitudes and non-PD excess current levels. It is concluded that the used methodology can serve as an important complement to the traditionally used pulse PD detection.