Tord Bengtsson

Influence of rise time on partial discharge extinction voltage at semi-square voltage waveforms

This work presents measurements of the partial discharge (PD) extinction voltage in three different types of test objects, using semi-square voltages with 2 ¿s and 100 ¿s rise time. A needle creating corona discharges, a twisted pair specimen commonly used for testing motor insulation and a paper/oil test object modelling the turn-to-turn insulation of a transformer winding were investigated, presenting extinction voltages between approximately 1 kV and 8 kV. For the twisted pair specimen the PD extinction voltage was significantly lower at the shorter rise time, whereas for the other test objects, just a small or no difference could be detected. The conclusion is that depending on what range of rise times and what insulation system that are studied, there may be an influence of the rise time of the applied voltage on the PD extinction voltage.

Dielectric response measurements utilizing semi-square voltage waveforms

Dielectric response measurements belong to the group of diagnostic tools used for quality evaluation of high voltage insulation systems. A measurement system called arbitrary waveform impedance spectroscopy (AWIS) is presented here which is capable of utilizing the harmonic content of repetitive voltages with a semi-square waveform and the corresponding current response to perform dielectric response measurements. Extensive circuit modeling and calibrations are required in order to perform accurate measurements. AWIS could offer new possibilities, especially for continuous monitoring of the dielectric properties of high voltage components and systems. The accuracy of the technique in both low and high voltage applications is verified by comparison with results obtained by means of the frequency domain spectroscopy (FDS) technique.

Dielectric heating in insulating materials subjected to voltage waveforms with high harmonic content

Dielectric heating is one potential aging mechanism active below partial discharge inception voltage in materials used as high voltage insulation. When exposed to voltage waveforms containing high amount of harmonics, the heat generation will be larger due to increased power losses as compared with power frequency excitation. This may result in a decreased life or even failure of insulation due to the increased operating temperature or to thermal runaway. An analysis of the power developed due to dielectric heating in two different materials subjected to voltage waveforms with high harmonic content is presented in this paper. By expressing the non-sinusoidal loss as an enhancement factor to the sinusoidal one, a geometry-independent formalism is derived. From dielectric response measurements at low voltage and at several temperatures the dielectric power loss in the material can be calculated for different voltage levels and waveforms. Two important material parameters can be extracted from the calculated dielectric power loss: (i) non-sinusoidal loss compared with sinusoidal loss with the same fundamental frequency (pfact) and (ii) change of loss with changing temperature (dpfact/dT). These two parameters could potentially be used to indicate the suitability of materials for use in applications where voltage waveforms contain high harmonic content.

Measurement of partial discharges at rapidly changing voltages

This work presents a measuring system for PDs at repetitive voltages with short rise times, based on a coupling device adapted for this kind of voltages and a method for removing contributions from the applied voltage. The potential of the system has been demonstrated through measurements of PDs around a needle in air at voltages of 5.5 kV - 9.5 kV with rise times down to 10 mus. The observations obtained with this PD source prove that the measuring technique described enables detailed studies of PD properties under repetitive rapidly changing voltage waveforms in the investigated range of voltages and rise times.

Repetitive fast voltage stresses-causes and effects

In this article, we have seen how high-frequency system resonances, triggered by power electronic devices, may enhance both electrical and thermal stress on an insulation system. An important lesson from this study is that even an insulation system that is very well designed according to present standards may fail if it is subjected to stresses that were never considered in the design. The root cause of such failures is thus not within the failed part, and it can only be found by studying the entire system where the failed part is used. With the examples given here, we hope that the reader may more easily recognize situations which may warrant a deeper study and perhaps some extra precautions.

Evaluation of a PD measuring system for repetitive steep voltage waveforms

Measurements of partial discharges (PDs) at high frequency voltages is important but not a straightforward task. In this paper a PD measuring system developed for use at repetitive, steep voltage waveforms is presented, which is based on electrical detection using capacitive decoupling of the PD signals. The system utilizes only a low order filter and allows the presence of significant remnants of the applied voltage in the signal used for PD detection. Analytical calculations that estimate the reachable sensitivity of this method are presented, and for an example with a test object of 100 pF capacitance, a sensitivity in the range of 1 - 10 pC/kV at 1 μs rise time was obtained. Further, the transfer function of the measuring system has been measured and used for reconstruction of the voltage across the test object, the latter consisting of both the applied square-like voltage and voltage drops due to PDs. This is an attractive possibility since the applied high voltage signal at the test object can be recalculated from the same signal as is used for PD detection. In addition, the reconstruction of the PD voltage drop could provide information about the PD processes, provided that the sampling frequency is high enough.

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.

Noise and Aliasing Aspects in a Multiharmonic-Dielectric-Response-Measurement System

Dielectric-response measurements are commonly performed with frequency-domain spectroscopy, polarization/depolarization-current measurements, or return-voltage measurements. These techniques operate in a frequency or time domain, and all have high requirements on the voltage source in order to acquire accurate results. This limits dielectric-response measurements to offline applications. A new technique, which is called arbitrary-waveform-impedance spectroscopy, has been developed, which makes use of the harmonics of any voltage waveform to perform dielectric-response measurements. The technique provides possibilities for online measurements facilitating the monitoring of materials and components in high-voltage applications. Here, the different aspects of the measurement system are presented, including circuit modeling, normalization, and discussions on aliasing and noise; all of them are necessary to control in order to perform accurate measurements.

Barrier Effects in a Rod/Rod Air-Gap Under DC Voltage

The effects of dielectric barriers on partial discharge and breakdown characteristics in a 50 mm rod/rod air-gap have been investigated under DC voltage. The results show that a barrier with large diameter and situated in a favourable position leads to a significant improvement in PD characteristics and a remarkable increase in the breakdown voltages. For example, the breakdown voltage of the air gap with a barrier of diameter 400 mm is 3 times as large as that without barrier. The effects of dielectric barriers depend on DC voltage polarity, size of barrier and position in the gap.

PD-source identification in solids

It is of great importance not only to detect but also to localize and identify the source of partial discharges (PDs). The intention is to see if there is any potential in identification of defects using an acoustic technique aided by a general theory. The idea is that PDs embedded in a solid material would excite different wave modes, depending on the structure of the defect. Standing waves or cavity resonances of different orders will then result in a different frequency spectra, depending on the geometry of the defect. The authors present some general arguments for the acoustic mode density analysis in flat, hole, or spherical cavities. The investigation demonstrated that the normal resonant acoustic modes of a gas-filled cavity were excited by partial discharges. There was a strong correlation between the frequency spectra experimentally obtained and the resonant modes predicted by the theory. It is concluded that the frequency spectra from acoustic detection of PDs can give information on the form of cavity in which it occurred.<<ETX>>