W. Pfeiffer

Partial discharge. VI: PD testing of printed circuit boards

For pt.V see ibid., vol.7, no.2, p.28 et seq. (1991). The results of a research program concerned with applying partial-discharge (PD) testing to the nondestructive evaluation of the dielectric characteristics of coated printed circuit board (PCB) systems that must meet the high-voltage dielectric test requirements of low-voltage equipment are reported. To insure that the resulting data were applicable to typical PCBs a standardized test board layout was used, and the various coatings applied to the board were subjected to a range of conditioning treatments. The measurement and test specimens are described. The PD inception voltage and extinction voltage data for seven coatings and four conditioning regimes are presented, for either parallel conductor traces or pads adjacent to conductor traces, as measured from either the component or the solder side of the board. The data indicate that PD measurements, in combination with suitable conditioning, provide a good tool for determining the quality of coated PCBs. If applied correctly, the PD test is nondestructive.<<ETX>>

Partial discharge measurements for testing of low-voltage equipment

The application of PD (partial discharge) measurements to solid insulation within electronic components and assemblies in the low voltage range (e.g. optocouplers, small transformers, and printed wiring boards with coating) for the verification of insulation integrity, is reported. Such measurements are of special interest if such insulation is used to reduce electrical separation. In order to simulate the aging process during the normal life of equipment, different aging procedures were applied. The results of PD measurements after aging are compared with the data obtained before.<<ETX>>

Diagnostics of prebreakdown phenomena in SF/sub 6/ in non-uniform gaps for very fast transient voltage stress

This paper deals with the predischarge development in SF/sub 6/ in strongly non-uniform gaps stressed by very fast transient voltages (VFT) of positive polarity. In our investigations the shape of the applied voltage simulates the voltages that may occur in gas insulated substations (GIS) due to switching operations. Therefore a sinusoidal voltage of 2.6 MHz was superimposed to a voltage step. The maximum voltage amplitude was 215 kV peak. The parameters that have been investigated, are the gap width and the gas pressure. The gap width varied from 15 mm to 25 mm and the gas pressure from 0.1 MPa to 0.3 MPa. For diagnostics both, optical and electrical measuring techniques were used to analyse the prebreakdown and breakdown phenomena. Optical investigations were performed with ultraviolet- and infrared-sensitive photomultiplier tubes. For spatial resolved optical measurements of the predischarge development a high speed camera system with an exposure time of 1 ns was used. The results presented here show that the predischarge development in SF/sub 6/ in strongly non-uniform gaps stressed by very fast transient voltages is based on the leader mechanism, even for a gap width of only 15 mm. Depending on the parameters gas pressure and gap width the leader development changes. The transition from streamer to leader occurs according to the Stielbuschel mechanism at low gas pressure or the precursor mechanism at high gas pressure. Furthermore, an increase in the leader radius with increasing gap width was observed.<<ETX>>

Long time exposure influence of partial discharge (PD) to coated printed circuit boards

The authors deal with the behavior of solid insulation under the long-term influence of partial discharges (PDs). As a very recent example for a low-voltage component with solid insulation, namely, the coated printed circuit board was investigated. The measurements necessary to evaluate the long-term influence of PDs were performed by computer-aided test equipment. The measurement equipment, the test specimen, the test cycles, and the statistical methods used for the analysis are described. The data that were received during the test cycles were condensed by statistical methods. To evaluate the degradation mechanism, microscopic investigations were performed. A number of conclusions are drawn from the study.<<ETX>>

Computer aided partial discharge testing of electrical motors and capacitors

The present methods for testing of low voltage equipment, in particular electrical motors and capacitors are discussed. It is demonstrated that those tests realising high voltage overstress of the object tested cannot guarantee reliable performance of solid insulation throughout the equipment life. Partial discharge (PD) measurement as an alternative is a true nondestructive test method providing detailed information about the duality of manufacturing of the equipment tested and the selection of suitable insulation materials. As mentioned, above two different types of test objects have been investigated. The measurements have been performed by means of a computerised partial discharge measuring system with particular software features which simplify its application. Based on experimental results it is demonstrated that PD tests are of great importance for quality assurance of electrical motors and capacitors. Moreover it seems to be possible to predict the quality of the final product by precise PD investigations and measurements of the materials and components used.

Insulating characteristics of SF/sub 6/ and SF/sub 6//N/sub 2/-mixtures for very fast transient voltages (VFT)

The time lags in SF/sub 6/ and SF/sub 6//N/sub 2/ mixtures for gaps with strong nonuniform field distribution, stressed by very fast transient voltages (VFTs), were investigated. The shape of the applied voltages was used to simulate the voltages that can occur by switch operation in gas-insulated substations (GISs). Sinusoidal voltages with different frequencies of up to 20 MHz were superimposed on a voltage step. The maximum amplitude was 160 kV/sub pp/. It is shown that the dielectric strength is not only a function of the frequency of the VFT; all the other parameters of the gap, i.e. the gap distance, the pressure of the gas, and the admixture of N/sub 2/ to SF/sub 6/, have to be taken into account. It was found that, under certain conditions for a 20-MHz VFT, the dielectric strength is higher than for a 8-MHz VFT. In most of the experiments, however, increasing the frequency will decrease the breakdown voltage.<<ETX>>

Performance testing of coated printed circuit boards by means of partial discharge measurements

Within a research program on the insulation performance of low-voltage equipment, an experimental setup for partial discharge (PD) measurements was developed. Various types of coatings were evaluated in the research program. To obtain data close to those encountered in practice, a standardized test board layout was used. In order to simulate the aging process during the normal life of coated boards, different aging procedures were used. The results of PD-measurements performed on different test boards after aging are compared with previously obtained data. Results are presented for parallel conductor tracks and pads covered with different types of coating. It is concluded that POD measurements are a reliable method for testing the quality of coatings on printed circuit boards. Comparing the results of conditioning for parallel conductor track specimens, it is found that there is no significant difference in the effects of the applied types of conditioning.<<ETX>>

Simulation of discharge development in SF/sub 6/ and SF/sub 6/-mixtures for homogeneous and inhomogeneous field distribution

A model has been developed for the simulation of electrical breakdown phenomena in compressed gaseous dielectrics taking into account electron-neutral-gas collisions and collective interactions among charged particles. Elastic, exciting, ionizing and attaching collisions between electrons and neutral gas molecules are included using a Monte Carlo technique. Space charge effects are considered by solving Poissons equation for one dimension and two dimensions. A method is presented for simulating the predischarge development in multicomponent gas mixtures. Using this model, it is possible to analyze the effect of corona stabilization under impulse voltage stress. The ionization and attachment coefficients are determined by directly solving the Boltzmann equation.<<ETX>>

Partial discharge measurement for the investigation of solid insulation ageing [in electric machines]

The paper deals with the use of partial discharge (PD) measurements for the investigation of solid insulation ageing caused by voltage and temperature stress. Different insulating materials such as insulating foils, insulated wires, insulating tapes and flexible insulating sleeving used in manufacturing of electrical machines were tested. Partial discharge data collected during a certain recording time have been accumulated. Comparisons between degradation caused by voltage stress, degradation caused by thermal stress and degradation due to both factors are given and discussed.

Digital partial discharge measuring system with interference recognition

This paper deals with a digital PD measuring system especially designed for reliable high-speed testing of large numbers of test specimens. The measuring process is based on a broadband measurement (f/sub c//spl ap/100 MHz) of PD in the time domain followed by a high speed A/D converter (f=1 GHz). Then the signal is transformed to the frequency domain by FFT and the frequency characteristics of the measured pulses are compared with a standard-PD-spectrum that has been generated before. So the automatic distinction between interference pulses and PD-signals is possible. The measuring system is described and typical examples of measurements given. The capability of the system to distinguish between PD and interference pulses is shown.