Traditionally high electrical stress of insulation occurs in HV equipment. Usually this stress is 50/60 Hz AC and rather seldom DC. Exceptional situations may occur in RF transmitters. This is very different in low voltage equipment. Due to the increased use of high-power semiconductors with high cutoff frequency, working voltages of rather high amplitudes and with frequencies >100 kHz occur frequently. Also, the distances through solid insulation may be rather low resulting in electrical fields in excess of those being typical for HV equipment. In combination with such HF working voltages, this means an extraordinary stress for the insulation. As there is a strong economical interest in further miniaturization, those problems have to be analyzed more precisely. The most important data about HF stress of insulation are summarized in this paper. >

High-frequency voltage stress of insulation. Methods of testing

Traditionally high electrical stress of insulation occurs in HV equipment. Usually this stress is 50/60 Hz ac and rather seldom dc. Exceptional situations may occur in rf transmitters. This is very different in low voltage equipment. Due to the increased use of high-power semiconductors with high cutoff frequency, working voltages of rather high amplitudes and with frequencies > 100 kHe occur frequently. Also, the distances through solid insulation may be rather low resulting in electrical fields in excess of those being typical for HV equipment. In combination with such hf working voltages, this means an extraordinary stress for the insulation. As there is a strong economical interest in further miniaturization, those problems have to be analyzed more precisely. The most important data about hf stress of insulation, which are readily available until now, are summarized in this paper.

http://ieeexplore.ieee.org/iel1/14/2583/00078324.pdf

High-frequency Voltage Stress of Insulation

A report about the behaviour of inhomogeneous air gaps stressed by high frequency voltages was published by Kampschulte and Lassen in 1930. The measurements had been performed within a frequency range of 50 Hz up to 100 kHz. It was the aim of that investigation to find out dimensioning criteria for the design of high power rf-transmitters. These investigations have already shown that the breakdown voltage of inhomogeneous gaps decreases significantly with the increase of the frequency. The values of the breakdown voltages even for those comparatively low frequencies may be only 50% of the 50 Hz breakdown voltages. Due to the continuous reduction of size of electrical equipment the field strength and therefore the electrical stress of installation has greatly increased. The probability of the occurence of PD is also increasing. Today high frequency voltage stress often occurs at printed circuit boards (PCB) due to the increasing use of converter circuits with high switching frequencies. This paper deals with the influence of the frequency on the PD inception voltage (U/sub i/) and PD-intensity on PCBs for a frequency range up to 400 kHz. The investigations are performed for normal climatic conditions. Additionally point-plane air gaps are investigated to evaluate the influence of the frequency on the PD-characteristics for very inhomogeneous fields.

Influence of the frequency on the partial discharge inception voltage

High voltage devices and systems operating in space environment, which is considered as a partial vacuum environment, are usually subject to partial discharges or corona discharges, which are major design problems. The corona or partial discharge initiation is a function of several design and environmental parameters. The most important factors can be listed as the operation pressure, the electrode geometry, and the frequency and voltage level of the applied power within a power system. Initial studies conducted in the laboratory suggest that the corona breakdown voltage levels at high frequency are indeed much lower than the dc breakdown voltages for varying pressures. Thus, this presentation will review the data available in the literature that can be readily implemented in the design of these power systems. Furthermore, It will also discuss current research work in high frequency break down in partial pressure environment.

Electrical insulation in space environment

The capabilities of modern materials and new technologies allow significant size reductions to be achieved particularly in the field of low-voltage equipment. The limits encountered are often due not so much to technical or physical factors, but to widely varying safety requirements determined in many cases by earlier technological capabilities and the limits and specifications based thereon. This paper reports further results obtained from an extensive research project. Exposure of a large number of test specimens to various environmental conditions has proved conclusively that spacings on the surfaces of insulating materials are affected only if contamination is significant. Under heavy contamination these so-called creepage distances form tracks leading sooner or later to failure. Comparatively clean creepage distances however are not affected by tracking and should therefore only be dimensioned on the basis of their short-term electric strength. The results of this extensive series of tests are a statistically verified basis for the dimensioning of rather clean, small creepage distances (>10 mm). They can be incorporated directly into the relevant standards. Other results from the research project will be reported on conclusion of current investigations. However, it is expected that this work will not be completed before 1988.

Electric Strength of Small Insulating Distances

The electric strength of insulating configurations of solid insulating materials mainly depends on the contamination of the creepage distance, the formation of creepage paths on the insulating material by creepage currents, and the micro-climate on the insulator surface. For the intended smaller and more precise dimensioning of insulation configurations for low voltage equipment, exact knowledge about the influence of these factors on the withstand voltage is necessary. Therefore test specimens which are made from insulating materials with different comparative tracking index were investigated. The test specimens have been exposed for some months under natural environmental conditions. During the period of exposure they were connected to ac or dc voltages in the 10 to 660 V range. The laboratory measurements of the withstand voltage are carried out with impulse voltage of the shape 1.2/50 ?s under well-defined climatic conditions. According to the measured withstand voltages, new dimensioning rules for creepage distances are proposed.

K. Richter

Papers

Electric Strength of Small Creepage Distances under Natural Environmental Conditions

Electric Strength of Small Insulating Distances