Frank Berger

Experimental Investigation of the Interaction of Interrupting Arcs and Gassing Polymer Walls

Gassing polymer walls are used in circuit breakers to improve their interrupting and current limiting performance. The energy of the arc will be partially absorbed by the polymer walls, causing chemical degradation and evaporation. The evaporated gas reaches the arc plasma changing its composition and influencing its burning conditions. The resulting increase on pressure and arc voltage contributes to current limiting and reduction of arcing time. This paper deals with experimental results of the investigation of the interaction between the arc and gassing polymer walls. The simultaneous detection of pressure in the arc chamber, of arc current and voltage and the use of high speed photography and SEM analysis contribute to the characterisation of the influence of the polymer gases on the arc behaviour. The results of the ex-periments are used to verify numerical models.

Interaction of pressboard with impregnating fluids in HVDC insulation systems

Pressboard is one of the key components in insulation systems for HVDC (high voltage direct current) equipment. The conductivity of pressboard is one of the most important properties of these systems. It is dependent on various parameters, e. g. time, temperature, electrical field strength, water content and impregnating fluid, which are varied in this paper. Pressboard manufacturing is described and results of conductivity measurements of pressboard according to the PDC method (step response measurement in time domain) are presented. In order to come to a physical understanding of pressboard conductivity in transformer insulation systems, conductivity of dry pressboard is examined under vacuum. Low electric field strength leads to a restriction in parameter variation (Paschens law) and therefore, pressboard impregnated with dry air is investigated. No significant influence of impregnating air on pressboard conductivity is found. Moreover, pressboard is impregnated with mineral oil and the change in pressboard conductivity by impregnating with different fluids is shown. Polarization and conduction processes in insulation systems are taken into account. With an understanding of these processes, transformer design can be improved.

Ageing-condition assessment of generator transformer bushings by means of dielectric simulation models

Diagnosis and ageing-condition assessment of electrical equipment are getting more and more important, as many of the units in use are close to the end of their designed service life or have even exceeded it already. There are several tools for diagnosis which are well known and proven for transformer monitoring and assessment. In principle, the same tools can also be used for bushing diagnosis. The most common bushing diagnosis method is the capacity and dissipation factor measurement at power frequency and ambient temperature. It can be used to detect partial breakdowns prior to a bushing failure, but it is insensitive for the detection of ageing processes. At ambient temperature, dielectric analyses like PDC (polarization-depolarization current) measurement is significantly better suited to get information about the aging condition related to water or conductive ageing products inside the dielectric (see also [1]). To evaluate PDC measurements, it is very important to have the possibility for comparison of the results with either older measurements of comparable units or with simulation results. For bushing simulation, a special model was developed and presented in [2]. This tool was now adapted to 400 kV OIP (oil-impregnated paper) bushings. Further it was improved to perform more detailed simulations. The simulation model then has been used for comparison with measurement data of service aged bushings as well as for investigation of the influence of temperature and water content by the use of different parameter settings gained from laboratory sample measurements.

Methods of Early Short-Circuit Detection for Low-Voltage Systems

Short-circuits in electrical networks imply extreme mechanical and thermal stresses to loads, systems, and protection devices. The paper discusses new approaches for low-voltage system protection. The three-dimensional locus curves criterion will be pointed out as one opportunity for short-circuit detection. Regression analysis, used to predict the amplitude of a sinusoidal function and therefore the prospective short-circuit current, provides another possible detection algorithm. Also a method based on combined three-phase quantities, e.g. phase currents, has been studied for network failure identification. A comparison and benchmark of the above algorithms for utilization in low-voltage networks has been presented. Moreover, disturbances from typical loads must be considered carefully to avoid nuisance tripping in addition to the aspired rapid failure detection. Locus curve methods increase hardware complexity against both short trip times and very good nuisance tripping avoidance. Instantaneous methods have superior nuisance avoidance and minimal hardware complexity, trading this off against longer detection times. Other possible techniques are the analysis of short-circuit depend voltage drops in the network or wavelet methods interpreting the network currents via dyadic filter banks of a multiresolution analysis.

Electric Arc in Low-Voltage Circuit Breakers: Experiments and Simulation

The aim of this paper is to present a further approach for analyzing the air electric arc in low-voltage circuit breakers (LVCBs). In order to achieve that, a new simulation model and experimental tests have been carried out. The simulation model has been designed using ANSYS CFX, a finite-volume method commercial software. This model has been defined as a 3-D geometry, with a high density structured hexahedral mesh, P1 radiation model and hot air characteristics for thermal plasma properties and transport coefficients. The model is applied to simulate the behavior of an LVCB for 50, 100, and 200 A with different numbers of splitter plates in the arc chamber and different locations for the arc ignition. As result, arc elongation and arc voltage increase have been observed when increasing the splitter plates number. Also faster arcs for higher ignition zones and greater expansion and diffusion for higher input currents have been obtained. These simulation results have been verified and validated. The verification process has been accomplished calculating the numerical errors, by means of the grid convergence index and Courant-Friedrichs-Lewis number. Thus, the most accurate mesh densities, time steps, and radiation models have been selected. Finally, the validation process has been achieved performing real experimental tests in the laboratory, proving that the results of the simulation model are close to real scenarios.

Two Different Experimental Approaches to Investigate the Influence of Gassing Polymer Walls on Switching Arcs in Miniature Circuit Breakers

Experiments with high current arcs confined in gassing polymer walls for two different experimental setups and characteristics of the current source were carried out. The first setup consists of a rotationally symmetric electrode configuration with the arc burning inside a small polymer tube. For the second setup conventional 16 A miniature circuit breakers were slightly modified by changing the geometry of the polymer gassing inserts and by drilling a small channel for the measurement of pressure inside the arc chamber. The signals of arc current and voltage and pressure inside the arc chamber using 9 different polymers and a not gassing ceramic were measured and compared. The paper presents the results of the different experimental approaches and evaluates the differences due to the corresponding setup. The influence of the polymers on the interrupting performance of the breakers is described.

Numerical Analysis of Low-Voltage Circuit-Breakers under Short-Circuit Conditions

Short-circuits in electrical networks come along with severe mechanical and thermal stresses not only to loads and systems, but also to the installed protection devices. Combined analytical simulations have been performed for analyzing and optimizing the behavior of low-voltage circuit-breakers. Especially circuit-breakers with electronic tripping units have been scrutinized.

Influence of board density on the conductivity of oil-impregnated pressboard for HVDC insulation systems

In HVDC (high voltage direct current) insulation systems, the conductivities of the insulating materials pressboard and oil are the relevant material parameters that determine the field distribution. The pressboard conductivity depends on various parameters, which can be categorized in service-parameters (e. g. time of energization, water content, impregnating fluid) and parameters during pressboard manufacturing (e. g pressboard density). Measurements of pressboard with different densities are performed by using a step response measurement in time domain. At first, the materials are impregnated with gas. It is found that electrical conductivity increases with an increase in pressboard density. The reason is that gas has a much lower conductivity than pressboard and therefore, a larger number of fibers per volume causes a higher conductivity than a smaller fiber number. Afterwards, pressboards with different densities are impregnated under vacuum with two types of mineral oil (a low-conductive oil and a high-conductive oil). For the low-conductive oil, the influence of the pressboard density is little, because conductivities and electric field stresses are similar, both for pressboard fibers and for oil. By impregnation with the high-conductive oil, the pressboard pores have a much higher conductivity than the surrounding pressboard fibers and thus the conductivity of the oil-impregnated pressboard depends on pressboard density again. In this case, a low-density pressboard has therefore a higher conductivity than a high-density pressboard corresponding to the different fiber volume and oil volume. The pressboard density can be modified during manufacturing and the designer can choose which pressboard density and which oil with a certain oil-conductivity should be used in a certain HVDC apparatus leading to desired conductivity values for the oil-pressboard system.

Field test results of serial DC arc fault investigationson real photovoltaic systems

DC arcing faults pose a safety risk in already existing photovoltaic systems. Due to the aging of the photovoltaic system components enhanced by environmental factors and the high DC voltages, long-lasting arc faults can occur and may cause serious damage. Conventional fault protection methods - like circuit breakers or fuses - are only able to clear faults if they carry a large fault current. Compared to arcing faults in AC installations DC arcing faults in photovoltaic systems have a bigger hazard potential. This is mainly based on the favorable conditions of existing. That is first of all missing zero-crossings of the current and the fact that a solar generator regarded as a source of energy cannot be turned off. Secondly a solar generator is a distributed energy systems with a big amount of potential places of origin for arc faults. Because of the non-linear current-voltage output characteristic of photovoltaic systems a lot of arc faults cannot be detected or even extinguished by common fault protection methods. Moreover it does not only ensure a higher arc-stability but also a wider range of stable arc operating points. Furthermore ambient conditions (e.g. solar radiation, temperature, etc.) and the arc length extend the range of stable arc operating points tremendously. For those reasons the characterization of DC arc faults in photovoltaic systems by means of measurement in real existing photovoltaic systems is indispensable. Especially under consideration of the increasing number of accidents with photovoltaic systems involved. This paper will include a general description of the infrastructure used for measuring and recording arc voltage and current. In addition, the experimental test setup used to insert a series arc fault within a PV system and to generate comparable results under safe conditions will be explained in detail. Furthermore the field test results of serial DC arc fault investigations in photovoltaic systems with different module technologies and taking into account a wide range of environmental conditions and a huge variety of different fault locations within the solar generator will be presented.

Influence of the electrical resistance and wire size on the current carrying capacity of connectors

The electrical-thermal behavior of an electrical connector is determined by heat generation due to Joule heating and heat absorption by conduction, convection and radiation. Heat flow from the connector to the wire is an important heat absorption mechanism for most electrical connectors. The temperature difference between the connector and the wire at infinity is proportional to the axial heat flow induced into the wire. The purpose of this study is to dimension the electrical resistance of a connector for power distribution by the heat flow into the wire. The heat flow is used as a design factor in order to define the maximum power loss for wires with different cross-section areas. With this approach the maximum acceptable electrical resistance for connectors with different sizes can be estimated in the early stages of the design process.