This paper analyses the electrocution hazard in three-phase electrical networks that operate with the neutral point isolated from ground. The following cases are analyzed: short low-voltage electrical networks, long low-voltage electrical networks and highvoltage networks. In high voltage electrical networks, touching a phase is always dangerous even if the insulation resistances are considered to be infinitely high. The analyzed cases show that in three-phase electrical networks with ground-isolated neutral point, the value of the current which passes through the human body depends on its electrical resistance as well as on the insulation resistances of the phases to earth.
The quality of electricity is a problem of major interest, making it necessary to analyze the factors and causes that result in the worsening of the electric energy quality. An important aspect of electricity quality is the introduction of current and voltage harmonics in the alternating current network by non-linear consumers. The paper analyzes the deforming regime introduced by the drive systems of a bucket wheel excavator within a technological line at an open pit mine. Methods. The paper presents the measurements of power quality in a laboratory experimental study and measurements of an upgraded technological line at a lignite open pit. In this sense, the present the study of a distorting regime introduced by the drive systems formed from a static frequency converter and an asynchronous motor, as well as the results of quality power measurements introduced by the drive systems in the electric network. Findings. The paper presents analysis of the voltage and current harmonics introduced in the alternating current network of static converter-asynchronous motor drive systems, which is necessary to establish the deformation factor. The values of the deforming regime are essential to obtain the harmonic compensation solutions. Originality. The originality of the paper consists in the approach to the measurements performed and the analysis of the deforming regime introduced by the electric drive systems. Practical implications. The values of the current and voltage harmonics were determined both by the laboratory measurements and by measurements made on the supply line of a bucket wheel excavator in different operating regimes.
Purpose. This article discusses how to choose the optimal number and load factor, respectively the economic power in the first year of mining power transformers operation. The analysis is carried out based on technical-economic criteria. In this regard, two economic criteria are proposed for a detailed analysis, namely the minimum updated total expenses criterion and the minimum power and energy losses criterion.
Methods.For determining the number and the optimal load factor, the paper presents mathematical models for the two eco-nomic criteria used. The results obtained by the presented methods are simulated using Matlab for several series of under-ground mining transformers. Also, it is assumed that the load remains constant over the year.Findings. The article confirms the possibility of using the analyzed economic criteria for establishing the optimal number of mining transformers as well as the optimal load factor, respectively the optimal power for the first year of operation. The difficulty of the research is related to the loss time assessment. Also, the paper presents the performed comparative analysis of the two implications.Originality. This research provides a novel approach, by the detailed presentation of the two criteria used for describing the objective functions which have to be minimized in order to gain the optimum, referring strictly to mining transformers, which represents a novelty for power engineering in mining.Practical implications. The methods described in the article can be successfully used in the case of new mining power networks which are going to be designed, and in the case of those currently in operation. Economic criteria analysed also provide results for the economical regime of mining transformers which corresponds to minimum energy loss. Therefore, this case also results in significant energy savings, i.e. lower economic criteria used.
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