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SPECIALIST EQUIPMENT


SWITCHED-ON THINKING L


Karol Bartodziej explains why power supply equipment is an underestimated link in the extraction process


ongwall shearers or roadheaders, powered roof supports, belt conveyors and sorting systems – these are the machines that are usually associated with the equipment used in the mining industry. However, an integral part of extraction systems is the electric power supply apparatus for heavy equipment. Few people realise why the power grid in mines are so different from those for domestic use and even from those used in other industries. Meanwhile, the devil is in the detail: without these devices, all of the massive mining machines would be of no use; and the quality of such devices leads to the overall safety level of a mine. Contrary to the everyday situation, where the energy quality is the responsibility of the supplier, in mines a considerable part of the power line is the concern of the user – starting with the national power grid connection on the ground, to the place of operation of the machine, which is often several kilometres away. A comparison of these situations is presented in the figure on the right, which shows a power line of a typical machine in a traditional manufacturing plant (e.g. numerically controlled machine tool) and for a machine used in mines (e.g. roadheader). Several kilometres of transmission lines with the powers of thousands of kW means that the user must equip this line with energy infrastructure that allows minimising voltage loss on the wiring itself. Terefore the user must install appropriate transformer stations and use suitable diameters of wires so that


28 www.engineerlive.com


the power supplied to the machine has the appropriate level of voltage for its operation.


SAFETY IS PARAMOUNT Another important aspect that must be taken care of by the mine itself is to ensure safe operation of these power lines. Te traditional power industry uses simple methods of securing lines, such as over- current protection or earth fault protection, in the case of lines installed in mines, risks such as gas or dust explosion occurring in such places must also be taken into account. Already at first glance it is clear that ensuring safe operation of electrical installations, where the typical voltages are 1kV or even 6kV, is not a trivial issue. One of the important aspects is the


protection response time, namely switching off the voltage on a given power cable in an


emergency situation. It must be sufficiently short so that, in the case of a sudden damage to the cable, the voltage is switched off before a spark is produced that has the energy capable of igniting an explosive gas mixture that can be present in the given workplace. Due to the increasing requirements


regarding digital protection modules or systems monitoring the operation of stations, the coexistence of precise electronic devices with current circuits and switching elements (contactors) in one electric equipment box has become everyday situation. Electronic devices made using digital technology are relatively sensitive to interference and therefore it should be borne in mind that proper operation of these devices requires their constructors to be particularly experienced


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