Karol Bartodziej explains why power supply equipment is an underestimated link in the extraction process
Longwall 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. Therefore the user must install appropriate transformer stations and use suitable diameters of wires so that 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. The 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 in dealing with the issue of resistance to electromagnetic interference. These issues are particularly important in times of the increasing use of inverter drives, which are the source of considerable EMC interference. The above-mentioned issues mean that even well-known companies from the energy industry do not offer products for mining and electric companies taking their first steps in the mining industry are facing considerable problems with meeting the functional and safety requirements of these devices.
Modern power stations offer their users a lot of useful functionalities, such as remote operation and remote monitoring systems for control over their status. The function of analysis of start-up currents on a given outlet is among the novelties available on the market that deserve particular attention. This function is especially valuable in difficult
cases of protection response diagnosis during the initial run of machinery. To perform such an analysis, the station is switched to the mode of increased frequency of current registration on a given outlet. In this mode, the recording is done with the measurement period equal to 20ms. This allows the observation of the current waveform distortion during machine start-up, which in turn, with a little bit of experience, allows assessing the causes of power shutdowns and deciding on the further actions to be taken (e.g. engine or power cable replacement).
Although the issues described here do not exhaust the subject of mine power systems, they introduce many of them, especially to those who do not normally come into contact with mining electrical engineering. Modern power supply equipment used in mines is one of the most technologically advanced pieces of equipment used in the mining extraction system. Therefore its proper operation may considerably influence the efficiency of the entire process and that is why it is so important to use high-quality equipment that allows easy diagnosis of the problems arising in connection with the supply of power to individual machines.
In the era of the efforts to optimise the process of extraction, it is extremely important the entire set of machinery and equipment was treated as a complete technological process. In addition, selecting equipment and maintaining it in good technical condition is essential for ensuring the safety and continuity of the extraction process.
Karol Bartodziej is control systems director at Famur