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Optimization of Power Supply Equipment for Modern Telecommunication Systems Christer Boije af Gennas The extensive use of semiconductor components in modern telecommunication systems has resulted in an ever increasing part of the telecommunication equipment being supplied with power via DC/DC converters mounted in the equipment racks. This means that the power supply plant, unlike the earlier type which consisted of a clearly defined central equipment, now comprises not only a central part but also a distribution part and integrated power units mounted in the racks of the telecommunication equipment. The article deals with the studies that have been carried out by LM Ericsson concerning alternative system solutions for power supply, and describes an optimization process in which the costs and performance of the rack converters and the distribution material are also included as parameters. The article also describes an example in which the optimization method has been applied for a power supply plant that supplies a computer-controlled telecommunication equipment having typical power supply data. Moreover sectioning of the power supply plants is presented as a means of limiting the cost of the distribution cabling in the case of large telecommunication plants that extend over a large area. UDC 621.311.4: 621.39 LME 781 General considerations The construction of modern telecommunication systems differs quite considerably from that of older systems. This naturally affects the design of the power supply for these systems. From the point of view of power, some of the most important development trends in modern telecommunication plants are that — electromechanical components are to a great extent being replaced by semiconductor electronics. — telecommunication systems are being computerized more and more, SPC (Stored Program Control) systems. — very large exchanges with high power consumption have become more usual, for example transit exchanges with very high traffic handling capacity. — larger telecommunication exchanges often means that power has to be distributed over long distances in such exchanges. This applies particularly if, for space reasons, they have been divided up on a number of different floors in the building. Increased electronics factor Electronic circuits require to be fed with other and more constant voltage levels than the electromechanical components. Consequently, because of the voltage drop and interference risks, it is not a good solution to distribute these voltages from the central power supply plant. Hence it is necessary to convert the distributed system voltage to voltages suitable for feeding the electronic, power-consuming units. This conversion is usually carried out in what are known as rack power units, i.e. DC/DC converters placed in the telecommunication equipment racks close to the units they are to provide with power. However, to a greater or lesser degree, modern exchanges still contain units supplied directly from a suitably selected system voltage. The ratio between the power that is distributed via the rack converters and the total distributed power is usually Fig. 1 The electronics factor a is the ratio between the power distributed via the rack converters and the total distributed power
43 Fig. 2 CHRISTER BOIJE AF GENNAS Power Supply Department Telefonaktiebolaget LM Ericsson A comprehensive knowledge of the whole power supply system is necessary in order to be able to effect an optimization called the electronics factor for the exchange, see fig. 1. As is clear from what has been said above the electronics factor for modern telecommunication systems is appreciably higher than for older systems. More stringent requirements on the quality of the supply voltages The introduction of SPC systems has led to an increase of requirements on voltage accuracy as well as freedom from transient overvoltages and undervoltages in the power supply. Undervoltages which are of such short duration that they are completely imperceptible in electromechanical systems can cause disturbances in SPC systems through the loss of stored information. Such short duration transient voltage deviations occur, for example, as a result of a short circuit and the consequent fuse blowing in the distribution network. In the LM Ericsson system this disturbance risk is prevented by feeding the power consuming units individually, direct from the central power plant via cables with suitably chosen resistance. This individual feeding is a characteristic of the distribution system that is usually called high-ohmic distribution, and which has been standardized by LM Ericsson for feeding SPC systems. The method has been described in an earlier number of this publication.' Total optimization of power supply plants When designing a power supply system for modern telecommunication plants the whole system must be taken into consideration when optimizing, that is to say the rack converters and distribution material as well as the central power plant. For example, minimization of only the cost of the central power equipment can often result in increased costs for other parts of the plant, and probably does not give the lowest total cost for the whole power supply plant. Thus an optimization process demands a very good overall knowledge of the parts included in the power supply equipment, from the incoming mains supply to the power consuming units in the racks of the telecommunication equipment, fig. 2. The following optimization parameters must be considered: • Equipment performance requirements. Towards the telecommunication equipment these are fixed by detailed specifications. On the other hand the internal performance requirements (voltage levels, tolerances etc.) within the power supply equipment can be varied. For example, it should be possible to compensate a reduced regulation capability in the central power units by an increase in the regulation range of the rack converters.
Page 1: ERICSSON REVIEW 3 1976 GEOSTATIONAR
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