Power

23.1-SOURCE, POWER-SYSTEM ELEMENTS A. J. T HUM Basic considerations in providing power for mine production and collateral operations are: 1. Mining operations of any size normally use electric motors for driving all equipment except mobile service units. 2. The electricity for power may be purchased or self-generated, depending on the location, size and specific operating circumstances of the individual property. 3. The major elements of the power system will be the source, either an incoming transmission line from a utility in the case of purchased power or the power plant if self-generated. Energy for the power plant may be water, hydrocarbon fuels, waste heat or nuclear, which will govern the type of prime mover selected. 4. The electricity normally is generated in the form of alternating current and must be distributed to the points of usage. This usually requires a primary substation for transformation to suitable voltages and secondary substations at points of usage throughout the operations, The power is delivered from the primary to the secondary substations over transmission lines or cables, depending on distances, voltages and local conditions. 5. At the points of use, there will be starting and protective devices for each motor. In some instances, direct current will be needed and rectifiers of various types can be used to provide it. 6. The entire electrical system requires suitable switching and protective devices to prevent damage to the equipment in case of power failues or faults; also, in most cases, sufficient metering to establish the demand and amount of electrical energy consumed. 23.2-PURCHASED POWER A. J. THULI 1. Where public-utility or government-generated power is available, it usually is advantageous for the mine owner to purchase rather than generate. Purchased power generally will be cheaper and more reliable than operating a small generating plant except in some cases where waste heat is available as the source of energy and can be used to supply all or a portion of the power requirements. 2. If it is decided to purchase power, the owner usually must participate in the cost of constructing the connecting line from the closest existing utility line to his point of usage. If the connected load is less than 1,000 lip, it probably is best to let the utility provide the primary substation. For larger connected loads the owner can obtain a more favorable "primary voltage" rate if he provides the primary substation to receive the incoming power and transform it to the distributing and usage voltages. 3. The cost of purchased power usually is made up of a demand charge, deter¬mined by the maximum amount of power the customer takes during any 15-min period throughout a month, and a graduated energy charge for the kilowatt-hours consumed-that is, so much for the first block of energy based on the demand,