100%(3)3 out of 3 people found this document helpful
This preview shows page 1 - 4 out of 11 pages.
HAND-OUTS IN VARIABLE LOAD PROBLEM FOR STUDENTS IN POWER PLANT ENGINEERING Compiled By: Prof. Rogelio A. Antenor, MSME, MEM
Load Curves and Selection of Generating Units Because demand is continuously varying and power cannot be stored, the power output of a utility system must equal demand unless outside power is purchased. This variation in demand is most easily seen on a load-demand curve. The load-demand curve is a graph of power output (in kilowatts) over a given period of time. A typical daily load-demand curve over a 24-hour period if extended to cover an entire year, it would be apparent that there are a number of components to the demand variation: 1.Diurnal variation: daily variation caused by normal changes in activities during a day. 2.Seasonal variation: effects of seasonal loads such as heating and air conditioning. 3.Random variation: variation caused by factors such as short-term weather changes, sales of power to neighboring systems, and equipment failure. 4.Long-term growthSince the load on a power station is seldom constant and it varies from time to time, hence, a single generating unit (i.e., alternator) will not be an economical proposition to meet this varying load. It is because a single unit will have very poor (The efficiency of a machine (alternator in this case) is maximum at nearly 75% of its rated capacity) efficiency during the periods of light loads on the power station. Therefore, in actual practice, a number of generating units of different sizes are installed in a power station. The selection of the number and sizes of the units is decided from the annual load curve of the station. The number and size of the units are selected in such a way that they correctly fit the station load curve.Once this underlying principle is adhered to, it becomes possible to operate the generating units at or near the point of maximum efficiency. Illustration. The principle of selection of number and sizes of generating units with the help of load curve is illustrated in Figure below. In Fig. (i), the annual load curve of the station is shown. It is clear from the curve that load on the station has wide variations; the minimum load being somewhat near 50 kW and maximum load reaching the value of 500 kW. It hardly needs any mention that use of a single unit to meet this varying load will be highly uneconomical.
As discussed previously, the total plant capacity is divided into several generating units of different sizes to fit the load curve. This is illustrated in Fig. (ii) where the plant capacity is divided into three units numbered as 1, 2 and 3. The three units employed have different capacities and are used according to the demand on the station. In this case, the operating schedule can be as follows: Time Units in operation From 12 midnight to 7 A.M. Only unit no.1 is put in operation.