1. LOAD CURVES:- Load curve or chronological curve is the graphical representation of load (in kW or MW or W) in proper time sequence and the time in( hours/ days/ month /yearly or annually). It shows the variation of load on the power station.When the load curve is plotted for 24 hours a day, then it is called daily load curve. If the one year is considered then, it is called annual load curve. The load curve of the power system is not same all the day. It differs from day to day and season to season. The load curve is mainly classified into two types, i.e., the summer load curve and the winter load curve.
The following are the information obtained from load curves.
1. Load duration curve determines the load variation during different hours of the day.
2. It indicates the peak load which determines the maximum demand on the power station.
3. The area under the load curve gives the total energy generated in the period under consideration.
4. The area under the curve divided by the total numbers of hours gives the load.
5. The ratio of the area under the load curve of the total area of the rectangle in which it is contained gives the load factor.
6. The ideal load curve is flat, but practically it is far from flat. For a flat load curve, the load factor will be higher. Higher load factor means the more uniform load pattern with fewer variations in load.
The following are the utility of the load curve.
1. Load curve decides the installed capacity of a power station.
2. It is helpful in choosing the most economical sizes of the various generating units.
3. The load curve estimates the generating cost.
4. It decides the operating schedules of the power station, i.e., the sequence in which the different generating units should run
2. LOAD DURATION CURVE :-
The load duration curve is defined as the curve between the load and time in which the ordinates representing the load, plotted in the order of decreasing magnitude, i.e., with the greatest load at the left, lesser loads towards the rights and the lowest loads at the time extreme right.
This curve represents the same data as that of the load curve. The load duration curve is constructed by selecting the maximum peak points and connecting them by a curve.The load duration curve plotting for 24 hours of a day is called the daily load duration curve. Similarly, the load duration curve plotted for a year is called the annual load curve.
Procedure for plotting load duration
From the data available from the load curve determines the maximum lose and the duration for which it occurs.
Now take the next load and the total time during which this and the previous load occurs.
Plots the loads against the time during which it occurs.
3. MASS CURVE :-
This curve is plotted with units ( kwh) as ordinate or X- axis and time as abscissa or Y-axis axis. Thus a mass curve gives the total energy consumed by the load up to a particular time in a day. This curve can be easily plotted from the load curve by summing up the energy consumed up to different times starting at the zero time.
This curve used in study variations between the rate of water flow and the electrical load and determination for the necessary storage.
4. Connected Load :- The sum of the continuous ratings of all electrical equipment connected to the supply system is known as connected load.
5. Maximum Demand :- It is not necessary that all the connected load be switched to a system at a time. The greatest of all “short time interval averaged” (15 minutes or 4 hour or ½ hour) during a given period (a day, a month or a year), on the power station is called the maximum demand. It is sometimes also called as system peak. It is to be clarified here that the maximum demand never means the greatest instantaneous maximum demand but the greatest short time average demand occurring during a long period of time under consideration.
Hence whenever the maximum demand is to be mentioned it is essential to indicate the short interval of time over which averaging has been carried out for the measurement of demand and the period of load under consideration. It is the maximum demand which determines the size and the cost of the installation.
6. Demand Factor :-
The ratio of actual maximum demand on the system to the total rated load connected to the system is called the demand factor. It is always less than unity.
The idea of a demand factor was introduced due to the fact that all the equipment connected to the system will not be worked at a time in practice and the kW or kVA maximum demand of a group of electricity consuming devices will be always less than the sum of the kW or kVA ratings or capacities of these devices.
The minimum capacity of the generating plant must be such as to meet the maximum demand, the value of demand factor, therefore, determines the capacity and hence cost of the power equipment required to serve a given load.
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