Sag (dip) a decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for durations of 0.5 cycles to 1 minute. Swell - an increase to between 1.1 pu and 1.8 pu in rms voltage or current at the power frequency durations from 0.5 to 1 minute.
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Overhead lines have 'sag' (not 'slack') because the conductors need to be able to expand and contract as the temperature changes. If sag isn't built into the design of the line then, when the conductors contract, they will put undue lateral strain on the supporting poles or towers. It is important that, with maximum sag, the minimum clearance distance between the line conductors and the ground below is always maintained. This safety clearance increases, of course, with line voltage. Allowing for the necessary sag part of the design-process for any overhead line, and achieving it becomes the responsibility of the linesmen when the line is eventually constructed.
An AC voltmeter is ideal. If you really want to get fancy and analytic about it, you could use data-acquisition equipment to constantly read the AC voltmeter, and store or record the data.
The tap changer changes the ratio of primary to secondary windings by physically adding or subtracting windings from the primary or secondary. An on load tap changer (LTC) allows dynamic voltage control, which is important when loading of transformers, and the system in general, varies (if the primary voltage sags, the LTC can be used to increase the secondary voltage so customers don't see this sag).
because of heat cables expand
sag means the deepest point of the conductorAnswerSag is defined as follows: 'Sag, under any system of conductor loading, is the distance measured in the direction of the resultant load, between the conductor and the midpoint of a straight line joining adjacent supports'.So, draw an imaginary line beween two adjacent supports (towers or poles) and, from the midway point, draw a vertical line to where it intersects the conductor, and that vertical line represents the sag.