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Sag in Overhead Transmission Line and Its Calculation

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Sag in overhead Transmission line conductor refers to the difference in level between the point of support and the lowest point on the conductor. As shown in the figure above, a Transmission line is supported at two points A and B of two different Transmission Towers. It is assumed that points A and B are at the same level from the ground. Therefore as per our definition of Sag, difference in level of point A or B and lowest point O represents the Sag. Sag in Transmission line is very important. While erecting an overhead Transmission Line, it should be taken care that conductors are under safe tension. If the conductors are too much stretched between two points of different Towers to save conductor material, then it may happen so that the tension is conductor reaches unsafe value which will result conductor to break. Therefore, in order to have safe tension in the conductor, they are not fully stretched rather a sufficient dip or Sag is provided. The dip or Sag in Transmission line i...
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Why Armature Winding on Stator in Synchronous Machine?

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In Synchronous Machine, the armature winding is provided on Stator and field winding on Rotor which is contrary to other Electrical Machines. In fact there are certain issues due to which we provide armature winding on Stator. The benefits of this are as follows: More Economical: It is more economical to provide Armature winding on Stator and Field winding on rotor. Let us consider a three phase Star connected 750 MVA, 21 kV Synchronous Machine. Line Current = (750x10 3 )/(1.732x21) = 20,620 A If the armature winding is placed on the Rotor, we need three slip rings each capable of handling 20,620 A. Only not this, we need to provide slip rings with an insulation of (21/1.732 = 12.12 kV). The star point for grounding is also to be taken out through the fourth slip ring. Now, suppose the power requirement for the Field is 2 MW at 500 V. Then Excitation / Field Current = 2000/0.5 = 4000A In this case only two Slip Rings each capable of handling 4000 A is re...
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df/dt Relay - Rate of Change of Frequency (ROCOF) Relay

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Rate of Change of Frequency (ROCOF or df/dt) relay is used for fast load shedding, to speed up operation time in over- and under-frequency situations and to detect loss of grid. For better understanding of role and operation of df/dt relay, let us first study the variation of frequency with load for a Grid. Graphical relationship between power and frequency of a Grid is shown in figure below. We can have two things to be noted from the above graph: a)     If the power available in the Grid increases i.e. in other words generation is more than the load, frequency will go up. b)     If the generation is less than load i.e. power deficient in the Grid then frequency will decreases. Large power grids (here large power grid means having large installed capacity in MW) are characterized by a very high stiffness constant which means that a large perturbation (load generation mismatch) is required to cause the grid frequency to change by 1...
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