Friday, August 12, 2011

Single Line Diagram of Power System

       The different elements of power systems are always required to be connected with each other so that the complete power system can be modelled. The supply system is normally three phase balanced system so while representing the system on of the three lines is shown with return through neutral. The representation of all three lines with neutral return is rarely required in practice. The representation of all power system elements along with their interconnection is done with the help of single line diagram of the system.
       The single line diagram can still be simplified by excluding the completed circuit through the neutral and the components are represented by their standard symbols rather than by their equivalent circuits. The circuit parameters are not shown while the transmission line is indicated by a single line between its two ends.
Note : Thus the single line diagram is nothing but the simplified representation of power system components with each other, with each component represented by its symbol.
       The standard symbols used for drawing single line diagram of a power system are shown below.
       The significant information about the power system can be obtained in concise from the single line diagram. The various features of the system are different based on the problem that is considered. The information to be induced in the single line diagram is dependent on the purpose for which the diagram is drawn.
       The location of circuit breakers and relays is not important when one undergoes load flow study of the system. In such case they are not shown on the single line diagram and the intension of single line diagram in such case to study the load flow.
       But if it is required to determine the stability of power system under transient conditions during fault then the role of relays and circuit breakers is vital. This is because the stability under transient conditions due to fault is dependent on speed with which relays and circuit breakers operate to isolate the faulty part. Under such case, information related to circuit breakers must be included in the single line diagram as it is important.
       The single line diagram may also include also the information about current and potential transformers which are either connected to the relays or are used for metering purpose. Hence it can be seen that the information obtained from single line diagram varies with its purpose and problem. It is also dependent upon the standard practice of the company which is preparing these single line diagrams.
       The various organisations such as ANSI (American National Standards Institute). IEEE (Institute of Electrical and Electronics Engineers) have published standard symbols for drawing electrical diagrams. It is expressed that these symbols must be used in all the diagrams everywhere in order to have simplicity, clarity and uniformity. If someone is not consistent with these diagrams and symbols then it is better to represent the machine with its basic symbol followed by information on its type and rating.
       The point at which the system is connected to ground is important as it helps in determining the amount of current flowing when an unsymmetrical fault occurs on the system involving ground. The current to the ground is limited by connecting either a resistance or reactors between the neutral of  Y and ground. The correct symbol for resistance or inductance should be added in the diagram when Y grounded connected transformer is shown. Most transformer neutrals in transmission system are solidly grounded. The generators neutrals are normally grounded through comparatively higher resistances or sometimes through inductance coils.
       The Fig. 1 shows a single line diagram of a simple power system.
Fig. 2   Single line diagram

       One generator grounded through reactor is connected to bus and through a transformer to the transmission line while two generators, one grounded through resistor and one grounded through reactor is connected to bus through a transformer to a transmission line at the other end. A load is connected on each of the bus. The ratings of the generators, transformers loads and reactances of different components of the circuit can also be represented on this diagram.
       The only limitation of single line diagram is it can not represent the conditions during unbalanced operation of a power system. Under the unbalanced operation of a power system, all three phases are to be shown for currents and voltages and single line diagram proves to be insufficient.

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