Impedance and Reactance Diagrams

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http://powerlearn.ece.vt.edu/modules/B4/html/sect3.html

To analyze the steady-state behavior of the system, or to analyze its response under faulted conditions, the per-phase equivalent circuit has to be obtained. The one line diagram described above is used to generate the per-phase equivalent circuit. In order to obtain the impedance diagram, the appropriate equivalent circuit of each component needs to be obtained. This aspect of the analysis will be covered in the subsequent chapters. In general the following representation is adopted for the various components. Rotating machines are represented by constant voltage sources in series with appropriate impedances. Transformers are represented by an equivalent circuit which has three branches. A series branch representing the primary winding impedance, a shunt branch representing the magnetizing current, and the effect of the no-load losses, and another series branch representing the effect of the secondary winding series impedance. In analysis commonly done, the effect of the magnetizing current is neglected. As a result the shunt branch is eliminated, and the transformer is represented by its series impedance reflected to any one side. Transmission lines are represented by appropriate models based on the length of the line. A commonly used model consists of a series impedance which includes the resistance and reactance, and a shunt capacitance at each end of the line equal to half the total capacitance of the line. Loads are modeled in a variety of ways. A common model used is the representation of the load by an equivalent shunt impedance [8].