Background of the Study
The basic scheme of differential relaying is explained with reference to the diagram below Fig.1.1:Basic Scheme of Differential Relaying The protected equipment may be a length of a circuit or a winding of a transformer or a portion of the bus etc. A current transformer (C.T.) is connected at either end of the protected equipment. Protected Equipment I1 Relay I2 I2 Direction of Current under fault Condition Direction of Power Flow Current Transformer Primary Side Current Transformer Secondary Side 4 The secondary of the circuits are interconnected as shown in figure 1.1 with a relay which may be an over current relay. Now suppose current flows through the protected equipment to an external load or to a fault at X. If the two circuits have an identical ratio, and are properly connected with respect to polarity, then the secondary induced currentI1 and I2 will circulate between the two circuits and no current flows through the relay. However if an internal fault should develop in the protected equipment or between the primaries of the two Circuits then the currents I1 and I2 will be different and the different in current (I1-I2) flows through the relay causing the relay to operate. This relay which operates on the vector difference of the current entering and leaving a protected equipment is called a differential relay and the scheme of protection as differential protection. A differential system can be arranged to cover the complete transformer, this is possible because of the high efficiency of transformer operation, and the close equivalence of ampere-turns developed on the primary and secondary windings. Figure 1.2 illustrates the principle of current transformers on the primary and secondary sides which are connected to form a circulating current system.
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