Thursday, September 15, 2011

High Impedance Differential Protection of Busbar

Feeder and Busbar protection (Part4)
       Another method to provide differential protection to busbar is based on sensing a voltage drop across a high impedance, under fault conditions. The scheme is shown in the Fig. 1.

       The basic principle remains same as differential scheme. Under normal conditions vector sum of the currents in the lines is zero. Hence If i.e. current flowing through high impedance ZH is zero and the relay is inoperative.
       During fault conditions, unbalanced current exists. Such an out of balance current If flows through ZH causing a high voltage drop Vz across it. It is given to a transformer. A measuring unit is connected to the secondary of this transformer which measures this drops and trips the relay accordingly. Main advantage is that as voltage drop is sensed, saturation of core of one of the current transformers has no effect on the protection scheme.

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