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Purpose of Negative phase sequence current relay; Type- CTN/CTNM 

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Negative phase sequence current in the stator of a generator, caused due to unbalanced loads or faults, it induces double frequency eddy current in the rotor. These currents, if allowed to persist, can cause serious overheating and the purpose of this relay is to disconnect the machine before such excess temperature is reached. The inputs from the current transformers, which are connected in each phase of the generator supply, are fed to a negative sequence filter which gives an AC output voltage proportional to the negative sequence current. This voltage is rectified and smoothed and fed into the squaring circuit of the main measuring element, the definite time delay circuit and the alarm element. The output from the squaring circuit is proportional to the square of the input voltage and is applied directly to the main timing circuit to give the required relationship between I2 2t and the relay operates time t. 

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