Over Heating Protection Scheme of 3 Phase Transformer:
Figure shows overheating protection scheme for 3 phase transformer. Wheatstone‟s bridge principle is used to detect overheating. P, Q and S are the fixed value resistances. The RTD is connected in one arm. This RTD is kept near each winding of transformer.
When temperature is within limits, points a and b are at same potential and relay coil does not carry any current i. e. bridge is balanced. When overheating occurs, RTD resistance changes causing imbalance and some potential difference is created across the points a and b. The relay coil is thus energized and relay operates the alarm circuit.
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