Protective Schemes for Various Electrical Equipments Project Report

Introduction to protective relaying:

Protective relaying is a system security technique which eliminates the problems occurred in the device and protects unaffected portion. If the effected portions are not damaged then it reenergizes them to work. The main objective of this technique is to minimize the service interruption when abnormal situations occur. In this we will discuss two protective schemes which are employed in power transformer and motor.

Power transformer protection:

The transformer is an expensive device the damage occurs due to fault conditions like voltage over flow, short circuits and due to over load. The transformer consists of many components which are to be monitored at every aspect to prevent damage. Buchhoktz relay is placed in pipe of the tank which gives a alarm and closes the block when the pressure levels change in the oil tank. In differential method wires are connected at the ends of the transformer which corrects the faults occurred due to phase difference. A bypassed differential relay is used monitor the transformer ratio. Harmonic restraint is used which employs a tuning technique for filtering the harmonic currents. If the differential protection scheme is not provided then it is provided with restricted over current and earth fault protection which serves the same purpose. To protect the transformer from thermal heating, thermo couples or resistor temperature detectors are placed in winding of the coils. Due to higher magnetic currents over heating of coil and insulation failure may occur which is prevented by over fluxing protection.

Motor protection:

Low voltage motors are protected by using HC fuses which prevents the faults due to short circuits and over currents. For higher voltage motors different protection schemes are used at various levels. By employing temperature detectors the bearings are protected from overheating. To reduce the loss of supply caused by back emf armature type relays are used. To protect the motor insulation from thermal stress thermostats are employed. By sing a bimetallic relay or a phase balance relay we can protect the circuit from unbalanced voltage conditions. Thus we can employ various protection schemes for electronic equipments from getting damaged from abnormal conditions.

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