Design and construction of a numerical over current relay

Abstract

The project aims at the design and the construction of a laboratory replica for a commercial numerical over current relay using microcontroller and micro CT. It will acquire, sample and covert into digital form the analog current signals from a power system in real time. Then it simulates the working principles of a conventional electromechanical over current relay in through a numerical (mathematical) model. Appropriate equations and algorithm have been used. When a current in a part of a power system exceeds a pre-set value either due to a fault or a large overload the relay will isolate the part through sending trip signal to a circuit breaker. In the developed laboratory model, the system is assembled by a lamp board and the circuit breaker is simulated by a hardware solid state (MOSFET) switch. The constructed relay has been tested extensively for various values of load current and two different tripping characteristics (Inverse Definite Minimum Time: IDMT and Definite Time Lag: DTL) have been verified. The project is expected to help students acquire skills in digital logic circuits design and implementation, and applying it in a power system for isolating the faulty part. As commercial numerical relay is expensive, so we have developed for lab demonstration a prototype using cheap and off-the-shelf components to show the working procedures of a numerical over current relay in a power system. The project will also help students develop hands-on skill on numerical relay and enrich their knowledge on the working principle of a relay-breaker combination circuit.