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    Currently I am doing simulation on digital power protection(busbar protection). After fault applied, is it I have to set myself when to trip the relay and data taken is input in the algorithm to discriminate internal and external fault? OR after fault occur, the algorithm itself make decision when to trip the relay and discriminate internal and external fault?


    Not sure whether you are looking at traditional overcurrent/short circuit protection on a busbar or busbar zone protection. They are two different things.

    If you are talking about detecting internal and external faults you are talking about zone protection. This works on a principle the principal of differential protection. Here you monitor the current coming in at the switchboard incomer and the current going out at each feeder. The relay sums all of these and the resultant should be zero when all is healthy. If a fault occurs outside the board the currents passing will be very high but the sum will still be zero and the relay will ignore it. If the sum is not zero it will indicate that there is some form of fault in the board and trip. You will need to take some care as to the time delay settings to take care of allowable transients.

    If you are looking at traditional overcurrent/short circuit protection there is a bit more to consider.

    The relay is a universal device and effectively has hundreds of algorithms to choose from. You will need to choose exactly the right one for your application. The right algoritm is selected through the relay settings which select particular current/time curves and time delay setings from the many that are available.

    To determine discrimination with upstream circuit breakers or protection relays you must know what the settings in those are.

    You then need to set the characteristics of your relay such that they are underneath those of the upstream relay/breaker. This means that your relay will be set to a lower trip current for any given time trip time. On the charateristic current/time tripping curves, this will mean that you relay curve must lie to the left of the upstream relay curve in the vertical or near vertical part of the curve (this is the inverse time part of the curve) and below the horizontal part of the upstream relay curve (this is the instantaneous trip part of the curve).

    The next check will be that the continuous current setting you have chosen will be above the rated current of the device you are protecting. This means that the vertical part if the current/time characteristic of your relay (this is the continuous current area where your relay will not trip if that current passes) must lay to the right of the current that the load will draw.

    A final check if you are starting an induction motor which will have a high staring current for a short time after which it falls back to the rated current, will be that the starting current value will lay to the laft or below the curve of the relay for the starting time of the motor.

    Once you have done this the algoritm will monitor the passing currents and trip when necessary.

    Whether it trips correctly or not will depend on youy skills in setting it correctly. It will no magically do the job for you straight out of the box.

    Hope that this helps

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