Computer operated coordinated over-current protection scheme

Over-current protection scheme has very wide applications in transmission and distribution lines. In order to maintain coordination between several points of protection especially in radial feeders inverse time graded scheme is employed. The coordination is very essential in order to maintain minimum time of operation for the relay concerned as well as to provide a backup in case of failure of the particular relay. An on-line computer operated over-current protection scheme has been developed at the laboratory. A scaled down power system network has been constructed and four points within the network are chosen as points of protection. A single computer is engaged in monitoring, taking decision and controlling the protection gears of the system. The set up has been tested in the laboratory by creating short circuits through fault impedance at different points of the network. A computer program has been written in order to read input sampled voltages corresponding to the line currents, compare the currents with setting currents, calculate the time of operation of circuit breaker for a fault and send the appropriate trip signal. The user may use the setting values decided by the software. For this the user is to provide the system configuration and parameters, CT ratios and desired minimum time of operation. The software calculates the setting values and shows the results for faults at different points of protection. Alternatively, the user may define the setting values before running the program. The values are current setting for different relay points and individual time multiplier. The user is also asked to define the relay characteristic. The operation of the computer operated protection system is found to be extremely accurate. Since, in this scheme relay parameters and relay characteristics can easily be modified from the software the user may observe the effect of any change on the operation of the relays. It is very convenient learning tool both for the students and the practising engineers.