When a fault occurs in any portion of the power system network, the primary relay must act first. But to be on the safer side a 'Back-Up' relay to the 'Primary' relay is also provided with a proper discrimination time in their operation. Discrimination time is provided so as to allow the primary relay to act first. In case if it fails, then the back-up relay acts. This is done so as to isolate the healthy portion from the faulty portion.
Time of operation is basically determined by the following relay settings:
1. Plug Setting Multiplier (PSM), and,
2. Time Dial Setting (TDS)
So for a given fault current we have a desired 'Time of Operation' for both the primary as well as back up relay.
But, since the settings available are often discrete, so obtaining desired time of operation is not always possible.Then by using trial and error we need to choose PSM and TDS, which is not always the optimum choice.
This small simple MATLAB code, finds the optimum set of PSM and TDS so as to give the best possible time of operation to the relay.
This code is only a representation to the algorithm I have used. I have considered 7SJ61 Numerical OC Relay settings as an example in the code.
The Inputs are stored in 'INPUTS.xls' and MATLAB reads data from it. The output is stored in 'OUTPUT.xls'
I have considered a small margin to get a flexibility in the choice of PSM. It is known as Over Load Factor. Also I have considered the discrimination time between primary and backup relay to be 0.3 seconds.
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