Grading of Overcurrent Relays

Grading of over-current is the adjustment of the settings of the over-current relays to ensure discrimination and selectivity.  Consider a radial feeder with multiple feeders in series.  An over-current protection relay is installed at every breaker location.  When a fault occurs at any given point, only the relay located closest to the fault should operate.   This is known as grading.

There are different types of grading.  They are
1)    Current Grading
2)    Time Grading
3)    Time-Current Grading

Current Grading
Current Grading refers to the discrimination achieved by reducing the current setting as we move towards the power source.  This ensures that the relay closest to the fault trips first.  The downside of this arrangement is that the fault  current does not always vary with the location.  Hence, it is not possible to accurately discriminate between the relays. 

Time Grading
Time Grading refers to the discrimination achieved by varying the time delay for the different relays.  In this method, the relay farthest from the source has the shortest time delay and the time delay increases as we move towards the source.  That is, the source breaker will have the highest time delay.  This will work in systems where the fault current is uniform across the system.  However, this type of grading will not be sufficient in systems where the fault current varies with the location of the fault. 

Time-Current Grading System
The Time current grading system is the most widely used method of Grading.  This method uses a combination of Time and Current grading to achieve discrimination.  In this method, the time setting varies with the fault current.  A severe fault will have a shorter time delay   while the delay will be more for a mild fault.

Selection of Current Setting
The current setting is determined by first calculating the current during a fault.  This is done by a procedure called the fault level calculation.  The current during a fault will depend on the number of power of upstream power sources.  Thus the fault current at minimum generation and the fault current at maximum generation should be calculated.  A three phase fault during maximum generation will cause maximum fault current while a fault between two phases during minimum generation will result in minimum fault current.

Each section of the distribution should serve as a backup for the immediate section downstream.  The setting such that the relay operates for a fault at the adjoining section during minimum generation.  The current setting is lowest at the feeder farther from the source and increases towards the source.