What is the advantage of using the current grading in relay systems? 

Grading of Overcurrent Relays

Grading of over-current is the adjustment of the settings of the over-current relays to ensure discrimination and selectivity. 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.

Types of grading system:

1.) Current grading

• Current Grading refers to the discrimination achieved by reducing the current setting as we move toward 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. 
• It overcomes the long-time delays occurring in graded time lag systems.
   

2.) 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. 
   

3.) Time-Current Grading

• 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.