STUB PROTECTION:
It is basically an in-built function of a distance relay. It falls under the auxiliary part of the distance relay.
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It is mostly used in one-and-half-bus bar schemes. As we already know from our previous article on the bus bar scheme in one-and-a-half bus bar schemes, we generally use 3 circuit breakers for two-line feeders.
Stub protection is an important function that provides protection to the section between two CTs and the line isolator when the main protection is not available because of the opening of the line isolator. The meaning of stub is a dead end.
WORKING PRINCIPLE
Let us suppose, that line 1 is under maintenance and its line isolator-89L is open. However, the associated circuit breakers are closed for feeding line 2 from Bus 1. As long as the associated circuit breakers of line 1 are open, we don’t have any problem as far as protection is concerned.
As soon as the breakers are closed let us say for feeding line 2 from bus1. The section in between the CT-1 and CT-2 becomes vulnerable as it loses protection. This is because the section is protected via Main-1 protection of line-1, that is distance relay-1.
As the isolator 89L of line1 is open therefore the distance relay does not get the voltage input from line-1’s CVT which is connected after the line isolator. This causes a problem. As we know the distance relay measures the impedance of the line, which is a function of voltage and current. The distance relay is now unable to provide the required protection to the section between both CTs because of the loss of voltage input from the line CVT.
Now to compensate for this lack of protection in this section, stub protection is introduced to the protection philosophy.
The only possible thing to protect the section in lack of voltage is the overcurrent protection philosophy.
Stub protection is an inherent part of the main-1 protection. It is an overcurrent element that is instantaneous in nature. It is capable of generating a trip signal instantly to the associated circuit breakers as soon as any discrepancies in the current parameter of any of the three phases are sensed. Thus providing the required protection to the unprotected section in case of opening of 89L.
As the status of 89L open reaches the distance relay, it then switches to stub protection mode or makes it enable while disabling the distance protection function to protect the section between two current transformers.
To ensure precise protection the fault current limit is set higher than the full load current of the line, preventing unintended tripping during normal service conditions.
Another reason why the fault current setting is not set lower than the full load current is that if due to any maloperation false isolator’s status goes in the relay, it will activate the stub protection and unnecessary tripping of the line will take place, which is not an ideal case.
Additionally, the time delay is to be set to zero or below the zone-2 time delay for swift fault isolation. This quick response is needed to ensure no propagation of faults and prevent cascading effect, which ensures the reliability and stability of the grid during critical emergencies.
SEQUENCE:
Opening of line isolator 89L triggers loss of voltage to distance protection
This status disables the line distance protection and enables STUB protection
If the current in the circuit reaches above the limiting value which is above full load current.
The stub protection in distance relay/ Main 1 protection triggers trip command to master trip relay 86
It in turn trips the associated circuit breakers and protects the section between two current transformers which is unprotected in the absence of stub protection.
It also blocks the auto-reclosure of tie CB and line CB
