Question

Define the terms related to Protective Relay: i) Selectivity ii) Sensitivity.

Answer 1

Sure!

i) **Selectivity**: This refers to a relay's ability to discriminate between a fault within its designated protection zone and faults outside of it. It ensures that only the relay closest to the fault will trip, leaving other parts of the system unaffected. This minimizes system disruption and isolates the fault efficiently.

ii) **Sensitivity**: This is the relay's capability to detect and respond to small fault currents or deviations from normal operating conditions. High sensitivity means the relay can identify and react to minor faults, ensuring early detection and protection of equipment before severe damage occurs.

Answer 2

Certainly! Protective relays are crucial components in electrical power systems used to detect faults and initiate appropriate actions to protect the system. The terms **selectivity** and **sensitivity** are fundamental in understanding how these relays function:

### I) Selectivity

**Selectivity** (or coordination) refers to the ability of a protective relay system to isolate only the faulty section of the system while keeping the healthy parts operational. In other words, selectivity ensures that the protection system can distinguish between faults occurring in different parts of the electrical network and isolate only the specific section where the fault has occurred.

**Key Aspects of Selectivity:**
- **Grading:** Protective relays are set up in a hierarchical manner with each relay protecting a specific section of the system. The grading or coordination ensures that the relay closest to the fault operates first, before any other relays further upstream.
- **Time Coordination:** This involves setting different time delays for relays at different levels of the protection hierarchy. The relay closest to the fault has the shortest time delay, while relays further away have progressively longer delays.
- **Current Coordination:** Relays are set with different pickup currents, so that relays closer to the fault have lower pickup settings compared to relays further away.

**Benefits:**
- **Minimized Disruption:** Proper selectivity ensures that only the affected part of the system is disconnected, minimizing the impact on the rest of the network.
- **Improved Reliability:** By isolating faults efficiently, the reliability and stability of the power system are enhanced.

### II) Sensitivity

**Sensitivity** refers to the ability of a protective relay to detect low-level faults or incipient faults that may not be immediately obvious but could potentially develop into more serious problems if not addressed. High sensitivity ensures that the relay can detect even small deviations from normal operating conditions, allowing for early detection of faults.

**Key Aspects of Sensitivity:**
- **Threshold Levels:** The sensitivity of a relay is determined by its ability to detect and respond to signals below a certain threshold level. A more sensitive relay can detect lower magnitude faults.
- **Detection of Subtle Changes:** Sensitivity allows relays to pick up on minor changes in parameters like current, voltage, or frequency that might indicate a developing fault.
- **Response Time:** A sensitive relay should also respond quickly to detected faults to prevent damage or further issues.

**Benefits:**
- **Early Fault Detection:** High sensitivity allows for early detection of potential faults, enabling timely intervention to prevent larger issues.
- **Enhanced Protection:** Sensitive relays can provide additional protection by identifying faults that might otherwise go unnoticed, thus improving overall system safety.

In summary, **selectivity** focuses on ensuring that only the affected section of the network is disconnected in case of a fault, while **sensitivity** is about the relay's ability to detect and respond to even minor faults. Both characteristics are crucial for an effective and reliable protection system.