22524 Switchgear and Protection Unit 3 Notes PDF

Protective Relays

Introduction:

How Protective Relays Work:

1. Sensing: Protective relays continuously monitor electrical quantities such as current, voltage, frequency, and temperature. They compare these quantities against preset limits.
2. Decision: When a monitored electrical quantity goes outside of its predetermined range (indicating a potential fault), the relay makes a decision to operate or "trip."
3. Actuation: Once the relay decides to trip, it sends a signal to circuit breakers to interrupt the current flow and isolate the faulted section of the network.

Types of Protective Relays:

1. Electromagnetic Relays:
• Operate on principles of electromagnetic attraction or induction.
• Are analog devices that have been widely used for decades.
2. Static Relays:
• Utilize solid-state components.
• Offer greater precision than electromagnetic relays and have no moving parts, resulting in less wear and longer operational lifespans.
3. Microprocessor-based/Digital Relays:
• Use microprocessors to analyze power system voltages, currents, or other process variables.
• Highly customizable and can be programmed to provide various functions.
4. Numerical Relays:
• A type of digital relay.
• Deploy complex algorithms and time-domain processing. They can manage multiple protection functions simultaneously.

Common Protection Schemes:

1. Overcurrent Protection: Activates when the current exceeds a predetermined level.
2. Distance/Impedance Protection: Measures the impedance of a line to detect faults. It is commonly used in transmission line protection.
3. Differential Protection: Compares the current entering and leaving a device (like a transformer or generator). If there's a difference, it indicates an internal fault.
4. Under/Over Voltage Protection: Monitors the voltage level. It will trip if the voltage goes beyond or falls below set thresholds.
5. Frequency Protection: Monitors the frequency of the electrical system, tripping the system if the frequency deviates from its normal range.
6. Directional Protection: Determines the direction of fault currents and is used in interconnected systems where the direction of power flow is crucial.

Advantages of Protective Relays:

1. Safety: By quickly isolating faulted sections, they prevent potential hazards and equipment damage.
2. System Reliability: They ensure that faults affect minimal parts of the system, keeping most of the network live and operational.
3. Equipment Protection: By isolating faults, they prevent damage to expensive machinery and equipment.

Conclusion: