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3. Protective Relays
Protective relays are devices used to monitor electrical circuits and detect abnormal conditions (such as overloads, short circuits, or faults). Once these abnormal conditions are detected, the relay activates a circuit breaker to disconnect the faulty section of the circuit.
3.1 Fundamental Quality Requirements
Protective relays must meet certain quality requirements to ensure they provide reliable protection. These quality requirements are:
3.1.1 Selectivity
- Definition: Selectivity refers to the relay’s ability to operate only for the fault condition in the specific area or zone where the fault occurs, without affecting the rest of the system.
- Explanation: A relay should only disconnect the faulty section of the system, leaving the rest of the system unaffected, so that power supply is not unnecessarily interrupted.
3.1.2 Speed
- Definition: Speed refers to how quickly the relay responds to a fault and triggers the protective action (e.g., opening a circuit breaker).
- Explanation: The faster the relay operates, the quicker the fault can be isolated, minimizing damage to equipment and ensuring safety.
3.1.3 Sensitivity
- Definition: Sensitivity refers to the relay’s ability to detect even small or low-level faults and respond accordingly.
- Explanation: A relay should be sensitive enough to detect fault conditions, even if they are not significant in magnitude.
3.1.4 Reliability
- Definition: Reliability refers to the relay’s ability to operate consistently and accurately over time without failure.
- Explanation: The relay should have a proven track record of functioning as expected, even in harsh operating conditions.
3.1.5 Simplicity
- Definition: Simplicity means that the relay’s operation and setup should be straightforward and easy to understand.
- Explanation: A simpler relay is easier to maintain and troubleshoot, reducing the possibility of errors in operation.
3.1.6 Economy
- Definition: Economy refers to the cost-effectiveness of the relay in terms of purchase price, installation, and maintenance.
- Explanation: A protective relay should provide good protection at a reasonable cost, balancing performance with budget constraints.
3.2 Basic Relay Terminology of Protective Relays (Concept Only)
Here are some essential terms used in the context of protective relays:
3.2.1 Relay Time
- Definition: Relay time is the time taken by the relay to respond after a fault condition is detected.
- Explanation: It includes the time taken to detect the fault and activate the corresponding protection mechanism (e.g., tripping a circuit breaker).
3.2.2 Pick-up
- Definition: Pick-up refers to the minimum value of the fault condition (e.g., current or voltage) at which the relay will start operating.
- Explanation: The relay "picks up" and begins to function when the fault condition reaches this threshold.
3.2.3 Reset Current
- Definition: Reset current is the level of current below which the relay returns to its normal condition after having operated.
- Explanation: After a fault is cleared, the current returns to a safe level, and the relay resets to resume normal operation.
3.2.4 Current Setting
- Definition: Current setting refers to the adjustment that sets the threshold level of current at which the relay operates.
- Explanation: This setting determines the level of current that will trigger the relay to operate and protect the circuit.
3.2.5 Plug Setting Multiplier (PSM)
- Definition: The Plug Setting Multiplier is a factor that multiplies the plug setting to determine the operating range of the relay.
- Explanation: It helps to fine-tune the relay’s operating characteristic to match the protection needs of the system.
3.2.6 Time Setting Multiplier (TSM)
- Definition: Time Setting Multiplier is a factor used to adjust the time delay settings for a relay.
- Explanation: It allows the relay’s time settings to be modified to provide appropriate protection based on the system’s characteristics.
3.3 Protective Relays: Principle of Working, Operation of Different Types
3.3.1 Electromagnetic Relay
Attracted Armature Type:
- Working Principle: This relay operates based on electromagnetic induction. When the fault current flows, it generates a magnetic field that pulls an armature towards a contact, closing the circuit and activating the protection mechanism.
- Operation: It is commonly used for overcurrent protection.
Solenoid Type:
- Working Principle: A solenoid creates a magnetic field when current passes through it. This field pulls a movable iron core, which operates the relay.
- Operation: Often used for overload or short circuit protection.
Watthour Meter Type:
- Working Principle: This relay uses a rotating disc that is proportional to the power consumption. If the power exceeds a certain threshold, the relay operates.
- Operation: Used in power metering and protection of circuits where power consumption needs to be monitored.
3.3.2 Thermal Relay
- Working Principle: Thermal relays operate based on the heating effect of the current. When current flows through the relay, it heats a bimetallic strip that bends due to thermal expansion, activating the protection mechanism.
- Operation: Commonly used for overload protection, where the relay responds to sustained high currents.
3.3.3 Static Relay
- Working Principle: Static relays use electronic components (like transistors and diodes) to sense fault conditions and activate a relay.
- Operation: These relays are faster and more reliable than electromagnetic or thermal relays and are widely used for modern protection systems.
3.4 Overcurrent Relay – Time-Current Characteristics
An overcurrent relay operates when the current exceeds a set threshold. Its time-current characteristics show how the relay's operating time decreases as the fault current increases.
- Inverse Time Characteristics: The relay operates faster when the fault current is higher.
- Definite Time Characteristics: The relay operates after a fixed time delay, regardless of the fault current.
3.5 Microprocessor-Based Overcurrent Relays
Microprocessor-based overcurrent relays use digital processors to provide advanced protection functions. These relays can be programmed with more complex characteristics, including time delays, fault detection, and coordination with other relays in the system.
- Advantages: Greater flexibility, accuracy, and ability to perform more functions than conventional relays.
- Applications: Commonly used in modern power systems for better control and protection.
3.6 Distance Relaying
Distance relays are used in transmission lines to measure the impedance (distance) between the fault and the relay location. The relay trips when the impedance reaches a preset value, indicating a fault within that distance.
- Types: Impedance relays, Reactance relays, and Mho relays.
- Application: Mainly used in long-distance transmission lines to provide reliable fault protection.
3.7 Directional Relay
A directional relay detects the direction of current flow in the system. It only operates when the current flows in the direction of the fault (towards the protected equipment), providing selectivity.
- Application: Directional relays are commonly used for protection in transmission lines, generators, and motors to distinguish between faults in the forward or reverse direction.
3.8 Operation of Current and Voltage Differential Relay
Current Differential Relay:
- Working: This relay compares the currents entering and leaving a protected zone. If the difference between these currents exceeds a threshold (indicating a fault within the zone), the relay operates.
- Application: Used in transformer and generator protection.
Voltage Differential Relay:
- Working: This relay compares the voltage at different points of the system. A significant difference in voltage indicates a fault in the system.
- Application: Used for protection of electrical equipment like transformers and generators.
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