Switchgear and Protection (2020503C).pptx

switch gear and protection basic

1. Switchgear and Protection (2020503C) Lecture-1

2. Switchgear: Switchgear refers to a combination of electrical disconnect switches, circuit breakers, fuses, and other protective devices used to control, protect, and isolate electrical equipment in a power system. Its primary functions include: Switching Operations: Switchgear allows operators to control the flow of electrical power by enabling the opening and closing of electrical circuits. This is essential for normal system operation and maintenance. Protection: Switchgear plays a crucial role in safeguarding electrical equipment and preventing damage due to overcurrent, short circuits, and other electrical faults. When a fault occurs, switchgear isolates the faulty section to prevent further damage.

3. Isolation: In case of maintenance, repair, or when electrical equipment needs to be disconnected from the power supply, switchgear provides a means to safely isolate the equipment, ensuring the safety of maintenance personnel. Fault Clearing: Switchgear devices like circuit breakers and fuses are designed to interrupt the flow of current during faults, thus protecting the equipment and preventing electrical fires or explosions. Switchgear can be classified into several types based on its application and voltage level: Low Voltage (LV) Switchgear: Used in distribution networks with voltages typically up to 1 kV. Medium Voltage (MV) Switchgear: Designed for voltages from 1 kV to 33 kV, typically used for regional distribution. High Voltage (HV) Switchgear: Used in transmission networks for voltages exceeding 33 kV.

4. Necessity and Functions of Protective System Necessity : Electrical systems can face abnormal conditions like short circuits or equipment failure. Protection systems are needed to detect and isolate these faults quickly to avoid damage, fires, or power outages. Functions : Detect faults (like high current or voltage). Isolate faulty parts using circuit breakers or fuses. Minimize damage and maintain safety of people and equipment. Maintain supply continuity by disconnecting only the affected part.

5. Normal Conditions These are the healthy operating states of an electrical system. Everything works as it should. Voltage , current , and frequency are within safe, standard limits. Electrical loads (like lights, motors) operate without trouble. There are no faults (like short circuits or overloads). Power flow is balanced and stable. Example : A house receives a steady 230V supply and all appliances work properly.

6. Abnormal Conditions These are unwanted or dangerous situations in the power system. They may lead to damage if not corrected quickly. Common Abnormal Conditions: Overload : Current is more than the equipment is rated for. Causes heating and may lead to insulation damage. Short Circuit : Two or more conductors come in contact directly (e.g., phase-to-phase or phase-to-ground). Very high current flows suddenly → can damage equipment or cause fires. Overvoltage or Undervoltage : Voltage is too high or too low due to faults or switching surges. Frequency Variations : Happens if load and generation are not balanced (mainly in generators). Open Circuit : One or more conductors break → may cause imbalance or equipment malfunction.

7. fault A fault in a power system is an abnormal electrical condition that disrupts the normal flow of current. It usually involves a short circuit , open circuit , or ground fault that can damage equipment or interrupt power supply. Causes of Faults: Lightning strikes Tree branches falling on lines Insulation failure Equipment aging or damage Human errors Animals coming in contact with conductors

8. Main Types of Faults: Symmetrical Faults (Balanced Faults) Asymmetrical Faults(Unbalanced Faults) Any abnormal operating state of a power system is known as FAULT. Faults in general consist of short circuits as well as open circuits. Open circuit faults are less frequent than short circuit faults, and often they are transformed in to short circuits by subsequent events

9. Type Description Occurrence Severity Line-to-Ground (L-G) One phase touches the ground Most common Less severe Line-to-Line (L-L) Two phases touch each other Medium Moderate Double Line-to-Ground (L-L-G) Two phases touch ground Medium High Open Circuit One or more lines break Happens in overhead lines Lower current but causes imbalance Types of Asymmetrical Faults:

10. Zones of protection/Protective Zones It refer to predefined areas or sections within an electrical network that are covered by specific protective devices and relays. These zones are strategically designed to ensure the reliable and safe operation of the power system by promptly detecting and isolating faults or abnormal conditions. Purpose : The primary purpose of establishing zones of protection is to minimize the spreading of faults and disturbances within the electrical grid. When a fault occurs, such as a short circuit or an equipment failure, the goal is to isolate the affected area while keeping the rest of the power system operational. This helps prevent widespread blackouts and ensures that power is available to as many customers as possible.

11. What is a Protective Zone? A protective zone is a specific area of a power system (like a transformer, generator, transmission line, etc.) that is covered by a protection scheme (relays, circuit breakers, CTs, etc.) designed to detect faults and isolate the affected part. Types of Protective Zones in a Power System: Generator Zone -Protects the generator and its auxiliaries. Busbar Zone -Covers busbars where multiple circuits are interconnected. Transformer Zone -Protects the transformer windings and connections. Transmission Line Zone -Detects faults on overhead lines or cables. Feeder Zone Motor Zone etc…

12. Power System Protection Zone

13. Protective Zone Each zone is bounded by CTs (Current Transformers) and CBs (Circuit Breakers). Zones are slightly overlapping to avoid any unprotected (dead) zones. Faults occurring within a zone are cleared by primary protection. Faults not cleared by primary protection are handled by backup protection . Notes- CTs monitor the current entering and leaving the zone. They provide inputs to the protection relays. If a fault occurs inside the zone, the CTs detect abnormal current and trigger the relay. CTs define the boundary of that zone.

14. Primary protection is the main protection scheme designed to detect and isolate faults within a specific protected zone of a power system. It operates quickly and selectively, ensuring minimal damage and disturbance. Key Features: First line of defense High speed and accuracy Covers a specific zone Operates only for internal faults Installed close to the protected equipment (e.g., transformer, generator) Example: Transformer differential relay that trips the breaker instantly if internal fault occurs

15. Backup protection is a secondary protection scheme that operates when the primary protection fails (e.g., due to relay malfunction, CT failure, breaker stuck, etc.). It acts as a redundant system to ensure fault clearance. Key Features: Operates with time delay Less selective, may isolate larger sections Can be local (same location as primary) or remote (from another substation) Ensures system reliability in case of primary failure Example: Overcurrent relay at a remote substation that trips after a delay if the local transformer protection fails to operate.

16. Functional Requirements of Power System Protection or Essential features of protective system 1.Selectivity and Discrimination 2.Speed 3.Sensitivity 4.Reliability 5.Simplicity 6.Economy

17. Selectivity is the ability of the protective system to select correctly the faulty part of the system and disconnect the faulty part without disturbing rest of the system. In order to provide selectivity to the system entire system is divided into several protection zones. This property of protective relaying enables it to distinguish between the normal condition and abnormal condition. The protective scheme should operate during abnormal condition and not during normal condition. Selectivity and Discrimination

18. Speed -The relay system should disconnect the faulty section as fast as possible. Sensitivity - it is the ability of the system to operate for lower values of actuating quantity. The relay should detect even small deviations or low-magnitude faults, Reliability -Reliability is the ability of a protective relay (or protection system) to perform its intended function correctly under all expected operating conditions — especially during faults. Economy (Cost-effectiveness)- The cost of protection should be justified by the value of equipment being protected.

19. Current Limiting Reactors A current limiting reactor is an inductive coil (with high reactance and low resistance) installed in a power system to limit the short-circuit current during fault conditions. Short-circuit currents can: Exceed the breaking capacity of circuit breakers Cause mechanical stress on equipment Lead to voltage dips, system instability, or equipment damage Increase thermal stress on cables, transformers, and busbars Thus, reactors are inserted to increase system impedance, which limits fault current to safe, manageable levels.

20. Functions of Current Limiting Reactors: Limit short-circuit current Protect switchgear and equipment Improve voltage stability under fault conditions Enable coordination between protection devices Reduce dynamic stresses during faults

21. Common Arrangements of Current Limiting Reactors: Bus-Bar Reactor Arrangement Feeder Reactor Generator Reactor Neutral Grounding Reactor Ring Main Reactor Generator Reactor Feeder Reactor Bus Bar Reactor