Fundamentals of power protection system.pptx
- 1. www.eucl.reg.rw www.eucl.reg.rw Introduction to Power System Protection December 2024
- 2. www.eucl.reg.rw General Protection Philosophy Basic Principles of System Protection The philosophy behind system protection is to ensure the safe, stable, and reliable operation of electrical systems, particularly in power plants, electrical grids, and distribution networks. Protection systems are designed to detect faults (such as short circuits, overloads, and ground faults), isolate affected sections, and minimize damage to equipment while ensuring that the overall system continues to operate efficiently. There are several key principles that guide the design and operation of protection systems. These principles ensure that protection devices, such as relays, circuit breakers, and fuses, perform optimally to safeguard both equipment and personnel.
- 3. www.eucl.reg.rw 1. ReliabilityDefinition: Reliability refers to the ability of the protection system to correctly detect faults and operate consistently under all conditions, ensuring that it functions as intended without failure. 2. Importance: A protection system must have a high degree of reliability to avoid false trips (incorrectly isolating healthy parts of the system) or failure to trip during actual fault conditions. Reliable protection ensures that faults are cleared quickly, preventing equipment damage, fire hazards, and personnel injury. Overview of Power Systems
- 4. www.eucl.reg.rw System redundancy and fail-safe mechanisms are often incorporated to enhance reliability, ensuring that if one component of the protection system fails, the system still operates correctly. Example: In a power plant, if a fault occurs in a transformer, a reliable protection system will correctly identify the fault and isolate the transformer from the rest of the system to prevent further damage. 2. Selectivity Definition: Selectivity (or discrimination) refers to the ability of the protection system to isolate only the faulty section of the system without affecting the rest of the network. It ensures that the protection system operates in a way that minimizes the impact of faults on the overall system.
- 5. www.eucl.reg.rw Importance: Proper selectivity ensures that only the faulty area is disconnected, allowing the rest of the system to continue normal operation. This is particularly important in complex systems with multiple interconnected components where an isolated fault could trigger cascading failures if not properly managed. How Selectivity Works: Protection devices are coordinated in such a way that the device closest to the fault operates first, while upstream devices remain unaffected. Time coordination and current settings are key to ensuring selective operation. For example, if a fault occurs in a distribution line, only the protection device nearest the fault should trip, leaving upstream equipment such as transformers and main distribution lines in service. Example: If a fault occurs in a motor circuit, the protection system should only disconnect the motor and not the entire plant or factory electrical network.
- 6. www.eucl.reg.rw 3. Speed Definition: Speed refers to the response time of the protection system to detect faults and take corrective actions (such as opening circuit breakers or triggering alarms). A fast protection system is essential to prevent further damage to equipment and avoid unsafe conditions. Importance :Quick fault detection and isolation reduce the duration of faults, minimizing damage to electrical equipment such as transformers, generators, and cables. Speed is particularly critical in preventing the escalation of faults. For example, a short circuit in a generator must be cleared quickly to prevent permanent damage to the machine.Protection relays need to be fast enough to operate before excessive currents or voltages cause irreversible damage to equipment.
- 7. www.eucl.reg.rw How Speed Works: The protection system is designed to measure fault conditions continuously and act swiftly when a fault is detected. For example, instantaneous protection relays may be used for high-speed operation to clear short circuits immediately, while time-delayed protection may be used in cases of overloads or transient conditions. Example: In a short circuit fault, the protection system must act within milliseconds to trip the circuit and prevent damage to transformers or generators.
- 8. www.eucl.reg.rw 4. Sensitivity Definition: Sensitivity refers to the ability of the protection system to detect small faults or abnormal conditions at an early stage and take action before they escalate into more significant issues. It ensures that the system responds to faults of varying magnitudes, from minor disturbances to major faults. Importance: High sensitivity ensures that even small deviations from normal operating conditions (such as slight overloads or minor ground faults) are detected early, preventing them from causing more severe damage to equipment. Sensitivity is especially important in ensuring sensitive equipment such as control panels, protection relays, and power electronic devices are safeguarded against abnormal conditions.
- 9. www.eucl.reg.rw Challenges: Protection systems must balance sensitivity and selectivity too sensitive a system may result in nuisance tripping (tripping for minor or non- harmful conditions), while too low sensitivity may cause delayed fault clearance or failure to detect faults. Advanced relays and signal processing techniques (like harmonic analysis or digital filtering) are often used to increase sensitivity while minimizing false tripping. Example: If a small ground fault occurs in a motor winding, a sensitive protection system will quickly detect it and trip the circuit before the fault causes significant damage or overheating.
- 10. www.eucl.reg.rw Voltage Transformers High Voltage Medium Voltage Note: Voltage transformers are also known as potential transformers
- 12. www.eucl.reg.rw Examples of Relay Panels Old Electromechanical Microprocessor- Based Relay
- 13. www.eucl.reg.rw 2.Protective devices Protective relays control the tripping of the circuit breakers surrounding the faulted part of the network Automatic operation, such as auto-reclosing or system restart, Automatic transfer to alternate power supplies Automatic synchronization if spinning reserve is available. Monitoring equipment which collects data on the system for data transmission control and post event analysis like SCADA. While the operating quality of these devices, and especially of protective relays, is always critical, different strategies are considered for protecting the different parts of the system. Very important equipment may have completely redundant and independent protective systems, while a minor branch distribution line may have very simple low-cost protection.
- 14. www.eucl.reg.rw How Do Relays Detect Faults? • When a fault takes place, the current, voltage, frequency, and other electrical variables behave in a peculiar way. For example: – Current suddenly increases – Voltage suddenly decreases • Relays can measure the currents and the voltages and detect that there is an overcurrent, or an undervoltage, or a combination of both • Many other detection principles determine the design of protective relays
- 15. www.eucl.reg.rw Main Protection Requirements • Reliability – Dependability – Security • Selectivity • Speed – System stability – Equipment damage – Power quality • Sensitivity – High-impedance faults – Dispersed generation
- 17. www.eucl.reg.rw Primary Protection Zone Overlapping Protection Zone B Protection Zone A To Zone B Relays To Zone A Relays R Protection Zone B Protection Zone A To Zone B Relays To Zone A Relays R
- 18. www.eucl.reg.rw 3.Types of protection: Generator sets – In a power plant, the protective relays are intended to prevent damage to alternators or to the transformers in case of abnormal conditions of operation, due to internal failures, as well as insulating failures or regulation malfunctions. Such failures are unusual, so the protective relays have to operate very rarely. If a protective relay fails to detect a fault, the resulting damage to the alternator or to the transformer might require costly equipment repairs or replacement, as well as income loss from the inability to produce and sell energy.
- 19. www.eucl.reg.rw Differential Protection Principle Internal Fault IDIF > ISETTING CTR CTR 50 Relay Operates Protected Equipment
- 20. www.eucl.reg.rw Problem of Unequal CT Performance • False differential current can occur if a CT saturates during a through-fault • Use some measure of through-current to desensitize the relay when high currents are present External Fault Protected Equipment IDIF ¹ 0 CT CT 50
- 21. www.eucl.reg.rw Possible Scheme – Percentage Differential Protection Principle Protected Equipment ĪR ĪS CTR CTR Compares: Relay (87) OP S R I I I | | | | 2 S R RT I I k I k ĪRP ĪSP
- 22. www.eucl.reg.rw • Current Transformer Ratio (CTR) in Differential Protection Relay The Current Transformer Ratio (CTR) plays a crucial role in the operation of differential protection relays, ensuring accurate fault detection and protection for equipment like transformers, generators, and busbars. Differential protection operates based on the principle of comparing the current entering and leaving a protected zone. In an ideal condition (no fault), the current entering equals the current leaving. Any significant difference (differential current) indicates a fault within the zone.
- 23. www.eucl.reg.rw • Role of CTR in Differential Protection In systems where current transformers (CTs) are used, CTR is the ratio between the primary current (on the power system side) and the secondary current (on the relay side). It ensures that: • Currents from both sides of the protected zone are comparable: Transformers and other equipment may have different current levels due to varying voltage ratings and capacities. The CTR adjusts these currents to a common base for accurate comparison. • The relay measures differential current correctly: Mismatched CTRs can cause errors, leading to nuisance tripping or failure to detect faults.
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- 27. www.eucl.reg.rw • Bus protection • Transformer protection (Types and Ratio of CT’s are to be proper) • Generator protection • Line protection ( If communications between two ends of the line are made available-GPS Relays) • Large motor protection • Reactor protection • Capacitor bank protection • Compound equipment protection Differential Protection Applications
- 28. www.eucl.reg.rw • The overcurrent differential scheme is simple and economical, but it does not respond well to unequal current transformer performance • The percentage differential scheme responds better to CT saturation • Percentage differential protection can be analyzed in the relay and the alpha plane • Differential protection is the best alternative selectivity/speed with present technology Summary of Differential Protection
- 29. www.eucl.reg.rw High voltage transmission network – Protection on the transmission and distribution serves two functions: Protection of plant and protection of the public (including employees). At a basic level, protection looks to disconnect equipment which experience an overload or a short to earth. Some items in substations such as transformers might require additional protection based on temperature or gas pressure, among others.
- 30. www.eucl.reg.rw Electric Power System Exposure to External Agents
- 31. www.eucl.reg.rw Damage to Main Equipment
- 32. www.eucl.reg.rw Short Circuits Produce High Currents Fault Substation a b c I I Wire Three-Phase Line Thousands of Amps
- 33. www.eucl.reg.rw Electrical Equipment Thermal Damage I t In Imd Damage Curve Short-Circuit Current Damage Time Rated Value
- 34. www.eucl.reg.rw Mechanical Damage During Short Circuits • Very destructive in busbars, isolators, supports, transformers, and machines • Damage is instantaneous i1 i2 f1 f2 Rigid Conductors f1(t)= k i1(t) i2(t) Mechanical Forces
- 35. www.eucl.reg.rw Earth fault – Earth fault protection again requires current transformers and senses an imbalance in a three- phase circuit. Normally the three phase currents are in balance, i.e. roughly equal in magnitude. If one or two phases become connected to earth via a low impedance path, their magnitudes will increase dramatically, as will current imbalance. If this imbalance exceeds a pre- determined value, a circuit breaker should operate.
- 36. www.eucl.reg.rw Distance (Impedance Relay)– Distance protection detects both voltage and current. A fault on a circuit will generally create a sag in the voltage level. If the ratio of voltage to current measured at the relay terminals, which equates to an impedance, lands within a pre- determined level the circuit breaker will operate. This is useful for reasonable length lines, lines longer than 10 miles, because its operating characteristics are based on the line characteristics. This means that when a fault appears on the line the impedance setting in the relay is compared to the apparent impedance of the line from the relay terminals to the fault.
- 37. www.eucl.reg.rw • Under Distance Protection scheme, Three Zone Protection using Directional mho relays are generally used. • If the relay setting is determined to be below the apparent impedance it is determined that the fault is within the zone of protection. When the transmission line length is too short, less than 10 miles, distance protection is becomes more difficult to coordinate. In these instances the best choice of protection is current differential protection.
- 38. www.eucl.reg.rw Back-up – At all times the objective of protection is to remove only the affected portion of plant and nothing else. Sometimes this does not occur for various reasons which can include: Mechanical failure of a circuit breaker to operate Incorrect protection setting Relay failures A C D E Breaker 5 Fails 1 2 5 6 11 12 T 3 4 7 8 9 10 B F Backup Protection
- 39. www.eucl.reg.rw A failure of primary protection will usually result in the operation of back-up protection. Remote back-up protection will generally remove both the affected and unaffected items of plant to clear the fault. Local back-up protection will remove the affected items of the plant to clear the fault. Low-voltage networks – The low voltage network generally relies upon fuses or low-voltage circuit breakers to remove both overload and earth faults.
- 40. www.eucl.reg.rw www.eucl.reg.rw THANK YOU FOR ATTENTION