power system protection fFinal

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A Seminar on POWER SYSTEM PROTECTION By Syed Osman Azeem 11311D4912:



INTRODUCTION Electrical Energy is generated at few kV and stepped up. Transmitted through AC and HVDC lines. Stepped down and distributed at load centers. Its natural mode of synchronous operation knits the system together .


POWER SYSTEM PROTECTION Branch of electrical power engineering that deals with the protection of electrical power systems from faults through the isolation of faulted parts .

Why do we need protection?:

Why do we need protection? Electrical apparatus operates at various voltage levels and may be enclosed or placed in open . Under abnormal operating conditions protection is necessary for safety of electrical equipments, and safety of human personnel.

Disturbances: Light or Severe:

Disturbances: Light or Severe Maintain acceptable operation 24 hours a day Voltage and frequency must stay within certain limits Small disturbances - variation in transformer or generator load Severe disturbances require a protection system They can jeopardize the entire power system They cannot be overcome by a control system

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Bad design of the protection system Overreaction of the protection system Loss of service in a large area or population region Hazard to human life May result in enormous economic losses

Damage to Main Equipment:

Damage to Main Equipment

Protection Systems Components :

Protection Systems Components Current and Voltage Transformers Protective relays Circuit breakers Batteries Fuses


INTRODUCTION TO RELAY A relay is a logical element which process the inputs ( mostly voltages and currents) from the system and issues a trip decision if a fault within its jurisdiction is detected. Inputs to a relay are Current from a current transformer. Voltage from a voltage transformer.

Protective Relays:

Protective Relays

Examples of Relay Panels:

Examples of Relay Panels Old Electromechanical Microprocessor- Based Relay

How Do Relays Detect Faults?:

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

Advantages of Relay’s:

Advantages of Relay’s Maximum flexibility. Provides multiple functionality. Self checking and communication facility. It can be made adaptive


CIRCUIT BREAKER A circuit breaker is an automatically operated electrical switch designed to protect the power system from damage caused by overload or short circuit It opens on relay command. It has to handle large voltages and currents. The inductive nature of power system results in arcing between the terminals of a CB. CBs are categorized based on the interrupting medium used.

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C ircuit breakers may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation. Circuit breakers can be classified by the medium used to extinguish the arc : Vacuum circuit breakers—With rated current up to 3000 A, these breakers interrupt the current by creating and extinguishing the arc in a vacuum container.

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Air circuit breakers—Rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. SF 6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas. They are most often used for transmission-level voltages

Components to be protected in power system:

Components to be protected in power system 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. High voltage transmission network – Protection on the transmission and distribution serves two functions: Protection of plant and protection of the public. Here protection means to disconnect equipment which experience an overload or a short to earth.

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Earth fault – Earth fault protection requires current transformers and senses an imbalance in a three-phase circuit. Normally the three phase currents are in balance. 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. 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, exceeds a pre-determined level, the circuit breaker will operate.

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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 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.


CONCLUSION The objective of a protection scheme is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible still in operation.



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