Relay and switchgear protection


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Relay and switchgear protection

  1. 1. Devjoti Das Shantanu Kumar Das Binit Kumar Das Presented By:
  2. 2. CONTENTS Relay Buchholz Relay Circuit Breakers Instrument Transformer
  3. 3. What is a relay? • Relay is a sensing device which senses abnormal voltage and current conditions in power system and sends signal to circuit breaker to remove faulty part from rest of the power system. • First line of defense is provide by the back up relaying equipment. Back up protection operates only when primary protection has failed.
  4. 4. Electromagnetic Attraction Type Relay
  5. 5. Electromagnetic Induction Principle • The relay operating on this principle can be used for A.C. quantity only. • In this relay, first alternating flux produces eddy currents in aluminium disk. • The second alternating flux interact with these eddy currents and thus produces torque. • Similarly, Eddy currents produced by second flux interacts with the first flux and torque is produced. • Net torque produces rotating torque in the disc.
  6. 6. • Over Current Relay1 • Directional Relay2 • Distance Relay6 TYPES OF RELAY
  7. 7. • Instantaneous Over-Current Relays 1 • Inverse-time Current Relay 2 • Inverse Definite Minimum Time Current Relay 3 • Very Inverse Relay 4 • Extremely Inverse Relay 5 TYPES OF OVER CURRENT RELAY
  10. 10. DISTANCE RELAY • Impedance Relay1 • Reactance Relay2 • Mho Relay6
  12. 12. Feeder can be long, medium or short transmission line and it may also be a distribution network. Feeder can be protected by various method: 1) Over Current Protection 2) Distance Protection 3) Pilot Wire Protection FEEDER PROTECTION
  13. 13.  Over Current Protection though simple and economical, require re-adjustment if change in circuit condition occurs.  Only two phase and one earth fault relay is required for complete protection of three phase line.  Distance protection is costly so far distribution network, electric utilities and on sub transmission lines over current protection is used.
  15. 15. Distance Protection • Whenever over-current relay is not selective or circuit requirements change, then distance protection is preferred. • Problem with over current relaying is that fault current depends upon ckt condition and generating capacity. • If these conditions changes, then overall protection needs to be changed.
  16. 16. Transformer relay • Transformers need to be protected against short circuits and over-heating. • For low KVA ratings, over current relays are used, but in case of large transformers with higher KVA ratings, differential protection is used. • For this purpose, Buchholz Relay is used.
  17. 17. Buchholz Relay • Gas actuated relay immersed installed in oil immersed transformer for protection against all kinds of faults. • In the field of electric power distribution and transmission, a Buchholz relay is a safety device mounted on some oil-filled power transformers and reactors, equipped with an external overhead oil reservoir called a conservator. • The Buchholz Relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment.
  19. 19. Circuit breaker is a piece of equipment which can:- Make or break a circuit either manually or by remote control under normal conditions Break a circuit automatically under fault conditions Make a circuit either manually or by remote control under fault conditions
  20. 20. Operating Principle  Consists of fixed and moving contacts called electrodes.  Under normal conditions, these contacts remain closed & will not open until and unless the system becomes faulty.  When faults occurs in any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit.
  21. 21. Arc Phenomenon  When a short circuit occurs, a heavy current flows through the contacts of CB before they are opened.  At the instant when the contacts begin to separate, the contacts area decreases rapidly and large current increases current density thus increasing the temperature.  Heat produced in the medium[Oil or Air] is sufficient to ionise.  The ionise air or vapour of oil acts as a conductor and an arc get sustain in the contacts.
  22. 22. Arc Extinction Factors responsible for sustenance of Arc.  Potential difference between the contacts.  Ionised particles between contacts. Methods of Arc Extinction • High Resistance Method • Low Resistance Method
  23. 23. Classification of Circuit Breakers:-  Oil Circuit Breakers.  Air- blast circuit breakers.  Sulphur hexafluroide circuit breakers.  Vacuum circuit breakers.
  24. 24. OIL CIRCUIT BREAKER  Transformer oil used as arc quenching medium  It insulates between phases and the ground, and it provides the medium for the extinguishing of the arc.  When electric arc is drawn under oil, the arc vaporizes the oil and creates a large bubble that surrounds the arc.  The gas inside the bubble is around 80% hydrogen, which impairs ionization.  The oil surrounding the bubble conducts the heat away from the arc and thus also contributes to deionization of the arc.
  25. 25. Advantages: • It absorbs the arc energy to decomposes the oil into gases which have excellent cooling properties • It acts as an insulator and permits smaller clearance between live conductors and earth components. • The surrounding oil presents cooling surface enclose proximity to the arc.
  26. 26. Disadvantages:  It is inflammable and there is a risk of a fire.  It may form an explosive mixture with air.  The arcing products (eg. Carbon) remains in the oil and its quality get deteriorated with successive operations.
  27. 27. Air-Blast Circuit Breaker  These breakers employ a high pressure air-blast as an arc quenching medium.  The contacts are opened in a flow of air-blast established by the opening of blast valve. The air-blast cools the arc and sweeps away the arcing products to the atmosphere. This rapidly increases the dielectric strength of the medium between contacts and prevents from re- establishing the arc.
  28. 28. Advantages:  The risk of fire is eliminated.  The arcing time is very small due to rapid movement of the contacts.  The arcing products are completely removed by the blast. Disadvantages:  The Air has relatively inferior arc extinguishing properties
  29. 29. Vacuum Circuit Breaker • When the contacts of the breaker are opened in vacuum (10−7 to 10−5 torr), an arc is produced between the contacts • Vacuum is used as an arc quenching medium. • Have greatest insulating strength. • The arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc rapidly condense on the surfaces of the circuit breaker contacts, resulting in quick recovery.
  30. 30. Advantages • Compact, reliable and have longer life. • No fire hazards. • No generation of gas during and after operation. • Can interrupt any fault current. • No noise is produced while operating. • Require less power for control operation.
  31. 31. The Sulphur hexaflouride SF6 is an electro-negative gas and has a strong tendency to absorb free electrons.  The contacts of the breaker are opened in a high pressure flow of SF6 gas and an arc is struck between them. SF6 has excellent insulating strength. When a free electron collides neutral gas molecule a negative ion is formed and the electron is absorbed by neutral gas molecule. SF6 + e SF6 - Negative ions formed are relatively higher compared to free electrons. Sulphur Hexaflouride (SF6) Breakers
  32. 32. ADVANTAGES OF SF6 C.B. Current chopping is minimized at low pressure and low velocity. Eliminates moisture problem. Gives noiseless operation. No carbon particle is formed during arcing. Atmospheric conditions does not affect Circuit breaker performance.
  33. 33. DISADVANTAGES OF SF6 C.B. SF6 breakers are costly due to the high cost of SF6 Since SF6 gas has to be reconditioned after every operation of the breaker, aditional equipment is required.
  34. 34. Comparison of circuit Breaker
  35. 35. Features of CB
  36. 36. Instrument Transformer • Instrument transformers convert the currents and voltages of the H.V. lines to values, which are measurable by meters and protections • Two types: – Current Transformer – Potential/Voltage Transformer.
  37. 37. • A current transformer (CT) is used for measurement of electric currents. Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are known as instrument transformers. • When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. • A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays