EE402-Protection and Switchgear Dept. of EEE

Transcription

1 UNIT II - ELECTROMAGNETIC RELAYS Part A 1. Name the different kinds of over current relays. Induction type non-directional over current relay, Induction type directional over current relay & current differential relay. 2. Define operating time of a relay. Operating time of a relay is defined as the time period extending from the occurrence of the fault through the relay detecting the fault to the operation of the relay. 3. Define resetting time of a relay. Resetting time of a relay is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset. 4. What is Time grading of relays. It is the setting of time of operation of various relays protecting the different sections of a line and so set that there by nearest to the fault location alone will operate first and clear the fault. 5. What are Over and Under current relays? Over current relays are those that operate when the current in a line exceeds a predetermined value. (e.g.: Induction type non-directional/directional over current relay, differential over current relay) whereas Undercurrent relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(e.g.: differential over-voltage relay) 6. What is biased differential beam relay? The biased beam relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an over-current balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force. 7. Give the limitations of Merz Price protection. Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes impossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral point, the voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CT s. In addition to this, there always an inherent phase difference between the primary and the secondary quantities and a possibility of current through the relay even when there is no fault. 8. Why is an under frequency relay required in a power system?(may/june 2012) (Nov/Dec 2013) An under frequency relay is one which operates when the frequency of the system (usually an alternator or transformer) falls below a certain value. This relay is used to sense the frequency of the entire power system. 9. What are the features of directional relay? High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal ratio; burden must not be excessive. 10. What is static relay? It is a relay in which measurement or comparison of electrical quantities is made in a static network which is designed to give an output signal when a threshold condition is passed which operates a tripping device. 11. What is a programmable relay? A static relay may have one or more programmable units such as microprocessors or microcomputers in its circuit. 12. What are the advantages of static relay over electromagnetic relay? (Nov/Dec 2011) (May/June 2014) (Nov/Dec 2014) i) Low power consumption as low as 1mW ii) No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of contacts iii) No gravity effect on operation of static relays. Hence can be used in vessels ie, ships, aircrafts etc. iv) A single relay can perform several functions like over current, under voltage, single phasing protection by incorporating respective functional blocks. This is not possible in electromagnetic relays v) Static relay is compact vi) Superior operating characteristics and SNS College of Technology 1

2 accuracy vii) Static relay can think, programmable operation is possible with static relay viii) Effect of vibration is nil, hence can be used in earthquake-prone areas ix) Simplified testing and servicing. 13. What are the different types of over current relays i) Definite time ii)inverse definite minimum time(idmt) iii)very Inverse iv)extremely Inverse. 14. What is earth fault protection? The protective scheme used for the protection of an element of a power system against earth faults is called as earth fault protection 15. List out the applications of static relays. (Nov/Dec 2012) i)protection of generators ii)protection of transformers iii)protection of transmission lines, and iv)protection of motors. 16. What is meant by directional relay? (May/June 2012) The relay which is able to sense the direction of power flow and act for a particular direction of power flow is called directional relay. 17. What is meant by differential relay? (May/June 2013) (Apr/May 2015) A differential relay is one that operates when the phasor difference of two or more similar electrical quantities exceeds a predetermined value. 18. What are the types of fuses? (Nov/Dec 2013) a)low voltage fuses i)semi-enclosed rewireable fuse ii)hrc fuse b)high voltage fuses i)cartridge type ii)liquid type iii)metal clad type 19. Write the function of under frequency relay. (Nov/Dec 2013) The frequency relay is designed to be used for the protection of power generators and other AC equipment against over frequency and under frequency. The operation of the relay can be based on the definite time principle, on the rate of frequency change principle, or on a combination of these two principles. 20. What is an under frequency relay. (Nov/Dec 2013) (Nov/Dec 2014) Under frequency relay trips the feeder on load at set value of frequency. So as to give relief to the generator, thereby saving the unit.under frequency relay thus aids load shedding programme to save the grid. 21. List out the different types of distance relay.(may/june 2014) Dependent on the ratio of V and I there are three types of distance relays which are i)impedance relay which is based on measurement of impedance Z ii)reactance relay which is based on measurement of reactance X iii)admittance or Mho relay which is based on measurement of component of admittance Y. 22. In what way distance relay is superior to over current protection? (Nov/Dec 2015) Distance relays are preferred to overcurrent reiays because they are not nearly so much affected by changes in short-circuit-current magnitude as overcurrent relays are, and, hence, are much less affected by changes in generating capacity and in system configuration. This is because distance relays achieve selectivity on the basis of impedance rather than current. 23. Where are negative sequence relays employed? It is necessary to provide the negative sequence protection to the generators against the unbalanced load condition. The negative phase sequence filter along with the over current relay provides the necessary protection against the unbalanced loads. 24. What are the three measurement operations that need to be done for relay operation? 1. Magnitude measurement, 2. Comparison, 3. Ratio measurement. 25. What is the operational principle of induction cup structure relay? Induction cup relay work in same principle of induction disc relay. The basis construction of this relay is just like four poles or eight pole induction motor. The number of poles in the protective relay depends SNS College of Technology 2

3 upon the number of winding to be accommodated. Actually when any one replaces disc of induction relay by a aluminum cup, the inertia of rotating system of relay is significantly reduced. Due to low mechanical inertia, the operating speed of induction cup relay is much higher than that of induction disc relay. Moreover, projected pole system is designed to give maximum torque per VA input. Part B 1. Describe the construction and operation of over current relay with directional Scheme.(June 2014) (Nov/Dec 2015). Diagram of the relay (8) Construction and working principle of relay (4) IDMT characteristics and explanation (4) 2. i)discuss the operating principle, constructional features area of applications of directional relay. ii) How do you implement directional feature in the over current relay. (Nov/Dec 2015) (Nov/ Dec 2012) Diagram (6) Operating principle, construction and application (8) Describe the addition of directional element (2) 3. Explain MHO relay characteristic on the R- X diagram. Discuss the range setting of various distance relays placed on a particular location. (Nov/ Dec 2012) Definition of mho relay (2) Diagram (4) Explanation with R X characteristics. (6) Range setting explain with zones of protection concept (4) 4. (i) What are the different inverse-time characteristics of over current relays and mention how to characteristics can be achieved in practice for an electromagnetic relay? (ii) Mention the advantages of static relays. (Nov/Dec 2014). Draw and Explain various IDMT characteristics (10) IDMT char for Electromagnetic relay. (2) Static relay definition (2) Advantages (min of 4) (2) SNS College of Technology 3

4 5. Explain the principle of percentage biased differential relay with necessary diagrams. Also discuss its application. (May/June 2012) The fault occurs at point X and the primary currents in the circuit are I 1 and I 2. The C.T. secondary currents are i 1 and i 2. The current flowing through the operation coil of the relay is (i 1 + i 2 )/2. This is because the operating coil is connected to the midpoint of the restraining coil. Thus if the number of turns of the restraining coil is N then i 1 flows through N/2 and i 2 flows through remaining N/2. The total ampere turns are i 1 N/2+ i 2 N/2 i.e N ((i 1 + i 2 )/2. This is as good as the flow of current ((i 1 + i 2 )/2 through the entire restraining coil. The operation characteristics of such a biased differential relay is shown in the Fig 2. The characteristics shows that except at low currents, ratio of differential operation current to average restraining current is a fixed percentage. Hence the relay is called the percentage differential relay. 6. i) With neat block diagram, explain the construction and operating principle of electromagnetic relay. ii) Describe the operation of over current relay with directional feature.(nov/dec 2013) Block diagram Relay diagram Construction and working in brief Overcurrent relay with directional feature. (Diagram + Explanation) 7. Describe the operating principles and characteristics of impedance and mho relays. (Nov/Dec 2013) SNS College of Technology 4

5 8. Explain the operation of i) Negative sequence relay ii) Static relay. iii) under frequency relay (Apr/May 2015) For each relay type Diagram SNS College of Technology 5

6 Working principle explanation SNS College of Technology 6

7 9. Explain in what way distance protection is superior to over current protection for the protection of transmission lines. (Nov/Dec 2013) (May/June 2014) (Nov/Dec 2014) Perhaps the most interesting and versatile family of relays is the distance-relay group. In the preceding chapter, we examined relays in which one current was balanced against another current, and we saw that the operating characteristic could be expressed as a ratio of the two currents. In distance relays, there is a balance between voltage and current, the ratio of which can be expressed in terms of impedance. Impedance is an electrical measure of distance along a transmission line, which explains the name applied to this group of relays. 10. i) Explain with the help of neat diagram the construction and working of induction type directional power relay (Nov/Dec 2015). ii) Draw and explain the block diagram of a static relay and state its advantages. Block Diagram Static Differential Relay Static Overcurrent Relay (May/June2013) SNS College of Technology 7

8 11. What is universal torque equation? Using this equation derive the following operating characteristics. i) Impedance relay ii) Reactance relay iii) Mho relay. (16) (May/June 2013) (Nov/Dec 2015) Refer to Part B Question no Draw and explain about differential protection of transmission lines. (Apr/May 2015) Merz Price Scheme for Tr Lines SNS College of Technology 8