SGP IMPORTANT QUESTIONS UNITWISE UNIT 1: 1(a) Derive an expression for Restriking voltage.(b)Show that the RRRV is proportional to the natural frequency of the circuit. 2(a) Describe briefly the arc phenomenon in a Circuit breaker. (b)In a short circuit test on a Circuit breaker, the following readings were obtained on single frequency transient: i.time to reach the peak restriking voltage, 50µsec ii.the peak restriking, 100kv Determine the average RRRV and frequency of oscillations. 3(a) Is it logical to express the breaking capacity of a circuit breaker in MVA? Discuss (b)In a short circuit test on a 3-phase, 132Kv C.B the following observations are made: pf of fault 0.4, the recovery voltage 0.90 times full line value, the breaking current symmetrical, the frequency of oscillations of restriking voltage 16k Hz. Assume that the neutral is grounded and the fault does not involve ground, determine the average rate of rise of restriking voltage. 6. Explain Slepian’s theory of arc interruption and discuss its limitations. How does energy Balance theory, Explain the process of arc interruption 7. Calculate the RRRV of a 220KV C.B with earthed neutral. The short Circuit test data obtained is as follows: The C.B is symmetrical and the restriking voltage has an oscillatory frequency of 15k Hz, The p.f of the fault is 0.2.Assume the short circuit to be an earthed fault. 8. Explain the effect of following terms on active recovery voltage (a) Power factor (b) Armature reaction (c) Circuit conditions UNIT 2: 1(a) Compare the arc rupture in oil and air circuit breakers and summarize the relative advantages and disadvantages of these types of switch gear (b)Explain the operating duty of a C.B 2. How does the current effect the arcing time in (a)Oil circuit breaker (b)Air Blast circuit breaker 9. 6(a) what are the favorable technical aspects of Air Blast circuit breaker? Why are pre-closing resistors necessary? (b)Compare the performance and characteristics of minimum oil breakers and air-blast breakers. Specify the connection you use for the directional element and justify by actually working of the connection and the maximum torque angle setting needed for the relay. Write short notes on (a)Reactance relay .B. 7(a) What are the advantages and disadvantages of Air Blast C. 5(a) Classify the types of C. 8.State its principle of operation. Assuming a 4 pole cup element for the directional element. With a neat sketch describe the difference between definite cahracterists and inverse characteristic of relays. explain the construction of a typical EHV Air Blast C.f of the current interrupted and the rate of recovery voltage after the rating.c and d.B’s. Explain the Differential protection.c? State its salient features. 5. Describe the principle of Air Blast C.3. Describe the various types of construction of attracted armature type relay. 2. Describe the construction of an induction disc relay.What are the advantages to induction relays. 3.How is the current setting and the time setting obtained.How does the p.B. Distinguish between Air Blast and SF6 circuit breakers 9. 4. Show that the torque on the disc of an induction disc relay is maximum when the phase difference between the two fluxes is 90.B’s when the arc quenching medium is the criterion (b)Mention the voltage range for which a particular type of C. Explain the principle of arc extinction and what the different methods of arc extinction are.Indicate the direction of rotation of the disc with reference to the fluxes under the poles.B is recommended. Specify also the phase shifting network to be used if the relay potential coil has impedance of 1000 60 Ω. 7. leakage in case of SF6 C. moisture.Why can they operate in a. Define the following terms and explain their significance in distance protection (a)Reach of a distance relay (b)under reach 4.With the help of neat sketch explain the principle of operation of Differential relays. UNIT:3 1. A cable circuit with an impedance angle of10 is to be protected by directional over current relay. 6.B with the help of neat sketches.And explain the arc quenching process in SF6 C. 8. (b)Describe how the KVA which a circuit has interrupted can be determined from an oscillogram of the voltage and currents’ occurring during the interruption process. State the various applications of Differential protection.B. Discuss the precautions to be taken to avoid dust. 2(a) Describe with a neat diagram of the percentage differential protection scheme to protect Y-∆ transformer (b) Describe with a neat diagram. Explain the protection schemes employed for generators against stator and rotor faults. 6. (a)Fault between phases or between 1-phase and earth. 4. (b) Fault between turns or one of the phase windings.3-phase. 3(a) what is the principle of harmonic restraint relay? Explain its applications. a circulating current protection scheme for a 3-phase. (b)A 3-phase Y-∆ connected 30MVA. 1MVA. 1000MVA generator is provided with differential scheme of protection. the application of the Merz-price circulating current system to the protection of alternators. An 11 KV. Describe with a neat diagram. What precautions must be taken in installing this system. 33/11KV transformer is protected by a simple differential relaying scheme. 2(a) Explain how the inclusion of a resistance in the neutral earthing circuit of an alternator affects the performance of the differential protection of the 3-phase stator.A 50MVA. Explain with a diagram.L-L synchronous generator is protected by an ordinary current balance protection scheme using 2000/5A C. Determine the value of resistance to be placed in the neutral to ground connection.33 KV. 7. and 11KV/400V delta-star transformer. The % of the generator winding to be protected against phase to ground fault is 80%.The neutral grounding resistor being 7. the operation of Buchholtz relay.Explain briefly with schematic diagrams of the protective gear for alternators connected on a grid for the following conditions. If the current transformer has a nominal secondary current of 5A. UNIT:4 1. The CT ratio on primary side is 500:5 and that on secondary side is . UNIT:5 1. 5(a) Enumerate the relaying schemes which are employed for the protection of a modern alternator (b)Describe with a neat sketch the percentage differential protection of a modern alternator.(b)Off-set mho relays (c)Mho relay 10.Calculate what percentage of the winding of each phase remains unprotected. calculate their ratio.The relay is said to operate when there is 50% out of balance current. Why are the differential relays more sensitive than over current relays. (b)Describe how protection is provided in large turbo-alternators against earth-fault in the rotor 3.T. Explain.5A.5Ω v and the minimum value of pick up current of relay being 0. 7. Draw the Sketch of complete scheme. The CTs on the LT side have a ratio of 420/5A.Show that the CTs on the HT side will have a ratio of 70:5/root 3. the general principle of operation of a distance protection scheme.Internal fault and ii.Ts for protecting a Y-∆ transformer.External fault. What is Buchholtz relay? Discuss its working principle? For what types of fault is it employed? 5(a) A 3-phase Y-∆ connected 66/11KV transformer is protected by Merz-price protection system. (b)How the protection system graded with respect to the time of operation of relays. (b)write a short notes on the following: i. 2(a) Explain over-current protection of feeder. 4(a)What are the requirements of protection of lines.Different transformer faults ii.6KV transformer is connected in Y-∆ and the protecting CT on the LV side has a ratio of 300/5A. Also calculate the relay current setting for fault drawing up to 200A of rated current. 4. (b) A 3-phase 33/6. Why? (b)What is back up protection for bus bar? .Biased differential protection for transformers iii. (b) A 100MVA ∆ /Y connected. Buchholtz relay. 8(a) The CT ratios of CTs on the 2 sides of a transformer will. (b)Explain the scheme of protection for ring mains.Translay scheme 5(a) Explain bus bar protection needs special attention. Explain in detail carrier current protection scheme. 11/220KV transformer is to be protected by a differential scheme. I. 9(a) Discuss the special factors that are to be considered while designing the protection scheme for a large Y-∆ power transformer. Write the scheme of protection for i. 3. 6(a) Discuss biased differential protection for transformers. The CT used are of ratio 6000/5 and 300/1 respectively. (b)Explain with reasons the connections of C. (b)Explain the principle of the percentage biased differential relay with harmonic restraint.Sketch the CT connection diagram for the relaying scheme. UNIT:6 1(a) Describe with a neat diagram. in general be different explain.What will be the ratio of the CT on the HV side.fault bus protection ii. Write short notes on the following.2000:5. 80% of the length of the wire (b)Write short notes on biased differential protection for transformer . and ii. 2(a) Discuss the advantages of grounding the neutral of the system. (b)Explain the phenomenon of Arcing grounds. (b)Explain why bus –coupler circuit breaker is used 8(a) with a neat sketch develop the duplicate bus-bar system (b) Explain why bus –coupler circuit breaker is used (c) Discuss why duplicate bus-bar system is used. 3-phase 50Hz over head line of 100Km length has a capacitance to earth of each line of 0. 50Hz network has a capacitance to neutral of 1.It is decided to use a ground fault neutralizer.2µF.0µF per phase. Calculate the reactance of an arc suppression coil suitable for the system to avoid adverse effect of arcing ground 3(a) what are the requirements of ground wire for protecting power conductors against a direct lightning stroke? Explain how it is achieved (b)Explain various methods of neutral grounding 4(a) Explain with the aid of circuit and phasor diagrams the function of Peter-son coil in a 3-phase system. the general principle of operation of a distance protection scheme. UNIT:7 1(a) what are the various methods of neutral grounding. A 132KV. What are permissible practical deviations from resonance in the tuning of the peter-son coil? (b)Explain various methods of neutral grounding 5(a) What are the reasons leading to the general practice of earthing the neutral point of a power system? Explain (b) Explain the phenomenon of Arcing grounds and discuss the method to minimize the effect of this phenomenon 6.6(a)What is meant by high impedance relay and what for it is used in differential protection of bus bars? (b)Discuss the necessity of bus bar protection 7(a) Describe with a neat diagram.100% of the length of the wire.01µF per Km.Determine inductance and KVA rating of the arc suppression suitable for this line 7(a) Discuss the advantages of neutral grounding (b)What is Tower-Footing Resistance? 8(a) A 50Hz over head line has the line to ground capacitance of 1. (b)A 33KV. Determine the reactance to neutralize the capacitance of i. (b)Describe the construction and operation of metal oxide surge arrestor.9. UNIT: 8 1(a) How over head transmission lines are protected from lighting strokes (b)Why ground wire is provided as the top lost conductor in high voltage transmission lines. 4(a) what is the importance of a ground wire in protecting power conductors against during lighting strokes. 2(a) Explain clearly how rating of a lighting arrestor is selected. Write short notes of the following (a)Causes of over voltages in a power system (b) Switching surges 8(a) Explain the principle of surge diverter (b)Describe the construction and working of a valve type lighting arrestor with neat diagram. 10(a) State the various causes of over voltages in a power system (b)Name the various devices used for protection against over voltage due to lighting.What is the best location of the lighting arrestor and why (b)Explain clearly why lighting arrestor is used. 9(a) Explain the phenomenon of insulating co-ordination in a power system (b)What is the necessity of protecting electrical equipment against traveling waves. . Write short notes of the following i. 7.Effective grounding ii . 3(a) what protective measures are taken against lighting over voltages.Resistance grounding iii. 11(a) Explain with neat sketches the mechanism of lighting discharge (b)How protection against switching surges are carried out in power systems. 5(a) what are volt time curves (b)What is their significance in power system studies? 6(a) what are the various methods of over voltage protection of over head transmission lines. (b)Explain clearly how the rating of a lighting arrestor is selected.What is the best location of a lighting arrestor and why. Explain.5µF.Reactance grounding 10(a) Derive an expression for the reactance of the Peter-son coil in terms of the capacitance of the protected line (b)Calculate the reactance of the coil suitable for a 33KV 3-phase transmission system of which the capacitance to earth of each conductor is 0. (b) What is the function of surge absorber and in what way it is different from lighting arrestor.