1.a) Latching current for an SCR inserted in between a dc voltage source of 200v and the load, is 100mA. Compute the minimum width of gate – pulse current required to turn –on the SCR in case the load consists of

a) L = 0.2H b) R = 20Ω in series with L = 0.2H

c) R = 20 Ω in series with L = 2H.

b) Discuss the conditions which must be satisfied for turning – on SCR with a gate signal. [8+8]

2.a) Which current rating of an SCR is the most important?

b) SCR’s with a rating of 1000v and 200A are available to be used in a string to handle 6kv and 1kA. Calculate the number of series and parallel units required in case derating factor is 0.1 and 0.2. [8+8]

3. Consider a single phase half-wave rectifier with R load purely and the delay angle is 90⁰. Determine

a) Rectification efficiency b) Form factor c) Ripple factor

d) Total utilization factor e) Peak inverse voltage of thyristor. [16]

4. Determine the expression for the following performance factors of a single – phase fully controlled bridge converter.

a) Input displacement factor b) Input current distortion factor.

c) DC voltage ratio d) Input power factor

e) Input harmonic factor f) Voltage ripple factor

g) Active power input h) Reactive power input. [16]

1. (a) What is the importance of Surge current rating of a thyristor, explain in detail.

(b) A thyristor has half-cycle surge current rating of 1000mA for a 50Hz supply.

Calculate its one-cycle surge current rating and I2t rating. [10+6]

2. A single phase fully controlled converter is connected to a load comprised of 2 ohms

resistance and 0.3H inductance. The supply voltage is 230V at 50Hz. Estimate the

average load voltage, average load current and input power factor for a firing angle

of 200. Assume continuous and ripple free load current, draw load voltage waveform.

1. The latching current of a thyristor with d.c. voltage source of 100V is 50mA.

Calculate the value of minimum width of the gate pulse current when connected

to a pure inductive load of 1H. Compute the effect, if a resistance of 10 ohms is

connected in series with the load. [8+8]

2. Explain the operation of a singe phase half wave converter for R-load with neat

circuit diagram and necessary waveforms. Also derive the output average voltage

and current for α = 300. [12+4]

1. A 200A thyristor operates in parallel with a 300A thyristor. Their ON state voltage

drops are respectively 1.5V and 1.0V. Calculate the value of the resistance to be

inserted in series with each thyristor so that they share a load of 500A in proportion

to their respective current ratings. [16]

2. (a) Compare mid-point converters and bridge type converters and bring out im-

portant features.

(b) Compare discontinuous and continuous current modes of operation of convert-

ers and bring out salient features. [8+8]

1.Explain in detail various voltage ratings and current ratings of a thyristor.

4. Two SCRs are connected back-to-back have a load resistance of 400 ohms and a

supply of 110V ac. If firing angle is 600, find

(a) the rms output voltage

(b) average power.

 1.a) Sketch the dynamic characteristics of SCR during turn-ON process. Write down the necessary conditions required to turn-ON SCR.

b) A thyristor is connected in series with an inductance of 0.1 H and a resistance of 20 ohms and a sinusoidal voltage source of 230 V, 50 Hz. The latching current of SCR is 25 mA. Calculate the minimum width of gate pulse required to ensure reliable triggering if it is gated at an angle of 30⁰ in every positive half cycle of supply voltage. [8+8]

2.a) How a thyristor is protected from high values of dv/dt and di/dt. Derive an expression for parameters of the circuit required to protect thyristor from high values of dv/dt .

b) Design a UJT relaxation oscillator circuit for triggering a thyristor. The UJT has the following parameters:

η=0.72, I_p= 60μA , V_v=2.5 V, I_v=4mA, V=15 V, R_BB= 5 k ohms

The leakage current with emitter open is 3mA.The triggering frequency is 1kHz and V_g(min) of thyristor is 0.3 V. Also calculate the minimum and maximum values of triggering frequency. [8+8]

3.a) What is meant by the term ‘phase control’? Draw the circuit of a single phase half controlled bridge rectifier with common anode configuration. Explain the sequence of firing of thyristor to obtain output voltage across R-L load without free wheeling diode. Derive an expression for its output voltage.

b) A single phase half controlled bridge rectifier is connected to a source of 115 V, 50 Hz and a load of 20 ohms. Determine the average load current at firing angle of i) 30 deg ii) 60 deg. [8+8]

4.a) Explain the operation of single phase full wave controlled rectifier feeding an inductive load. Consider the effect of source inductance and derive an expression for output voltage in terms of source inductance and firing angle.

b) A single phase fully controlled rectifier is operated from 120 V, 50 Hz supply. It has a load of having R= 0.5 ohm and inductance of 6.5 mH and E=10 V. The firing angle is 60 deg. Calculate the average load current, average and rms values of thyristor current. [8+8]

1.a) Sketch static V-I characteristics of SCR. Explain how SCR could be operated as a switch with the help of the characteristics.

b) For an SCR the gate-cathode characteristics are given by a straight line passing through origin with gradient of 16 V/A. The turn-on time is 4 μsec and gate current required is 500 mA. Given the gate source voltage of 15 V, calculate

i) Gate power dissipation

ii) The resistance to be connected in series with gate.

2.a) Draw the two transistor analogy of SCR and explain why gate loses control once thyristor is fired when anode current is greater than latching value. (8)

b) Derive an expression for dynamic resistance to be connected across each thyristor in string of n number of series connected SCRs. [8+8]

3.a) Derive an expression for average and rms values of output voltage in a single phase half wave controlled rectifier with R-L load.

b) An SCR is connected between a source of 330 Sinωt and a load of 10 ohms and a battery with emf of 165 V. When it fired with a continuous dc signal, calculate

i) Average current ii) power supplied to battery iii) power dissipated in resistor.

[8+8]

4.a) Explain the operation of full wave controlled rectifier with input supply derived from a single phase transformer with centre tapped secondary.

b) Carry out harmonic analysis of input current in a fully controlled rectifier feeding an R-L load without free wheeling diode.

1.a) A BJT is acting as a switch, the forward current gain β is in the range 8 to 40. The load resistance is R_C = 11Ω. The dc supply voltage is V_CC=200v and the input voltage to the base circuit is V_B=10v. Find

a) The value of R_B that results in saturation with an over drive factor of 5.

b) The β forced

c) The power loss P_T in the transistor.

b) What are the various applications of an IGBT? [8+8]

2.a) Compare a BJT, MOSFET, IGBT with respect to the following:

a) Basel Gate control variable

b) Control characteristic

c) Switching frequency

d) On-stage voltage drop

e) Max voltage rating

f) Max current rating.

b) Write down the advantages and limitations of BJT and MOSFET. [8+8]

3.a) Explain the features of phase angle control.

b) A 230v, 50Hz one pulse SCR controlled converter is triggered at a firing angle of 40⁰ and the load current extinguishes at an angle of 210⁰. Find the circuit turn off time, average output voltage and the average load current for

a) R = 5Ω, L = 2mH

b) R = 5Ω, L = 2mH, E = 110v. [8+8]

4. Determine the expression for the following performance facts of single-phase fully controlled bridge converter

a) Input displacement factor b) Input power factor

c) DC voltage ratio d) Input current distortion factor

e) Input harmonic factor f) Voltage ripple factor

g) Active power input h) Reactive power input.

4. Explain only the operation of a single phase fully controlled bridge converter for firing angle α for a R load (do not derive any expressions).

Derive the following expressions:

The converter is connected to a 120v, 50Hz supply. The load current I_a is continuous and its ripple content is negligible. The turns ratio of the transformer is unity. Express the input current in a Fourier series and determine the HF of the input current. [16]

                                      

                                   TIRUMALA ENGINEERING COLLEGE, BOGARAM.                                        

                                          II B.Tech II Semester II Mid Examinations, March-April 2011                         Max Marks: 10

                                                                     POWER SYSTEMS-I                                                         Time: 1 hour                                                

           Answer any TWO Questions     (Electrical & Electronic Engineering)              All Questions carry equal marks

1.       A synchronous motor improves the pf of a load from 0.7 to 0.9 lagging. The losses and other mechanical load on it amount to (250 +200) KW, while the load for which power factor is considered is 1,500 KW. Find out the input to the synchronous motor in KVA and its power factor.

2.       Classify substations by considering the constructional features?

3.       A factory has a maximum demand of 500 KW, the load factor being 60 % during working hours.

The following two tariff's are available:

      ( i ) Rs 8 per KW of maximum demand plus 3 paisa per KWh            ( ii ) A flat rate of Rs 0.1 per KWh. 
      Determine the working hours per week above which tariff  will be cheaper.

4.       Compute the generation cost per KWh from the following data:  Installed capacity = 200MW;

capital cost = Rs 3000 per KW; Interest and depreciation = 12%; Fuel consumption= 0.9 Kg/KWh;

Fuel cost = Rs 70 per 1000Kg; other operating costs = 30% of fuel costs; load factor = 80%; peak load=170 MW.

                                   

                          TIRUMALA ENGINEERING COLLEGE, BOGARAM.                                         

                                          II B.Tech II Semester II Mid Examinations, March-April 2011                         Max Marks: 10

                                                                     POWER SYSTEMS-I                                                         Time: 1 hour                                                 

           Answer any TWO Questions     (Electrical & Electronic Engineering)              All Questions carry equal marks

1. The load connected to a station is 100 MW and the demand factor of the consumers is 1.25. Estimate the capacity of the generating station when load factor is 60%, capacity factor is 50%, diversity factor of distribution is 1.33 and that of transmission is 1.5. Take the efficiency of transmission and distribution as 90%.
2. A consumer is offered power at Rs 50 per annum per KVA of maximum demand plus 5 paise per unit. He proposes to install a motor to carry his estimated maximum demand of 300 b.h.p. The motor available has a power factor of 0.83 at full load. How many units will be required at 30 % load factor and what will be the annual bill? The motor efficiency is 90%
3. A consumer has an average demand of 400 KW at a p.f of 0.8 laging and annual load factor of 50%. The tariff is Rs 50 per KVA of maximum demand per annum plus 5 paise per KWh. If the power factor is improved to 0.95 lagging by installing phase advancing equipment, calculate the capacity of phase advancing  equipment.
4. Write a short notes on the construction aspects of a Gas Insulated Substation.

                                        TIRUMALA ENGINEERING COLLEGE, BOGARAM.                                         

                                          II B.Tech II Semester II Mid Examinations, March-April 2011                         Max Marks: 10

                                                                     POWER SYSTEMS-I                                                         Time: 1 hour                                                 

           Answer any TWO Questions       (Electrical & Electronic Engineering)              All Questions carry equal marks

1. Draw the layout & schematic connection of a pole mounted substation
2. A synchronous motor improves the pf of a load from 0.7 to 0.9 lagging. The losses and other mechanical load on it amount to (250 +200) KW, while the load for which power factor is considered is 1,500 KW. Find out the input to the synchronous motor in KVA and its power factor.

     3.  A factory has a maximum demand of 500 KW, the load factor being 60 % during

working hours. TH e following two tariffs’ are available:

            ( i ) Rs 8 per KW of maximum demand plus 3 paisa per KWh

            ( ii ) A flat rate of Rs 0.1 per KWh.

            Determine the working hours per week above which tariff ( i ) will be cheaper.

 4.   Compute the generation cost per KWh from the following data:          Installed capacity = 200MW;

        capital cost = Rs 3000 per KW; Interest and depreciation = 12%; Fuel consumption= 0.9 Kg/KWh;

        Fuel cost = Rs 70 per 1000Kg; other operating costs = 30% of fuel costs; load factor = 80%; peak load=170 MW.

                                    

                          TIRUMALA ENGINEERING COLLEGE, BOGARAM.                                       

                                          II B.Tech II Semester II Mid Examinations, March-April 2011                         Max Marks: 10

                                                                     POWER SYSTEMS-I                                                         Time: 1 hour                                                 

           Answer any TWO Questions         (Electrical & Electronic Engineering)              All Questions carry equal marks

1. The load connected to a station is 100 MW and the demand factor of the consumers is 1.25. Estimate the capacity of the generating station when load factor is 60%, capacity factor is 50%, diversity factor of distribution is 1.33 and that of transmission is 1.5. Take the efficiency of transmission and distribution as 90%.
2. A consumer is offered power at Rs 50 per annum per KVA of maximum demand plus 5 paise per unit. He proposes to install a motor to carry his estimated maximum demand of 300 b.h.p. The motor available has a power factor of 0.83 at full load. How many units will be required at 30 % load factor and what will be the annual bill? The motor efficiency is 90%

.     3.  a) Write a short notes on the construction aspects of a Gas Insulated Substation.

           b) Write short notes on the advantages of a Gas Insulated Substations.

           4.  A consumer has an average demand of 400 KW at a p.f. of 0.8 lagging and annual load    factor of 50%. The tariff is

                Rs 50 per KVA of maximum demand per annum plus 5 paisa per KWh. If the power factor is improved to 0.95
        lagging by installing phase advancing equipment, calculate the capacity of phase advancing equipment.

STLD IMP QUESTIONS

1.      Simplify this by using TABULAR METHOD.

f(A,B,C,D)= ABCD + ABD + BCD + ABD + ACD

2.      Design a 4:1 MUX with neat diagram.

3.      Implement the following by using a 4:1 MUX and 2:1 MUX.

F(A,B,C)=∑m(0,2,3,6,7)

4.      Design 1:4 DEMUX and design the combinational circuit for even and odd parity bit generators.

5.      Design the combinational circuit which converts the given BCD code into GRAY CODE.

6.      Explain about the classification of sequential circuits and different methods of triggering.

7.      Draw the truth tables and excitation tables of various flip-flops.

8.       Design a 5 bit ring counter and 6 bit Johnson counter.

9.      Design serial binary adder.

10.  Design sequence detector.

11.  Explain about the mealy and Moore models of seq.circuits.

12.  Design a mod-7 syn counter by using T- flip flops.

13.  Explain about the state diagram and draw the state diagrams of J-K ff, S-R ff, T-ff and D-ff.

14.   Get the characteristic equations for J-K ff, T-ff and D-ffs.

15.  Design the combinational circuit for 3 bit magnitude comparator.