ee1301 - two marks q & a new

P.RATHNAVEL, LECTURER/ EEE DHANALAKSHMI COLLEGE OF ENGINEERING, CHENNAI

UNIT I POWER SEMICONDUCTOR DEVICES

1. What are the classifications of power semi converter devices? The classifications of power semi converter devices are,

1. Uncontrolled turn- on and off (diode) 2. Controlled turn- on and uncontrolled turn off ( SCR) 3. Controlled turn on and turn off ( BJT, power MOSFET, SITH, IGBT, SIT and

MCT) 4. Continuous gate signal requirements ( BJT, MOSFET, IGBT, SIT) 5. Pulse gate requirements ( SCR, GTO, MCT) 6. Bipolar voltage withstanding capability ( SCR, GTO) 7. Unipolar voltage withstanding capability ( BJT, MOSFET, IGBT) 8. Bidirectional current capability ( TRIAC, RCT) 9. Unidirectional current capability ( SCR, GTO, BJT, MOSFET, IGBT, SIT,

SITH, power diode)

2. Mention some of the thyristors family Some of the thyristors are,

1. SCR ( silicon controlled rectifier) 2. TRIAC 3. DIAC 4. SCS ( silicon controlled switch) 5. GTO ( gate turn off) thyristor

3. What is a thyristor?

The term of thyristor denotes a family of silicon based semiconductor devices used for power control in DC and AC system.

The thyristor has the combination of THYRatron characteristics and transISTOR construction, so the name thyristor is derived from the bold letters (THYRISTOR).

4. IGBT is a voltage controlled device. Why?

IGBT is a voltage controlled device because the controlling parameter is gate-emitter voltage.

5. Why IGBT is very popular nowadays?

IGBT is very popular to nowadays because it has, 1. Lower hate requirements 2. Lower switching losses 3. Smaller snubber circuit requirements


6. What are the different methods to turn on the thyristor? The different methods to turn on the thyristor are,

1. Forward voltage triggering 2. Gate triggering 3. dv/dt triggering 4. Temperature triggering 5. Light triggering

7. What is the difference between power diode and signal diode? The difference between power diodes and signal diodes are,

Sl.No.

Power diode

Signal diode

1

Constructed with n-layer, called drift region between p+ layer and n+ layer.

Drift region is not present.

2

The voltage, current and power ratings are higher.

The voltage, current and power ratings are Lower

3

Power diodes operate at high speeds.

Operates at higher switching speed.

8. IGBT is a voltage controlled device. Why? IGBT is a voltage controlled device because the controlling parameter is gate-

emitter voltage.

9. Power MOSFET is a voltage controlled device. Why? Power MOSFET is a voltage controlled device because the output (drain) current

can be controlled by gate-source voltage.

10. Power BJT is a current controlled device. Why? Power BJT is a current controlled device because the output (collector) current

can be controlled by base current.

11. What is the relation between α and β? The relation between α and β is given by,

where, β = forward current gain α = current gain


12. What are the different types of power MOSFET? Two types of power MOSFET are,

1. N-channel MOSFET 2. P-channel MOSFET

13. How to turn off the thyristor? A thyristor which is in the on state can be turned off by reducing the forward current to a level below the holding current.

14. Define: latching current The latching current is defined as the minimum value of anode current which it must attain during turn on process to maintain conduction when gate signal is removed.

15. Define: holding current

The holding current is defined as the minimum value of anode current below which it must fall to for turning off the thyristor.

16. Define: SCR turn off time ( tq) Turn off time (tq) is the minimum value of time interval between the instant when the on state current has decrease to zero and the instant when the thyristor is capable of withstanding forward voltage without turning on.

17. What is the turn off time for converter grade SCR? The turn off time for converter grade SCR is 50-100 ms.

18. What is the turn off time for inverter grade SCR?

The turn off time for inverter grade SCR is 3-50 ms.

19. What is a snubber circuit? A snubber circuit is a protection circuit. It consists

of a series combination of a resistor and a capacitor in parallel with the thyristors. It is mainly used for dv / dt protection.

20. What losses occur in a thyristor during working conditions? The losses occur in a thyristor during working condition are,

1. Forward conduction losses 2. Loss due to leakage current during forward and reverse blocking 3. Switching losses at turn-on and turn-off 4. Gate triggering loss. 21. Define: hard-driving or over-driving

Hard-driving or over-driving defined as, when gate current is several times higher than the minimum gate current required, a thyristor is said to be hard-driving or over-driving.


22. Define: circuit turn off time

The turn off time is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process. 23. Why circuit turn off time should be greater than the thyristor turn -off time?

Circuit turn off time should be greater than the thyristor turn-off time for reliable turn-off, otherwise the device may turn-on at an undesired instant, a process called commutation failure.

24. What is meant by secondary break down?

The secondary breakdown is a destructive phenomenon which result from the current flow to a small portion of the base, producing localized spots is sufficient, the excessive localized heating may damage the transistor. Thus secondary breakdown is caused by a localized thermal runaway, resulting from high current concentrations.

25. What are the operational difference between thyristor family and transistor family of devices? The operational difference between thyristor family and transistor family of devices are,

1. Once a SCR is turned on by a gate signal, it remains latched in on state due to internal regenerative action. However a transistor must be given a continuous base signal to remain in on state.

2. In order to turn off a thyristor , a reverse voltage must be applied across its anode cathode terminals. However a transistor turns off when its base signal is removed.

26. What are the advantages of GTO over BJT?

The advantages of GTO over BJT are,

1. Higher voltage blocking capability.

2. It is majority carrier Devices.

3. High ratio of peak surge current to average current.

4. Operates as a switch always hence on state losses are reduced

27. What is reverse recovery time?

The reverse recovery time is the time for which negative current flows through the SCR. During this period, all the carriers inside the SCR are removed and junctions, J1 and


J2 achieve their forward blocking state. At the end of reverse recovery time, anode current becomes zero.

28. What is meant by secondary break down?

The secondary breakdown is a destructive phenomenon, result from the current flow to a small portion of the base, producing localized spots is sufficient, the excessive localized heating may damage the transistor. Thus secondary breakdown is caused by a localized thermal runaway, resulting from high current concentrations.

29. What are the different types of power transistor?

Types of power transistors are,

1) Bipolar junction transistor (BJT)

2) Metal – oxide semiconductor field – effect transistors (MOSFET)

3) Insulated gate bipolar transistor (IGBT)

30. What are the advantages of TRIAC?

The advantages of TRIAC are:

1) It can be triggering with positive or negative polarity voltage.

2) It needs a single fuse for protection, which also simplifies the construction 3) It needs a single heat sink of slightly larger size, whereas antiparallel thyristor pair needs two heat sinks.

4) In some dc applications, SCR is required to be connected to be connected with a parallel diode to protect against reverse voltage, where a triac used may work without diode as safe breakdown in either direction is possible.

31. In TRIAC which of the modes the sensitivity of gate is high?

The more sensitive of the TRIAC is greatest in the first quadrant when turned on with positive gate current and also in the third quadrant when turned on with negative gate current.

32. Define: pinch off voltage of MOSFET

The pinch off voltage of MOSFET defined as the gate source voltage VGS at which, the channel will be completely depleted, offering a high value of RDS and there will be no current flow from the drain to source, IDS=0.


33. Distinguish between MOSFET and BJT?

The distinguish between MOSFET and BJT are,

34. What are the factors that influence the turn off time of a SCR?

The factors are influence the turn off time of a SCR,

1. Recovery process

2. Recombination process

35. What is meant by forward break over voltage?

The forward break over voltage is the forward voltage at which the junction J2 will have an avalanche breakdown.

MOSFET BJT

1. Power MOSFET has lower switching losses.

2. It has more conduction losses.

3. It is a voltage controlled device.

4. It is a unipolar device.

5. Powers MOSFET operate at switching frequency in the MHz range.

6. MOSFET has positive temperature coefficient.

7. Secondary breakdown does not occur in MOSFET.

8. MOSFETs are available with ratings upto 500V and 140V

1. BJT has higher switching losses

2. It has low conduction losses

3. It is current controlled device.

4. It is a bipolar device.

5. BJT operate at switching frequencies in KHz range.

6. BJT has negative temperature coefficient.

7. Secondary breakdown occur in BJT

8. BJTs are available with rating upto 1200V and 800V.


36. Distinguish between holding current and latching current of SCR.

The distinguish between holding current and latching current of SCR are,

Holding current Latching current

1. The thyristor can be turn off or the thyristor can be forward blocking state only , if the forward current falls below a low level current called the holding current.

2. Holding current is associated with turn off process.

3. Holding current is less then the latching current.

1. The latching current is defined as the minimum value of anode current which it must attain during turn on process to maintain conduction when gate signal is removed.

2. Latching current is associate with turn on process.

3. Latching current is the 2 to 3 times of holding current.

37. What are the terminals in thyristor?

The terminals in thyristor are,

1. anode

2. cathode

3. gate

38. Define: β and α

α is a current gain which is the ratio of collector current to base current.

β is a forward current gain which is the ratio of collector current to the

emitter current.

Mathematically, α = ∆IC / ∆IB

β = ∆IC / ∆IE

39. Define: delay time

The delay time period define as, base emitter voltage VBE is applied, the base current IB rises to IBS and the collector current IC is equal to zero or collector emitter leakage current ICBO. The time required to change the base emitter capacitance to VBES= 0.7V.


40. Define: rise time

The rises time period define as, collector current IC rises to steady state values ICS and the collector emitter voltage falls from VCC to VCES. The rise time depends on the input capacitance.

41. Mention some disadvantages of TRIAC.

The disadvantages of TRIAC are,

1) It has low dv/dt rating compared to SCR.

2) Since TRIAC can be triggered in either direction, a trigger circuit with TRIAC needs a careful consideration.

3) SCR is available in large rating compared to TRIAC.

4) Reliability is less than that of SCR.

42. What is meant by reverse break over voltage?

The reverse break over voltage means, when cathode is positive with respect to anode with gate circuit open, the junction I1 and J3 are reverse biased but J2 junction is forward biased. When the reverse voltage is increased the junction J1 and J3 will have an avalanche breakdown at a voltage. This voltage is called critical break over voltage Vbr.

43. Mention the advantages of GTO over SCR.

The advantages of GTO over SCR are,

1) Elimination of commutative components in forced commutation, resulting in reduction in cost, weight and volume

2) Reduction in acoustics and electromagnetic noise due to the elimination of commutation chokes

3) Faster turn off, permitting high switching frequencies

4) Improved efficiency of the converter


44. Mention the disadvantages of GTO over SCR.

The disadvantages of GTO over SCR are,

1) Magnitude of latching and holding current is more in a GTO.

2) ON state voltage drop and the associated loss is more I a GTO

3) Due to multicathode structure of GTO, triggering gate current is higher than that required foe the conventional thyristor.

4) Gate drive circuit losses are more.

5) Its reverse voltage blocking capability is less than its forward blocking capability.


UNIT 2

PHASE CONTROLLED CONVERTER

1. What is meant by phase controlled rectifier? Phase controlled rectifier converts constant AC voltage into variable DC output voltage.

These rectifiers use line voltage for their commutation, as such these are also called line commutated or naturally commutated AC to DC converter.

2. Mention some of the applications of controlled rectifier.

The applications of controlled rectifiers are, 1. Steel rolling mills, printing press, textile mills and paper mills employing dc motor

drives. 2. DC traction 3. Electro chemical and electro-metallurgical process 4. Portable hand tool drives 5. Magnet power supplies 6. HVDC transmission system

3. What is the function of freewheeling diodes in controlled rectifier?

The function of freewheeling diodes in controlled rectifier: 1. It prevents the output voltage from becoming negative. 2. The load current is transferred from the main thyristors to the freewheeling diode,

thereby allowing all of its thyristors to regain their blocking states.

4. Mention the advantages of freewheeling diodes in a controlled rectifier? The advantages of freewheeling diodes in a controlled rectifier are,

1. Input power factor is improved. 2. Load current waveform is improved and thus the load performance is better.

5. Define: firing angle α

The firing angle is defined as, the angle between the zero crossing of the input voltage and the instant the thyristor is fired.

6. What are the advantages of single phase bridge converter over single phase mid-point

converter? The advantages of single phase bridge converter over single phase mid-point converter are,

1. SCRs are subjected to a peak-inverse voltage o f 2Vm in a fully controlled bridge rectifier. Hence for same voltage and current ratings of SCRs, power handled by mid- point configuration is about of that handled by bridge configuration. 2. In mid-point converter, each secondary winding should be able to supply the load power. As such, the transformer rating in mid-point converter is double the load rating.


7. What are the different methods of firing circuits for line commutated converter? The different methods of firing circuits for line commutated converter are,

1. Using UJT 2. The cosine wave crossing pulse timing control 3. Digital firing schemes

8. Give an expression for average output voltage of single phase semiconverters.

The expression for average output voltage of single phase semi converters Average output voltage is,

Vdc = (Vm / π) (1 + cos α).

9. Define: input power factor in controlled rectifier The input power factor is defined as the ratio of the total mean input power to the

total RMS input volt-amperes. p.f = V1 I1 cosΦ1/ Vrms Irms

where, V1 =Vrms= phase voltage, I1 = fundamental components of the supply current Φ1 = input displacement angle Irms=supply rms current 10. What is meant by commutation?

The commutation means, the process of changing the direction of current flow in a particular path of the circuit. This process is used in thyristors for turning it off.

11. What are the types of commutation?

Two types of commutations are, 1. Natural commutation 2. Forced commutation

12. What is meant by natural commutation?

Natural commutation means, the current flowing through the thyristor goes through a natural zero and enables the thyristor to turn off.

13. What is meant by forced commutation?

Forced commutation means, the current flowing through the thyristor is forced to become zero by external circuit.

14. What is meant by half controlled rectifier? A half controlled rectifier or semiconverter uses a mixture of diodes and thyristors and

there is a limited control over the level of dc output voltage. It is also known as one-quadrant converter. Here, the output current and output voltage is always positive.

15. What is meant by full controlled rectifier?

A fully controlled converter or full converter or full converter uses thyristor only and there is a wider control over the level of dc output voltage. It is also known as two quadrant converter.


16. Define: two pulse converter

Two pulse converter is defined as, two triggering pulse or two sets of triggering pulses are to be generated during every cycle of the supply to trigger the various SCRs.

17. Define: voltage ripple factor Voltage ripple factor is defined as, the ratio of the net harmonic content of the output voltage to the average output voltage.

18. How is power factor of semiconverter better than that of full converter? When supply given to load, the semiconverter receives less reactive power due to freewheeling action when compared with full converter. Therefore, the power factor is better in semi converter.

19. What is meant by rectification mode in single phase full controlled converter? In single phase full converter, α < 90°, the voltage at the dc terminals is positive. Therefore, power flows from source to load and the converter operates as a rectifier is source voltage VS and source current IS positive. This is known as rectification mode.

20. What is mean by inversion mode? The inversion mode means, in a single phase full converter, α > 90°, the voltage at the dc terminals is negative. Therefore, power flows from load to source and the converter operates as a line commutated inverter is source voltage VS is negative and source current is positive. This is known as inversion mode or synchronous inversion.

21. Define: harmonic factor of the input current The harmonic factor of the input current is defined as, the ratio of the total harmonic current to the fundamental component. IH = ( I2rms – I21) / I1

22. Define: displacement factor The input displacement factor is defined as, the angler displacement between the fundamental component of the AC line current and the association line to neutral voltage.

23. Mention the advantages of three phase controlled rectifiers? The advantages of three phase controlled rectifiers are,

1. Output voltage increase 2. Ripple content decreases. 3. By using higher number of pulses, the output voltage should be

smoother.

24. Mention the advantages of six pulse converter? The advantages of six pulse converter are,

1. Commutation is made very simple 2. Distortion on the AC side is lower order harmonics 3. Inductance required in series is considerably reduced


25. What are the effects of source impedance in the controlled rectifiers?

The effects of source impedance in the controlled rectifiers are, 1. The average output voltage should be reduced. 2. Reduced displacement factor. 3. Output current waveform also changed.

26. Define: commutation angle or overlap angle

The commutation period, when both outgoing and incoming thyristor are conducting is known as the overlap period. The overlap period, both devices share conduction is known as commutation angle or overlap angle (µ).

27. What are all the types of three phase controlled converter? The types of three phase controlled converter are,

1. Half wave controlled converter 2. semi converter 3. full converter

28. What are the advantages of three phase converters over single phase converter?

The advantages of three phase converters over single phase converter are, 1. In three phase converters, the ripple frequency of the converter output

voltage is higher than in single phase converter. Consequently, the filtering requirements for smoothing out the load current are less.

2. The load current is mostly continuous in three phase converters. The load performance when the three phase converters are used is therefore superior as compared to when single phase converters are used.

29. Mention the modes of operation of single phase full converter.

The modes of operation of single phase full converter are, 1. Forward motoring 2. Regenerative braking

30. Why power factor is better in single phase semi converter over full

converter? Power factor is better in single phase semiconverter over full converter is, power delivered to load is greater in semiconverter than full converter, because in semiconverter there is no negative conduction.

31. Mention the functional modes of dual converters? The functional modes of dual converter are,

1. Non- circulating current mode 2. Circulating current mode


32. What are the effects of source inductance?

The effects of source inductance are, 1. Due to source inductance, the average output voltage should be reduced. 2. Reduce the displacement factor. 3. Output current waveform also changed.

33. What are the two methods of control in AC voltage controller?

The two methods of control in AC voltage controller are, 1. ON- OFF control 2. Phase control

34. Define: ON-OFF control and phase control

ON-OFF control ON-OFF control defined as, the thyristors are employed as switched to connect the load circuit to the source for a few cycle of the source voltage and disconnect it for another few cycles Phase control The phase control defined as, thyristor switches connect the load to the AC source foe a position of each cycle of input voltage.


Unit III

DC TO DC CONVERTER

1. Define: dc chopper A dc chopper defines as a high speed static switch used to obtain variable dc voltage from a constant dc voltage. 2. What are the applications of dc chopper?

The applications of dc chopper are, 1. Battery operated vehicles 2. Traction motor control in electric traction 3. Trolley cars 4. Marine hoists 5. Mine haulers 6. Electric braking.

3. What are the advantages of dc chopper?

The advantages of dc choppers are, 1. High efficiency 2. Smooth acceleration 3. Fast dynamic response 4. Regeneration

4. What are the types of dc chopper?

The types of dc chopper are, 1. Step up chopper 2. Step down chopper

5. What are the different types of chopper configuration?

Depending on the direction of current and voltage, chopper can be classified into the following types,

1. Type A chopper or first quadrant chopper 2. Type B chopper or second quadrant chopper 3. Type C chopper or two quadrant type A chopper 4. Type D chopper or second quadrant type B chopper 5. Type E chopper or four quadrant chopper

6. Define step-up and step-down chopper?

In a step- down chopper or Buck converter define as, the average output voltage is less than the input voltage. i.e., Vo < Vs In a step- up chopper or Boost converter define as, the average output voltage is more than the input voltage. Vo > Vs


7. Write down the expression for average output voltage for step down chopper. Average output voltage for step down chopper V0 = α Vs, α is the duty cycle.

8. Write down the expression for average output voltage for step up chopper.

Average output voltage for step up chopper V0 = Vs / (1- α), α is the duty cycle.

9. What is meant by duty-cycle? Duty cycle is defined as the ratio of the on time of the chopper to the total time period of the chopper. It is denoted by α.

10. What are the two types of control strategies? 1. Time Ratio Control (TRC) 2. Current Limit Control method (CLC)

11. What is meant by TRC?

In time ratio control (TRC) means, the value of Ton / T is varied in order to change the average output voltage.

12. What are the two types of TRC? 1. Constant frequency control 2. Variable frequency control

13. What is meant by FM control in a dc chopper?

The frequency modulation control means, the chopping frequency ‘f’ is varied and either on time Ton is kept constant or off time Toff is kept constant. This method of controlling is called frequency modulation scheme.

14. What is meant by PWM control in dc chopper?

The PWM control method means, the on time Ton is varied but chopping frequency ‘f’ is kept constant, the width of the pulse is varied and this type of control is known as Pulse Width Modulation (PWM).

15. What are the different types of chopper with respect to commutation process? The different types of chopper with respect to commutation process are,

1. Forced commutation i. Voltage commutated chopper. ii. Current commutated chopper.

2. Load commutated chopper.

16. What is meant by current commutation? The current commutation means, a current pulse is made to flow in the reverse direction through the conducting thyristor and when the net thyristor current becomes zero, it is turned off.


17. What is meant by voltage commutation? The voltage commutation means, a charged capacitor momentarily reverse biases the conducting thyristor and turn it off.

18. What is meant by forced commutation? The forced commutation means, when the current flowing through the thyristor is forced to become zero to turn the thyristor off .

19. What is meant by load commutation? The load commutation means, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting thyristor.

20. What are the advantages of current commutated chopper? The advantages of current commutated chopper are,

1. The capacitor always remains charged with the correct polarity 2. Commutation is reliable as load current is less than the peak commutation

current ICP 3. The auxiliary thyristor TA is naturally commutated as its current passes

through zero value.

21. What are the advantages of load commutated chopper? The advantages of load commutated chopper

1. Commutating inductor is not required 2. It is capable of commutating any amount of load current 3. It can work at high frequencies in the order of kHz 4. Filtering requirements are minimum

22. What are the disadvantages of load commutated chopper? The disadvantages of load commutated chopper

1. For high power applications, efficiency becomes very low because of high 2. Switching losses at high operating frequencies. 3. Freewheeling diode is subjected to twice the supply voltage. 4. Peak load voltage is equal to twice the supply voltage. 5. The commutating capacitor has to carry full load current at a frequency of

half chopping frequency. 6. One thyristor pair should be turned-on only when the other pair is

commutated. This can be realized by sensing the capacitor current that is alternating.

23. What is meant SMPS? SMPS is switched mode power supply. It is based on a dc chopper with a rectified and possibly transformed output. The output voltage amplitude is controlled by varying the mark space ratio of the chopper. This may be achieved by pulse with control or frequency variation with constant pulse width.


24. What are the types of SMPS? The types of SMPS are,

1. Buck converter 2. Boost converter 3. Buck – Boost converter 4. Cuk converter

25. What are the applications of SMPS?

The applications of SMPS are, 1. Computer 2. Television receiver 3. Battery charger

26. Define: buck converter

The buck converter defined as, a converter used to step down an unregulated DC input voltage to produce a regulated DC output voltage.

27. Define: boost converter The boost converter defined as, a converter used to step up an unregulated DC input voltage to produce a regulated DC output voltage.

28. Define: buck- boost converter The buck- boost converter defined as, a converter used to either buck or boost function. The polarity of the output voltage is opposite to the input voltage. This also known as fly back converter.

29. Define: cuk converter The cuk converter is similar to buck- boost converter which can provide an output voltage greater or lesser then the input voltage and also the polarity of the output is opposite to the input.

30. What are the characteristics of buck converter? The characteristics of buck converter are,

1. The output voltage is less than or equal to the input voltage 2. Input current is pulsating 3. The output current is non- pulsating 4.

31. What are the characteristics of boost converter? The characteristics of boost converter are,

1. The output voltage is greater than or equal to the input voltage 2. Input current is non- pulsating 3. The output current is pulsating 4. The efficiency is high due to the transistor 5. The peak transistor current is high


UNIT 4

INVERTERS

1. What is meant by inverter? A inverter is a device that converts dc power into ac power at desired output voltage and frequency is called an inverter.

2. What are the applications of inverter? The applications of inverter are,

1. Adjustable speed drives 2. Induction heating 3. Stand-by aircraft power supplies 4. UPS 5. HVDC transmission

3. What are the main classifications of inverter?

The main classifications of inverter are, 1. Voltage Source Inverter 2. Current Source Inverter

4. Why thyristors are not preferred for inverters? Thyristors require extra commutation circuits for turn off which results increased complexity of the circuit. For these reasons thyristors are not preferred for inverters.

5. How output frequency is varied in case of a thyristor? The output frequency is varied by varying the turn off time of the thyristors in the inverter circuit, i.e. the delay angle of the thyristors is varied.

6. Give two advantages of CSI. The advantages of CSI are,

1. CSI does not require any feedback diodes. 2. Commutation circuit is simple as it involves only thyristors.

7. Why diodes should be connected in antiparallel with the thyristors in inverter

circuits? For RL loads, load current will not be in phase with load voltage and the diodes connected in antiparallel will allow the current to flow when the main thyristors are turned off. These diodes are called feedback diodes.

8. What is meant by series inverter? The series inverter means an inverter in which the commutating elements are connected in series with the load.


9. What is meant by parallel inverter? An inverter in which the commutating elements are connected in parallel with the load is called a parallel inverter.

10. What are the applications of a series inverter? The applications of a series inverter are, The thyristorised series inverter produces an approximately sinusoidal waveform at a high output frequency, ranging from 200 Hz to 100kHz. It is commonly used for fixed output applications such as

1. Ultrasonic generator 2. Induction heating 3. Sonar Transmitter

11. How is the inverter circuit classified based on commutation circuitry?

The inverter circuits classified based on commutation circuitry are, 1. Line commutated inverters 2. Load commutated inverters 3. Self commutated inverters 4. Forced commutated inverters.

12. What is meant by McMurray inverter?

The McMurray inverter means a relies on LC circuit and an auxiliary thyristor for commutation in the load circuit.

13. What are the applications of a CSI? The applications of a CSI are,

1. Induction heating 2. Lagging VAR compensation 3. Speed control of ac motors 4. Synchronous motor starting.

14. What is meant by PWM control?

PWM control means, a fixed dc input voltage is given to the inverter and a controlled ac output voltage is obtained by adjusting the on and off periods of the inverter components. This is the most popular method of controlling the output voltage and this method is termed as PWM control.

15. What are the advantages of PWM control? The advantages of PWM control are,

1. The output voltage can be obtained without any additional components 2. Lower order harmonics can be eliminated or minimized along with its output

voltage control. As the higher order harmonics can be filtered easily, the filtering requirements are minimized.


16. What are the disadvantages of the harmonics present in the inverter system? The disadvantages of the harmonics present in the inverter system are,

1. Harmonic currents will lead to excessive heating in the induction motors. This will reduce the load carrying capacity of the motor.

2. If the control and the regulating circuits are not properly shielded, harmonics from power ride can affect their operation and malfunctioning can result.

3. Harmonic currents cause losses in the ac system and can even some time produce resonance in the system. Under resonant conditions, the instrumentation and metering can be affected.

4. On critical loads, torque pulsation produced by the harmonic current can be useful.

17. What are the methods of reduction of harmonic content? The methods of reduction of harmonic content are,

1. Transformer connections 2. Sinusoidal PWM 3. Multiple commutation in each cycle 4. Stepped wave inverters

18. Compare CSI with VSI.

Comparisons of CSI with VSI are, Sl.No VSI CSI 1 2 3 4 5

Input voltage is maintained constant The output voltage does not depend on the load The magnitude of the output current and its waveform depends on the nature of the load impedance It requires feedback diodes Commutation circuit is complicated i.e. it contains capacitors and inductors.

Input current is constant but adjustable The output current does not depend on the load The magnitude of the output voltage and its waveform depends on the nature of the load impedance It does not requires feedback diodes Commutation circuit is simple i.e. it contains only capacitors.


19. What are the disadvantages of PWM control? The disadvantages of PWM control are,

1. Logic circuit is relatively complex 2. Operation above 120- 150 Hz is difficult 3. SCRs are expansive as they must possess low turn on and turn off

times

20. What are the different types of PWM control? The different types of PWM control are,

1. Single pulse width modulation 2. Multi pulse with modulation 3. Sinusoidal pulse width modulation.

21. What is single pulse width modulation?

In single pulse width modulation, there is only one pulse per half cycle and the output voltage is varied by varying the width of the pulse.

22. What is the disadvantage of the single pulse width modulation? The harmonic content is more at lower pulse width is the disadvantage of the

single pulse width modulation.

23. Define: amplitude modulation index The amplitude modulation index defined as the ratio of reference voltage ER to the control voltage EC and it is denoted by M.


UNIT V

APPLICATIONS

1. What is the expansion of UPS? The expansion of UPS is uninterruptible power supply.

2. Mention the various configurations of UPS. The various configurations of UPS are,

1. Online UPS 2. Offline UPS 3. Line interactive UPS

3. What is the need for UPS?

In application of such as medical care systems, chemical plant process control, safety monitors or a computer installation, where even a temporary loss of supply could have a severe consequence, there is need to provide an uninterruptible power supply system which can maintain the supply under all conditions.

4. Mention the applications of UPS. The applications of UPS are,

1. Computers 2. Data processors 3. Data transmitters 4. Microwave relay stations 5. Nuclear reactor controls 6. Communication links

5. What is the function of UPS?

The function of UPS is, to provide an interrupt free supply of power to the AC load, which cannot be directly fed from DC source and Dc source is required to be converted into AC.

6. Which material is used for installation of UPS? The installation of UPS uses either nickel cadmium or lead acid batteries to provide backup. Nickel cadmium batteries have the advantages that their electrolyte is non corrosive, do not emit an explosive gas while charging and they cannot be damaged by discharging. Their cost is two to three times that of lead acid batteries.

7. Mention the applications of online UPS. The applications of online UPS are,

1. Induction motor drive and similar other motor control applications 2. Intensive care units, medical equipments.


8. Mention the advantages of online UPS. The advantages of online UPS are,

1. It provides isolation between main supply and load. 2. Since inverter is always on, the quality of load voltage is free from

distortion. 3. All the disturbance of supply such as blackout, brownout, spikes etc

are absent in the output. 4. Voltage regulation is better 5. Transfer time is practically zero since inverter is always on.

9. Mention the disadvantages of online UPS.

The advantages of online UPS are, 1. Overall efficiency of UPS is reduced since inverter is always on 2. The wattage of the rectifier is increased since it has to supply power to

inverter as well as charge battery. 3. Online UPS is costlier than other UPS system

10. Mention the advantages of offline UPS.

The advantages of offline UPS are, 1. Offline UPS has high efficiencies, since charger is not continuously on 2. The power handling capacity of charger is reduced 3. Offline UPS are cheap 4. Offline UPS are internal control is very simple

11. Mention the disadvantages of online UPS.

The disadvantages of online UPS are, 1. The output voltage contains spikes, brownouts, blackouts 2. There is finite transfer time from mains to inverter when mains supply

fails 3. Output of offline UPS is not perfectly reliable

12. Mention the application of online UPS.

The applications of online UPS are, 1. Computers, printers and scanners etc., 2. Emergency power supplies

13. Mention the parts of UPS.

The parts of UPS are, 1. Rectifier 2. Inverter 3. Filter 4. Battery charger 5. Battery 6. Switches


14. What is HVDC? HVDC is high voltage direct current transmission system.

15. What is the function of static switch in UPS system? The functions of static switch in UPS system are,

1. It is used to provide an alternative supply to the load in case of inverter failure

2. It is used to provide a higher capacity main supply to the load if a load fault occurs since the switch is required to operate within 10ms.

16. Mention the advantages of HVDC. The advantages of HVDC are,

1. Resistive drop is less 2. Volume of copper used is less 3. There is no inductive and capacitive drop 4. There is no corona loss 5. There is no skin and proximity effect.

17. Mention the types of HVDC links.

The types of HVDC links are, 1. Monopolar link 2. Bipolar link 3. Homopolar link 4. Back to back HVDC coupling system 5. Multiterminal HVDC system

18. What are the standard voltage for HVDC system?

The standard voltages for HVDC system are, • Vindhychal – 500MW • Chandrapur – 2* 500MW • Vishakhapatnam – 500MW • Sasaram – 500MW

19. What are the disadvantages of HVDC transmission?

The disadvantages of HVDC transmission are, 1. The power transmission with HVDC is not economical if the length of

transmission is less than 500 km as HVDC system additionally requires converters, inverters and filters.

2. Overload capacity of HVDC converter is low 3. The maintenance cost of insulator in HVDC system is more


20. What are the components used in HVDC system? The components used in HVDC system are,

1. Generator 2. Step up transformer 3. Rectifier 4. Inverter 5. Step down transformer 6. Circuit breaker and relays

21. What is the value of firing angle for inverter and rectifier operation?

The firing angle is 0o to 90o in rectifier operation and 90ᵒ to 180o for inverter operation.

22. Define: overlap angle Overlap angle defined as the lasts for definite duration time. This time in electrical degrees is called angle of overlap.

23. What is FACTs? FACT is flexible AC transmission. FACT is power electronic based system.

24. Where the first FACT device is established? In 1995 between Sulivan substation and the rest of the network.

25. Mention the advantages of FACTs. The advantages of FACTs are,

1. It controls line impedance angle and voltage which helps in controlling the power flow in transmission lines

2. The power flow in the transmission lines can be made optimum 3. It helps in damping out the oscillation and avoids damage of various

equipments. 4. It limits the impacts of faults and equipment failures

26. What are the main objectives of FACTs?

The main objectives of FACTs are, 1. The power transfer capability of transmission systems is to be

increased. 2. The power flow is to be kept over the designated routes.

27. Mention some of FACT devices.

Some of the FACT devices are, 1. Static VAR compensator 2. Static synchronous generator 3. Static synchronous compensator 4. Thyristorised switching capacitor 5. Static VAR system 6. Unified power flow controller.


28. What are the types of FACTs? The types of FACTs are,

1. Series controller 2. Shunt controller 3. Combined series – series controller 4. Combined series – shunt controller

29. Compare SVC and STATCOM.

Comparison of SVC and STATCOM are, Sl.no

SVC

STATCOM

1. Generation of more harmonics

Generation of less harmonics

2. Acting as a variable susceptance

Acting as voltage source behind a reactance

3. Operates mainly in capacitive region Operates in both inductive and capacitive regions

4. Sensitive to transmission system harmonic resonance.

Insensitive to transmission system harmonic resonance.

30. What is TSCS?

TSCS is thyristor controlled series compensator. It is a capacitive reactance type of compensator consisting of a series capacitor bank connected in parallel with a thyristor controlled reactor, so as to provide smooth variable capacitive reactance.

31. Mention the advantages of TSCS. The advantages of TSCS are,

1. It is based on thyristor without gate turn- off capability 2. It helps in limiting fault current foe a firing angle of 90o

32. What is the drawback of STATCOM?

STATCOM needs a gate turn- off thyristors which are costly as comparared to normal thyristors.

ee1301 - two marks q & a new

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