EE6503 - POWER ELECTRONICS UNIT IV – INVERTERS

PART- A 1. Define harmonic distortion factor? (N/D15) Harmonic distortion factor is the harmonic voltage to the fundamental voltage. 2. What is CSI? (N/D12) In Current Source Inverter, the current is nearly constant. The voltage changes with respect the the load change. 3. What are the difference between VSI and CSI? (M/J12) (M/J16) 4. What is the necessity of PWM within an inverter? (N/D12) To control voltage and harmonics PWM is used. In PWM, output voltage is controlled by varying the width of pulse. Percentage harmonic can be controlled by changing number of pulse per half cycle. 5. Define the term inverter gain. (M/J12) Inverter gain is defined as the ratio of output voltage to the input voltage of an inverter. 7. What is harmonic elimination by PWM? (M/J15) Pulse width modulation is used to eliminate harmonics by comparing reference signal and carrier signal and when reference signal is greater than carrier signal triggering pulses are generated for switches in the inverter. 8. In a CSI, if frequency of output voltage is ‘f’ Hz, what is the frequency of voltage input to CSI? (M/J13) The input frequency is 50Hz. 9. What is Space Vector? (M/J13) Space vector modulation (SVM) is an algorithm for the control of pulse width modulation (PWM). It is used for the creation of alternating current (AC) waveforms; most commonly to drive 3 phase AC powered motors at varying speeds from DC using multipleclass-D amplifiers. There are various variations of SVM that result in different quality and computational requirements. 10. What is the advantages of 120 degree conduction mode of inverter over 180 degree conduction mode? (N/D13)

In 120ᵒ conduction mode safe commutation is obtained on complementary switches in same leg.

VSI CSI Uses voltage source. Uses current source. Uses current commutation. Uses voltage commutation Diodes are connected in parallel. Diodes are connected in series. Can be used for only 1208 and 1808 Can be used for only 1208 Output voltage is quasi square wave. Output current is quasi square wave.

11. List the various advantages of using PWM control to inverters? (N/D13)(M/J14) PWM inverters produce high-quality sine waves. The harmonic levels are very low and

can be lower than those of common domestic appliances. 12. What is meant by voltage source inverter?(Dec 2014)(DEC 2015)

Voltage source inverters (VSIs) are named so because the independently controlled output is a voltage waveform. Voltage Source Inverters (VSI) feed the output inverter section from an constant-voltage source. 13. Write the advantages of resonant converters?(Dec 2014) Resonant converters are used to reduce/eliminate the switching losses and the output waveform is more sinusoidal. There are three basic types of the resonant DC/DC converters: 1. Series resonant converter 2. Parallel resonant converter 3. Series-Parallel resonant converter 14. Draw the circuit diagram of single phase CSI ?(May 2015)

15.What are the applications of inverter? (M/J 16) The inverter is an electrical device, which is used to convert the DC power to an AC power. The inverter for home is used for emergency backup power and used in some aircraft systems to convert a portion of the aircraft DC power to AC. Inverters are used to control the speed of AC motors. 16. What are the application of CSI? (N/D16) i)it can be used for the speed control of ac, specially induction, motors subject to variation in load torque. ii)used in induction heating application. 17. Define modulation index and what its use. (N/D16) Modulation index is the ratio of peak magnitudes of the modulating waveform and the carrier

waveform. M= VmVc

Modulation index used for control the output of inverter.

PART-B

1. Explain the operation of series Resonant Inverter. (M/J12)(M/J14) (N/D14)

2. Explain the operation of 180° Conduction of three phase inverter. (N/D14)(M/J15)

3. Explain the operation of 120° Conduction of three phase inverter. Also obtain the epression for rms value of output voltage (N/D12) (M/J13) (M/J14)(N/D15)

Derive the mathematical expression. Average line voltage expression Average current expression

Compare with 180 degree mode States advantages of 120 degree mode States disadvantages of 120 degree mode Explain the working principle Explain the different mode of operation Output waveform of 3 phase inveter in 120 degree conduction mode

4. Explain different PWM techniques. (N/D12) (M/J12) (M/J14)(N/D15)

Multi-Pulse width modulation The harmonic content can be reduced by using several pulses in each half-cycle of output voltage. The generation of gating signals for turning on and off transistors is shown in Figure (3). The gating signals are produced by comparing reference signal with triangular carrier wave. The frequency of the reference signal sets the output frequency (fo) and carrier frequency (fc) determine the number of pulses per half cycle, The variation of modulation index (M) from 0 to 1 varies the pulse from 0 to π/p and the output voltage from 0 to Vm. Uniform pulse width modulation (UPWM) Figure (1) shows the uniform pulse width modulation (UPWM). It can be note the reference voltage is square wave changed from the Vm to –Vm in frequency = fo. The carrier frequency is triangle wave with amplitude equal to Vc in frequency = fc. Sinusoidal Pulse Width modulation Instead of maintaining the width of all pulses the same as in the case of uniform pulse width modulation, the width of each pulse is vary in proportion to the amplitude of a sine wave evaluated at the center of the same pulse.

The distortion factor and lower-order harmonics are reduced scientifically. The gating signals as shown in Fig.(2) are generated by comparing a sinusoidal reference signal with a triangular carrier signal of frequency fc. This type of modulation is commonly used in industrial applications and abbreviated as SPWM. The frequency of reference signal (fr), determine the converter output frequency (fo) and its peak amplitude (Ar) controls the modulation index, M. The number of pulses per half cycle depends on the carrier frequency.

5. Explain space vector pulse width modulation techniques

The space vector modulation technique is somewhat similar to the Sine+3rdharmonic PWM technique but the method of implementation is different. Before going into details of this technique, it would be useful to explore the concept of voltage space-vector, in analogy with the concept of flux space-vector as used in three-phase ac machine. The stator windings of a three-phase ac machine (with cylindrical rotor), when fed with a three-phase balanced current produce a resultant flux space-vector that rotates at synchronous speed in the space. The flux vector due to an individual phase winding is oriented along the axis of that particular winding and its magnitude alternates as the current through it is alternating. The magnitude of the resultant flux due to all three windings is, however, fixed at 1.5 times the peak magnitude due to individual phase windings. The resultant flux is commonly known as the synchronously rotating flux vector. Now, in analogy with the fluxes, if a three phase balanced voltage is applied to the windings of a three-phase machine, a rotating voltage space vector may be talked of. The resultant voltage space-vector will be rotating uniformly at the synchronous speed and will have a magnitude equal to 1.5 times the peak magnitude of the phase voltage. Fig. 38.2 (a) shows a set of three-phase balanced sinusoidal voltages. Let these voltages be applied to the windings of a three-phase ac machine as shown in Fig. 38.2(b). Now, during each time period of the phase voltages six discrete time instants can be identified, as done in Fig. 38.2(a), when one of the phase voltages have maximum positive or negative instantaneous magnitude. The resultants of the three space-voltages at these instants have been named V1 to V6. The spatial positions of these resultant voltage space-vectors have been shown in Fig. 38.2(b). At these six discrete instants, these vectors are aligned along the phase axes having maximum instantaneous voltage. As shown in Fig. 38.2(a) the magnitude of these voltage vectors is 1.5 times the peak magnitude of individual phase voltage. The instantaneous voltage output from a 3-phase inverter, discussed in earlier lessons, cannot be made to match the three sinusoidal phase voltages of Fig. 38.2(a) at all time instants. This is so because the inverter outputs are obtained from rectangular pole voltages and contain, apart from the fundamental, harmonic voltages too. However, the instantaneous magnitudes of the inverter outputs and the sinusoidal voltages can be made to match at the six discrete instants (talked above) of the output cycle. At these six discrete instants one of the phase

voltages is at its positive or negative peak magnitude and the other two have half of the peak magnitude. The polarity of the peak phase-voltage is opposite to that of the other two phasevoltages. A similar pattern is seen in the instantaneous phase voltages output by a 3-phase inverter and is explained below. Fig. 38.3 shows a three-phase voltage source inverter whose output terminals are fed to the three terminal of a three-phase ac machine (in fact to any three phase balanced load). Fromthe knowledge of 3-phase voltage source inverters, it may be obvious that the two switches ofeach inverter pole conduct in a complementary manner. Thus the six switches of the three poles will have a total of eight different switching combinations. Out of these eight combinations, two combinations wherein all the upper switches or all the lower switches of each pole are simultaneously ON result in zero output voltage from the inverter. These two combinations are referred as null states of the inverter. The remaining six switching combinations, wherein either two of the high side (upper) switches and one of the low side (lower) switch conduct, or vice-versa, are active states. During the six active states the phase voltages output by the inverter to a balanced 3-phase linear load are as detailed in Sec.35.1 of Lesson 35. Accordingly instantaneous magnitude of two of the phase voltages are 1/3 rd Edc and the third phase voltage is 2/3rdEdc (where E dc is the dc link voltage). The voltage polarities of the two phases getting 1/3rdEdcare identical and opposite to the third phase having 2/3 rd Edc. Fig. 38.3 also shows, in a tabular form, the instantaneous magnitudes of the three load-phase voltages (normalized by the dc link voltage magnitude) during the six active states of the inverter. The switching states of the inverter have been indicated by a 3-bit switching word. The 1 st(MSB) bit for leg ‘A’, 2ndbit for leg ‘B’ and 3rdbit for leg ‘C’. When a particular bit is 1, the high (upper) side switch of that leg is ON and when the bit is 0, the low side switch is ON. Thus a switching word 101 indicates that high side switches of legs ‘A’ and ‘C’ and lowside switch of leg ‘B’ conduct. The resulting voltage pattern isidentical to the voltage pattern of space voltage vector V 1 of Fig. 38.2 provided 2/3rdEdcequals the peak magnitude of phase voltage in Fig. 38.2. The table given in Fig. 38.3 shows how six active states of the inverter produce space voltage vectors V1 to V6 that can be identified on one to one basis with the six voltage vectors.

There are some important differences between the resultant space voltage vectors due to the sinusoidal phase voltages of Fig. 38.2 and the space voltage vectors formed by the inverter output voltages.

6. Explain with waveform multiple pulse width modulation inverter(M/J15) 7. Explain the basic operation of single phase current source inverter. (M/J12) (N/D13)

The circuit of a Single-phase Current Source Inverter (CSI) is shown in Fig. 39.1. The type of operation is termed as Auto-Sequential Commutated Inverter (ASCI). A constant current source is assumed here, which may be realized by using an inductance of suitable value, which must be high, in series with the current limited dc voltage source. The thyristor pairs, Th1 & Th3, and Th2 & Th4, are alternatively turned ON to obtain a nearly square wave current waveform. Two commutating capacitors − C1 in the upper half, and C2 in the lower half, are used. Four diodes, D1–D4 are connected in series with each thyristor to prevent the commutating capacitors from discharging into the load. The output frequency of the inverter is controlled in the usual way, i.e., by varying the half time period, (T/2), at which the thyristors in pair are triggered by pulses being fed to the respective gates by the control circuit, to turn them ON, as can be observed from the waveforms (Fig. 39.2). The inductance (L) is taken as the load in this case, the reason(s) for which need not be stated, being well known. The operation is explained by two modes.

Mode I: The circuit for this mode is shown in Fig. 39.3. The following are the assumptions. Starting from the instant, , the thyristor pair, Th−=0t2 & Th4, is conducting (ON), and the current (I) flows through the path, Th2, D2, load (L), D4, Th4, and source, I. The commutating capacitors are initiallycharged equally with the polarity as given, i.e., . This mans that both capacitors have right hand plate positive and left hand plate negative. If two capacitors are not charged initially, they have to precharged. At time, t = 0, thyristor pair, Th1 & Th3, is triggered by pulses at the gates. The conducting thyristor pair, Th2 & Th4, is turned OFF by application of reverse capacitor voltages. Now, thyristor pair, Th1 & Th3, conducts current (I). The current path is through Th1, C1, D2, L, D4, C2, Th3, and source, I. Both capacitors will now begin charging linearly from () by the constant current, I. The diodes, D0CV−2 & D4, remain reverse biased initially. The voltage, across D1Dv1, when it is forwardbiased, is obtained by going through the closed path, abcda as It may be noted the voltage across load inductance, L is zero (0), as the current, I is constant. As the capacitor gets charged, the voltage across D1Dv1, increases linearly. At some time, say t1, the reverse bias across D1 becomes zero (0), the diode, D1.starts conducting. An identical equation can be formed for diode, D3also. Actually, both diodes, D1

& D3, start conducting at the same instant, t1. The time t1

for which the diodes, D1& D3, remain reverse biased is obtained.

At time, t = 0, thyristor pair, Th1 & Th3, is triggered by pulses at the gates. The conducting thyristor pair, Th2 & Th4, is turned OFF by application of reverse capacitor voltages. Now, thyristor pair, Th1 & Th3, conducts current (I). The current path is through Th1, C1, D2, L, D4, C2, Th3, and source, I. Both capacitors will now begin charging linearly from () by the constant current, I. The diodes, D0CV−2 &D4, remain reverse biased initially. The voltage, across D1Dv1, when it is forward biased, is obtained by going through the closed path, abcda as it may be noted the voltage across load inductance, L is zero (0), as the current, I is constant. As the capacitor gets charged, the voltage across D1Dv1 increases linearly. At some time, say t1, the reverse bias across D1becomes zero (0), the diode,D1.starts conducting. An identical equation can be formed for diode, D3also. Actually, both diodes, D1& D3, start conducting at the same instant, t1. The time t1 for which the diodes, D1 & D3, remain reverse biased is obtained by equating,. The capacitor voltages, appear as reverse voltage across the thyristors. MODE2

8. Discuss the principle of working of a three phase bridge inverter with an appropriate circuit diagram.

Draw the output phase and line voltage waveforms on the assumption that each tryistor conducts for 180ᵒ and resistive load is star connected. The swquence of firing of various SCR should also be indicated. (Nov 2013)

9. Explain the basic operation of load commutated current source inverter In the last lesson, ASCI mode of operation for a single-phase Current Source Inverter (CSI) was presented. Two commutating capacitors, along with four diodes, are used in the above circuit for commutation from one pair of thyristors to the second pair. Earlier, also in VSI, if the load is capacitive, it was shown that forced commutation may not be needed. The operation of a single-phase CSI with capacitive load (Fig) is discussed here. It may be noted that the capacitor, C is assumed to be in parallel with resistive load (R). The capacitor, C is used for storing the charge, or voltage, to be used to force-commutate the conducting thyristor pair as will be shown. As was the case in the last lesson, a constant current source, or a voltage source with large inductance, is used as the input to the circuit

The power switching devices used here is the same, i.e. four thyristors only in a full- bridge configuration. The positive direction for load current and voltage, is shown in Fig. 40.1. Before t = 0, the capacitor voltage is , i.e. the capacitor has left plate negative and right plate positive. At that time, the thyristor pair, Th1VvC−=2&Th4 was conducting. When (at t = 0),vthe thyristor. through load resistance, R has the same nature as that of (Fig. 40.2). Similarly, when (at), the thyristor pair, Thpair, Th1 & Th3 is triggered by the pulses fed at the gates, the conducting thyristor pair, Th2 & Th4 is reverse biased by the capacitor voltage 1VvC−=, and turns off immediately. The current path is through Th1, load (parallel combination of R & C), Th3, and the source.

Various current and voltage waveforms during one cycle, are shown in Fig..

10. Explain multiple pulse width modulation and explain sinusoidal pulse width modulation. (June 2013) (Nov 2013) Sinusoidal Pulse width modulation The switches in the voltage source inverter (See Fig. 1)can be turned on and off as required. In the simplest approach, the top switch is turned on If turned on and off only once in each cycle, a square wave waveform results. However, if turned on several times in a cycle an improved harmonicprofile may be achieved.

In the most straightforward implementation, generation of the desired output voltage is achieved by comparing the desired reference waveform (modulating signal) with a highfrequency triangular ‘carrier’ wave as depicted schematically in Fig.2. Depending on whether the signal voltage is larger or smaller than the carrier waveform, either the positive or negative dc bus voltage is applied at the output. Note that over the period of one triangle wave, the average voltage applied to the load is proportional to the amplitude of the signal (assumed constant) during this period. The resulting chopped square waveform contains a replica of the desired waveform in its low frequency components, with the higher frequency components being at frequencies of an close to the carrier frequency. Notice that the root mean square value of the ac voltage waveform is still equal to the dc bus voltage, and hence the total harmonic distortion is not affected by the PWM process. The harmonic components are merely shifted into the higher frequency range and are automatically filtered due to inductances in the ac system. When the modulating signal is a sinusoid of amplitude Am, and the amplitude of the triangular carrier is Ac, the ratio m=Am/Ac is known as the modulation index. Note that controlling the modulation index there for controls the amplitude of the applied output voltage. With a sufficiently high carrier frequency (see Fig. 3 drawn for fc/fm = 21 and t = L/R = T/3; T = period of fundamental), the high frequency components do not propagate significantly in the ac network (or load)due the presence of the inductive elements. However, a higher carrier frequency does result in a larger number of switching’s per cycle and hence in an increased power loss. Typically switching frequencies in the 2-15 kHz range are considered adequate for power systems applications. Also in three-phase systems it is advisable to use

so that all three waveforms are symmetric.