58 CHAPTER 4 MODIFIED H- BRIDGE MULTILEVEL INVERTER USING MPD-SPWM TECHNIQUE 4.1 INTRODUCTION Conventional voltage source inverter requires high switching frequency PWM technique to obtain a quality output voltage with a minimum amount of ripple content. These inverters have some limitations in operating at high frequency mainly due to switching losses and constraints of device ratings. This chapter presents a new topology of multilevel inverter with less number of switching devices. Operation of the modified H-bridge inverter is presented in section 4.2. Modified Phase Disposition Sinusoidal Pulse Width Modulation (MPD-SPWM) technique is explained in section 4.3. Performance parameters of the multilevel inverters are given in section 4.4. MATLAB/Simulink model of three phase modified H-bridge multilevel inverter with equal DC source is presented in section 4.5. Performance parameters are analysed for three phase modified H-bridge multilevel inverter. 4.2 PROPOSED MODIFIED H-BRIDGE MULTILEVEL INVERTER TOPOLOGY Modified H-bridge multilevel inverter topology separates the output voltage into two parts. One part is named as level generation part and it is responsible for generating levels. The other part is called as polarity
59 generation part and it is responsible for generating the polarity of the output voltage. Positive levels are generated by the level generation part and the output of this part is fed to the polarity generation part in order to generate a complete multilevel output voltage. This will eliminate many of the semiconductor switches which were responsible to generate the output voltage levels in positive and negative polarities. Figure 4.1 Proposed modified H-bridge seven level inverter topology The new multilevel inverter topology for seven levels is shown in Figure 4.1. It requires ten controlled power semiconductor switches and three equal isolated DC sources for the generation of seven level output voltage. Polarity generation part transfers the required output level to the load with the same direction or opposite direction. It reverses the voltage direction when the voltage polarity is required to be changed.
60 It has modular structure and it can be extended to higher levels by adding the middle part (V dc, S 3, & S 6 ) of level generation part. Modified H-bridge multilevel inverter topology with h number of equal DC sources generates the output voltage levels (N L ) and maximum output voltage as per the Equations (4.1) and (4.2). This inverter has less number of switches when compared with the standard cascaded H-bridge multilevel inverter. Hence, it is advisable to suggest this topology to replace the existing multilevel inverters in the industries. N L = 2h+1 (4.1) V omax = hv dc (4.2) 4.2.1 Number of Components Cost of the switching device is directly proportional to the switching frequency. So high switching frequency devices and its driver circuits are very expensive than low switching frequency devices. As the number of high frequency components is increased, the reliability of the inverter is decreased. Number of components in the system is indirectly proportional to the reliability of the system as per the MIL-HDBK-217F standard. Switching components required for different multilevel inverters are given in Table 4.1.
61 Table 4.1 Comparison of switching components for different multilevel inverter topologies Inverter type NPC Multilevel Inverter Flying capacitor Multilevel Inverter Cascaded H-bridge Multilevel Inverter Proposed Modified H- bridge Multilevel Inverter Main Switches 6(N L -1) 6(N L -1) 6(N L -1) 3((N L -1)+4) Main diodes 6(N L -1) 6(N L -1) 6(N L -1) 3((N L -1)+4) Clamping Diodes 3(N L -1) (N L -2) 0 0 0 DC bus Capacitors /isolated supplies (N L -1) (N L -1) 3(N L -1)/2 3(N L -1)/2 Flying capacitors 0 3/2 (N L -1) ( N L -2) 0 0 Switching components required for different levels are compared with the modified H-bridge multilevel inverter which is shown in Figure 4.2. Modified H-bridge multilevel inverter requires fewer components compared with the conventional multilevel inverters. Thus the reliability of this multilevel inverter is highly improved and expenses associated with this system are less. It can be extended to any number of levels as other topologies. It is clear that the modified H-bridge multilevel inverter uses less number of devices. Number of components required decreases tremendously for higher levels.
62 TOTAL NUMBER OF COMPONENTS 1000 900 800 700 600 500 400 300 200 100 0 5 7 9 11 13 15 17 NPC 88 168 272 400 552 728 928 FC 70 123 188 265 354 455 568 CMLI 54 81 108 135 162 189 216 MHMLI 54 69 84 99 114 129 144 LEVELS NPC FC CMLI MHMLI Figure 4.2 Comparison of the proposed modified H- bridge multilevel inverter with the conventional multilevel inverters for different levels 4.2.2 Switching Sequences Switching sequences for this proposed multilevel inverter are easier than its conventional topologies. Full bridge converter reverses the output voltage. So, it does not require generating negative pulses for negative cycle. Required level is produced by the level generation part of the inverter. Then this level is translated to negative or positive according to output voltage requirements. The switching sequences for the positive levels are shown in Figure 4.3. This topology is redundant and flexible in the switching sequence.
63 Figure 4.3 Switching sequences for positive levels By considering the minimal switching transitions during each levels, the switch sequences (2-3-4), (2-3-5), (2-6-5) and (1-5) are chosen for 0,1,2,3 positive levels respectively. Seven levels of output voltage are generated as follows. 1) Zero level output: This level can be produced by two switching combinations, controlled switches S 2, S 3, S 4, S 8 and S 9 are ON or controlled switches S 2, S 3, S 4, S 7 and S 10 are ON and all other controlled switches are OFF. Load terminals are short circuited and the voltage applied to the load terminals are zero.
64 2) One-third level positive output (V dc ): The controlled switches S 2, S 3 and S 9 are ON, connecting the load positive terminal to the lower DC source V dc, and controlled switches S 8, S 5 are ON, connecting the load negative terminal to ground. All other controlled switches are OFF. Voltage applied to the load terminal is V dc. 3) Two-third level positive output (2V dc ): The controlled switches S 2, S 6, S 9 are ON, connecting the load positive terminal to upper DC sources, and the switches S 8, S 5 are ON, connecting the load negative terminal to ground. All other controlled switches are OFF. Voltage applied to the load terminal is 2V dc. 4) Maximum positive output (3V dc ): The controlled switches S 1, S 9 are ON, connecting the load positive terminal to all DC sources and the controlled switches S 8, S 5 are ON, connecting the load negative terminal to ground. All other controlled switches are OFF. Voltage applied to the load terminal is 3V dc. 5) One -third level negative output (-V dc ): The controlled switches S 2, S 3 and S 7 are ON, connecting the load negative terminal to lower DC source V dc and controlled switches S 10, S 5 are ON, connecting the load positive terminal to ground. All other controlled switches are OFF. Voltage applied to the load terminal is -V dc. 6) Two-third level negative output (-2V dc ): The controlled switches S 2, S 6, S 7 are ON, connecting the load negative terminal to upper DC sources, and the switches S 10, S 5 are ON, connecting the load positive terminal to ground. All other controlled switches are OFF. Voltage applied to the load terminal is -2V dc.
65 7) Maximum negative output (-3V dc ): The controlled switches S 1, S 7 are ON, connecting the load negative terminal to all DC sources, and the controlled switches S 10, S 5 are ON, connecting the load positive terminal to ground. All other controlled switches are OFF. Voltage applied to the load terminal is -3V dc. The efficiency of the overall multilevel inverter is also decided by the number of switches that conduct current. The conventional seven level cascaded inverter requires twelve switches and in each instance, six switches conduct the current. In the proposed topology, the number of switches that conduct current varies from four to five switches, out of which two switches are from the polarity generation part. Therefore the number of switches that conduct current in the topology is lesser compared with the conventional topology. Hence, it has better efficiency and reliability. Producing gating signal for the polarity generation part is simple and is synchronous with line frequency. Fundamental switching frequency scheme is used to generate the switching pulse for the polarity generation part. Low frequency H-bridge inverter operates in forward and reverse modes to generate polarity. Switches S 8 and S 9, conduct in forward mode to generate positive direction. Switches S 7 and S 10, conduct in reverse mode to generate negative direction. 4.3 MODIFIED PHASE DISPOSITION SINUSOIDAL PULSE WIDTH MODULATION Modified SPWM technique is introduced to generate the switching signals. It requires half of the conventional carriers for SPWM controller. SPWM technique for conventional cascaded H-bridge seven level inverter requires six carriers. Three carriers are sufficient for the modified topology.
66 Proposed multilevel inverter generates only positive polarity whereas the negative levels are generated by the polarity generation part. Pulses for the switching devices are generated by comparing the reference and three carrier signals, which are shown in Figure 4.4. Three triangular carrier signals are compared with an unidirectional sinusoidal reference signal. The carrier signals have the same frequency and amplitude and are in phase. Figure 4.4 Carrier and reference waveform for MPD-SPWM Carriers in this method do not have any coincidence and they have definite offset from each other. If the reference signal exceeds the lowest carrier (Carrier A), middle carrier (Carrier B) is compared with the reference signal until it exceeds the peak amplitude of the middle carrier signal. Then the reference signal is compared with the highest carrier (Carrier C) until it reaches zero. It is possible to generate six switching patterns for switches S 1 to S 6. Pulses for the polarity generation part are generated with the line frequency of 50 Hz.
67 4.4 PERFORMANCE PARAMETERS The output of the multilevel inverter contains undesirable harmonics and the quality of the multilevel inverter is normally evaluated in terms of several distortion criteria and the performance parameters. Concerning to voltage and current harmonics amplitudes, the parameter used in the comparative analysis is Harmonic Factors of n th harmonic (HF n ), Total Harmonic Distortion (THD), Distortion Factor (DF), Weighted Total Harmonic Distortion (WTHD) and Lowest Order Harmonics (LOH) (Muhammed Rashid 2003). 4.4.1 Harmonic Factor of n th Harmonic The harmonic factor of the n th harmonic which is a measure of individual harmonic contribution is in Equation (4.3). HF n Vn V 1 for n > 1 (4.3) where V 1 is the rms value of the fundamental component and V n is the rms value of the n th harmonic component. 4.4.2 Total Harmonic Distortion Total harmonic distortion is a measure of closeness in shape between a waveform and its fundamental component and is given in Equations (4.4) and (4.5). THD 1 V 1 n 2,3 V 2 n 1/ 2 (4.4) THD 1 I 1 I 2 n n 2,3 1/2 (4.5)
68 In expressions (4.4) and (4.5), V 1, I 1 and n are the fundamental voltage, current and harmonic order respectively. According to standards on harmonic distortion the summation operator is evaluated from n=2 to the harmonic order considered by a specific requirement. THD formula in (4.4) assumes that there is no DC content in the output voltage. Linear loads like resistor, inductor and capacitor do not create new harmonic frequencies. In nonlinear loads, inter-modulation products cause new harmonic and sub-harmonic components that are added in the spectrum by superposition. THD value of a voltage signal can be very different from that of a current signal. This means that harmonic current and harmonic voltages must be measured individually for the nonlinear loads. 4.4.3 Distortion Factor THD gives the total harmonic content but it does not indicate the level of each harmonic component. Higher order harmonics would be attenuated more effectively when a filter is used at output of the inverters. Knowledge of both the frequency and magnitude of each harmonic is important. The DF indicates the amount of harmonic distortion that remains in a particular waveform after the harmonics of that waveform have been subjected to a second order attenuation (divided by n 2 ). Thus, DF is a measure of effectiveness in reducing unwanted harmonics without having to specify the values of a second order load filter and is defined by Equation (4.6). 1 n 2,3 n 2 2 1/2 1 V DF (4.6) V n Equation (4.7). The DF of an individual or n th harmonic component is given in
69 V DF for n > 1 (4.7) V n n 2 1 4.4.4 Weighted Total Harmonic Distortion Weighted total harmonic distortion is superior to THD as a figure of merit for a non-sinusoidal inverter waveform in which lower portion of the frequency spectrum is weighted heavily, accurately portraying the expected harmonic current of an inductive load. The WTHD uses spectral weighting factor and it is calculated using (4.8). WTHD is usually preferred for the highly inductive loads such as induction or synchronous machines. WTHD 50 n 2,3 V V n 1 n 2 (4.8) 4.4.5 Lowest Order Harmonic LOH is the harmonic component whose frequency is closest to the fundamental one and its amplitude is greater than or equal to 3% of the fundamental component. All the above distortion criteria are used to validate the results concerning with harmonic performance. 4.5 SIMULATION STUDY Simulation of modified H-bridge multilevel inverter is done using MATLAB/Simulink environment. Simulation diagram of a three phase modified H-bridge seven level inverter is shown in Figure 4.5. The IGBT switches are used as switching devices because of their lower switching loss and smaller snubber circuit requirement. All the switches used in the simulation are considered to be ideal. The frequency of output voltage is 50Hz. The DC voltage sources used in this topology are equal in magnitude.
70 The single phase structure of modified H-bridge seven level inverter is presented in Figure 4.6. Figure 4.5 Simulation diagram of the three phase modified H-bridge seven level inverter using MPD-SPWM Figure 4.6 Subsystem of three phase modified H-bridge seven level inverter for phase A
71 This topology consists of level generation part and polarity generation part. In this modified inverter, only ten switches are required to obtain the seven level output voltage for each phase. More switches are required to achieve the same output voltage in the symmetric type where equal DC sources are used. MPD-SPWM is applied to the proposed topology. Level generation part uses high switching frequency to generate required levels. Polarity generation part requires fundamental switching frequency to generate required polarity in modified H-bridge seven level inverter. Figure 4.7 Simulation circuit of level generation part Simulation circuit of level generation part is shown in Figure 4.7. Six semiconductor switches and three equal DC sources are used in the level generation part. The simulation parameters of the modified three phase seven level inverter are furnished in Table 4.2.
72 Table 4.2 Simulation parameters of the proposed modified H-bridge seven level inverter using MPD-SPWM Description Parameter Value DC Supply DC Voltage Sources(V dc ) 107 V Level Generation Part Switching Frequency 5 khz Polarity Generation Part Switching Frequency 50Hz Three phase Squirrel cage induction motor Load Rated Voltage 400V Rated Current 10A Rated Power 5.4HP(4.03KW) Rated Speed 1430rpm Rated Torque 26.9 N-m Stator Resistance 1.405 Rotor Resistance 1.395 Number of Poles 4 Moment of Inertia (J) 0.0131 kg.m 2 Friction co-efficient (B) 0.002985 N-m-s The switching patterns for MPD-SPWM are produced by comparing three carrier signals against the unidirectional sinusoidal reference signal. This comparing process produces switching signals for switches S 1 to S 6. Switching frequency used for these switches is 5000 Hz. Fundamental switching frequency is used to produce the switching pulses for switches S 7 to S 10. The value of fundamental switching is 50 Hz. The switches connected in the same arm of the H-bridge should not be turned ON at the same time. So that the switches S 7, S 10 are turned ON at the positive half cycle and the switches S 8, S 9 are turned ON at the negative half cycle. These switching pulse waveforms are shown in Figure 4.8.
73 (a) Switching pulse waveform for switches S 1 -S 6 (b) Switching pulse waveform for switches S 7 -S 10 Figure 4.8 Switching pulse waveforms of seven level inverter using MPD-SPWM
74 Figure 4.9 Three phase voltage waveform of level generation part using MPD-SPWM Figure 4.10 Three phase voltage waveform of modified H-bridge seven level inverter using MPD-SPWM
75 The simulation results of level generation part are shown in Figure 4.9. Three positive levels such as 107 V, 214 V and 321 V are generated using level generation part. Switching frequency of 5000 Hz is used in this level generation part. Output of the level generation part is fed to the polarity generation part to produce the desired seven level output voltage. Switching frequency of 50 Hz is used in the polarity generation part. Simulation results of phase voltage are shown in Figure 4.10. It can be seen that each phase is phase shifted by 120 o. Seven levels of output voltage are generated using the modified multilevel inverter. It can be shown that from the Figure 4.10, semiconductor switches in the modified topology which uses modified MPD-SPWM scheme is turned ON and OFF many times. Figure 4.11 Phase current waveform of seven level inverter using MPD-SPWM Stator current of the induction motor when it is fed by three phase modified H-bridge seven level inverter using MPD-SPWM scheme is given in Figure 4.11. From Figure 4.10 and Figure 4.11, it is clear that the output current waveform is smoother than the output voltage waveform. It is evident
76 from Figure 4.11 that the currents fluctuate upto 0.1 sec and after that they reach a constant value. The extremely high value of current at the starting points of the simulation is due to the fact that before rotor gains some speed, the motor acts like a transformer with short circuited secondary winding. The fluctuations in the stator and rotor current values die out at about 0.15 sec and the currents reach a fairly constant value at the full speed of 1430 rpm. Figure 4.12 Electromagnetic torque of induction motor fed by modified H-bridge seven level inverter Electromagnetic torque characteristic of the induction motor when supplied by three phase modified seven level inverter using MPD-SPWM method is shown in Figure 4.12. Torque has an oscillating characteristic at the starting instant. A nearly constant electromagnetic torque is obtained after a time of 0.3 sec. It is observed that as the stator currents settle to a constant value the electromagnetic torque also attains a fairly constant value. It is seen that the torque has more oscillations while using this scheme. Torque attains a mean value of about 26.9N-m at steady state.
77 Rated speed of 1430 rpm is reached at 0.3 sec. The speed curve shows that the motor is started from stall and very quickly reaches its rated speed of about 1430 rpm at 0.3 sec. The speed then settles down to a fairly constant value at its rated speed. Observation also shows that the stator current is quite noisy. The noise introduced by the inverter is also observed in the electromagnetic torque waveform shown in Figure 4.12. However the motor inertia prevents this noise from appearing in the motor speed waveform. (a) Rotor Speed at no load (b) Rotor Speed at 50% of full load
78 (c) Rotor Speed at full load Figure 4.13 Rotor Speed of induction motor fed by modified H-bridge seven level inverter Torque applied to the three phase induction motors are gradually increased from no load to full load. Induction motors operated with no load have low power factor and also consumes more reactive power. So in the industries, induction motors are always operated with load. The speed characteristic of the induction motor fed by modified H-bridge seven level inverter is shown in Figure 4.13. THD of the inverter is determined from the line voltages. The simulation result of line voltages and their corresponding harmonic spectrum of modified inverter fed by MPD-SPWM are presented in Figures 4.14 and Figure 4.15. From the harmonic analysis shown in Figure 4.15, it is observed that the magnitudes of lower order harmonics are very low and the magnitudes of higher order harmonics are nearly equal to zero. From the harmonic analysis of line voltage waveform, it is seen that the triplen harmonics get cancelled automatically in three phase system. It also shows that the THD content of line voltages are 0.34%, 0.37% and 0.37% respectively. These numerical values are found to be minimum and within the
79 permissible limit of 5% as per IEEE 519 standard. Settling time of the three phase induction motor operating at different loading is given in Table 4.3. From the table 4.3, it is observed that the settling time increases with load. Table 4.3 Settling time of modified H-bridge seven level inverter fed induction motor for different loading conditions Load Settling Time No load 0.2s 50% of full load 0.25s Full load 0.3s Figure 4.14 Line voltage waveform of modified H-bridge seven level inverter using MPD-SPWM
80 (a) Harmonic analysis of line voltage V AB (b) Harmonic analysis of line voltage V BC (c) Harmonic analysis of line voltage V CA Figure 4.15 Harmonic analysis of line voltages for modified H-bridge seven level inverter using MPD-SPWM
81 The simulation results of line current and their corresponding harmonic spectrum of the modified inverter, when fed by MPD-SPWM scheme are presented in the Figure 4.16 and Figure 4.17. From the normalized harmonic analysis shown in Figure 4.17, it can be seen that the magnitude of low order harmonics are minimized and the magnitude of higher order harmonics are equal to zero. From the harmonic analysis of line current waveform, it is seen that the triplen harmonics get cancelled automatically in three phase system. It also shows that the THD content of line currents are 0.41%, 0.39% and 0.39% respectively. The improved harmonic performance which satisfies the IEEE 519 standard is achieved using the MPD-SPWM scheme with the modified topology. Figure 4.16 Line current waveform of modified H-bridge seven level inverter using MPD-SPWM
82 (a) Harmonic analysis of line current I A (b) Harmonic analysis of line current I B (c) Harmonic analysis of line current I C Figure 4.17 Harmonic analysis of line current waveform for modified H-bridge seven level inverter using MPD-SPWM
83 Table 4.4 Performance parameters of modified H-bridge seven level inverter Parameters Modified H-Bridge Seven Level Using MPD-SPWM Phase Voltage Analysis Line Voltage Analysis THD (%) 0.39 0.34 V 1 in volts 321.2 556.1 WTHD (%) 0.017 0.0203 DF (%) 0.003 0.005 HF 3 (%) 0.016 0.015 HF 5 (%) 0.063 0.050 HF 7 (%) 0.033 0.011 HF 9 (%) 0.029 0.023 HF 11 (%) 0.021 0.025 HF 13 (%) 0.019 0.020 HF 15 (%) 0.070 0.062 HF 17 (%) 0.022 0.026 HF 19 (%) 0.049 0.009 LOH NIL NIL The performance parameters of modified H-bridge seven level inverter, operated by MPD-SPWM method is shown in Table 4.4. Odd triplen harmonics such as HF 3, HF 9 and HF 15 cancelled automatically in three phase system is demonstrated. For the modified seven level inverter, the value of THD is very less. It is found that there is no lowest order harmonic in the modified H-bridge seven level inverter, when it is fed by modified MPD- SPWM.
84 4.6 SUMMARY In this chapter, a detailed analysis of modified H-bridge seven level inverter topology operated by MPD-SPWM is presented. Switching sequences of the modified multilevel inverter are clearly explained. A combination of high switching frequency and fundamental switching frequency schemes is proposed in this chapter. Performance parameters of the modified H-bridge seven level inverter and simulation results of proposed topology with MPD-SPWM scheme are discussed.