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Comparative studies of Cascaded and Modular Multilevel Inverters
1Lipika Nanda, 2U.K.Rout, 3A.Dasgupta
1,2,3KIIT University, School of Electrical Engineering, Patia Square, Bhubaneswar, Odisha, India Email: 1[email protected], 2[email protected], 3[email protected]
Abstract : The concepts of multilevel inverters don’t depend on just two levels of voltage to create an AC signal.
Several voltage levels are added to each other to create smooth stepped waveform with lower dv/ dt and lower harmonics distortion. More voltage levels in the inverter the waveform it creates become smoother, but many levels the design become more complicated, with more components controller for the inverter is needed.
Index Terms—Cascaded MLI, Modular MLI,PWM switching schemes
I. INTRODUCTION
Conventional two level inverters are mostly used today to generate an AC voltage from a DC voltage. The two level inverter can only create two different output voltage for the load, VDC/2 or –VDC/2 when the inverter is fed with VDC. To build up an AC output voltage these two voltages are switched with PWM[1]. In case of conventional inverter series and parallel combination of power switches in order to achieve the power handling voltages and currents. It produces THD level around 60% even under normal operating condition which are undesirable and cause more losses[2]. For the high current handling, the switches are connected in parallel, because of un even switching characteristics the load current is not shared equally. Higher power dissipation, thus increasing the junction temperature, thus decreasing the internal resistance, thus damaging the devices permanently[3]-[5].
In case of 3-level inverters, every phase leg can create the three voltages VDC/2, 0, -VDC/2 can be seen in the first part which is same as the conventional two level inverter but there is as twice as many values in each phase leg.
Figure1. A three level waveform, A five level waveform, A seven level waveform switched at
fundamental frequency
In between the upper and the lower two values there are diodes, called the clamping diode connected to the neutral midpoint in between two capacitors. Capacitors
build up the DC-bus which is charged with the voltage VDC/2[6]. Line to line voltage can be obtained on increasing the number of levels together with another phase legs. To create the zero voltage, two switches closest to the midpoint are switched on and the clamping diodes hold the voltage to zero.
In case of 3-level inverter, it generates output voltages which has very low distortion and it generates smaller common node voltage and it operate with lower switching frequency.
1.1 A. MULTILEVEL POWER CONVERTER
Some medium voltage motor drives and utility application require medium voltage and mega watt power level. For a medium voltage grid, it is troublesome to convert only one power semiconductor switch directly. A multilevel power converter structure has been introduced as an alternatives in high power and medium voltage situation. A multilevel converter not only achieves high power rating but also enables the use of renewable energy resources.
The concept of a multilevel to achieve higher power is to use a series of power semiconductor switches with several lower voltage dc sources to perform the power conversion by synthesizing a staircase voltage waveforms[7]. The commutation of the power switches aggregate these multiple dc sources in order to achieve high voltage at the output; however the rated voltage of the power semiconductor switches depends only the rating of the dc voltage sources to which they are connected.
A multilevel converter has several advantages over a conventional two level converter that uses high switching frequency pulse width modulation (PWM)[8].
SALIENT FEATURES OF MULTILEVEL
INVERTERS:
Synthesis of higher voltage level using power devices of low voltage ratings.
Increased number of voltage levels which leads to better voltage waveforms and reduced total harmonic distortion in output voltage.
Reduced switching stresses on the devices due to the reduction of step voltages between the levels.
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It solves harmonic and EMI problems, but also avoids possible high frequency switching stress dv/dt.
Low switching losses and better electromagnetic compatibility for high power applications.
II. CASCADED MULTILEVEL INVERTER
It uses cascaded full bridge inverter in a modular setup, with separate DC sources to create stepped waveform.
This topology consists of series power conversion cells, the voltage and power level can easily be scaled. The cells can be supplied by phase-shifted transformers in medium voltage system in order to provide high power quality at the unity connection. The inverter consists of H-bridge sometimes referred to as full bridge cells in a cascaded connection. Since each cell can provide three voltage levels(zero, positive DC voltage and negative DC voltage), the cells are themselves multilevel inverters.
H-bridge can supply both positive and negative voltages contributing to line to ground voltage, a switching state is defined for H-bridge cells SaHi, that has negative values.
The general relationship for cells i are : Vagi = (Tali-TaRi)vdci
Iadci=(Tali-TaRi)ias
The per cell switching state may be expressed in terms of transistor signals by
Sahi=Tali-Tari-1
Figure 2:Cascaded MLI
The a-phase line to ground voltage, the sum of the series cells or,
Vag = Pi=1Vagi for p series power cells[9].
In the first implementation of the series H-bridge inverter, the isolated DC source were equal to an overall switching state of
SaH = Pi=1SaHi
Separate DC sources is connected to single phase full bridge or H bridge inverter. Each inverter level can generate three different voltage level outputs, by connecting the DC source to the AC output by the different combinations of four switches S1, S2, S3, S4. To
obtain VDC, switches S1 and S4 are turned on whereas VDC can be obtained by turning on S2 and S3. By turning on S1 and S2 or S3 and S4, the output voltage is 0.
The AC outputs of each of the different full bridge inverter levels are connected in series such that the voltage waveform is the sum of the inverter output. The number of output phase voltage levels „m‟ in a cascaded inverter is defined by m=2s+1, where s is the number of separate DC sources.
III. MODULAR MULTILEVEL INVERTER, M2I
The Modular Multilevel Inverter was introduced in 2002. It uses a setup of sub modules, which are either connected or by passed for generation of output voltage level. Every phase-leg consists of two arms where each arms has „n‟ number of sub-modules. In every sub module, there is a charged DC capacitor with the voltage VM2I = VDC/m-1.So for a number of voltage levels „m‟
the inverter needs „m-1 = n number of sub modules per arm. The two inductors, in each arm consider the voltage difference when the modules are switched in and out.
S1
S2
S3
S4 Vdc
R
C1
C2
S1
S2
S3
S4 Vdc
R
C1
C2
C3
C4 S5
S6
S7
S8 Vdc1
Vdc2
Vdc3
Vdc4
Figure 3: Modular MLI
To activate a certain module in leg arm, the switch S1 is switched on and S2 is switched off. Again to bypass any module reverse switching has to be done that is switches S1 is turned off and switches S2 is switched on. The connection of the sub-module in a particular arm is determined by the positive or negative voltages which helps in balancing modulation. The order of connection can be changed on the basis of the charged stored in the capacitor and on current direction. Modular multilevel inverter requires half bridge, a DC capacitor and every sub-module there is two switches, two diodes and one capacitor. There are two inductors at the midpoint in the phase leg to take up the voltage difference between states. The voltage value in both arms is VDC, and the no. of sub-modules as a function of voltage levels is m- 1, such that it can withstand total DC voltage of VDC/m- 1.
IV. MODULATION TECHNIQUE
In case of multilevel modulation two types of methods are used : modulation with fundamental switching frequency and high switching frequency PWM. In both the cases a stepped waveform is achieved, but in case of
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high switching frequency method, the steps are modulated with PWM.
To generate a sinusoidal output wave, Multilevel PWM methods uses high switching frequency carrier waves than reference waves. For reducing the harmonic distortions in the output signal phase-shifting techniques are used. The number of switches used to be controlled in the inverter decides the number of carrier waves to be used. Two well known alternative methods that will be discussed in the report are : Alternative Position Opposition Disposition(APOD) and Phase Opposition Disposition(POD).
Figure 4: PWM reference and triangular carrier wave
Figure 5: The output voltage wave A. CASCADED H-BRIDGE INVERTER Case 1: 3-level Inverter
S1 S2
S3 S4
Vdc Vdc Vdc
Figure 6: 3-level Cascaded h-bridge Inverter In cascaded H bridge inverter separate DC source is connected to single phase full bridge. There are three different voltage level outputs generated, +VDC, 0, -VDC by connected dc source to the ac output by the different combination of four switches S1, S2, S3 and S4. Switches S1 and S4 are made on for the positive half cycle.
Switches S2 and S3 are made on for the negative half cycle. For 0th level, two cases arises:
Case 1: Switches S1 and S2 are made on.
Case 2: Switches S3 and S4 are made on.
The ac outputs are connected in series such that the voltage waveform is the sum of the inverter output.
Table 1: Switching States of 3-level Inverter Serial
Number
Switching State Output Voltage 1 S1 = ON, S4 = ON +VDC
2 S1 = ON, S2 = ON S3 = ON, S4 = ON
0 3 S2 = ON, S3 = ON -VDC Case 2: 5-level Inverter
Figure 7: 5-level Cascaded inverter
The total output voltage is the sum of the outputs of all the full-bridge modules in the inverter and every full- bridge can create the three voltages VCMC, 0, and –VCMC. To change one level of voltage in the phase output the CMCI turns one switches on and one off in one full- bridge module. For a full bridge module to add the voltage VCMC the switches S1 and S4 are turned on, for – VCMC the switches S2 and S3 are turned on. When there is current flowing through the full-bridge the 0 voltage is achieved by turning on the two switches on the upper halves of the full bridge S1 and S3 or the two switches on the lower halves S2 and S4. Together with several full- bridges a stepped waveform can be generated. The maximum output voltage is m-1/2VCMC = sVCMC = VDC/2 and minimum voltage m-1/2(-VCMC) = -VDC/2, where
„m‟ is the number of levels and „s‟ is the number of full- bridge modules.
B.MODULAR MULTILEVEL INVERTER Case 1: 3-level Modular Inverter
S1
S2
S3
S4 Vdc
R
C1
C2
Figure 8: 3-level Modular Inverter
In modular 3-level inverter, switches A and B are ON for the first half cycle i.e. for VDC/2. Switches AA and BB are made ON for the second half cycle i.e. for – VDC/2. And for 0th level, two cases arises:
Case 1: AA and B are made on.
Case 2: A and BB are made on.
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Table 2: Switching State of 3-level Inverter Serial
Number
Switching States Output voltage 1 A = ON, B = ON +VDC/2
2 AA = ON, B = ON
A = ON, BB = ON 0 3 AA = ON, BB = ON -VDC/2 Case 2: 5-level Inverter
S1
S2
S3
S4 Vdc
R
C1
C2
C3
C4
S5
S6
S7
S8
Vdc1
Vdc2
Vdc3
Vdc4
Figure 9: 5-level Modular Inverter
To activate a certain sub-module in an phase-leg arm to make its voltage source contribute to the output voltage the switch S1 is switched on and S2 is switched off. To bypass a sub-module the switches S1 is turned off and S2
is turned on in that certain sub-module. The arm in which a sub-module is to be connected is determined by if the wanted voltage is positive or negative and which sub-module in the arm is determined by the balancing modulation. The balancing modulation is the program that chooses which modules that are to be activated for each state to achieve voltage balance in all modules. To keep the sources in the sub-modules balanced the outer in which they are connected can be changed. One sub- modules has more charged stored in its capacitor it can be polarized to be activated first or last, depending on current direction, to balance the sub-module voltages.
The M2I topology hence has a redundant setup of switching states.
Figure 10: 3-level Cascaded inverter THD The three Cascaded Multilevel Inverter is simulated and the result is analyzed to determine the THD. By applying FFT the THD of the output voltage waveform is determined to be 43.08%.
Figure 11: 5-level Cascaded inverter THD The five-level cascaded multilevel inverter is simulated and the result is analyzed to determine the THD. By applying FFT the THD of the output voltage waveform is determined to be 34.87%.
Figure 13: 3-level Modular Inverter THD The three-level modular multilevel inverter is simulated and the result is analyzed to determine the THD. By applying FFT the THD of the output voltage waveform is determined to be 43.68%.
Figure 14: 5-level Modular Inverter THD The five-level modular multilevel inverter is simulated and the result is analyzed to determine the THD. By applying FFT the THD of the output voltage waveform is determined to be 23.59%.
V. CASCADED
H-BRIDGE INVERTER
Figure 12: 3-level Cascaded Inverter Case 1: 3-level inverter
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The 3-level cascaded h-bridge inverter is simulated. It consists of 12 switches and 3 DC sources. In this chapter we will discuss about the output waveform of line voltage, phase voltage and current waveform of 3-level inverter.
A. Line voltage
The three phase three level Cascaded H-bridge Multilevel inverter is simulated and the output voltage (line to line) waveform is obtained.
Figure 15:3-level line voltage output waveform B. Phase Voltage
The three phase three level Cascaded H-bridge Multilevel inverter is simulated and the output voltage (phase) waveform is obtained.
Figure 16: 3-level phase voltage output waveform C. Current waveform
Figure 17: 3-level current output waveform Case 2: 5-level Inverter
Figure 18: 5-level Cascaded Inverter
The 5-level cascaded h-bridge inverter is simulated. It consists of 24 switches and 6 DC sources. In this chapter we will discuss about the output waveform of line voltage, phase voltage and current waveform of 5-level inverter.
A. Line Voltage
The three phase five level Cascaded H-bridge Multilevel inverter is simulated and the output voltage (line to line) waveform is obtained
Figure 4.19: 5-level line voltage output waveform B. Phase Voltage
The three phase five level Cascaded H-bridge Multilevel inverter is simulated and the output voltage (phase) waveform is obtained.
Figure 4.20: 5-level phase voltage output waveform C. Current Waveform
Figure 4.21: 3-level current output waveform MODULAR MULTILEVEL INVERTER Case 1: 3-level inverter
Figure 22: 3-level Modular Inverter
The 3-level modular multilevel inverter is simulated. It consists of 4 switches and 2 sub-modules. In this chapter we will discuss about the output waveform of line voltage, phase voltage and current waveform of 3-level inverter.
A. Line voltage
The three phase three level Modular Multilevel inverter is simulated and the output voltage (line to line) waveform is obtained.
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-100 -80 -60 -40 -20 0 20 40 60 80 100
Time(sec)
Voltage(volts)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-80 -60 -40 -20 0 20 40 60 80
Time(sec)
Voltage(volts)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-15 -10 -5 0 5 10 15
Time(sec)
Current(Amps)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-300 -200 -100 0 100 200 300
Time(sec)
Voltage(volts)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-200 -150 -100 -50 0 50 100 150 200
Time(sec)
Votage(volts)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-15 -10 -5 0 5 10 15
Time(sec)
Current(Amps)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-100 -80 -60 -40 -20 0 20 40 60 80 100
Time(sec)
Voltage(volts)
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Figure 23: 3-level line voltage output waveform B. Phase Voltage
The three phase three level Modular Multilevel inverter is simulated and the output voltage (phase) waveform is obtained.
Figure 24: 3-level phase voltage output waveform C.Current Waveform
Figure 25: 3-level current output waveform Case 2: 5-level Inverter
Figure 26: 5-level Modular Inverter
The 5-level modular multilevel inverter is simulated. It consists of 12 switches and 6 sub-modules. In this chapter we will discuss about the output waveform of line voltage, phase voltage and current waveform of 3- level inverter.
A. Line voltage
The three phase five level Modular Multilevel inverter is simulated and the output voltage (line to line) waveform is obtained.
Figure 27: 5-level line voltage output waveform B. Phase Voltage
The three phase five level Modular Multilevel inverter is simulated and the output voltage (line to line) waveform is obtained.
Figure 28: 5-level phase voltage output waveform C. Current Harmonics
Figure 29: 5-level current output waveform
VI. THEORITICAL OUTCOME
The different topologies of multilevel inverter was studied and in the table given below the theoretical values of diode clamped, flying capacitor cascaded and modular multilevel inverter for the given voltage of 100 V.
Table 3: Theoretical analysis of the different topologies Type of
topology
Phase to neutral voltage (vro)
Phase to line voltage
(vll)
Phase voltage (vph) Cascaded
(h-bridge)
N level 50%, 0%, -50%
(2N-1) level 100%, 50%, 0%, -50%,-100%
(4N-3) level 66%, 50%, 33%, 16.5%, 0%, - 16.5%, -33%, - 50%, -66%
Modular N level 50%, 0%, -50%
(2N-1) level 100%, 50%, 0%, -50%,-100%
(4N-3) level 66%, 50%, 33%, 16.5%, 0%, - 16.5%, -33%,- 50%, -66%
VI. SIMULATED OUTCOME
The different topologies of 3-level multilevel inverter was studied, simulated and analyzed and in the table given below the practical values of diode clamped, flying capacitor cascaded and modular multilevel inverter for the given voltage of 100 V is obtained.
Table 4: Simulated output of 3-level multilevel inverter.
Type of Topology
Pole Voltage
Line Voltage
Phase Voltage
Current Harmonics Cascaded
h bridge 50, 0, -50
99.6, 49.8, 0.1, -49.5, - 99.7
66.3, 49.7, 33.1, 16.7, 0,
-15.8, -33, - 49.1, 65.8
0.48%
Modular 50, 0, -50
99.6, 49.8, 0.1, -49.5, - 99.7
66.3, 49.7, 33.1, 16.7, 0,
-15.8, -33, - 49.1, 65.8
9.47%
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-80 -60 -40 -20 0 20 40 60 80
Time(sec)
Voltage(volts)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-15 -10 -5 0 5 10 15
Time(sec)
Current(Amps)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-300 -200 -100 0 100 200 300
Time(sec)
Voltage(volts)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-200 -150 -100 -50 0 50 100 150 200
Time(sec)
Votage(volts)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-15 -10 -5 0 5 10 15
Time(sec)
Current(Amps)
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The different topologies of 3-level multilevel inverter was studied, simulated and analyzed and in the table given below the practical values cascaded and modular multilevel inverter for the given voltage of 100 V is obtained.
Table 5: Simulate output of 5-level inverter.
Type of Topology
Phase Voltage Line Voltage Current Harmonics Cascaded
h bridge
49.5, 40.6, 29.95, 21.7, 12.3, 0,
-12.3, -21.7 , - 29.95, -40.6, 0- 49.5
72.3, 47.1, 23.9, 0,
-23.9, - 47.1, -72.3
0.28%
Modular 49.5, 40.6, 29.95, 21.7, 12.3, 0,
-12.3, -21.7 , - 29.95, -40.6, 0- 49.5
72.3, 47.1, 23.9, 0,
-23.9, -47.1, -72.3
10.56%
Cascaded h-bridge inverter
The 3-level and 5-level Multilevel Inverter was simulated, analyzed and the values of line voltage and phase voltage was determined
Table 6: Comparison of 3-level VS 5-level Topology 3-level 5-level Line Voltage 42.60% 34.87%
Phase Voltage 43.08% 34.89%
5.7 Modular Multilevel Inverter
The 3-level and 5-level Multilevel Inverter was simulated, analyzed and the values of line voltage and phase voltage was determined
Table 7: Comparison of 3-level VS 5-level Topology 3-level 5-level Line Voltage 43.24% 27.46%
Phase Voltage 43.68% 28.40%
Table8: Comparison of the different topologies of 3- level inverters
Type of Topology
Line Voltage
Phase Voltage
Current Harmonics Cascaded h
bridge
42.55% 43.08% 0.48%
Modular 43.15% 43.68% 9.54%
Table 9: Comparison of the different topologies of 5- level inverters
Type of
Topologies
Line Voltage
Phase Voltage
Current Harmonics Cascaded h-
bridge
34.87% 34.89% 0.28%
Modular 27.46% 28.40% 10.58%
VII. CONCLUSION
Different converter topologies, of a three-level inverter have been simulated using “ Sine PWM Modulation Technique.” The THD of output voltage waveform was found to be very nominal for all but cascade have little bit high value compare to other. With increase of level THD of output waveform as well as filtering problem decreases .The theoretical results were validated by simulations. It is found that the voltage harmonics reduces on increasing the levels but the current harmonics increases. Different converter topologies, of a five-level inverter have been simulated using “ Sine PWM Modulation Technique. With increase of level THD of output waveform as well as filtering problem decreases. Diode clamped and Cascaded h bridge 5-level inverter was also simulated and analysed. It is found that the voltage harmonics reduces on increasing the levels but the current harmonics increases. Modular multilevel Inverter was simulated using sine pwm techniques.
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