Abstract- In this paper, a bypass diode technique is introduced to the conventional H-bridge multilevel inverters topology which reduces the number of controlled switches used in the system. Only one H-bridge is required for the single phase system, plus a switch and a diode for each voltage source. Due to involvement of high number of switches thereby the harmonics, switching losses, cost and the total harmonics distortion is increased. This proposed topology increases the level with less number of switches. It dramatically reduces the switches for high number of levels that reduces the switching losses; cost and low order harmonics and thus effectively improves Total harmonics distortion.
Keywords- Cascaded and Hybrid multilevel inverter, H-bridge multilevel inverter, THD, PWM.IGBT.
Numerous industrial applications have began to require higher power apparatus in recent years. Some medium voltage motor drives and utility applications require medium voltage and MW power level. For a medium voltage grid, it is troublesome to connect one power semiconductor switch directly -. The application of ac variable frequency speed regulations are widely popularized , high power and medium voltage inverter has recently become a research focus so far as known there are many problems in conventional two level inverter in the high power application. Multilevel inverter have been gained more attention for high power application in recent years which can operate at high switching frequencies while producing lower order harmonic components -.
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A multilevel inverter not only achieves high power ratings, but also enables the use of renewable energy sources. Renewable energy sources such as photovoltaic, wind, and fuel cells can be easily interfaced to a multilevel inverter system for a high power application. There are several topologies such as neutral point clamped inverter, cascaded multilevel inverter, diode clamped multilevel inverter, flying capacitor based multilevel inverter, and hybrid multilevel inverter .
The main disadvantage still exists in diode clamped multilevel inverter topology, which restricts the use of it to the high power range of operation. The first topology introduced is the series H-bridge design -, in which several configurations have been obtained. This topology consists of series power conversion cells which form the cascaded H- bridge multilevel inverter and power levels may be scaled easily. An apparent disadvantage of this topology is the large number of isolated voltage required to supply each cell. The proposed topology for multilevel inverter has a high number of steps associated with a low number of power switches. In addition for producing all the levels (odd and even) at the output voltage, a procedure for calculating the required dc voltage source is proposed .In this topology Pulse width modulation technique is used.
II. HYBRID MULTILEVEL INVERTER
The general structure of the hybrid multilevel inverter for single phase is shown in fig .1. Each of the separate voltage source Vs1, Vs2, Vs3 connected in cascade with other sources via a special H-bridge circuit associated with it. Each of the circuit consists of four active switching elements that can make the output voltage source in positive or negative polarity; or it can be simply zero volts depending on the switching condition of the switches in the circuit. A conventional multilevel power inverter topology employs multiple/link voltage of equal magnitudes. It is fairly easy to generalize the number of distinct levels  - .
Fig .1 Topology for Hybrid Multilevel Inverter
The S number of or stages and the associated number output level can be written as follows
N level =2s+1-1 (1)
For example if S=3, the output wave form has 15 levels (±7, ±6, ±5, ±4, ±3, ±2, ±1 and 0). The voltage on each stage can be calculated by using the equation,
V=2s-1.Vdc (i=1, 2, 3……….) (2)
The number of switches used in this topology is expressed as,
N switch=4 S (3)
The advantages of the hybrid multilevel inverter are series H-bridges for modularized layout and packaging. This will enable the manufacturing process to be done more quickly and cheaply. The draw back of this topology needs a separate dc source for each of the H-bridges and involves high number of semiconductor switches. Fig. 2 shows the output voltage waveform of a 7-level hybrid multilevel inverter with 2 separate dc sources.
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Fig .2 Typical output waveform for hybrid multilevel inverter
IV. MODIFIED HYBRID MULTILEVEL INVERTER
The proposed method multilevel inverter has a general structure of the hybrid multilevel inverter is shown in fig. 3.Each of the separate voltage source (Vs1, Vs2, Vs3) connected in cascade with other sources via a special circuit associated with it. Each stage of the circuit consists of only one active switching element and one bypass diode that can make the output voltage source only in positive polarity with several levels.
The basic operation is to turn on S1 (S2 and S3 turn off) and the output voltage is +1VS, turning on S2 (S1 and S3 turn off) producing output +2VS. Similarly other step can be achieved by turning on the suitable switches at particular intervals, Table.1 shows the operation clearly.
Only one H-bridge is connected to get both positive and negative polarity. The main advantage of modified hybrid multilevel inverter is high number of levels with reduced number of stages and dc sources. The S number of dc source or stages and the associated number output level can be calculated by using the equation
Fig .3 Topology for modified hybrid multilevel inverter
N level =2s+1-1 (4)
For example if S=3, the output wave form has 15 levels (±7, ±6, ±5, ±4, ±3, ±2, ±1 and 0), voltage on each stage can be calculated by using the equation
The number switches used in this topology is given by the equation
N switch=S +4 (6)
BASIC OPERATION OF HYBRID MULTILEVEL INVERTER
Current flow path
S2 , S3
S1 , S3
S1 , S2
S1, S2 , S3
Fig.4 Simulation diagram for 15 level Modified Hybrid Multilevel Inverter for Induction Motor Drive
V. PWM FOR HARMONICS REDUCTION
PWM technique is extensively used for eliminating harmful low-order harmonics in input and output voltage and current of static power. In PWM control, the inverter switches are turned ON and OFF several times during a half cycle and output voltage is controlled by varying the pulse width.
VI. SIMULATION RESULT ANALYSIS
The performance of the proposed modified hybrid multilevel inverter for induction motor drive is verified through the simulation results. It can be seen from fig.3 input voltages for each succeeding voltage source is 2s-1 Vdc. The total rms voltage is 228 V for 15 levels and fig.4 shows the simulation diagram of three phase modified hybrid multilevel inverter for induction motor drive.
Fig.5 Three phase line to line output voltage waveform for 15-level modified hybrid multilevel inverter
Fig.6 Speed curve of modified hybrid multilevel inverter fed induction motor drive
Fig.7 Torque curve of modified hybrid multilevel inverter fed induction motor drive
Fig. 5, 6 and 7 shows the MATLAB simulation output wave form of three phase line to line wave form, speed curve, torque curve respectively. From the FFT analysis window when the numbers of levels are increased, the harmonics and total harmonic distortion is reduced. For 15 levels inverter from fig.8, THD value is 7.63%.
Fig.8 FFT analysis for 15-level Modified Hybrid Multilevel inverter
VII. COMPARISON RESULTS
The modified hybrid multilevel inverter involves only seven number of switches compare to the conventional hybrid multilevel inverter table2 shows clearly. Therefore the proposed modified hybrid multilevel inverter has less switching losses than conventional hybrid multilevel inverter
COMPARISON OF HYBRID AND MODIFIED HYBRID MULTILEVEL INVERTER
Name of the Topology
Voltage level on each stage(S)
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Number of output level
Number of switches used
Number of switches used for 15 Level
Modified Hybrid MLI
2 S+1 -1
This paper has demonstrated the state of the art of modified multilevel inverter topologies with reduced number of switches. This multilevel inverter structure and its basic operations have been discussed. A procedure for calculating the required voltage level on each stage has been described. In the conventional methods as the number of levels are increased the required number of switches also increased. Due to involvement of high number of switches thereby increasing the harmonics, switches losses, cost and the total harmonics distortion the proposed method dramatically reduces the switches for high number of levels, switching losses, cost, and low order harmonics and effectively improves the total harmonics distortion.
Rotor Type : Wound
Nominal Power : 3730 W
Voltage : 420 Volts
Frequency : 50 Hz
Stator Resistance : 1.115Ω
Stator Inductance : 0.005974Ω
Rotor Resistance : 1.083Ω
Rotor Inductance : 0.005947Ω