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In cascaded H-bridge multilevel inverter, a variable frequency inverted sine PWM technique is modeled for hybrid electric vehicles. It has a particular advantage of increasing power which is achieved using series connection of H-bridge and also this topology is capable to produce superior spectral quality with considerable improvement of fundamental voltage. The variable frequency inverted sine PWM technique produces lesser torque ripple and enhances the fundamental output voltage mainly at lower modulation index ranges. The topologies of multilevel inverter are flying capacitor, diode clamped and cascaded inverter. In the paper, we will discuss about the cascaded multilevel inverter based on inverted sine PWM technique. The two switching strategies widely used to control multilevel inverters are constant frequency inverted sine PWM (CF-ISPWM) and variable frequency inverted sine PWM (VF-ISPWM). This implies that switch utilization substantially reduces 32.35% of the constant frequency inverted sine PWM switching technique. The performance of the technique is validated in terms of Total Harmonic Distortion (THD) and Torque ripple which significantly reduces when compared to constant frequency ISPWM. The analysis of conventional triangular PWM inverter and inverted sine PWM inverter using constant and variable switching scheme is done in MATLAB Simulink and verified experimentally by FPGA Spartan 3E processor.

Power Electronics based inverters are widely used in many industrial, aerospace and military applications because of their incredible concert on control capability and energy saving. Multi Level Inverter (MLI) is an attractive topology for high voltage DC-AC conversion. The reliability of power electronics system is directly related to the security of motor drive systems. The function of a multilevel inverter is to synthesize a desired staircase output waveform from several levels of dc input voltages that can be batteries, fuel cells, etc. [

Hussein Ashram M G, Fathi S H and Gharehpetian B have suggested selecting the suitable power semiconductor devices for asymmetric multi-level inverter which is very reliable and safe [

The paper is organized as follows: Section 2 introduces hybrid cascaded multilevel inverter and Section 3 explains variable frequency inverted sine PWM technique. Simulation results and Experimental results have been presented in Sections 4 and 5 respectively. Finally Section 6 concludes this paper.

Multiple dc sources are required for many applications which increases the cable length and this could lead to voltage unbalance among the dc sources [

where, n is the number of H-bridge cells per phase.

The hybrid cascaded multilevel inverter consists of two H-Bridges. The first H-Bridge H_{1} consists of dc source 1V_{dc} whereas the second H-Bridge H_{2} consists of dc source 3V_{dc} is as shown. Each dc source is connected to a single phase inverter. Each inverter level can generate three different voltage outputs +V_{dc}, 0 and −V_{dc} by various combinations of the four switches S_{1}, S_{2}, S_{3} and S_{4}. When switches S_{1} and S_{4} are ON, the output is +V_{dc}, when switches S_{2} and S_{3} are ON, the output is −V_{dc}, when either pair S_{1} and S_{2} or S_{3} and S_{4} are ON, the output is

0 [_{1} can be made equal to −1V_{dc}, 0, or 1V_{dc}, similarly the output voltage of H_{2} can be made equal to −3V_{dc}, 0, or 3V_{dc} by opening and closing its switches appropriately. Therefore, the output voltage of the inverter have the values−4V_{dc}, −3V_{dc}, −2V_{dc}, −V_{dc}, 0, 4V_{dc}, 3V_{dc}, 2V_{dc}, V_{dc} can be designed, as shown in _{1} and the second H-bridge is represented by V_{2}. The output voltage is V = V_{1} + V_{2}.

The proposed control strategy replaces triangular carrier waveform by variable frequency inverted sine wave so it is known as Inverted sine PWM techniques. The main objective of the proposed method deals THD minimization and switch utilization in a considerable manner for the progress of efficiency factors in MLIs. The proposed PWM strategy replaces the conventional constant frequency carrier waveform by variable frequency inverted sine wave [

The significance of ISPWM methods are:

1) Spectral quality is better and fundamental component is higher when compared to the conventional sinusoidal PWM.

2) The ISCPWM strategy enhances the fundamental output voltage particularly at lower modulation index ranges.

3) Reduction in THD.

4) Low switching losses.

5) Due to improvement of THD in the lower range of modulation index, this is applicable for low speed drive application.

Total Harmonic Distortion (THD) decreases and switching losses increases with increase in switching fre-

Voltage | S_{1} | S_{2} | S_{3} | S_{4} | S_{5} | S_{6} | S_{7} | S_{8} |
---|---|---|---|---|---|---|---|---|

4V_{dc} | On | Off | Off | On | On | Off | Off | On |

3V_{dc} | On | On | Off | Off | On | Off | Off | On |

2V_{dc} | Off | On | On | Off | On | Off | Off | On |

1V_{dc} | On | Off | Off | On | On | On | Off | Off |

−1V_{dc} | Off | On | On | Off | Off | Off | On | On |

−2V_{dc} | On | Off | Off | On | Off | On | On | Off |

−3V_{dc} | Off | Off | On | On | Off | On | On | Off |

−4V_{dc} | Off | On | On | Off | Off | On | On | Off |

quency and switching loss. The graph is plotted with THD, switching losses and switching frequency to obtain a low value of THD and switching losses. The optimum frequency is found to be 3950 Hz and the corresponding THD and switching loss is found from the graph shown in

In

where θ = Switching angle.

The calculation of the slope values for the four bands is shown below:

In our proposed method, the strategy spotlighted on Phase Disposition (PD) based inverted sine PWM (ISPWM) for nine-level inverter. The parameters chosen for simulation using the proposed PWM technique is as shown in

From the _{dc} = 300 V. As we shown, the proposed VFISPWM technique always claims lower THD than CFISPWM technique and the Load voltage waveform shows that the top and bottom levels of VFISPWM technique has less number of switching compared to the CF-ISPWM technique.

From the above results, it is also observed that VFISPWM gives an enhanced fundamental voltage and reduced total harmonic distortion. To verify the robustness of the proposed scheme, a simulation model for a single phase hybrid cascaded MLI with Induction motor load is implemented. The motor specifications are revealed in

The rotor speed for 1ϕ induction motor is shown in _{s}. The relation is given by [

S. No | Parameters | Values |
---|---|---|

1 | Main DC Source Voltage (V_{dc}) | 300 V |

2 | Modulation Index (ma) | 0.9 |

3 | New Carrier Frequency | 3950 Hz, 3645 Hz, 3043 Hz, & 1366 Hz |

4 | Frequency Modulation Ratio (mf) | mf1 = 79, mf2 = 73, mf3 = 61 & mf4 = 27 |

5 | RL Load | R = 10 Ω; L = 0.024 mH |

6 | Rated Output Frequency | 50 HZ |

7 | Reference Voltage | 300 V |

Parameter | CFISPWM | VFISPWM |
---|---|---|

Nsw | 68 | 46 |

V-THD | 18.27% | 10.02 % |

I-THD | 3.07 % | 2.90 % |

Parameter | Constant Frequency | Variable Frequency | ||
---|---|---|---|---|

Triangular PWM | Inverted Sine PWM | Triangular PWM | Inverted Sine PWM | |

V-THD | 12.76% | 18.27% | 8.69% | 10.02% |

I-THD | 2.93% | 3.07% | 2.93% | 2.90% |

Parameters | Values |
---|---|

Rating | 186.5 W, 110 V, 50 Hz, 1440 rpm |

Stator Resistance (Rs) | 2.02 Ω |

Rotor Resistance (Rr) | 4.12 Ω |

Inductance | Ls = 0.278 H, Lm = 0.177 H |

Moment of Inertia | 0.0146 J |

Pole Pairs | 2 |

where N_{s} = synchronous speed [rpm]; f = frequency of the source [Hz]; p = number of poles. The rotor turn at slightly less than synchronous speed and the full-load slip is typically 3 percent to 5 percent for fractional horsepower motors [

The electromagnetic torque and the stator flux for a 1ϕ induction motor are shown in

From the speed and torque curves, it is seen that rated speed quickly achieved within 0.2 ms and the torque is quickly settled at 0.25 ms. Therefore the proposed hybrid multilevel inverter can be used for variable speed drive applications which can be obtained by varying the frequency of multilevel inverter. The harmonics in induction motor sometimes exhibit to run stably as low as 1/7 of the synchronous speed. This could be avoided by reducing the harmonics which are shown in

where (V_{0})_{n} is the rms value of harmonic component and (V_{0})_{1} is the rms value of fundamental component.

The number of possible output voltage levels is twice the number of dc sources so the complexity in the cir-

cuits for higher order levels with high power loads can be reduced and the series of H-bridge makes for modularized layout and packaging. This will make the manufacturing process to be done in quickly and cheaply. This structure is favorable for high power application since its produces higher voltage at higher modulation frequencies with low switching frequency. The multilevel inverter can be implementing for Hybrid Electric Vehicles (HEV) and Electric Vehicles (EV). The HEV combines a conventional IC engine, battery pack and an electric motor whereas the EV includes rechargeable batteries and an electric motor. The multilevel inverter that drives the electric motor is an input device for HEV and EV [

Moreover, the performance parameters measured for evaluating the proposed modulation strategy are: Crest factor, Distortion factor, WTHD and HSF.

Crest factor is a measure of a waveform, such as alternating current or sound, showing the ratio of peak value to average value. Crest factor value 1 indicates no peaks, but higher crest factors indicate peaks.

where, V_{peak} is the Peak Voltage and V_{rms} is the R.M.S Voltage.

The distortion factor is the result from a mathematical equation which resembles the geometrical means. The intensity of the nonlinear distortions is measured.

where, V_{1} is the Fundamental voltage, V_{n} is the total harmonics voltage and n is the order of the harmonics.

The weighted total harmonic distortion (WTHD) is a commonly used to evaluate the quality of pulse width modulation (PWM) inverter waveforms. The WTHD weights the voltage harmonics is inversely with its frequency. Whereas this is tolerable for some inductor type loads, the commonly employed induction motor load has significant effects resulting from eddy currents in the rotor bars not included in the WTHD [

Harmonic Spread Factor is the deciding factor to indicate noise generation in the motor. The harmonic spread factor is calculated for evaluating the quality of voltage spectrum of inverters.

where, H_{j} is the value of j_{th} harmonic and H_{0} is the average value of all N Harmonics.

From

Moreover, with variable frequency, enhanced fundamental output is obtained. It has to be emphasized that less torque ripple for variable frequency PWM technique leads to better stability operation with minimum mechanical noise.

S. No | Parameters | CF ISPWM | VF ISPWM |
---|---|---|---|

1 | V_{rms} (V) | 251.1 | 255.1 |

2 | V_{peak} (V) | 355.1 | 360.7 |

3 | Crest Factor ( CF) | 1.414 | 1.414 |

4 | Distortion Factor ( DF) | 0.017 | 0.0055 |

5 | Weighted THD (WTHD) | 0.257 | 0.083 |

6 | Harmonic Spread Factor (HSF) | 3.191 | 2.247 |

7 | Torque Ripple (%) | 17.9 | 7.9 |

To validate the single-phase asymmetric hybrid cascaded H-bridge multilevel inverter, a prototype module is built using CT60AM18F smart power module (SPM) as the switching devices and is experimented as shown in

The inverter was first controlled by using constant frequency ISPWM with the following parameters: ma = 0.9 and mf = 79. The prototype inverter was tested using the proposed VFISPWM with different carrier frequencies which balances the switching actions and gives a lower value of THD compared to the CFISPWM. Hardware set-up for Single phase power circuit with optocoupler is exposed in

The specifications for semiconductor device are: Part Number: CT60AM-18F, Voltage rating = 900 V and Current rating = 60 A.

PWM generators incorporated within microcontrollers are not normally flexible enough to generate switching pulse, so that Field Programmable Gate Array (FPGA) based controller is employed. The pulse produced by FPGA processor is transmitted through an optocoupler to the multilevel inverter. The core processor of the system is a Xilinx FPGA, Spartan 3E programmed through a Xilinx EEPROM. The FPGA program is downloaded from Personal Computer through a parallel cable to the EEPROM using master serial mode and the stored program in the electrically erasable programmable ROM (EEPROM) is reloaded to the FPGA once it is reset [

The proposed strategy is employed for single phase RL load which have the value of R = 10 ohms and L = 0.024 mH. The inductance value is small, so that the load voltage and load current waveforms are identical. The simulation is carried out for the motor load in order to calculate the performance of the MLI with Variable frequency ISPWM. This will be helpful to design the hardware for induction motor load.

From the above

Insulated Gate Bipolar Transistor (IGBT) is a very popular device among power semiconductor due to its easy switching and handles high level of power demand by Hybrid Electric Vehicles (HEV) motor drives. In this work, Cascaded H-Bridge Multilevel Inverter can be interfaced with electric drives of HEVs because of following features.

1) When compare to diode clamped and flying capacitor MLI, the number of components is less.

2) It is suitable for high current and low voltage rating electric drives which are needed for Hybrid Electric Vehicles.

3) Cascaded H-Bridge Multilevel Inverter can be switched at low frequency, so that noise can be suppressed which is comfortable for driving HEVs [

The three single-phase multilevel inverter are connected in parallel to add up the voltage. The output of the three single phase inverter needs to be synchronized with separation of 120 degree between each phase. So that we can implement for three phase circuit.

Parameter | CFISPWM | VFISPWM | ||
---|---|---|---|---|

V-THD | I-THD | V-THD | I-THD | |

Simulation Result | 18.27% | 3.07% | 10.02% | 2.90% |

Experimental Result | 6.28% | 6.25% | 4.45% | 5.95% |

Thus the simulation and experimental results demonstrated the ability of the proposed methodology to effectively be used to increase the performances of the MLIs. A comparative evaluation between CFISPWM & VFISPWM methods has been presented in terms of output voltage quality, power circuitry complexity and THD. From this observation VFISPWM technique provides an enhanced fundamental voltage and reduced THD compared to other conventional strategies. To overcome the marginal boost in the magnitude of lower order harmonics and unbalanced switch utilization, variable frequency inverted sine carrier signals are employed. Now, it is proved that the VFISPWM method gives a better performance and THD for a chosen modulation index. Further, the proposed MLI with reduced number of switches can also be employed for electric vehicle applications.

M. Sudhakaran,R. Seyezhai, (2016) Modeling and Analysis of Variable Frequency Inverted Sine PWM Technique for a Hybrid Cascaded Multilevel Inverter. Circuits and Systems,07,2633-2650. doi: 10.4236/cs.2016.79228