Simulation Analysis of PV Based Single Phase Modified H-Bridge Eleven Level Inverter

This study presents the simulation analysis of a PV based single-phase modified H-bridge eleven-level inverter. Multilevel inverters offer high power capability, associated with lower output harmonics and lower turn-off losses. This study informs a PV based multilevel inverter using a H-bridge output stage with four bidirectional auxiliary switches. The inverter is capable of producing eleven levels of output-voltage levels (+Vdc/5, +2Vdc/5, +3Vdc/5, +4Vdc/5, +Vdc, ‘0’ level, -Vdc/5, -2Vdc/5, -3Vdc/5, -4Vdc/5 and -Vdc) from the DC supply. Theoretical predictions of the proposed PV based single-phase modified H-bridge eleven-level inverter with MPPT are validated using simulation in MATLAB SIMULINK.


INTRODUCTION
Due to the need, constant increase of costs of fossil fuels and its huge negative impact on the environment, renewable energy sources have recommended and developed, in recent years.It is estimated that the electrical energy generation from renewable sources is increased from 19%, in 2010, to 32%, in 2030, because of innovation in power electronics techniques and control strategies (European Commission, 2010).Among various types of renewable energy sources, solar photovoltaic energy is one of the fastest growing and promising, owing to pollution free.In Photovoltaic (PV) systems, solar energy is converted into electrical energy by PV arrays.PV arrays are very popular since they are clean, inexhaustible and require little maintenance.Photovoltaic systems require interfacing power converters like dc-dc converter and dc-ac inverter between the PV arrays and the grid.The generated power from PV system can be distributed to power system networks through grid-connected inverters.
A single-phase grid-connected inverter is usually used for residential or low-power applications of power ranges that are less than 10 kW (Calais and Agelidis, 1998).Types of single-phase grid-connected inverters have been investigated (Kjaer et al., 2005).A wellknown topology of this inverter is full-bridge threelevel.Multilevel inverters are promising; they have nearly sinusoidal output-voltage waveforms, output current with better harmonic profile, less stressing of Fig. 1: Single-phase MHBELI for PV system electronic components due to reduced voltages, switching losses that are lower than those of conventional two-level inverters, a smaller filter size and lower EMI, all of which make them cheaper, lighter and more condensed (Hinga et al., 1994;Cheng et al., 2006).
Over the years, different types of multilevel inverter topologies have been introduced.Familiar ones are diode-clamped, flying capacitor or capacitor clamped, cascaded H-bridge and simplified H-bridge multilevel inverters (Rodriguez et al., 2002;Ceglia et al., 2006).This study illustrates the development of a single-phase modified H-bridge eleven-level inverter that has four bidirectional switches with embedded diodes.The proposed single-phase MHBELI for PV system is shown in Fig. 1.
Photovoltaic (PV) arrays were attached to the inverter via a dc-dc boost converter.The power generated by the inverter is to be delivered to the power set-up, so the utility grid, rather than a load, was used.
The dc-dc boost converter was required because the PV arrays had a voltage that was lower than the grid voltage.High dc bus voltages are necessary to ensure that power flows from the PV arrays to the load.The inverter used in the power stage offers a significant enhancement in terms of lower component count and condensed design complexity when compared with the other existing eleven-level inverters.

METHODOLOGY PV array with MPPT:
PV array: A solar PV system is powered by many crystalline or thin film PV modules.Individual PV cells are interconnected to form a PV module.This takes the form of a panel for easy installation.PV cells are made of light-sensitive semiconductor materials that use photons to dislodge electrons to drive an electric current.
A PV cell basically is a p-n semiconductor junction.When exposed to the light, a DC current is generated.The generated current varies linearly with the solar irradiance.The equivalent electrical circuit of a practical PV cell can be treated as a current source parallel with a diode shown in Fig. 2.
The I-V characteristics of the equivalent circuit of PV can be determined by following equations.The current through diode is given by: While, the PV cell output current: where, I = PV cell output current (A) I PV = Light generated current (A) (Short circuit value assuming no series/ shunt resistance) I O = Diode saturation current (A) Incremental conductance MPPT with boost converter: When a PV module is directly coupled to a load, the PV system operating point will be at the intersection of its I-V curve and the load line which is the I-V relationship of the load.In other words, the impedance of load dictates the operating condition of the system.Moreover, since the capacitors are connected in parallel with the main dc power supply, no significant capacitor voltage swing is produced during normal operation, avoiding a problem that can limit operating range in some other multilevel configurations.

Power circuit description:
The proposed single-phase Modified H-Bridge Eleven-Level Inverter (MHBELI) was developed from the five-level inverter reported in (Ceglia et al., 2006;William and Ramesh, 2012).It includes (Christopher et al., 2012;Rahim et al., 2011), a single-phase conventional H-bridge inverter which has four main switches S1, S2, S3 and S4, a capacitor voltage divider formed by five capacitors C 1 , C 2 , C 3, C 4 and C 5 , as shown in Fig. 4.
The modified H-bridge multilevel inverter topology is obviously cost-effective compare to other topologies, i.e., it requires less number of power switches, power diodes and less capacitor for inverters of the same number of levels.

Mode I operation:
The switch S1 is ON, connecting the load positive terminal to Vdc and S4 is ON, connecting the load negative terminal to ground.Remaining switches S2, S3, S5, S6, S7 and S8 are OFF; the voltage across the load terminals ab is Vdc.

Mode II operation:
The bidirectional switch S5 is ON, connecting the load positive terminal and S4 is ON, connecting the load negative terminal to ground.Remaining switches S1, S2, S3, S6, S7 and S8 are OFF; the voltage across the load terminals ab is 4Vdc/5.

Mode III operation:
The bidirectional switch S6 is ON, connecting the load positive terminal and S4 is ON, connecting the load negative terminal to ground.Remaining switches S1, S2, S3, S5, S7 and S8 are OFF; the voltage across the load terminals ab is 3Vdc/5.

Mode IV operation:
The bidirectional switch S7 is ON, connecting the load positive terminal and S4 is ON, connecting the load negative terminal to ground.

Mode V operation:
The bidirectional switch S8 is ON, connecting the load positive terminal and S4 is ON, connecting the load negative terminal to ground.Remaining switches S1, S2, S3, S5, S6 and S7 are OFF; the voltage across the load terminals ab is Vdc/5.

Mode VI operation:
This mode of operation has two possible switching combinations, Either switches S1 and S2 are ON, remaining switches S3, S4, S5, S6, S7 and S8 are OFF or S3 and S4 are ON, remaining switches S1, S2, S5, S6, S7 and S8 are OFF.In both switching combinations terminal ab is short circuited, hence the voltage across the load terminals ab is zero.

Mode VII operation:
The bidirectional switch S5 is ON, connecting the load positive terminal and S2 is ON, connecting the load negative terminal to Vdc.

Mode VIII operation:
The bidirectional switch S6 is ON, connecting the load positive terminal and S2 is ON, connecting the load negative terminal to ground.Remaining switches S1, S3, S4, S5, S7 and S8 are OFF; the voltage across the load terminals ab is -2Vdc/5.

Mode IX operation:
The bidirectional switch S7 is ON, connecting the load positive terminal and S2 is ON, connecting the load negative terminal to ground.Remaining switches S1, S3, S4, S5, S6 and S8 are OFF; the voltage across the load terminals ab is -3Vdc/5.

Mode X operation:
The bidirectional switch S8 is ON, connecting the load positive terminal and S2 is ON, connecting the load negative terminal to ground.Remaining switches S1, S3, S4, S5, S6 and S7 are OFF; the voltage across the load terminals ab is -4Vdc/5.

Mode XI operation:
The switch S2 is ON, connecting the load negative terminal to Vdc and S3 is ON, connecting the load positive terminal to ground.Remaining switches S1, S4, S5, S6, S7 and S8 are OFF; the voltage across the load terminals ab is -Vdc.
In the eleven-level inverter circuit five capacitors in the capacitive voltage divider are connected directly across the dc supply voltage V dc and since all switching combinations are activated in an output cycle, the dynamic voltage balance between the five capacitors is automatically restored.

Simulation model:
The simulation model of singlephase MHBELI for PV system using MPPT is developed using MATLAB SIMULINK tool box, is illustrated in Fig. 6.
This model contains of photovoltaic module, DC-DC boost converter with incremental conductance MPPT algorithm and finally the proposed single-phase MHBELI.
The PWM signal for each of the switching devices in the MHBELI circuit comes from the pulse generator block.This block includes all the PWM signals required for switches are multiplexed on a single bus to the eleven-level inverter power circuit.

PV module characteristic:
The simulation of 250 W PV module for the following specifications has been simulated is shown in Table 2.
The simulated V oc -I sc characteristic is shown in Fig. 7.The x-axis represents the open circuit voltage (V oc ) of 37.47V and y-axis represents the corresponding short circuit current (I sc ) of 8.76A.
The simulated P-V characteristic is shown in Fig. 8.The x-axis represents the open circuit voltage (V oc ) of 37.47V and y-axis represents the corresponding maximum output power of 250 W.

Converter:
The DC-DC Boost converter input power is taken from PV module and the output of the converter is maximized using Incremental conductance algorithm is fed to the single-phase MHBELI through dc-dc boost converter.The PV module has been tested for load change due to change in solar radiation.The incremental conductance algorithm was implemented in the dc-dc boost converter.The output of the MPPT is the duty-cycle function.As the dc-link voltage Vdc was controlled in the dc-ac single-phase MHBELI, the change of the duty cycle changes the voltage at the output of the PV panels.
The incremental conductance method is based on the fact that the slope of the PV array power curve is zero at the MPP, positive on the left of the MPP and negative on the right, as given:   The MPP can thus be tracked by comparing the instantaneous conductance (I/V) to the incremental conductance (∆I/∆V).Vref is the reference voltage at which the PV array is forced to operate.At the MPP, Vref is equal to VMPP.Once the MPP is reached, the operation of the PV array is maintained at this point unless a change in ∆I is noted, indicating a change in atmospheric conditions and the MPP.

Single-phase MHBELI:
The switching sequence required for the single-phase MHBELI circuit is shown in Fig. 9.
Figure 10 show the simulated output voltage waveform taken from the load terminals.It is clearly visible that the simulated output is very close to the ideal output defined for a eleven-level inverter circuit.
The Total Harmonic Distortion (THD) of the single-phase MHBELI is observed that 7.82% without filter circuit and the fundamental voltage is observed that 234.3V (50 Hz) that has been illustrated in Fig. 11.

CONCLUSION
The modeling of PV based single-phase MHBELI was done and simulated using MATLAB SIMULINK.The development of simulation model has been successfully simulated and the results of PV characteristics, Incremental conductance MPPT, switching sequence for single-phase MHBELI and output voltage waveform were obtained.The concurrence between the theoretical forecasts and simulated results show clearly that the developed simulation model works as expected PV based singlephase MHBELI and provides a broad approach on solar energy based multilevel inverters.The proposed model has obvious advantage of using single phase supply and it can be implemented using an embedded system.This approach can be used for grid connected photovoltaic energy applications.

Fig. 2 :
Fig. 2: Equivalent electrical circuit of a PV cell

Table 1 :
Switching combinations required to generate the eleven-level output voltage waveform Vo

Table 2 :
PV module specifications