A Novel Asymmetrical Single-Phase Multilevel Inverter Suitable for Hybrid Renewable Energy Sources

This paper introduces a novel asymmetrical single-phase multilevel inverter suitable for hybrid renewable energy sources. The proposed inverter consists of two isolated DC sources and six power semiconductor controlled switches. The suggested inverter is capable of generating seven level output when the input DC voltage is take in the ratio of 1:2. The higher magnitude DC source is fed from Photo Voltaic (PV) panels, where as the lower magnitude DC source is fed from Wind turbine (WT) driven permanent magnet DC (PMDC) generator. Both the renewable energy sources are connected to the inverter via two DC-DC boost converters connected in cascade (i.e. one for maximum power point tracking and another for DC-link voltage control). The proposed hybrid renewable energy source inverter is connected to single-phase grid via proper control systems. The complete system is simulated using MATLAB/SIMULINK and the results are presented in detail.


INTRODUCTION
convert DC into AC signal as required by the grid [3].
In recent years, Renewable Energy Sources (RES) are renewable energy systems single stage boost inverters gaining more importance over the globe because of the are proposed in the literature [1-4].Single stage power exhausting nature of the conventional energy sources, conversion technique reduces the losses there by rise in earth's temperature due to carbon dioxide increases the efficiency, but it suffers from drawbacks like emissions, ever increasing oil price, non availability of poor total harmonic distortion [THD] at the output power supply in the rural areas etc [1-4].The energy voltage which eventually increases the filter size, thereby generated from RES depends upon the environmental increasing the cost and size of the total system [5].conditions [5]

(i.e. energy generated from solar energy
To improve the harmonic profile of the output conversion system depends upon the solar irradiation and voltage of the inverter Multilevel Inverters [MLI] are energy generated from wind energy conversion system suggested in [1][2][3][4][5][6].MLIs have nearly sinusoidal output depends upon the wind speed), whereas the electrical grid voltage and current waveforms with improved harmonic requires constant voltage and frequency.Hence proper profile, less stress in power electronic switches due to power electronic interface must be provided between the reduced voltages, lower switching losses when compared renewable energy and the grid for stable operation [6].
to conventional three level inverters, smaller filter size and In order to connect RES to the grid, two stages of reduced electromagnetic interference [5].power conversion are used.First stage is to boost up the In recent years various MLIs are proposed in the low voltage output of RES and to track its Maximum literature.Among those commonly used types are diode-Power Point (MPP), where as the second stage is used to clamped MLI [7,8], capacitor clamped MLI [8], cascaded To increase the efficiency of the grid connected H-bridge MLI [9][10][11][12] and modified H-bridge MLI [12-13].
MLIs are further classified into symmetrical and asymmetrical types.Asymmetrical MLIs (ASMLI) are capable of producing more levels for the given power electronic devices when compared with symmetrical MLIs (SMLI) [14].This paper introduces a modified ASMLI topology suitable for renewable energy sources.The proposed inverter is capable of generating seven levels with two isolated DC sources and six power semiconductor controlled switches.The two isolated DC sources are of different in magnitudes of voltages with the ratio of 1:2.Among the six power semiconductor devices, two devices are bidirectional and four devices are unidirectional devices.The proposed inverter has many advantages like simple in structure, adaptable for integrating RES with the grid, lower THD and lesser number of semiconductor Fig. 1: Proposed Asymmetrical MLI Topology switches.
The remaining part of this paper is structured as symmetrical in nature.The asymmetrical MLI proposed in follows.The operation and description of the modified this paper requires only two bidirectional switches to ASMLI topology along with switching logic are detailed generate seven levels further the problem of capacitor in Section II, the suggested ASMLI fed from RES along voltage balancing does not exist.Fig 1 shows circuit with proper control system are described in Section III, topology of the proposed asymmetrical MLI topology.Section IV details the simulation results obtained from The proposed topology of ASMLI consists of two MATLAB/SIMULINK and the performance of the system isolated DC sources with the ratio of 1: 2, six power is recapitulated at the conclusion.
semiconductor devices in which two devices are Circuit Topology: The proposed topology consists of a connected between the middle of the Leg 1 and to the two bidirectional switches added to the conventional H middle of the two isolated DC sources, where as the bridge inverter.The proposed topology has been derived switch AS is connected between the middle of the leg 2 from the topology proposed in [5].The topology and to the middle of the DC sources.Fig 2 shows the proposed in [6] has only one bidirectional switch which is modes of operation of the proposed inverter.In fig 2 the capable of generating only five levels and is of conducting paths are shown in dark black lines whereas symmetrical type.Whereas the topology proposed in [6] the non-conducting paths are shown in the light grey has two bidirectional switches which is capable of colors.Table 1 shows the switching table of the proposed generating seven levels, but it has the problem of ASMLI, which is capable of generating seven levels.capacitor voltage balancing when fed to high power loads Further the proposed ASMLI is capable of generating [8].Further the MLI proposed in [10]

Proposed Inverter Fed from Renewable Energy Sources:
Characteristics of TATA BP 180W panel is shown in The proposed inverter is a best fit for grid connected figure 6 for various irradiations and cell temperature.It is renewable energy applications.The figure 5 shows the very clear from the figure 6 that the location of the proposed inverter fed from solar photovoltaic and wind maximum power point varies when the environmental turbine driven PMDC generator.The upper DC source is condition varies (i.e.irradiation and module temperature).fed from Solar PV, where as the lower DC source is fed Similarly the wind power vs. turbine speed characteristics from WT driven PMDC generator.The operating point of for various wind velocities is shown in figure 7. It can be solar PV module is decided by solar radiation, temperature observed from figure 7 if wind velocity varies MPP also of PV module and the resistance of the load.For a given varies.Hence MPPT is essential in the case of PV arrays cell temperature and solar radiation, there is an exclusive and WT.The details of PV module and WT coupled operating point of the PV array in its PV curve with PMDC machine is given in table 2. There is lot of MPPT maximum output power.Hence Maximum Power Point algorithms proposed in the literature [1]- [8].Among the Tracking (MPPT) is essential in PV arrays in order to draw various MPPT algorithms perturb and observe and maximum power from it irrespective of the climatic and Incremental conductance algorithms are most popular load conditions [11]    Where Y is the PV module current and X is the PV module Voltage.Since the sliding surface equation is passing Similarly the sliding surface of WT is given by through the maximum power points, the result of the above equation should be zero to ensure MPP.Hence PV Y = 0.00454X -1.7X+179.

WT and PMDC parameters ------------------PV Module -------------------------------Wind Turbine ----------------------------------PMDC ----------------
(2) module current and voltage are sensed and equation ( 1) is calculated instantaneously.When the result of the Where Y is the power output of PMDC generator and X above equation is greater than zero, the switch S is the WT rotor speed.When the result of the above pv_b1 (shown in figure 5) is turned ON, else it is turned OFF.equation is greater than zero, the switch S (shown in figure 5) is turned ON, else it is turned OFF.In order to grid integration.Hence another Boost converter is evaluate the proposed sliding mode algorithm, step connected in cascade with MPPT boost converter.The change in the wind speed is given and the corresponding main purpose of the second boost converter is to maintain power output is measured and compared with actual MPP constant voltage in the DC link. Figure 12 shows the power as shown in figure 10.
output voltage obtained from the second boost converter, By comparing figure 9 with data points of figure 6(a), which remains constant at 130 volt. it is very clear that steady state error is very minimum and Similarly the output voltage of MPPT boost also it has good dynamic response.Similar comparison converter of WT also varies when there is a variation in can be made between Figure 10

CONCLUSION
In this paper a novel asymmetrical multilevel inverter is proposed.The operation of the proposed inverter is discussed, simulated in MATLAB/SIMULINK environment and the results are presented.The proposed inverter is fed from renewable energy sources through two boost converter connected in cascade one for tracking the MPP and another for voltage control.Further the proposed inverter is connected to the single phase grid through proper control structure.The complete system is simulated and the results are presented.

Fig. 2 :
Fig. 2: Modes of operation of the proposed ASMLI

Fig. 5 :
Fig. 5: System Configuration of the Proposed Inverter fed from PV arrays and WT driven PMDC Generator.

Fig. 9 :Fig. 10 :
Fig. 9: MPPT of PV module using Sliding Mode algorithm Fig. 11: Output voltage of MPPT boost converter of PV figure5) is turned ON, else it is turned OFF.In order to grid integration.Hence another Boost converter is evaluate the proposed sliding mode algorithm, step connected in cascade with MPPT boost converter.The change in the wind speed is given and the corresponding main purpose of the second boost converter is to maintain power output is measured and compared with actual MPP constant voltage in the DC link.Figure12shows the power as shown in figure10.outputvoltage obtained from the second boost converter, By comparing figure9with data points of figure6(a), which remains constant at 130 volt. it is very clear that steady state error is very minimum and Similarly the output voltage of MPPT boost also it has good dynamic response.Similar comparison converter of WT also varies when there is a variation in can be made between Figure10and figure 7 to evaluate the wind speed.Hence the Voltage control boost the performance of sliding mode control of WT. converter is connected in cascaded to have stiff voltage at the DC link of the inverter.Figure 13 shows the output Cascaded Boost Converters: Two boost converters are voltage of voltage control boost converter of WT.It is connected in cascade between the RES and the inverter evident from figure 13 that the output voltage of WT DC sources as shown in figure 5.The output of the MPPT voltage control boost converter is 260 volts.boost converter of PV module is shown in figure 11.Since the primary task of the MPPT boost converter is track the Grid Integration: The proposed inverter fed from MPP, the output voltage of the MPPT boost converter renewable energy sources is connected to grid through also varies when there is a change in environmental control components as shown in figure 5.The control conditions as shown in figure 11.But the DC link voltage block diagram of the grid integration is shown in the of the inverter should be maintained constant for stable figure 14.

Fig. 14 :
Fig. 14: Control Block diagram of Grid Integration Figure 16 is set to 130 volts where as the lower DC link voltage is shows the capability of the proposed inverter when set to 260 volts.The output voltage of the proposed there is sudden change in the grid voltage (i.e.Grid inverter with seven levels is shown in figure 15(a) for Disturbance).It can be seen in the figure 16 that when the modulation index of 0.95.When the modulation index is grid voltage falls the inverter modulation index is adjusted reduced to 0.6, the inverter is capable of generating only itself so that the inverter voltage also reduces[13-15].(a) (b) Fig. 16: (a) Output Voltage of the proposed inverter due to grid disturbance (b)Reference waveform of the inverter due to grid disturbance.

Figure 17
Figure 17 shows the grid voltage and grid current Converters as a Utility Interface for Renewable waveform during the time of grid disturbance.It is very Energy Systems, IEEE Power Engineering Society clear from figure 17 that the grid voltage and grid current Summer Meeting, IEEE, 2: 1271-1274.are in phase and the power factor is almost unity.It can be 4. Calais, M., V.G.Agelidis and M. Meinhardt, 1999.seen from figure 18 that the reference current from MPPT Multilevel Converters for Single-Phase Grid controllers and the inverter current are one over the other Connected Photovoltaic Systems: An Overview, and tracks very perfectly.Sol.Energy, 66(5): 325-335.

Table 1 :
is again of higher levels if the basic blocks are connected in cascade.Switching logic of the Basic Block of the Proposed ASMLI bidirectional (i.e.AS and AS ).The switch AS is