Design of Flying Capacitor Multilevel Inverter for Solar Energy Applications

. The continuous rise in electricity demand leads to adoption of renewable energy sources such as sun, wind for power generation. In order to utilize the electric power obtained from renewable solar energy, inverters are required to inject the electricity into the grid. This paper presents single phase flying capacitor multi-level inverter with help of SPWM technique. The comparison of single phase three-level, five-level and seven-level flying capacitor multilevel inverters is carried out with respect to waveform pattern, output current, output voltage, active power and reactive power waveforms. The simulations are conducted in MATLAB/SIMULINK environment.


Introduction
In order to get better utilization of renewable energy sources [1][2] the electricity generated is to be connected with the grid. The DC power obtained from solar photovoltaic panels is to be inverted into AC at 50 Hz frequency with constant voltage matched with the grid voltage [3][4][5][6]. The power should be harmonics free and sinusoidal. This requires DC to AC multilevel inverters [7][8]. These inverters offer low interference, with high efficiency. In literature various control strategies such as cascaded inverter, hysteresis current control etc have been proposed [9][10].
The multi-level inverter is a system or device which utilizes.various low level dc voltages as an input to achie ve optimalalternating voltage at the output. It consists of many switches and arrangement of switches are very important. A multilevel inverter enables higher voltages with less noise at output. It is required to obtain higher output power from sources like battery and solar arrays. Types of inverters are: Cascaded H-bridge inverter (CHB), Flying capacitor inverter (FC) and Diode clamped inverter. In this work, various topologies of flying capacitor inverter and their performances has been compared [11]. The section 2 presents Flying capacitor multilevel inverter and Sinusoidal Pulse Width Modulation (PWM) schemes, modelling of FC inverter is presented in section 3, simulation results are elaborated in section 4 followed by conclusion at the end.

Multilevel Inverter
The Multi-level inverter can be classified on basis of source i.e. common DC source and separate DC source inverter. Common dc source inverter includes FC and diode clamped inverter. Separate DC source includes CHB inverter. In CHB, arrangement of switches together with voltage is termed as H-bridge cell. H-bridge cell supply three separate voltages such as 0, +ve DC and -ve DC voltages. Each H-bridge cell has a different dc source in this inverter, which requires a wide range of sources and hence its applications is limited. In FC inverter topology condenser are used.

Flying Capacitor Multilevel Inverter
In this type of multilevel inverter topology capacitors are used. The condensers send the restricted amount of voltage to electrical equipment. Switching states of FC inverter and diode clamped inverter are identical [12]. In this form of inverter, clamping diodes are not used. The peak voltage that can be achieve from this inverter cannot greater than half the input voltage. The active and reactive power flow may also be regulated by FC inverter. Switching losses are attributed to the high frequency switching in this form of inverter [13][14].

Sinusoidal Pulse Width Modulation (PWM)
In this technique sinusoidal signal for PWM is used. With help of this technique gating signal for the inverter switches can be produced by comparing a sinusoidal reference wave with triangle carrier wave and width of each wave which varies according to amplitude of sine wave measured in middle of same wave [15][16][17][18].

Modelling of FC Inverter
In this type of multilevel inverter topology capacitors are used. The 3-level, 5-level and 7-level FC inverter models are shown in Fig. 1-3 respectively. The maximum voltage obtained from it cannot be more than half of input voltage. Power Flow can be controlled by capacitor. Switching losses will take place in this type of inverter due to high frequency switching.
In multilevel inverter the output voltage is generated at high frequency and there is low switching frequency with low distortions. Hybrid multi-level inverter is utilised to produce high voltage at output which decreased THD, dv/dt stress and common mode voltage.       In single phase 5 level FC inverter, inductive load is connected at output and current lags inductive voltage. The rms value of current and voltage waveform are 29.12 A and 164.1 V respectively. Active and reactive power waveform value is 4065 W and 1075 VAR respectively. The PF of load is 0.850 and phase angle is 31.715 degree.   Thus, higher levels of multilevel inverter show better performance as compared to lower levels of multilevel inverters and noise in the output side of inverter is decrease with increase in levels of multilevel inverter.

Conclusion
Due to depleting conventional energy sources the use of renewable energy sources is increasing day by day. However, the power produced by solar and wind energy sources is to be connected with the power grid to improve its utilization. This requires harmonics free power with constant voltage and frequency matched with the grid supply. Thus there is a need for suitable power electronic inverters. In this paper, by using the SPWM technique, the single phase three level, five level and seven flying capacitor multilevel inverter are simulated. It is observed that higher levels of flying capacitor multilevel inverter show better performance as compared to lower levels of multilevel inverters It is also observed that noise at output side is decrease with increase in levels of FC inverter, hence it is concluded that for dc to ac conversion for renewable energy applications higher level flying capacitor multilevel inverter should be preferred.