Grid tied PV system using modular multilevel inverter

Received Mar 12, 2019 Revised Apr 20, 2019 Accepted Jun 4, 2019 A grid tied photovoltaic system using modular multilevel inverter topology is proposed in this paper. Basic unit structure of modular multilevel inverter used in this system is capable of converting DC power from PV array to AC power for feeding power to the household loads or utility grid. The proposed modular multilevel inverter structure has lesser power electronic devices compared to the existing multilevel inverter topologies. The proposed system generates a nearly sinusoidal signal and achieves better output profile with low total harmonic distortion. Simulation of the proposed system is carried out in MATLAB/Simulink software and the results are presented.


INTRODUCTION
Photovoltaic (PV) systems are booming in the renewable energy market due to its clean and abundantly available nature [1,2]. Also, solar energy is easily tapped and converted into electricity which is the added advantage of PV array over other renewable energy sources [3]. Due to the intermittent nature of the PV array, power electronic interface is needed in the standalone and grid tied PV systems for obtaining the desired output [4]. For grid tied PV system, power electronic inverters are utilized as the interface to convert the generated dc power from the PV array into ac power to feed the utility grid [5][6][7][8][9].
Basic inverter generates a squarewave output which is considered to have higher harmonic content. Thus, large size filter has to be employed to obtain pure sinusoidal waveform inorder to feed the PV generated power to the grid. Also, this inverter has high dv/dt stress and harmonic distortion in the output waveform. To overcome these problems, multilevel inverters were developed.
Multilevel inverter (MLI) is popular among industrial and research sectors due to its advantages like improved output voltage profile with low dv/dt and low common mode voltages [10,11]. The output voltage of MLI has low distortion and low harmonic content which reduces the size and cost of the filter and the level of electromagnetic interference (EMI) [12]. The three basic MLI topologies are diode clamped MLI, flying capacitor MLI and cascaded MLI [13][14][15]. As the number of levels of output voltage increases, these topologies require increased number of switches, diodes and capacitors which eventually increases the cost, volume and control complexity [16][17][18][19][20].
To overcome these disadvantages, modular multilevel inverters (MMLI) are developed [21,22] with two parts viz. level generator and polarity changer. Level generator creates unipolar multilevel output voltage waveforms using high frequency switches and polarity changer converts this unipolar waveform into a bipolar waveform using low frequency switches at line frequency. In this MMLI, the number of switches, conduction loss and switching loss are reduced compared to that of the conventional multilevel inverter for the same output levels [23]. Thus, the modular multilevel inverter is employed in the proposed system to feed the PV array power to the utility grid. The contributions of the proposed grid tied PV system are summarized as follows:  This paper proposes an improved system which employs modular multilevel inverter topology in order to feed PV array generated power to utility grid with lesser number of components and reduced losses.  The proposed inverter operates with lesser voltage and current THD in order to facilitate smooth grid integration of the system.

WORKING OF THE PROPOSED GRID CONNECTED SYSTEM
The block diagram of the proposed system shown in Figure 1 consists of PV array, modular multilevel inverter, controller and utility grid. Basic unit structure of MMLI shown in Figure 2(a) consists of Level generator and Polarity changer. This basic unit structure can be expanded to the modular structure as shown in Figure 2(b) to increase the number of output voltage levels. The level generator of the MMLI basic unit consists of 3 PV array sources, VPV1, VPV2 and VPV3 and 4 power switches, S1, S2, S3 and S4. The polarity changer is an H-bridge inverter consisting of 4 switches, S5, S6, S7 and S8. The controller in the proposed system generates the gate pulses to the switches of level generator to produce a unipolar staircase waveform and generates the gate pulses to the polarity changer switches to convert the unipolar signal into a bipolar signal. Basic structure of MMLI with 3 symmetrical voltage sources generates the output waveform with seven levels of equal widths. The switching of level generator is explained in this section for different voltage levels. In this level, all the switches in the level generator are in OFF condition and the switches, S5 and S7 are ON in the polarity changer resulting a zero level output and hence, there is no flow of current in the system. The equivalent circuit for this level is shown in Figure 3(a).

Level 1
In this level, switches S4, S5 & S8 are turned ON, keeping other switches OFF and the dc voltage, VPV3 appears across the output terminal and the current flows from VPV3 to the output terminals through the path S5, S8 and S4 as shown in Figure 3(b).

Level 2
Switches S2, S3, S5 and S8 are turned ON and the other switches are turned OFF in this level. As a result, the inverter output voltage terminal is (VPV2 + VPV3). The current flows from VPV2 and VPV3 to the output terminal through the path S2, S3, S5 and S8 as shown in Figure 3(c).

Level 3
Switches S1, S5 & S8 are turned ON, keeping other switches OFF in this level. As shown in Figure 3(d), (VPV1 + VPV2 + VPV3) appears across output terminal of the inverter and the current flows from VPV1, VPV2 and VPV3 to the output terminal through the switches S1, S5 and S8.
After achieving the peak voltage of (VPV1 + VPV2 + VPV3), the voltage levels are gradually decreased from level 3 to level 0 to generate seven level output voltage waveform at input terminals of polarity changer  Figure 4 and the corresponding switching states of the MMLI are shown in Table 1. The number of output levels increases with the increase in number of this basic unit of MMLI.

SIMULATION STUDIES
Simulation model of the proposed system shown in Figure 5 is constructed using power MOSFET switches, pulse generator and utility grid available in the SimPowerSystem Blockset library of MATLAB/simulink software and PV array is modeled using the classical equation as follows [24]: where, Iph is the light generated current, I0 is the reverse saturation current of the PV array, Rse is the series resistance of the PV array and V(th) is the thermal voltage. The proposed 7 level system is simulated with the symmetrical PV array sources at an irradiation of 900 W/m 2 (each source is constituted with 2 series connected PV modules each rated 250 W with open circuit voltage, Voc of 37.8 V and short circuit current, Isc of 8.8 A). Figure 6 shows the waveforms of PV array voltages, VPV1, VPV2 & VPV3 of 62.08 V, 61.96 V & 62.82 V respectively and the generated dc link voltage, Vdclink of 85.14 V and dc link current, Idclink of 5.33 A.  2018 output voltage and current without filter is found to be 22.16 % and 2.7 % and THD of the grid voltage and current with filter is reduced to 1.74 % and 2.18 % respectively which is well within the IEEE 519 standard as shown in Figure 8.
The proposed system is simulated and analyzed for various irradiation conditions of PV array and the corresponding power fed to the grid and efficiency is plotted in Figure 9. From Figure 9, it is evident that the grid power and efficiency is increasing with the increase in PV array irradiations. Thus, modular multilevel inverter topology feed the maximum power to the utility grid from the PV array efficiently at higher irradiation conditions with low voltage and current THD.
A comparsion between the components count of the existing topologies and the modular multilevel inverter toplogy presented in this paper are provided in Table 2. From the table, it is evident that the modular multilevel toplogy presented in this paper has less number of components which reduces the cost and size of the system.

CONCLUSION
Grid tied photovoltaic system using modular multilevel inverter is proposed in this paper. MMLI used in the proposed system produces a 7 level output with minimum number of switches active at any instant. Generation of output levels are explained with the corresponding equivalent circuit in this paper. MATLAB/Simulink software is used for the simulation analysis of the proposed system and the results are furnished. From the simulation results, it is evident that the proposed system is capable of feeding power to grid from the symmetrical PV array sources with low voltage and current THD of 1.74 % and 2.18 % respectively. The number of output levels can also be increased, if more number of basic units of this MMLI is added.