Implementation of Micro-Inverter for PV Based

Stand-Alone Applications

Conan Michael Reynolds

M.Tech. Power Electronics and Drives

VIT University Chennai Campus

Chennai, India

conanreynolds@yahoo.co.in

Dr. Febin Daya J. L.

Associate Professor,SELECT

VIT University Chennai Campus

Chennai, India

febindaya.jl@vit.ac.in

Abstract—Electrical power obtained from solar panels need

to be converted to AC in order to supply the loads. There are

various converter topologies available for this purpose. One of

these types of converters can be directly attached to the back of

a panel. These are known as micro-inverters. This paper shows

the design and implementation of the inverter stage of a single-

phase, transformerless micro-inverter topology used for stand-

alone PV applications. The proposed topology is designed for a

230W solar panel.

Keywords—micro-inverter; stand-alone PV; solar panel

I INTRODUCTION

There is an increasing demand for the supply of electrical energy, globally. Due to the adverse effects of increased dependence on fossil fuels for this production, the importance of exploring renewable energies is gaining momentum. Among the variety of renewable energy resources available, due to its numerous advantages, photovoltaic sources are estimated to become the largest contributors to electric energy generation. Solar cells are used to convert this energy into electrical energy, but majority of the cost involved in the generation depends on the solar inverters used.

Solar inverters can be broadly classified as string inverters and micro-inverters. String inverters operate on a string of solar modules, such that they are treated as one big module. The main disadvantage associated with these types of inverters is that the maximum power output of the entire string depends on the weakest module in it. In case of shading for one module in the entire string, the maximum power point of that module is tracked and the other solar modules are made to operate at that power level, thereby resulting in a low power output of the system. Another disadvantage of string inverters is that they’re available in a limited range of power ratings. This means that a solar array uses an inverter of the next higher rating than the power output of the string, if the inverter is not available for the same rating. This also prevents future expansion of the string, in case the demand is increased, without changing the inverter. These disadvantages are overcome in the case of a micro-inverter.

A micro-inverter is an electrical converter circuit attached to the back of a single solar module, which converts

the DC output of the panel to AC, which is either synchronized and supplied to a grid, or used for stand-alone photovoltaic applications. In its most basic form, it can be viewed as a single unit consisting of, (i) a DC-DC converter circuit using a maximum power point tracking (MPPT) control algorithm to optimize the output of the solar panel, and (ii) a DC-AC inverter along with required filters to obtain the required sine-wave output with reduced harmonics. In the case of a PV system with multiple modules, the performance of each module is not affected by the defects or shading or other abnormalities of the other modules. The disadvantage in using micro-inverters is the increased installation costs and complexity, but has a better system efficiency and power output compared to the string inverter approach.

In this paper, the inverter stage of a proposed topology of a micro-inverter is studied and designed for a 230W solar panel. The designed inverter is the simulated using MATLAB and then a hardware prototype of the same is implemented.

Fig. 1. Block diagram of a micro-inverter

II MICRO-INVERTER TOPOLOGY AND OPERATING MODES

Fig. 2. shows the circuit diagram of the micro-inverter. When the output current is positive, all the four inductors will work; when the output current is negative, only inductors L02 and L03 will work. L01 and L04 are wave shaping inductors, whereas, L02, L03 and C form an L-C-L filter for harmonic elimination.

Switches S1, S4 and S5 operate during the positive half cycle and S2, S3 and S6 work during the negative half cycle. S5 and S6 are polarity selection as well as freewheeling switches when the respective switches are off in that half

cycle. PWM is used to operate switches S1 to S4. The carrier frequency used is 10 kHz with a reference frequency of 50 Hz.

Fig. 2. Proposed micro-inverter topology

There are four modes of operation in every cycle of

output power. These can be seen in Fig. 3. During the

positive half cycle S5 stays on while S1and S4 are switched

at a high frequency. When S1 and S4 are off, the current

freewheels through S5 and D5. During the negative half

cycle S6 stays on while S2 and S3 are switched at a high

frequency. When S2 and S3 are off, the current freewheels

through S6 and D6.

Fig. 3(a). Positive half cycle with S1, S4 and S5 ON

Fig. 3(b). Positive half cycle with S1, S4 OFF and freewheeling current through S5 and D5

Fig. 3(c). Negative half cycle with S2, S3 and S6 ON

Fig. 3(d). Negative half cycle with S2, S3 OFF and freewheeling current

through S6 and D6

III SYSTEM DESIGN

The micro-inverter was designed for an Eldora 230 solar panel, with input voltage as 30V and current of 7A. The ratings of the panel are as specified in Table 1.

TABLE 1. ELDORA 230 RATINGS

Parameters Value

Type ELV 230

Nominal power PMPP 230

Nominal voltage VMPP(V) 29.8

Nominal current IMPP(A) 7.8

Open circuit voltage VOC(V) 37

Short circuit current ISC(A) 8.4

Module efficiency (%) 14.3

Inductors L01 and L04 are wave shaping inductors of value 60uH each. The output L-C-L filter is designed as shown below.

Inductor Design

(1)

Where, i = current ripple in Ampere

and , i = 20% of Irated

Irated = 7A

Vdc = Vrated = DC input voltage = 30V

fSW = Switching frequency = 10 kHz

(2)

L = 2.34 mH

Capacitor Design

(3)

(4)

Where, fn = 50Hz

= 27°

C = 8.6 x 10-4

F

C ≈ 10 x 10-4

F

IV MATLAB SIMULATION OF SYSTEM

The micro-inverter was designed and simulated using MATLAB Simulink. A DC voltage source was applied as input and the output taken across a resistor of 5Ω. The output AC voltage and current were obtained. Along with this, an FFT analysis was done with and without the filter, to show harmonic elimination. The Simulink model of the inverter is shown in Fig. 4. The results that were obtained are shown in Fig. 5. The FFT analysis can be seen in Fig. 6.

Fig. 4. MATLAB model of micro-inverter

Fig. 5. MATLAB simulation output

Frequency to voltage calculation

Fig. 6(a). MATLAB FFT analysis without filter

Fig. 6(b). MATLAB FFT analysis with filter

V HARDWARE IMPLEMENTATION

A 230W, 30V inverter circuit has been designed and fabricated to verify the simulation results. The PWM is applied to the switches using an 8051 microcontroller, P89V51RD2. A TLP250 driver circuit has been implemented to amplify the microcontroller output for the switches as well as to provide isolation of both circuits. The pulses from the microcontroller and the driver circuit can be seen in Fig. 7.

Fig. 7(a). Microcontroller pulses for PWM and S5 and S6

Fig. 7(b). Driver circuit for the six switches

Fig. 7(b). Driver circuit output

The switches used in the micro-inverter circuit are IRF540 MOSFET switches. The inductors used for wave shaping and as filters are toroid iron core wound inductors. The hardware circuit of the micro-inverter can be seen in Fig. 8.

Fig. 8. Micro-inverter circuit

The hardware experimental setup as well as output

results for a 24V DC input can be seen in Fig. 9, without the

use of filter.

Fig. 9(a). Experimental setup of Micro-inverter

Fig. 9(b). Output of Micro-inverter without filter

VI CONCLUSION

The micro-inverter was designed successfully.

Neglecting the real-time constraints, the model was

simulated and the required results have been obtained. The

hardware design of the same has been successfully

implemented and tested without the use of the filter.

VII REFERENCES

[1] Baifeng Chen, Bin Gu, Jih-Sheng Lai, Wensong Yu, “A High Efficiency and Reliability Single-Phase Photovoltaic Micro-Inverter with High Magnetics Utilization for Nonisolated AC-Module Applications”, August 2013

[2] Timothy CY Wang, Zhihong Ye, Gautam Sinha, Xiaoming Yuan, “Output Filter Design for a Grid-interconnected Inverter”, March 2003

[3] W. Yu, J.-S. Lai, H. Qian, C. Hutchens, J. Zhang, "High-efficiency inverter with H6-type configuration for photovoltaic non-Isolated AC module applications," in Proc. of IEEE Applied Power Electronics Conference and Exposition, Palm Springs, CA, Feb. 2010, pp. 1056-1061

[4] W. Yu, J. –S. Lai, H. Qian, C. Hutchens, J. Zhang, “ High-efficiency inverter with H-6 type configuration for PV non-isolated AC module applications”, in Proc. of IEEE Applied Power Electronics Conference and Exposition, Palm Springs, CA, Feb 2010, pp. 1056-1061

[5] B. Gu, J. Dominic, J. –S. Lai, C –L. Chen, T, LaBella, and B. F. Chen, “ High reliability and efficiency single phase transformerless inverter for grid connected photovoltaic applications”, IEEE Trans. Power Electron, vol.28, no. 5, pp.2235-2245. May, 2013

[6] Pekik A. Dahono, Agus Purwadi, and Qamaruzzaman , “An LC Filter Design Method for Single-phase PWM Inverters”, Laboratorium Penelitian Konversi Energi Elektrik, Jurusan Teknik Elektro, Institut Teknologi , IEEE Catalogue No.95TH8025

[7] Antoni M. Cantarellas1, Elyas Rakhshani1, Daniel Remon1, Pedro

Rodriguez, “Design of Passive Trap-LCL Filters for Two-Level Grid Connected Converters”