Design and Implementation of Three Phase Micro Inverter

Journal of Green Engineering (JGE) Volume-9, Issue-1, April 2019

Design and Implementation of Three Phase Micro Inverter Based PV Module

S.Subramaniam1, B.Akash

2, R.Vignesh

3 and R.Mahalakshmi

4

1,2,3,4

Department of Electrical and Electronics Engineering, Amrita School of Engineering, Bengaluru, Amrita Vishwa Vidyapeetham, India. E-mail:1

[email protected], [email protected].

Abstract

Generation of Electricity from solar energy is very popular as there is an

exponential growth in energy consumption. This paper focuses on the

operational performances of two kinds of configurations of solar modules.

Conventionally series-parallel connected solar panels use the centralized

single inverter named as DC Module where single module failure in whole

unit cannot be solved easily. In order to have more efficient PV module,

new AC module called as three phase micro inverter based PV module is

proposed. Each single module will have embedded inverter along with the

boost converter. Thereby failure in single module will not affect the whole

system performance and is more efficient than conventional topology by

10% to 20% in whole. Detailed Simulink modelling and development of

hardware implementation of prototype model of standalone three phase

micro inverter is discussed. Also, Centralized inverter based PV system is

compared with micro inverter based PV module.

Keywords: Renewable Energy Sources, Solar Energy, Micro Inverter,

Centralized Inverter, Boost Converter, Maximum Power Point Technique.

Journal of Green Engineering, Vol.9_1, 34-59. Alpha Publishers

This is an Open Access publication. © 2019 the Author(s). All rights reserved

35 R.Mahalakshmi et al

1 Introduction

The research interest on the conversion of solar to electricity is

becoming more popular. The uniqueness of this form of energy is that for

the generation of energy, there is no turbine involved no moving parts and

freely available in nature. This helps in reducing the machinery. Solar

energy systems can also be installed at lower cost and lesser time would be

required [1-2]. This form of energy has got certain limitations such as Low

efficiency of panels which is only 18% of the energy can be extracted at

maximum power point condition, not reliable at night as sunlight would not

be available, any objection of sunlight falling on the panel would reduce the

output such as deposition of dust and it requires circuitry of higher

performance due to the lower efficiency of extracting energy from the

panel.

To extract the maximum amount of energy from the panel at any given

point of time of the day for the particular value of irradiance, MPPT

(Maximum Power Point Technique) controller is used. There are many

MPPT techniques are proposed to extract the maximum power from the

solar array. The commonly used MPPT techniques are Perturb and Observe

(P & O) method and Incremental Conductance method. The P & O method

is the iterative method [3-4] to trace the Maximum Power Point (MPP).

This is commonly used and considered to be efficient when the irradiance

level is not varied much. The classic Perturb and observe (P & O) method

has the disadvantage of poor efficiency at low irradiation. The

shortcomings in P & O method is overcome by incremental conductance

method. The accuracy of this method depends on the iteration size. In

general, the size is maintained constant for the traditional Incremental

Conductance method. The increased step size used for improving the MPP

tracking speed, however, accuracy is decreased. Likewise, reducing the step

size improves the accuracy, but detriments the speed of convergence of the

algorithm. [5] In both the methods, the true MPP is not achieved. For vast

variations in the weather conditions, both the methods are inefficient and

also, they have a complex circuit. The shortcomings of incremental

conductance method, P & O method can be overcome by constant voltage

and current method. It assumes a linear dependence between voltage and

corresponding to maximum power and short circuit current. The constant

voltage method is one of the simplest and fastest MPPT technique. The

operating point of the solar array is kept near the MPP by regulating the

output voltage of the solar panel to the reference voltage Vref which is

nothing but the maximum voltage (Vmpp) of the solar panel. it is observed

that the constant voltage MPPT technique is effective than either the P &

O, or the Incremental Conductance algorithm [6-8]. The constant voltage

MPPT method for the boost converter is implemented in this paper [9]. It

also mimics the action of feed forward control. In the present scenario,

Design and Implementation of Three Phase Micro Inverter based PV Module 36

number of solar panels are connected in series and parallel to obtain the

required amount of load voltage and current. A single solar panel module

consists of boost converter and it outputs the DC voltage. To obtain

required load voltage, single modules are concatenated, so that the boost

converter DC output voltages are added up, then finally it is given to

centralized inverter and is called as DC Module.

The commercially used module is centralized inverter module (DC

module). Since it has a drawback that total outage will be reduced even

there is a shading effect on a single module itself. This drawback is being

tweaked by using individual inverters for each of the power converters to

keep up the maximum efficiency of the individual panels. In this case

shading effect on the any of the panels would reduce the output of that

particular panel whereas the output of other panels leaving undisturbed.

This topology is known as Micro inverter topology. This topology is

otherwise known as AC module (ACM). ACM consists of a commercially

available photovoltaic module and small inverter that is designed for fixing

directly on a standard PV module and for grid connection. Centralized

inverter would be severely affected as all the switches are under

stress for the condition specified above. Benefit of AC module is that not

all the switches are under higher stress as the modules with lower irradiance

would be affected from that.

The basic idea of micro inverter is dealt in [10-11] in which the

intelligent PV module concept, a low-cost high-efficiency dc–dc converter

with Maximum Power Point Tracking (MPPT) functions and control are

discussed in detail. The correction on single point of failure due to DC

module is discussed and is resolved using micro inverter based solar PV

panels [12]. The advantages of micro inverter over traditional PV system

are discussed in [13-14]. The various converter topology with different

types of efficient MPPT techniques are proposed in many literatures [15-

20].

This paper focuses on standalone three phase micro inverter-based PV

modules and is applied for the three phase load voltage applications.

Comparison of centralized and micro inverter-based PV systems is done

with respect to irradiance level change and Total Harmonic Distortion

(THD) of the total output. System model is well explained in section 2.

Section 3 describes the design of each subsystem followed by

discussion on MPPT algorithm is presented in section 4. Section 5 and 6

explain the simulation and hardware results respectively.

2 System Model

This section explains the different topology of PV standalone system

namely centralized inverter based and micro inverter based PV system. In

both the configurations, 15 series connected 20W, 24V PV panels are

interconnected for three phase load applications.


2.1 Centralized Inverter

Fig.1 shows the block diagram of PV array with centralized inverter for

three phase load application. This type of configuration is conventionally

used for generation of electricity from solar energy. The whole system

comprises of 15 PV units. Each unit consists of PV panel embedded with a

boost converter operated with MPPT technique. Positive terminals of each

unit’s boost converters are connected in series and is finally connected to

the single three phase centralized inverter. The centralized inverter

topology is shown in Fig.1. The drawback of this system is, if there is a

failure in a single module due to change in weather conditions and

reduction in irradiance level, the performance of the whole system is

disturbed and the efficiency of the total system decreases.

Figure 1 Centralized Inverter PV module Scheme


Figure 2 Micro Inverter PV module Scheme

2.2 Micro Inverter

Fig.2 shows the configuration of standalone micro inverter-based

PV module. This is otherwise known as AC module. Each module is

embedded with the boost converter and three phase inverter. Inverter

outputs are concatenated and connected to a resistive load through isolation

transformers as shown in Fig.2.The micro inverter would be fixed on the

rear end of the solar panel along with the power converter. Therefore,

individual module would generate alternating voltage, whereas individual

module in centralized inverter would generate direct voltage. The output of

centralized inverter drastically reduces and loses its capacity to withstand

its connected load when there is shading effect or malfunctioning even in a

single module. In micro inverter, single module failure affects the output

AC voltage of the same PV module alone, other modules get unaffected.

Hence withstands its connected load. The microinverter configuration of

single solar panel module is shown in Fig.3. The irradiance level is varied

between 500 to 1000Watts/m2. The concatenation of DC voltage from all

series connected panels are easier. In this microinverter configuration, the

output of the single panel is AC. The addition of AC voltages is done with

the help of multi winding transformer as shown in Fig.2 and 3.


Figure 3 Micro Inverter single solar panel module Scheme

2.3 Comparison Between Micro Inverter and Centralized Inverter Topology

The advantage of micro inverter module over conventional module

is discussed through the Fig.4 and 5 by considering three panels in series.

Fig. 4 and 5 show the micro inverter and conventional centralized inverter

topology for the solar array of having three panels. There is a shading effect

on the first solar panel due to the leaf and this reduces the irradiance level

by 30%. In the case of micro inverter, only the individual panel is getting

affected. Whereas the remaining two panels keeps working at its fullest

performance with 100%. In the case of the centralized inverter, the shading

effect on one panel effects the output voltage of the other panels also.

Hence total efficiency and performance are reduced.


Figure 4 Micro Inverter solar panel module with the shading

Figure 5 Centralized Inverter solar panel module with the shading

Micro inverter has higher reliability, as it has no single point of

failure, and deals with the less DC voltage (Solar panel output voltage).

Mounting of an AC system is easier than DC. Whereas in the case of


centralized inverter is less reliable, the inverter is a single point of system

failure, difficult to install as high voltage DC and AC are involved. Hence

the installation cost of the micro inverter module is lesser. Also, the

installation can be faster in AC module than DC modules [14].

3 Design and Implementation of Sub Systems

3.1 Solar Panel

The Table 1 shows the rating of solar panel and is operated at 25°C

temperature.

Table 1 solar panel rating

The characteristics of 20W solar panel are given in Fig.6 and 7.

Figure 6 VI characteristics of 20W panel

Parameters Rating

Rated Power 20W

Rated Voltage(Voc) 24V

Open Circuit current(Ioc) 0.83A

Short Circuit current 1.27A

Maximum Voltage Vmpp 17.15V

Maximum current Impp 0.83A


Figure 7 PV characteristics of 20W panel

3.2 Boost Converter

The boost converter is designed for the expected output voltage of 24V

for an input voltage range of 10V to 20V. The following equations are used

for the design of boost converter.

(1)

(2)

(3)

(4)

(5)

IL=

(6)

(7)

(8)


C=

(9)

(Assumption: ∆V0/Vo =0.01)

Where D is the duty cycle, Vs and Is are the input voltage and current

respectively. Vo and Io are output voltage and current respectively. R is the

load resistance. L and C are inductor and capacitors of boost converter.

Assumed switching frequency is 25kHz. From the equations (1) to (8) it is

found that resistance is 2.85Ω, inductor is 8.33mH, Capacitor Value is

given by 19.79µF and inductor ripple current ratio is chosen as 0.019.

3.3 Inverter

The open loop three phase sine PWM technique is used for the three

phase IGBT based six pulse configuration inverter circuit. Inverter which

is embedded in micro inverter module is designed to deliver an AC output

voltage of 24V. The three phase AC voltages of all the 15 panels are added

through the transformers. For the generation of open loop pulses, carrier

wave frequency of 20 kHz and the reference sine wave frequency of 50 Hz

is used. The pulses are generated by keeping frequency modulation index as

400 and amplitude modulation value as 1. The single inverter in centralized

inverter module is designed to get an output voltage magnitude of 230V per

phase and is fed to the load. Input of the centralized inverter is total DC

output from the series connected fifteen converters.

3.4 Filter

The filter acts as a low pass filter and it converts the alternating square

wave waveform into sinusoidal waveform. The filter is designed using the

following Eqn. (10)

√ (10)

where fc is the cut off frequency, Lf is the inductance of filter and Cf is

the capacitance of filter and chosen capacitor value is 100 µF. Hence the

value of Lf is 5mH. For the concatenation of each unit, single module uses

isolation three phase transformer. This transformer helps in protection, also

it can be used to step up the boost converter output voltage


3.5 MPPT Technique

The constant voltage MPPT technique is adopted. As discussed in many

literatures, constant voltage MPPT technique is simple, fast and effective

than other algorithms. The technique assumes that solar panel variations,

such as temperature and irradiation are not significant, and the constant

reference voltage is adequate to achieve performance close to the MPP. The

reference voltage value is set to the voltage at the maximum power point

(Vmpp) of the characteristic photovoltaic array. For this reason, in practice,

the Constant Voltage algorithm may never exactly locate the MPP. During

installation, it is usually necessary to gather data to establish the constant

voltage reference, as this may change from one location to another. In low

insolation conditions, for various values of input voltage of the boost

converter (solar output), the duty cycle required to boost the voltage to 24V

is found from the Eqn. (1) and the MATLAB embedded function is

programmed as per the calculation of duty cycle for varying solar output

voltage. The algorithm senses the voltage output from the solar panel and

accordingly sends out the pulses with required duty cycle. Hence the boost

converter delivers the output voltage of 24V. The constant voltage MPPT

control method needs detection of the variation in output voltage of the PV

array with respect to its open-circuit voltage and adjusting the duty cycle to

move the system operating point towards the MPP.

4 Simulation Results

The simulation results are discussed in this section for a change in

irradiance level. The irradiance level is changed from 1000W/m2 to

800W/m2 at 0.5 sec and at 1 sec again, it is reduced to 700W/m

2 in step

manner. The decrease in irradiance level decreases the solar output and

hence reduces the boost converter output. With the adopted MPPT

technique, though there is decrease in irradiance level, boost converter

output voltage is nearly maintained at a constant voltage level of 24V. Fig.

8 and 9 show the output voltage of solar panel and boost converter

respectively. MPPT of the converter brings the converter output voltage to

24V. The solar output and boost converter output current are shown in Fig.

10 and 11.


Figure 8 Solar output voltage without boost converter

Figure 9 Solar output voltage with boost converter


Figure 10 Output current without Boost converter

Figure 11 Output current with Boost converter

The Fig. 12 shows the output voltage of boost converter with MPPT

technique algorithm and without MPPT logic i.e) open loop pulses are sent

to boost converter switch. Hence the output is not maintained at constant

value.


Figure 12 Output voltage of the Boost converter with and without MPPT

Figure 13 Output voltage of the micro inverter when irradiance level decreases

The Fig.13 shows the three-phase output voltage from the micro

inverter-based PV system after interconnecting all 15 panels and filter

circuit. At 0.4s the irradiance level is changed from 1000W/m2 to 500W/m

2

for all the 15 solar panels. As per the IV characteristics of the given solar

panel, the MPPT tracks the MPP and delivers the output voltage. Without

the use of MPPT at boost converter, the magnitude of the voltage is

reduced drastically to 184V(peak) after the reduction in irradiance level.

Fig.14 shows the output current of the micro inverter with the filter for

the varying irradiance level from 1000W/m2 to 500W/m

2 at 0.4S.


The efficiency and effectiveness of the micro inverter over centralized

inverter can be analyzed by drastically reducing the irradiance level in

five panels to 500W/m2 from 1000 W/m

2 the output voltage and current of

centralized inverter configuration is getting disturbed with more harmonics

as shown in Fig. 15 and 16. Fig.17 and 18 show the output voltage and

current of the micro inverter based PV system when the radiance level in

five panels are reduced to 500W/m2.

Figure 14 Output current of the micro inverter

The ripples are less in micro inverter topology output. The THD of the

centralized inverter during shading effect is 22.71% whereas for the micro

inverter is 5.39% with the LC filters. Huge improvement in THD is

observed in micro inverter topology though there is a shading effect in 5

panels. THD level of an output voltage of the centralized inverter and micro

inverter topology are shown in Fig.19 and 20. When the irradiance level of

the panels is not decreased and is maintained constant at 1000W/m2. then

the obtained THD level with the same values of filter components is less

than 3.42% and is shown in Fig.21. Fig. 22 shows the waveform of the

single phase voltage without filter with the shading effect in 5 panels out of

15 panels. From the results of %THD, it is observed that the micro inverter

is more efficient and effective than centralized module.


Figure 15 Output current of the centralized inverter-shading effect on 5 Panels

Figure 16 Output voltage of the centralized inverter-shading effect on 5 Panels


Figure 17 Output voltage of the micro inverter-shading effect on 5 Panels

Figure 18 Output current of the centralized inverter-shading effect on 5 Panels


Figure 19 % THD of an output of micro inverter

Figure 20 % THD of an output of centralized inverter


Figure 21 % THD of an output of micro inverter

Figure 22 Output current of the inverter-shading effect on 5 Panels without filter

5 Hardware Results

Hardware implementation of the single micro inverter module is

developed which comprises of solar panel, boost converter and three phase

inverter with resistive load. The hardware setup is shown in Fig. 23


Figure 23 Hardware setup of the prototype model

Figure 24 Output voltage of the boost converter


Figure 25 Boost Converter pulses for 65% duty cycle for Vs= 8.4V

Figure 26 Pulses to the three phase inverter (1 and 4)


Figure 27 Single phase output voltage of the three-phase inverter

(between phase a and b)

Fig. 24 shows the output voltage of the boost converter for the

prototype model. The input voltage to the boost converter is 8.4V. With

65% duty cycle the boost converter approximately produces the voltage

level of 24V. The pulses of 25 kHz with 65% ON period is shown in Fig.

25 are obtained from Arduino and the driver IC used is TLP250. The pulses

generated from Arduino is fed to the three-phase inverter circuit and is

shown in Fig. 26. Fig.27 shows the single-phase output voltage of the three-

phase inverter.

6 Conclusions

As there are various issues on centralized inverter such as single point

failures, impedance mismatch during shading conditions, delivering lower

power when any one of the panels receives irradiance of lower values

compared to other panels. Therefore, to overcome these issues of

centralized inverter, also to sustain the energy as the efficiency of the

panels are less, implementation of micro inverter is tested. Micro inverter

topology proved to be a better topology than the centralized inverter

topology as the voltage dips have significantly reduced. The lower ripples

at the output and better THD results during reduction in irradiance level

have been observed. This would also reduce the usage of bulky

equipment’s as the AC Module comes integrated with the PV panel at its

rear.


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Biographies

S.Subramaniam received his B.Tech degree in Electrical and Electronics

Engineering from Amrita School of Engineering, Amrita Vishwa

Vidyapeetham Bengaluru, India in the year 2018. He is current working in

Power Research & Development Consultants (PRDC) in Bengaluru. His area

of interests includes Renewable Energy, Power Electronics etc.,

B.Akaash, received his B.Tech degree in Electrical and Electronics


Vidyapeetham Bengaluru, India in the year 2018. He is current working in

Infosys Chennai. His area of interests includes Renewable Energy, Power

Electronics etc.,

R.Vignesh received his B.Tech degree in Electrical and Electronics


Vidyapeetham, Bengaluru, India in the year 2018. He is current working in

Control and Automation Systems, Hosur, Tamil Nadu, His area of interests

includes Renewable Energy, Power Electronics PLC programming etc.,


R.Mahalakshmi received her B.Tech degree in Electrical and Electronics

Engineering in 2003 from Government College of Engineering, Salem, Tamil

Nadu, India. She received her M.Tech degree in 2012 in Power Electronics

from Dayanada Sagar College of Engineering, Visvesvaraya Technological

University, Bengaluru, Karnataka, India. She is currently working as an

Assistant professor in the Department of Electrical and Electronics

Engineering, Amrita Vishwa Vidyapeetham, Bengaluru, India. She is pursuing

her PhD in Amrita Vishwa Vidyapeetham, Bengaluru, India. Her research

interest includes Grid Integration issues in Renewable Energy Sources,

Application of Power Electronics in Power Systems and Flexible AC

Transmission Systems.

Documents

Design and Implementation of Three Phase Micro Inverter