Experimental investigation of distributed maximum power ... · electronic converter interface for dc energy sources. Three specific examples of such dc energy sources that will have

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Experimental investigation of distributed maximum power point operation for solar PV system

Nitesh Y* B Malakondareddy* S Senthil kumar* Anand I* [email protected] [email protected] [email protected] [email protected]

*Dept. of EEE, National Institute of Technology, Tiruchirappalli, Tamil Nadu, India-620015

Abstract – The major issues in PV systems are the effect of module dissimilarities and of partial shading of the PV panels. In this paper a distributed maximum power point tracking (DMPPT) technique is used to track the maximum power under partial shading condition. In this paper, both series and parallel connected DC-DC converters are considered to track the MP from panels by use of a DMPPT algorithm. DMPPT is a good technique which improved system efficiency and reliability. Modelling and design of the DMPPT system exceptionally difficult than implementing the conventional MPPT system. The DMPPT system implemented for PV modules is proposed and analysed employing micro power converter. To observe steady state analysis and stability of system a small signal model is derived. Finally, the efficacy of the proposed system is verified with MATLAB/Simulink simulations and validated through dSPACE 1103 controller-based laboratory experimental setup. Key words- DMPPT, PV systems and Micro power converter.

I. INTRODUCTION

With an increasing worldwide interest in sustainable energy production and use, there is renewed focus on the power electronic converter interface for dc energy sources. Three specific examples of such dc energy sources that will have a role in distributed generation and sustainable energy systems are the photovoltaic (PV) panel [1]. The effective utilization of solar energy for such application like standalone and grid connected, it became very much needed for local loads where generating stations are not available [2]. The solar energy is generating from Photo-Voltaic (PV) cell which directly converts the solar radiation into the electricity. It became most prominent renewable energy source. So, the usage of the solar energy is becoming most important but enhancing the maximum power from the PV is remarkable task due to its nonlinear characteristics changes according the environment. Due to this, different types of algorithms to enhance the maximum power such as perturb and observe, incremental conductance method, etc., are presented in [3], [4].

Under partial shading conditions, the power output from the PV field decreases effectively due to decrease in the current generated in shaded panels in a series connected PV panels in string. This shaded module forces the non-shaded module to operate with lower current, this stops the non-shaded modules in string from generating full power. Partial shading cause local hotspots. To avoid this local hotspot bypass diodes are used which will cause multiple peaks in the PV characteristics of the string. Therefore, conventional PV system is failed to track the maximum power point [5]-[7]. In past many researchers worked to enhance the maximum power from the PV field by tracking the global peak point among the local peak points but this is complex. Based on above motivation, in this paper, new DMPPT technique with module dedicated micro converters and DMPPT algorithm is recommended to trace the PV curve under partial

shading conditions and observed behaviour of the proposed system in real time implementation using dSPACE 1103 controller. The small signal model of single controlled photovoltaic module (CPVM) is done to analyse steady state performance and stability of system with respect system variables [7]-[11].

II. ARCHITECTURE FOR DISTRIBUTED MAXIMUM POWER POINT

TRACKING

Based on connection of the PV panels to converters, the converters classified into two types micro converter and micro inverter architecture. In micro converters the PV panels are connected to the converters and the output of the converters are connected in series or parallel depend upon the application. The string’s output is connected to either DC grid or load, otherwise for AC loads or grid by connecting with central inverter. In this study a series and parallel connected DC-DC converter is considered to study the controller response under partial shading conditions. The complete control system configuration as shown in Fig. 1(a) and (b).

III. SMALL SIGNAL ANALYSIS OF THE CONTROLLED PV

MODULE

The Schematic diagram of the individual CPVM as shown in the Fig. 2 it represents the exhaustive study of the CPVM including the control part to realizing the feedback regulation to input voltage. The changes in the converter output voltage there will be reflect in the input voltages. Due to this system may lose stability or degradation of the performance. Here the boost converter is considered for the input voltage regulation. The P and O control variable of such system isV , the control loop system is fast responding, the interrupts at the output side converter model not disturb the operation of the single CPVM. In this study i, v and d small letters are indicated to represent the large variable values of the currents, voltage and duty ratios respectively; I, V and D capital letters are representing the constant values of the same variables, and small signal changes around their operating point of steady state are indicated by the small letters with hat. Given two parallel connected impedences z and z , the symbol z ∥ z indicates there equivalent impedence. The negative symbol in the transfer function of the negtive feedback operational amplifier .

A. Transfer function of an open loop CPVM

The subsequent transfer functions are describing the small

signal model. G OL (s)= ( )( ) ( ̂ ( ) = 0; V ( ) = 0) (1)

Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India

G OL (s)= ( )( ) ( ̂ ( ) = 0; d( ) = 0) (2)

The control of the single CPVM is done the input voltage regulation by the opeartional amplifier with negative feedback. And transfer function describes regarding its dynamics.

G = ( )( ) (3)

Where

Z (s) = (R + )∥ (4)

Z (s) = (R + )∥ R (5) M (s)= ( ). . ( ). (6)

B. Closed loop transfer function of CPVM

The circuit diagram of Fig. 3 describes the active intractions among the input and output parameters of the CPVM and allows computation of the transfer function of input to output closed loop CPVM G CL (s) =

( )( ) ( ̂ ( ) = 0; V ( ) = 0)

= ( )( ) (7)

IV. STATE SPACE MODEL OF THE CPVM

A. State space model for open loop system

The Fig. 4 repersents the PV connected boost converter. To verify the stability of the system, the frequency analysis performed by deriving the state space model. The required equations to derive the state space model of the system as follows.

Fig.4 PV connected boost converter

From above circuit it is evident that there are three state variables = , = , = .

During ON state:

Fig.5 circuit diagram during ON state

Fig.2 small signal model of single CPVM

Fig.3 Loop representation of small signal model of CPVM

(a) (b) Fig.1 DMPPT solar PV system by means of micro converters (a)Series Configuration (b)Parallel Configuration


= ∗ − + ∗ (8)

= (9)

= ∗ + (10)

From above equations the matrix obtained is as follows =

∗ 00 00 0 ∗ +

∗ 0 00 0 00 0

During OFF state:

Fig.6 Circuit diagram during OFF state

= ∗ − + ∗ (11)

= − (12)

= ∗ + + (13)

From the above equations obtained matrix during OFF state is

as follows =

∗ 000 ∗ +

∗ 0 00 0 00 0

The average or overall system state space model obtained

=

∗ 00 ( )0 ( ) ∗ +

∗ 0 00 00

Y = 0 0 1

Therefore, the transfer function from the above state space

models are derived and stability of the system verified as follows G OL (s)= ( )( ) ( ̂ ( ) = 0; d( ) = 0) (14)

The phase margin and gain margin of the open loop system are plotted by using transfer function obtained to know the system stability. The root locus and bode plot are shown in Fig. 7.

Fig.7 Root locus and bode plot of the PV connected boost converter From Fig. 7 it is evident that Gain Margin (GM) and Phase Margin (PM) are positive. Therefore, the system is stable but it is very near to imaginary axis, so it will limits the operating region. Gain margin is determined by the point in phase margin where it crosses the -180 line which is phase crossover frequency and phase margin is determined by gain crossover frequency . The GM is 3.45dB and the PM is21.5 .

B. State space model for closed loop system

The Fig. 8 shows the PV connected boost converter with a controller in the feedback. The transfer function of this system is derived from the following equations

Fig. 8 self-controlled PV module G CL (s) =

( )( ) ( ̂ ( ) = 0; V ( ) = 0)

= ( )( ) (15)

Fig.9 Root locus and Bode plot of closed loop CPVM

From the Fig. 9, it is observed that GM and PM are positive. Therefore, the system is stable, but it is a bit away from the


imaginary axis compared to the open loop root locus. Gain margin is determined by the point in phase margin where it crosses the -180 line which is phase cross over frequency and phase margin is determined by gain crossover frequency . The GM is 45.9dB and the PM is infinity because of gain cross over frequency not exists.

V. SIMULATION RESULTS

The series and parallel connected boost converter are simulated under different conditions using MATLAB/Simulink Software. SW 250 solar panel is considered for the simulations. The specifications of the panels are as follows: = 37.6 V, = 8.81 A, = 30.5 V and = 8.27 A. the behaviour of the system is done under different conditions, by changing the irradiation at different instants of time as no shading (1000 W/m ), moderate shading (800 W/m ) and dark shading (600W/m ). The series connected boost converter system performance is observed from the Fig.10 and it shows the input and output parameters of the V, I and P. In this condition the panel I is considered as unshaded and panel II is varying with irradiation at different instants that is at t = 0.5s the irradiation is S = 800W/m and at t = 1s irradiation is S = 600W/m . Therefore, at t = 0.5s irradiation changes from that instant control algorithm takes time of 0.25s to reach the maximum power point. At t = 1s further irradiation reduced and the time taken by control algorithm to reach the maximum power point is ts = 0.4s. From this figure, it is observed that the control settling time is more under low irradiation.

The parallel connected boost converter system performance

is observed from the Fig. 11 and it shows the input and output parameters of V, I and P. The panel I is considered as unshaded and panel II is varying with irradiation at different instants that is at t = 0.5s the irradiation is S = 800W/m and at t = 1s irradiation is S = 600W/m . Therefore, at t = 0.5s irradiation changes from that instant control algorithm takes time of 0.075s to reach the maximum power point. At t = 1s further irradiation reduced and the time taken by control algorithm to reach the maximum power point is ts = 0.075s. From this figure, it is observed that in parallel configuration the convergence speed of controller is high. The simulation results demonstrated that working of the controller at different shading conditions. The power output, voltage and current are changing according to the change in irradiation and temperature. Initially at non-shaded condition the powers and voltages are matching with the exact MPP but when the irradiance is changes the power and voltages are extract to the reduced.

VI. EXPERIMENTAL RESULTS

The performance of the controller is validated through experimentation under different irradiation condition. The complete experimental setup shown in Fig. 12. Two PV panels are connected to two dedicated DC/DC converter and the output of the this converters terminals are connected in in series. The experimental system parameters are shown in Table. 1

Table 1. System parameters parameters values

Switches (N-channel MOSFER)

IRF540N

Input capacitor 1000 µF Output capacitor 1000 µF Filter inductor 1.5 mH PV panel ratings 30 W

The complete experimental setup is built in laboratory and controller is implemented in dSPACE 1103 real time controller board.

Fig.10 Simulation results: dynamic response of series connected boost converter under different irradiation

Fig.11 Simulation results: of dynamic response of parallel connected boost converter under different irradiation


To verify the performance of the controller, two cases are considered. (i) Uniform irradiation throught the simulation (ii) Non-unifrom irradiation at panel II (Partail shaded)

A. Uniform irradiation

Intially both panels are under non shading condition, for this condition MPPT controller extracting the maximum power from the PV modules. Fig. 13 and 14 shows the simulation results of PV panels voltage, current and power. From Fig. 13 and 14, it is observed that the controller effectively tracking the maximum power under uniform irradiation condition.

The values of the voltage, current and power are = 14V, = 13.9V and = 1.78A, = 1.77A and = 30 W and = 31 W and Pout= 58 W

Fig.15.Output power of proposed system

B. Non-Uniform irradiation

The main purpose of the controller is to enhance the maximum power in partial shading conditions. Fig. 16 shows the simulation results of the PV panels voltage and current under partial shaded condition. From Fig. 16, it is obserevd that the panel I volatge and current are maintain constant and panel II voltage and current is changing due to change of irradiation in panel II.

Fig.16 and 17 shows the input and output parameters of the syatem under partial shaded condition. From Fig. 16 and 17, it is obserevd that up to t = t1 s both panels are unshaded at that instant shading is introduced the voltage and current of panel II is decreased therefore power extracted is decreased. According to atmospheric conditions the controller is enhancing the maximum power from PV field. Therefore the output power regarding the input parameters is given as below. The output power is changed correspondingly to the change in input parameters.

Fig.12 Experimental set up

Fig.13 . Input paramerters (a) voltages (b) currents

(a) (b)

Fig.14.Input powers of proposed system

Voltage 10 V/div

t1

Current 2 A/div

t1

Fig.16. Voltage and current of the PV panels

Power 10 W/div

t1

Power 10 W/div

t1

Fig.17. Power of (a) PV panels (b) converter output

(a) (b)


VII. CONCLUSIONS

In this study, implementation of DMPPT method in solar PV system is studied. A small signal analysis is dervied to observe the performance of the controller in terms of steady-state, dynamic behaviour and also the stability of the controller with system.

The performance of the DMPPT for solar PV system has been validated using simulation (MATLAB/Simulink) and experimention (dSPACE 1103 real time controler board) under various operating conditions. From both the simualtion and experimental results, it is observed that the controller is effectively tracking the maximum power under uniform and non uniform irradiation conditions.

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Experimental investigation of distributed maximum power ... · electronic converter interface for dc energy sources. Three specific examples of such dc energy sources that will have