Design and Development of Power Electronic Controller for Grid-connected PV Array

International Journal of Applied Power Engineering (IJAPE)
Vol. 1, No. 2, August 2012, pp. 87~92
ISSN: 2252-8792 87
Journal homepage: http://iaesjournal.com/online/index.php/IJAPE
Design and Development of Power Electronic Controller for
Grid-Connected PV Array
T.Shanthi
Dept. of Electrical & Electronics Engineering, Kumaraguru College of Technology, Coimbatore- 641 049, India.
Article Info ABSTRACT
Article history:
Received May 20, 2012
Revised Aug 20, 2012
Accepted Aug 26, 2012
Design and simulation of a simple power electronic interface for grid-
connected PV array has been proposed using boost converter and line-
commutated inverter with maximum power point tracking (MPPT)
controller. The output of PV array varies with irradiation, and hence the duty
cycle of the PI controller is adjusted automatically to supply a constant DC
voltage to the inverter circuit, the output of which is directly connected to the
grid. The MPPT controller extracts maximum power from the solar array and
feeds it to the single-phase utility grid. The proposed scheme has been
modeled in the MATLAB 7.1 software and the complete system has been
simulated for open loop and closed loop configurations. The active power fed
to the grid is taken for different firing angles in open loop mode and the
firing angle for maximum power has been determined. This is compared with
the firing angle obtained from the closed loop mode and found that both
results agree with each other.
Keyword:
Boost converter
Line commutated inverter
Maximum power point tracking
PI controller
Copyright © 2012 Insitute of Advanced Engineeering and Science.
All rights reserved.
Corresponding Author:
T. Shanthi
Assistant Professor (Senior Grade), Dept. of Electrical & Electronics Engineering,
Kumaraguru College of Technology, Coimbatore- 641 049, India, Ph:9443820420.
E-mail id: shanthits@gmail.com
1. INTRODUCTION
The rapid trend of industrialization of nations and increased interest in environmental issues has led
recently to consideration of the use of renewable forms such as solar energy. Photovoltaic (PV) generation is
gaining increased importance as a renewable source due to its advantages such as the absence of fuel cost,
little maintenance, no noise and wear due to absence of moving parts etc. Photovoltaic sources are used today
in many applications such as battery charging, water pumping, home power supply, swimming pool heating
systems, satellite power systems etc. They have the advantage of being maintenance and pollution free but
their installation cost is high and in most applications, they require a power conditioner for load interface.
Since PV modules still have relatively low conversion efficiency, the overall system cost can be reduced
using high efficiency power conditioners which, in addition are designed to extract the maximum possible
power from the PV module. A controlled power interface between solar cells and grid is required to vary the
power fed to the grid [4] as per the load conditions. But in India, the rural areas suffer from the scarcity of
electrical power throughout the day. This paper discusses one such scheme wherein a controller consisting of
a boost converter and a line commutated inverter has been used to interface the PV array to grid with
provision to track maximum power from the solar array. Hence a control circuit is needed for maximum
power transfer from the solar cells to the grid all the time [3], [7].
2. PROPOSED SCHEME
The block diagram schematic of the proposed scheme is shown in Figure 1. It consists of an array of
solar panels interfaced to the utility grid through a boost converter and single phase SCR bridge circuit. The

ISSN: 2252-8792
IJAPE Vol. 1, No. 2, August 2012 : 87 – 92
88
DC voltage from the PV array is sensed and given to the boost converter which steps up the voltage to the
required DC level. The duty cycle of the boost converter is controlled through a PI controller in the feedback
for giving constant output voltage. The raised DC voltage from the boost converter is converted to AC using
SCR bridge circuit to interface with the grid.
Figure 1. The block diagram schematic of the proposed scheme
The SCR bridge circuit is operated as line commutated inverter by keeping the firing angle above
90°. One input to the PI controller is from the actual output voltage from the boost converter and the
reference voltage (260V) is given as another input. Based on this reference voltage the modulation of duty
cycle is carried out. The inverter is provided with controller for feeding maximum power to the grid all the
time. The actual power Po and the reference power Pref are compared and the difference between these two
powers is given as input to the controller. The output of the controller modifies the firing angle to minimize
the error. The PV array in the proposed scheme consists of two solar panels of 80W, 21V, 5A each,
connected in series. A single-phase utility grid of 230V, 50Hz is considered, to which the output of the
inverter is connected.
2.1. Boost converter
The boost converter shown in Figure 2. is used to step-up the voltage obtained from the PV panel to
equal the reference voltage (260V). The output voltage of the boost converter is given by
1
Vo = Vd  (1)
(1-δ)
where δ = duty cycle of the boost converter and Vd = input DC supply.
By varying the duty cycle, the output voltage can be varied above Vd. The introduction of the boost
converter eliminates the step-up transformer between the inverter and the grid thereby reducing the cost of
the system and also eliminating the losses produced by the transformer leakage inductance.[5]
Figure 2. Circuit of the boost converter

IJAPE ISSN: 2252-8792
Design and Development of Power Electronic Controller for Grid-Connected PV Array (T.Shanthi)
89
2.2. Controller for boost converter
For getting the constant output voltage (260V) from the boost converter, the duty cycle of the
converter is automatically adjusted using PI controller in the closed loop.The actual output voltage from the
converter Vact is compared with the reference voltage Vref. The error voltage Verror is used for the variation of
the duty cycle δ of the converter.The error voltage is given to the PI controller. The PI controller output V0 is
compared with the sawtooth wave (500KHz) in a comparator which gives the gate pulses for the converter.
The PI controller output is given by
KI
V0 = KP +  ( Vref – Vact) (2)
S
where Kp and Ki are proportional and integral gains respectively.
In the proposed scheme, the proportional and integral controller gains (KP = 0.25 and KI = 1) have
been chosen by trial and error method for getting the output voltage of 260V from the boost converter.
2.3. Analysis of Line Commutated Inverter
A single-phase fully controlled bridge converter shown in Figure 3. can be operated in two modes
namely rectifier and inverter modes. When the firing angle α is between 0° and 90°, the converter is said to
be in rectification mode and when the firing angle α is between 90° and 180°, it is said to be in inversion
mode. In the proposed scheme, the converter operates as an inverter. The thyristors T1 and T2 are fired at
firing delay angle of α° and the thyristors T3 and T4 are fired at (α+180)°. The direction of power flow can be
reversed by reversal of the DC voltage, the current direction being unchanged. The delay angle α must be
greater than 90°. In the present case, no extra effort is required to synchronize the inverter output frequency
with that of the grid supply[2]. This of course is possible only with SCR converters. The average output
voltage Edc is hence given by,
1 π+α 2
Edc =  ∫ Em sin θ dθ =  Em cosα (3)
π α π
Figure 3. Circuit of the single-phase fully controlled bridge converter
2.4 Controller for Maximum Power Tracking
For extracting maximum power from the solar array, the firing angle of the inverter is adjusted in closed
loop.The reference power from the solar array Pref is given by[1]
Pref = Vmp x Imp W (4)
Here Vmp = K1 x Voc (5)
Imp = K2 x Isc (6)

ISSN: 2252-8792
90
where Voc is the open-circuit voltage and Isc is the short-circuit current of the PV array. K1 and K2 are
constants taken as 0.76 and 0.8 respectively.
The actual grid power Pgrid is compared with the reference power and any mismatch is used to
change the firing angle α of the inverter as follows:
α = (Pref – Pgrid)[KP+KI/S] (7)
The optimum values for KP and KI have been arrived by trial and error method. In the proposed
scheme, the P and I controller gains are chosen to be 0.3 and 7 respectively.
3. SIMULATION RESULTS
The complete model of the proposed scheme is modeled in MATLAB 7.1/simulink software. It
consists of blocks of solar array, boost converter, line commutated inverter, utility grid and closed loop
controllers.The DC link inductance together with boost converter acts as load on the line commutated
inverter. Dr.S.Arul Daniel and Dr.N.Ammasai Gounden [6] have proposed a simulink model for the solar
array. This model has been used in the proposed scheme. The parameters of solar array are Voc = 21.2V and
Isc = 5.17A; inductance L1 = 3.4mH; for the boost converter; L = 1mH, C = 1000µF, R = 50Ω; for DC link
inductance R2 = 0.2Ω, L2 = 20mH;
The SCRs in the inverter are triggered in open loop mode for different firing angles and the readings
of active power fed to the grid, DC link voltage, DC link current and grid current were noted. From these
readings, the firing angle corresponding to the maximum power is determined. This is compared with the
closed loop readings and the results are furnished in Table 1. It is seen that both results agree with each other.
Table 1. Comparison of open loop and closed loop results
Parameters Open loop Closed loop
Firing angle at which maximum
power occurs, α
160° 160.2°
Direct voltage, Vdc (in V) -108 -119
Direct current, Idc (in A) 0.9 1
Grid current, Igrid (in A) 1 0.4
Active power fed to grid,
Pgrid (in W)
-70.5 -72.62
The grid voltage and grid current waveforms are shown in Figure 4. The waveforms of DC link voltage
and the DC link current are shown in Figure 5. The waveform for the power fed to the grid is shown in
Figure 6.
Figure 4. Grid voltage and grid current

IJAPE ISSN: 2252-8792
Design and Development of Power Electronic Controller for Grid-Connected PV Array (T.Shanthi)
91
Figure 5. DC link voltage and DC link current
Figure 6. Active and Reactive power fed to the grid
4. CONCLUSION
A simple power electronic interface employing a boost converter and line commutated inverter has
been developed for interfacing PV array with the single-phase utility grid. The simulation studies have been
carried out to get the parameters such as active and reactive powers, DC link voltage and current and the
firing angle corresponding to the maximum power. Due to losses in the inductor, the output power fed to the
grid is somewhat less. This can be increased by selecting the inductor with low losses.
Nomenclature
Edc DC link voltage (V)
Idc DC link current (A)
Kp Proportional gain
KI Integral gain
L1 Inductance (H)
L2 DC link inductance (H)
Vd Input voltage to the boost converter (V)
Vo Output voltage from the boost converter (V)
α Firing delay angle for SCRs (deg)
δ Duty cycle of the boost converter
REFERENCES
[1] Dong-Yun Lee, Hyeong-Ju Noh, Dong-seok Hyun and Ick Choy, ‘An Improved MPPT Converter Using Current
Compensation Method for Small Scaled PV-Applications’, IEEE conference, pp. 540-545, 2003.
[2] Emil S. “Inverters for utility interactive photovoltaic power” Electrotechnical Conference, 1989. Proceedings.
'Integrating Research, Industry and Education in Energy and Communication Engineering', MELECON '89.

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[3] Henry Shu-Hung Chung, Member, IEEE, K.K.Tse, Member, IEEE, S.Y.Ron Hui, Fellow IEEE, C.M.Mok, and
M.T.Ho, Student Member, IEEE, ‘A novel Maximum Power Point Tracking Technique for solar Panels Using a
SEPIC or Cuk Converter’, IEEE transactions on Power Electronics, Vol.18, No.3, pp. 717-724, May 2003.
[4] Koosuke Harada and Gen Zhao, ‘Controlled power interface between solar cells and ac source, IEEE transactions
on Power Electronics, Vol.8, No.4, pp. 654-662, October 1993
[5] M. Veerachary, T. Senjyu and K. Uezato, ‘Maximum Power Point Tracking Control of IDB converter supplied PV
system’, IEE proceedings – ElectroPower.Appl. Vol.148, No.6, pp. 494-502, Nov 2001.
[6] S.Arul Daniel, N.Ammasai Gounden, ‘A Novel Hybrid Isolated Generating System Based on PV Fed Inverter-
Assisted Wind-Driven Induction Generators’, IEEE transactions on energy conversion, Vol.19, No.2, pp. 416-422,
June 2004.
[7] S.Yuvarajan and Shanguang Xu, ‘Photo Voltaic Power Converter With a Simple Maximum-Power-Point-Tracker’,
IEEE conference, pp. 399-402, 2003.
BIOGRAPHY OF AUTHORS
T.Shanthi received her bachelor’s degree from Institute of Road and Transport Technology,
Erode, Tamilnadu, India in Electrical and Electronics Engineering during 1999. She received her
Master’s degree from National Institue of Technology, Tiruchirappalli, India during 2007. She
is currently working as Assistant Professor at the department of Electrical and Electronics
Engineering in Kumaraguru College of Technology, Coimbatore, India.

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