AN ACCURATE MPPT SCHEME FOR SMALL SCALE PV SYSTEMS Juline Shoeb and S. Yuvarajan

1
AN ACCURATE MPPT SCHEME FOR SMALL SCALE PV
SYSTEMS Juline Shoeb and S. Yuvarajan
Department of Electrical and Computer
Engineering North Dakota State University FARGO,
ND 58105 NOVEMBER 1, 2007
2

• CONTENTS
• Introduction
• Maximum Power Point Tracking of Photovoltaic
  Panels
• Different Maximum Power Point Tracking (MPPT)
  Methods.
• Development of An Accurate MPPT Method
• Introduction.
• Methods.
• Results.
• Discussion.
• Conclusions

3
Introduction

• Solar energy is an important energy source since
  it is abundant, clean, pollution
• free and recyclable.
• Drawbacks of Photovoltaic (PV) Systems
• a) High manufacturing cost.
• b) The electric power generated by a solar module
  is greatly influenced by
• irradiation and temperature.
• Maximum Power Point Tracking
• Using different algorithms, the maximum-power
  operating point of the system is
• tracked and the system is forced toward this
  optimal operating point.

4
Solar Cell Basics

• A solar cell is a special p-n junction
• which absorbs sunlight and converts
• light energy into electric energy.
• Energy Conversion in a Solar Cell
• Photovoltaic process converts light
• into electricity. The equivalent circuit
• of a solar cell has four components
• a light induced current source, a diode,
• a series resistance, and a parallel
• resistance. Ohmic losses occur due to
  heating in
• the series and shunt resistances.

5
PV Characteristics

• The I-V characteristic of a PV panel is given
  by
• where VPV is the voltage across the PV module,
  IPV is the output current of
• the module, ISC is the short-circuit current,
  and IS is the dark saturation
• current, and K is a constant dependent on
  temperature and cell arrangement.
• The short-circuit current ISC of the PV panel is
  approximately equal to the
• light generated current and the effect of
  illumination on ISC is significant. The
• short-circuit current ISC increases with
  irradiation.
• The open-circuit voltage, VOC of a PV panel
  increases linearly with a
• decrease in temperature and vice versa.

6
Maximum Power Point Tracking (MPPT) Methods
The widely used MPPT algorithms can be broadly
classified as 1) Perturbation and Observation
(PO) Method (a) Conventional PQ Method
(b) Incremental Conductance Method 2)
Linearity-based Methods (a) Short-circuit
current method (b) Open Circuit Voltage
Method 3) Switching Frequency Modulation
Method 4) Ripple Correlation Control Method
7
Perturbation and Observation (PO) Method

• Conventional PO Method
• This method can be described as follows
• If the operating point of the PV panel is
  perturbed in a given direction and the
• power drawn from the panel increases, it
  means that the operating point is
• moving toward the maximum power point.
  Therefore the operating point
• should be further perturbed in the same
  direction. If the power drawn from the
• panel decreases, then the operating point
  has to be perturbed in the
• opposite direction to reach the MPP.
• b) Incremental Conductance (INC) Method
• To reach the MPP, this algorithm has to satisfy
  (dIPV / dVPV IPV / VPV) 0.
• Algorithm performs well under rapidly varying
  atmospheric conditions.
• K. H. Hossein, I. Mota, T. Hshino, and M.
  Osakada, Maximum photovoltaic power tracking
  An algorithm for rapidly changing atmospheric
  condition, Proc. Inst. Eng. Vol. 142, pt. G, no.
  1, pp. 59-64, Jan. 1995.

8
Short-circuit Current and Open Circuit Voltage
Methods

• (a) Short circuit Current Method method exploits
  the assumption of linear
• relationship between the cell current
  corresponding to the maximum
• power (IMP) and the cell-short circuit
  current (ISC). This relationship can
• be expressed as IMP Mc . ISC where Mc is
  called the current factor.
• (b) Open Circuit Voltage Method employs the
  assumption of linear
• relationship between the cell voltage
  corresponding to the maximum
• power (VMP) and the cell-open circuit
  voltage (VOC). This relationship
• can be expressed as VMP Mv .VOC, where
  Mv is called the voltage
• factor.
• M. A. S. Masoum, H. Dehbonei, and E. F. Fuchs,
  Theoretical and experimental analysis of
  photovoltaic system with voltage and
  current-based maximum power point tracking,
  IEEE Trans. Energy Conversion, vol. 17, pp.
  514-522, Dec. 2002.

9
Switching-Frequency-Modulation Scheme (SFMS)

• A small signal sinusoidal perturbation is
  injected into the switching frequency
• of the converter and the MPP is located by
  comparing the ac component and
• the average component of the panel terminal
  voltage.
• Panel terminal voltage vi has an average value of
  Vi and a small variation .
• The error term e is defined as
  , where and are,
• respectively, the peak value of and
  the scaling factor for Vi .
• The converter matches the panel if e 0. The
  required control-adjustment
• direction of the converter duty cycle is
  decided by the sign of e.
• K. K. Tse, B. M. T. Ho, H. S. Chung, and S. Y. R.
  Hui, A comparative study of maximum-power-point
  trackers for photovoltaic panels using
  switching-frequency modulation scheme, IEEE
  Trans. Industrial Electron., vol. 51, pp.
  410-418, April 2004.

10
Ripple Correlation Control (RCC) Method

• This does not require any external signal
  injection instead, it uses the natural
• disturbances already present in the PV
  system.
• The output of the PV panel is connected to the
  dc link of the single phase
• voltage source inverter (VSI), and the
  inverter output is connected to the grid
• through a link inductor.
• Complicated control scheme and not feasible for
  converters with DC loads.
• D. Casadei, G. Grandi, and C. Rossi,
  Single-phase single-stage photovoltaic
  generation system based on a ripple correlation
  control maximum power point tracking, IEEE
  Trans. Energy Conversion, vol. 21, pp.
  1281-1291, June 2006.

11
Development of an Accurate MPPT Method

• Features
• The proposed approach ensures maximum
  electrical power transfer under all
• environmental conditions and it does not use
  complex DSP boards or
• microprocessors for computation.
• The MPPT is realized by sensing the short
  circuit current and the open circuit
• voltage and adjusting the duty cycle of the
  buck-boost converter and hence the
• converter output current such that the MPPT
  equation holds.
• The methodology is based on a maximum power
  point tracking (MPPT)
• equation, which is derived from the
  expression for the output current of a PV
• Panel.
• The developed algorithm is verified using
  MATLAB and it is seen that this new
• algorithm works extremely well over wide
  temperature and illumination ranges.

12
Development of an Accurate MPPT Method

• Method
• The approximate expression for the output
  current IPV of a PV Panel is given by
• IPV ISC - IS exp (KVPV).
• The Power output of the PV panel is given by
  PPV VPV .IPV.
• Combining the above two equations we get K PPV
  IPV ln ( (ISC I PV) / IS).
• Above equation is differentiated with respect to
  IPV and equated to zero. At the
• maximum power point the following result is
  obtained
• where IMP is the current at the maximum power
  point.
• IS strongly depends on temperature.

13
Development of an Accurate MPPT Method

• Method
• Exact Approach
• The dark Saturation current of the PV Panel in
  can be expressed as
• where Ior is cell saturation current at a
  reference temperature Tr, T is cell
• temperature in deg Kelvin, Ego is the band
  gap energy for Silicon, q is electron charge,
• B is ideality factor, and k is Boltzmanns
  constant.
• If the above relationship is used in the
  expression of IMP, an accurate
• algorithm is obtained. However,
  implementation becomes very complex.
• S. Liu and R. A. Dougal, Dynamic multiphysics
  model for solar array, IEEE Trans. Energy
  Conversion, vol. 17, no. 2, pp. 285-294, June
  2002.

14
Development of an Accurate MPPT Method

• Method
• b) Approximate Approach
• The term in the expression of IS can be
  approximated to 1 in the
• practical temperature range of the PV Panel
  and thus the term
• 
  describes the variation of IS at different
• temperature levels.
• Equating in the expression for IS to 1
  and then substituting into the
• expression for IMP yields
• where B and C are constants.

15
Development of an Accurate MPPT Method

• Method
• b) Approximate Approach
• Here temperature T can be expressed as a
  function of open circuit voltage
• using VOC (T) VOC (Tr) a (T-Tr)
• where VOC (T) is the open-circuit voltage at
  temperature T, VOC (Tr) is the open-
• circuit voltage at a reference temperature Tr,
  a is the temperature coeff. of VOC.
• Using the above relationship, the expression for
  IMP becomes
• where B1 and C1 are constants and VOC (T) is the
  open circuit voltage of the
• solar panel at temperature T. Hereafter this
  will be called the MPPT-Equation .

16
Development of an Accurate MPPT Method

• Control Circuit
• The MPPT control circuit will force the PV
  system to operate at the optimal
• current IMP so that the load will receive the
  maximum power from the PV panel.
• Both the short circuit current and the open
  circuit voltage of the panel have to
• be sensed.
• The sampled values of VOC, ISC, and IPV are fed
  into various analog
• computation blocks such as the natural
  logarithmic amplifier, multiplier, summer
• and divider.
• The right hand side and the left hand side of
  MPPT Equation are both currents
• and the duty cycle of the buck-boost converter
  is adjusted until these two
• currents are equal.

17
Development of an Accurate MPPT Method
18
Development of an Accurate MPPT Method

• Results
• The maximum power PMP for the photovoltaic
  module BP 3160 is obtained as
• The value of IMP can be calculated from exact
  or approximate approach for a
• specific temperature T and illumination level
  (specific short-circuit current ISC).
• The value PMP of is calculated for a wide
  temperature range (-20 C to 50 C)
• and short circuit current range (0.25A to 5A)
  using IMP values from both the
• exact and the approximate approach and plotted
  in this using MATLAB.
• Percentage error in the approximate PMP is less
  than 3 for all the
• temperatures and short circuit currents in the
  specified range. Around the
• normal temperature range, the percentage of
  error is less than 1.

19
Development of an Accurate MPPT Method
20
Development of an Accurate MPPT Method
21
Development of an Accurate MPPT Method

• Discussion
• At a temperature 25oC, the INC algorithm has
  roughly 1.25 error in the PMP
• with respect to the actual PMP . The proposed
  algorithm was simulated at 35oC,
• and the percentage of error in the PMP is
  around 1.2. At 25oC, the percentage
• of error in PMP is much lower than 1.
• The PO method tracks 15 more power than the
  simple open circuit method
• and short circuit method.

22
Conclusions

• All the maximum power point tracking algorithms
  which have been used for
• decades are summarized.
• A cost-efficient maximum power-point tracking
  for small scale PV systems utilizing the exact
  equation of a solar panel is presented.
• The method works well under varying temperature
  and insolation.
• Control circuit for implementation of proposed
  method is given.
• Percentage error in maximum power is computed
  using MATLAB and shown to be very small.

23
References

• K. H. Hossein, I. Mota, T. Hshino, and M.
  Osakada, Maximum photovoltaic power tracking
  An algorithm for rapidly changing atmospheric
  condition, Proc. Inst. Eng. Vol. 142, pt. G, no.
  1, pp. 59-64, Jan. 1995.
• M. A. S. Masoum, H. Dehbonei, and E. F. Fuchs,
  Theoretical and experimental analysis of
  photovoltaic system with voltage and
  current-based maximum power point tracking,
  IEEE Trans. Energy Conversion, vol. 17, pp.
  514-522, Dec. 2002.
• Y. Chen and K. M. Smedley, A cost-effective
  single-stage Inverter with maximum power point
  tracking, IEEE Trans. Power Electron., vol. 19,
  Sept. 2005. K. K. Tse, B. M. T. Ho, H. S. Chung,
  and S. Y. R. Hui, A comparative study of
  maximum-power-point trackers for photovoltaic
  panels using switching-frequency modulation
  scheme, IEEE Trans. Industrial Electron., vol.
  51, pp. 410-418, April 2004.
• K. K. Tse, B. M. T. Ho, H. S. Chung, and S. Y. R.
  Hui, A comparative study of maximum-power-point
  trackers for photovoltaic panels using
  switching-frequency modulation scheme, IEEE
  Trans. Industrial Electron., vol. 51, pp.
  410-418, April 2004.
• D. Casadei, G. Grandi, and C. Rossi,
  Single-phase single-stage photovoltaic
  generation system based on a ripple correlation
  control maximum power point tracking, IEEE
  Trans. Energy Conversion, vol. 21, pp.
  1281-1291, June 2006.
• S. Liu and R. A. Dougal, Dynamic multiphysics
  model for solar array, IEEE Trans. Energy
  Conversion, vol. 17, no. 2, pp. 285-294, June
  2002.
• S. Yuvarajan and Juline Shoeb, Lighting System
  With an LED String Fed from PV Panel, Proc. Of
  Power Electronic Technology Conference, Oct. 2006.