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conventional boost converter. The efficiency and reliability of single-phase PV inverter systems can be improved using
transformer less topologies and maximum power point tracking methods.
In this paper, a PV array is modeled and simulated using MATLAB/Simulink .This PV model is coupled to a
high step up DC-DC boost converter. By changing the duty cycle of the DC-DC booster the system implements the most
popular MPPT method to extract maximum power. Different MPPT methods have been studied and verified like perturb
& observe method, Incremental conductance method, Fractional short circuit current method, Fuzzy logic control method
etc [3]. Incremental Conductance algorithm is most efficient MPPT tracking method because it responds rapidly with the
changes in atmospheric conditions [4]. The model is then tested under various conditions, irradiance and temperature
comparing it to the values provided by the manufacture’s datasheet. The system is then connected to a Single Phase
Inverter implemented in Simulink. Finally the grid synchronization of this system is done through current control.
2. HIGH STEP UP BOOST MICRO INVERTER
Fig. 2. Proposed high step up micro inverter
The proposed high step up DC -DC converter based micro inverter topology for grid connected photovoltaic
system is depicted in Fig.2. Two power processing stages are decoupled here. In the front end DC-DC converter, a
conventional boost converter is modified in such a way to obtain high step up transformation ratio. A single phase full
bridge inverter using PWM control serves as the DC-AC conversion stage. The control mechanisms for both power
conversion stages are important
2.1 High Step Up DC- DC Converter
Solar panel with conventional boost interface circuit is simulated to verify the step up ratio and it is found to be
small about 2 to 3 %. If this converter is used for inverter grid connection it is difficult for ac module to reach the higher
efficiency. In order to achieve higher efficiency high conversion ratio boost converter is used instead of conventional
converter. High step up boost converter is shown in Fig.3.
Fig.3. High step up boost converter
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The main part of this converter is a coupled inductor which is denoted by T1. N1 is the primary and N2 is the
secondary of coupled inductor. N2/N1 gives the turns ratio (n). Advantage of this converter is that with small turns ratio
high step up conversion is possible. Primary of coupled inductor N1, C1, D1 along with switch S1 perform the function
of conventional boost converter. Also C2, D2, N2 form another pair which is connected in series with N1which boost up
the voltage to a double value. Rectifier Diode D3 connected to the output capacitor C3 and load. Some assumptions are
made to analyses the converter. All components are ideal expect leakage inductance (Lm) of coupled inductor. All
capacitors are large enough to keep voltage across them constant. Parasitic resistance of T1 and equivalent series
resistance (ESR) of capacitors are neglected. Lk1, Lk2 are the leakage inductance of N1and N2 respectively. This
converter will works in two modes of operations, continuous conduction mode and discontinuous conduction mode. For
the micro inverter operation only continuous conduction mode is considered.
Mode 1
Initially the magnetizing inductor Lm charges the capacitor C2 through diode D2. When the switch S1 is ON the
input voltage Vin is series connected with N2, C1, and C2 to charge output capacitor C3 and load R. magnetizing
inductor Lm is also receiving energy from Vin. The current through the Lm, Lk1, and D3 are increasing because the Vin
is crossing Lk1, Lm, and primary winding N1. Lm and Lk1 are storing energy from Vin. Vin is also serially connected
with secondary winding N2 of coupled inductor T1, capacitors C1, and C2, and then discharges their energy to capacitor
C3 and load R. So the output voltage is the sum of the voltage across these series connected elements.
Vout=Vin + nVin + (D/1-D) Vin+ (nd/1-D) Vin (1)
Mode 2
During this mode S1 is turned OFF, secondary leakage inductor Lk2 keeps charging C3 when switch S1 is OFF.
The leakage inductor Lk1 charges the capacitor C1 instantly when S1 is OFF. Also Lk2 is series connected with C2 to
charge output capacitor C3 and the load and iLm is increasing because magnetizing inductor Lm is receiving energy from
Lk1. The energy stored in magnetizing inductor Lm is released to C1 and C2 simultaneously. Currents iLk1 and iD1 are
continually decreased because the leakage energy still flowing through diodeD1 keeps charging capacitor C1. The Lm is
delivering its energy through T1 and D2 to charge capacitor C2. The energy stored in capacitor C3 is constantly
discharged to the load R.
Fig .4. simulink model of proposed converter
2.2 High Step Up Boost Converter Control
PV voltage is instantaneously regulated by the MPPT block. The MPPT block performs the incremental
conductance algorithm which is the most efficient tracking method. MPPT function block in the boost converter system
periodically track the PV voltage and modifies the duty cycle of the converter. (1) Indicate that the output voltage Vout
of converter is changing dynamically in accordance with duty cycle D. The charging and discharging of capacitor causes
the uneven voltage distribution and that may saturate the magnetizing inductance of the coupled inductor. This can be
prevented by designing MPPT block which has the simple control and low cost.
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Fig.5. I-V, P-V curves of PV module
For simplicity, consider that the PV module is working under standard irradiance (1000W/m2) and room
temperature (25 ̊c) and the characteristics of PV module under this condition is plotted and it is shown in fig.5 .In
incremental conductance method terminal voltage of module is always adjusted according to the MPP voltage and it is
based on the incremental and instantaneous conductance of the PV module [5]. The slope of P-V curve gives the change
in power with respect to the voltage. At the maximum power point (MPP) the slope dP/dV is zero. If the slope is greater
or lesser than zero then the operating point is at the left or right side of the MPP respectively.
dP/dV=I + V* dI/dV (2)
From (2) the tracking point can be determined.
dI/dV = -I/V at MPP, (3)
dI/dV > -I/V at left side of MPP (4)
dI/dV < -I/V at right side of MPP (5)
The MPPT regulate the PWM control pulses to the boost converter until the condition for MPP is satisfied. The
Flow chart of incremental conductance algorithm is shown in Fig.6. The controller tracks the changes in the PV output
and adjusts the duty ratio of the converter to operate it at the MPP.
2.3 Grid Synchronization
As over mentioned distributed generation systems are the solution for the increased demand of energy. PV
technology has so many advantages like long lifetime, low maintenance requirements and environmentally friendly
power generation. Transformer less PV inverter systems are important when it works as a part of distributed generation
system. Grid synchronization of PV system is done by a single phase full bridge inverter with current control method.
The PV module and inverter must be capable of adapting to the grid frequency and phase. When the inverter is connected
to the utility, the grid controls the frequency and amplitude of the inverter output voltage and the inverter itself operates
in the current control mode. The inverter current is compared to a reference current and the error is fed back through a
proportional controller [7]. Its output is scaled and added to a feed forward loop with the final output of the duty given by
(6).
D=.5+ Vinv/2Vdc + [Ki(Iref-Iinv)*f*L] / (2Vc) (6)
where, V inv is the inverter voltage, V dc is the input DC voltage, I ref is the reference current, I inv is the
inverter output current, Ki is the proportional gain, L is filter inductance set to 500 mH, and f is the a switching
frequency equal to 10 kHz.
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Fig.6. flow chart of MPPT
3. SIMULATION RESULTS AND ANALYSIS
The modeling of the high step up boost converter and grid connected inverter is done in Matlab/Simulink and the
simulink model is shown in Fig.7. The PV module is the input of simulink model and the simulation result of the PV
module is shown in Fig.8. & Fig.9. The proposed converter with an inverter is the interface circuit of PV-grid system.
Simulink model of proposed converter is shown in Fig.4.
Fig.7 simulink model of grid connected PV system
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(a) (b)
Fig.8 (a) I-V chara (b) P-V chara of PV module
Fig .8 (a) shows the I-V chara and (b) shows the P-V chara of the PV module. From (b) the MPP is corresponds to a
voltage of 19 V
(a) (b)
Fig.9 (a) output voltage (b) output power of PV panel
The output voltage and output power of PV panel is shown in Fig.9. From the simulation the output voltage of PV panel
is obtained as 24V.
(a)
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(b)
Fig. 10. Output Voltage of (a) conventional (b) modified boost converter
Fig .11. Output of grid connected inverter
The output voltage of both conventional (a) and modified boost converter is shown in Fig.10. From the result
the output voltage of conventional converter is 44V and that of modified converter is 240 V. Both of them have the input
of 24V. So it is clear that the step up conversion ratio of modified converter is 10 times higher than the conventional
boost converter. Grid connected inverter output is shown in Fig.11. The amplitude of the grid voltage is 100 volt. So the
grid connected inverter output voltage has amplitude of 100 V.
4. CONCLUSION
A novel high step up boost converter based micro inverter for grid connected photovoltaic system has been
presented in this paper. An incremental conductance MPPT algorithm for converter and current controller for grid
synchronization of inverter was developed. The proposed converter achieves high step up conversion without using
numerous turns ratio. Simulation results are shown to verify the circuit operation principles, current control and MPPT
method. The proposed PV micro inverter system with its control implementations will be a competitive method for grid
connected photovoltaic applications.
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