13-09-2013, 04:12 PM
An Improved Maximum Power Point Tracker for Photovoltaic Energy Systems
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Abstract:
In this paper, A combined low cost, high efficiency inverter and peak power tracker has been
presented. Interfacing of photovoltaic and fuel cell energy systems requires a wide operating
range of DC to AC power conversion to utilize the available power in these energy sources. The
maximum power point tracker system consists of DC-DC boost converter and PWM voltage
source inverter as a utility interface unit. PWM generates high quality sinusoidal line current. By
the data supplied to the control system, it will generate a control signal to the PI controller to
generate suitable duty ratio for the boost converter and suitable value for the modulation index of
PWM inverter. The suitable duty ratio for the boost converter will force the PV to work around
the optimum voltage. The control system adjusts the modulation index of PWM inverter to
transfer the maximum power available from PV to the electric utility. Simulation results from
PSIM computer program has been presented in this paper. Results from analysis shows the
superiority of the Maximum Power Point Tracker system and clean power utility interface has
been achieved.
Introduction
The Power Voltage (P-V) characteristics and the locus of maximum power points are shown in
Fig.1. From this figure it is clear that when the radiation varies, the operating voltage varies
linearly with it. If the PV array forced to operate around maximum power point, 20 to 30%
increase in the output power from Photovoltaic arrays [1]. Maximum Power Point Tracker
(MPPT) has been used to force the PV array to work around the maximum power point. For this
reason, the MPPT is required to track the maximum power available in the PV array. The MPPT
operates by periodically incrementing the terminal voltage of the PV array and continuously seek
the peak power point as shown in Fig.2.
System Configurations
The proposed system consists of PV array, DC-DC boost converter, PWM inverter and the
MPPT as shown in Fig.3.The radiation and temperature are used to calculate the maximum PV
array output power and PV array terminal voltages. The MPPT operates by periodically
incrementing the terminal voltage of the PV array and continuously seek the peak power point as
shown in Fig.3. The control flowchart of the MPPT is shown in Fig.4. The control system adjusts
the boost converter as well as the PWM inverter taking into account to seek maximum power
point of PV array. A comparison between the terminal voltages (actual and optimum) will
control the duty ratio of boost converter. Changing the modulation index of PWM converter
according to the error signal between the maximum and actual power will pass the maximum
power available from PV to the electric utility.
Simulation Results
The proposed control scheme has been simulated using PSIM Program linked with visual C
language. The radiation and temperature were fed to the PV simulator (Fig.3) to calculate the
optimum voltage and the maximum power. The error signal between the actual and optimum
value of PV terminal voltage were used to control the boost converter. To increase the system
stability, the error signal between the actual power and the maximum power output from PV
simulator were used to change the modulation index. The control system of the proposed
approach is shown in Fig.1. Fig.4 shows the terminal voltage of the PV arrays. Fig.6 shows the
actual and maximum power obtained from PV simulator. From this figure, we can see the control
system efficiently tracks the maximum power at any time. Fig.6 shows the terminal voltage of
the PV arrays (in Volt). Fig.7 shows the DC-link voltage. Fig.8 shows the three phase utility line
current and its FFT components which presents high quality utility line current. Fig.9 shows the
utility line current and the utility phase voltage.
Conclusions
In this paper a combined low cost, high efficiency inverter and peak power tracker has been
proposed. This converter operates close to the maximum power point of the photovoltaic array
and forms a DC to AC inverter. Simulation and experimental results are shown. This system
shows a wide operating rage of DC to AC power conversion to utilize the available power in the
photovoltaic and fuel cell energy systems.