Photovoltaic (PV) energy is one of the most important energy sources since it is clean and inexhaustible. It is important to operate PV energy conversion systems in the maximum power point (MPP) to maximize the output energy of PV arrays. An MPPT control is necessary to extract maximum power from the PV arrays. In recent years, a large number of techniques have been proposed for tracking the maximum power point. This paper presents a comparison of different MPPT methods and proposes one which used a power estimator and also analyses their suitability for systems which experience a wide range of operating conditions. The classic analysed methods, the incremental conductance (IncCond), perturbation and observation (P&O), ripple correlation (RC) algorithms, are suitable and practical. Simulation results of a single phase NPC grid connected PV system operating with the aforementioned methods are presented to confirm effectiveness of the scheme and algorithms. Simulation results verify the correct operation of the different MPPT and the proposed algorithm.

Renewable sources of energy are a hot topic acquiring a growing importance in the world due to its consumption and exhaustion of fossil fuel. The photovoltaic (PV) power system is becoming increasingly important as the most available renewable source of energy since it is clean with little maintenance and without any noise.

Nevertheless, PV systems have problems, such as the conversion with low radiation (in general less than 17%), as well as the nonlinear characteristic that depends on irradiation and temperature in its operation which change the amount of electric power generated [

Since a PV array is an expensive system to build, it is necessary to exploit all of the available output power. Multilevel converters are particularly interesting for high power applications.

The location of the maximum power point can be determined using different algorithms. Maximum Power Point Tracking (MPPT) techniques are used to maintain the PV array’s operating point at its maximum power point (MPP) and extract the maximum power available in PV arrays.

Different MPPT techniques have been proposed in the literature such as the perturbation and observation (P&O) technique [

These techniques vary in complexity, cost, speed of convergence, sensors required, hardware implementation, and effectiveness.

Due to the various MPPT methods, different research in PV systems has presented a comparative analysis of MPPT techniques. Indeed, some papers present comparative study among only few methods and others present a comparison of several MPPT methods, based on simulations, under the energy production point of view. The MPPT techniques are evaluated considering different irradiation and tempreature variation and calculation of the energy supplied by the complete PV array.

In this paper, the attention will be concentrated on PSIM simulation comparisons between some of these techniques and a proposed method will be discussed using the short circuit current and the open circuit voltage. Consequently, the purpose of this work is to compare several widely adopted MPPT algorithms between the different MPPT techniques in order to understand which technique has the best performance with the proposed method as well as injecting the high quality sinusoidal current to the grid.

In the simulations, the MPPT techniques have been implemented strictly following the description indicated in the references excluding the proposed algorithm. In particular, we will focus our attention on a grid connected photovoltaic system constructed by connecting a single phase neutral point clamped inverter (SP-NPCI) with an inductor as an output filter, using different embedded MPPT algorithms.

The SP-NPCI output current is controlled in order to follow a sinusoidal reference waveform, synchronized, and in phase with the grid voltage.

Figure

Scheme of the control system including the MPPT.

Using a phase-locked loop (PLL) algorithm, we can extract a unitary signal in phase with the grid voltage. The amplitude of the reference current

The switches

The maximum power supplied by the photovoltaic panels is not always stable and fixed in the same operating point; it varies with the weather conditions, such as solar irradiation, shadow, and temperature. To extract the maximum power, it is necessary to implement an MPPT algorithm that dynamically adjusts the extraction of the power. Convergence speed is one of the most important features among all different MPPT algorithms. Any improvement in the rise time of MPPT improves the reliability of the system and increases the power extraction and efficiency of the whole system.

Perturb and observe algorithm is simple and does not require previous knowledge of the PV generator characteristics or the measurement of solar intensity and cell temperature and is easy to implement with analogue and digital circuits. It perturbs the operating point of the system causing the PV array terminal voltage to fluctuate around the MPP voltage even if the solar irradiance and the cell temperature are constants [

Moreover, it is the most widely used and workhorse MPPT algorithm because of its balance between performance and simplicity. However, it suffers from the lack of speed and adaptability which is necessary for tracking the fast transients under varying environmental conditions [

Flowchart of perturbation and observation.

The incremental conductance method is based on the principle that the slope of the PV array power curve is zero at the MPP, so that

Considering that

The algorithm increments or decrements the reference until the condition

Flowchart of the incremental conductance algorithm.

If the grid current

From (

The ripple correlation control uses the average function to find out the average term of the instantaneous PV power

The knowledge of the instantaneous operating point region makes it possible to change the output reference current in order to approach the maximum power operating point. This method presents very fast dynamics converging asymptotically to the MPP, and it achieves convergent speeds at a rate similar to the switching converter frequency. Figure

Flowchart of ripple correlation MPPT algorithm.

The proposed MPPT is a combination of the open circuit voltage and the short circuit current methods with a variable step size (VSSOCV) in the gain parameters which is multiplied by the open circuit voltage to have the maximum power which can be extracted from the PV panels. In addition to that, we proposed a power estimator, to measure the power from the DC side of the PV system.

In case the grid current is in phase with the grid voltage, the instantaneous PV power, upon neglecting power losses in NPC inverter, is given by

Figure

Variation of the open circuit voltage according to the temperature.

The SCC of the PV panel depends on the irradiation. This relationship can be described by

Figure

Variation of short circuit current according to the irradiation.

However, the optimum operating current for maximum output power is proportional to the short circuit current under various irradiation [

The proposed method needs using two sensors for the temperature and irradiation measurements.

This method needs also knowing the characteristics of the PV panels to determine the constant of the proposed algorithm in Figure

Flowchart of the proposed MPPT algorithm.

For this purpose, a low-cost temperature sensor is adopted and is maintaining the right track of MPP but, in practical implementations, can be a problematic issue due to irregular distribution of PV array temperature, which can be avoided in small PV converters.

In order to obtain a good characterization of the proposed topology, simulations were performed using PSIM software. The system was simulated under different operating conditions, in steady state and during transient state caused by solar radiation variations.

The P&O and IncCond techniques are the most widely used because of InCond and P&O simplicity to implement. But, as it can be seen in Table

Efficiency of the different MPPT.

MPPT | Efficiency ( |
THD (%) | |||
---|---|---|---|---|---|

1000 W/m^{2} |
800 W/m^{2} |
600 W/m^{2} |
400 W/m^{2} |
||

VSSOCV | 99.95 | 1.6 | 1.5 | 2.0 | 3.0 |

IncCond | 99.94 | 4.6 | 5.7 | 7.7 | 24.0 |

P&O | 99.93 | 1.7 | 1.9 | 7.1 | 9.0 |

RC | 99.86 | 2.1 | 3.8 | 2.0 | 8.0 |

The proposed method is efficient and extracts the maximum power but it needs two sensors to measure the irradiation and temperature variation. This method needs only the knowledge of the

IncCond has the highest rise time as it can be seen in Figure

Power extracted from PV panels in steady state at 1000 W/m^{2}.

Power extracted from PV panels during the start-up with different MPPT techniques.

Table ^{2}. Table

The techniques are not equivalent concerning the costs and the software complexity but all of them require a microcontroller or DSP with higher performances due to the necessity of high computation capability.

The RC technique has an efficiency lower than the P&O and IncCond techniques, but its THD of the injected current to the grid is better than the P&O and IncCond.

Figures

Maximum power extracted and theoretical power during a fluctuation in solar radiation from 1000 W/m^{2} to 200 W/m^{2} passing through 600 W/m^{2} and 400 W/m^{2}.

Maximum power extracted and theoretical power during a fluctuation in solar radiation from 1000 W/m^{2} to 800 W/m^{2}.

DC-link voltage with different MPPT.

Injected current into the grid with different MPPT algorithms.

The results show that the best MPPT technique is the proposed one (VSSOCV).

The purpose of MPPT is to extract the high level power from PV systems. This paper presents a comparison of MPPT methods with a proposed one which is the most effective on the basis of MPPT efficiency and considers their suitability for systems which experience a wide range of operating conditions. From this, it is clear that each MPPT method has its own advantages and disadvantages. In addition, the MPPT should be capable of minimizing the ripple around the MPP. Different MPPT methods are compared based on simulations in the PSIM environment in terms of the dynamic response of the PV system and efficiency and implementation considerations. Therefore, the two techniques incremental conductance (IncCond) and P&O algorithms are simple to implement but they have some inconvenience in the DC-link and the output inverter current. Otherwise, the RC perturbs itself without an external perturbation. These three methods have been evaluated by simulating a grid connected PV system, utilizing a single phase NPC inverter to connect the PV panel to the grid. In particular, the performance of each method has been considered over a wide range of different irradiation conditions. It is shown that the proposed system is able to always extract the maximum power available from the solar PV panels with the proposed MPPT.

The authors declare that they have no competing interests.