1. Introduction

Today, the major energy sources used for generating electricity are fossil fuel resources [1]. To reduce the dependency on fossils based energy it necessary for power generation to move towards the renew- able resources including solar energy, wind power, hydroelectricity, and biomass. In literature, most of the distributed production sys- tems are uniquely focused on one type of sustainable resource, e.g., a wind power like in [2] or solar power like in [3].

The combination and the penetration of the renewable resources, face considerable challenges due to growing power industry. The injection into the grid of the energy yielded by wind and solar photo- voltaic generators, is proven to be one of the technological solution [4, 5]. Their combination offers additional benefits such as increas- ing the overall operational efficiency and power reliability, optimal use of lands resources, and improves the capital investments [5]. Grid-connected wind/Photovoltaic (PV) cogeneration is not largely discussed [6]–[7], whereas in literature, most of wind-PV hybrid systems are proposed for standalone off-grid applications [5]. The optimization method of generation of power using hybrid system used for battery charging is developed in [8], to increase the stability

and efficiency. In [9], the author have proposed a small scale hybrid solar- wind power generation system to reduce Electricity shortage in Egypt.

Much research was conducted to track maximum power point (MPP) of the wind using methods based on Perturb and Observe (P&O) algorithm [6], neural network technique, or other approaches based on fuzzy logic. Due to the nonlinear voltage-current proper- ties of PV solar panels, the necessity of MPP tracking has become apparent. MPP tracking schemescan be accounted for as perturb and observe (P&O), incremental conductance method, Fractional Open- Circuit Voltage, Neural Network, Fuzzy Logic Control, Current Sweep [10]. The combination of multilevel inverter and renewable energy sources are advantageous. In [11], the authors have proposed a hybrid cascaded nine level inverters for photovoltaic applications. The proposed system in [12] are based on Maximum Power Point Tracking (MPPT) combined with multilevel inverter to achieve the maximum power. In [6], the system comprises back-to-back voltage source converters with no extra dc/dc conversion stages to connect wind and photovoltaic generators to the power grid. Moreover, the control of the currents on the machine- and grid- side are carried out using hysteresis regulators, introducing higher harmonic contents from the variable switching frequency. The objective in [13] is to The mechanical equation is as shown below:

by employing a new control technique in order that it is possible to transfer maximum active power from the PV to the grid without injecting harmonics. The study proposes in [14, 15] a control strat- egy for a single-phase, three-phase photovoltaic system connected to a distribution network. The proposed technology is based on the modification of the conventional control in current mode to adjust the power factor of the photovoltaic system and control the voltage of the converter link.

This paper extends the work originally presented in ICEIT’4 [2], where the modeling and simulation of a wind turbine string, based on a vector-controlled five-level inverter, was presented. Its objec- tive was the optimization of the energy produced, adjust the DC link and control power transferred into the grid. Motivated by advan-

Where: R_s and L_s are stator-winding resistance and inductance, re- spectively, I_d and I_q are the stator currents, Ω is Rotational speed of the PMSG, ϕ is stator phase magnetic flux, f_c and J are the friction coefficient and motor inertin, respectively. ϕ_f : the flux generated by the permanent magnets across stator windings, C_m is the mechanical torque .

2.3    Photovoltaic Generator

The model of the PV generator is presented in Appendix whereas it is modeled as following:

tages of hybrid production systems, presented paper covers a new topology to interface both is Permanent Magnet Synchronous Generator (PMSG)-based wind energy and PV generators with power

grid. Note a blocking diode is series connected with each PV string to avoid reverse flow of current. Two P&O algorithms, namely P&O Wind and P&O PV, are used to optimize the energy extracted by turbine and solar system, respectively. Note that perturbations could impact correct operation of the receivers attached to the grid if the voltage output wave is not sinusoidal. To avoid this problem, we have implemented in this chain a two-stage rectifier followed by a five-stage Neutral Point Clamped (NPC) inverter regulated by the Space Vector Pulse Width Modulation (SVPWM) technique, with- out additional DC/DC conversion stages[16]. The converter control strategy is detailed in which the presented study was treated using Matlab/Simulink. Simulation results proved efficiency provided by this controlling approach: decrease distortion of harmonic and increased power factor.

2. Wind Photovoltaic Generator Side Model

2.1        Wind turbine model

Mechanical energy absorbed by a wind turbine is defined by [2]:

where: C_p is the coefficient of the rotor that is a non-linear function of the tip speed ratio λ and the pitch angle of blades β, R and V_v are the radius of blades and the wind speed, respectively.

2.2        PMSG model

PMSG in the dq marker is modelised using these equations [2]:

The I_op–V_op characteristics of the PV array, there is an optimal operating point (I_op–V_op) corresponding to the maximum generated PV power, at any solar irradiance level.

3. Control Strategy

The suggested wind-PV system appliances two appropriate con- trol techniques for machine-side Voltage Source Rectifier Control (VSRC) and grid-side Voltage Source Inverter Control (VSIC).The proposed system operates efficiently despite solar insolation changes and wind speed variability. The next section deals with control strategies.

3.1        Voltage Source Rectifier Control (VSRC)

Figure 1 visualizes the VSRC schema. Momentary torque is regu- lated through the application of the vector control schema. Vector Control technology treats the technique of control of AC machines. This control is based on eliminate the coupling between the inductor and the induced by dissociating the static current into two com- ponents (i_sd,i_sq). So that, the reference direct axis current (i_sd) is controlled relative to desired flux, while reference quadrature axis current (i_sq) is adjusted according to the desired torque. Which allows for a function comparable to a dc machine, with separate excitement [4].

The Voltage Source Rectifier (VSR) equivalent circuit is pre- sented in Figure 1. The control of the VSR is split over two parts. The first is the direct and quadrature reference current generation (i_{sdre f} and i_{sqre f} ) and generation of switching signals for VSR.

• Generation of i_{sdre f} and i_{sqre f}

In proposed system, the electromagnetic torque is controlled by controlling the current i_{sqre f} while the current i_{sdre f} is set to zero.

For each wind speed, the speed of the rotor has only one value for maximum wind power. Using MPPT P&O algorithm, the refer- ence speed Ω_{re f} is generated.

Speed is regulated by the proportional integral (PI) speed controller, as illustrated in Figure 1, and its output is considered as isqre f .

Figure 1: The proposed Wind-PV System Configuration

Commutation times are determined by the equations below:

With: K_p and K_i are the proportion and integral coefficients of PI speed controller.

• Generation of switching signals for VSR

The PI current controller is employed to ensure stator currents gen- erated by PMSG correspond to references in (6), as shown in Figure 1 and (7).

3.2        Voltage Source Inverter Control (VSIC)

Figure 9 shows the Voltage Source Inverter (VSI) commutation configuration, uses the SVPWM strategy. VSI control is structured With: K_p and K_i are proportion and integral coefficients of PI current controller.

Three phase reference stator voltages (V_{sare f} , V_{sbre f} , V_{scre f} ) are generated from V_{sdre f} and V_{sqre f} using inverse Parks transform. They fed the SVPWM controller.

• Generation of i_{dgre f} and i_{qgre f}

VSI system is regulated by using vector control scheme in which a phase locked loop (PLL) is applied to converter to synchronize it with power grid. Active and reactive power regulation are expressed by:

Voltage reference is determined using:

This control has the advantage of decoupling active and reactive.

The rectifier delivers eight states, consisting of two zero vectors and six active vectors. The complex diagram is split by the active vectors in six sectors with 60 degrees forming each hexagon, with control, whereas to regulate DC bus voltage V_dc, current i_dg is used and to regulate reactive power transited Q, current in quadrature i_qg is used. . voltage reference vector corresponding to a pair of two active adja- cent vectors (T_a, T_b) and a zero vector (T₀) of commutation time T_s

Figure 2: Decomposition of the five-level inverter vector diagram

We can therefore, through the imposition of reference currents, provide referencing for active power P_{re f} and reactive power Q_{re f} fixed in zero that maintains a unity power factor (PF), we have:

This approach is founded on geometrically splitting of a vector diagram to several hexagons that represent low-level vector dia- grams, as shown in figure 2 [18].

To attain this objective, two actions need to be completed:

– Depending of voltage reference vector position V_{re f} , one of six – V      is moving to center of the hexagon selected in the first action.

The DC link reference (V_{dcre f} ) is obtained by the PV P&O MPPT algorithm as shown in figure 1, corresponding to its maximum PV power output voltage at various irradiation levels. The regulation of DC bus voltage is performed by PI regulator and its output is considered as the active power component i_{dgre f} . The i_{dgre f} is given as,

With: K_p and K_i are the proportion and integral coefficients of DC-bus voltage controller PI, respectively.

• Generation of switching signals for VSI

The i_{dgre f} and i_{qgre f} are compared with i_dg and i_qg, respectivly, to obtain the currents error and are given as,

Following these actions two times would reduce the five-level in- verter’s vector diagram to the two-level inverter’s vector diagram. Next, we will employ the classical method of vector modulation of the two-level inverter, discussed in the section 3.1.2.

4. Simulation results

For the hybrid system, a simulation model shown at figure 1 has been modulated with Matlab/Simulink R in order to validate and evaluate system performance. Simulation parameters are as shown in the appendix A. Penetration of wind and PV energy is studied under different weather conditions.As shown in Figure 3 and 4, the wind speed increases from 20 to 8, and finally drops to 5 m/s, at t= 0.3 and 0.7s, respectively. Solar irradiance level increases from 400 to 1000, than drops at 500, and increases to 300 W/m, at t= 0.3, 0.5, and 0.7s, respectively.

The three phase reference grid voltages (V_{agre f} , V_{bgre f} , V_{cgre f} ) are generated from V_{dgre f} and V_{qgre f} using inverse Parks transform.

Figure 3: Cp(λ,β)curves typical for different inclination angles

Figure 4: I_pv V_pv photovoltaic characteristics at different irradiation levels

Figure 5: Generator rotation speed

Figure 6: DC-link voltage

Figure 7: wind power

Optimal Ω_{re f} and V_{dcre f} are generated by Wind and PV MPPT P&O, respectively. As depicted in Figure 5 and 6, the corresponding variables are then regulated.

Under these conditions, Ω_{re f} and V_{dcre f} are both refracted on the corresponding PV and wind powers, as shown at figures 7 and 8.

Figure 8: PV power

Three-phase grid voltages before filtering are shown in figure 9, representing five-level inverter output voltage waveform obtained from capacitors. Economically, it is not interesting to use a rectifier for each capacitor, so it is necessary to ensure the balance of the continuous bus, as depicted in Figure 10 .

Figure 9: Output voltages of the three-phase inverter at five levels

Figure 10: Capacitor voltage

Figure 11: Grid current

Figure 12: Reactive power

Figure 13: THD of grid currents injected

The AC current injected into grid is indicated in Figure 11. Dur- ing entire operating range, the reactive power is set to zero, then a unity PF is maintained, as shown in in Figure 12.

The figure 13 illustrates the THD currents injected in the grid. According to THD analysis above, the total harmonic distortion at Fc=1KHz given THD=0.80%. Therefore, as the number of levels is increased, THDs are minimized in comparison with classical multi-level inverters [19].

5. Conclusion

In this paper, the modeling and simulation of wind-power hybrid systems connected to the grid, using vector control systems were presented.A hybrid system based on wind and photovoltaic energy was set up with a five-level inverter. The VSRC’s purpose consists of extracting the maximum wind power, and the VSIC is respon- sible for extracting maximum PV power, and balance of dc-link capacitor in order to transfer produced power to utility-grid. This way, reactive power is fixed at zero, then a unity Power Factor PF is maintained. The proposed system has enhanced efficiency and reliability via a hybrid renewable energy system, the maximum power is extracted from independent MPPT using P&O algorithm, and reduces current THD. The results of the simulations showed the correspondence of the quantities assessed with the reference quantities and allowed the validation of the mathematical models.

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