photovoltaic systems are an excellent candidate to replace other existing polluted energy sources [1-4]. The grid-connected PV system supplies the active power.
Improving the quality of energy in grid connected photovoltaic systems B. Boukezata1, A. Chaoui2, J.-P. Gaubert3 and M. Hachemi4 1 Departement d’elctrotechnique, Université de Sétif 1, Algeria 2 Laboratoire d’Electronique de Puissance et Commande Industrielle (LEPCI), Université de Sétif 1, Algeria 3 Laboratoire d’Informatique et d’Automatique pour les systèmes (LIAS-ENSIP), Université de Poitiers, France 4 Laboratoire d’Automatique de Sétif (L.A.S), Université de Sétif 1, Algeria
Abstract— This paper proposes the effective utilization of active power filter (APF) for interconnecting the PV modules to the grid using direct power control (DPC) method. Its main feature is the capability to compensate the reactive power and harmonic currents drawn by nonlinear loads and simultaneously inject the maximum power available from the PV array into the grid. The reference current containing PV maximum power and harmonic components is developed based on direct power control via an integral-proportional (IP) controller, exploiting optimal solar energy was extracted by an algorithm of maximization MPPT is the INC-COND. The whole system (single stage converter) presents increased efficiency when compared to the conventional system. Simulation results on MATLAB/Simulink of the proposed system have been done and the obtained results prove the effectiveness of using a shunt active power filter as the interfacing unit for grid integrated renewable energy system. Keywords – Direct Power Control, Active Power Filter, Photovoltaic system, MPPT
I. INTRODUCTION Photovoltaic energy has great potential to supply energy with minimum impact on the environment, since solar energy is clean, pollution-free, negligible maintenance and zero noise, air pollution and inexhaustible. With the decrease in the price of PV modules and the increase in the price of traditional petrochemical fuels for generation energy, the photovoltaic systems are an excellent candidate to replace other existing polluted energy sources [1-4]. The grid-connected PV system supplies the active power from the PV array to the grid via an inverter. Today, nonlinear loads are widely used in residential and office buildings. If the grid-connected PV system is applied directly to non-linear loads, the power quality is relatively poor, because of the active power supply by the PV array only. To solve this problem, the grid-connected PV system should not only provide active power to the system via MPPT, but also improve the power quality (low THD and unity power factor) [1, 5].
In this paper, a transformerless grid-connected photovoltaic (PV) system with a direct power (active and reactive) control algorithm is proposed. This system increases the conversion efficiency (single stage PV system) and operating as power supply as well as harmonic and reactive power compensator when the sun is available. At low irradiation, the system operates only as harmonic and reactive power compensator (Active Power Filter-APF). The aim is that the system can operate as an inverter of distributed generation or/and as a shunt APF independently or simultaneously. In order to verify the proposed system, the PV system is simulated using Simpower of Matlab/SimulinkTM. II.
DESCRIPTION OF THE GRID-CONNECTED PV SYSTEM
The system that has been simulated consists of a photovoltaic array connected through a DC bus to a threephase inverter that is connected to a grid through a simple filter and nonlinear load, as shown in Fig. 1.The MPP tracker is integrated in the inverter control (Fig. 3), as there is a one stage Grid connected PV system. The inverter is used to transfer the power from PV module, it also assure the compensation of the harmonic currents, reactive power and unbalanced current. The load is represented by three phase of rectifier with R L load.
Figure 1. Three-phase grid connected to the PV module
III. PV ARRAY MODEL The PV array used in the proposed system is MSX60 and it is simulated using a model based on [5, 6]. In this model, a PV cell is represented by a current source in parallel with a diode and a series resistance as shown in Fig. 2(a). The basic current equation is given in the following equation:
qv 1 akT
I I pv ,cell I ,cell exp
(1)
where is the current generated by the incident light (directly proportional to sun irradiation), the leakage current of the diode, q the electron charge 1.60217646e10-19 C, k the Boltzmann constant, T the temperature of the PN junction, and a is the diode ideality constant. But practically the PV array comprised with many PV cells connected in series and parallel connection. This makes some additional parameters to be added with the basic Eq. (1). The modified equation is shown in the following equations:
V Rs I V Rs I 1 Rp Vt a
I I pv I exp
I pv I pv ,n K I T
(2)
G (3)
Gn
A practical PV array consists of several connected PV modules formed by Ns solar cells connected in series and parallel. Therefore, (1) which presents a single PV cell should be amended into (4) to represent a PV array [7].
V IRs N ss N pp 1 V aN t ss
I N pp I pv N pp I exp
(b)
V IRs N ss N pp
(4)
(c) Figure 2. (a) Equivalent circuit of a PV cell, (b) I-V characteristic, (c) P-V characteristic
IV. INCREMENTAL CONDUCTANCE (INC-COND) MPPT This algorithm uses the instantaneous conductance of the PV module which is the current divided by the voltage ⁄ , and the incremental conductance which is the variation of current divided by the variation of voltage ⁄ and compares them in order to obtain the MPP [8, 9]. This method is based on the fact that the slope of the PV array power curve (Fig. 2 c) is zero at the MPP, positive on the left of the MPP, and negative on the right, as given by
R p N ss N pp
Where:
dP dV 0
at MPP
dP dV
0
left of MPP
dP dV
0
right of MPP
is the number of PV modules connected in parallel. is the number of PV modules connected in series. The PV model is simulated using Solarex MSX60, 60W PV module. The simulated I-V and P-V characteristics of the Solarex PV module at standard test conditions are shown in Fig. 2(b) and Fig. 2(c) respectively.
(5)
Since
dP dV
d IV dV
I V
dI dV
I V
I V
(6)
The main advantage of this algorithm is its fast power tracking process but it might be unstable when the solar intensity is low due to the low current differentiation [10]. The flow chart of incremental conductance algorithm is shown in Fig. 3.
(a)
ss ps jqs ps vsa isa vsb isb vsc isc qs
1
vsa vsc isa vsc vsa isb vsa vsb isc
(7)
3
Figure 3. Inc-Cond algorithm
V. ACTIVE POWER FILTER Active power filters (APF) are basically power electronic devices that are used to compensate the current or voltage harmonics and the reactive power flowing in the power grid caused by a three-phase diode bridge rectifier load, followed by an inductor in series with a resistor [11, 12]. The APF may be used as a controlled current source and it has to supply a current wave as close as possible to current reference. It consists of a full bridge voltage source pulse width-modulation inverter, a dc-side capacitor and ac-side high-frequency inductors that are required to shape the compensator input currents .The basic compensation principle of APF is explained in Fig. 4.
Figure 5. Scheme of DPC with source voltage sensors
The digitized variables , and the line voltage vector ( ⁄ ) form a digital word, which by position accessing the address of lookup table selects the appropriate voltage vector according to the switching table. For this purpose, the stationary coordinates are divided into 12 sectors, as shown in Fig. 6, and the sectors can be numerically expressed as:
n 2
6
n n 1
n 1, 2,...,12.
(8)
6 4
5 6 v
3 t
v
7
2
v 1
8 9
12 10
11
Figure 6. Sectors on stationary coordinates Figure 4. Three Shunt active power filter configuration
A. Direct Power Control The bloc scheme in Fig. 5 gives the configuration of direct power control where the commands of reactive power (set to zero for unity power factor) and active power (delivered from the outer integral-proportional (IP) DC voltage controller) are compared with the calculated and values given by (7), in reactive and active power hysteresis controllers, respectively.
The digitized error signals , and digitized voltage phase are input to the switching table in which every switching state , and of the converter is stored, as shown in Table 1. By using this switching table, the optimum switching state of the converter can be selected uniquely in every specific moment according to the combination of the digitized input signals. The selection of the optimum switching state is performed so that the power errors can be restricted within the hysteresis bands [11].
Table 1: The switching table. dp dq 1 0 1 0 0 1
θ1 v6 v7 v6 v1
θ2 v7 v7 v1 v2
θ3 v1 v0 v1 v2
θ4 v0 v0 v2 v6
θ5 v2 v7 v2 v6
θ6 v7 v7 v6 v4
θ7 v3 v0 v6 v4
θ8 θ9 v0 v4 v0 v7 v4 v4 v5 v5
θ10 v7 v7 v5 v6
θ11 v5 v0 v5 v6
θ12 v0 v0 v6 v1
(b) VI. RESULTS AND DISCUSSION To evaluate the performance of the combined operation of APF with PV generation system, the proposed control system and the on-grid operation mode have been modeled and simulated under Matlab/SimulinkTM and power system blockset environment. The source current waveforms of the simulation results have been analyzed to obtain their THD under varying climatic conditions, as shown in Fig. 7. An irradiation ramp change was used. It starts from 0 W/m2, stops at 1000 W/m2, waits at this level for 0.25 s, and decreases again back to 200 W/m2. The temperature is considered constant during the simulation. Simulation results shows in first part between 0s to 0.4s in Fig. 8 (b) that the active power filter (with null irradiation) ensures grid power quality improvement and source current becomes quasi-sinusoidal with a THD of 1.08%. And from 0.4s to 1.2s it works with PV generator. The total harmonic distortion becomes 1.40% (Fig. 9). for t = 0.4s the irradiation (solar power and solar current Fig. 8 (f, g)) increases with a constant slope causes a reduction in source current amplitude and decrease of grid active power Fig. 8 (h). In this case the flux of solar energy is injected towards the network through the continuous bus of APF, and ensures an optimal quality of energy in the same time, then source current (active power) is stabilized with a constant value until the decrease of irradiation, after that solar energy decrease, power grid increases to compensate this missing energy.
(c)
(d)
(e)
In Fig. 8 (a, c) show curves of source voltage and load current under varying climatic conditions. (f)
Figure 7. Irradiation profile
(a)
(g)
(h) Figure 8. (a, b) Voltage and current source, (c, d) Load and filter current, (e) DC bus voltage, (f,g) Solar current and power and (h)Active and reactive power source.
We can see in Fig. 10 source, load and filter currents with and without PV Generator at permanent mode.
VII. CONCLUSION This paper presents a solar energy transformation chain connected to the grid with the functionality of compensation harmonics due to the nonlinear load. Management of energy (active power and reactivate) using a direct power control method has been made and the effectiveness of the algorithm was proven. INC-COND MPPT is chosen to maximize solar power with minimum power fluctuation around the MPP compared to P&O algorithm. Finally, the results show that the system can correct the power factor to values close to unity, verify the performance of the combined (540 W PV array) PV–APF and the INC-COND MPPT method for all the cases tested.
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(b) Figure 9. Source current spectrum: (a) APF alone, (b) APF-PV
(a)
APF
(b)
APF PV
Figure 10. Source, load and filter currents (a) off PV, (b) on PV
