HARMONIC POLLUTION COMPENSATION BY CONNECTED PHOTOVOLTAIC SYSTEMS USING THE INSTANTANEOUS POWER METHOD

Youssef ait el kadi, fatima zahra baghli and yassine lakhal. Laboratory of Engineering and Applied Technologies (LITA) Higher School of Technology Sultan Moulay Slimane University -Beni Mellal Morocco. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 06 September 2018 Final Accepted: 08 October 2018 Published: November 2018

This paper discusses the potential for photovoltaic systems to compensate the grid harmonics pollution by using the instantaneous power method. The objective is to develop an optimal algorithm of the control allowing both the active power transfer to the electrical grid and an optimal compensation of the harmonic pollution resulting from unbalanced non-linear loads as well as the compensation of the reactive energy involved. The studied system includes a photovoltaic generator (PV), a DC-DC converter that steps up the PV output to the DC link voltage level with maximum power point tracking (MPPT) control and an inverter that links the system to the grid with a variety of non-linear loads. This method consists in extracting the AC components of the active and reactive instantaneous powers which are related to the harmonics and to optimize the algorithm of the inverter control in order to compensate the disturbing currents caused by these powers. The study was realized according to different regimes that are related to both: the harmonic rate caused by the non-linear charges and the level of solar power received. The obtained results show the benefits of such optimization of the active filter compensation method, in order to improve the quality of the energy (THD ≤2.02%), while limiting the repercussions of the filter on the photovoltaic station and by automatically adapting to the variation of the solar irradiation and the unbalanced load without risk of resonance with the impedance of the grid that can be caused by the passives filters.

…………………………………………………………………………………………………….... Introduction:-
The electrical grid pollution by harmonic currents is an unavoidable consequence of the increasing utilization of nonlinear loads which cause many disturbances, such as generating the distortion of the voltage within the electrical grid and contributing to poor quality of the energy supplied to consumers. To overcome this problem, the use of photovoltaic systems; in order to substitute the conventional filtering systems at different points of the grid; proves to be an adequate and efficient solution. Indeed, solar energy captured using photovoltaic modules represents a viable alternative energy that will allow; in addition to the injection of active energy into the grid; to compensate the different disturbances present in this grid.

ISSN: 2320-5407
Int. J. Adv. Res. 6(11), 526-536 527 In this work, we propose to optimize the control of an array of photovoltaic generators connected to the distribution grid in order to ensure; in addition to the injection of the active power; an optimal compensation of the harmonic pollution (both voltage and current), the voltage dips and reactive energy and within minimizing the effects of the filter on the photovoltaic station for various operating conditions.

Studied configuration
The studied system in this work (figure 1) consists of two stages conversion system, the first one includes a DC-DC converter that steps up the photovoltaic output voltage to the DC link voltage level using the maximum power point tracking (MPPT) control, the second stage is an inverter that connects the compensation system to the (380V / 50Hz) distribution grid through a shock self-inductance. During sufficient irradiation, the proposed compensation system acts as an active shunt filter with active power supplying in the electrical grid. During low irradiation, it performs the function of a reactive power compensator.
According to various operating conditions imposed by the irradiation fluctuation on the one hand, and by the variation of the load on the other hand, an analysis of the power transits is made at the level of the photovoltaic source, the load and the grid. The system adopted to transfer the photovoltaic power into the grid is used also; without any supplementary investment; to fulfill the role of an active filter in order to improve the quality of energy at several connection points on the grid. This is thanks to a control algorithm of the voltage inverter, which is adapted to ensure simultaneously the compensation of harmonic currents, the reactive power, the imbalanced loads effects and the transfer of the active power supplied by photovoltaic array into the distribution grid.  The mathematical model for the current-voltage characteristic of the cell is:

System model and control approach Photovoltaic system modelling
where: : Photo generated current, : the saturation current, : Boltzmann"s constant, : Absolute temperature of the cell, m: Ideality factor of the diode [ ] with typical value .
[1] For a cell with a good quality (great value of ), the current-voltage equation (2) can be reduced to: The short-circuit current h and the voltage at open-circuit constitute two important characteristics of the cell, this voltage is given by: where is the thermodynamic voltage. Then, the current-voltage equation (3) of the cell becomes: Either for a module of serial cells: The equations giving the variation of the open-circuit voltage and the short-circuit current according to the irradiation and the cell temperature , relatively to standard conditions , are given by : [2].
[ ] The coefficient represents the correction factor of the open-circuit voltage according to the temperature. The typical value of this factor equals : (10) The cell temperature depends on the irradiation and the ambient temperature [2]. (11) where is approximately .
The characteristic of a photovoltaic cell, strongly dependon the solar irradiance and the temperature . Figure  3 shows that the current of a module is significantly influenced by irradiation variation while the voltage remains approximately constant. On the other hand, when the temperature changes, one can observe that the voltage varies considerably but the current remains almost constant under normal conditions (figure 4).  To locate and track this maximum power point, the method used in this work is the control by incremental conductance method [4] with proportional integral (PI) controller. The MPPT is obtained when: The term represents the instantaneous conductivity of the photovoltaic array, and represents the incrementterm of conductance.
The PI regulator minimizes the error and elaborates an adjustment of the duty cycle to control the DC-DC converter.
530 The estimation of the DC-link voltage reference value is based on the maximum power point search technique of the photovoltaic generator. The objective is to maintain a constant voltage on the DC-link under stable irradiation conditions or during changes of these atmospheric conditions. The MPPT algorithm adapts the reference voltage depending on the meteorological conditions. [5] Identifying harmonic currents by the instantaneous power method. To compensate the harmonic currents generated by the non-linear loads and the unbalanced currents [6] caused by the unbalanced loads, the technique used is aimed at controlling the three-phase inverter with detection of these polluting currents by the instantaneous power method. The first stage is to bring back the grid voltages and currents to the coordinate plane using the Concordia transformation.
According to equations (14) and (15), the instantaneous active powers and reactive are calculated: These instantaneous powers and can be composed in two components: the continuous components (DC) related to the fundamental components ̅ ̅ and the alternative components (AC) related to the harmonics ̃ ̃ as following [7]: A power filter is used to separate the harmonic"s power component from the fundamental power component. After separating these components, the disturbing currents in the reference is calculated by using expressions (16) and (17):

System without photovoltaic compensator
In a first stage, the system operates without compensation filter, the grid supplies to the non-linear loads an active power of and a reactive power of . Figures 8 and 9 illustrate the phase current waveform, the instantaneous active and reactive powers absorbed without photovoltaic compensator. The grid current with the RMS value matches the non-linear load current, the latter is characterized by a shape distortion, a frequency spectrum containing only odd order harmonics (non-multiple of three) and a total harmonic distortion rate ( figure 11).
532 Figure 9:-Instantaneous powers evolution without compensation  Following the start of the compensation system under an irradiation of , we observe that after a transitional time , the grid current (figure 11) becomes perfectly sinusoidal and is characterized by a phase-opposition with the corresponding voltage. This phase-opposition between voltage and current indicates the injection of active energy into the supply grid without exchange of the reactive power. This latter is completely compensated by the filter (figure 12).
For a load absorbing an active power of and a reactive power , the active power injected into the grid (marked by negative sign) becomes stable in a steady state at a value of while the reactive energy tends towards the zero value. This energy is perfectly compensated by the photovoltaic filter which generates all the active power absorbed by the load and that supplied into the grid (Figure 12).

System with photovoltaic filter under variable irradiation
In this part, an irradiation fluctuation cycle is applied and the behavior of the whole system is evaluated. Figure 15 shows the evolution of the voltage and the phase current, load current and compensator current during the variation of the solar irradiation. At constant and sufficient irradiation, the current provided to the grid remains almost sinusoidal before undergoing a decrease at due to the fact that the power supplied by the photovoltaic generator to the grid has decreased. In the case of low solar irradiation, the load current is supplied by the grid, it resumes its initial value when the irradiation is at its initial value.

Conclusion:-
The objective of this study is to optimize the performances of a compensation system made up of a parallel active filter and a photovoltaic generator connected to the distribution grid in order to compensate all disturbances generated by non-linear loads, such as the harmonics currents, the reactive currents and the unbalanced currents while transferring to the grid the power produced by the photovoltaic array.
The obtained results show the robustness of the control technique based on the identification of the different disturbances using the instantaneous powers method. This method allows a better control of the active and reactive instantaneous powers, as well as a significant improvement of the total harmonic distortion rate of the current even in the presence of several drifts such as the variation of the solar irradiation and the load.
Compared with other control techniques such as the hysteresis current control method that is characterized by its nonlinearity, this instantaneous power method is an excellent one to improve the quality of the energy in the electrical distribution grids while limiting the filter's repercussions on the photovoltaic station for various exploitation regimes.