Optimization the Performance of a Synchronization Controller For a 3- Phase Photovoltaic Grid-Connected System Using the PSD Algorithm

In a distributed generation system, divers renewable agents are connected to the low voltage 3 phase utility grid by an inverter which is used as power condition and must assurance the higher efficiency of the renewable agent. To achieve this level of efficiency, a unitary power factor between the utility grid voltages and the inverter currents is necessary, and a synchronization algorithm is required for the perfect synchronization between the 3-phase utility grid and the renewable agent. The aim of this paper is to present the optimization of the performance of a Synchronization controller for a 3-phase photovoltaic grid-connected system, assessing its accuracy under different conditions and studying their drawbacks and advantages. A grid connected photovoltaic system with a nominal power of 5 kW is used so as to assess the behavior of the synchronization algorithm when the 3 phase utility grid is affected by some disturbances such as voltage unbalances Key-Words: PV Array, PSD, PLL, Inverter, MPPT,...


Introduction
Solar energy has become the most popular renewable energy source where in energy is extracted directly from sun employing photovoltaic modules [1][2][3][4]. The power electronics converters, variable nonlinear loads and voltage regulators inject harmonics to the grid and are accountable for serious power quality problems [5]. Indeed, there are a number of outstanding problems mainly related to the power quality, such as reactive power compensation, power factor, harmonics and voltage regulation in a photovoltaic system connected to grid [6]. The global performance of the total system gets affected and it becomes a serious concern for the final users. Any integration of renewable energy sources to the grid has to meet standard power quality requirements. The power quality expected from distributed generations has been the subject of discussion and standardization. In a Photovoltaic system, the signal quality is of particularly preoccupied due to high proportion of nonlinear and single-phase loads. moreover, switching of a singlephase load can depict a large transient leading to sag and swell of voltage signal. generally, the grid connected Photovoltaic system fails to control harmonic currents and the reactive power drawn by the nonlinear load. mostly passive filters are used to control the harmonics currents generated. But due to divers disadvantages like series/parallel resonance, these filters have been changed and also these filters are not suitable for certain loads [7]. Active power filter is a better choice which ameliorates harmonic compensation features of the passive filter. In this works new control algorithms have been designed focusing on increasing the performance of the connection of primary renewable energy agents to the low voltage 3 phase utility grid. It is essential an appropriated control of the power factor of the inverter grid connection to get the maximum efficiency in the photovoltaic agent, and the synchronization algorithm will be one of the aim modules in detecting the phase angle of the 3 phase utility grid voltages with optimal dynamic response. the Synchronous Reference Frame Phase Locked Loop (dqPLL method) is the classical synchronization algorithm, in view of it is easy to implement, but it is also very sensible to grid voltage unbalances leading to errors when the frequency and phase are detected. For this, a big amount of studies has been carried out in this area in order to discover a solution to this fact, as may be found in [8][9][10][11][12][13][14], the most of them showing a perspective of how to solve this issue when the detection of the frequency is conducted.

GRID Connected Photovoltaic System
This document focuses on the photovoltaic systems granted to the network, an "intelligent" voltage control system integrated in PV inverters is proposed. It ensures that the voltage inverters of PV inverters can withstand the voltage dips on the network. The proposed system decrease connection costs and upgrade the performance of gridconnected PV inverters. On the other hand, a control / control system for limiting overvoltages of the DC bus of the PV inverters is developed. This system avoids failures and connections of PV systems in the event of a short circuit [3].

Power Subsystem
The power subsystem is constituted by the Photovoltaic array, a converter (DC-DC) controller by MPPT, and inverter (DC-AC) and the LCL filter. ensuing, a brief description of each block is developed.

Photovoltaic modules
A photovoltaic generator consists of an elementary PV cell assembly mounted in series and / or parallel to obtain desired electrical characteristics such as power, short-circuit current and open voltage. This generator allows a suitable conversion of solar energy into solar electric energy in the form of tesion and direct current, variable according to the influence of temperature and illumination [4].

The converter controller by MPPT
The boost converter is used not only to uplift the Photovoltaic array output voltage, but also to realize Perturb and Observe (P&O) scheme in Maximum Power Point Tracking (MPPT) [5,25]. The boost converter connected to Photovoltaic array, forever works in a continuous current mode and the current ripple is reduced by using a large value of inductor. A low value of capacitor at output acts as filter.

The inverter
The inverter acts as a control of the output current and indirectly of the DC bus voltage. The control diagram of the inverter is shown in Figure 1  (1) Where vp, ip are the voltage and the output current of the photovoltaic generator, respectively, PPV is the power available for a specific irradiation, Vcc is the DC bus voltage, link ic is the current through the link capacitor Cclink, And icc is the current delivered to the phase 3 VSI (which is a function of the line currents iu, iv, iw and the states of the power poles Su, Sv, Sw (1: -upper Pole, If the lower pole in the 3-phase VSI.) as a result of the fact that the voltage in the capacitor is roughly the same as the voltage in the three-phase grid, the dynamics in the ac of the inverter expressed. in a vector the following way is the following: (2) Where U is the voltage of the inverter, i: line current of the inverter, Uac: vectors of the service grid voltage space, L: line inductance, R: resistance. By expressing the last vector equation with its components d and q in the dq axes using the transformation of the Park vector [9,16], the instantaneous active power (p) and the instantaneous reactive power (q) can be expressed as follows [8,16]: Where UACd, UACq, identifies the d-q mechanisms of the three-phase voltages and currents, respectively, permitting decoupled control of the instantaneous active and reactive powers if the vector Uac is aligned with the d-axis (UACq = 0).

LCL Filter
In order to be able to connect the voltage inverter in parallel with the mains, and to make it work as a current source, it is necessary to use an inductive connection filter (L or LCL). Whatever the flter used for the connection, one will always have the same schema equivalent: a controlled source (discontinuous alternative in the case of the topology L, is quasi sinusoidal with the topology LCL) which is known to the network through an inductor [10][11][12][13][14][15][16].

EMI filter
In a grid-connected renewable agent, it is essential to take into consideration, the harmonic pollution due to the Electromagnetic Interference (EMI). These EMIs are generate, by the commutation of semiconductor electronic devices (IGBTs and diodes) [20] and an EMI filter is necessary to reduce it. There are various methodologies to design an appropriate EMI filter, some of them are founded on trial an error [17,18], and some novel methodologies are cited in several publications, including [17,19].

Control Subsystem
The control subsystem is formed by the PI regulators, PSD synchronization, and the and the Uuvw generation.

PI REGULATORS
A three-phase PV system is modeled by a current injector with its power regulation. The control system regulates the power injected by the PV system into the connection node as a function of the sunshine.
The operation of this model can be designated by (FIG. 1): from the voltages and currents measured at the point of connection of the injector, the active and reactive powers which regulate it are determined. These powers are controlled by simple Proportional-Integral type correctors (Kp + Ki / p). The current references are then calculated in the Park repository by the formula: (5) Where P and Q are the reference powers of the PV system. Vdet Vq are the direct and quadrature components of the voltage, measured at the connection point of the PV system, in the Park repository. Idet Iq are the direct and inquadrature components of the reference product current by the PV system on the network to which it is connected. These currents therefore depend on the power requirements and on the voltage measured at the point of connection of the production. A Phase Locked Loop (PLL) is used to synchronize the Park transformation to the pulsation of the measured voltage across the network. Thus, when the system is in steady state, the direct component Vden output from the Park transformation is an image of the amplitude of the measured voltage and the quadratic component Vq is zero. These currents are then converted into the three-phase frame of reference [20]. The amplitude and the phase shift of the currents injected into the network will thus regulate the powers to their set value. The limit for the component Id is chosen as a function of the maximum output current of the inverter and the power limit of the DC source (for example Idmax = 1.1 In). The limit for the Iq component is chosen accordingly, so as not to exceed the selected reactive power limit (eg Q / P = 0.4). Two PI correctors are in charge of regulating the active and reactive powers to their setpoint. Thus, there are two loops: the loop for regulating the active power and the loop for reactive power. Considering that the selected Park reference frame rotates at the voltage pulse, then it is possible to set Vq = 0 and Vd = Vmax. Thus, the active power control loop can be modeled as shown in Figure 2 (b) with ε which represents the difference between the setpoint power and the measured power (error term). By considering Vq = 0 and Vd constant [21].
where Kp is the proportional gains employed PI regulator, Ki: integral gains of the employed PI regulator. Eq. (7) is a second order transfer function, similar to (8).
The PLL method is very sensitive to the grid voltage unbalance [22], which also produces second order harmonics in d-q synchronous reference frame due to the effect of the inverse sequence, in fact, the sensors to be utilized can introduce second order harmonics due to accuracy errors. moreover, the 3 phase low voltages of the utility grid could be contaminated with harmonics and assumed by variations of the fundamental frequency. A resolution to the problems caused by the unbalance 3 phase utility grid voltages is adding a PSD block, which is founded on the symmetrical components method [23]. In order to have a good performance of the PLL algorithm, it is possible to decompose the unbalanced 3-phase utility network voltages into negative, positive and zero sequences. In time domain, the instantaneous positive sequence component V+abc of a voltage vector is given by [24]: where S90 is a 90-degree phase-shift operator can be designed with the following transfer function: HS90 (S) = (1-(s / ω0)) / (1+ (s / ω0)) (15) By adding the PSD block with Eqs. (11)-(13) to the dqPLL structure, a PSD + dqPLL synchronization algorithm able to extract the positive sequence of the 3-phase utility grid voltagesis obtained, and then, a reliable detection of the positive sequence of the frequency and phase will be achieved when voltage unbal-ances occur. A possible inconvenience, of the (a) bloc diagram of PSD (b) bloc diagram of Vdc regulator (c) bloc diagram of current regulator PSD can be observed in Equation (14). The S90 phase-shift operator has been implemented using a non-adaptive nominal angular frequency ω0, making this filter sensibleto frequency variations of the utility grid voltages, which will induce to a small degradation.

Vuvw REFERENCE GENERATOR
The reference voltage signal for generating the pulses is generated in Vuvw reference generator diagram, which is shown in figure 4 below: Two control loops are used to control the voltage source converter t: an internal loop which allows regulated Iq and Id, and an external loop which controls the DC link voltage to +/-250 V. Iq is set to zero to maintain the unit power factor.
Pulse generators of DC-DC and VSC converters use a fast sample time of one microsecond in order to obtain an appropriate resolution of PWM waveforms.  The boost converter amplifies the DC voltage from 273.5V to 500 V. To obtain the maximum power we used an MPPT command based on the P & O algorithm, we varied D. In order to control the system, two control loops have been used, an external loop which makes it possible to regulate the direct voltage to the grid currents Id and Iq. The output of the external controller represents an identity current reference. Iq is the current reference set to zero to maintain the unit power factor.The control system as well as the synchronization unit uses a sampling time of 100 controller.

Results and Analysis
Running Simulink on the MATLAB platform for a period of 3 seconds the following is observed (Figure 6).  From t=1.5 s to 3 s various irradiance changes are applied in order to illustrate the nice performance of the MPPT controller.
In Figure 7 present the reponse of voltage source converter. A typical situation in Photovoltaic systems is a variation of the solar irradiance over the Photovoltaic modules due to clouds or a sunny day: Figure 8 and figure 9. shows the time simulation of a variation in the incoming irradiance. for that, a step in the output current of the Photovoltaic generator is exerted at 0.05 s from a 50% up to nominal conditions with constant dc bus voltage reference.
FIG . 9 shows the evolution of the utility network current time in phase 1, thus increasing the current can be obseved.
The simulation of the time evolution of the detected frequency and phase, utilizing the dqPLL and the PSD + dqPLL synchronization algorithms are shown in Figure11. The rms value of the 3-phase utility grid voltage is Vrms= 500 V (phase-to-phase) and a step of frequency from 50 Hz to 60 Hz is exerted at 0.05 s. The frequency detection by two algorithms is shown in Figure 11; even though the S90 filter has been elabored for a nominal frequency of 50 Hz, a similar response to the dqPLL is atteinted. admissible phase detection is attained by the PSD + dqPLL, but it must be pointed out that a small lag between the detected phases can be observed.