optimizationtheperformanceofasynchronizationcontrollerfora3- … · 2018-05-21 · synchronization...

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

GHITA BENNIS*, MOHAMMED KARIM*, AHMED LAGRIOUI**, MERYEM FELJA*

*LISTA Laboratory, Faculty of Sciences Dhar El Mahraz, Sidi Mohammed Ben Abdellah UniversityFès, MAROCCO

**Department of Electrical Engineering And Computer Engineering, National School of Arts andMétiers, University Moulay Ismail Meknes – MAROCCO

Abstract: - In a distributed generation system, divers renewable agents are connected to the low voltage 3 phaseutility grid by an inverter which is used as power condition and must assurance the higher efficiency of therenewable agent. To achieve this level of efficiency, a unitary power factor between the utility grid voltagesand the inverter currents is necessary, and a synchronization algorithm is required for the perfectsynchronization between the 3-phase utility grid and the renewable agent. The aim of this paper is to presentthe optimization of the performance of a Synchronization controller for a 3-phase photovoltaic grid-connectedsystem, assessing its accuracy under different conditions and studying their drawbacks and advantages. A gridconnected photovoltaic system with a nominal power of 5 kW is used so as to assess the behavior of thesynchronization algorithm when the 3 phase utility grid is affected by some disturbances such as voltageunbalances

Key-Words: - PV Array, PSD, PLL, Inverter, MPPT,...

1 Introduction

Solar energy has become the most popularrenewable energy source where in energy isextracted directly from sun employing photovoltaicmodules [1-4]. The power electronics converters,variable nonlinear loads and voltage regulatorsinject harmonics to the grid and are accountable forserious power quality problems [5]. Indeed, thereare a number of outstanding problems mainlyrelated to the power quality, such as reactive powercompensation, power factor, harmonics and voltageregulation in a photovoltaic system connected togrid [6]. The global performance of the total systemgets affected and it becomes a serious concern forthe final users. Any integration of renewable energysources to the grid has to meet standard powerquality requirements. The power quality expectedfrom distributed generations has been the subject ofdiscussion and standardization. In a Photovoltaicsystem, the signal quality is of particularlypreoccupied due to high proportion of nonlinear andsingle-phase loads. moreover, switching of a single-phase load can depict a large transient leading to sag

and swell of voltage signal. generally, the gridconnected Photovoltaic system fails to controlharmonic currents and the reactive power drawn bythe nonlinear load. mostly passive filters are used tocontrol the harmonics currents generated. But due todivers disadvantages like series/parallel resonance,these filters have been changed and also these filtersare not suitable for certain loads [7]. Active powerfilter is a better choice which ameliorates harmoniccompensation features of the passive filter. In thisworks new control algorithms have been designedfocusing on increasing the performance of theconnection of primary renewable energy agents tothe low voltage 3 phase utility grid. It is essential anappropriated control of the power factor of theinverter grid connection to get the maximumefficiency in the photovoltaic agent, and thesynchronization algorithm will be one of the aimmodules in detecting the phase angle of the 3 phaseutility grid voltages with optimal dynamic response.the Synchronous Reference Frame Phase LockedLoop (dqPLL method) is the classicalsynchronization algorithm, in view of it is easy toimplement, but it is also very sensible to gridvoltage unbalances leading to errors when the

WSEAS TRANSACTIONS on SYSTEMS and CONTROLGhita Bennis, Mohammed Karim,

Ahmed Lagrioui, Meryem Felja

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frequency and phase are detected. For this, a bigamount of studies has been carried out in this area inorder to discover a solution to this fact, as may befound in [8–14], the most of them showing aperspective of how to solve this issue when thedetection of the frequency is conducted.

2 GRID Connected Photovoltaic System

This document focuses on the photovoltaic systemsgranted to the network, an "intelligent" voltagecontrol system integrated in PV inverters isproposed. It ensures that the voltage inverters of PVinverters can withstand the voltage dips on thenetwork. The proposed system decrease connectioncosts and upgrade the performance of grid-connected PV inverters. On the other hand, a control/ control system for limiting overvoltages of the DCbus of the PV inverters is developed. This systemavoids failures and connections of PV systems in theevent of a short circuit [3].

3 Power Subsystem

The power subsystem is constituted by thePhotovoltaic array, a converter (DC-DC) controllerby MPPT, and inverter (DC-AC) and the LCL filter.ensuing, a brief description of each block isdeveloped.

3.1. Photovoltaic modules

A photovoltaic generator consists of anelementary PV cell assembly mounted in series and/ or parallel to obtain desired electricalcharacteristics such as power, short-circuit currentand open voltage.This generator allows a suitable conversion of solarenergy into solar electric energy in the form oftesion and direct current, variable according to theinfluence of temperature and illumination [4].

3.2. The converter controller by MPPT

The boost converter is used not only to uplift thePhotovoltaic array output voltage, but also to realizePerturb and Observe (P&O) scheme in MaximumPower Point Tracking (MPPT) [5,25]. The boostconverter connected to Photovoltaic array, foreverworks in a continuous current mode and the currentripple is reduced by using a large value of inductor.A low value of capacitor at output acts as filter.

3.3. The inverter

The inverter acts as a control of the output currentand indirectly of the DC bus voltage. The controldiagram of the inverter is shown in Figure 1, itconsists of two parts with two different roles:control of the output current (internal loop) andcontrol of the DC bus voltage (external loop) via thecommand of The output power of the inverter [6].The SVM and PWM techniques are employed tocontrol the grid signals of the commutator accordingto the average voltage and the current references [7].

Figure 1. Block diagram of the power and control subsystems for the3-phase grid-connected PV system.

ip = iclink+ icc

icc= Su.iu + Sv.iv + Sw.iw (1)

iclink = cclink (dVcc/st)

Ppv= ip Vp

Where vp, ip are the voltage and the output currentof the photovoltaic generator, respectively, PPV isthe power available for a specific irradiation, Vcc isthe DC bus voltage, link ic is the current through thelink capacitor Cclink, And icc is the currentdelivered to the phase 3 VSI (which is a function ofthe line currents iu, iv, iw and the states of thepower poles Su, Sv, Sw (1: -upper Pole, If the lowerpole in the 3-phase VSI.) as a result of the fact thatthe voltage in the capacitor is roughly the same asthe voltage in the three-phase grid, the dynamics inthe ac of the inverter expressed. in a vector thefollowing way is the following:

(2)

Where U is the voltage of the inverter, i: line currentof the inverter, Uac: vectors of the service gridvoltage space, L: line inductance, R: resistance. Byexpressing the last vector equation with its



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components d and q in the dq axes using thetransformation of the Park vector [9,16], theinstantaneous active power (p) and the instantaneousreactive power (q) can be expressed as follows [8,16]:

P= UACd id + UACq iq (3)

q = UACd id - UACq iq (4)

Where UACd, UACq, identifies the d-q mechanismsof the three-phase voltages and currents,respectively, permitting decoupled control of theinstantaneous active and reactive powers if thevector Uac is aligned with the d-axis (UACq = 0).

3.4. LCL Filter

In order to be able to connect the voltage inverter inparallel with the mains, and to make it work as acurrent source, it is necessary to use an inductiveconnection filter (L or LCL). Whatever the flterused for the connection, one will always have thesame schema equivalent: a controlled source(discontinuous alternative in the case of thetopology L, is quasi sinusoidal with the topologyLCL) which is known to the network through aninductor [10-16].

3.5. EMI filter

In a grid-connected renewable agent, it is essentialto take into consideration, the harmonic pollutiondue to the Electromagnetic Interference (EMI).These EMIs are generate, by the commutation ofsemiconductor electronic devices (IGBTs anddiodes) [20] and an EMI filter is necessary to reduceit. There are various methodologies to design anappropriate EMI filter, some of them are founded ontrial an error [17,18], and some novel methodologiesare cited in several publications, including [17,19].

4 Control Subsystem

The control subsystem is formed by the PIregulators, PSD synchronization, and the and theUuvw generation.

4.1. PI REGULATORS

A three-phase PV system is modeled by a currentinjector with its power regulation. The controlsystem regulates the power injected by the PVsystem 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 atthe point of connection of the injector, the activeand reactive powers which regulate it aredetermined. These powers are controlled by simpleProportional-Integral type correctors (Kp + Ki / p).The current references are then calculated in thePark repository by the formula:

(5)

(6)

Where P and Q are the reference powers of the PVsystem. Vdet Vq are the direct and quadraturecomponents of the voltage, measured at theconnection point of the PV system, in the Parkrepository. Idet Iq are the direct and inquadraturecomponents of the reference product current by thePV system on the network to which it is connected.These currents therefore depend on the powerrequirements and on the voltage measured at thepoint of connection of the production. A PhaseLocked Loop (PLL) is used to synchronize the Parktransformation to the pulsation of the measuredvoltage across the network. Thus, when the systemis in steady state, the direct component Vden outputfrom the Park transformation is an image of theamplitude of the measured voltage and the quadraticcomponent Vq is zero. These currents are thenconverted into the three-phase frame of reference[20]. The amplitude and the phase shift of thecurrents injected into the network will thus regulatethe powers to their set value. The limit for thecomponent Id is chosen as a function of themaximum output current of the inverter and thepower limit of the DC source (for example Idmax =1.1 In). The limit for the Iq component is chosenaccordingly, so as not to exceed the selectedreactive power limit (eg Q / P = 0.4). Two PIcorrectors are in charge of regulating the active andreactive powers to their setpoint. Thus, there are twoloops: the loop for regulating the active power andthe loop for reactive power. Considering that theselected Park reference frame rotates at the voltagepulse, then it is possible to set Vq = 0 and Vd =Vmax. Thus, the active power control loop can bemodeled as shown in Figure 2 (b) with ε which



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represents the difference between the setpoint powerand the measured power (error term). Byconsidering Vq = 0 and Vd constant [21].

Figure 2. (a,b,c,) bloc diagram of controller for 3-phase VSI for PVsystem

The design of the Phase Locked Loop gain is acritical point within this process. The closed looptransfer function of the dqPLL is giving by:

H(s)= ( KpS+Ki) / (S² + KpS+Ki) (7)

where Kp is the proportional gains employed PIregulator, Ki: integral gains of the employed PIregulator. Eq. (7) is a second order transfer function,similar to (8).

G(s)= (2ξω0S + ω0²) / (S²+2ξω0S + ω0²) (8)

where ω0 is the natural angular frequency, ξ: is thedamping factor. from equating (7) and (8):

Kp = 9.2 / Ts (9)ω0 = Kp / 2ξ (10)Ki = ω0² (11)

In this work, the dqPLL gains are calculated so as toobtain TS= 0.05 ms (one 50 Hz cycle) allowing afast response of the algorithm when a variation ofthe nominal frequency occur, and a damping factorξ= 0.871 is chosen. Equating (9)– (11), anothervariable can be computed: Kp= 60, ω0= 37.41 rad/sand Ki= 1400. The value give of ω0 guarantees atrade-off between the dynamic, the stability andharmonic rejection capability of the dqPLLalgorithm.

The PLL method is very sensitive to the grid voltageunbalance [22], which also produces second orderharmonics in d–q synchronous reference frame dueto the effect of the inverse sequence, in fact, thesensors to be utilized can introduce second orderharmonics due to accuracy errors. moreover, the 3phase low voltages of the utility grid could becontaminated with harmonics and assumed byvariations of the fundamental frequency. Aresolution to the problems caused by the unbalance3 phase utility grid voltages is adding a PSD block,which is founded on the symmetrical componentsmethod [23]. In order to have a good performance ofthe PLL algorithm, it is possible to decompose theunbalanced 3-phase utility network voltages intonegative, positive and zero sequences. In timedomain, the instantaneous positive sequencecomponent V+abc of a voltage vector is given by[24]:

Figure 3. Block diagram of the PSD + dqPLL synchronizationalgorithm.

Va+(t) = 0.5 Vc(t)-((1/6)(Vb(t)+Vc(t))-(1/2√3)Sgg(Vb(t)-Vc(t)) (12)

Vb+(t) =-( Va+(t) + Vc+(t) ) (13)Vc+(t) = 0.5 Vb(t)-((1/6)(Va(t)+Vb(t))-(1/2√3)Sgg(Va(t)-Vb(t)) (14)where S90 is a 90-degree phase-shift operator can bedesigned with the following transfer function:HS90 (S) = (1- (s / ω0)) / (1+ (s / ω0)) (15)

By adding the PSD block with Eqs. (11)– (13) to thedqPLL structure, a PSD + dqPLL synchronizationalgorithm able to extract the positive sequence ofthe 3-phase utility grid voltagesis obtained, and then,a reliable detection of the positive sequence of thefrequency and phase will be achieved when voltageunbal-ances occur. A possible inconvenience, of the

(a) bloc diagram of PSD

(b) bloc diagram of Vdc regulator

(c) bloc diagram of current regulator



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PSD can be observed in Equation (14). The S90phase-shift operator has been implemented using anon-adaptive nominal angular frequency ω0,making this filter sensibleto frequency variations ofthe utility grid voltages, which will induce to a smalldegradation.

4.2. Vuvw REFERENCEGENERATOR

The reference voltage signal for generating thepulses is generated in Vuvw reference generatordiagram, which is shown in figure 4 below:

Figure 4. Vuvw reference generator diagram

5 Case of Study

A Photovoltaic grid connected system of 5 kW ofnominal power at standard conditions (1000 W / m2and 25◦C) will be studied in order to evaluate thecontrol algorithms performance.MPPT is connected in the boost converter using thePerturb and Observe 'P & O' technique. Theelaborate model includes the following components:

• Photovoltaic array delivering a maximum of5 kW at 1000 W/m^2 sun irradiance.

• Boost converter (DC-DC)increasing voltagefrom Photovoltaic natural voltage(273 V DC atmaximum power) to 500 V DC.

Two control loops are used to control the voltagesource converter t: an internal loop which allowsregulated Iq and Id, and an external loop whichcontrols 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 usea fast sample time of one microsecond in order toobtain an appropriate resolution of PWMwaveforms.

6. Results and Analysis

Figure 5 shows the SIMULINK model of theproposed sys-tem, where a discrete model is usedfor the Plant with a sample time of 50 us.

Figure 5. SIMULINK model of the grid-connected PV system.

The boost converter amplifies the DC voltage from273.5V to 500 V. To obtain the maximum power weused an MPPT command based on the P & Oalgorithm, we varied D. In order to control thesystem, two control loops have been used, anexternal loop which makes it possible to regulate thedirect voltage to the grid currents Id and Iq. Theoutput of the external controller represents anidentity current reference. Iq is the current referenceset to zero to maintain the unit power factor.Thecontrol system as well as the synchronization unituses a sampling time of 100 controller.

Running Simulink on the MATLAB platform for aperiod of 3 seconds the following is observed(Figure 6).

From t=0 s to t= 0.05 s, pulses to DC-DC and VSCconverters are blocked. PV voltage corresponds toopen-circuit voltage (Nser*Voc=5*64.2=321 V, seeV trace on Scope Boost Figure 6). The three levelbridge function as a diode rectifier and DC linkcapacitors are charged above 500 V (see Vdc_meastrace on Scope VSC).

At t=0.05 s, Boost and VSC converters are deregulated at Vdc=500V. Duty cycle of DC-DCconverter is fixed (D= 0.5 figure 7) and sunirradiance is fixed to 1000 W/m2.

Stable at t=0.25 s. Resulting Photovoltaic voltage istherefore V-PV = (1 0.5)*500=250 V (see V traceon Scope Boost). The Photovoltaic array outputpower is 4.5 kW (see Pmean trace on Scope Boostconverter).

At t = 0.4 s, the maximum power point tracker isactivated. At time D = 0.453. At t = 0.6 s, the MPPis obtained (Pmax = 5KW).



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From t = 0.7 s to t = 1.2 s, the irradiance of the sunis reduced from 1000 W / m2 to 250 W / m2. theMaximum power point tracker continues trackingmaximum power. At t=1.2 s when irradiance hasdiminished to 250 W/m2, duty cycle is D=0.485.Corresponding PV voltage and power are Vmean=255 V and Pmean=1.2 kW. Note that the MPPTcontinues tracking maximum power during this fastirradiance.

From t=1.5 s to 3 s various irradiance changes areapplied in order to illustrate the nice performance ofthe MPPT controller.

In Figure 7 present the reponse of voltage sourceconverter. A typical situation in Photovoltaicsystems is a variation of the solar irradiance over thePhotovoltaic modules due to clouds or a sunny day:Figure 8 and figure 9. shows the time simulation ofa variation in the incoming irradiance. for that, astep in the output current of the Photovoltaicgenerator is exerted at 0.05 s from a 50% up tonominal conditions with constant dc bus voltagereference.

FIG. 9 shows the evolution of the utility networkcurrent time in phase 1, thus increasing the currentcan be obseved.

The simulation of the time evolution of the detectedfrequency and phase, utilizing the dqPLL and thePSD + dqPLL synchronization algorithms areshown in Figure11. The rms value of the 3-phaseutility grid voltage is Vrms= 500 V (phase-to-phase)and a step of frequency from 50 Hz to 60 Hz isexerted at 0.05 s. The frequency detection by twoalgorithms is shown in Figure 11; even though theS90 filter has been elabored for a nominal frequencyof 50 Hz, a similar response to the dqPLL isatteinted. admissible phase detection is attained bythe PSD + dqPLL, but it must be pointed out that asmall lag between the detected phases can beobserved.

Figure 6. Waveforms of Boost Converter: Ir, Pmean,Vpv, D

Figure 7. Response of Voltage Source Converter

Figure 8. Response of Voltage Source Converter Waveform forModulation Index and Inverter

Figure 9. grid voltage and curent

Figure 10. Synchronized Grid Power

Figure 11. Time evolution of the detected frequency and phase when astep of frequency is exerted.



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7 Conclusion

In the present study, we have modeled andsimulated all the equipment representing the PVsystem connected to the electrical network, controland connection of the electrical network is ensuredby the PSD algorithm plus the synchronizationalgorithm (dqPLL) The performance of thisalgorithm evaluates the synchronization behavior,introducing several perturbations in the three-phaseservice grid.The proposed Photovoltaic system can provideenhanced active power smoothing and expandedreactive power compensation. A developed dual-stage DFT PLL was developed and implemented bythe Photovoltaic System.The validation of the results is obtained by theresponses of the tensions, currents and powerswhich are obtained by simulation which are close tothose presented in the literature, which makes itpossible to validate our work.

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