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A Current-Forced Line-Commutated Inverter for Single-Phase Grid-Connected Photovoltaic Generation Systems.

I. INTRODUCTION

In the past, most houses using PV systems had not been connected to the local grid utility. In recent years, however, the number of houses having PV systems connected to the utility grid has increased significantly. The grid-connected PV systems can transfer generated the dc power directly to utility system consequently it usually does not require the battery bank to store energy on the contrary stand-alone PV systems [1]. This is accomplished by means of grid-connected inverters. Therefore, inverter technology has dramatically improved during the last decade [2]. This is reason that the studies concentrated on new, cheap and innovative inverter solutions, and so, the various the inverters and new system configurations have been proposed [3]. Two main tasks of inverter interfacing PV module for grid-connected applications are to convert the dc current obtained from the PV module to the good-quality sinusoidal current for grid and to control the output of PV modules so as to tracking maximum power point [4]. The THD of the injected current must be within the limits defined by the IEC61727 [5].

Grid-connected inverters can be categorized with respect to commutation type as line-commutated and self-commutated. The self-commutated inverter is generally composed of H-bridge and uses pulse-width modulated (PWM). It is achieved by using higher switching frequencies in order to form sinusoidal current lower harmonic distortion [6]. The self-commutated inverter computes their turn-on and turn-off timing by using possessing high-frequency switching devices such as MOSFETs and IGBTs [3]. The total losses of this inverter are more in comparison to the line- commutated inverters due to higher switching frequencies. However, the forward voltage drop of the IGBTs available on the market is higher than in comparison to the same ratings SCRs and thus the power handling capability and reliability of the LCI and are quite high with respect to MOSFET/IGBT [7]. Consequently, the LCI inverter would be more efficient than self-commutated inverter particularly in higher power ratings.

In contrast to self-commutated inverter, line-commutated inverter (naturally commutated converters) is realized by means of SCRs and, therefore, it does not require computing turn-off timing switching devices. Because when one SCR is turn-on; it cannot be turn-off unless the current flow in to be zero, turning-on another SCR only forced to its current to be zero. The line-commutated inverters were based on the technologies used in electrical drives from the beginning of the 1980s and generally used for induction motors [1]. Although they were popular in the past, nowadays they are not preferred due to the requirement of additional filters. The LCI is equipped with by four SCRs for single-phase system. The major advantages are having simple structure, cheapness and robustness. In addition to advantages of this inverter, it is automatically synchronized with utility grid thanks to the native advantage of self-latching property of SCRs that is possible only with SCR converters [8]. However, the main disadvantage of this system is to inject poor-quality current, which contain quite large harmonic content to the grid.

In recent years, studies that deal with LCI techniques in grid-connected PV generation systems are very few. In these papers, the power transfer from PV array to the grid has been accomplished by using the line-commutated inverter. However, they have some common drawbacks. The conventional LCI circuit has been used in these studies is explained in Section II and shown in Fig. 1. The PV array is connected through dc-link inductor to SCR bridge in the conventional LCI method. It is a very simple structure but its injected current waveform approximates a square wave and therefore the total harmonic distortion of output current ([THD.sub.i]) is quite high due to large harmonic content [8-14]. The THD of the current values are far higher than that determined by the international standards [6]. Consequently, the standard can be satisfied only by adding huge passive filters, which increases the size, weight, and cost of the system. Besides, the power factor cannot be kept at the constant value because the firing angle is used as control signal for tracking the maximum power point [8, 9, 11, 13, 14].

In the present paper, a current-forced LCI power electronic interface is developed and implemented for single-phase grid-connected PV generation systems. The proposed current-forced LCI is able to inject a sinusoidal current to the utility grid. This means that it is reduced the THD of the injecting current. It is succeeded that the THD of the injecting currents for the different firing angles/power factors and reference currents is about 5% or lower than 5%. THD values obtained satisfy the standard IEC61727 [5]. Thus, it is achieved without filter equipment in contrast to the conventional LCI system. The reference current signal is utilized to control power transfer from the PV array to the grid at the constant firing angle/power factor. Hence, the proposed system allows to maximum power tracking via the reference current instead of the firing angle. In addition, power factor can be controlled by changing firing angle of SCRs. Thus, the proposed inverter overcomes the drawbacks of the conventional LCI. It is connected to output of the PV array without dc/dc converter and has been accomplished to operate at the maximum power point of PV by controlling the reference sinusoidal current.

II. THE CONVENTIONAL LINE-COMMUTATED INVERTER

The conventional LCI circuits that reported in the literature have been used to transfer the dc power available at PV panels to the grid. The conventional LCI circuit for a single-phase is shown in Fig. 1. In this circuit, the firing angle [alpha] is kept at between 90[degrees] and 180[degrees] (90 < [alpha] < 180[degrees]), thus the average output voltage of SCR bridge [V.sub.d] becomes positive that is computed as given in (1). In the present case, the converter operates as a line-commutated inverter. This means that the power can be transferred from the PV generator to the grid.

[mathematical expression not reproducible] (1)

Where; [V.sub.m] is the peak value of grid phase voltage and [alpha] is firing angle.

These methods employ the firing angle as a control parameter in order to extract maximum power from PV array when that is directly connected to LCI. In two stage configurations, it is used a boost dc/dc converter at the output of the PV array. In that case, maximum power tracking can be performed at the side of dc/dc converter for constant firing angle/power factor. In order to power transfer to grid, the firing angle [alpha] has to be above 90[degrees]. Optimally, firing angle [alpha] can stretch up to 180[degrees], but practically, it should be less than 180[degrees] because of the turning off or commutation problem of SCR. This firing angle [alpha], experimentally stretch up to 165[degrees] for inversion operation to facilitate the commutation voltage for SCR. Thus, it is a slightly lagging power factor. However, the main drawback of the conventional line-commutated inverter is to inject non-sinusoidal current to the grid. The injecting current is approximately square-shaped that contains high harmonic content and furthermore, this current is uncontrolled. In this LCI circuits, the THD of the injecting current having square wave, is can be calculated as 48.4% for single-phase systems.

III. THE PROPOSED CURRENT-FORCED LCI

The proposed current-forced LCI circuit for single-phase grid-connected PV system consists of PV panels/array, sinusoidal current injection module, dc-link inductor, current sensor, a line-commutated inverter module, a controller, and step-up transformer, which is shown in Fig. 3. Moreover, operating principle with switching intervals of the circuit given Fig. 3 and the schematic diagram of the overall system scheme are shown in Fig. 4 and Fig. 5 respectively. The main characteristics of the proposed grid-connected PV generation system and PV panel specifications are given in Table I. The main function of the proposed LCI system is to control the current injected into the grid by means of hysteresis controller signals through the switches of the sinusoidal current injection module.

The proposed circuit improves the wave shape of injecting current the conventional LCI circuits. It controls dc-link inductor current [I.sub.L] in order to force the current injected into the grid [I.sub.grid]. To achieve this, [I.sub.L] has to follow a rectified sinusoidal reference waveform [I.sub.Lref] that is [alpha] out of phase with respect to grid voltage [V.sub.grid]. In this way, the current flow in dc link inductor [I.sub.L] can be regulated by switching S1 and S2. Thus, it can be injected the current in sinusoidal waveform and adjusted the magnitude of the current by controlling the reference current. Furthermore, it provided that maximum power tracking via the reference current instead of the firing angle. However, power factor can be adjusted by changing firing angle of SCRs.

Consequently, the proposed circuit, with hysteresis current controller, reduces the THD values of the injecting current without filtering equipment that used in the conventional LCI circuits in order to suppress harmonics. Furthermore, the desired power can be easily tuned and transferred into the utility grid.

The PV array can be connected to the grid through a step-up transformer or directly connected to the utility grid with large PV array. If the large PV array is used, it reduces the number of stages in grid-connected PV generation system [15]. In the proposed system, a step-up transformer (16.8/220 V) is preferred to allow study lower power. The firing angle is kept at a constant value for each SCR in fully controlled SCR bridge. A key component of the proposed current-forced LCI inverter is the controller. The controller generates the reference current and firing pulses for triggering circuit of the SCR. Moreover, it performs hysteresis current controller to inject a desired sinusoidal current to the grid and generates signals for the driving circuit of MOSFET. The proposed system is tested at the different firing angles. The software interrupt is used to keep the firing angle constant. The proposed current-forced LCI provides sinusoidal current comply with grid interface requirements.

A. PV array module

PV array module consists of PV array, a dc-link capacitor [C.sub.dc] and blocking diode. The parameters are given in Table I related to this module. The PV array consists of two 80-W panels (Sharp NE-80EJEA). The surface temperature of the panel is maintained at around 25 [degrees]C during the experiment. The PV array is obtained by means of connecting of PV panels in serial. Series connection of a PV panels in an array is necessary to get needed voltage ([V.sub.pv]), and for this study two panels is adequate because it should be higher the peak value of the voltage input of step-up transformer in order to inject desired grid current. The PV panels are exposed the different insolation conditions and P-V characteristics at this insolation conditions are given in Fig. 2. Subsequently, in order to verify the proposed current-forced LCI grid-connected PV generation system, simulations are performed by connecting the PV array that built in MATLAB/Power System Toolbox to input of the proposed power converter under these insolation conditions.

B. Relationship between the PV array power and injected power to the grid

The capacitor [C.sub.dc] provides energy storage necessary to stabilize instantaneous power transferred to the grid. It works as a buffer for absorbing the difference between PV array power and injected power to the grid [16]. The output voltage of PV array [V.sub.pv] ripples at twice the grid voltage frequency (2 x50=100 Hz) and it varies from [V.sub.pvmin] to [V.sub.pvmax] [17]. The magnitude of the voltage ripple [[DELTA]V.sub.pv] depends on the average power [P.sub.grid] and capacitance [C.sub.dc]. The average power generated from PV array [P.sub.pv] is calculated in (2) and if grid current [i.sub.grid] (t) is a out of phase with respect to grid voltage [v.sub.grid](t), the instantaneous value of active power injected into the grid [p.sub.grid] (t) is calculated according to (3),

[P.sub.pv]=[V.sub.pv][I.sub.pv] (2)

[mathematical expression not reproducible] (3)

[P.sub.pv]=[P.sub.grid]+[P.sub.losses] (4)

[P.sub.pv] [congruent to] [p.sub.grid] (5)

where [V.sub.grid] is the rms grid voltage, [I.sub.grid] is the rms fundamental grid current and a, which is defined by firing angle, is the displacement angle between grid voltage and current. Besides, [V.sub.mgrid] and [I.sub.mgrid] are the amplitude of grid voltage and the amplitude of the injected current, respectively. If it is neglected [P.sub.losses] in (4), then (5) is obtained.

The power oscillations are due to the energy that is charged and discharged in the capacitor. Thus, if [P.sub.pv] is higher than [p.sub.grid](t), capacitor [C.sub.dc] is charge up [V.sub.pvmax] from [V.sub.pvmin]. And in this interval, the instantaneous power associated with the capacitor [p.sub.C](t) is as given in (6) [18] and energy supplied to the [C.sub.dc] is calculated in (7) and (8),

[mathematical expression not reproducible] (6)

[mathematical expression not reproducible] (7)

[mathematical expression not reproducible] (8)

The voltage ripple at the PV array output [[DELTA]V.sub.pv] can be limited to a predefined value by connecting appropriate [C.sub.dc]. The proper [C.sub.dc] value can be calculated according to (10). Therefore, it is achieved approximate constant power extracted from the PV array, by keeping the output voltage of PV array [V.sub.pv] at the constant value with small ripples.

[mathematical expression not reproducible] (9)

[mathematical expression not reproducible] (10)

C. Sinusoidal current injection module

This module includes two MOSFETs (S1 and S2) and two power diodes (D1 and D2) as depicted in Fig. 3. S1 and S2 are switched in order to the sinusoidal current injected to the grid to follow a sinusoidal reference current waveform. So this module is operated in a current-regulated mode at high frequency on the purpose of that drawing current from PV array current to be same reference current ([I.sub.Lref]). In this module, the switches S1and S2 are operated at high switching frequency. The switching frequency varies with tolerance bandwidth. A DsPIC30F3011 microcontroller has been programmed for driving the MOSFETs via TLP250 optocoupler MOSFET driver circuit. It is depicted operating principle of the proposed system and shows switching signals for SCRs and MOSFETs in Fig. 4.

D. LCI module

The LCI module consists of a fully controlled SCR and step-up transformer as shown in Fig. 3. This module is operated in inverter mode by keeping the firing angle above 90[degrees] ([alpha]>90[degrees]). The circuit components of the LCI module are (T1, T2, T3, T4) SCRs (IXYS CS35) and MOC3021 optoisolator triac driver is used for SCR trigger circuit. The firing pulses for SCRs are generated in DsPIC30F3011 as a software interrupt and send to T1, T2, T3 and T4 SCRs via driving and isolation circuit with MOC3021.

E. ZCD (Zero Crossing Detection)

It is required to synchronize with the grid voltage in order to power transfer to the utility grid. To achieve this, the zero-crossing detection (ZCD) and a phase locked loop (PLL) techniques have been employed to sense zero-crossings of the grid voltage. In this study, the ZCD circuit is used to obtain reference signal with respect to the grid voltage because it is simpler than PLL [19]. This reference signal is exploited to generate reference inductor current [I.sub.Lref] and compute timing of firing pulses for SCRs. The fundamental sinusoidal waveform (internal tables) is produced in controller according to ZCD signals. It is summarized working of the complete system with simplified block diagram of the proposed system in Fig. 5. In this study, the ZCD signal has been applied to the hardware interrupt of DsPIC30F3011 in order to obtain the accurate reference current signal.

F. System controller

The aim of the system controller is to ensure the current injected into the grid by means of appropriate control signals. In grid-connected inverters, the current injected to the grid mainly is characterized by the inner current feedback loop. The controller has been programmed to build the reference current. In the controller, the current flow in dc-link inductor [I.sub.L] is forced to follow a rectified sinusoidal reference waveform [I.sub.Lref] that is [alpha] out of phase with respect to grid voltage [V.sub.grid].

The DsPIC30F3011 microcontroller is used as a system controller component. The switching times are defined in this controller according to hysteresis controller that it compares reference current and the real current measured from the line on dc-link inductor ([I.sub.L]). Thereafter, it generates appropriate control signals to the switches in the sinusoidal current injection module (S1 and S2). The current through dc-link inductor [I.sub.L] is shaped by means of switching MOSFETs (S1 and S2). The reference signal is utilized to control power transfer from the PV array to the grid as well as ensuring sinusoidal current. This means that the active power injected to the grid can be adjusted at the constant firing angle [alpha] of the LCI.

The reference current [I.sub.Lref] is internally self-generating a rectified sinusoidal output current based on internal tables in microcontroller as summarized in Fig. 5. [I.sub.Lref] is produced as a rectified sinusoidal waveform and [alpha][degrees] out of phase with [V.sub.grid]. The actual current through inductor [I.sub.L] is sampled and compared with the reference current [I.sub.Lref], and then it is determined the MOSFETs should be turned on by hysteresis controller. Following the reference current is achieved by using hysteresis controller. Building reference current, hysteresis controller and generating driving signals for the SCRs and MOSFETs are implemented in a DsPIC30F3011.

Hysteresis current controller reads actual inductor current [I.sub.L] and compares with reference inductor current [I.sub.Lref], then computes the difference. S1 and S2 are switched on and off at a high frequency to achieve the inductor current [I.sub.L] is restricted within a certain tolerance band that is predetermined is between [I.sub.Lref] - [[DELTA].sub.I] and [I.sub.Lref] + [[DELTA].sub.I]. Thus, the current through dc-link inductor [I.sub.L] can track rectified sinusoidal reference current by means of switching MOSFETs.

The switching frequency is a function of the hysteresis tolerance bandwidth. If tolerance band is chosen large, harmonic level in injecting sinusoidal current is lower but it needs high frequency switching. Here, to track the reference inductor current is applied the positive/negative dc-link voltage. Consequently, the inductor current [I.sub.L] restricted at predefined limit by controlling the switches S1 and S2. The hysteresis controller is simple so that it can be performed with a few lines of C-language program codes in the microcontroller. Accordingly, it needs less computation and allows save time to do other duties for microcontroller.

IV. SYSTEM PERFORMANCE

A. Simulation results

The simulation results are obtained using the proposed current-forced LCI grid-connected PV generation system MATLAB model. Fig. 6 shows the simulated grid voltage and grid / injected current waveforms of [V.sub.grid] and [I.sub.grid] at the different insolation levels and firing angles.

B. Experimental results

The proposed current-forced LCI grid-connected PV generation system has been built and investigated. The experimental setup of the proposed system is depicted in Fig. 8. It consists of PV array, a single-phase full controlled SCR-bridge, dc-link inductor, two switches MOSFETs (S1 and S2) and two the power diodes, a single-phase step-up transformer, the ZCD circuit, MOSFET and SCR driving circuits and the DsPIC30F3011 controller. The PV array consists of two 80-W panels (Sharp NE-80EJEA) connecting in series is connected input side of the proposed inverter that it can be seen from Fig. 8. The output of the proposed inverter is connected to a grid supply of 220 [V.sub.rms] and 50 Hz via 16.8/220 [V.sub.rms] step-up transformer.

The hysteresis controller has been implemented in the proposed system. The system controller generates driving signals for MOSFETs and firing signals for SCRs accordingly. The SCRs and MOSFETs switch via optical isolator. The proposed inverter is tested for different firing angle values that are chosen above 90[degrees] in order to facilitate inverter operation. The reference currents are computed with respect to insolation levels to extract maximum power from PV array. Finally, for different reference currents and firing angles are obtain results as in the simulation results and summarized in Table II. The experimental results are given in Fig. 7.

The experimental results show the grid voltage and the grid / injected current, [V.sub.grid] and [I.sub.grid] for the different the insolation levels and firing angles. It can be seen that experimental the results are in close agreement with the simulated results under the different insolation levels and the injected currents possess a high-quality sinusoidal wave. For instance, the THD of the injecting current to the grid for the [I.sub.Lref] = 5.05 A and [alpha] = 155[degrees] is obtained as 2.89% as can be seen in Table II. The reduction in the harmonic content of injected current can be observed in the frequency spectrum of Fig. 9. This current complies with the restrictions established by IEC61727.

The all of the THDs and power factors obtained for all reference currents/insolation conditions are listed in Table II. The THD values are about 5% or below 5% and thus they meet the grid-connected standards. In these results, the power factor values are lagging (inductive) since the firing angles are between 90[degrees] and 180[degrees]. The firing angle [alpha] should be between 180[degrees] and 270[degrees] in order to obtain leading (capacitive) power factor. The injected current [I.sub.grid] and grid voltage [V.sub.grid] have been measured by means of a Fluke 225CS scope meter and the harmonics of [I.sub.grid] is analyzed in FlukeView[R] Software.

V. CONCLUSION

In this paper, a simple power electronic interface based on the LCI for single-phase grid-connected PV generation systems was proposed. The proposed current-forced LCI is able to inject a sinusoidal current to the utility grid. This means that the harmonic contents were reduced in the injecting current. Hence, the obtained THD values of injected currents for the different firing angles and reference currents were about 5% or less than 5%. These values are below the limits determined by the IEC61727 without filter equipment in contrast to the conventional LCI system. The proposed inverter without dc/dc converter was connected to output of PV array. The reference sinusoidal current signal was utilized to control power transfer. Therefore, this allows the possibility of tracking the maximum power point of PV to extract maximum power from PV.

The entire grid-connected PV generation system has been modeled and simulated in MATLAB/Simulink. Moreover, the proposed system experimentally tested by a laboratory prototype. From simulation and experimental results, it is clear that the proposed inverter shows good performance under different insolation conditions and lagging power factors. It is observed from the experimental results in Table II that the THD of the injected current is very low without any harmonics compensation even at low power. The proposed single-phase LCI inverter topology is simple and a good alternative to the power transfer from PV generation systems to the utility grid.

Note that the proposed system, with current controller, is able to limit the dc-side current in the event that a fault occurs in the grid. However, in the future work, it is aimed to investigate the influence of the grid voltage variation, overcurrent and shortcurrents, etc.

ACKNOWLEDGMENT

This work is supported by the Scientific Research Projects Unit of Kocaeli University (grant no. 087-2010).

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Murat UNLU, Sabri CAMUR, Ersoy BESER, Birol ARIFOGLU

University of Kocaeli, 41380, Turkey

muratunlu@kocaeli.edu.tr

Digital Object Identifier 10.4316/AECE.2015.02011
TABLE I. MAIN CHARACTERISTIC OF THE PROPOSED SYSTEM

Parameter                                   Value

     The rated power of the PV panel        [P.sub.m] = 80 W
           Dc-link Capacitance              [C.sub.dc] = 22 mF
Photovoltaic panel (2 x series connection)  Sharp NE-80EJEA
 Short circuit current (1000 W/[m.sup.2],   [I.sub.sc] = 5.15 A
              25 [degrees]C)
 Open circuit voltage (1000 W/[m.sup.2],    [V.sub.oc] = 21.6 V
              25 [degrees]C)
       Maximum power point voltage          [V.sub.m] = 17.3 V
    (1000 W/[m.sup.2], 25 [degrees]C)
       Maximum power point current          [I.sub.sc] = 4.63 A
    (1000 W/[m.sup.2], 25 [degrees]C)
  Kv (Open circuit voltage coefficient)     -0.077/[degrees]C
  Ki (Short-circuit current coefficient)     0.003/[degrees]C
             Dc-link Inductor               L = 5 mH
 Internal resistance of dc-link inductor    r = 0.4 [OMEGA]
           Step- up transformer             16.8 / 220 V

TABLE II. EXPERIMENTAL RESULTS

              Parameter                [alpha]=130[degrees]

   Insolation level [W/[m.sup.2]]      280
           PV Current [A]                1.146
           PV Voltage [V]               36.1
 Active Power Generated from PV [W]     41.3
Active Power Transfer to the Grid [W]   26.7
  Injection Current to the Grid [A]      2.56
            Power Factor                 0.62
                                       (lagging)
     THD of the Grid Current [%]         5.18

              Parameter                [alpha]=145[degrees]

   Insolation level [W/[m.sup.2]]      450
           PV Current [A]                2.082
           PV Voltage [V]               35.4
 Active Power Generated from PV [W]     73.7
Active Power Transfer to the Grid [W]   50.9
  Injection Current to the Grid [A]      3.90
            Power Factor                 0.79
                                        (lagging)
     THD of the Grid Current [%]         2.90

              Parameter                [alpha]=150[degrees]

   Insolation level [W/[m.sup.2]]      330        560
           PV Current [A]                1.428      2.512
           PV Voltage [V]               36.0       36.9
 Active Power Generated from PV [W]     51.4       92.6
Active Power Transfer to the Grid [W]   34.5       61.7
  Injection Current to the Grid [A]      2.52       4.62
            Power Factor                 0.83       0.83
                                       (lagging)  (lagging)
     THD of the Grid Current [%]         5.30       2.08

              Parameter                [alpha]=155[degrees]

   Insolation level [W/[m.sup.2]]      640
           PV Current [A]                2.878
           PV Voltage [V]               36.9
 Active Power Generated from PV [W]    106.1
Active Power Transfer to the Grid [W]   76.0
  Injection Current to the Grid [A]      5.05
            Power Factor                 0.89
                                       (lagging)
     THD of the Grid Current [%]         2.89
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Author:Unlu, Murat; Camur, Sabri; Beser, Ersoy; Arifoglu, Birol
Publication:Advances in Electrical and Computer Engineering
Date:May 1, 2015
Words:5178
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