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Design and simulation of a unified power quality conditioner fed by solar energy
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 1
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Design and simulation of a unified power quality conditioner fed by solar energy Yahia Bouzelata a, Erol Kurt b,*, Rachid Chenni a, Necmi Altın b a
University Constantine, Faculty of Technology, Department of Electrotechnic, MoDERNa Laboratory, Constantine 25000, Algeria b Gazi University, Faculty of Technology, Department of Electrical and Electronics Engineering, 06500 Teknikokullar, Ankara, Turkey
article info
abstract
Article history:
Presently, quality problems in grid-integrated applications take great interest because of
Received 7 January 2015
the growing applications in power electronics. The elimination of the harmonics in the grid
Received in revised form
and the usage of clean energy resources in the power electronics applications become
15 February 2015
popular world widely. In the present paper, a unified power quality conditioner fed by solar
Accepted 21 February 2015
energy which can also export active power to the grid is proposed. The conditioner uses a
Available online xxx
photovoltaic (PV) system and its topology is made up of a hybrid active power filter combination. This combination bases on a parallel active power filter, which shares a common
Keywords:
DC voltage assured by the photovoltaic system with a serial active power filter. According
Power quality
to the analyzes, the proposed unified power quality conditioner eliminates both the supply
Unified power quality conditioner
current distortion caused by a non-linear load and the load voltage distortion introduced
PV
after adding fifth and seventh harmonics to the AC main voltage. In addition, the proposed
THD active power filter
unified power quality conditioner exports the photovoltaic power to the grid using a boost converter, perturbed and observed maximum power point tracking algorithm, compensates the reactive power and filters the current and voltage harmonics confirmed by the total harmonic distortion values, such as 4.76% and 3.86%, respectively. The design and the analyses have been performed with MATLAB/Simulink software. The simulation system determines the performances of such system and offers future perspectives on unified power quality conditioners. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The renewable energy resources are the best solutions for the clean energy requirement of the recent communities. Due to its wide application areas, easy construction and maintenance, the photovoltaic (PV) solar energy is the most
promising green energy field [1e3]. Thus, the numerous examples of successfully deployed PV systems are already available for commercial usages and also technical researches [4e6]. The PV based solar energy explorations have been improved further by focusing on their grid integration. In fact, the improvements in power electronics technology have accelerated the usage of PV supplied inverter systems as
* Corresponding author. E-mail address: [email protected] (E. Kurt). http://dx.doi.org/10.1016/j.ijhydene.2015.02.077 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Nomenclature Ipv I0 q kB THD Vm u p p p~ PWM IGBT T a Rs Rsh Vdc K * q q q~ GTO LPF
photocurrent, A diode saturation current, A coulomb constant, 1.602 101 9C Boltzmann's constant, 1.381 1023 J/K total harmonic distortion maximum voltage, V modulation which is given by 2pf active power DC component of p AC component of p pulse-width modulation insulated-gate bipolar transistor cell temperature, 25 C ¼ 298 K PeN junction ideality factor series resistance of the cell, U shunt resistance of the cell, U DC voltage, V gain which is given by 1/Vm reference quantities reactive power DC component of q AC component of q gate turn-off thyristor low-pass filter
much as grid integration capabilities efficiently [7]. However, the proliferation of nonlinear loads due to their compact size and better controllability such as static power converters has deteriorated power quality by switching actions. Whenever, these systems are connected to the utility, they cause certain harmonics, subharmonics or superharmonics in voltage and current patterns [8,9]. They also pose themselves as loads having poor displacement, which draw considerable reactive power from the utility [10], thereby these problems can cause a malfunctioning in the electrical equipment. Different types of passive filters have been proposed to filter these harmonics and to compensate reactive power. On the other hand, a numerous drawbacks of the passive filters exist such as causing larger system size and resonant problems, being effective for only specified harmonic ranges such as 5th, 7th, 11th and 13th and for specific loads [10e12]. Therefore that gives a good motivation to carry out researches in the area of passive filter. In order to enhance the quality of power by considering voltage and current distortion limits for non-linear loads, many systems have been proposed [8e11]. In addition to suppress the current/voltage distortion, one may also handle other problems for instance, swells, sags, current/voltage imbalance, flickers, surges, reactive currents, frequency oscillation, interruptions [13]. Mainly the systems are classified into three categories: First is the unified power flow controller (UPFC) which performs power flow control, voltage regulation and reactive power compensation as an equipment for compensation at the system frequency. Second is the unified power quality conditioner (UPQC) which combines a
series active filter for harmonics-voltage compensation with a shunt active filter for harmonic-current compensation as a new equipment controlled to perform current and voltage compensation. Third is the universal active power line conditioner (UPLC) which aggregates the functions of the UPFC and the UPQC into a signal power conditioner [9]. To avoid an additional cost and hardware for the UPQC system, some studies discussed these systems with the PV. This latter injects active power to the grid and filters the load current/voltage harmonics. Furthermore, in one of our previous papers [6], a shunt active power filter (APF) fed by a PV system has been confirmed in terms of performance. In the present study, a UPQC has been adopted and modeled to improve the power quality influenced by a harmonic disturbance and to export active power to the grid. The proposed system is supplied by the PVs containing a combination of two active power filters, in parallel and series shared a common DC voltage generated by the PVs. While the parallel APF has been controlled by the instantaneous reactive power theory in order to compensate the current distortion caused by a non-linear load, it is also export energy generated by PV system to the grid. The series APF has used the unit vector templates generation to filter the load voltage distortion in the power supply. The PVs have also exported a certain amount of active power to the grid through the parallel APF, the boost converter and the maximum power point tracking (MPPT) unit. The latter have defined the current reference signal to control the active power injected to the grid. The paper is organized as follows: The configuration of the UPQC fed by the PV will be presented in Section 2. It also underlines the required units such as power source, nonlinear load, UPQC and the control technique algorithms. Later, the main simulation results, current and voltage waveforms and THD analyses will be discussed in Section 3. Finally, the paper will be completed by the concluding remarks.
Configuration of a UPQC supplied by PV Description of PV system Despite of high specific costs, PV systems present an attractive solution for the electricity supply especially for remote locations because of low maintenance requirement, high reliability, long lifetime and stability with unrotating units [5e7]. Frequently, the application areas of PVs are dominated by the stand-alone systems such as household electrification and water pumping, however the grid-connected applications of PVs also flourish in cities as well as rural areas. In order to provide a concrete definition for a PV connected system, one should handle some components like PV cells, inverters, MPPT units and grid connection elements. The basic equation of the PV current for a PV cell is defined by [14,15], I ¼ Ipv I0 e
qðVþRs IÞ akB T
! 1
V þ Rs I ; Rsh
(1)
In order to improve the efficiency of the electricity generation by the PV system, several methods have been suggested
Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Table 1 e The common MPPT methods and their relative features. MPPT technique Open circuit voltage Short circuit current Curve fitting Look-up table Perturb & Observe (P&O) Incremental conductance Parasitic capacitance Feedback voltage or current One cycle control Differentiation Ripple correlation control Sliding mode control Current sweep Fuzzy logic control Artificial Neural Network
PV array dependency
Implementation complexity
Parameters for sensing
Tracking speed
Direct or indirect
Yes Yes Yes Yes No No No Yes
Simple Simple Simple Simple Low Medium Simple Simple
Voltage Current Voltage Voltage, Current Voltage, Current Voltage, Current Voltage, Current Voltage or Current
High High High High Varies Varies Varies High
Indirect Indirect Indirect Indirect Direct Direct Direct Indirect
Yes No No
Medium High Medium
Current Voltage or Current Voltage or Current
High Medium High
Indirect Direct Direct
No Yes No Yes
High High High High
Voltage or Current Current Voltage and/or Current Voltage and/or Current
High Medium High High
Direct Direct Direct Direct
in the literature [15e23]. Among them, the maximum power point tracking of a PV module connected with a DC-DC converter can be counted. The MPPT is changing operation point of the PV system according to PV module and load parameters to obtain maximum output power in any case. Several MPPT methods have been introduced in the past literature. These methods can be categorized into two groups: indirect methods and direct methods. Indirect methods use some parameters of the PV module or use a database which is obtained from some predefined PV module parameters. Since these methods use some assumptions to determine the maximum power point of the PV system, they only estimate the maximum power point. However, they have very fast responses. Curve fitting method, look-up table method, open circuit voltage method, short circuit current method, plot cell method, one cycle control are the most common indirect methods [19e23]. Direct methods measure and use the real data continuously and track the real maximum power point of the PV system all the time. Thus, changes in irradiation and temperature do not affect these methods. Among the direct MPPT techniques the perturb & observe (P&O) method is the most
popular one. It mainly considers the current and voltage of the PV module and calculates the PV power. The process continues by perturbing the PV voltage or duty cycle of converter. Then the PV power is calculated. The next step is to compare these two power values in order to apply the technique. The P&O is an effective perturbation technique; however there exists an oscillation at the steady state operation and this situation decreases the efficiency of the MPPT. This oscillation can be reduced by the use of small step size in the algorithm; however this lowers the MPPT speed. For that reason, variable step size P&O techniques have also been proposed in the literature. On the one hand, the incremental conductance technique is another popular MPPT technique. In this technique, the slope in the P-V curve from the PV module is calculated. At the MPP, this slope becomes zero. While the positive slope indicates that the operation point is at the left hand-side of the MPP, the negative slope indicates that the operation point is at the right-hand side of the MPP. In this manner, the step size is important in the incremental conductance technique, too. Strictly speaking, the bigger step size gives a good tracking speed and the oscillations also increase [20e25]. The advanced Extremum Seeking control is proposed to overcome the limitations about step size, by using an adaptive gain for the dither amplitude based on the
Fig. 1 e Basic block diagram of unified power quality conditioner.
Fig. 2 e Block diagram of the sinusoidal template vector algorithm.
Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Fig. 3 e Block diagram for the peq theory.
fundamental component of the PV power. In addition this method can be applied for both PV system and fuel cell [26e28]. The differentiation technique and the parasitic capacitance technique which are similar to the incremental conductance method, and the ripple correlation control technique can be mentioned as other direct methods. The direct and indirect methods can also be combined in order to achieve the fast response of the indirect method and accuracy of direct methods [20e22]. In addition to above, intelligent techniques can also be used for MPPT. For instance, the fuzzy logic based MPPT technique does not need any characteristic parameter of the PV module that provides fast response and less oscillations especially under rapidly changes in the atmospheric conditions. The artificial neural network (ANN) based MPPT technique is another intelligent based method. It indeed bases on the black box modeling of
the PV module. Although this technique is an effective one, it is a module dependent technique. Because an input-output data set obtained from experiment or simulation are performed in the learning process of the neural network. Besides, the genetic algorithm, the particle swarm optimization, the ant-colony optimization and the firefly algorithm based MPPT techniques are among the other intelligent MPPT techniques [20e25]. On the other hand these intelligent techniques are also used to optimize other MPPT methods. The step size of the P&O method or incremental conductance method is calculated by artificial neural network continuously. In addition, genetic algorithm is used for tuning artificial neural network or fuzzy logic based MPPT method [23]. Usually, the maximum power point tracking is carried out by a DCeDC converter. If the PV generated power is converted to AC, these systems require two power conversion stages. In
Fig. 4 e Overall circuit diagram configuration of UPQC fed by PV System. Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Table 2 e Optimized parameters of the power circuit. Parameters
Numerical values
Power supply
RMS voltage Frequency (f) Line impedance ðRs ; Ls Þ
230 V 50 Hz (0.5 mU, 19 mH)
Shunt active filter
ðLfp Þ ðRfp Þ
1.5 mH 2 mU
Series active filter
ðLfs Þ ðRfs Þ Switching frequency
1.05 mH 2 mU 10 kHz
Loads
Resistance RL Inductance LL Capacitor CL Lf
6.7 U 0.5 mH 0.1 F 1.5 mH
DC output voltage of PVs
Vdc
650 V
addition, the MPPT process and the DCeAC conversion are carried out via one power converter in the single stage power conversion scheme. In this kind of system, the conventional MPPT methods require some modifications. Indeed, some new and modified MPPT methods are proposed for the single stage systems. The main features and comparisons of the popular MPPT methods are presented in Table 1 [6,20e25]. In this study, the P&O method has been used as in Ref. [19] because of its certain advantages. A detailed study of the proposed PV system was explained in one of our earlier papers [6]. A control loop was formed to regulate the DC output voltage of the PV panel at 650 V. In order to increase the active power generated by the proposed PVs, 64 PV panels are used from SPR-315 E model panel consisting of 6 strings which consists of 8 serial connected PV modules.
Design of the proposed unified power quality conditioner (UPFC) The inclusion of solar energy to the existing power system presents some technical challenges and provides the consideration of voltage regulation, stability and power quality problems [29]. In this manner, many factors like harmonic disturbances due to the non-linear loads, voltage and current flickering, swell and sag etc. can be encountered as power quality problems. It is also known that one of the most powerful compensators is the use of a combined system of
5
shunt and series active filters namely, the UPFC in order to compensate the currents and voltages distortions simultaneously. The UPFC has two main units as shown in Fig. 1 [9,30]: The first unit is the power circuit formed by the series and shunt PWM converters. The series PWM converter behaves as a controlled voltage source; thereby it behaves as a series active filter, being responsible to mitigate the supply side disturbances such as voltage unbalance harmonics, flickers and voltage sage/swells. The shunt PWM converter behaves as a controlled current source in order to compensate the harmonic current disturbance caused by the non-linear load. The combined series and shunt active filter made by two IGBT or GTO inverters are connected back-to-back and share a common DC voltage generated by a capacitor Cdc . However, in the present paper, the PV system is proposed in place of Cdc in order to supply the power circuit of the UPQC by a stable DC voltage supplied by PV system. The second unit is the control circuit of the UPQC (Fig. 1). It is responsible to generate the exact reference compensating voltage and current of the shunt APF and series APF. In our case, a control strategy based on the extraction of the unit vector templates from the distorted power supply voltage is performed to control the series active power filter, while the instantaneous reactive power theory is introduced to control the shunt active power filter. There exist different harmonic current/voltage identification algorithms in the literature. They are used to extract the exact harmonic components caused by non-linear load and distorted supply respectively. A proper selection of the harmonic identification algorithm is realized for the compensation of the harmonics disturbances. In addition, a precise dimensioning of the control circuit is required for the formation of the switching orders. The control strategy of the load voltage harmonics identification algorithm can be found in Ref. [11]. It extracts the unit vector templates from the distorted input supply. Then, the templates are expected to be equivalent to ideal sinusoidal signal with unity amplitude. The operating principle of this proposed algorithm is show in Fig. 2. The distorted supply voltages are measured and divided by the peak amplitude of fundamental input voltage Vm given by [31]: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 V þ V2sb þ V2sc Vm ¼ 3 sa
(2)
A three-phase phase locked loop (PLL) is utilized in order to generate the sinusoidal unit vector templates with phase
Fig. 5 e Distorted load voltage of phase-a before the insertion of UPQC-PV system. Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Fig. 6 e Harmonic spectrum of load voltage before the insertion of UPQC-PV system.
Fig. 7 e Distorted supply current of phase-a before the insertion of UPQC-PV system.
delay by the use of sinus function. The reference load voltage signals are found by multiplying the unit vector template with the peak amplitude of the fundamental input voltage Vm . A hysteresis controller can also be adopted to generate the required gate signals for series APF. In order to have a perfect compensation of current harmonics disturbances caused by the non-linear loads, many algorithms have been used in the literature. In this paper, the famous instantaneous reactive power theory “peq theory” is handled due to its numerous advantages [9,32]. The voltages and currents in Cartesian abc coordinate can be transformed to Cartesian ab coordinate as in [9]:
va vb
2
3 va ¼ ½C32 4 vb 5; vc
ia ib
2
3 ia ¼ ½C32 4 ib 5 ic
(3)
½C32 ¼
rffiffiffi 2 1 p 1=2 1=2 ffiffiffi pffiffiffi 3 2 3 2 3 0
(4)
The instantaneous active and reactive power statements of the loads are given by, pðtÞ ¼ va ðtÞia ðtÞ þ vb ðtÞib ðtÞ
(5)
qðtÞ ¼ va ðtÞib ðtÞ þ vb ðtÞia ðtÞ
(6)
From Eqs. (5) and (6), the vector space of the reactive power is introduced by Akagi as follows [9]: ! ! ! v b i a: q ¼! va i b þ!
(7)
In addition, the DC and AC components of the active and reactive powers are given by,
Fig. 8 e Harmonic spectrum of supply current before the insertion of UPQC-PV system. Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Fig. 9 e Harmonic voltage of phase-a generated by the series APF.
Fig. 10 e Load voltage of phase-a after the insertion of UPQC-PV system.
p ¼ p þ p~
(8)
q ¼ q þ q~
(9)
The currents in Cartesian a b coordinate read as,
ia ib
1 v va p v þ a 2 vb va 0 vb v2a þ vb iap iaq ¼ þ ibp ibq ¼
v va
0 q
i*a i*b
# ¼
v2a
1 va 2 v þ vb b
vb va
p~ : ~ qþq
the reference harmonics currents are given by,
i* 6 a* 4 ib i*c
2 .pffiffiffi 3 3 2 1 0 rffiffiffi6 1. 2 7 pffiffiffi pffiffiffi 7 i* 0 26 7 6 7 4 i 1 2 1=2 3 2 7 a 5; 5¼ 36 4 .pffiffiffi pffiffiffi 5 i*b 1 2 1=2 3 2 3
1 i0 ¼ pffiffiffi ðia þ ib þ ic Þ; 3 (10)
The reference signals of the shunt active filter must ~ in order to guarantee a double compensation ~; q and q contain p of the harmonics currents and reactive power. If the reference currents can be found from, "
2
(11)
(12)
(13)
Fig. 3 summarizes the explanations about the peq theory given by the equations above.
Configuration of the proposed UPQC supplied by PV system Fig. 4 shows the general power circuit configuration of the proposed UPQC fed by the PV system. The system consists of two active power filters, in series and parallel topologies connected back-to-back and sharing a common DC link generated by the overall PV system. It is expected to improve the power quality through a double compensation of the
Fig. 11 e Harmonic spectrum of load voltage after the insertion of UPQC-PV system. Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Fig. 12 e Load current of phase-a after the insertion of UPQC-PV system.
Fig. 13 e Harmonic spectrum of load current after the insertion of UPQC-PV system.
distortions from the power supply such as the current and voltage harmonics and resulting reactive power caused by the increasing usage of the non-linear loads. According to the proposed UPQC system, the supply voltage Vs is itself already distorted containing the 5th and 7th harmonic components. This distorted supply voltage is applied to the critical non-linear load which injects harmonic currents into the grid-line. The series active filter connected to the grid in series through a transformer provides a harmonic isolation between the power distribution system and the proposed system. This series active power filter is capable to compensate the voltage imbalance. Thus, the regulation of the voltage is ascertained at the point of common coupling (PCC) [33]. The series APF which should inject voltage to the grid through the series line transformer acts as a controlled voltage generator, and an output voltage of the inverter VAFS includes only the 5th and 7th harmonics components. Hence, a unit vector templates algorithm is applied at the series APF to determine the reference sinusoidal load voltages and to generate the required gate pulses. The inverter unit connected in parallel is operated as shunt APF, which should be equal to the non-linear load [34]. The
shunt APF can compensate all undesirable current harmonics by injecting its inverse image through an inductor at the gridline. In this manner, the instantaneous reactive power theory is applied to the shunt APF for the extraction of the harmonic components generated by the nonlinear load and also for the hysteresis controller. Both series APF and shunt APF share a common DC link generated by the proposed PV system containing 8 strings with 6 series PV panels with a boost converter and a PI controller in order to regulate the DC output voltage. In addition, this system exports the active power to the network. The second PI controller and MPPT algorithm determine the fundamental component of shunt active filter compensation current. Thus compensation current combines the harmonic components determined by the peq theory and fundamental component determined by MPPT algorithm and PI controller.
Results and discussion To demonstrate the efficiency of the proposed UPQC fed by PV system with a back-to-back combination of the series APF and shunt APF, the power circuit given in Fig. 4 has been
Fig. 14 e Harmonic current of phase-a generated by the shunt APF. Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Fig. 15 e Supply current of phase-a after the insertion of UPQC-PV system.
Fig. 16 e Harmonic spectrum of supply current after the insertion of UPQC-PV system.
established with MATLAB/Simulink software. Table 2 summarizes the optimized parameters of the power circuit. Before the application of the proposed system, the shape of the load voltage has been totally distorted as shown in Fig. 5. Here the signal includes the 5th and 7th harmonic components. The analysis proves that high value of total harmonic distortion (THD) exists (i.e. THDV ¼ 24.58%), after the calculation of the harmonic spectrum in Fig. 6. The shape of the supply current has also been influenced by the harmonics perturbation showing a critical deformation, which caused by the non-linear load as described in Fig. 7. Analysis of this current gives the total harmonic distortion value of THDi ¼ 23.42% as shown in Fig. 8. In order to test the effectiveness of the series APF and shunt APF, a timer has been programmed as in Figs. 9 and 14. Initially, the series APF starts the operation at t1 ¼ 0.1 s. Later
on, the shunt APF starts to operate at t2 ¼ 0.2 s. Both the load voltage and the supply current have been totally distorted before time t1, when the series APF and the shunt APF are not in operation. At time t1, the series APF starts the operation by injecting the sum of 5th and 7th harmonic voltage through line transformer as shown in Fig. 9. At this time, a great compensation is occurred in load voltage with perfectly sinusoidal signal, when the distorted signal has been totally filtered from the 5th and 7th harmonic voltage as described in Fig. 10. This improvement is confirmed by the detailed analyses on the harmonic spectrum which proves that THD has been decreased to THDV ¼ 3.86% according to Fig. 11. In addition, the series APF also helps to enhance the load current by filtering a small amount of harmonics currents. On the other hand, the load current has been less distorted as
Fig. 17 e Output currents of the PV system (black), output current of the peq theory (red) and the reference harmonic current (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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appear in Fig. 12. At this moment, the harmonic spectrum analysis indicates a decreasing value of THDi from 23.42% to 20.45%, as illustrated in Fig. 13. Fig. 14 shows the harmonic current generated by the inverter after the operation of the shunt APF at instant t2 which has been injected to the grid through the inductor. When the APF is activated, the supply current has been filtered effectively and become nearly sinusoidal with a switching frequency on it as demonstrated in Fig. 15. The improvement in the supply current is also confirmed by the harmonics analysis (Fig. 16). It gives a satisfactory value of THDi , which is calculated as THDi ¼ 4.76%. According to the IEEE 519-1992 standards, lower THD values than 5% is acceptable for the signal recovery. Thus the proposed system has an acceptable result in terms of the relevant standard. The PV system supplies the DC bus and generates the active power in order to export to the grid-line via shunt APF. The output current of the shunt APF (i*ha ) is the combination of the harmonic compensation current (iha ) determined by peq theory and the sinusoidal current component generated by the MPPT algorithm and current controller of the PV system (iPVa ). The output current of the shunt APF (iha ) is shown in Fig. 17. According to results above, it is proven that a good improvement has been obtained in the load voltage and the supply current being nearly sinusoidal confirmed by the total harmonics distortion values THDV ¼ 3.86% and THDi ¼ 4.76%, respectively. In other words, the proposed UPQC fed by PV system has perfectly achieved to suppress the voltage and harmonic disturbances and prevent the grid-line from the distortions. In addition, the active power generated from the PV system has been sent to the grid as the green energy option.
Conclusions A new unified power quality conditioner (UPQC) supplied by photovoltaic system has been realized at the limits of standards. In the proposed system, the overall PVs with the 64 PV panels, boost converter, PI controllers and MPPT algorithm have been designed to generate the maximum active power, which is exported to the network through the shunt APF function of the UPQC. The UPQC is composed of back-to-back connected series APF and shunt APF, which are share a common DC voltage generated by the PVs. Thus, the PV system does not provide only the active power to the grid but also guarantees a regulated DC link to the power circuit of the UPQC. The series APF has been controlled by the unit vector templates algorithm for determining the reference compensation voltage signals, which have been used in the carrierbased modulation PWM technique for launching the switching orders in order to counter the voltage harmonics disturbances. In the system, the instantaneous reactive power theory (peq theory) has been utilized with hysteresis technique in order to control the shunt APF providing a perfect compensation of the current harmonics perturbations caused by the non-linear load. The simulation results performed by MATLAB/Simulink proves that the proposed UPQC fed by PVs has offered a promising way to annihilate the current and
voltage harmonics at the order of THDV ¼ 3.86% and THDi ¼ 4.76%, when the series APF and the shunt APF is activated, respectively. Consequently, the proposed UPQC fed by PVs can be considered as a new method for the power quality conditioner and some laboratory works are required for the realization of the new system as future work.
references
[1] Chenni R, Matagne E, Khennane-Benbitour M. Study of solar radiation in view of photovoltaic systems optimization. Smart Grid Renew Energy 2011;2:367e74. [2] Kurt E, Gor H, Demirtas M. Theoretical and experimental analyses of a single phase permanent magnet generator (PMG) with multiple cores having axial and radial directed fluxes. Energy Convers Manag 2014;77:163e72. [3] Uzun Y, Demirbas S, Kurt E. Implementation of a new contactless piezoelectric wind energy harvester to a wireless weather station. Elektron Elektrotech 2015;20(10):35e9. [4] Von Appen J, Stetz T, Schmiegel A. Local voltage control strategies for PV storage systems in distribution grids. Smart Grid, IEEE Trans 2014;5:1002e9. [5] Sefa I, Altin N. Grid interactive photovoltaic inverters. a review. J Facul Eng Archit Gazi Univ 2009;24:409e24. [6] Bouzelata Y, Kurt E, Altin N, Chenni R. Design and simulation of a solar supplied multifunctional active power filter and a comparative study on the current-detection algorithms. Renew Sustain Energy Rev 2014;43:1114e26. [7] Silvestre S, Aires da Silva M, Chouder A, Guasch D, Karatepe E. New procedure for fault detection in grid connected PV systems based on the evaluation of current and voltage indicators. Energy Convers Manag 2014;86:541e9. [8] Ulinuha A, Masoum MAS, Islam S. Hybrid genetic-fuzzy algorithm for volt/var/total harmonic distortion control of distribution systems with high penetration of non-linear loads. Generat Transm Distrib IET 2011;5:425e39. [9] Akagi H, Watanabe EH, Aredes M. Instantaneous power theory and application to power conditioning. IEEE Press Ser Power Eng 2007;31. [10] Sefa I, Altin N. A novel approach to determine the interphase transformer inductance of 18 pulse rectifiers. Energy Convers Manag 2009;50:2595e603. [11] Khadkikar V, Aganval P, Chandra A, Bany AO, Nguyen TD. A simple new control technique for unified power quality conditioner (UPQC). Proc. of HQP Sep. 2004:289e93. [12] Colak I, Demirbas S, Sefa I, Irmak E, Kahraman HT. Remote controlling and monitoring of HVAC system over Internet. J Sci Ind Res 2008;67(9):680e4. [13] Menti A, Zacharias T, Milias-Argitis J. Harmonic distortion assessment for a single-phase grid-connected photovoltaic system. Renew Energy 2011;36:360e8. [14] Mahammad AK, Saon S, Chee WS. Development of optimum controller based on MPPT for photovoltaic system during shading condition. Proc Eng 2013;53:337e46. [15] Ozdemir S, Altin N, Sefa I. Single stage three level grid interactive MPPT inverter for PV systems. Energy Convers Manag 2014;80:561e72. [16] Sera D, Mathe L, Kerekes T, Spataru SV, Teodorescu R. On the perturb-and-observe and incremental conductance MPPT methods for PV systems. Photovoltaics. IEEE J 2013;3:1070e8. [17] Zhou Z, Holland PM, Igic P. MPPT algorithm test on a photovoltaic emulating system constructed by a DC power supply and an indoor solar panel. Energy Convers Manage 2014;85:460e9.
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[18] Muthuramalingam M, Manoharan PS. Comparative analysis of distributed MPPT controllers for partially shaded stand alone photovoltaic systems. Energy Convers Manage 2014;86:286e99. [19] Shuhui L, Timothy A Haskew, Dawen L, Fei H. Integrating photovoltaic and power converter characteristics for energy extraction study of solar PV systems. Renew Energy 2011;36:3238e45. [20] Esram T, Chapman PL. Comparison of photovoltaic array maximum power point tracking techniques. IEEE Trans Energy Convers 2007;22(2):439e47. [21] Logeswaran T, SenthilKumar A. A review of maximum power point tracking algorithms for photovoltaic systems under uniform and non-uniform irradiances. Energy Proc 2014;54:228e35. € [22] Sefa I, Ozdemir S. Multifunctional interleaved boost converter for PV systems. In: IEEE International Symposium on Industrial Electronics (ISIE 2010); 2010. p. 951e6. [23] Subudhi B, Pradhan R. A comparative study on maximum power point tracking techniques for photovoltaic power systems. IEEE Trans Sustain Energy 2013;4(1):89e98. [24] Ozdemir S, Altin N, Sefa I, Bal G. PV supplied single stage MPT inverter for induction motor actuated ventilation systems. Elektron Elektrotech 2014;20(5):116e22. [25] Altin N, Ozdemir S. Three-phase three-level grid interactive inverter with fuzzy logic based maximum power point tracking controller. Energy Convers Manag 2013;69:17e26.
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[26] Bizon N. Energy harvesting from the PV hybrid power source. Energy 2013;52:297e307. [27] Bizon N. Energy harvesting from the FC stack that operates using the MPP tracking based on modified extremum seeking control. Appl Energy 2013;104:326e36. [28] Bizon N. FC energy harvesting using the MPP tracking based on advanced extremum seeking control. Int J Hydrogen Energy 2013;38(4):1952e66. [29] Trillo-Montero D, Santiago I, Luna-Rodriguez JJ, Real-Calvo R. Development of a software application to evaluate the performance and energy losses of grid-connected photovoltaic systems. Energy Convers Manag 2013;76:893e8. [30] Karanki K, Geddada G, Mishra MK, Kumar BK. A modified three-phase four-wire upqc topology with reduced dc-link voltage rating. Ind Electron IEEE Trans 2012;60:3555e66. [31] Djeghloud H, Benalla H, Bentounsi A. Supply current and load voltage distortion suppression using the unified power quality conditioner. In: Proc. of SSD; 2008. p. 978e84. [32] Babu PC, Dash SS. Design of unified power quality conditioner (UPQC) to improve the power quality problems by using P-Q theory. In: Int. Conf. Computer Communication and Informatics (ICCCI); Jan. 2012. p. 1e7. [33] Ajami A, Armaghan M. Fixed speed wind farm operation improvement using current-source converter based UPQC. Energy Convers Manag 2012;58:10e8. [34] Fujita H, Akagi H. Voltage-regulation performance of a shunt active filter intended for installation on a power distribution system. Power Electron IEEE Trans 2007;22:1046e53.
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Design and simulation of a unified power quality conditioner fed by solar energy Yahia Bouzelata a, Erol Kurt b,*, Rachid Chenni a, Necmi Altın b a
University Constantine, Faculty of Technology, Department of Electrotechnic, MoDERNa Laboratory, Constantine 25000, Algeria b Gazi University, Faculty of Technology, Department of Electrical and Electronics Engineering, 06500 Teknikokullar, Ankara, Turkey
article info
abstract
Article history:
Presently, quality problems in grid-integrated applications take great interest because of
Received 7 January 2015
the growing applications in power electronics. The elimination of the harmonics in the grid
Received in revised form
and the usage of clean energy resources in the power electronics applications become
15 February 2015
popular world widely. In the present paper, a unified power quality conditioner fed by solar
Accepted 21 February 2015
energy which can also export active power to the grid is proposed. The conditioner uses a
Available online xxx
photovoltaic (PV) system and its topology is made up of a hybrid active power filter combination. This combination bases on a parallel active power filter, which shares a common
Keywords:
DC voltage assured by the photovoltaic system with a serial active power filter. According
Power quality
to the analyzes, the proposed unified power quality conditioner eliminates both the supply
Unified power quality conditioner
current distortion caused by a non-linear load and the load voltage distortion introduced
PV
after adding fifth and seventh harmonics to the AC main voltage. In addition, the proposed
THD active power filter
unified power quality conditioner exports the photovoltaic power to the grid using a boost converter, perturbed and observed maximum power point tracking algorithm, compensates the reactive power and filters the current and voltage harmonics confirmed by the total harmonic distortion values, such as 4.76% and 3.86%, respectively. The design and the analyses have been performed with MATLAB/Simulink software. The simulation system determines the performances of such system and offers future perspectives on unified power quality conditioners. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The renewable energy resources are the best solutions for the clean energy requirement of the recent communities. Due to its wide application areas, easy construction and maintenance, the photovoltaic (PV) solar energy is the most
promising green energy field [1e3]. Thus, the numerous examples of successfully deployed PV systems are already available for commercial usages and also technical researches [4e6]. The PV based solar energy explorations have been improved further by focusing on their grid integration. In fact, the improvements in power electronics technology have accelerated the usage of PV supplied inverter systems as
* Corresponding author. E-mail address: [email protected] (E. Kurt). http://dx.doi.org/10.1016/j.ijhydene.2015.02.077 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Ipv I0 q kB THD Vm u p p p~ PWM IGBT T a Rs Rsh Vdc K * q q q~ GTO LPF
photocurrent, A diode saturation current, A coulomb constant, 1.602 101 9C Boltzmann's constant, 1.381 1023 J/K total harmonic distortion maximum voltage, V modulation which is given by 2pf active power DC component of p AC component of p pulse-width modulation insulated-gate bipolar transistor cell temperature, 25 C ¼ 298 K PeN junction ideality factor series resistance of the cell, U shunt resistance of the cell, U DC voltage, V gain which is given by 1/Vm reference quantities reactive power DC component of q AC component of q gate turn-off thyristor low-pass filter
much as grid integration capabilities efficiently [7]. However, the proliferation of nonlinear loads due to their compact size and better controllability such as static power converters has deteriorated power quality by switching actions. Whenever, these systems are connected to the utility, they cause certain harmonics, subharmonics or superharmonics in voltage and current patterns [8,9]. They also pose themselves as loads having poor displacement, which draw considerable reactive power from the utility [10], thereby these problems can cause a malfunctioning in the electrical equipment. Different types of passive filters have been proposed to filter these harmonics and to compensate reactive power. On the other hand, a numerous drawbacks of the passive filters exist such as causing larger system size and resonant problems, being effective for only specified harmonic ranges such as 5th, 7th, 11th and 13th and for specific loads [10e12]. Therefore that gives a good motivation to carry out researches in the area of passive filter. In order to enhance the quality of power by considering voltage and current distortion limits for non-linear loads, many systems have been proposed [8e11]. In addition to suppress the current/voltage distortion, one may also handle other problems for instance, swells, sags, current/voltage imbalance, flickers, surges, reactive currents, frequency oscillation, interruptions [13]. Mainly the systems are classified into three categories: First is the unified power flow controller (UPFC) which performs power flow control, voltage regulation and reactive power compensation as an equipment for compensation at the system frequency. Second is the unified power quality conditioner (UPQC) which combines a
series active filter for harmonics-voltage compensation with a shunt active filter for harmonic-current compensation as a new equipment controlled to perform current and voltage compensation. Third is the universal active power line conditioner (UPLC) which aggregates the functions of the UPFC and the UPQC into a signal power conditioner [9]. To avoid an additional cost and hardware for the UPQC system, some studies discussed these systems with the PV. This latter injects active power to the grid and filters the load current/voltage harmonics. Furthermore, in one of our previous papers [6], a shunt active power filter (APF) fed by a PV system has been confirmed in terms of performance. In the present study, a UPQC has been adopted and modeled to improve the power quality influenced by a harmonic disturbance and to export active power to the grid. The proposed system is supplied by the PVs containing a combination of two active power filters, in parallel and series shared a common DC voltage generated by the PVs. While the parallel APF has been controlled by the instantaneous reactive power theory in order to compensate the current distortion caused by a non-linear load, it is also export energy generated by PV system to the grid. The series APF has used the unit vector templates generation to filter the load voltage distortion in the power supply. The PVs have also exported a certain amount of active power to the grid through the parallel APF, the boost converter and the maximum power point tracking (MPPT) unit. The latter have defined the current reference signal to control the active power injected to the grid. The paper is organized as follows: The configuration of the UPQC fed by the PV will be presented in Section 2. It also underlines the required units such as power source, nonlinear load, UPQC and the control technique algorithms. Later, the main simulation results, current and voltage waveforms and THD analyses will be discussed in Section 3. Finally, the paper will be completed by the concluding remarks.
Configuration of a UPQC supplied by PV Description of PV system Despite of high specific costs, PV systems present an attractive solution for the electricity supply especially for remote locations because of low maintenance requirement, high reliability, long lifetime and stability with unrotating units [5e7]. Frequently, the application areas of PVs are dominated by the stand-alone systems such as household electrification and water pumping, however the grid-connected applications of PVs also flourish in cities as well as rural areas. In order to provide a concrete definition for a PV connected system, one should handle some components like PV cells, inverters, MPPT units and grid connection elements. The basic equation of the PV current for a PV cell is defined by [14,15], I ¼ Ipv I0 e
qðVþRs IÞ akB T
! 1
V þ Rs I ; Rsh
(1)
In order to improve the efficiency of the electricity generation by the PV system, several methods have been suggested
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Table 1 e The common MPPT methods and their relative features. MPPT technique Open circuit voltage Short circuit current Curve fitting Look-up table Perturb & Observe (P&O) Incremental conductance Parasitic capacitance Feedback voltage or current One cycle control Differentiation Ripple correlation control Sliding mode control Current sweep Fuzzy logic control Artificial Neural Network
PV array dependency
Implementation complexity
Parameters for sensing
Tracking speed
Direct or indirect
Yes Yes Yes Yes No No No Yes
Simple Simple Simple Simple Low Medium Simple Simple
Voltage Current Voltage Voltage, Current Voltage, Current Voltage, Current Voltage, Current Voltage or Current
High High High High Varies Varies Varies High
Indirect Indirect Indirect Indirect Direct Direct Direct Indirect
Yes No No
Medium High Medium
Current Voltage or Current Voltage or Current
High Medium High
Indirect Direct Direct
No Yes No Yes
High High High High
Voltage or Current Current Voltage and/or Current Voltage and/or Current
High Medium High High
Direct Direct Direct Direct
in the literature [15e23]. Among them, the maximum power point tracking of a PV module connected with a DC-DC converter can be counted. The MPPT is changing operation point of the PV system according to PV module and load parameters to obtain maximum output power in any case. Several MPPT methods have been introduced in the past literature. These methods can be categorized into two groups: indirect methods and direct methods. Indirect methods use some parameters of the PV module or use a database which is obtained from some predefined PV module parameters. Since these methods use some assumptions to determine the maximum power point of the PV system, they only estimate the maximum power point. However, they have very fast responses. Curve fitting method, look-up table method, open circuit voltage method, short circuit current method, plot cell method, one cycle control are the most common indirect methods [19e23]. Direct methods measure and use the real data continuously and track the real maximum power point of the PV system all the time. Thus, changes in irradiation and temperature do not affect these methods. Among the direct MPPT techniques the perturb & observe (P&O) method is the most
popular one. It mainly considers the current and voltage of the PV module and calculates the PV power. The process continues by perturbing the PV voltage or duty cycle of converter. Then the PV power is calculated. The next step is to compare these two power values in order to apply the technique. The P&O is an effective perturbation technique; however there exists an oscillation at the steady state operation and this situation decreases the efficiency of the MPPT. This oscillation can be reduced by the use of small step size in the algorithm; however this lowers the MPPT speed. For that reason, variable step size P&O techniques have also been proposed in the literature. On the one hand, the incremental conductance technique is another popular MPPT technique. In this technique, the slope in the P-V curve from the PV module is calculated. At the MPP, this slope becomes zero. While the positive slope indicates that the operation point is at the left hand-side of the MPP, the negative slope indicates that the operation point is at the right-hand side of the MPP. In this manner, the step size is important in the incremental conductance technique, too. Strictly speaking, the bigger step size gives a good tracking speed and the oscillations also increase [20e25]. The advanced Extremum Seeking control is proposed to overcome the limitations about step size, by using an adaptive gain for the dither amplitude based on the
Fig. 1 e Basic block diagram of unified power quality conditioner.
Fig. 2 e Block diagram of the sinusoidal template vector algorithm.
Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Fig. 3 e Block diagram for the peq theory.
fundamental component of the PV power. In addition this method can be applied for both PV system and fuel cell [26e28]. The differentiation technique and the parasitic capacitance technique which are similar to the incremental conductance method, and the ripple correlation control technique can be mentioned as other direct methods. The direct and indirect methods can also be combined in order to achieve the fast response of the indirect method and accuracy of direct methods [20e22]. In addition to above, intelligent techniques can also be used for MPPT. For instance, the fuzzy logic based MPPT technique does not need any characteristic parameter of the PV module that provides fast response and less oscillations especially under rapidly changes in the atmospheric conditions. The artificial neural network (ANN) based MPPT technique is another intelligent based method. It indeed bases on the black box modeling of
the PV module. Although this technique is an effective one, it is a module dependent technique. Because an input-output data set obtained from experiment or simulation are performed in the learning process of the neural network. Besides, the genetic algorithm, the particle swarm optimization, the ant-colony optimization and the firefly algorithm based MPPT techniques are among the other intelligent MPPT techniques [20e25]. On the other hand these intelligent techniques are also used to optimize other MPPT methods. The step size of the P&O method or incremental conductance method is calculated by artificial neural network continuously. In addition, genetic algorithm is used for tuning artificial neural network or fuzzy logic based MPPT method [23]. Usually, the maximum power point tracking is carried out by a DCeDC converter. If the PV generated power is converted to AC, these systems require two power conversion stages. In
Fig. 4 e Overall circuit diagram configuration of UPQC fed by PV System. Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Table 2 e Optimized parameters of the power circuit. Parameters
Numerical values
Power supply
RMS voltage Frequency (f) Line impedance ðRs ; Ls Þ
230 V 50 Hz (0.5 mU, 19 mH)
Shunt active filter
ðLfp Þ ðRfp Þ
1.5 mH 2 mU
Series active filter
ðLfs Þ ðRfs Þ Switching frequency
1.05 mH 2 mU 10 kHz
Loads
Resistance RL Inductance LL Capacitor CL Lf
6.7 U 0.5 mH 0.1 F 1.5 mH
DC output voltage of PVs
Vdc
650 V
addition, the MPPT process and the DCeAC conversion are carried out via one power converter in the single stage power conversion scheme. In this kind of system, the conventional MPPT methods require some modifications. Indeed, some new and modified MPPT methods are proposed for the single stage systems. The main features and comparisons of the popular MPPT methods are presented in Table 1 [6,20e25]. In this study, the P&O method has been used as in Ref. [19] because of its certain advantages. A detailed study of the proposed PV system was explained in one of our earlier papers [6]. A control loop was formed to regulate the DC output voltage of the PV panel at 650 V. In order to increase the active power generated by the proposed PVs, 64 PV panels are used from SPR-315 E model panel consisting of 6 strings which consists of 8 serial connected PV modules.
Design of the proposed unified power quality conditioner (UPFC) The inclusion of solar energy to the existing power system presents some technical challenges and provides the consideration of voltage regulation, stability and power quality problems [29]. In this manner, many factors like harmonic disturbances due to the non-linear loads, voltage and current flickering, swell and sag etc. can be encountered as power quality problems. It is also known that one of the most powerful compensators is the use of a combined system of
5
shunt and series active filters namely, the UPFC in order to compensate the currents and voltages distortions simultaneously. The UPFC has two main units as shown in Fig. 1 [9,30]: The first unit is the power circuit formed by the series and shunt PWM converters. The series PWM converter behaves as a controlled voltage source; thereby it behaves as a series active filter, being responsible to mitigate the supply side disturbances such as voltage unbalance harmonics, flickers and voltage sage/swells. The shunt PWM converter behaves as a controlled current source in order to compensate the harmonic current disturbance caused by the non-linear load. The combined series and shunt active filter made by two IGBT or GTO inverters are connected back-to-back and share a common DC voltage generated by a capacitor Cdc . However, in the present paper, the PV system is proposed in place of Cdc in order to supply the power circuit of the UPQC by a stable DC voltage supplied by PV system. The second unit is the control circuit of the UPQC (Fig. 1). It is responsible to generate the exact reference compensating voltage and current of the shunt APF and series APF. In our case, a control strategy based on the extraction of the unit vector templates from the distorted power supply voltage is performed to control the series active power filter, while the instantaneous reactive power theory is introduced to control the shunt active power filter. There exist different harmonic current/voltage identification algorithms in the literature. They are used to extract the exact harmonic components caused by non-linear load and distorted supply respectively. A proper selection of the harmonic identification algorithm is realized for the compensation of the harmonics disturbances. In addition, a precise dimensioning of the control circuit is required for the formation of the switching orders. The control strategy of the load voltage harmonics identification algorithm can be found in Ref. [11]. It extracts the unit vector templates from the distorted input supply. Then, the templates are expected to be equivalent to ideal sinusoidal signal with unity amplitude. The operating principle of this proposed algorithm is show in Fig. 2. The distorted supply voltages are measured and divided by the peak amplitude of fundamental input voltage Vm given by [31]: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 V þ V2sb þ V2sc Vm ¼ 3 sa
(2)
A three-phase phase locked loop (PLL) is utilized in order to generate the sinusoidal unit vector templates with phase
Fig. 5 e Distorted load voltage of phase-a before the insertion of UPQC-PV system. Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Fig. 6 e Harmonic spectrum of load voltage before the insertion of UPQC-PV system.
Fig. 7 e Distorted supply current of phase-a before the insertion of UPQC-PV system.
delay by the use of sinus function. The reference load voltage signals are found by multiplying the unit vector template with the peak amplitude of the fundamental input voltage Vm . A hysteresis controller can also be adopted to generate the required gate signals for series APF. In order to have a perfect compensation of current harmonics disturbances caused by the non-linear loads, many algorithms have been used in the literature. In this paper, the famous instantaneous reactive power theory “peq theory” is handled due to its numerous advantages [9,32]. The voltages and currents in Cartesian abc coordinate can be transformed to Cartesian ab coordinate as in [9]:
va vb
2
3 va ¼ ½C32 4 vb 5; vc
ia ib
2
3 ia ¼ ½C32 4 ib 5 ic
(3)
½C32 ¼
rffiffiffi 2 1 p 1=2 1=2 ffiffiffi pffiffiffi 3 2 3 2 3 0
(4)
The instantaneous active and reactive power statements of the loads are given by, pðtÞ ¼ va ðtÞia ðtÞ þ vb ðtÞib ðtÞ
(5)
qðtÞ ¼ va ðtÞib ðtÞ þ vb ðtÞia ðtÞ
(6)
From Eqs. (5) and (6), the vector space of the reactive power is introduced by Akagi as follows [9]: ! ! ! v b i a: q ¼! va i b þ!
(7)
In addition, the DC and AC components of the active and reactive powers are given by,
Fig. 8 e Harmonic spectrum of supply current before the insertion of UPQC-PV system. Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Fig. 9 e Harmonic voltage of phase-a generated by the series APF.
Fig. 10 e Load voltage of phase-a after the insertion of UPQC-PV system.
p ¼ p þ p~
(8)
q ¼ q þ q~
(9)
The currents in Cartesian a b coordinate read as,
ia ib
1 v va p v þ a 2 vb va 0 vb v2a þ vb iap iaq ¼ þ ibp ibq ¼
v va
0 q
i*a i*b
# ¼
v2a
1 va 2 v þ vb b
vb va
p~ : ~ qþq
the reference harmonics currents are given by,
i* 6 a* 4 ib i*c
2 .pffiffiffi 3 3 2 1 0 rffiffiffi6 1. 2 7 pffiffiffi pffiffiffi 7 i* 0 26 7 6 7 4 i 1 2 1=2 3 2 7 a 5; 5¼ 36 4 .pffiffiffi pffiffiffi 5 i*b 1 2 1=2 3 2 3
1 i0 ¼ pffiffiffi ðia þ ib þ ic Þ; 3 (10)
The reference signals of the shunt active filter must ~ in order to guarantee a double compensation ~; q and q contain p of the harmonics currents and reactive power. If the reference currents can be found from, "
2
(11)
(12)
(13)
Fig. 3 summarizes the explanations about the peq theory given by the equations above.
Configuration of the proposed UPQC supplied by PV system Fig. 4 shows the general power circuit configuration of the proposed UPQC fed by the PV system. The system consists of two active power filters, in series and parallel topologies connected back-to-back and sharing a common DC link generated by the overall PV system. It is expected to improve the power quality through a double compensation of the
Fig. 11 e Harmonic spectrum of load voltage after the insertion of UPQC-PV system. Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Fig. 12 e Load current of phase-a after the insertion of UPQC-PV system.
Fig. 13 e Harmonic spectrum of load current after the insertion of UPQC-PV system.
distortions from the power supply such as the current and voltage harmonics and resulting reactive power caused by the increasing usage of the non-linear loads. According to the proposed UPQC system, the supply voltage Vs is itself already distorted containing the 5th and 7th harmonic components. This distorted supply voltage is applied to the critical non-linear load which injects harmonic currents into the grid-line. The series active filter connected to the grid in series through a transformer provides a harmonic isolation between the power distribution system and the proposed system. This series active power filter is capable to compensate the voltage imbalance. Thus, the regulation of the voltage is ascertained at the point of common coupling (PCC) [33]. The series APF which should inject voltage to the grid through the series line transformer acts as a controlled voltage generator, and an output voltage of the inverter VAFS includes only the 5th and 7th harmonics components. Hence, a unit vector templates algorithm is applied at the series APF to determine the reference sinusoidal load voltages and to generate the required gate pulses. The inverter unit connected in parallel is operated as shunt APF, which should be equal to the non-linear load [34]. The
shunt APF can compensate all undesirable current harmonics by injecting its inverse image through an inductor at the gridline. In this manner, the instantaneous reactive power theory is applied to the shunt APF for the extraction of the harmonic components generated by the nonlinear load and also for the hysteresis controller. Both series APF and shunt APF share a common DC link generated by the proposed PV system containing 8 strings with 6 series PV panels with a boost converter and a PI controller in order to regulate the DC output voltage. In addition, this system exports the active power to the network. The second PI controller and MPPT algorithm determine the fundamental component of shunt active filter compensation current. Thus compensation current combines the harmonic components determined by the peq theory and fundamental component determined by MPPT algorithm and PI controller.
Results and discussion To demonstrate the efficiency of the proposed UPQC fed by PV system with a back-to-back combination of the series APF and shunt APF, the power circuit given in Fig. 4 has been
Fig. 14 e Harmonic current of phase-a generated by the shunt APF. Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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Fig. 15 e Supply current of phase-a after the insertion of UPQC-PV system.
Fig. 16 e Harmonic spectrum of supply current after the insertion of UPQC-PV system.
established with MATLAB/Simulink software. Table 2 summarizes the optimized parameters of the power circuit. Before the application of the proposed system, the shape of the load voltage has been totally distorted as shown in Fig. 5. Here the signal includes the 5th and 7th harmonic components. The analysis proves that high value of total harmonic distortion (THD) exists (i.e. THDV ¼ 24.58%), after the calculation of the harmonic spectrum in Fig. 6. The shape of the supply current has also been influenced by the harmonics perturbation showing a critical deformation, which caused by the non-linear load as described in Fig. 7. Analysis of this current gives the total harmonic distortion value of THDi ¼ 23.42% as shown in Fig. 8. In order to test the effectiveness of the series APF and shunt APF, a timer has been programmed as in Figs. 9 and 14. Initially, the series APF starts the operation at t1 ¼ 0.1 s. Later
on, the shunt APF starts to operate at t2 ¼ 0.2 s. Both the load voltage and the supply current have been totally distorted before time t1, when the series APF and the shunt APF are not in operation. At time t1, the series APF starts the operation by injecting the sum of 5th and 7th harmonic voltage through line transformer as shown in Fig. 9. At this time, a great compensation is occurred in load voltage with perfectly sinusoidal signal, when the distorted signal has been totally filtered from the 5th and 7th harmonic voltage as described in Fig. 10. This improvement is confirmed by the detailed analyses on the harmonic spectrum which proves that THD has been decreased to THDV ¼ 3.86% according to Fig. 11. In addition, the series APF also helps to enhance the load current by filtering a small amount of harmonics currents. On the other hand, the load current has been less distorted as
Fig. 17 e Output currents of the PV system (black), output current of the peq theory (red) and the reference harmonic current (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077
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appear in Fig. 12. At this moment, the harmonic spectrum analysis indicates a decreasing value of THDi from 23.42% to 20.45%, as illustrated in Fig. 13. Fig. 14 shows the harmonic current generated by the inverter after the operation of the shunt APF at instant t2 which has been injected to the grid through the inductor. When the APF is activated, the supply current has been filtered effectively and become nearly sinusoidal with a switching frequency on it as demonstrated in Fig. 15. The improvement in the supply current is also confirmed by the harmonics analysis (Fig. 16). It gives a satisfactory value of THDi , which is calculated as THDi ¼ 4.76%. According to the IEEE 519-1992 standards, lower THD values than 5% is acceptable for the signal recovery. Thus the proposed system has an acceptable result in terms of the relevant standard. The PV system supplies the DC bus and generates the active power in order to export to the grid-line via shunt APF. The output current of the shunt APF (i*ha ) is the combination of the harmonic compensation current (iha ) determined by peq theory and the sinusoidal current component generated by the MPPT algorithm and current controller of the PV system (iPVa ). The output current of the shunt APF (iha ) is shown in Fig. 17. According to results above, it is proven that a good improvement has been obtained in the load voltage and the supply current being nearly sinusoidal confirmed by the total harmonics distortion values THDV ¼ 3.86% and THDi ¼ 4.76%, respectively. In other words, the proposed UPQC fed by PV system has perfectly achieved to suppress the voltage and harmonic disturbances and prevent the grid-line from the distortions. In addition, the active power generated from the PV system has been sent to the grid as the green energy option.
Conclusions A new unified power quality conditioner (UPQC) supplied by photovoltaic system has been realized at the limits of standards. In the proposed system, the overall PVs with the 64 PV panels, boost converter, PI controllers and MPPT algorithm have been designed to generate the maximum active power, which is exported to the network through the shunt APF function of the UPQC. The UPQC is composed of back-to-back connected series APF and shunt APF, which are share a common DC voltage generated by the PVs. Thus, the PV system does not provide only the active power to the grid but also guarantees a regulated DC link to the power circuit of the UPQC. The series APF has been controlled by the unit vector templates algorithm for determining the reference compensation voltage signals, which have been used in the carrierbased modulation PWM technique for launching the switching orders in order to counter the voltage harmonics disturbances. In the system, the instantaneous reactive power theory (peq theory) has been utilized with hysteresis technique in order to control the shunt APF providing a perfect compensation of the current harmonics perturbations caused by the non-linear load. The simulation results performed by MATLAB/Simulink proves that the proposed UPQC fed by PVs has offered a promising way to annihilate the current and
voltage harmonics at the order of THDV ¼ 3.86% and THDi ¼ 4.76%, when the series APF and the shunt APF is activated, respectively. Consequently, the proposed UPQC fed by PVs can be considered as a new method for the power quality conditioner and some laboratory works are required for the realization of the new system as future work.
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Please cite this article in press as: Bouzelata Y, et al., Design and simulation of a unified power quality conditioner fed by solar energy, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.077