(IJEECS) International Journal of Electrical, Electronics and Computer Systems. Vol: 18 Issue: 01, 2014
Multilevel PV-AF System Hamza Tédjini#1, Youcef Meslem#2, Mustapha Rahli*3 #
Lobratory of electrical engineering and plasmas, University Ibn Khaldoun, Tiaret 14000, Algeria 1
[email protected] 2
[email protected] * University of sciences and technologies UST MB Oran31000, Algeria 3
[email protected]
Abstract— Active filters (AF) have been rapidly expanding with the advancement of power electronics technology. The purpose of the active filters installed by individual consumer is to compensate current harmonics and/or current imbalance of their own harmonic-producing loads. But it requires additional costs. A Grid-connected inverter used in the PV system outputs a sinusoidal current to inject an active power to the utility. So, the PV system combined with the function of the active filter system can be useful for the application in the power distribution system. In this paper, PV-AF system confirmed that it is possible to combine the AF theory to the three phase PV system connected to the utility. Keywords—Photovoltaic (PV), Grid connection, Harmonics, Active filters (AF), PV-AF system, Three levels NPC inverter.
I. INTRODUCTION The photovoltaic (PV) power systems has been regarded as the most promising future energy source among all other alternative energy sources due to advantages such as the absence of fuel cost, low maintenance, no noise and free of charge. However, two important factors still limit the implementation of PV systems i.e. low efficiency in energy conversion and high cost. For these factors, two major methods can be used. First, the PV modules should be operated at maximum power point (MPP), the PV systems are supposed to draw maximum power from the arrays continuously, regardless of weather conditions or load voltage. Secondly, the efficiency of the power converter should be increased, because it is necessary to interface the PV arrays with the power system and load [1]. On the other hand, with significant development of power electronics technology, the proliferation of nonlinear loads such as static power converters has deteriorated power quality in power transmission/ distribution systems. Notably, voltage harmonics caused from current harmonics produced by the nonlinear loads have become a serious problem in many countries. In order to compensate harmonics in power transmission/ distribution systems, Active Filter (AF) systems have been rapidly expanding with power electronics technology. The main purpose of the active filters installed by individual consumers is to compensate current harmonics and current imbalance of their own harmonic-producing loads. But the installation of the active filter is accompanied by some additional costs. And in order to compensate those harmonics, authors just add the function of the active filter to the conventional system without any kind of hardware upgrade. Because the PV generation system includes the DC-AC converter, the concept of PV-AF system
does not need the additional hardware. And in the proposed PV-AF system, it is possible to add the function of the AF into the power inverter of the PV power generation system [2], [3], [4], [5]. The problem of PV-AF system is that structure need to use a special LC inverter filter, and it consume a part of PV system power to cancel harmonics. In this paper, we propose the uses of three levels inverter which have a voltage form closer to the sinusoid, and as a solution to climb inverter power of PV-AF system. II. SOLAR MODULES CHARACTERISTICS Photovoltaic cell changes solar energy to electric energy using photovoltaic effect and the output power depends on irradiance and temperature of photovoltaic cell. Even though irradiance and surface temperature condition are the same, the output characteristic of photovoltaic cell is determined differently by output voltage and current. The building block of PV arrays is the solar cell, which is basically a p-n semiconductor junction, shown in Fig. 1. Where V and I represent the output voltage and current of the PV, respectively; Rs and Rsh are the series and shunt resistance of the cell; q is the electronic charge; ISC is the light-generated current; Io is the reverse saturation current; n is a dimensionless factor; k is the Boltzman constant, and Tk is the temperature in °K.
Fig. 1 Equivalent circuit of PV array
Equation (1) was used in computer simulations to obtain the output characteristics of a solar cell, which shows that the output characteristics of a solar cell are non-linear and are crucially influenced by solar radiation, temperature and load condition. Each curve has an MPP, at which the solar array operates most efficiently.
q (V + R S I ) V + R S I I = I Ph − I 0 exp − 1 − R Sh nKT K
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(1)
(IJEECS) International Journal of Electrical, Electronics and Computer Systems. Vol: 18 Issue: 01, 2014 To avoid the problems of this type of grouping, we start to use the multilevel inverters. For the three levels PWM inverter, the condition of control implies that the transition between the configurations do not depend any more of the internal commands (electric quantities) but only the transistors commands (external order) [8] [9].
Fig. 2 I-V characteristic of a solar cell (T=25°C) irradiance change
Fig. 4 Three levels bridge VSI NPC structure
Introducing the functions of connection of the half arm we obtain: b F b10 U AB 0 − 1 0 F 11 b b U = 0 F 21 U C1 − F 20 U c 2 1 − 1 BC F b 30 U CA − 1 0 1 F b 31 (2)
Fig. 3 I-V characteristic of a solar cell (103 W/m2) temperature change
When temperature and irradiance are constant, output power of photovoltaic cell follows the V-I curve as shown in Figure. 2 and 3. A Moving point which generates Maximum Power is called the Maximum Power Point (MPP). According to the function of MPP, the power obtained from the photovoltaic cell can be changed; it has an effect on stability of entire system including inverter. This system is controlled under a constant voltage control which was proposed to overcome the tracking failure and accelerated the controllability [6] [7]. When temperature and irradiance are constant, output power of photovoltaic cell follows the V-I curve as shown in Figure. 2 and 3. A Moving point which generates Maximum Power is called the Maximum Power Point (MPP). According to the function of MPP, the power obtained from the photovoltaic cell can be changed; it has an effect on stability of entire system including inverter. This system is controlled under a constant voltage control which was proposed to overcome the tracking failure and accelerated the controllability [6] [7]. III. THREE LEVELS THREE PHASE CONVERTER The converters conveying the high powers through the components which constitute them undergo considerable constraints during the control of the opening and closing as well. The idea is to keep a power raised without to oversize the switches and to associate structures in series of the part concerning DC-AC conversion. Association in series of converters, certainly, make the analysis of interaction related phenomenon more complex but it offers in return. A reasonable distribution and acceptable constraints, in this configuration, the constraints due to the phenomena of commutation requesting the switches are decreased by half.
The outputs voltages are given as follows: b b − 1 − 1 F 11 − F 10 V A 2 V = 1 U − 1 2 − 1 F b 21 − F b 20 B 3 C b b − 1 − 1 2 F 31 − F 30 V C
(3)
A. PWM triangular- sinusoidal control with two carrying This strategy exploits the fact that a three levels inverter is equivalent to two levels inverters in series. We can use two identical carrying dephased a half period of chopping one compared to the other from one another in order to improve harmonic rate of outputs voltages. IV. SHUNT ACTIVE FILTER FUNCTION Active power compensators or active filters are modern applications of power electronic converters. These converters can be such controlled to generate ac waveforms (voltage/current), which can then be combined with the distorted waveform to produce an ideal waveform. Active power compensators are often used to cancel the harmonics by generating a harmonic current or voltage in opposite phase to the harmonic current or voltage to be corrected. There are basically two types of filter configurations, parallel compensation and series compensation [10]. In a parallel compensation system, the compensator is normally connected to the circuit in parallel and injects a compensation current at the connection point. On the other hand, a series compensator is connected into the circuit in series, usually through a transformer, to insert a compensation voltage into the circuit. Generally, there are two kinds of loads, linear and non linear, the thyristor converter discussed in the paper is
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(IJEECS) International Journal of Electrical, Electronics and Computer Systems. Vol: 18 Issue: 01, 2014 considered as a harmonic current source; naturally, the parallel compensation system is the choice consumers. The control techniques of active filters have been studied by many researchers. Various methods based on both frequency domain and time domain have been developed and reported in literature, such as the instantaneous power theory, the synchronous rotating reference frame method, and the fast Fourier transform (FFT)-based control method [11][12]. These control schemes produce a current reference waveform (IAF*), which is compared with the measured current. Then the active filter is driven by the difference between the two waveforms to generate the desired compensation waveform. The use of the instantaneous power theory based method is reported in this paper. In the instantaneous power theory [10], with the use of Concordia transformation and low pass filter, the instantaneous real and reactive power and can be decoupled as dc components, p and q , and ac components, p% and q% . The fundamental
system would be very helpful for the improvement of power quality for individual consumers rather than for utility system [2] [14] [15]. For the purpose of stable control, constant voltage control is applied. And, any kinds of MPPT control can be applied to PV-AF system, and the output terminal of the PV array is connected to the smoothing capacitor interfacing with the PV-AF inverter, Fig.6 shows the control block and the main circuit diagram of the PV-AF system.
p and q , and % and q% . the harmonics correspond to the ac components p power is represented by the dc components
Fig. 6 Control block diagram of the PV-AF system
Fig. 5 Block diagram control of active filter
B. Harmonic Elimination Only The three phase compensation currents (IAF*) are % and q% , which is to be eliminated. calculated from p C. Reactive Power Plus Harmonic Compensation The three phase compensation currents (IAF*) are % and q + q% . Then the compensator will calculated with p compensate the harmonics and reactive power. Consequently, this scheme requires a higher compensator rating. V. PVAF SYSTEM The conventional PV system connected to grid without transformer, fed a linear load has three functions modes [13]: • Mode 1: the load consuming power superior than the power product by the PV generator.
VI. SIMULATION RESULTS A 220V, 50Hz, distribution utility connected to a 35kW PV-AF system, based DC-DC MPPT booster, sourced a series inductance battery. and a three phases DC-AC PWM (m=21, r=0.8) inverter with LC filter, which is controlled with the feedback loops of the output current of inverter, and optimal values of PI gains and filter constants are tuned to obtain proper responses, In order to confirm the effectiveness of the precedent system, a shunt load in two cases: a three phase linear load; a three phase thyristor rectifier as non linear load. A. PVAF sourced linear load "Compensation role"
• Mode 2: the PV generator is not operational (light absence). • Mode 3: the load consuming power inferior than the power product by the PV generator. The basic system PV-AF is similar to the general PV generation system. The compensation theory of the AF system is adapted to inverter control. Including the function of AF in the PV power generation system connected to utility
Fig. 7 Simulation model
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(IJEECS) International Journal of Electrical, Electronics and Computer Systems. Vol: 18 Issue: 01, 2014
Fig. 8 MPPT Power control
Fig. 12 Inverter voltage
Fig. 9 Voltage at the end of booster
Fig. 13 Load current with its two composites
Fig.13 shows PVAF current and the current of network, then the load current present its sum, the PV-AF is under action of mode 1. B. PVAF sourced non linear load "Filtrate role"
Fig. 10 Battery voltage
MPPT control confirms the power adaptation of inverter input as it shown in Fig.8. Fig.11 and Fig.12 present the PWM control of three levels inverter and the output voltage respectively.
Fig. 14 Simulation model
Fig. 11 PWM control
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(IJEECS) International Journal of Electrical, Electronics and Computer Systems. Vol: 18 Issue: 01, 2014
Fig. 19 Voltage at the end of booster
Fig. 15 Load currents
The PV-AF system performs DC voltage control as it shown in Fig.20, this loop generate a reference (IPV) current which is associated with the generated IAF current of instantaneous power algorithm, formed the global reference current of PV-AF as it shown in Fig.22, its control sourced the PWM inverter with the references voltages (Fig.23).
Fig. 16 Load currents in (α, β) plan
Fig. 20 PI DC voltage control
Fig. 17 Spectral analysis of load currents
The spectral analysis shows the presence of many harmonics in the current. The suppression of those harmonics is the fundamental objective of the proposed PV-AF system. Fig. 21 Generate instantaneous power algorithm current IAF
Fig. 18 MPPT Power control
Fig. 22 PI control of PV-AF current
The PVAF action impeached harmonics to transport from non linear load current to network current and in same time compensate it as it shown in Fig.24. And the voltage form is disturbed disrupted following the shape of the reference voltage in Fig.24.
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(IJEECS) International Journal of Electrical, Electronics and Computer Systems. Vol: 18 Issue: 01, 2014 The multilevel structure reduce LC filter value problem and give a solution for transformerless central inverter conversion over 5 kV. REFERENCES [1]
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Fig. 23 Generate reference voltage VPVAF in PWM control
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[6] [7] Fig. 24 Inverter voltage [8]
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[12] Fig. 25 Network currents [13]
The spectral analysis in Fig.26 shows the suppression of harmonics in network current. [14]
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B. Jigjid, "Photovoltaics: An Energy Option For Sustainable Development", 3rd World Conference on Photovoltaic Energy Conversion 11-18 May, 2003 Osaka, Japan, pp. 2518-2521. G. Kim, H. Seo, S. Jang, S. Park, M. Park, I. Yu "Comparison of EMTDC™ based Simulation with Real Experimental Results of PVAF System", Pulse the Manitoba HVDC research center journal, October 2008 H. Seo, G. Kim, M. Hasan Ali, "A study on the Performance Analysis of the Grid-Connected PV-AF System" Proceeding of International Conference on Electrical Machines and Systems, Seoul, Korea, 8-11 Oct. 2007, pp.371-375. F.R. Islam, H. R. Pota, "Design a PV-AF system using V2G technology to improve power quality ", IECON 2011 - 37th Annual Conference on IEEE Industrial Electronics Society, 7-10 Nov. 2011, pp. 861-866. S. Khani, L. Mohammadian, S. H. Hosseini, "Controlling a 4-wire PV-AF system in existence of unbalanced and distorted supply voltages", 20th Iranian Conference on Electrical Engineering (ICEE), 2012, pp. 473- 478. Y. Hsiao, C. Chen, "Maximum Power Tracking for Photovoltaic Power System", IEEE trans. 2002, pp1035-1040. P. Sanchis, J. López, A. Ursúa, L. Marroyo, "Electronic Controlled Device for the Analysis and Design of Photovoltaic Systems", IEEE Power Electronics Letters, Vol. 3, No. 2, June 2005, pp.57-62. M. Saitou, T. Shimizu, "A Novel Strategy of The High Power PWM Inverter With The Series Active Filter", ISIE’2000, Cholula, Puebla, Mexico, pp.67-72. H.Tédjini, " Differentes strategies de la commande d'un convertisseur dc-ac triphase pour des applications aux systemes electriques ", thésis of Doctorat,University of sciences and technologies UST-MB Oran, 2012. A. Laxmi, G. Ram Das, K. Rao , K. Sreekanthi, K. Rayudu, "Different control strategies for Unified Power Quality Conditioner at load side", IEEE ICIEA, 2006. C. Wekesa, T. Ohnishi, "Utility Interactive AC Module Photovoltaic System with Frequency Tracking and Active Power Filter Capabilities", IEEE PCC, Osaka 2002, pp. 316-321. Z. Chen, "Compensation Schemes for a SCR Converter in Variable Speed Wind Power Systems", IEEE transactions on power delivery, vol. 19, no. 2, April 2004, 813-821. S. B Kjaer, J.K. Pedersen, F. Blaabjerg, "A Review Of Single-Phase Grid-Connected Inverters For Photovoltaic Modules", IEEE transactions on industry applications, Vol. 41, No. 5, September 2005, pp. 1292-1306. Y.Thiagarajan, T.S.Sivakumaran, P.Sanjeevikumar, "Design and Simulation of Fuzzy Controller for a Grid connected Stand Alone PV System", IEEE International Conference on Computing, Communication and Networking ICCCN, 2008. T. Wu, H. Nei, C. Shen, G. Li; "A Single-Phase Two-Wire GridConnection PV Inverter with Active Power Filtering and Nonlinear Inductance Consideration", IEEE trans. 2004, pp. 1566-1571.
Hamza.TEDJINI Received the M.Sc. degree from the university of Ibn Khaldoun –Tiaret- Algeria in 2007, and the Ph.D. degree the University of Sciences and Technology of Oran (USTO) in 2012, he is a member of LGEP laboratory of Tiaret and a Professor of Electrical engineering in Bechar university. Youcef MESLEM Professor of electrical engineering at University of Ibn Khaldoun –Tiaret- Algeria, director of laboratory of electrical engineering and plasmas.
Fig. 26 Spectral analysis of network currents
VII. CONCLUSION The PV-AF system assured that it is possible to combine the AF theory to the three phase PV system connected to utility. Through the test analysis of the proposed control strategy of PVAF system, the simulation results show the effectiveness of the proposed PV-AF system.
Mustapha RAHLI Professor in USTO, director of Laboratory of Networks Optimization LOR. Oran, Algeria.
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