Performance Analysis of Grid-Connected PV System

Transcription

1 Minia University From the SelectedWorks of M. S. Hassan Winter December 27, 2016 Performance Analysis of Grid-Connected PV System Dr. M. S. Hassan, Minia University Available at:

2 Performance Analysis of Grid-Connected PV System Adel A. Elbaset (1) M. S. Hassan (1) Hamdi Ali (2) (1) Electrical Engineering Department, Faculty of Engineering, Minia University, El-Minia 61517, Egypt (2) Department of Electrical and Computer Engineering, El-Minia High Institute for Engineering and Technology, El-Minia, Egypt Abstract The electricity power generated from photovoltaic (PV) array depends mainly on climate conditions. So, the PV solar grid connected inverters should equip with control system to meet fast response of solar irradiance change. This paper describes steady state performance of the PV grid-connected system at different solar irradiances. The proposed system model is built on MATLAB/Simulink, including the PV array with a modified perturb and observe (MP&O) tracker connected to DC-DC boost converter, the three-phase three level electronic power inverter that is connected to the utility grid (UG) through low pass filter and coupling transformer and synchronizing control system of PV inverter and UG. The proposed system is simulated under daily weather conditions to test its operating performance. The simulation results of the proposed system satisfy requirements grid performance with high power quality. Index Terms: Photovoltaic power system, Utility Grid, Performance analysis. I. INTRODUCTION Increasing environmental concerns regarding the inefficient use of electrical energy from fossil oil, have directed attention to the importance of producing electric power from renewable energy resources such as solar and wind energies which are environmental friendly [1]. Solar energy plays a major role since it is globally available, flexible with regard to the system size and because it can fulfill the needs of different countries since it offers on-grid and off-grid solutions. Normally, the PV installation is static, does not need strong high towers, produces no vibration, and does not need cooling systems. In addition, it is environmentally friendly, safe, and has no gas emissions. So, the use of PV systems in electricity generation started in the seventies of the 20 th century and today is currently growing rapidly around worldwide in spite of high capital cost [2-4]. Grid-connected PV systems currently dominate the PV market, especially in Europe, Japan and USA. With utility interactive systems, the public electricity grid acts as an energy store, supplying electricity when the PV system cannot. The performance of a PV system largely depends on solar radiation, temperature and conversion efficiency. Although, PV systems have many advantageous, they suffered from changing of system performance due to weather variations, high installation cost, and low efficiency that is hardly up to 20% for module [5-6]. Therefore, the modeling of PV system is an important aspect to describe the performance of the PV systems at different weather conditions to meet the actual system operation during seasonal weather variations. Photovoltaic modules have a low efficiency as compared to other renewable energy resources. As a consequence, it is mandatory to adopt an intermediate conversion stage, interfacing the PV array with the power system to maximize the output power from PV array through a MPPT algorithm. There were several research works in the literature that aims to extract maximum power MPP, control of active and reactive power, and increase power quality by reducing harmonic current distortion [7-9]. Various MPPT methods are presented in literature [10-17]. Among all the MPPT methods, Perturb & Observe (P&O) are most commonly used because of their simple implementation; they have faster time to track the MPP and also other economic reasons. A modified P&O algorithm based on weather change at constant load has the advantage of being independent of the knowledge of the PV generator characteristics, so that the MPP is tracked regardless of the irradiance level, temperature and degradation, thus ensuring high robustness and reliability [18-19]. MPP tracker is done with the aid of DC-DC converters that matches the PV load impedance with the source PV impedance by varying duty cycle D of DC-DC converter. The purpose of DC- DC converters is to convert unregulated DC input voltage to regulated or controlled DC output voltage at a desired voltage range. In such systems the input voltage is often fluctuating due to variation of solar irradiance. Normally, the DC-DC converter adjusts its operation according to the optimal Power of the PV module [20-21]. Also, a high DC capacitor is connected across the PV panel terminals to reduce the DC voltage ripple of PV array. There are two control strategies for three-phase VSI such as current control and voltage control. The VSI voltage-controlled use the phase angle between the inverter output voltage and the grid voltage to control the power flow [22]. The control approach in designing the grid connected PV inverter employs two control loops: an outer control loop that is used to regulate the output power from the PV array to the grid, and an inner control loop that is used to regulate the injected current to the grid and keep it in phase with voltage to achieve unity power factor operation [23]. Many control mechanisms have been proposed to regulate the inverter output current that is injected into the UG [23-24]. The developed control technique for the three phase inverter includes a method for DC link voltage control that can stabilize the voltage at the inverter input which gives the inverter to function efficiently. This paper describes PV system performance during seasonal weather at all day to obtain optimal operation of PV three-phase distributed generator that is suited for large /16/$ IEEE

3 building such as faculties in universities, hospitals or large shopping center. The PV system is connected generally with medium voltage network through power transformer to confirm power sharing between PV system and grid. This system is generally designated for operation with public network which has three-phase line voltage is 20kV±10%. II. SYSTEM MODELING The proposed model of a grid-connected PV system for general rooftop building is shown Fig. 1. The system consists of PV arrays of Solar panel Sun Power SPR-305-WHT, its data sheet is shown in Table.1, the DC-DC boost converter for boosting the PV voltage to a level that is adequate for the inverter to produce a maximum output voltage, a three-phase VSI connected between DC link capacitor (i.e. acts as a temporary power storage device to provide the VSI with a steady flow of power) and LC filter to prevent harmonics from propagating into the UG. The normal PV voltage is 274 V dc at a solar irradiance of 1000 W/m 2, which sets up to 500 V dc via DC-DC boost converter and then converted into AC voltage by a three level VSI up to 20 kv through a step up transformer to inject to UG. A. Modeling of PV System The modules of PV array are connected in series and parallel manner to give rated power as given in Table 1. Fig. 2 illustrates the electrical circuit of PV array based on accurate two diode model. The output current (I PV ) of the PV array is given by [25]: I PV =N p I ph -N p I exp 1.0 V PV I PVR s -1 - N s N p N p I exp 1.0 V PV I PVR s -1 N s N p 1 R sh V PV N s I PVR s N p (1) Fig. 2: Equivalent circuit of single-diode model for PV array [25 ] Where: I s1 is the saturation current of diode D 1, I s2 is the saturation current of diode D 2, V T is the thermal voltage (V T =N s KT/q), K is the Boltzmann constant, T is the absolute temperature in degrees kelvin, q is the electronic charge (1.6 * C) a 1 is ideality factor of diode D 1, a 2 is ideality factor of diode D 2, V PV is the output voltage of PV array, R s, and R sh are series and shunt resistances of PV cells N s and N p are the number of series and parallel cells respectively The two-diode model parameters of photo current (I ph ), saturated currents (I s1, I s2 ), series and shunt resistances (R s, R sh ), and ideality two-diode factors (a 1, a 2 ) are given in Table. 1. B. Modeling of DC-DC Boost Converter The output and input relations of the DC-DC boost converter shown in Fig. 3 during continues mode operation are given as follows [21,26-27]: (2) ( ) (3) (4) (5). Where: Fig. 1: Basic block diagram of PV three- phase grid connecter inverter

4 Module Item Sun Power SPR-305-WHT Table. 1: Sun Power SPR-305-WHT modules Parameters of Two-diode model SPR-305-WHT Two-diode model values DC-DC Boost Converter DC-DC boost parameter values P max W Iph A C B 100 μf V OC 64.2 V I s x10-11 A L boost 5x10-3 H I sc 5.96 A I s x10-11 A C DC 6000 μf V mpp 54.7 V R s Ω f s 5000 HZ I mpp 5.58 A R sh Ω LC filter Parameter values Efficiency 16.4 % a L f 250 μh Number of cells 96 cell a C f 130 μf Where: is the amplitude of ripple boost inductor current 1.4 %, f s is the switching frequency of boost converter power switch, is the main voltage across the DC capacitor = 500V, is the amplitude of ripple PV output voltage, 0.4% is the rated power of PV system = 100 kw, ω g is the utility grid angular frequency in rad/sec = rad/s, is the amplitude of ripple capacitor voltage,10%, The DC-DC boost converter parameters are computed from Eqs. (2:5) and listed in Table 1. Fig. 3: DC-DC boost converter with its controller C. Power Filter The UG interface contains filters to reduce the harmonics generated by the inverter and to neutralize spikes coming from the UG [28]. Model of power filter considered in this system is shown in Fig.4 which is a three-phase passive filter of second order low pass filter. The filter is installed at the output of threephase VSI. The selection of the capacitor is a trade-off between reactive power supplied by the capacitor at fundamental frequency and coil inductance. The increasing capacitance value will reduce the efficiency of the inverter, while reducing capacitance value of LC filter will increase the inductor and its voltage drop across network. Normally the reactive power of capacitor is suggested less than 15% of the rated inverter power. In such design, the reactive power is chosen as 10% of the rated power of the inverter [29]. P PV C f =10%. 2 3 * 2π * V (6) g The selection of filter inductance L f depends on resonance frequency of filter that is should be greater than or equal to 10 th of grid frequency to avoid resonance with grid network. So, the filter inductance is expressed as: L f (7) ( ) The LC filter parameters are listed in table.1 D. Modelling of VSI Voltage source inverter (VSI) is used to convert the DC PV power into AC power that is injected to the UG. The DC-AC power conversion process is carried out with the aid of three level voltage source inverter (3L-VSI). The advantages of such inverters are for [30-32], improving voltage quality [33], reducing conduction loss and switching frequency, lowering blocking voltage and voltage tresses (dv/dt) on power switches. The simplified schematic diagram of a single leg of a threelevel capacitor clamped VSI is shown in Fig. 5. V dc 2 N C 1 C a S a1 S a2 D a1 D a2 A S a3 D a3 Fig. 4: LC power filter model connected at the output of PV inverter - V dc 2 C 2 S a4 D a4 Fig. 5: Simple single leg of a three-level VSI

5 Table 2 lists the output voltage levels for one phase of the inverter. The state condition 1 means the switch is ON, and 0 means the switch is OFF. Table 2: Switching states of 3L-VSI Pole voltage, Va0 Switch State Sa1 Sa2 Sa3 Sa4 Vdc/ Vdc/ III. CONTROL SCHEMES A. MPPT control system The MPPT controller uses the MP&O MPPT algorithm of the PV panel voltage and current based on reference. Fig. 6 illustrates the flowchart of the MPPT P&O modified algorithm [18-19]. The flowchart of Fig. 6 show that, at each sampling period, the MPP can be tracked by comparing the change in power, voltage and load demand with respect to zero in order to get the correct direction for perturbing the PV array voltage, to locate the MPP quickly where, equals to at MPP. Once the MPP is reached, the operation of the PV array is maintained at this point unless there is change in, which indicates a change in solar radiation or weather condition. The algorithm decreases or increases to track the new MPP [18-19]. This method provides a better tracking of the MPP under fast changing atmospheric conditions as compared with the conventional P&O method. B. Control of Grid-Connected Inverter Figure 7 shows control scheme of VSI. A PLL is used to synchronize the inverter with the grid, where it takes the grid voltage and gives the frequency and phase angle of the grid voltage correctly even with distortion in the grid voltage [34]. The system control is composed of voltage and current regulators to improve power factor and satisfy synchronization requirements of inverter with the UG in synchronous frame. The Voltage Regulator minimizes variation in the DC voltage due to change weather conditions via the PI controller. Voltage regulator generates command current (I d ) * to process of its current regulator through comparing measuring DC voltage with setting DC voltage (Vdc) *. The current regulator consists of PI controllers for both i d and i q currents. The command current (I d ) * is drawn from Voltage regulator and compared with the grid current (I d ). The compared signal (ΔI d ) is processed through the PI controller for minimizing the error and producing adding signal with generated voltage measuring signal (V d ) to compare with ω L to produce (V d )* command. Also, the command current (I q ) * is equal to zero to improve power factor of inverter up to unity. The signal (ΔI q ) is processed through the PI controller to produce adding signal with generated voltage (V q ) and ω L to produce (V q )* command. The outputs of the PI controllers are processed through hysteresis band to limit the errors between upper and lower limits. The dq0, is converted into the three phase (abc) to PWM. 12 Pulses SPWM * v abc abc dq0 θ Fig. 6: Flowchart of the P&O MPPT algorithm V d ' ' Vq v dc v dc * Current Regulator PI Controller I PI d - * I d - R - ωl ωl PI PI Controller PI I d * actual to PI Controller p.u. values R I q - I q * =0 V d V q Three-phase PLL θ abc I q dq0 abc dq0 θ actual to p.u. values Fig. 7: Control scheme for three-phase grid-connected VSI IV. SYSTEM SIMULATION AND RESULTS The MATLAB/Simulink PV power inversion connected grid is shown in Fig. 8. The system is composed of PV array of 100 kw; a modified P&O MPP tracker equipped with boost converter, 3-phase-three level inverter with LC Filter, step-up transformer and control system to inject active power to 20 kv utility grids. To satisfy the results, the PV module is simulated with residential load during variable weather conditions. Fig. 9 shows the hypothetical solar radiation distribution over a specified period ranging between 200 W/m 2 and 1000 W/m 2. The irradiance figure shows that the peak irradiance at sunny period, while the roughness of irradiance levels is due to clouds. PLL v abc i abc

6 Fig. 8: System configuration of PV grid-connected system and its control A. Input Inverter DC Results Figure 9 shows the maximum extracting PV power, using a modified P&O algorithm during irradiance variation shown in Fig. 9-a. From this figure, it can be seen the maximum output power reaches a value of kw at irradiance of 1000 W/m 2, Fig.9-b. The P&O algorithm follows the irradiance variation to extract maximum power and keeping DC voltage at constant value of 500 V as in Fig.9-d. Fig.9-c shows the DC current that is the change in its value is according to irradiance levels. B. Output Inverter AC Results Fig. 10-a, shows the simulated phase A voltage of 3L-VSI before filtering by LC filter. Figs. 10(b-c) show the AC line voltage of the inverter at the input and output of filter LC. The output line voltage before LC filters is more distorted and content more harmonic distortion. The LC passive filter reduced harmonic voltage distortion and filtered the inverter to the sinusoidal waveform as shown in Fig.10.c. C. Grid Side Results Figs.11 (a-b) shows the three-phase line voltage and current waveforms at B2 of the grid side with zoom versions to have an insight view of these waveforms. From these figures, it can be seen that the VSI produces a balanced sinusoidal three-phase voltage waveforms in synchronism with grid voltage for injecting three-phase sinusoidal current inverter current. Figure 11(c-d) depicts the injected three-phase current waveforms at B2, with a zoom version of these waveforms. From these figures, it was observed that the inverter sinusoidal output current has the same form as the hypothetical solar radiation distribution over the day. Fig. 11-e shows the simulated both voltage and current profiles of phase A at bus B2 before the coupling transformer. From this figure, it can be seen that the voltage and injected current are in phase which means unity power factor. This can be seen from zero quadrature axis current component i q which represents the reactive component as shown in Fig. 12. Fig. 13 shows the real power injected to the UG. D. THD content studies A major power quality problem in power electronic converter applications is the harmonics in the voltage/current provided by the inverters. From simulation, the harmonic spectrums of the 3L- VSI is shown in Fig. 14(a-b) for phase A voltage at the input and output LC filter at Bus B2. From these figures, it can be observed that the installation of LC filter reduces the amount of THD for 3L-VSI topologies. The harmonics generated by the inverter is reduced through the passive filter. The THD for 2L- VSI was improved for 3L-VSI was improved from 41.06% to be 1.01%.

7 (a) (b) (b) (c) Fig. 10: AC inverter voltage before and after LC filter (c) (a) Three- phase inverter voltage at grid side of B2 (d) Fig. 9: DC Results for input of three-phase 3-level inverter (a) (b) Zoom of three-phase voltage at B2 (c) Three- phase inverter injected current at B2

8 DC voltage at constant value of 500 V DC. The system performance has good agreement with UG. (d) Zoom of three-phase injected current (e) Phase angle between voltage and current Fig. 11: Inverter voltage and injected current at the UG (a) Fig. 12: Quadrature axis current component Fig. 13: Active and reactive powers injected to UG V. CONCLUSION This paper studies the performance of PV-grid connected system composed of a PV array, a modified P&O tracker and three level three-phase inverter equipped with LC filter and setup transformer. The control system based on synchronous frame of utility grid synchronous inverter with grid utility according to grid code and improved inverter power factor to unity. The LC filter improved power quality of the inverter for reducing harmonic distortion. The MPP tracker follows irradiance levels for extracting maximum power and stabilizing (b) Fig. 14: Harmonic spectrum of phase A voltage at the input and output LC filter REFERENCES [1] R. Y. Georgy and A. T. Soliman, Mediterranean and National Strategies for Sustainable Development Priority Field of Action 2: Energy and Climate Change, Energy Efficiency and Renewable Energy, Egypt's National Study, Final Report, Plan Bleu, Regional Activity Centre, March [2] REN21, "Renewables Global Futures Report, in Renewable Energy Policy Network for the 21 st Century 2013" Paris, France [3] Walid Omran, Performance Analysis of Grid-Connected

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