Paper 6

‫ﻣﺆﺗﻤﺮ اﻷزهﺮ اﻟﻬﻨﺪﺳﻲ اﻟﺪوﻟﻲ اﻟﺜﺎﻣﻦ‬

AL-AZHAR ENGINEERING EIGHTH INTERNATIONAL CONFERENCE December 24 - 27, 2004

Code: S 08

COMPUTER SIMULATION OF PHOTOVOLTAIC POWER SYSTEM INTERCONNECTED WITH UTILITY GRID H. H. EL-TAMALY, ADEL A. EL-BASET MOHAMMED Faculty of Engineering, Elminia University, Elminia, Egypt.

Abstract Photovoltaic, PV is a green power source, which can convert sunlight to electricity. There are two modes of PV system operation. Stand alone PV system with battery storage and grid connected PV system without battery storage. This paper focus in grid connected PV system without battery storage taking into account all radiation and variation of the load demand during the day. It introduces a complete computer simulation program, analysis, and design of a power conditioning unit, PCU. The PCU consists of an inverter and filter. The filter has been designed to make the output of the inverter suitable and has the power quality requirement for the interconnection with utility grid, UG. The proposed computer simulation uses hysteresis current control and instantaneous p-q (real- imaginary) power theory. A computer simulation program has been designed to simulate phase voltage of the inverter leg, phase-to-phase voltage of the inverter leg, current in each IGBT's, DC input current to the inverter, AC output current of the inverter that injected to the load/grid, load current, grid current, power output of the inverter and finally power factor of the inverter. The DC input current represents the output of PV solar cell array for all sunlight conditions. The computer simulation program is confirmed on a realistic circuit model implemented in the simulink environment of Matlab. © 2004 Faculty of Engineering, Al-Azhar University, Cairo, Egypt. All rights reserved.

Keywords : Photovoltaic power system, DC/DC converter , DC/AC Inverter, Grid.

57 Al-Azhar University Engineering Journal, AUEJ Vol. 8, No. 7, Jan. 2005

INTRODUCTION Because of global environmental concerns and increasing energy demands, more attention is being concentrated on the renewable energy technologies. Compared to the traditional energy resources, the PV system, which converts sunlight into electric power, releases no pollutants for environmental concern. Because there is no negative impact to the global environment, the PV system becomes an emerging technology that gives promise as a future energy resource in tropical areas [1].

Recently, Egypt

government has formulated strategies and goals for use of PV to be interconnected to UG. A part-solar power plant is planned for Kuraymat, with 30 MW of solar power out of the total planned capacity of 150 MW. Egypt enjoys excellent solar radiation, the annual global solar radiation is between 900-2600 kWh/m2 [2]. The solar cell array produces only a small amount of current and voltage. So, in order to meet a large load demand, the solar cell array have to be connected into modules and the modules connected into arrays. The output voltage from PV array have been changed with solar radiation. So in order to connect to the UG the output voltage from PV array should be fixed and converted to AC voltage which can be done by an inverter, as shown in Figure 1. The PV converter and inverter have the task to guarantee safe and efficient operation, to track the maximum power of the PV solar cell array and to maintain good quality of the UG current. Design of PV system interconnected to UG is already well described in the literature [3]. The aim of this paper is to introduce a complete modeling and simulation of whole PV system connected to UG taking into account all variation of the radiation and the load demand during the day.

PHOTOVOLTAIC POWER

SYSTEM MODELING

The electrical power generated and terminal voltage of PV module depend on solar radiation and ambient temperature. The mathematical equation describing the I-V characteristics of a PV solar cell array is illustrated

in literatures [4,5]. From simulink of the 500 kW PV solar cell array, the output voltage and

output current due to solar radiation are shown in Figures 2 ,3 respectively. The output voltage from solar cell array is the input to the boost converter. The boost converter was fully designed and simulated with the use of Matlab software. The model's input is taken from solar cell array and the output of the model is fed to the inverter. The system have been tested for load change due to change in solar radiation. The load was 12kW at 9.30 A.M. for solar radiation 0.2 kWh/m2, 200 kW at 11.00 A.M. for solar radiation 0.6 kWh/m2 and finally to 500 kW at 1.00 A.M. for solar radiation 1.0 kWh/m2 and then back to 200 kW then 12 kW as shown in Figure 4.

Vol. 8, No. 7, Jan. 2005 58

Utility grid Lb

500 kW PV Solar cell array

0.4/11kV 500 kVA

Lf

Vinva Vinvb

Cb

Vinvc

IGBTb DC Link

Set-Up Transformer

Cf

Load 380 v, 50 Hz

Figure 1 Schematic Diagram of the PV Power System Connected to the Utility Grid

Figure 2 DC Output voltage from PV solar cell array

Figure 3 DC Output Current from PV solar cell

due to change of radiation.

array due to change of radiation.

Figure 4 Load Power curve during the Day. I. THE PROPOSED SYSTEM MODEL

The system model shown in Figure 1 demonstrates PV solar cell array connected to a 50 Hz, 380 V UG through a DC/DC boost converter and DC/AC inverter. The 600 Vdc obtained from DC/DC converter is applied to an IGBT's inverter. The task of the boost DC/DC converter drains the power from the PV solar cell array and feeds the DC link capacitor with a maximum power point tracker, MPPT control. The sensed variables for the controller are PV solar cell array current IPV, PV solar cell array voltage, VPV, DC link voltage, Vdc, inverter filter output currents Ifa, Ifb,Ifc , load phase currents Ila, Ilb,Ilc and UG phase voltages Va, Vb, Vc. The DC link voltage, Vdc must be controlled to be higher than the peak phase to phase voltage of the UG. To provide the active filtering function, the filter output currents Ifa, Ifb, and Ifc are controlled to ensure Vol. 8, No. 7, Jan. 2005 59

that the utility line currents are sinusoidal and in phase with the phase voltage. The filter output currents are also controlled to pass power from the PV solar cell array to the load and/or UG. The proposed system control scheme for the system under study usually use the instantaneous reactive power theory, IRPT [6]. The accuracy of this tracking process is constrained by the maximum allowable switching frequency of the converter power switches [7]. The load currents and load voltages are sampled and transformed into the twoaxis αβ-coordinate system and then into the rotating dq-coordinate system. IRPT uses the park transformation which given in Eq. 1 to generate two orthogonal rotating vectors α and β from the three-phase vectors a, b and c. This transformation is applied to the voltages and currents and so the symbol x is used to represent volt or current. IRPT assumes balanced three-phase loads and does not use the x0 term. x   o x α  = x   β

1 / 2 1 / 2 2 1 −1 / 2  0 3/2 3 

1/ 2 −1 / 2 − 3/2

 x   a   x b   x   c 

(1)

The instantaneous active and reactive powers p and q are calculated from the transformed voltage and current. Then the reference compensating currents have been determined as given in Eq. 2.

i *  1 α = i *   β  V 2+ V 2 α

 Vα  Vβ

β

− Vβ   Ppv 









Vα  Q   pv

(2)

In a balanced three-phase system with linear load, the instantaneous real power p and imaginary power q are constant and equal to the three-phase conventional active power P3Φ and reactive power Q3Φ respectively. So, the inverse park transformation is applied to iα* and iβ* and this gives the output currents in standard threephase form, as shown in Eq. 3.

i  i a  =  b i c 

 1 2 − 1 / 2  3 − 1 / 2 

 * 3 /2  i α   i *  - 3 /2   β   0

(3)

There are two modes of operation: • Mode 1 : When the generated power from PV solar cell array is lower than the load demand then the deficit power will be supplied from the UG. Presumably, the power factor will be within the allowed limits. • Mode 2 : When the generated power from PV solar cell array is greater than the load demand then the surplus power will be transmitted to the UG. In this condition, the power factor of the ac source will deteriorate.

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To solve this problem, the coupling converter should also supply or absorb active power and reactive power simultaneously. Then a variable reactive power reference Q* should be included in the inverter control. Thus, the AC source can operate at the allowed or unity power factor. That is, if Q* is made equal to QL, the source power factor can be kept equal to unity under different load conditions. Then the proposed control strategy is supposed to be capable of generating any output imaginary power, that is, the source power factor may be set at any desired value. In case of choosing a particular value for the source power factor, the imaginary power reference Q* should no longer be set to QL, but to the following value [6,8] :

(

)

Q * = Q L − PL − P * tan (Φ )*

(4)

where, Φ* is the source desired reference displacement angle, cos (Φ)* is the reference power factor.

SIMULATION RESULTS

Figure 5 shows Simulink block diagram for the simulated of PV solar cell array interfaced with UG through a sinusoidal pulse width modulation, SPWM voltage source inverter and its control. The parameters of the simulated circuit are as follows: DC input voltage 600 V, Three-phase line voltage 380 V, 50 Hz. Output filter Lf=1.8833 mH, Cf=70 µF and sampling time 2µ sec. The output power from PV solar cell array as shown in Figure 6 is applied to the inverter to feed the load. The total power load level is 300 kW with 455.8 A per phase load current for a duration 0.3 Sec. After 0.3 sec. the load have been changed from 300 kW to 100 kW with 151.93 A per phase load current for a duration from 0.3 Sec. to 0.5 Sec. as shown in Figure 6. The following Figures show simulation results of the proposed control strategy. Figure 7 displays the simulated phase voltage of the inverter leg, while Figure 8 shows the phase-to-phase of the inverter leg. Due to the small width of the hysteresis band, the voltage generated by the proposed model is nearly sinusoidal when seen at this bus. Figure 9 shows the waveform of the current following in one branch of IGBTs. On the other hand, Figure 10 shows the line current injected by the PV solar cell array with total harmonic distortion 1.3 %. The line current of the load demand is shown in Figure 11. From Figure 6 it can be seen that there is a surplus power in the period from 0.2 Sec. to 0.4 Sec. So the surplus power will be injected to the UG for this periods. On the other hand there is a deficit power in the period of 0.2 sec and in the period from 0.4 sec to 0.5. So, the UG will supply the load demand in cooperated with PV solar cell array for these periods. These can be seen in Figure 12 and Figure 13, where Figure 12 shows the simulated of grid line current with total harmonic distortion of 0.9% that injected to or drawn from grid and Figure 13 displays the simulated power factor of the grid. Also, from these Figures 12 and 13 it can be seen that the power factor is leading in the period of surplus power and lagging in the period of the deficit power. The input current iα(t) and iβ(t) and their corresponding load voltage vα(t) and vβ(t) are in phase, thus guaranteeing operation with a power factor very close to one as shown in Figure 14 for the simulated power factor of the inverter. From these Figures it can be seen that the proposed model is very excellent.

Vol. 8, No. 7, Jan. 2005 61

Figure 5 Schematic Diagram of the PVES Connected to the Utility Grid

Figure 8 simulated phase-to-phase voltage of the inverter leg, Vab

Figure 6 Simulated of generated power from PV, Load demand and UG power from/to UG.

Figure 7 Simulated phase voltage of the inverter leg.

Figure 9 Simulated switch current in IGBT's

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Figure 10 Simulated of inverter current injected to the

Figure 12 simulated grid current .

load/UG

Figure 11 simulated of load current

Figure 13 Simulated power factor of the grid.

Figure 14 simulated of the inverter power factor.

CONCLUSIONS From the results obtained above, the following are the salient conclusions that can be drawn from this paper: • A novel of PV interface with the UG for solving modeling and simulation problems by using Matlab/Simulink environment have been proposed. • Detailed modeling and simulation of a DC/DC and DC/AC converter connected to UG have proposed. Vol. 8, No. 7, Jan. 2005 63

• Designing the DC/DC, DC/AC converter, designing the control circuit for the converter and designing of the LC filter for all radiation have been studied and proposed. • The total harmonic distortion at the local bus is within acceptable limits and reached to 1.3 % for the inverter current and 0.9 % for the grid current. • Perform the necessary preliminary studies before investing and connecting PV power system to the grid where purchased and sold power from UG have been calculated.

REFERENCES [1] Wei-Fu Su, Shyh-Jier Huang and Chin-e. Lin, "Economic Analysis for Demand-side Hybrid PV and Battery Energy Storage System", IEEE Transactions on Industry Applications, Vol. 37, No. 1, Jan./Feb. 2001, pp.171-177. [2] New & Renewable Energy Authority, "Implementation of Renewable Energy TechnologiesOpportunities and Barriers- Egypt Country Study", Egypt, UNEP Collaborating Center on Energy and Environment, Risφ National Laboratory, Denmark, 2001, ISBN: 87-550-3011-4. [3] H. H. El-Tamaly, A. M. El-Tamaly, and Adel A. El-Baset, "Design and Control Strategy of Utility Interfaced PV/WTG Hybrid System", The Ninth International Middle East power System Conference, MEPCON'2003, Vol. 2, Dec. 16-18,2003, pp.699-704. [4] Chihchiang Hua, Jongrong Lin and Chihming Shen, " Implementation of a DSP-controlled Photovoltaic System with Peak Power Tracking", IEEE Trans. Industrial Electronics, Vol. 45, No. 1, Feb. 1998, pp.99-107. [5] T. Hiyama and K. Kitabayashi, " Neural Network Based Estimation of Maximum Power Generation From PV Module Using Environmental Information", IEEE Trans. Energy conversion, Vol. 12, No. 3, Sept. 1997, pp.241-247. [6] Barbosa P. G. et al., “Novel Control Strategy for Grid-Connected DC/AC Converters with Load Power Factor and MPPT Control ”, Congresso Brasileiro de Eletrônica de Potência, COBEP '95-III, Paulo 1995. [7] R Sharma, “Switching Frequency Filter Design for Utility Connected PV Inverters”, Australasian Universities Power Engineering Conference (Aupec2002), Melbourne, Australia, 29th Sept. to 2nd October 2002 pp. 146-152. [8] G. Grandi, D. Casadei, C. Rossi, "Dynamic Performance of a Power Conditioner Applied to Photovoltaic Sources", 10th International Power Electronics and Motion Control Conference, EPE-PEMC 2002, Dubrovnik Croatia, Sept. 9-11, 2002.

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