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INTERNATIONAL OF ELECTRICAL &– International Journal of JOURNAL Electrical Engineering and TechnologyENGINEERING (IJEET), ISSN 0976 6545(Print), ISSN 0976 – 6553(TECHNOLOGY Online) Volume 3, Issue 2, July- September (2012), © IAEME (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online ) Volume 3, Issue 2, July – September (2012), pp. 164-173 © IAEME: www.iaeme.com/ijeet.html Journal Impact Factor (2012): 3.2031 (Calculated by GISI) www.jifactor.com
IJEET ©IAEME
SIMULATION ANALYSIS OF 100KW INTEGRATED SEGMENTED ENERGY STORAGE FOR GRID CONNECTED PV SYSTEM (1)
(2)
M.Sujith , R.Mohan , P.Sundravadivel
(3)
(1)
Assistant professor, Vidyaa Vikas College of Engineering and Technology,Tiruchengode-637214 Technology,Tiruchengo de-637214 Email ID:
[email protected] (2) Assistant professor, Vidyaa Vikas College of Engineering and Technology,Tiruchengode-637214 Technology,Tiruchengo de-637214 Email ID:
[email protected] [email protected] (3) Assistant professor, K.S.R. College of Engineering,Tirucheng Engineering,Tiruchengode-637214 ode-637214 Email ID:
[email protected] [email protected] ABSTRACT
The present a single-phase photovoltaic (PV) system integrating segmented energy storage (SES) using cascaded multilevel inverter. The system is designed to coordinate power allocation among PV, SES, and utility grid, mitigate the overvoltage at the Point of common point (PCC), and achieve wide range reactive power compensation. The power allocation principle between PV and SES is described by a vector diagram. An appropriate reactive power allocation coefficient (RPAC) is designed to avoid duty cycle saturation and over modulation so that wide range reactive power compensation and good power quality can be achieved simultaneously. The self-regulating power allocation control system integrating the preferred RPAC and an advanced active power control algorithm are developed to achieve the aforesaid objective. Simulation results are provided to demonstrate the effectiveness of the proposed cascaded PV system integrating SES. Key Words : Photovolatic, Segmented Energy Storage, Reactive power Allocation Coefficient, Point of common Point I INTRODUCTION
Energy Storage (ES) elements such as batteries ES have been applied to gridconnected residential PV systems for peak power shavings and backup power. Recently, it is being looked at as a possible solution for improvement of the power quality of the grid. Research in proves that integration of small energy storage can effectively reduce the overvoltage caused by reverse power flow. Moreover, battery-integrated PV systems can improve grid quality by introducing reactive power compensation and harmonics cancellation. ���
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International Journal of Ele trical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553 Online) Volume 3, Issue 2, July- September (201 ), © IAEME
Traditionally, two ki ds of system configurations have been used in batteryintegrated PV systems: ac-link system and dc-link system. The ac-link system has separate dc/ac converters for he PV array and battery. The dc-link system has a common dc/ac converter for the PV array and battery. Although each configuration has its own advantages, they both require two conversion stages, i.e., dc/dc and dc/ac st ge, between the battery and the grid. However, it is reported that the efficiency of c rrent power conditioning system with ES is 8% lower than the traditional PV system without ES. Another disadvantage is that high switching frequency must be implemented for all the converters in order to achieve lower voltage total harmonic distortion (THD). II PV-GRID CONNECTE
SYSTEM
The configuration of a single phase grid connected PV system is illustrated in Fig.1. It consists of solar PV array, input capacitor, single phase inverter, and low pass output filter and grid voltage source. The solar PV modules are connected in a seriesparallel configuration to mat h the required solar voltage and power ratin . The direct current (DC) link capacitor maintains the solar PV array voltage at a certain level for the voltage source inverter. The ingle phase inverter with the output filter con erts the DC input voltage into AC sinusoi al voltage by means of appropriate switch signals and then the filter output pass through an isolation step up transformer to setup the filter output voltage to 220 VRMS required by the electric utility grid and load. The system also consists of a battery bank for supplying the electrical loads of the clinic in ca e of electric grid failure. Photovoltaic power s stems are generally classified according to their functional and operational requirements, their component configurations, and how the quipment is connected to other power so rces and electrical loads. The two principal classifications are grid-connected or utility-i teractive systems and stand-alone systems.
Fig.1 S hematic Diagram of PV-Grid System (a) CIRCUIT OPERATION The PV module is connected to the grid through an H-bridge inve ter. The ES devices are integrated throug cascaded H-bridge cells. The proposed system can operate in stand-alone and grid-connected mode through a static transfer switch (ST ). Although the cascaded multilevel inv rter is usually adopted for high-power and high-voltage ���
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553( Online) Volume 3, Issue 2, July- September (2012), © IAEME
applications, this research revealed the following advantages of applying this topology. First, the cascaded multilevel converter with separate dc source is ideal for connecting PV and SES. The SES can be controlled and maintained individually which improves the system reliability. Second, this topology integrates ES charge/discharge control and dc/ac power conversion. Therefore, there is only one conversion stage from ES to grid, which leads to higher efficiency, lower cost, and lighter weight. Third, the wide range reactive power compensation and proper active power allocation can be achieved simultaneously to improve power quality. In the proposed topology, the power allocation strategy between PV and SES plays the key role since the power allocation and output voltage generation are coupled with each other. An RPAC is then selected by plot analysis under different conditions. The self-regulating power allocation control system is developed to achieve active power control between PV and SES, and wide range reactive power compensation. (b) Battery Active Power Control Algorithm The battery active power control algorithm includes battery active power reference generation and active power control. Depending on the system operation conditions, the active power dispatch among PV, load, grid, and batteries may come into five operation states as follows. Operation state 1:if “P_main
0.2,”no power will be delivered to grid. Batteries will provide power to meet the load requirement. Each battery is controlled to provide half of (P_load−P_main) power. Operation state 2: if “P_mainP_load” and Vpcc is not over the upper limit Vpcc limit, the excess active power from PV will be delivered to grid, that is (P_grid = P_main−P_load). In this case, there is no batteries energy exchange. So, P_auxi1_ref andP_auxi2_ref are zero. Operation state 4: if “P_main>P_load,” “Vpcc>Vpcc limit” and “SOC <0.9,” the excess active power from PV will be delivered to grid and batteries. P_grid is limited to the upper power limit P_grid_limit. Each battery is controlled to absorb half of [P_grid_limit− (P_main−P_load)] power. Operation state 5: if “P_main>P_load,” “Vpcc>Vpcc limit,” but “SOC >0.9,” the MPPT for PV module cannot be achieved. P_main is limited to the upper power limit P_main_limit. P_grid is limited to the upper power limitP_grid_limit. Batteries are not allowed to absorb power. (c) Power Allocation Analysis The flexible active and reactive power allocation among PV, SES (ES1 and ES2), and utility grid. In this paper, a battery is used as SES. Due to the PV power variation under different operation conditions, SES will be charged or discharged to meet the load/grid requirement so as to improve power quality and maintain system stability. In addition, the low-order harmonic voltages being i ncluded in the quasi-square-wave of the main inverter output voltage can be cancelled by the equivalent negative harmonic voltage generated from auxiliary inverters. The proposed PV system with SES is able to operate in both stand-alone mode and grid-connected mode through an STS.
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International Journal of Ele trical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553 Online) Volume 3, Issue 2, July- September (201 ), © IAEME III SIMULATION ANALYSI It is a detailed model o a 100-kW array connected to a 25-kV grid via a C-DC boost converter and a three-phase thre -level Voltage Source Converter (VSC). Maximu Power Point Tracking (MPPT) is implemented in the boost converter by means of a Simulink m del using the “Incremental Conductance + Int gral Regulator” technique. The detailed model contains: • PV array delivering a m ximum of 100 kW at 1000 W/m2 sun irradiance. • 5-kHz boost converter ( range blocks) increasing voltage from PV natural v ltage (272 V DC at maximum power to 500 V DC. Switching duty cycle is optimized by the MPPT controller that uses the “Incremental Conductance + Integral Regulator” tec nique. • 1980-Hz (33*60) 3-leve l 3-phase VSC (blue blocks). The VSC converts the 500 V DC to 260 V AC and keeps unity power factor. • 10-kvar capacitor bank iltering harmonics produced by VSC. • 100-kVA 260V/25kV th ree-phase coupling transformer. • Utility grid model (25-k distribution feeder + 120 kV equivalent transmission systems). In the average model the b ost and VSC converters are represented by equi alent voltage sources generating the AC volt ge averaged over one cycle of the switching freq ency. Such a model does not represent harmo nics, but the dynamics resulting from control system and power system interaction is preserved. This model allows using much larger time steps (50 microsecs), resulting in a much faster simulation. Note that in the average mo el the two PV-array models contain an algebraic loop. Algebraic loops are required to get an iter tive and accurate solution of the PV models when large sample times are used. These algebraic loops are easily solved by Simulink. (a) PV Array The 100-kW PV array of the de tailed model uses 330 Sun Power modules (SPR-3 5). The array consists of 66 strings of 5 series connected modules connected in parallel (66*5*30 .2 W= 100.7 kW). Open the PV-array block enu and look at model parameters. Manufacturer specifications for one module are: Number of series-connected cells : 96 Open-circuit voltage: Voc= 64.2 V Short-circuit current: Isc = 5.96 Voltage and current at maximu power: Vmp =54.7 V, Imp= 5.58 A The PV array block menu allow you to plot the I-V and P-V characteristics for one module and for the whole array. The characteristics of the SunPower-SPR3 5 array are reproduced below.
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International Journal of Ele trical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553 Online) Volume 3, Issue 2, July- September (201 ), © IAEME
Fig.2 I-V and P-V characteristics of PV array Red dots on blue curves indicate module manufacturer specifications (Vo , Isc, Vmp, Imp) under standard test cond itions (25 degrees Celsius, 1000 W/m2). (b) Boost converter In the detailed model, the boost converter (orange blocks) boosts DC oltage from 273.5 V to 500V. This converter uses a MPPT system which automatically varies the duty cycle in order to generat the required voltage to extract maximum pow r. Look under the mask of the “Boost Converter Control” block to see ho the MPPT algorithm is implemented. For details on various MPPT techniques, refer to t e following paper: Moacyr A. G. de Brit , Leonardo P. Sampaio, Luigi G. Jr., Guilherm A. e Melo, Carlos A. Canesin “Comparative Analysis of MPPT Techniques for PV A plications”, 2011 International Conferenc on Clean Electrical Power (ICCEP). (c) VSC converter The three-level VSC blue blocks) regulates DC bus voltage at 500 and keeps unity power factor. The cont ol system uses two control loops: an external control loop which regulates DC link voltage to +/- 250 V and an internal control loop wh ch regulates Id and Iq grid currents (active and reactive current components). Id current reference is the output of the DC voltage external controll r. Iq current reference is set to zero in order to maintain unity power factor. Vd and Vq voltage outputs of the current controller are converted to three modulating signals U ef_abc used by the PWM three-level pulse generator. The control system ses a sample time of 100 ms for voltage and current controllers as well as for th PLL synchronization unit. In the detailed odel, pulse generators of Boost and VSC converters use a fast sample time of 1ms in or er to get an appropriate resolution of PW waveforms. 1. Run the photo.mdl for 3 econds and observe the following sequence f events on Scopes. From t=0 sec to t= 0. 5 sec, pulses to Boost and VSC converters are blocked. PV • voltage corresponds to open-circuit voltage (Nser*Voc=5*64.2=321 V, see V trace on Scope Boost). The three-level bridge operates as a diode rectifier and DC link capacitors are charged above 500 V (see Vdc_meas trace on Sco e VSC).
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International Journal of Ele trical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553 Online) Volume 3, Issue 2, July- September (201 ), © IAEME •
•
•
•
At t=0.05 sec, Boost and VSC converters are de-blocked. DC link voltage is regulated at Vdc=500 . Duty cycle of boost converter is fixed (D= .5 as shown on Scope Boost) and un irradiance is set to 1000 W/m2. Steady sta e is reached at t=0.25 sec. Resulting PV voltage is therefore V_PV = (1-D)*Vdc= (10.5)*500=250 V (see V trace on Scope Boost). The PV array output power is 96 kW (see Pmean trace on Scope Boost) whereas maximum power ith a 1000 W/m2 irradiance is 1 0.7 kW. Observe on Scope Grid that phase a voltage and current at 25 kV bus are in phase (unity power factor). At t=0.4 sec MPPT is nabled. The MPPT regulator starts regulating V voltage by varying duty cycle in order to extract maximum power. Maximum power (100.7 kW) is obtaine when duty cycle is D=0.453. At t=0.6 sec, PV mean voltage =274 V as expected from PV module specifications (Nser*V p=5*54.7= 273.5 V). From t=0.7 sec to t=1.2 sec, sun irradiance is ramped down from 100 W/m2 to 250 W/m2. MPPT continues tracking maximum power. At t=1.2 sec hen irradiance has decreas d to 250 W/m2, duty cycle is D=0.485. Corres onding PV voltage and power are Vmean= 255 V and Pmean=22.6 kW. Note tha the MMPT continues tracking ma imum power during this fast irradiance change . From t=1.5 sec to 3 se various irradiance changes are applied in order to illustrate the good per ormance of the MPPT controller.
Fig. 3 Simulation Diagram for 100KW Grid Connected PV Arra
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553( Online) Volume 3, Issue 2, July- September (2012), © IAEME
Fig.4. Waveforms of Boost Converter
Fig.5. Waveform for Modulation Index and Inverter
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553( Online) Volume 3, Issue 2, July- September (2012), © IAEME
Fig.6. Response of Voltage Source Converter
Fig.7. Synchronized Grid Power
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553( Online) Volume 3, Issue 2, July- September (2012), © IAEME
Fig. 8. Grid Voltage and Current
IV CONCLUSION
This paper has addressed the development of the cascaded PV system integrating SES. The proposed PV system can provide enhanced active power smoothing and expanded reactive power compensation. A developed dual-stage DFT PLL method was verified to be able to achieve the active and reactive power separation and improve the dynamic performance of the PV system. A coordinated power allocation strategy based on the proposed dual-stage DFT PLL can effectively allocate the active and reactive power between PV and SES. An appropriate reactive power allocation coefficient k 2 was derived from RPAC analysis under different conditions to achieve wide range reactive power compensation without degrading power quality. The particular battery active power control algorithm was conducted to deduce the active power allocation coefficient k 1 and improve the system stability and reliability. Overvoltage of PCC caused by reverse power flow is eliminated by appropriately dispatching PV power to SES. The simulation results confirmed the validity of the proposed power allocation control. V REFERENCES
1. Dr P.S. Bimbhra (2012) ‘Power Electronics’, Khanna publishers, Fourth edition, pp.127-198. 2. Moacyr A. G. de Brito, Leonardo P. Sampaio, Luigi G. Jr., Guilherme A. e Melo, Carlos A. Canesin “Comparative Analysis of MPPT Techniques for PV Applications”, 2011 International Conference on Clean Electrical Power (ICCEP). 3. Gopal k. Dubey (2007) ‘Fundamentals of Electric Drives’, Narosa publishing house, Second edition, pp.385-397.
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4. Juan Manuel Carrasco, et. al, ‘ Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey ’, IEEE Trans. On Industrial Electronics, Vol. 53, No. 4, 2006, pp. 1002- 1016. 5. Kjaer SB, Pedersen JK, Blaabjerg F., A review of singlephase grid-connected inverters for photovoltaic modules. IEEE Trans Ind Appl Sep./Oct. 2005;41(5):1292–306 6. M. Liserre, A. Dell’Aquila, and F. Blaabjerg, “Stability improvement of an LCLfilter based three-phase active rectifier,” in Proc. Power Electronics Specialist Conference, vol.3, pp. 1195-1201, 2002. 7. S. R. Bull, “Renewable energy today and tomorrow,” Proc. IEEE , vol. 89, no. 8, pp. 1216–1226, Aug. 2001. 8. Maish, “Defining requirements for improved photovoltaic system reliability,” Prog. Photovoltais: Res. Appl., vol. 7, pp. 165-173, 1999. 9. J. S. Lai and F. Z. Peng, “Multilevel converters—A new breed of power converters,” IEEE Trans. Ind. Applicat., vol. 32, pp. 509–517, May/June 1996. 10. R. W. De Doncker, D. M. Divan, and M. H. Kheraluwala, “A three phase softswitched high-power-density dc/dc converter for high-power applications,” IEEE Trans. Ind. Appl., vol. 27, no. 1, pp. 63–73, Jan./Feb,1991.
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