INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET)

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1 INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 ISSN (Print) ISSN (Online) Volume 3, Issue 2, July September (2012), pp IAEME: Journal Impact Factor (2012): (Calculated by GISI) IJEET I A E M E SIMULATION ANALYSIS OF 100KW INTEGRATED SEGMENTED ENERGY STORAGE FOR GRID CONNECTED PV SYSTEM M.Sujith (1), R.Mohan (2), P.Sundravadivel (3) (1) Assistant professor, Vidyaa Vikas College of Engineering and Technology,Tiruchengode ID: msujitheee@yahoo.co.in (2) Assistant professor, Vidyaa Vikas College of Engineering and Technology,Tiruchengode ID: mohanraju@yahoo.co.in (3) Assistant professor, K.S.R. College of Engineering,Tiruchengode ID: sundarkmp@gmail.com 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. 164

2 6545(Print), ISSN ( (Online) Volume 3, Issue 2, July- September (2012), IAEME Traditionally, two kinds of system configurations have been used in batterysystem has integrated PV systems: ac-link system and dc-link system. The ac-link separate dc/ac converters for the 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 stage, between the battery and the grid. However, it is reported that the efficiency of current 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 CONNECTED 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 series- The direct parallel configuration to match the required solar voltage and power rating. current (DC) link capacitor maintains the solar PV array voltage at a certain level for the voltage source inverter. The single phase inverter with the output filter converts the DC input voltage into AC sinusoidal voltage by means of appropriate switch signals and then the filter output pass throughh 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 case of electric grid failure. Photovoltaic power systems are generally classified according to their functional and operational requirements, their component configurations, and how the equipment is connected to other power sources and electrical loads. The two principal classifications are grid-connected or utility-interactive systems and stand-alone systems. Fig.1 Schematic Diagram of PV-Grid System (a) CIRCUIT OPERATION The PV module is connected to the grid through an H-bridge inverter. The ES devices are integrated throughh cascaded H-bridge cells. The proposed system can operate in stand-alone and grid-connected mode through a static transfer switch (STS). Although the cascaded multilevel inverter is usually adopted for high-power and high-voltage 165

3 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<P_load and SOC >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_main<P_load and SOC <0.2, grid will provide power with (P_load P_main) to load. In this case, batteries are not allowed to release energy. Operation state 3: if P_main>P_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 included 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. 166

4 6545(Print), ISSN ( (Online) Volume 3, Issue 2, July- September (2012), IAEME III SIMULATION ANALYSISS It is a detailed model of a 100-kW array connected to a 25-kV grid via a DC-DC boost converter and a three-phase three-level Voltage Source Converter (VSC). Maximumm Power Point Tracking (MPPT) is implemented in the boost converter by means of a Simulink model using the Incremental Conductance + Integral Regulator technique. The detailed model contains: PV array delivering a maximum of 100 kw at 1000 W/m2 sun irradiance. 5-kHz boost converter (orange blocks) increasing voltage from PV natural voltage (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 technique Hz (33*60) 3-levell 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 filtering harmonics produced by VSC. 100-kVA 260V/25kV three-phase coupling transformer. Utility grid model (25-kV distribution feeder kv equivalent transmission systems). In the average model the boost and VSC converters are represented by equivalent voltage sources generating the AC voltage averaged over one cycle of the switching frequency. Such a model does not represent harmonics, 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 model the two PV-array models contain an algebraic loop. Algebraic loops are required to get an iterative 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 detailed model uses 330 Sun Power modules (SPR-305). The array consists of 66 strings of 5 series-connectekw). Open the PV-array block menu and look at model parameters. Manufacturer specifications modules connected in parallel (66*5*305.2 W= for one module are: Number of series-connected cells : 96 Open-circuit voltage: Voc= 64.2 V Short-circuit current: Isc = 5.96 A Voltage and current at maximumm power: Vmp =54.7 V, Imp= 5.58 A The PV array block menu allows you to plot the I-V and P-V characteristics for one module and for the whole array. The characteristics of the SunPower-SPR305 array are reproduced below. 167

5 6545(Print), ISSN ( (Online) Volume 3, Issue 2, July- September (2012), IAEME Fig.2 I-V and P-V characteristics of PV array Red dots on blue curves indicate module manufacturer specifications (Voc, Isc, Vmp, Imp) under standard test conditions (25 degrees Celsius, 1000 W/m2). (b) Boost converter In the detailed model, the boost converter (orange blocks) boosts DC voltage from V to 500V. This converter uses a MPPT system which automatically varies the duty cycle in order to generatee the required voltage to extract maximum power. Look under the mask of the Boost Converter Control block to see how the MPPT algorithm is implemented. For details on various MPPT techniques, refer to the following paper: Moacyr A. G. de Brito, Leonardo P. Sampaio, Luigi G. Jr., Guilhermee A. e Melo, Carlos A. Canesin Comparative Analysis of MPPT Techniques for PV Applications, 2011 International Conferencee on Clean Electrical Power (ICCEP). (c) VSC converter The three-level VSC (blue blocks) regulates DC bus voltage at 500 V and keeps unity power factor. The control system uses two control loops: an external control loop which regulates DC link voltage to +/- 250 V and an internal control loop which regulates Id and Iq grid currents (active and reactive current components). Id current reference is the output of the DC voltage external controller. 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 Uref_abc used by the PWM three-level pulse generator. The control system uses a sample time of 100 ms for voltage and current controllers as well as for the PLL synchronization unit. In the detailed model, pulse generators of Boost and VSC converters use a fast sample time of 1ms in order to get an appropriate resolution of PWM waveforms. 1. Run the photo.mdl for 3 seconds and observe the following sequence of events on Scopes. From t=0 sec to t= 0.05 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 Scope VSC). 168

6 6545(Print), ISSN ( (Online) Volume 3, Issue 2, July- September (2012), IAEME At t=0.05 sec, Boost and VSC converters are de-blocked. DC link voltage is regulated at Vdc=500V. Duty cycle of boost converter is fixed (D= 0.5 as shown on Scope Boost) and sun irradiance is set to 1000 W/m2. Steady state is reached at t=0.25 sec. Resulting PV voltage is therefore V_PV = (1-D)*Vdc= (1-0.5)*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 with a 1000 W/m2 irradiance is 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 enabled. The MPPT regulator starts regulating PV voltage by varying duty cycle in order to extract maximum power. Maximum power (100.7 kw) is obtained when duty cycle is D= At t=0.6 sec, PV mean voltage =274 V as expected from PV module specifications (Nser*Vmp=5*54.7= V). From t=0.7 sec to t=1.2 sec, sun irradiance is ramped down from W/m2 to 250 W/m2. MPPT continues tracking maximum power. At t=1.2 sec when irradiance has decreased to 250 W/m2, duty cycle is D= Corresponding PV voltage and power are Vmean= 255 V and Pmean=22.6 kw. Note thatt the MMPT continues tracking maximum power during this fast irradiance change. From t=1.5 sec to 3 sec various irradiance changes are applied in orderr to illustrate the good performance of the MPPT controller. Fig. 3 Simulation Diagram for 100KW Grid Connected PV Array 169

7 Fig.4. Waveforms of Boost Converter Fig.5. Waveform for Modulation Index and Inverter 170

8 Fig.6. Response of Voltage Source Converter Fig.7. Synchronized Grid Power 171

9 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 k2 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 k1 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 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

10 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 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): 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 , S. R. Bull, Renewable energy today and tomorrow, Proc. IEEE, vol. 89, no. 8, pp , Aug Maish, Defining requirements for improved photovoltaic system reliability, Prog. Photovoltais: Res. Appl., vol. 7, pp , J. S. Lai and F. Z. Peng, Multilevel converters A new breed of power converters, IEEE Trans. Ind. Applicat., vol. 32, pp , May/June 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 , Jan./Feb,