2 Topology and Control Schemes

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1 AC Motor Drive Fed by Renewable Energy Sources with PWM J. Pavalam 1 ; R. Ramesh Kumar 2 ; R. Mohanraj 3 ; K. Umadevi 4 1 PG Scholar, M.E-Power electronics and Drives Excel College of Engineering and Technology pavalam.eee@gmail.com 2 PG Scholar, M.E-Power electronics and Drives Excel College of Engineering and Technology pbrameshkumar@gmail.com 3 Associative Professor, Head of the Department/ EEE, Excel College of Engineering and Technology ciemohanraj@gmail.com 4 Excel College of Engineering and Technology Uma1raj@gmail.com Abstract - In this fast approaching nature of technology the need of Electricity becomes a mandatory in developing technology. The need of Electricity increases the power demand where the power demand met by the conventional sources of energy has some disadvantage of pollution, this disadvantage can be decreased by the use of the Renewable energy sources like Fuel Cell and available solar energy. When a FUEL cell produces AC power, basically two stages are required for conversion first a boosting stage and second is inversion stage. In this paper the Boost inverter topology is achieved where in the conventional methods the normal DC - AC power conversion method is used where as in this paper the PWM based DC - AC inverter has been used which is useful in reducing the harmonics in the output of the Inverter. The voltage controlled output is produced in the boost inverter the current controlled output is taken from dc-dc bidirectional converter. The Fuel cell cannot be relied as a whole so a Solar PV module is connected across the Load so while the Sunlight days the PV arrays generate power and in the night time the Fuel cell is used to generate power for the load. Since, the Fuel cell and PV arrays can generate power in Partial load they are preferred than any other sources. When the output from the Solar PV array is low or when the sunlight available is not efficient in generating the power a automatic switch over is provided in the junction between the Solar PV array and Fuel cell so that whenever it happens the switch automatically switch over to another source. The simulation results are presented to confirm the operational feature of the proposed system. Key Words - PWM Boost-inverter; fuel cell; fuel cell power conditioning system; renewable energy. 1 Introduction Electricity is the most well-known energy transporter. An energy transporter is a substance or, system that moves power in a functional form from one position to one more. Electricity was generated in influence plants, in which a principal energy source is changed into electrical power. Case of widely 41

2 International Journal of Basic and Applied Science, Pavalam at. al. vigor sources are fossil fuels, falling or flowing dampen and nuclear fission. An important drawback of generating electricity from fossil fuels and nuclear fission. Most common primary energy sources for electricity generation worldwide is the adverse environmental impact, such as the greenhouse effect caused by the increase of the CO 2 concentration in the earth s atmosphere and the nuclear waste problem. Further, fossil fuel and uranium reserves are finite [1]. The output of the FUEL cell would be low more over the FUEL cell response would be very low when compared to the other conventional sources. Thus, a battery backup unit is added with the FUEL cell for improving the response time of the FUEL cell as well as to protect the FUEL cell from the over voltage or Transient Voltages. The double stage power production in FUEL cell is considered to be a bulky one thus it contains a separate circuit for Boosting process and also it contains separate circuit for Inverting process. Thus, to replace the Bulkier circuits with a simpler one the Single stage power processing is used in the FUEL cell. The objective of this paper is to propose an FC energy system with the lowest possible harmonics. In particular, the proposed system, based on the boost-inverter with a backup energy storage unit, solves the aforementioned problems, i.e., the low and variable output voltage of the FC and its slow response. The boost-inverter utilizes two identical bidirectional boost converters with a PWM and delivers in a single-stage boosting and inversion functions [2]. This results in a high power conversion efficiency, reduced converter size, and low cost. Additionally, the backup unit supplies the low-frequency current harmonics, hence minimizing the stresses on the FC, if it were to supply such currents. The control of the boost-inverter is moderately complex to handle, and sliding-mode control or double-loop voltage and current control schemes may be adopted in this system.[2] 2 Topology and Control Schemes In our conventional method of Boost inverter the controlling method or triggering method used is the normal pulse generation method. In this method of triggering the output may contains many harmonics which may leads to poor power factor and also to a very low efficiency of the operating FUEL cell. Even though the FUEL cell processing stage is reduced, due to the presence of harmonics in the output the output voltage becomes inefficient. The conventional equivalent circuit of the FUEL CELL is shown in the Fig.1 The popular FC energy system consists of two power converters: the main boost-inverter (Normal) and the extra backup unit, as shown in Figs. 2 and 3. The output of the boost-inverter is connected to the load, while the input side is supplied by the FC and the backup unit, and both are connected to the same unregulated dc bus. The backup unit incorporates a current-mode-controlled bidirectional boost converter with battery-based energy storage to support the FC power generation and two voltagecontrolled boost converters making up the boost-inverter stage. The FC energy system must dynamically adjust to varying input voltage while maintaining constant power operation. Voltage and current limits, which have been provided from manufacturer in Fig. 4, need to be imposed at the input of the converter to protect the FC from damage due to excessive loading and transients. 42 Insan Akademika Publications

3 Pavalam and Kumar International Journal of Basic and Applied Science, Fig.1: Equivalent circuit of FUEL CELL The power has to be ramped up and down so that the FC can react appropriately, avoiding transients and extending its life. The converter also has to meet the maximum ripple current requirements of the FC. 3 Modes of Control There are two modes of control available practically for robust control of the Boost inverter. Namely: 1. Sliding mode control 2. Double loop control scheme. The Sliding mode control theory is applied to a sinusoidal output voltage boost inverter with linear load. The boost inverter can be used in UPS design, where a second power conversion stage is not needed. [8] A sliding mode controller applied to the DC - AC boost converter achieved stability with respect to load parameter variation and good static behavior. The controller has a fast dynamic response, since all control loops act concurrently, and the robustness inherent to sliding mode control. The fig 5 shows the sliding mode control of Boost Inverter. In this case, the boost inverter operate with variable frequency, switching frequency varies depending on the working point. By means of this controller, the converter generates a sinusoidal output voltage with a total harmonic distortion lower than 2% [7]. 43

4 International Journal of Basic and Applied Science, Pavalam at. al. Fig.2: FC energy system consisting of the boost-inverter and a backup unit. Boost DC-AC inverter naturally generates in a single stage an AC voltage whose peak value can be lower or greater than the DC input voltage. The main downside of this construction deals with its influence. Boost inverter consists of Boost DC-DC converters that have to be controlled in a variableoperation point condition. The sliding mode control has been planned as an option to control the output voltage of the inverter. Nevertheless, it do not openly control the inductance averaged-current. The double-loop regulation scheme that consists of a new inductor current control inner loop and an also new output voltage control outer loop. These things contain compensations in order to survive with the boost variable operation point condition and to achieve a high robustness to both input voltage and output current disturbances. The double loop regulation control strategy achieves a very high unswerving performance, even in complicated momentary situations such as nonlinear masses, quick load changes, short circuits, etc., which sliding mode be in charge of cannot handle with. The Figs. 6 and 7 shows the current controlled loop and Voltage controlled loop of Double loop control scheme respectively. Fig. 3: Block Diagram of Existing FC system with Normal Boost Inverter and Battery Backup unit 44 Insan Akademika Publications

5 Pavalam and Kumar International Journal of Basic and Applied Science, Fig.4: Illustration of the beginning of life (BOL) polarization characteristics of the Nexa1.2-kW PEMFC power module: voltage-current and power-current characteristics with parasitic power graph Fig.5: Sliding mode control of Boost Inverter Fig.6: Current controlled Double loop control scheme 45

6 International Journal of Basic and Applied Science, Pavalam at. al. Fig.7: Voltage controlled Double loop control scheme 4 Proposed FC System 4.1 Description of the system The proposed FC system consists of two power converters: the main PWM Boost inverter and extra Back up unit. The input of the PWM Boost inverter is connected to the output of the FC system. The Output of the FC system consists of a Battery backup unit in parallel to it where it is used for increasing the output response of the FC system. The proposed Fuel Cell has the power regulation by the action of the PWM Boost Inverter. The Boost inverter present in the output side of the FC would consists of two inverter where the one converter is Boost converter and another one is normal DC - AC inverter. The input of the PWM inverter consists of 30 V as input and 250 V as an output. The PWM inverter is used for reducing the ripples present in the output of the FC. 4.2 Backup Unit The functions of the backup unit should be divided into two parts. First, the backup unit is designed to support the slow response of the FC and is shown in Fig. 3. Second, in order to protect the FC system, the backup unit provides low-frequency AC current that is required from the boost-inverter operation. The backup unit comprises a current-mode-controlled bidirectional boost converter and a battery as the energy storage medium. For instance, when a 1-kW load is added from a no-load condition, the backup unit immediately provides the 1-kW power from the battery to the load as shown in Table I. On the other hand, when the load is disconnected suddenly, the surplus power from the FC could be recovered and stored into the battery to increase the overall efficiency of the energy system. 46 Insan Akademika Publications

7 Pavalam and Kumar International Journal of Basic and Applied Science, Table 1: Backup unit operation P3 increase P3 Decrease Normal P1+P2 P3 P1 P2+P3 P1=P3 Discharge Charge Normal Charge Normal Normal Two generic 12-V lead acid batteries are introduced in this unit for energy storage to deal with the need to provide fast response and a relatively low cost solution. [10] The proposed backup unit performs properly not only the support function for the FC module during transients but also is used as storage when any surplus power delivered by the FC is recovered. In order to control the output current of the backup unit, the inner current control loop of the boostinverter is used. The reference of ILb1 is taken from Idc through a high-pass filtering and the demanded current Idemand relating the load change. Fig. 8: Proposed Block Diagram 47

8 International Journal of Basic and Applied Science, Pavalam at. al. Fig.9: Proposed Circuit The proposed system uses the output of the FC which is guarded by the Backup unit and the output of the FC is fed to the Unregulated DC Bus. [15] The output of the FC system is always a DC one so to convert it to AC the Boost inverter is used with the SPWM technique. Fig.10: Proposed Simulation Model 4.3 PWM inverter In this method, pulse over a half cycle, of unequal widths is generated. Pulse thickness is a sinusoidal function of the pointed location of each sequence. This is completed by comparing a sinusoidal signal of the same occurrence as inverter yield against a triangular transporter occurrence wave [50, 51, 52, and 53]. This practice is primarily used because of its plainness and easiness of realization. The 48 Insan Akademika Publications

9 Pavalam and Kumar International Journal of Basic and Applied Science, amount of signal per round is being decisive by the ratio of the triangular transporter occurrence to that of the modulating sinusoid, fr. This is shown in the figure 11. This method is known as SPWM. [1] The waveform in figure 10 is a 2 - level waveform with the inverter output changing from +V to -V Fig.11: Sinusoidal modulation The fundamental component of the PWM output waveform with N chops per quarter cycle is: = 4 Π [2 ( 1) 1] 4.4 Sismulation Result Fig.12: Output waveform The output of the simulation results is shown in the above figure 12. In this the x axis consists of Time and the Y axis consists of Voltage. 49

10 International Journal of Basic and Applied Science, Pavalam at. al. Fig.13: Current and Voltage Output waveform Fig.14: Circuit output waveforms The voltage V1, V2 are the voltages measured across the capacitor C1 and C2 and the Input voltage is 30 V and the output observed in the FC system is about 230 V Fig.15: Motor Torque and Speed 50 Insan Akademika Publications

11 Pavalam and Kumar International Journal of Basic and Applied Science, 5 Conclusion In this paper the efficiency in FC is increased by the action of SPWM technique. In this the sinusoidal signal is taken as reference signal and the sample signal is taken into account and compared for the production of the firing angle for the Boost Inverter. By using this power generated from FC can be increased for considerable amount. The technique used here would increase the output of the FC and also increases the efficiency of the FC. The system proposed here can be used to reduce the power demand in the society and thus reducing the economic set back to the nation. Due to the increase in the power demand the output from the FC can be combined with the output of the PV arrays in the grid where the power generated from both the sources can be coupled and used for power generation. The PV arrays used as a source can be used while the solar energy is sufficient and the FC can be used to produce power when PV array fails or when PV arrays efficiency is low. The inverter design for the PV arrays and the FC would be different so the inverters used in the model would be designed separately for each source. Reference [1] Kjaer.S.B., Pedersen.J.K., and Blaabjerg.F., A review of single-phase grid-connected inverters for photovoltaic modules, IEEE Trans. Ind. Appl., vol. 41, no. 5, pp , Sep./Oct [2] Kalaivani.B., Chinnaiyan.V.K., and Jerome.J., A novel control strategy for the boost dc-ac inverter, in Proc. India Int. Conf. Power Electron. (IICPE 06), India, Dec , pp [3] Caceres.R.O. and Barbi. I., A boost dc-ac converter: Analysis, design, and experimentation, IEEE Trans. Power Electron., vol. 14, no. 1, pp , Jan [4] Sethakul.P., Rael.S., Davat. B., and Thounthong.P., Fuel cell high-power applications, IEEE Ind. Electron. Mag., vol. 3, no. 1, pp , Mar [5] Mazumder. S. K., Burra. R. K., and Acharya. K., A ripple-mitigating and energy-efficient fuel cell power-conditioning system, IEEE Trans. Power Electron., vol. 22, no. 4, pp , Jul [6] Blanco. A. V., Aguilar-Castillo. C.,Abarca. F. C., and Arau-Roffiel. J., Two-stage and integrated fuel cell power conditioner: Performance comparison, Proc. IEEE APEC, vol , Feb., pp [7] Jin. K., Ruan. X., Yang. M., and Xu. M., Power management for fuel-cell power system cold start, IEEE Trans. Power Electron., vol. 24, no. 10, pp , Oct [8] Sanchis. P., Alonso. O., Marroyo. L., Meynard. T., and Lefeuvre. E., A new control strategy for the boost dc-ac inverter, in Proc. IEEE PESC 01 Conf., Vancouver, Canada, Jun , pp [9] Hodel. A. S. and Hall. C. E., Variable-structure PID control to prevent integrator windup, IEEE Trans. Ind. Electron., vol. 48, no. 2, pp , Apr [10] Thounthong. P., Rael. S., and Davat. B., Utilizing fuel cell and supercapacitors for automotive hybrid electrical system, in Proc IEEE Appl. Power Electron. Conf. Expos. (APEC05), Texas, Mar. 6-10, 2005, pp [11] Sethakul. P., Rael. S., Davat. B., and Thounthong. P., Fuel cell high-power applications, IEEE Ind. Electron. Mag., vol. 3, no. 1, pp , Mar

12 International Journal of Basic and Applied Science, Pavalam at. al. [12] Schenck. M. E., Lai. J.-S., and Stanton. K., Fuel cell and power conditioning system interactions, in Proc. APEC 2005,Mar. 6-10, vol. 1, pp [13] Lai. J.-S., Power conditioning circuit topologies, IEEE Ind. Electron. Mag., vol. 3, no. 2, pp , Jun [14] Nexa. TM Power Module User Guide, MAN , Ballard Power System, Inc., Burnaby, BC, [15] Yu. X., Starke. M. R., Tolbert. L. M., and Ozpineci. B., Fuel cell power conditioning for electric power applications: A summary, IET-Electr. Power Appl., vol. 1, no. 5, pp , Sep Insan Akademika Publications