See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/287587872 Microgrid test-bed design with renewable energy sources Article · December 2014 DOI: 10.1109/EPEPEMC.2014.6980622 CITATIONS READS 19 189 3 authors: Ersan Kabalcı Eklas Hossain Nevşehir Hacı Bektaş Veli University Oregon Institute of Technology 134 PUBLICATIONS 1,845 CITATIONS 131 PUBLICATIONS 1,282 CITATIONS SEE PROFILE R. Bayindir Gazi University 186 PUBLICATIONS 2,191 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Microgrid Test-bed Design with Renewable Energy Sources View project Renewable Energy View project All content following this page was uploaded by Eklas Hossain on 22 February 2017. The user has requested enhancement of the downloaded file. SEE PROFILE 16th International Power Electronics and Motion Control Conference and Exposition Antalya, Turkey 21-24 Sept 2014 Microgrid Test-bed Design with Renewable Energy Sources Ersan Kabalci1 Eklas Hossain2 Ramazan Bayindir3 kabalci@nevsehir.edu.tr; shossain@uwm.edu; bayindir@gazi.edu.tr 1 Nevsehir University, Faculty of Engineering, Department of Electrical & Electronics Engineering, Nevsehir-Turkey 2 University of Wisconsin-Milwaukee, Department of Mechanical Engineering, Milwaukee, WI-53211, USA 3 Gazi University, Faculty of Engineering, Department of Electrical & Electronics Engineering, 06500, Ankara-Turkey Abstract—This paper is dedicated to microgrid design by using several renewable energy sources (RES) together. The generator side contains the most widely used three types of RESs that are wind energy, solar energy, and fuel cells. The wind turbine is designed according to parameters of a 6.5kW commercial permanent magnet synchronous generator (PMSG). The solar plant that has 15kW rated power is constituted with string connection of 96 solar panels that has 170Wp rated power for each. The last generator of system is modelled according to parameters of a fuel cell that generates 10.5kW peak power. All the generation units are coupled over dc bus-bar by adjusting the voltages to 48Vdc. The dc loads are directly connected to bus-bar while the ac loads are supplied by ac bus-bar that is built by a three-phase 48V/380V-50Hz inverter. An RLC load with 10kW/250VA/250VAR power is connected to the output of inverter. The proposed test-bed can be improved by adding several generator and load types, and additional controllers, breakers, and contactors. In its current state, the proposed testbed is considered as a preliminary design. Keywords-wind turbine, solar panel, fuel cell, microgrid, renewable energy. I. INTRODUCTION The renewable energy sources (RES) are one of the most extensively studied area among power electronics and energy engineering. Since the limited reserves of fossil based fuels and increased costs, the researchers and scientists are focused on alternative energy sources that may tackle the cost and sustainability problems [1-3]. The distributed generation (DG) concept enables service providers and consumers to eliminate the dependency to a uniformed energy generation met in centralized systems. The DG plants are intended to generate the energy where it is located at near-site/on-site to consuming sites. A microgrid consists of several on-site DG sources together [4-6]. The thermal energy sources can also be integrated to electrical sources in these improvements. The actual microgrid studies cover DG planning, power flow controls, power sharing issues, and grid integration criteria. There are numerous methods developed to manage the load sharing, and related communication methods [6]. The control methods of microgrid are classified as centralized, namely hierarchical and decentralized regarding to requirements itself. The decentralized control promotes the distributed control mechanisms and allows managing the complex infrastructures by providing autonomous systems. However, it involves significant coordination among related parts of the entire system [7]. On the other hand, the hierarchical control is based on a master-controller that tracks the generating and consuming parts in terms of real and reactive power flows. The reactive power of voltage and real power of frequency yields the required real and reactive power of microgrid. Therefore, the master controller manages the droop characteristics of frequency or voltage in order to control the power flow [7,8]. The study proposed in this paper covers a microgrid improvement with centralized control features for each generator where the microgrid model consists of a wind turbine, solar plants, and a fuel cell stack. The powers generated by the sources are used to form a dc coupling that is interfaced with a unique inverter instead of configuring ac coupling. The wind turbine is modeled by using the parameters of a 6.5kW PMSG in Simulink. The solar plants that provide 15kW total power are configured with serial and parallel connection of 96 separate solar panels that each generates 170W peak power itself. The wind energy conversion system (WECS) consists of uncontrolled rectifier and buck converter to transfer the generated power to dc bus. The solar plants and fuel cell are connected to dc bus over dc-dc converters since they do not contain any ac waveform. All the converters are controlled with PI loops, and maximum power point tracking (MPPT) algorithm in solar system. The details of entire microgrid designed are introduced in the following sections such as the proposed design of microgrid testbed along with the mathematical model of DGs has illustrated in section II; in section III, the verification and analyses of designed on-site generation section will be found and finally this research paper will be concluded future direction of work in section IV. II. THE DESIGNED MICROGRID TESTBED The microgrid configurations are based on numerous sources generating energy that enables to enhancing the energy efficiency and improving the total yield. The microgrid has a robust structure depending on the “plug and play” characteristic of energy sources. This definition stands for the facility of any source integrated to existing DG system without requiring any configuration. There are many ways to integrate different renewable energy generation sources to form a hybrid system that are dc-coupling, ac-coupling, and hybrid coupling [1, 9]. The proposed design is based on dc-coupling integration as shown in Fig.1. The parameters of the generators are given in Table 1 where the modelling studies of each generation unit 978-1-4799-2060-0/14/$31.00 ©2014 IEEE PEMC 2014 907 16th International Power Electronics and Motion Control Conference and Exposition Antalya, Turkey 21-24 Sept 2014 v= Velocity of wind (m/s) The efficiency of rotor Cp varies with the tip-speed ratio (TSR) which defines as follows for a given wind speed, TSR ZR (2) v where, R= rotor radius, Ȧ= angular speed (rad/s) Fig. 1. Schematic diagram of the modelled dc-coupled microgrid. are introduced in this section. The cut-in speed of wind turbine is 3m/s while the cut-off speed is 20m/s. The rated power output is obtained at 9m/s where the rotational speed is around 100 rpm. TABLE I. PARAMETERS OF GENERATION UNITS Wind Turbine Generator Type Pole Nominal Voltage Nominal Speed Rated Power Stator phase resistance Phase inductance PMSG 24 226V 230 rpm 6500W 1.2451ȍ 18.52mH Solar Panel Rated Power 170V Open Circuit Voltage (Voc) 21.6V Maximum Power Voltage (Vpm) 18.2V Short Circuit Current (Isc) 7.94A Maximum Power Current (Ipm) 7.23A Number of Series Panel 8 Number of Parallel Panel 12 Fuel Cell Stack DC Voltage at 135A 77.6V Cell Count 120 Maximum Current 160A Nominal stack efficiency (%) 85 Fuel composition Hydrogen, nitrogen blend 80% H2 A. Wind Turbine and Power System The output power of wind turbine vary depending on mechanical parameters as wind speed, air pressure, pitch angle and coefficient.The mechanical power of wind turbine is determined using (1); Pm 1 C p (O , E ) U A v 3 2 where, Pm= mechanical output power of wind turbine, Cp= Coefficient of performance (the rotor efficiency), Ȝ= Pitch angle (degree), ȕ= Peak velocity ratio, ȡ= Air density (kg/m3), A= Rotor swept area (m2), PEMC 2014 The PMSG are widely used in small-scale wind turbines for variable speed configurations [10]. The efficiency that is one of the most important features of PMSG is provided by its self-excitation property. It does not require any external excitation source and the maintenance cost is reduced since the brushes, slip rings, and rotor windings are removed. The technical details and equations of PMSG can be found in textbooks [17]. Therefore, they will not be repeated in this section. The output current and voltage waveforms supplied by wind turbine at 6 m/s are shown in Fig.2. The generator voltage is measured at 116V while the current is around 10A. B. Solar Panel and Power System The solar panel model is built in Simulink by using the analytical equation of a photovoltaic (PV) cell that is shown in (3). Io §ª §V I R I L I Rs ¨ «exp ¨ O o s ¨ © K IVT ©¬ · º VO I O RS ¸ 1» Rsh ¹ ¼ · ¸ ¸ ¹ (3) in where; Io: output current of panel, IL: current generated by irradiation value, IRs: current of output resistor, VT: thermal voltage, Vo: output voltage, ȘI : diode ideality factor The electrical power obtained from a PV cell depends on the short-circuit current (Isc), the ideality factor of diode (ȘI), the shunt resistance (Rsh), and the series resistance (RS) parameters [1]. The solar panel module is constituted with series and parallel connection of PV cells in the required (1) Fig. 2. Current and voltage waveforms of wind turbine. 908 16th International Power Electronics and Motion Control Conference and Exposition (a) (b) Fig. 3. I-V and P characteristics of modelled PV module under various irradiation conditions, (a) I-V characteristic when irradiation increases from 200 W/m2 to 1000 W/m2, (b) Power characteristic under same conditions. number where the similar connection of panels are also performed to design the solar plants that yield 15kW rated power to microgrid. The verification of a solar panel model is done by visualizing the current-voltage (I-V) and power curves according to various irradiation values. The curves has shown in Fig.3 are obtained by applying irradiation from 200 W/m2 to 1000 W/m2 to solar panel models. The MPPT algorithm of the solar plant is implemented adding PI control to conventional perturb&observe algorithm where the response time is decreased. The MPPT algorithm provides dynamic gate triggering signal to switch of buck converter that stabilizes the output voltage to 48V. C. Fuel Cell Stack and Power System The fuel cell stack model is designed regarding to commercial features of a polymer electrolyte membrane (PEM) fuel cell that has 10.5kW rated power. Differing from other designs, the fuel cell model is provided by Simulink libraries where it can be configured according to demanded parameters. The stack design is modelled with the parameters shown in Table 1. A PEM fuel cell converts the chemical energy to electrical energy during an electrochemical reaction of hydrogen and oxygen. The reaction of anode, cathode, and total electrical energy generated by a cell are calculated as shown in (4)-(6) respectively [11-13]. H 2 o 2H 2e ? E 0 0V O2 2 H 2e o H 2O ? E 0 1.229V 2 H2 O2 o H 2O ? E 0 1.229V 2 (4) (5) (6) The configured stack generates 77.6V nominal output while supplying 135A to the load. The maximum output values are 100V and 160A for the transient peak times. The voltage, current, and efficiency curves of the stack are shown in Fig.4, respectively. The output voltage is applied to a buck converter that adjusts the dc line voltage owing to its PI feedback controller. PEMC 2014 Antalya, Turkey 21-24 Sept 2014 Fig. 4. Waveforms of fuel cell stack. D. DC-DC Converter and Inverter The converters utilized in each generating unit are modeled in buck converter topology that is well established in power electronics area. The switching signals are updated in each cycle of feedback comparison and are separately generated at 30 kHz for each converter. The feedback control scheme is illustrated in Fig.5. The reference voltage of dc line V*dc is set to 48V and the instant output voltage of Vdc is measured to compare and minimize the error. Fig. 5. Basic feedback control scheme of buck converter The inverter is modelled in full-bridge topology of 3-phase configuration with MOSFET switches. The modulator is improved regarding to sinusoidal pulse width modulation (SPWM) where the detailed information can be found in [3]. The input voltage of inverter is supplied at 48Vdc by dccoupled busbar, and the rms value of output voltage is obtained around 40Vac. A line transformer that increases the line voltage to 380Vac at 50Hz line frequency drives the output of inverter [14-16]. III. VERIFICATION AND ANALYSES OF THE TESTBED The verification of the modeled microgrid system is performed by analyzing the generated power and related waveforms. The output voltages of each generation unit are shown in Fig.6 respectively to fuel cell stack, solar plants, and wind turbine where the last [Fig. 4] curve illustrates the coupled dc busbar voltage. The voltages generated by fuel cell and solar plants are adjusted to desired dc bus voltage instantly since they do not involve any mechanical and moving parts means the response time and time constant of entire system is tiny. However, the PI controller of wind turbine acts slower according to others because of mechanic response of wind turbine retards the PI controller. The busbar voltage that couples of each generator tolerates the spikes in power level fluctuations and harmonic noise seen in wind turbine waveforms in section II, and drives the dc bar of inverter as most of the resources is intermitting nature. 909 16th International Power Electronics and Motion Control Conference and Exposition The switching frequency of inverter is set to 5kHZ by the SPWM signals that are generated with 0.8 modulation index. The prescribed operating conditions of inverter converts dc Antalya, Turkey 21-24 Sept 2014 busbar voltage to 3-phase 40Vac (Fig.7) that is later amplified to 380V/50Hz sinusoidal voltages. The phase voltages of inverter are shown in Fig. 8 at 220V. Fig. 6. Voltage waveforms of each generator and dc busbar. Fig. 7. Output phase voltages of inverter Fig. 8. Output phase voltages of line transformer with respect to time in second PEMC 2014 910 16th International Power Electronics and Motion Control Conference and Exposition IV. CONCLUSION This study introduces a microgrid testbed design consisting wind turbine, solar plants, and fuel cell stack. The modeling and analyses operations are performed in Simulink focusing on coupling various generators on a dc busbar. Therefore, this paper is considered to analyze point of common coupling (PCC) control. There are several points of the design can be examined in detail according to different aspects of microgrid requirements. 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