Co-Ordination Control and Analysis of Wind/Fuel Cell based Hybrid Micro-Grid using MATLAB/Simulink in Grid Connected Mode

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1 IJIRST International Journal for Innovative Research in Science & Technology Volume 1 Issue 12 May 2015 ISSN (online): Co-Ordination Control and Analysis of Wind/Fuel Cell based Hybrid Micro-Grid using MATLAB/Simulink in Grid Connected Mode Vidyashree M S Department of Electrical & Electronics Engineering Adichunchanagiri Institute of Technology, Chikmagalur, Karnataka, India T. R. Narasimhe Gowda Department of Electrical & Electronics Engineering Adichunchanagiri Institute of Technology, Chikmagalur, Karnataka, India Abstract Micro grid comprises of micro-energy resources, loads together with energy storage devices with single controllable system to provide power to small area. A hybrid grid consists of both DC and AC grid, where AC grid includes AC power sources, AC loads and DC grid includes DC power sources, DC loads. Both DC and AC network are connected together by bi-directional power electronics converters with common energy storage devices. This paper proposes co-ordination control and operational analysis of Hybrid Micro grid consisting of Wind turbine (DFIG) and Fuel cell stack (SOFC) with single controllable system which in turn reduces process of multiple conversions in an individual DC or AC grid i.e., DC-AC-DC or AC-DC-AC. Proposed Hybrid micro grid operates in grid tied or in grid connected mode. Co-ordination control mechanisms are implemented for power electronic converters for smooth power exchange between DC and AC links and for stable operation under various resources and load conditions. Proposed small hybrid micro grid is considered, modeled simulated and analyzed using MATLAB/ Simulink. Keywords: Micro-Hybrid grid; Fuel cell stack; WTG; Grid connected mode; Bidirectional power electronics converter I. INTRODUCTION A. Typical Hybrid Ac/Dc Microgrid: Fig. 1: the typical structure of a hybrid AC/DC microgrid Fig. 1 shows the typical structure of a hybrid AC/DC micro grid. It consists of DC grid and AC grid and are interconnected with the help of three phase bidirectional (main converter) DC/AC converter and a transformer. In the hybrid grid the three phase AC network is connected through a transformer and a circuit breaker to the utility grid. AC power sources such as, wind turbine generator (WTG), small diesel generator, and tidal power plant are connected to the AC All rights reserved by 190

2 grid. Flywheels which are AC energy storage devices are connected to the AC grid through converters. AC loads can be connected to the AC network and DC power sources such as, fuel cell stacks, solar panels are connected to the DC grid through converters. All type of DC loads and energy storage devices such as super capacitors, electric vehicles, batteries can be connected to the DC grid through DC/DC power electronic converters. II. METHODOLOGY A. Grid Configuration: Fig. 2: representation of the proposed hybrid micro grid Fig. 2 shows the representation of the proposed hybrid micro grid. The proposed system is modeled and simulated using MATLAB/Simulink to simulate the system operation and for co-ordination control. In this system, as a DC power source 50 kw fuel cells stacks are used and it is connected through a DC/DC boost converter to the DC grid. To reduce the ripples of the fuel cell output voltage the capacitor is used. Doubly Fed Induction Generator (DFIG) WTG system of 20 kw is taken as AC power source and it is connected through a back to back AC/DC/AC converter to the AC grid. As an energy storage device battery is connected to the DC network through a bidirectional DC/DC converter. DC loads and AC loads are connected correspondingly to DC and AC grids. B. Modeling of the Fuel Cell: Fig. 3: Schematic of solid oxide fuel cell A fuel cell is a device that converts chemical energy from fuels into electric energy with the help of anode, cathode and an electrolyte. In this paper, Solid Oxide Fuel Cell (SOFC) is used and it uses solid oxide or ceramic as an electrolyte. The output voltage of the fuel cell is given by the equation. V fc =E fc -V act,fc -V ohm,fc -V conc,fc Where,V fc and E fc are the fuel cell output voltage and internal voltage. V act,fc, V ohm,fc and V conc,fc are activation, ohmic and concentration voltage drops inside the fuel cell. All rights reserved by 191

3 Fig. 4: Block diagram for dynamic model of SOFC The total power generated by the fuel cell stack is represented as: P fc =N 0 V fc I fc Where, Pfc is the total power generated by the fuel cell stack. N 0 is the number of cells in the stack series, I fc is the stack current. C. Modeling Of Wind Turbine Generator Fig. 5: Modeling Of Wind Turbine Generator There are different types of wind turbine generator and one of the commonly used WTG is the DFIG. The voltage equations of an induction motor and the power output from the WTG is represented as: 3 P m =0.5 ρ A C p (, ) V w Where P m is the output power, V w is the wind speed, ρ is the air density, A is the rotor swept area, C p (, ) is the power coefficient and it is function of speed ratio ( ) and pitch angle ( ) Cp(λ,β) = c1*(c2/λi-c3β-c4)*exp(-c5/λi) +c6λ Where c1, c2,...c6 are constants. c1 =.5176; c2=116; c3=.4;c4=5;c5=21;c6=.0068 The voltage equations of the induction motor in rotating d-q coordinates and the parameters used to simulate DFIG wind turbine. The doubly fed induction generator is connected before the transformer. The stator of DFIG is connected to the primary side and the rotor also fed back to the same line. To control the stator and rotor current of DFIG AC/DC/AC converter is connected to the DFIG. The initial torque we have to give to protect the machine from excess current at the starting state. Based on the sign of torque we can analyze the machine operation. If Tm is positive the machine is working as a motor. If Tm is negative the machine is working as a generator. To provide the torque we need a wind turbine. D. Modeling of Battery: Based on nonlinear voltage source in series with the internal resistance the battery is modeled and the output voltage mainly depends on the state of charge (SOC) of the battery, which is a nonlinear function of current and time. State of a battery is represented by two parameters i.e., state of charge and terminal voltage. All rights reserved by 192

4 Terminal voltage is represented by equation. V b =V o + R b i b k SOC of a battery is represented as: Fig. 6: Model of a battery SOC = 100 ( Where, R b is the internal resistance of the battery, i b is the battery charging current, V o is the open circuit voltage of the battery, Q is the battery capacity, K is the polarization voltage, A is the exponential voltage, B is the exponential capacity. In the equation term (K ) represents the non-linear voltage which changes with state of charge of battery and the magnitude of current. III. SIMULATION ) A. Boost Converter: The fuel cell stack provides input voltage. In fuel cell stack to provide high voltage the number of fuel cells are arranged in stack manner. The boost converter boost up the output voltage of fuel stack. Based on the equations below designing of boost converter is done, K= 1-(Vs/Va) where, K is duty cycle. Vs- input voltage and Va-output voltage L= k(1-k)r/(2f) R=Va/Ia C= k/2fr where,f is switching frequency. ΔI= Vsk/fL where ΔI is initial current through inductor. ΔV=Iak/fC where ΔV is initial voltage across the capacitor. We can design the boost converter By using these equations. All rights reserved by 193

5 B. VSC Main Controller Fig. 7: Vsc Main Controller The control signal for the main inverter is produced by VSC main controller. For control signal creation the primary voltage and current of transformer are taken as an input. The three phase voltage is changing into voltages of direct and quadrature axis(vd and Vq) by using following equations based on park and Clarke transformation. Vd=2/3[Va*sinώt+Vb*sin(ώt2*pi/3)+Vc*sin(ώt+2*pi/3)] Vq=2/3[Va*cosώt+Vb*cos(ώt2*pi/3)+Vc*cos(ώt+2*pi/3)] V0= 1/3(Va+Vb+Vc) Id=2/3[Ia*sinώt+Ib*sin(ώt2*pi/3)+Ic*sin(ώt+2*pi/3)] Iq=2/3[Ia*cosώt+Ib*cos(ώt2*pi/3)+Ic*cos(ώt+2*pi/3) I0= 1/3(Ia+Ib+Ic) C. V DC Regulator: Fig. 8: V dc Regulator Measured voltage from boost converter is compared with reference voltage.the error signal is given to the PI controller to rectify the error. The output is controlled as an current reference (Id_ref).Depending upon the id value the power flow should be change. (i.e) if Idc is positive the power flow will be from fuel cell to grid. If Idc is negative power flow will be from grid to fuel cell. D. Current Regulator Fig. 9: Current Regulator Id ref is taken from the voltage regulator and the Iq_ref is taken as 0 (because of minimum reactive power). By comparing measured voltages with current ref to get the converter voltages. All rights reserved by 194

6 IV. GRID OPERATION AND CONTROL There are five types of converters used in the hybrid grid and for supplying reliable power to AC and DC loads these converters have to be coordinately controlled under variable resource and load conditions. Hybrid micro grid can be operated in two modes namely grid-connected mode and autonomous (isolated) mode. In this paper, grid connected mode of operation is modeled, simulated and analyzed for the proposed system. In the WTG the AC/DC/AC converter is used to extract maximum power from wind turbine, to regulate the rotor side current and to synchronize with the AC grid. In this mode, the utility grid is connected to the hybrid grid and the utility grid will acts as a swing bus and any power demand is balanced by the utility grid. The excess power will be transferred to the utility grid, in case, of any power surplus on the DC side. At this condition the main converter will acts as inverter. In grid connected mode, the energy storage system function is to prevent the frequent power transfer between DC and other grids. The main function of the bidirectional (main converter) is to maintain smooth power transfer or exchange between DC and AC grids and to provide stable DC voltage. The basic coordination control mechanisms for the operation of battery converter and for the WTG have been used and studied here to coordinately control the overall operation of the hybrid micro grid. Fig. 7: PQ control scheme for controlling the main converter Fig. shows the control scheme for controlling the main converter during grid tied mode of operation. For smooth power exchange between the AC and DC grids and to supply the necessary reactive power, PQ control is used for controlling the main converter. The DC bus voltage is maintained to constant value through PI controller. Output of the DC-link voltage loop through a PI controller is set as an active current reference (id *) and the reactive current reference (iq*) is set to zero. The active current (id) and the reactive current (iq) is calculated from the line current through Clarke and Park transformation. When there is an excess power on the DC grid, the main controller is controlled to transfer the power from DC grid to AC grid. In this situation, the sign of active current (id) and its reference (id *) are both positive. If there is any sudden increase in DC loads, there will be a power shortage on the DC grid and so the main converter is controlled to supply power from AC grid to DC grid. In this situation, the sign of active current (id) and its reference (id *) are both negative. V. SIMULATION OUTPUT A. Fuel Cell Output: All rights reserved by 195

7 B. Battery Output: VI. CONCLUSION The proposed architecture, operation,co-control and analyses of the hybrid microgrid consisting of DFIG wind turbine generator and fuel cell stack is studied,modelled and simulated using MATLAB/Simulink. For smooth power exchange between AC and DC grids Converters are coordinately controlled.. Although power rating of the SOFC is high, it can be effectively without any interruption used along with WTG to supply the loads. For small isolated industrial plants the hybrid microgrid is one of the best options as the major power supply. All rights reserved by 196

8 REFERENCES [1] Kodanda Ram, M. Venu Gopala Rao Operation and Control of Grid Connected Hybrid AC/DC Microgrid using Various RES International Journal of Power Electronics and Drive System (IJPEDS) Vol. 5, No. 2, October 2014 [2] Abinash Singh, Balwinder Singh Surjan MICROGRID: A REVIEW IJRET: International Journal of Research in Engineering and Technology Volume: 03 Issue: 02 Feb-2014 [3] Bariya Ketan Ratanbhai and Mr. H. B. PATEL Modeling and Simulation of Grid Connected Solid Oxide Fuel Cell Using Matlab / Simulink IJSR - International journal of scientific research, Volume : 3 Issue : 1 January 2014 ISSN No [4] Yanbo CHE, Jian CHEN Research on Design and Control of Microgrid System Przegląd elektrotechniczny (Electrical Review), ISSN , R. 88 NR 5b/2012 All rights reserved by 197