1.4850475

Simulation of a new grid-connected hybrid generation system with Stirling engine andwind turbineH. Shariatpanah, M. Zareian Jahromi, and R. Fadaeinedjad

Citation: Journal of Renewable and Sustainable Energy 5, 063128 (2013); doi: 10.1063/1.4850475 View online: http://dx.doi.org/10.1063/1.4850475 View Table of Contents: http://scitation.aip.org/content/aip/journal/jrse/5/6?ver=pdfcov Published by the AIP Publishing

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Simulation of a new grid-connected hybrid generationsystem with Stirling engine and wind turbine

H. Shariatpanah,1,a) M. Zareian Jahromi,2,b) and R. Fadaeinedjad1,c)1Department of Electrical and Computer Engineering, Graduate University of AdvancedTechnology, Kerman, Iran2Department of Electrical Engineering, Amirkabir University of Technology, Tehran,Iran

(Received 9 July 2013; accepted 2 December 2013; published online 20 December 2013)

A detail model including all mechanical and electrical aspects is necessary to fully

study hybrid grid operation. In this paper, a new grid-connected hybrid generation

system with a Stirling engine and a wind turbine, which are connected to a grid

through a common dc bus, is presented. The Stirling is more efficient than photo

voltaic array and its combination with the wind turbine can create an efficient hybrid

system. Fatigue, Aerodynamics, Structures, and Turbulence and Simulink/MATLAB

are used to model the mechanical parts of the wind turbine, Stirling engine, and

electrical parts. Field oriented control method is developed on voltage source

converter. Power signal feedback method is implemented to determine generators

reference shaft speed in hybrid system. Permanent magnet synchronous generator is

used in the wind turbine and Stirling engine. Simulation results show that a new

hybrid generation system with Stirling and wind turbine can work like other

hybrid system and has suitable performance. VC 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4850475]

NOMENCLATURE

Ac cold heat transfer areaAh heat transfer areaAl the area of the leakAp the area of the power pistonAsc the heat transfer surface area of the cylinderB damping constant (total mechanical friction)Cp constant pressure specific heatCpb; k turbine performance coefficientdcon dish aperture diameterisdq dq components of the generator currentidq dq components of the utility grid currentI the inertia of the flywheelIir sunlight irradiancekloss overall loss coefficientLsd; Lsq dq components of the stator winding inductancem the molar mass of the airNe the number of mols of air in the cylinderp the number of pole pairsPa ambient pressure

a)Electronic mail: [email protected])Electronic mail: [email protected])Electronic mail: [email protected]

1941-7012/2013/5(6)/063128/17/$30.00 VC 2013 AIP Publishing LLC5, 063128-1

JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 5, 063128 (2013)


Po air pressure in the cylinderPpc the perimeter of the power pistonPm;Pe;Pn mechanical, electrical, and delivery power_QIir the rate of the transferred heat to the receiver from the concentrator_Qloss the rate of the absorber losses_Qh the rate of transferred heat to the Stirling engineR mass gas constant airRe linkage pivot radius on the flywheelRs the resistance of the stator windingsRbld the radius of the bladesRf ; Lf filter resistance and inductanceso the specific entropy of the air at T 300 (K)Sc heat sink transferred entropy from the airSe the total entropy of the air in the cylinderSh heat source transferred entropy to the airT temperature (K)Ta ambient temperatureTe air temperature in the engineTo starting temperature of the air in the cylinderTavg temperature that characterizes the heat loss in the absorberV volume m3Vc the volume of the air cylinderK constant proportional to the receiver dimensionsq material density kg=m3qa air densityh the angular position of the flywheel_h the angular velocity of the flywheelh the angular acceleration of the flywheelkm core magnetic fluxkd; kq dq components of the stator fluxl the heat transfer constant of the cylinder, this was calculated as the thermal con-

ductance of steel with the cylinder wall thickness

se the mechanical torque of the Stirling engineselec electromagnetic generator torquegm mirror reflectivityvsdq dq components of the generator voltagevndq; vdq dq components of the PCC and VSC output voltagexr;xe;xn rotor, electrical, and network angular speed

I. INTRODUCTION

In recent years, growing energy demands and rising environmental concerns make the

world energy consumption go toward renewable energy (RE). Due to network power distribu-

tion and transmission constraints, utilization of distributed generation (DG) units is growing to

supply downstream loads.1 Recently, renewable energy conversion systems (RECSs) are used

as DGs in utility grid because they require no fuel, low maintenance, and produce no pollution.

Wind and solar are the most common types of RE that can replace the traditional fuels. But

due to intermittent nature of wind and solar, their produced powers have higher disturbance and

fluctuations compared to conventional power sources. The hybrid renewable resources can be

used to improve RECS reliability and network power quality.2

There are many methods for different RE resources aggregation. The conventional classifi-

cation methods include dc-coupled, ac-coupled, and hybrid-coupled.3 In dc-coupled configura-

tion, a common dc bus integrates all powers that are extracted from RE resources and injects to

063128-2 Shariatpanah, Jahromi, and Fadaeinedjad J. Renewable Sustainable Energy 5, 063128 (2013)


ac load or utility gird by a common dc/ac converter. The dc-coupled scheme is simple, and it

does not require synchronization and complex control to integrate different energy source.

Different research is accomplished on the hybrid generation system with wind turbine

(WT) and photo voltaic (PV) array. The WT and PV are often used in grid-connected or stand-

alone configurations. To have a hybrid system with proper performance in grid-connected or

standalone configuration, it should have a comprehensive control scheme because the hybrid

system controls different parameters in standalone mode (voltage and frequency control) and

grid-connected mode (real and reactive power control).4,5 Also, with increasing the number of

the RE resources, power dispatch is an important issue in system power management. To

improve hybrid system performance, efficiency, and security constraints, supervisory control

should regulate the power generation of the RE resources.68

The Stirling engines are highly reliable and efficient compared to PV array. The Stirling

engine was reported to be the cheapest for solar electric generation in the range of 1 to

100 kW.9,10 With average efficiencies of over 20% and the record measured peak efficiency of

nearly 30%, Dish-Stirling systems currently exceed the efficiency of any other solar conversion

technology. Also, the WT with permanent magnet synchronous generator (PMSG) is an effi-

cient configuration for variable speed WTs. This configuration can extract maximum power

from wind.

In this paper, a PMSG WT and a Stirling engine are developed. They are associated to-

gether by the common dc link. The PMSG WT is modeled by three software, including

TurbSim, FAST (Fatigue, Aerodynamics, Structures, and Turbulence), and Simulink. The elec-

trical, mechanical, and aerodynamic aspects of the PMSG WT are considered in the model.

Also, the Stirling engine model is developed by considering the solar, thermal, mechanical, and

electrical aspects of the Stirling based power generation system. The hybrid system has a pre-

cise model in all aspects. The electrical controllers are designed based on field oriented control

(FOC) theory. In Sec. II, the overall components and association structure are described. The

Dish-Stirling engine model and its various components are explained in Sec. III. The various

electromechanical aspects of the WT have been discussed in Sec. IV. The electrical parts and

controller scheme are described in Sec. V. Then the simulation results are shown in Sec. VI

and hybrid system performance is evaluated in the different conditions. The simulation results

show system performance in different conditions.

II. HYBRID SYSTEM CONFIGURATION

In Fig. 1, the simulated system components are shown. In this research, a typical solar-

powered Dish-Stirling engine power generation system and a PMSG WT are modeled and stud-

ied. The solar heat is converted into the electricity by the proposed energy conversion system

in three stages that are solar to thermal, thermal to mechanical, and mechanical to electrical

conversion stages. Also, the PMSG WT extracts wind power and converts to electrical power.

The produced electrical powers are injected to the utility grid through the common dc link. The

generator current with variable frequency is rectified to direct current by rectifier, then it is con-

verted to alternative current with network frequency by an inverter. The storage device is not

considered in this research, since it is assumed that the hybrid system works in grid connected

mode.

III. STIRLING ENGINE SYSTEM

The Stirling engine can be used to convert the delivered heat by a solar collector into me-

chanical power as an energy conversion system. In this research, the temperature is assumed

variable for the solar collector (hot end in the Stirling engine) that is subjected to the solar radi-

ation. The modeling of the Dish-Stirling engine is performed based on some solar, thermal, and

mechanical equations that are given in this section. A Dish-Stirling engine, shown in Fig. 2,

consists of the following components: concentrator, receiver, and power conversion unit (PCU)

that contains the Stirling engine with control system. The parabolic concentrator consists of

mirrors that concentrate the sunlight irradiance and redirect it to the receiver which acts as an



interface between the concentrator and the PCU.9,11 The solar dish components and the Stirling

engine operation procedure are described in Secs. III AIII D.

A. Concentrator

The parabolic concentrator is a collection of the mirrors used to concentrate the sunlight to

the receiver which is located at the dish focal point. The concentrated beam radiation is

absorbed by the receiver which is used to heat a gas (air) or a fluid. Assuming perfect sunlight

tracking, the rate of heat transfer to the receiver from the concentrator is achieved using the fol-

lowing equation:11

_QIir pIirgmdcon2

2: (1)

As Eq. (1) shows the rate of transferred heat depends on the irradiance (Iir), mirror reflec-tivity (gm), and the dish diameter (dcon).

B. Receiver

The receiver, as an interface between the concentrator and the Stirling engine, is designed

to transfer the maximum possible amount of heat to the Stirling engine and minimize the ther-

mal losses. The absorber, as an internal part of the receiver, consists of a group of tubes that

FIG. 1. The overall structure of simulated system.

FIG. 2. The Dish-Stirling and its components.



carry the working gas of the Stirling engine. This flowing gas passes through the group of tubes

to absorb the receivers inside heat to provide the required power for the Stirling engine. The

system losses, related to the receiver, are consequences of thermal radiation, reflection, convec-

tive heat transfer into the atmosphere, and conduction through the receiver material.13

The temperature of the absorber is an important factor for the receiver operation. In order to

achieve the maximum possible efficiency of the Stirling engine, the temperature should be main-

tained as high as possible, considering the receiver material thermal limits. Two common meth-

ods, variable stork and variable pressure control, are used to control the receiver temperature.

Since the variable pressure control method is more common, it is used to control the receiver

temperature in this research. Applying the energy balance to the absorber results in Eq. (2),

qcpV _T _QIir _Qloss _Qh: (2)

The rate of absorber losses is relevant to the overall loss coefficient, ambient temperature,

and absorber temperature. It can be expressed by14

_Qloss klossKTavg Ta: (3)

C. Stirling engine

In this section, the Stirling engine operation concept is explained. It is also described how

a set of algebraic and differential equations is used to model the Stirling engine.

1. Stirling engine operation concept

The Stirling engines are external combustion machines working theoretically in the Stirling

cycle. The modified systems compressible fluid (such as air or Helium) is used as the working

fluid.10 The Stirling engines are powered by the expansion of a gas when heated, followed by

the compression of the gas when cooled. The Stirling engine contains a predetermined amount

of gas that is transferred back and forth between the two cold and hot ends. As shown in

Fig. 3, the Stirling engine includes a displacer piston and a smaller piston named the power pis-

ton. The displacer piston (bigger one) moves the gas between the two ends and the power pis-

ton changes the internal volume as the gas expands and contracts. Air in the engine is cyclically

heated and expands to push the power piston to the right. As the power piston moves to the

right, the linkage forces the loose-fitting and moves the piston to displace air to the cooler side

of the engine. By losing the heat on the cooler side, the air contracts and pulls the power piston

FIG. 3. The schematic of Stirling engine.



to the left. The air is again displaced, sending it back to the heated compartment of the engine,

and the cycle repeats. The power piston acts upon the linkage to a flywheel and the back and

forth motion of the power piston is converted to the rotational motion of the flywheel.12

2. Stirling engine modeling

In order to model the Stirling engine, the operational procedure of the Stirling engine

should be implemented using a set of relations that are explained in this part. The absorber is

considered as a heat source for the Stirling engine. The heat source (absorber) transfers entropy,

Sh, to the air in the cylinder through a variable resistance. The heat sink is considered as a con-stant temperature effort source, Tc, which also transfers entropy, Sc, from the air in the cylinderthrough a different variable resistance. The air in the cylinder is modeled as a multi-port capac-

itor. To consider the leakage from the cylinder, one port on the multi-port capacitor tracks the

mass loss through a resistance to ambient conditions, modeled as a constant pressure effort

source. The entropy variations are considered by a second port on the capacitor. The variations

are caused by mass flow, entropy flow from heat source, and entropy flow to heat sink. The

final port on the capacitor is associated with the volume change. The pressure in the cylinder

acts upon the power piston which is modeled as a constant ratio transformer. The piston then

acts upon the linkage to the flywheel, modeled as a modulated transformer. The flywheel is

modeled as an inertia, while all of the friction losses in the system are modeled as a resistor

with damping b. The major modeling assumption used in this model are12

uniform temperature for air in engine, lumped friction element to govern engine speed, no power transfer through the heat piston, mass-less pistons, uniform temperature sources, all leakage from engine through power cylinder, the motion of the heat piston is sinusoid 90 ahead of power piston.

For this research, the Stirling engine is modeled using the following set of algebraic and

differential equations that are implemented in Simulink environment:12,15,16

x Re1 sin h; (4)Ah Asc1 cos h; (5)

Ac Asc1 cos h Ppcx; (6)

_Sh AhlTh TeTe

; (7)

Qh mcpTh Te; (8)

_Sc AclTe TcTe

; (9)

_Ne Al2qePe Pa

p; (10)

_Sa SeNe

_Na; (11)

_Se _Sh _Sc _Sa; (12)Ve Vc Apx; (13)

ve VemNe

; (14)



Te To vevo

RCv

expse soCv

; (15)

Pe Po vevo

RCv1exp

se soCv

; (16)

Fe Pe PaAp; (17)se FeRe cos h; (18)

se selec b _h Ih: (19)

The thermo-mechanical operation of the Stirling engine is presented through Eqs. (4) to (17).

Equation (18) shows the generated mechanical torque of the Stirling engine as a function of the

radius of linkage pivot on the flywheel.

It is important that the interaction between the mechanical and electrical dynamics of the

system be accurately considered in the model, as the Stirling engine and the synchronous gener-

ator are coupled through a mechanical shaft. The Stirling engine torque, se, and the generatortorque, selec, are applied to the same shaft and the Eq. (19) can be used to calculate the rotationvelocity. This equation also represents the mechanical dynamics of the electrical generator.

D. Temperature control system

Since the temperature varies with the sunlight irradiance, the temperature control system

(TCS) should be used to maintain the absorber temperature at its maximum possible tempera-

ture while considering the thermal limits of the absorber and receiver materials. The pressure of

the working gas inside the Stirling engine controls the absorber temperature. When the irradi-

ance is high, the absorber temperature increases and the pressure in the Stirling engine

increases. In order to regulate the absorber temperature, the Stirling engine pressure should be

decreased. The gas pressure can be decreased in the Stirling engine by pumping it back to a

high pressure storage tank17 or using an external low pressure storage tank18 where the gas can

be flowed naturally out of the engine by opening a control valve. On the other hand, when the

irradiance is low, the absorber temperature decreases and the pressure in the Stirling engines

decreases. In order to regulate the absorber temperature, the Stirling engine pressure should be

increased. An external high pressure storage tank is used to add gas to the Stirling engine. The

detailed model of this control system can be found in Ref. 17.

IV. WIND TURBINE SYSTEM

A. Wind profile

The wind profile, generated by TurbSim software,19 has special characteristics such as, dif-

ferent velocity for each point of blades, wind vector generation in desired accuracy, and wind

confirmation with environmental conditions. This software produces wind vectors based on sta-

tistics data that is obtained from environmental data collection. Due to, altitude, average wind

speed, wind turbulent intensity, hub height, and other environmental conditions, it creates spe-

cial wind profile. FAST software20 reads the TurbSim output data.

B. Wind turbine model

FAST is comprehensive software to model the mechanical parts of the WT. The WT is com-

posed of different parts that are connected together and each part can move and influence on the

other parts. The simple model cannot consider the structural details but FAST considers the most

important WT motions and models mechanical parts such as nacelle, tower, blades, and shafts.

Against other mechanical software like ADAMS (Automatic Dynamic Analysis of Mechanical

Systems), it considers a few detail of the WT motions, therefore, it needs less time to run compare



with ADAMS. The translational, rotational, lateral, and longitudinal motions of the WT can be stud-

ied by FAST. FAST considers 24 degrees of freedoms (DOFs) for three blade horizontal axis WT

(HAWT) also, according to field research, it can neglect some DOF. Different types of the WT have

been modeled by FAST such as up-and down-wind, two and three bladed, pitch and stall controlled.

Some of these models are built based on real WTs. Before manufacturing a new WT, the prelimi-

nary design can be tested by FAST; therefore, the new design is evaluated and cost is reduced.

FAST uses AeroDyn21 to calculate the WT aerodynamic forces. The aerodynamic loads are

described by dynamic equations that are solved by AeroDyn sub code. It calculates the mechan-

ical torque, deflection, and velocities of the mechanical configuration based on the aerodynamic

forces for each point of the blades. To use this sub code, some input data files should be pre-

pared including wind profile, airfoil lift and drag coefficients, and some definable parameters.

As the WT equations are modeled in the dynamic form, it is possible to examine interactions

between the mechanical and the electrical systems.

In this paper, FAST is used to model an upwind, three-bladed rotor small HAWT with a

rigid hub and foundation. The WT DOFs include the first and second flapwise blade mode,

edgewise mode, yaw angle, and tail furl. This WT uses the furling control technique, as power

control method, to limit captured power at high wind speed. This structure permits rotating of

rotor and drive train around the yawing portion of structure on top of the tower, whereas tail

furl DOF allows the tail motion about the yawing portion of the structure on top of the tower.20

This technique cannot be used in large scale WTs because of the enormous gyroscopic loads on

the WT construction. In high wind speed, the rotor thrust and aerodynamic moments are con-

trolled by tuning the nacelle around the yaw axis.

The tail-furl technique maintains the WT rotor aligned with the wind below rated wind

speed. In this mode, tail vane helps WT to balance the lateral aerodynamic forces. Therefore,

WT can extract more power from wind. The furling mechanism turns the tail vane, connected

to back of the nacelle side, and unbalance the lateral aerodynamic forces to protect WT against

excessive power generation and rotor speed in high wind speed. The tail-furl and yaw-furl axes

are shown in Fig. 4. This controller is verified and validated by test data from small wind

research.22 The controller of the furling mechanism is designed based on fuzzy logic controller

that its inputs are the shaft speed, generated power, and wind speed.

FIG. 4. The tail furl operation in side view.



V. ELECTRICAL PARTS

A. PMSG model

The PMSG is selected to use in the WT and Stirling because it is simple, efficient, and

without dc excitation. The flux established by the permanent magnets in the stator is assumed

sinusoidal. The PMSG is modeled using below equations in d q reference frame. These equa-tions are based on stator currents and voltages,

vsd Rsisd dkddt

xekq; (20)

vsq Rsisq dkqdt

xekd; (21)

kd Lsdisd km; kq Lsqisq; (22)

selec 32pkmisq Lsq Lsdisqisd; (23)

Pe xmselec: (24)

B. Electrical controller scheme

The electrical controller composing of network side converter (NSC), Stirling generator

side converter (GSC), and WT GSC controller are designed in d q reference frame. The NSCcontroller can control the dc link voltage and injected reactive power to the network (or point

of common coupling (PCC) bus voltage). The NSC controls dc link voltage and there is no

need that it is controlled by two controllers (Stirling GSC and WT GSC) and when the WT is

off and irradiance is zero, NSC can support the connected network.

The generators active power that is extracted from wind and solar is controlled by GSC con-

trollers. These controllers make the hybrid system working at highest efficiency, where the set point

of generators shaft speed is determined by the maximum power point tracking (MPPT). The speed

set points are calculated based on the generator output power and hybrid parameters, when the WT

and Stirling are operating below rated power. The calculation of the speed set point is sometimes

summarized as a look up table of x P curve in the WT or Stirling. For Stirling engine, x Pcurve is obtained based on consecutive tests. According to wind speed and irradiation, the generator

shaft speed set points are changed and desired power is extracted from wind and solar.

1. NSC control

The main aim of the NSC controller, shown in Fig. 5, is to keep dc link voltage at constant.

The cascaded control scheme, designed based on the network current d q components, is usedto control dc link voltage and injected reactive power to the network. The q-component of thenetwork current can control dc link voltage, whereas, the d-component is used to control reactivepower. The outer loop sets the reference for inner loop (id) and the inner loop tracks this refer-ence. In normal condition, the reactive power set point is regulated at zero to reach unity power

factor by second loop. The NSC model is implemented using the following equations:

vd Rf id Lf diddt

xnLf iq vnd; (25)

vq Rf iq Lf diqdt

xnLf id vnq: (26)

Inner loop (current loop) is designed based on Eqs. (25) and (26). This controller includes

three standard proportional integral (PI) with anti wind up.23 Equation (27) shows the PI con-

troller transfer function,



Gs Kp KpTis

: (27)

The controller coefficients (Kp and Ti) are determined using the methods, explained inRefs. 2426. The dc link should be modeled by dynamical equation to design outer loop con-

troller. The average model is used to model dc link using the below equation,

dVdcdt

1CVdc

Pe 1CVdc

Pn: (28)

As can be seen in Eq. (28), the dc link voltage will remain constant if the WT output

power and delivery power to the network are equal.

2. GSC control

The generator side converter control scheme is indicated in Fig. 6. This controller is com-

posed of two control loops that are designed based on the generator stator current d q compo-nents and work independently. The first loop controls d-component of stator current. The d-component of the generator current is usually set at zero to decrease power loss and stator cur-

rent. The second control loop includes two cascade loops (outer and inner loop) where outer

loop regulates shaft speed to maximize the extracted power according to the reference, deter-

mined by MPPT. The GSC is modeled by Eqs. (29) and (30) that can be used to design current

control loops:

FIG. 5. Network-side converter control scheme.

FIG. 6. Generator-side converter control scheme.



vsd Rsisd Lsd diddt

xeLsqisq; (29)

vsq Rsisq Lsq diqdt

xeLsdisd xekm: (30)

Equations (29) and (30) show relation between the generator stator voltages and currents in

the d q reference frame. To determine generator shaft speed, MPPT is implemented for theWT and Stirling. In this paper, the power signal feedback (PSF) method is used to determine

the WT and Stirling shaft reference speed as MPPT. The main idea of this method is utilization

of the WT and Stirling x P curves, where x and P are shaft speed and output power. Thesecurves depend on the WT and Stirling inherent characteristics. The WT x P curve can beobtained from Eq. (14) but continual test is used to obtain the Stirling curve. The relation

between shaft speed, irradiance, and output power is nonlinear. The Stirling is tested in

FIG. 7. The Stirling MPPT curves.

FIG. 8. The effect of irradiance on absorber temperature.



different irradiance and different shaft speed to obtain Stirling inherent parameters and define

the power behavior. The effect of irradiance on the temperature, shaft speed, and power is

shown in Figures 7 and 8. Fig. 7 shows that the maximum power in various irradiation occurs

about shaft speed of 800 rad/s. This set point has been chosen to the Stirling generator shaft

speed. Equation (31) is used to determine the WT shaft reference speed,

xref Pmk

3

r; k 0:5qapCpmaxR

5bld

kopt: (31)

The performance coefficient, Cpb; k, is a nonlinear function, depending on the tip-speed-ratio, k, and the pitch angle of the blades, b.

FIG. 9. The load scenario.

TABLE I. The Stirling engine parameters.

Parameters Value

dcon 2.4m

gm 94%

Re 1.25 cm

Asc 40 cm2

Ppc 4.9 cm

Ah Variable

Ac Variable

l 10 000W=m2

Al 0.06 mm2

Ap 1.9 cm2

Vc 40 cm3

M 29 kg=kmol

R 287 J=kg

so 2800 J=KKgTo 300K

Po Pa 1 105 PaCv 717 J=kgKB 0:7 103 N=rad=sI 4 kg cm2Cp 5.19 kJ=kgKLs 0.02682H

km 0:1717wb

Rs 18:7X

P 2



FIG. 10. The wind profile.

TABLE II. The wind turbine data.

Parameters Value

Rotor diameter 6.17m

Cut-in wind speed 3.1m/s

Cut-out wind speed 25m/s

Rated wind speed 13m/s

Hub height 37m

Nacelle mass 260.5 kg

Blade mass 10.847 kg

Blade number 3

Air foil blade type SH3052

Gearbox ratio 1

Tail boom mass 86.8 kg

Tail boom inertia about tail-furl axis 264.7 kgm2Rs 0.5 X

Ls 0.00448 H

km 0:39V sGenerator inertia 25 kg m2p 19

TABLE III. The electrical parts parameters.

Parameters Value

DC link capacitance 2500lf

Network rated voltage(line to line) 380V

3-Phase short-circuit level 10 MVAX/R ratio 5

Network frequency 50 Hz

Rf 0.012 X

Lf 0.002H



VI. SIMULATION RESULTANALYSIS

In this section, the hybrid system operation is studied considering load variation, shown in

Fig. 9. A precise model, consisting of the thermal, mechanical, and electrical parts, is consid-

ered to simulate the hybrid system. Simulation results allow for study of the interaction between

electrical and mechanical parts. The main parameters of the WT, generator, and Stirling engine

are listed in Tables IIII.

Due to RCES and load uncertainty, the hybrid system should be connected to utility grid

or storage devices for system power management. In this research is assumed that the hybrid

system is connected to the utility grid and utility grid controls the system frequency.

The wind profile, with mean speed 13m=s, is generated by TurbSim (see Fig. 10). The irra-diance is considered 1000W=m2, assumed constant in simulation time.

FIG. 12. The power, torque, and shaft speed of the Stirling.

FIG. 11. The power, torque, and shaft speed of the WT.



FIG. 13. The power of PCC bus and grid bus.

FIG. 14. The dq currents of hybrid system and grid.

FIG. 15. The dc link voltage.



The electrical power, torque, and shaft speed of the WT and Stirling are shown in

Figures 11 and 12. Due to wind speed fluctuations, the variation of WT shaft speed and output

power is justified. The summation of the WT and Stirling output powers is injected to the PCC

bus through the common dc link.

The impact of load variation is studied on the hybrid system operation, changing the local

load in three steps. In the beginning, the hybrid system supplies the local load and injects its

excess power to utility grid. At t 5 s, second load is connected to the system with 7 kWpower, therefore; the delivery power to the grid is reduced. Connecting the third load to the

network with 3 kW power consumption, utility grid injects power to supply the local load. As

can be seen in Fig. 13, the delivery power to the utility grid is negative after t 10 s. Fig. 14indicates the current d q components of the grid and hybrid system. The q-component of gridcurrent is set at zero by NSC controller; therefore, the delivery reactive power to network will

be zero to maximize the active power transfer capacity. The dc link voltage and PCC bus volt-

age are depicted in Figures 15 and 16. They show that the dc link and PCC voltages are in an

acceptable range and the hybrid system maintains its stability in different load conditions.

VII. CONCLUSION

In this research, a hybrid system, including the WT and Stirling, was modeled and studied.

This model used TurbSim and FAST to simulate wind and WT direct drive. Also, Simulink

was used to model Stirling engine and electrical controllers. These controllers control reactive

power and real power independently. The dc link voltage is also maintained constant by the

controller. The PSF method was used to implement MPPT for the WT and Stirling. The hybrid

system reaction was evaluated in different load conditions and was shown that the new hybrid

system can work like other hybrid system with high performance. The feature of this model is

precision modeling of mechanical and electrical parts that are not considered in other works.

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FIG. 16. The network voltage at PCC bus.



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