IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 4, APRIL 2011 1141
Isolated Wind–Hydro Hybrid System UsingCage Generators and Battery Storage
Puneet K. Goel, Bhim Singh, Fellow, IEEE, S. S. Murthy, Life Senior Member, IEEE, and Navin Kishore
Abstract—This paper deals with a new isolated wind–hydrohybrid generation system employing one squirrel-cage induc-tion generator (SCIG) driven by a variable-speed wind turbineand another SCIG driven by a constant-power hydro turbinefeeding three-phase four-wire local loads. The proposed sys-tem utilizes two back-to-back-connected pulsewidth modulation-controlled insulated-gate-bipolar-transistor-based voltage-sourceconverters (VSCs) with a battery energy storage system at theirdc link. The main objectives of the control algorithm for the VSCsare to achieve maximum power tracking (MPT) through rotorspeed control of a wind-turbine-driven SCIG under varying windspeeds and control of the magnitude and the frequency of theload voltage. The proposed wind-hydro hybrid system has a ca-pability of bidirectional active- and reactive-power flow, by whichit controls the magnitude and the frequency of the load voltage.The proposed electromechanical system using SCIGs, an MPTcontroller, and a voltage and frequency controller are modeledand simulated in MATLAB using Simulink and Sim Power Systemset toolboxes, and different aspects of the proposed system arestudied for various types of linear, nonlinear, and dynamic loads,and under varying wind-speed conditions. The performance of theproposed system is presented to demonstrate its capability of MPT,voltage and frequency control (VFC), harmonic elimination, andload balancing.
Index Terms—Battery energy storage system (BESS), smallhydro, squirrel-cage induction generator (SCIG), wind-energy-conversion system (WECS).
I. INTRODUCTION
R ENEWABLE energy sources have attracted attentionworldwide due to soaring prices of fossil fuels. Renew-
able energy sources are considered to be important in improvingthe security of energy supplies by decreasing the dependenceon fossil fuels and in reducing the emissions of greenhousegases. The viability of isolated systems using renewable en-ergy sources depends largely on regulations and stimulationmeasures. Renewable energy sources are the natural energyresources that are inexhaustible, for example, wind, solar,geothermal, biomass, and small hydro generation [1]. Amongthe renewable energy sources, small hydro and wind energyhave the ability to complement each other [2]. For power
Manuscript received April 13, 2009; revised June 25, 2009; acceptedAugust 24, 2009. Date of publication December 4, 2009; date of current versionMarch 11, 2011.
P. K. Goel is with the Ministry of Power, Government of India, New Delhi11001 India, and also with the Indian Institute of Technology Delhi, New Delhi110016 India.
B. Singh and S. S. Murthy are with the Indian Institute of Technology Delhi,New Delhi 110016 India.
N. Kishore is with the Corporate Planning Department, Central Power Utility(NTPC Ltd.), New Delhi 110003 India.
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TIE.2009.2037646
generation by small or microhydro as well as wind systems,the use of squirrel-cage induction generators (SCIGs) has beenreported in literature [3]–[18].
Although the potential for small hydroelectric systems de-pends on the availability of suitable water flow, where theresource exists, it can provide cheap clean reliable electricity.Hydroelectric plants convert the kinetic energy of a waterfallinto electric energy. The power available in a flow of waterdepends on the vertical distance the water falls (i.e., head)and the volume of flow of water in unit time (i.e., discharge).The water powers a turbine, and its rotational movement istransferred through a shaft to an electric generator [1]. WhenSCIG is used for small or microhydro applications, its reactive-power requirement is met by a capacitor bank at its statorterminals. The SCIG has advantages like being simple, lowcost, rugged, maintenance free, absence of dc, brushless, etc.,as compared with the conventional synchronous generator forhydro applications [3], [4].
As regards wind-turbine generators, these can be built eitheras constant-speed machines, which rotate at a fixed speedregardless of wind speed, or as variable-speed machines inwhich rotational speed varies in accordance with wind speed.For fixed-speed wind turbines, energy-conversion efficiency isvery low for widely varying wind speeds. In recent years, wind-turbine technology has switched from fixed speed to variablespeed. The variable-speed machines have several advantages.They reduce mechanical stresses, dynamically compensatefor torque and power pulsations, and improve power qualityand system efficiency [12]. The grid-connected variable-speedwind-energy-conversion system (WECS) based on SCIG useback-to-back connected power converters [13], [15]. In suchsystems, the power converter decouples the SCIG from the grid,resulting in an improved reliability.
In the case of grid-connected systems using renewable energysources, the total active power can be fed to the grid. For stand-alone systems supplying local loads, if the extracted power ismore than the local loads (and losses), the excess power fromthe wind turbine is required to be diverted to a dump load orstored in the battery bank. Moreover, when the extracted poweris less than the consumer load, the deficit power needs to besupplied from a storage element, e.g., a battery bank [19]–[21].
In the case of stand-alone or autonomous systems, the issuesof voltage and frequency control (VFC) are very important. In[16]–[18], the authors have addressed the issues of VFC forstand-alone systems using SCIGs. Some work has also beenreported for stand-alone WECSs using doubly fed inductiongenerator [22], [23]. In [18], a battery-based controller isproposed for control of voltage and frequency in the isolated
1142 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 4, APRIL 2011
Fig. 1. Schematic diagram of wind–hydro hybrid system.
WECS. However, maximum power tracking (MPT) could notbe realized in this battery-based isolated system employingSCIG operated at fixed speed. In [3], Singh et al. have proposedan electronic load controller for VFC at the stator terminals,and the controller transfers excess power from the hydropowergenerator to a dump load, whenever the load is less than thegenerated power.
In this paper, a new three-phase four-wire autonomous (orisolated) wind–small hydro hybrid system is proposed forisolated locations, which cannot be connected to the grid andwhere the wind potential and hydro potential exist simultane-ously. One such location in India is the Andaman and Nicobargroup of islands [24]. The proposed system utilizes variable-speed wind-turbine-driven SCIGw (subscript w for wind), anda constant-speed/constant-power small hydro-turbine-drivenSCIGh (subscript h for hydro). For the rest of this paper, thesubscript w is used to denote the parameters and variables ofthe wind-turbine generator, and the subscript h is used to denotethe parameters and variables of the hydro-turbine generator.
A schematic diagram of a three-phase four-wire autonomoussystem is shown in Fig. 1. Two back-to-back-connected pulse-width modulation (PWM)-controlled insulated-gate-bipolar-transistor (IGBTs)-based voltage-source converters (VSCs) areconnected between the stator windings of SCIGw and the statorwindings of the SCIGh to facilitate bidirectional power flow.The stator windings of the SCIGh are connected to the loadterminals. The two VSCs can be called as the machine (SCIGw)side converter and the load-side converter.
The system employs a battery energy storage system(BESS), which performs the function of load leveling in thewake of uncertainty in the wind speed and variable loads.The BESS is connected at the dc bus of the PWM converters.The advantage of using BESS on the dc bus of the PWMconverters is that no additional converter is required for transferof power to or from the battery. Further, the battery keepsthe dc-bus voltage constant during load disturbances or loadfluctuations. An inductor is connected in series with the BESSto remove ripples from the battery current.
A zigzag transformer is connected in parallel to the loadfor filtering zero-sequence components of the load currents.
Further, the zigzag windings trap triplen harmonic (third, ninth,fifteenth, etc.) currents. As shown in Fig. 1, the zigzag trans-former consists of three single-phase transformers with a turnratio of 1 : 1. The zigzag transformer is to be located as near tothe load as possible. The neutral terminal of the consumer loadsis connected to the neutral terminal of the zigzag transformer.
For the hybrid system, a new control algorithm is proposedthat has the capability of MPT, harmonic elimination, loadleveling, load balancing, and neutral current compensationalong with VFC. The objectives of the machine (SCIGw) sideconverter are to provide the requisite magnetizing current tothe SCIGw and to achieve MPT. In the conventional control ofvariable-speed SCIGs, the objective of the load-side converter(called as grid-side converter in the grid-connected systems) isto maintain the dc-bus voltage constant at the dc link of twoback-to-back connected VSCs. Because in the proposed systemthe dc-bus voltage is kept constant by the battery, the controlobjective of the load-side converter is different, i.e., to maintainan active power balance in the system by transferring the excesspower to the battery or for providing deficit power from thebattery. Further, the load-side converter provides the requisitereactive power for the load. The reactive-power requirement ofthe SCIGh is provided by the excitation capacitors connected atits stator terminals.
A novel control strategy using indirect current control isproposed for the load-side converter. The control signals forswitching of the load-side converter are generated from theerror of the reference and the sensed stator currents of SCIGh
rather than by the errors of the load-side converter currents.With this control strategy, the switching of the load-side con-verter is controlled to make the SCIGh currents balancedand sinusoidal at the nominal frequency. Any unbalance andharmonics in the load currents are compensated by the zigzagtransformer and the load-side converter.
The proposed control algorithm for load-side converter re-quires sensing of the load voltage and stator currents of SCIGh.For the control purpose, sensing of load-side-converter cur-rents and load currents is not required, thus reducing therequirement of current sensors for the control of load-sideconverter.
GOEL et al.: ISOLATED WIND–HYDRO HYBRID SYSTEM USING CAGE GENERATORS AND BATTERY STORAGE 1143
Fig. 2. Coefficient of performance (Cp) versus tip speed ratio (λ) for windturbine.
For the proposed system, there are three modes of operation.In the first mode, the required active power of the load isless than the power generated by the SCIGh, and the excesspower generated by the SCIGh is transferred to the BESSthrough the load-side converter. Moreover, the power generatedby the SCIGw is transferred to the BESS. In the second mode,the required active power of the load is more than the powergenerated by the SCIGh but less than the total power generatedby SCIGw and SCIGh. Thus, portion of the power generated bySCIGw is supplied to the load through the load-side converterand remaining power is stored in BESS. In the third mode, therequired active power of the load is more than the total powergenerated by SCIGw and SCIGh. Thus, the deficit power issupplied by the BESS, and the power generated by SCIGw andthe deficit met by BESS are supplied to the load through theload-side converter.
The rest of this paper is organized as follows. In Section II,the principle of operation of the proposed hybrid system isgiven. In Section III, the control algorithm is presented for theproposed hybrid system. In Section IV, a design procedure ispresented for selection of various components of the proposedsystem. In Section V, the developed MATLAB-based simula-tion is discussed for the proposed system. In Section VI, thesimulation results for the proposed system under linear load,nonlinear load, mixed load, balanced load, unbalanced load,and variable wind-speed conditions are presented and discussedverifying the validity of the proposed methodology. Finally,an appraisal of the proposed hybrid system is presented in theSection VII.
II. PRINCIPLE OF OPERATION
As already stated, the proposed system uses two back-to-back-connected PWM-controlled IGBT-based VSCs. TheseVSCs are referred to as the machine (SCIGw) side con-verter and load-side converter. The objectives of the machine(SCIGw) side converter are to provide the requisite magnetiz-ing current to the SCIGw and to achieve MPT, and the objectiveof the load-side converter is VFC at the load terminals bymaintaining active- and reactive-power balance.
To achieve MPT, the SCIGw is required to be operated atoptimal tip speed ratio as shown in Fig. 2. The tip speed ratiodetermines the SCIGw rotor-speed set point for a given wind
Fig. 3. Mechanical power output of the wind turbine versus SCIGw speed fordifferent wind speeds.
Fig. 4. Phasor diagram of rotor flux oriented control of SCIG.
speed, and the mechanical power generated at this speed lies onthe maximum power line of the turbine, as shown in Fig. 3. Theoperating principle of the controller for the machine (SCIGw)side converter is based on the decoupled control of d- andq-axes stator currents of the SCIGw with the d-axis alignedto rotor flux axis as shown in Fig. 4. The reference value forthe d-axis or reactive component of the SCIGw stator currentis generated from the required magnetizing flux for the SCIGw.The reference value for the q-axis or active component of theSCIGw stator current is generated from error of the desiredspeed and the sensed SCIGw rotor speed.
As the wind speed varies, the rotor-speed set point changes,and the difference in the reference rotor speed and the sensedrotor speed is fed to the controller for the machine (SCIGw)side converter, also referred to as the speed controller.
The output of the speed controller gives the referenceq-axis stator current for SCIGw. The reference d − qSCIGw
stator currents are transformed to the reference three-phaseSCIGw stator currents and compared with the sensed three-phase SCIGw stator currents to generate control signals for themachine (SCIGw) side converter.
The load-side converter is controlled for the regulation ofload-voltage magnitude and load frequency. Further, for main-taining the load-frequency constant, it is also essential that anysurplus active power in the system is diverted to the battery.Alternatively, the battery system should be able to supply anydeficit in the generated power. Similarly, the magnitude of theload voltage is maintained constant in the system by balancingthe reactive-power requirement of the load through the load-side converter.
The detailed control algorithm for the machine (SCIGw) sideconverter and the load-side converter is given in Section III.
1144 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 4, APRIL 2011
Fig. 5. Control scheme of machine-side converter.
III. CONTROL ALGORITHM
As already mentioned in Section II, the objectives of themachine (SCIGw) side converter are to achieve MPT and toprovide the required magnetizing current to the SCIGw, and theobjective of the load-side converter is to control the magnitudeand the frequency of the load voltage. The detailed controlalgorithm for the two converters is described in the followingsections.
A. Control of Machine (SCIGw) Side Converter
The objectives of the machine (SCIGw) side converter are toachieve optimum torque for MPT for SCIGw and to providethe required magnetizing current to the SCIGw. The controlstrategy for the machine (SCIGw) side converter control isshown in Fig. 5.
1) Speed-Control Loop for MPT and Reference q-axisSCIGw Stator-Current Generation: In the proposed algorithm,the rotor position (θrw) of SCIGw and the wind speed aresensed. The rotor speed (ωrw) of SCIGw is determined fromits rotor position (θrw). The tip speed ratio (λw) for a windturbine of radius rw and gear ratio ηw at a wind speed of Vw isdefined as
λw =ωrwrw
ηwVw. (1)
For MPT in the wind-turbine-generator system, the SCIGw
should operate at the optimum tip speed ratio (λ∗w) as shown
in Fig. 2. Thus, the reference rotor speed (ω∗rw) for MPT is
generated using (1) as
ω∗rw = λ∗
wVwηw/rw. (2)
The reference rotor speed of SCIGw is compared with ωrw
to calculate the rotor-speed error (ωrwer) at the nth samplinginstant as
ωrwer(n) = ω∗rw(n) − ωrw(n). (3)
The aforementioned error is fed to the speed proportional-integral (PI) controller. At the nth sampling instant, the outputof the speed PI controller with proportional gain Kpω andintegral gain Kiω gives the reference q-axis SCIGw statorcurrent (I∗qsw) as
I∗qsw(n) = Iqsw(n−1)
+Kpω
(ωrwer(n) − ωrwer(n−1)
)+ Kiωωrwer(n). (4)
2) Reference d-axis SCIGw Stator-Current Generation:The reference d-axis SCIGw stator current (I∗dsw) is determinedfrom the rotor flux set point (ϕ∗
drw) at the nth samplinginstant as
I∗dsw(n) = φ∗drw/Lmw (5)
where Lmw is the magnetizing inductance of SCIG.3) Generation of PWM Signal for Machine-Side Converter:
For generation of three-phase reference SCIGw stator currents(i∗swa, i∗swb, and i∗swc), the transformation angle θrotorfluxw isgenerated as (Fig. 4)
θrotorfluxw = θslipw +(pw
2
)θrw (6)
where θslipw is the slip angle, which is generated by integratingthe slip frequency (ωslipw) as
θslipw(n) =∫
ωslipw(n)dt. (7)
ωslipw at the nth sampling instant is generated as [25]
ωslipw(n) =(RrwI∗qsw(n)
) /(LrwI∗dsw(n)
)(8)
where Lrw is the rotor self-inductance and Rrw is the rotorresistance of SCIGw.
The references for d − q components of SCIGw stator cur-rents (I∗dsw and I∗qsw) are converted to three-phase referenceSCIGw stator currents (i∗swa, i∗swb, and i∗swc) by d − q to abctransformation using angle θrotorfluxw as
The three-phase reference SCIGw stator currents (i∗swa, i∗swb,and i∗swc) are then compared with the sensed SCIGw statorcurrents (iswa, iswb, and iswc) to compute the SCIGw stator-current errors, and these current errors are amplified with gain(K = 5) and the amplified signals are compared with a fixed-frequency (10 kHz) triangular carrier wave of unity amplitudeto generate gating signals for the IGBTs of the machine-sideVSC. The sampling time of the controller is taken as 50 μs, asthis time is sufficient for completion of calculations in a typicalDSP controller.
GOEL et al.: ISOLATED WIND–HYDRO HYBRID SYSTEM USING CAGE GENERATORS AND BATTERY STORAGE 1145
Fig. 6. Control scheme of load-side converter.
B. Control of Load-Side Converter
The objectives of the load-side converter are to maintainrated voltage and frequency at the load terminals irrespec-tive of connected load. The power balance in the system ismaintained by diverting the surplus power generated to thebattery or by supplying power from the battery in case of deficitbetween generated power and load requirement. Similarly, therequired reactive power for the load is supplied by the load-sideconverter to maintain constant value of the load voltage. Thecontrol strategy for the load-side converter control is shown inFig. 6.
1) Generation of Reference Three-Phase SCIGh Currents:The reference voltages (v∗
an, v∗bn, and v∗
cn) for the control of theload voltages at time t are given as
v∗an =
√2Vt sin(2πft) (12)
v∗bn =
√2Vt sin(2πft − 1200) (13)
v∗cn =
√2Vt sin(2πft + 1200) (14)
where f is the nominal frequency, which is considered as50 Hz, and Vt is the rms phase-to-neutral load voltage, which isconsidered as 240 V.
The load voltages (van, vbn, and vcn) are sensed and com-pared with the reference voltages. The error voltages (vanerr,vbnerr and vcnerr) at the nth sampling instant are calculated as
vanerr(n) ={
v∗an(n) − van(n)
}(15)
vbnerr(n) ={
v∗bn(n) − vbn(n)
}(16)
vcnerr(n) ={
v∗cn(n) − vcn(n)
}. (17)
The reference three-phase SCIGh currents (i∗sha, i∗shb, i∗shc)
are generated by feeding the voltage error signals to PI volt-
age controller with proportionate gain Kpv and integral gainKiv as
The reference three-phase SCIGh currents are then comparedwith the sensed SCIGh currents (isha, ishb, and ishc) to com-pute the SCIGh current errors as
ishaerr = i∗sha − isha (21)
ishberr = i∗shb − ishb (22)
ishcerr = i∗shc − ishc. (23)
These current errors are amplified with gain (K =5), and theamplified signals are compared with a fixed-frequency (10 kHz)triangular carrier wave of unity amplitude to generate gatingsignals for IGBTs of the load-side converter. The sampling timeof the controller is taken as 50 μs, as this time is sufficient forcompletion of calculations in a typical DSP controller.
IV. DESIGN OF SCIG-BASED
WIND-HYDRO HYBRID SYSTEM
The system is designed for an isolated location with theload varying from 30 to 90 kW at a lagging power factor (PF)of 0.8. The average load of the system is considered to be60 kW. The following subsections describe the procedure forselection of ratings for SCIGs, battery voltage, battery capacity,machine-side converter, load-side converter, specifications ofwind turbine, and gear ratio.
A. Selection of Rating of SCIGs
The wind-hydro hybrid system being considered has a windturbine of 55 kW and a hydro turbine of 35 kW. Both turbinesare coupled to SCIGs. The rating of the SCIGw is equal to therating of the wind turbine, which is 55 kW. The rating of theSCIGh should be equal to the rating of the hydro turbine, whichis 35 kW. Commercially available SCIG whose rating is closeto 35 kW is of 37.3 kW rating. Hence, the rating of SCIGh istaken as 37.3 kW. The parameters of the turbines and SCIGs aregiven in the Appendix.
B. Selection of Voltage of DC Link and Battery Design
The dc-bus voltage (Vdc) must be more than the peak of theline voltage for satisfactory PWM control as [26]
Vdc >{
2√
(2/3)Vac
}ma (24)
where ma is the modulation index normally with a maximumvalue of one and Vac is the rms value of the line voltage on the
1146 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 4, APRIL 2011
ac side of the PWM converter. In this case, there are two PWMconverters connected to the dc bus; therefore, the constraint onthe dc-bus voltage is from the ac voltages of both the converters.The maximum rms value of the line voltage at SCIGw terminalsas well as the rms value of the line voltage at the load terminalsis 415 V. Substituting this value in (24), Vdc should be morethan 677.7 V. The voltage of the dc link and the battery bank isselected as 700 V.
Considering the ability of the proposed system to supplyelectricity to a load of 60 kW for 10 h, the design storage capac-ity of the battery bank is taken as 600 kW · h. The commerciallyavailable battery bank consists of cells of 12 V. The nominalcapacity of each cell is taken as 150 A · h.
To achieve a dc-bus voltage of 700 V through series-connected cells of 12 V, the battery bank should have(700/12) = 59 number of cells in series. Since the storagecapacity of this combination is 150 A · h, and the total amperehour required is (600 kW · h/700 V) = 857 A · h, the numberof such sets required to be connected in parallel would be(857 A · h/150 A · h) = 5.71 or 6 (selected). Thus, the bat-tery bank consists of six parallel-connected sets of 59 series-connected battery cells.
Thevenin’s model is used to describe the energy storage ofthe battery in which the parallel combination of capacitance(Cb) and resistance (Rb) in series with internal resistance (Rin)and an ideal voltage source of voltage 700 V are used formodeling the battery in which the equivalent capacitance Cb
is given as [27]
Cb =(kW · h∗3600∗1000)
0.5 (V 2ocmax − V 2
ocmin). (25)
Taking the values of Vocmax = 750 V, Vocmin = 680 V, andkW · h = 600, the value of Cb obtained is 43 156 F.
C. Selection of Rating of Machine (SCIGw)Side Converter
The maximum active-power flow through the machine-side converter Psw = 55 kW, and the maximum reactive-power flow provided from the machine-side converter (Qsw) iscalculated as
Qsw ={V 2
msc/(2πfLm)}
= 18.4 kvar
where Vmsc is the maximum line voltage generated at theSCIGw terminals, which is 415 V, at a frequency (f) of50 Hz generated at a wind speed of 11.2 m/s. The V A rat-ing (V Amsc) of the machine-side converter is calculated asV Amsc =
√P 2
sw + Q2sw =
√552 + 18.42 = 58 kVA, and the
maximum rms machine-side converter current as
Isw = V Amsc/(√
3Vmsc) = 80.7 A.
The voltage and current ratings of the switching devices(IGBTs) are decided by the maximum voltage across the deviceand the maximum current through it. In view of (24), thevoltage rating of the switching devices is decided by the dc-linkvoltage, whose maximum value is 750 V. Taking a 25% margin,
the voltage rating of the switching devices of the machine-sideconverter should be more than 1.25 ∗ 750 V, i.e., 937.5 V.
The maximum current through the switching device is1.25{Ir(p−p)msc + I(peak)msc}[26], where I(peak)msc is thepeak line current through the machine-side converter, andIr(p−p)msc is the peak-to-peak ripple current in the machine-side converter, and 1.25 is the safety margin taken for design.For design purpose, the ripple in the machine-side convertercurrent is assumed to be 5% of I(peak)msc. Thus, the maximumcurrent through the switching device is 1.25{0.05 ∗ (
√2) ∗
80.7 + (√
2) ∗ 80.7} A = 149.8 A.From the previous calculation, the maximum voltage across
the switching devices is 937.5 V, and the maximum currentthrough the devices is 149.8 A. The ratings of the commerciallyavailable device (IGBT) higher than these values are 1200 Vand 200 A, and the same are selected for the design purpose.
D. Selection of Rating of Load-Side Converter
The rating of the load-side converter is determined by thecase when the connected load is at its maximum value, i.e.,90 kW at 0.8 lagging PF. The reactive power of the load issupplied by the load-side converter. Hence, the reactive-powerflow through load-side converter (Qlsc) is equal to the reactive-power demand of the load (QL). At a load of 90 kW at 0.8 lag-ging power factor, Qlsc = QL = (90/0.8) ∗ 0.6 = 67.5 kvar.Therefore, the kVA rating of the load-side converter (kVAlsc)is calculated as
kVAlsc =√
P 2lsc + Q2
lsc = 112.5 kVA.
The maximum rms current through the load-side converter(Ilsc) is calculated as
Ilsc = VAlsc/(√
3Vlsc) = 156.5 A
where Vlsc is the rms voltage on the ac side of the load-side converter, which is considered as 415 V. The maximumcurrent through the switching devices in the load-side converteris 1.25{Ir(p−p)lsc+I(peak)lsc
}, where I(peak)lsc is the peak linecurrent through the load-side converter, which is equal to(√
2) ∗ 156.5 A = 221.3 A, and Ir(p−p)lsc is the peak-to-peakripple current in the load-side converter, which is consideredas 5% of the peak line current and is calculated as 11.1 A.For the design purpose, safety margin is taken as 1.25. Thus,the maximum current through the switching device = 1.25 ∗(11.1 + 221.3) = 290.5 A.
In view of (24), the voltage rating of the switching devices isdecided by the dc-link voltage, whose maximum value is 750 V.Taking a 25% margin, the voltage rating of the switchingdevices of the load-side converter should be more than 1.25 ∗750 V, i.e., 937.5 V.
From the previous calculation, the maximum voltage acrossthe device may be 937.5 V, and the current through the de-vice may be 290.5 A. The commercially available rating forswitching device (IGBT) higher than 937.5 V and 290.5 A is1200 V and 300 A, respectively. Therefore, the rating of theswitching devices (IGBTs) of the load-side converter is decidedto be 1200 V and 300 A.
GOEL et al.: ISOLATED WIND–HYDRO HYBRID SYSTEM USING CAGE GENERATORS AND BATTERY STORAGE 1147
E. Selection of Rating of AC Inductor and RC Filter on ACSide of Load-Side Converter
An inductor is used on the ac side of the load-side converterfor boost function. For 5% ripple in the current through theinductive filter, inductance (Lf ) of the inductive filter can becalculated as [26]
Lf ={
(√
3/2)maVdc/(6afsIr(p−p)lsc
)}(26)
where fs is the switching frequency and is equal to 10 kHzand Ir(p−p)lsc is the peak-to-peak ripple current in the load-sideconverter and inductive filter. During transients, the current inthe inductive filter is likely to be more than the steady-statevalues. For calculation of inductance, current rating of 120%(a = 1.2) of steady-state current is taken; modulation index ma
is taken as one. Thus, the value of inductance of the filter is
Lf ={
(√
3/2)∗700/(6∗1.2∗10 000∗11.1)}
= 0.76 mH.
The rounded-off value of 0.8 mH is selected for investigation.A high-pass first-order filter tuned at half the switching
frequency is used to filter out the noise from the voltage atthe load terminals. The time constant of the filter should bevery small compared with the fundamental time period (T ), orRC � T/10. When T = 20 ms, considering, C = 5 μF, R canbe chosen as 5 Ω. This combination offers a low impedance of8.1 Ω for the harmonic voltage at a frequency of 5 kHz (halfof the switching frequency) and 637 Ω for the fundamentalvoltage.
F. Selection of Specifications of Wind Turbine and Gear Ratio
The wind turbine is designed for 55 kW at 11.2 m/s, whichis considered as rated wind speed. For wind speeds below therated wind speed, the mechanical power Pm captured by theturbine is a function of wind speed Vw, radius of turbine rw,density of air ρ, and coefficient of performance Cp, and is givenas [28]
Pm = 0.5Cpπr2ρV 3w . (27)
The relationship between the coefficient of performance and tipspeed ratio for a typical wind turbine is shown in Fig. 3. Themaximum coefficient of performance (Cpmax) is achieved atoptimum tip ratio (λ∗
w). The values of Cpmax and λ∗w obtained
from the Fig. 3 are 0.4411 and 5.66, respectively. Substituting,Pm = 55 kW, Cp = 0.4411, wind speed Vw = 11.2 m/s, anddensity of air ρ = 1.1544 kg/m3 in (27), the radius of the windturbine rw is obtained as
rw =√
[55 000/{0.5∗0.4411∗1.1544∗π∗(11.2)3] = 7.5 m.
At 11.2 m/s wind speed, the generator rotor speedis considered as 100 rad/s. Substituting the value oftip speed ratio = 5.66, radius of the wind turbine = 7.5 m,
wind speed = 11.2 m/s, and generator speed = 100 rad/s, thegear ratio is obtained as
ηw =ωrwrw
λwVw=
100∗7.55.66∗11.2
= 11.811 ∼= 12 (selected).
In (27), Cp is a function of tip speed ratio λ and blade-pitchangle β [28] as
cp(λ, β) = 0.73(
151λi
− 0.002∗β − 13.2)
e−(18.4/λi) (28)
where
1λi
=1
(λ + 0.08β)− 0.035
β3 + 1. (29)
Equations (27)–(29) are used to simulate wind turbine. In thereal turbines above the rated wind speed, the blade-pitch controlcomes in operation, and the turbine blades are pitched slightlyout of the wind to limit power. Conversely, the blades are turnedback into the wind whenever the wind drops again.
The ratings and the specifications of the selected componentsof the hybrid system based on the aforementioned designprocedure are used for simulation purpose.
G. Computation of Controller Gains
The gains of the controllers are obtained usingZeigler–Nichols step-response technique [29]. A step input ofamplitude (U) is applied, and the response is obtained for theopen-loop system. The maximum gradient (G) and the pointat which the line of maximum gradient crosses the time axis(T ) are computed. The gains of the controller (Kp and Ki) arecomputed using the following equations:
Kp =∣∣∣∣1.2U
GT
∣∣∣∣ (30)
Ki =∣∣∣∣0.6U
GT 2
∣∣∣∣ . (31)
The gain values for both the PI controllers are computed andgiven in the Appendix.
V. MATLAB-BASED MODELING
A simulation model is developed in MATLAB usingSimulink and Sim Power System set toolboxes. The simula-tion is carried out on MATLAB version 7 with ode3 solver.The electrical system is simulated using Sim Power System.The different loads are modeled using resistive and inductiveelements and diode-rectifier-fed resistive loads combined withan LC filter. The unbalanced load is modeled using breakersin individual phases. The developed MATLAB model for thewind-hydro hybrid system is shown in Fig. 7.
VI. RESULTS AND DISCUSSION
The performance of the wind-hydro hybrid system with theproposed control algorithm is demonstrated under differentdynamic conditions as shown in Figs. 8–12. Moreover, per-formance of the wind-hydro hybrid system is studied with
1148 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 4, APRIL 2011
Fig. 7. MATLAB simulation diagram of wind–hydro hybrid system.
Fig. 8. Performance of hybrid system with balanced linear load at wind speedof 11 m/s.
various electrical loads, i.e., balanced linear load (Fig. 8),unbalanced linear load (Fig. 9), balanced/unbalanced nonlinearload (Fig. 10), and mixed load consisting of linear, nonlinear,and dynamic loads (Fig. 11). The performance of the systemis also studied under varying SCIGw rotor speeds due to wind-speed variations (Fig. 12). It is observed that under all theseconditions, the wind-hydro hybrid system performs in the desir-able manner. For nonlinear load under balanced and unbalancedconditions, SCIGw current, SCIGh stator current, and the loadvoltage are balanced, and the total harmonic distortion (THD)in the SCIGw current, SCIGh current, and the load voltage arewithin the desired limit, as shown in Table I. The SCIGw isable to run at speeds corresponding to the MPT with varyingwind speeds. The simulated transient waveforms of the SCIGw
stator current (isw), SCIGh stator current (ish), load-side con-verter current (iC), three-phase load voltage (vL), three-phaseload current (iL), single-phase load currents (iLa, iLb, andiLc), zigzag transformer currents (ita, itb, itc, and itn), loadfrequency (fL), rms value of phase load voltage (Vt), SCIGw
stator frequency (fw), battery current (Ib), battery voltage(Vdc), SCIGw stator power (Pw), SCIGh stator power (Ph),load power (PL), battery power (Pb), coefficient of power(Cp), SCIGw rotor speed (ωrw), and wind velocity (Vw) areshown for different operating conditions.
A. Performance of Wind–Hydro Hybrid System With BalancedLinear Load
In Fig. 8, the performance of the wind-hydro hybrid systemis shown with balanced linear load at wind speed of 11 m/s.The corresponding rotor speed set point for SCIGw is at99.6 rad/s, and its stator frequency is 47.08 Hz. At this speed,the mechanical power corresponding to maximum coefficientof performance is 52 kW. The input mechanical power tothe SCIGh is taken as 35 kW, and the power generatedthrough SCIGh is 33.3 kW. Thus, the total power generated is(52 + 33.3) kW = 85.3 kW. The system is feeding electrically
GOEL et al.: ISOLATED WIND–HYDRO HYBRID SYSTEM USING CAGE GENERATORS AND BATTERY STORAGE 1149
Fig. 9. Performance of proposed system with balanced/ unbalanced linearloads at wind speed of 8 m/s.
balanced three single-phase linear loads (each of 20 kW and10 kvar). Since the power generated by the system is more thanthe required active power for the electrical loads (60 kW), thebattery is absorbing the surplus power to maintain the frequencyof the load voltage constant. Further the reactive power requiredby the load is supplied by the load-side converter to maintainthe magnitude of the load voltage constant. Thus, under theseconditions, both the magnitude and the frequency of the loadvoltage are maintained constant.
B. Performance of Wind–Hydro Hybrid System WithUnbalanced Linear Load
In Fig. 9, the performance of the wind-hydro hybrid systemis shown with unbalanced linear load at wind speed of 8 m/s.The corresponding rotor-speed set point for SCIGw is at
Fig. 10. Performance of proposed system with balanced/ unbalanced nonlin-ear loads at wind speed of 10 m/s.
72.45 rad/s and the stator frequency is 34.17 Hz. At this speed,the mechanical power corresponding to a maximum coefficientof performance is 20 kW. The input mechanical power to theSCIGh is taken as 35 kW, and the power generated throughSCIGh is 33.3 kW. Thus, the total power generated is (20.0 +33.3) kW = 53.3 kW. The system is started with electricallybalanced three single-phase linear loads (each of 30 kW) con-nected between each phase and neutral terminal. Because thepower generated by the system is less than the required powerfor the electrical loads (90 kW), the battery is supplying thedeficit power. At 3.2 s, an unbalance is created by disconnectinga load of 30 kW by opening phase “a” of the load. Now, thetotal active power of the load on the system is reduced from
1150 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 4, APRIL 2011
Fig. 11. Performance of proposed system with combined load of balancedlinear, nonlinear, and dynamic loads at wind speed of 9 m/s.
90 to 60 kW, and, therefore, the battery power (discharging)is decreased to maintain the load frequency constant. At 3.5 s,the phase “b” of the load is also opened. Now, the total activepower of the load on the system is reduced from 60 to 30 kW,and the battery now absorbs the surplus power to maintain thefrequency of the load voltage constant. When the linear loads inthe three phases are balanced, the currents through the zigzagwindings are zero. However, during the unbalanced linear-loadconditions, the currents in the zigzag windings are nonzero.Further, the currents in all the three zigzag windings are thesame, indicating that the zero-sequence currents are flowingthrough these windings. Under the balanced/unbalanced linear-load conditions, the SCIGw stator currents, SCIGh stator cur-rents, and the load voltages are balanced, even though the loadcurrents and the load-side converter currents are unbalanced.Under these conditions, both the magnitude and the frequencyof the load voltage are maintained constant.
C. Performance of Wind–Hydro Hybrid System WithBalanced/Unbalanced Nonlinear Load
In Fig. 10 and Table I, the performance of the wind-hydrohybrid system is shown with balanced/unbalanced nonlinear
Fig. 12. Performance of proposed system with balanced linear loads atvariable wind speeds.
load at wind speed of 10 m/s. The corresponding rotor speedset point for SCIGw is at 90.57 rad/s, and the stator frequencyis 42.58 Hz.
At this speed, the mechanical power corresponding to amaximum coefficient of performance is 40 kW. The inputmechanical power to the SCIGh is taken as 35 kW, and thepower generated through SCIGh is 33.3 kW. Thus, the totalpower generated is (40 + 33.3) kW = 73.3 kW. The system isstarted with three single-phase diode bridge-rectifier loads, eachof 16 kW, and three single-phase linear loads, each of 5 kW.The loads are made unbalanced by disconnecting nonlinear loadfrom phase “a” at 3.1 s and disconnecting nonlinear load fromphase “b” at 3.2 s. In this case, during the period of simulation,the active power load (63 kW) is less than the power generatedby the system. Hence, the frequency at the load end is main-tained constant by diverting the surplus power to the battery.Under these conditions, it is observed that the magnitude andthe frequency of the load voltage are maintained constant. Dur-ing the balanced/unbalanced nonlinear-load conditions, the cur-rents in the zigzag windings are nonzero. Further, the currents
GOEL et al.: ISOLATED WIND–HYDRO HYBRID SYSTEM USING CAGE GENERATORS AND BATTERY STORAGE 1151
TABLE IPERCENTAGE THD OF GENERATOR VOLTAGE, CURRENT, AND CONSUMER LOAD CURRENT UNDER BALANCED/UNBALANCED NONLINEAR LOAD
in all the zigzag windings are the same, indicating that the zero-sequence currents are flowing through these windings.
To demonstrate the harmonic-elimination capability of thesystem under these conditions, the THDs of the SCIGw statorcurrents, SCIGh stator currents, load voltages, and the loadcurrents are given in Table I. The THDs of the load voltages,SCIGw stator currents, and the SCIGh stator currents are wellwithin the limit of 5% as per the IEEE-519 standard, eventhough the THDs of the load currents are on the order of 35%.Under the unbalanced nonlinear-load conditions, the SCIGw
stator currents, SCIGh stator currents, and the load voltagesare balanced, even though the load currents and the load-sideconverter currents are unbalanced.
D. Performance of Wind–Hydro Hybrid System With MixedLoad Consisting of Linear-, Nonlinear-, and Dynamic Loads
In Fig. 11, the performance of the wind-hydro hybrid systemis shown with balanced mixed load with a wind speed of9 m/s. The corresponding rotor-speed set point for SCIGw isat 81.5 rad/s, and the stator frequency is 38.36 Hz. At thisspeed, the mechanical power corresponding to the maximumcoefficient of performance is 29 kW. The input mechanicalpower to the SCIGh is taken as 35 kW, and the power generatedthrough SCIGh is 33.3 kW. Thus, the total power generated is(29 + 33.3) kW = 62.3 kW. The system is feeding electricallybalanced three single-phase linear loads (each of 3.3 kW) andbalanced three single-phase nonlinear loads (each of 8 kW).Because the power generated by the system is more than therequired active power for the electrical loads (33.9 kW), thebattery is absorbing the surplus power to maintain the frequencyof the load voltage constant. At 3.05 s, an induction motorof 15 kW is started, which takes large starting current. Aftera few cycles, the starting transients settle down. At 3.6 s, amechanical load of 40 N · m is applied at the motor shaft, andthe motor starts drawing current to meet the requirement ofthe mechanical load. As a result, the battery charging currentreduces to maintain the frequency of the load voltage constant.Thus, under these conditions, the system performs in the de-sired manner.
E. Performance of Wind–Hydro Hybrid System Under VaryingWind Speeds With Constant Balanced Linear Load
In Fig. 12, the performance of the wind-hydro hybrid systemis shown under varying wind-speed conditions. The wind-hydrohybrid system is started with a wind speed of 7 m/s andthree single-phase loads totaling to 45 kW and 15 kvar. Thecorresponding rotor-speed set point for SCIGw is at 63.4 rad/sand the stator frequency is 30 Hz. At this speed, the mechanical
power corresponding to a maximum coefficient of performanceis 13 kW. The input mechanical power to the SCIGh is takenas 35 kW, and the power generated through SCIGh is 33.3 kW.Thus, the total power generated is (13 + 33.3) kW = 46.3 kW.Since the generated power is almost equal to the active powerof the load, the battery power is zero. At 3.15 s, the windspeed is increased from 7 to 8 m/s. The rotor-speed set pointcorresponding to the wind speed of 8 m/s is at 73.3 rad/s, andthe stator frequency is 34.5 Hz. At this speed, the mechanicalpower corresponding to a maximum coefficient of performanceis 20 kW. Thus, the total power generated is (20 + 33.3) kW =53.3 kW. Now, the generated power is more than the activepower of the load; therefore, the surplus power is used forbattery charging. At 3.45 s, the wind speed is decreased from8 to 6 m/s. The rotor-speed set point corresponding to windspeed of 6 m/s is at 54.4 rad/s and the stator frequency is25.7 Hz. At this speed, the mechanical power correspondingto a maximum coefficient of performance is 7.529 kW. Thus,the total power generated is (7.529 + 33.3) kW = 40.829 kW.Now, the generated power is less than the active power of theload; therefore, the deficit power is delivered by the battery.During variable speed operation, the wind-turbine generatoris able to maintain its coefficient of performance of 0.4411irrespective of the wind speed.
VII. CONCLUSION
Among the renewable energy sources, small hydro and windenergy have the ability to complement each other. Further, thereare many isolated locations which cannot be connected to thegrid and where the wind potential and hydro potential existsimultaneously. For such locations, a new three-phase four-wire autonomous wind-hydro hybrid system, using one cagegenerator driven by wind turbine and another cage generatordriven by hydro turbine along with BESS, has been modeledand simulated in MATLAB using Simulink and Sim PowerSystem tool boxes. The design procedure for selection ofvarious components has been demonstrated for the proposedhybrid system. The performance of the proposed hybrid systemhas been demonstrated under different electrical (consumerload variation) and mechanical (with wind-speed variation)dynamic conditions. It has been demonstrated that the proposedhybrid system performs satisfactorily under different dynamicconditions while maintaining constant voltage and frequency.Moreover, it has shown capability of MPT, neutral-currentcompensation, harmonics elimination, and load balancing.
APPENDIX
1) Parameters of 55-kW 415-V 50-Hz, Y-connectedsix-pole SCIGw: Rs = 0.059 Ω, LS = 0.687 mH,
1152 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 4, APRIL 2011
Rr = 0.0513 Ω, Lr = 0.867 mH, Lm = 0.0298 H, andInertia = 1.5 kg · m2.
2) Parameters of 55-kW wind turbine: wind-speed range =6.0−11.2 m/s, speed range = 43−81 r/min,I = 13.5 kg · m2, r = 7.5 m, Cpmax = 0.4412, andλ∗ = 5.66.
3) Parameters of 37.3-kW 415-V 50-Hz Y-connected four-pole SCIGh: Rs = 0.09961 Ω, LS = 0.867 mH, Rr =0.058 Ω, Lr = 0.867 mH, Lm = 0.030369 H, andInertia = 0.4 kg · m2.
4) PI Controllers: Voltage controller: Kpv = 15 and Kiv =0.05.
5) Transformer Specifications: Three single-phase trans-formers of 15 kVA 138/138 V, connected in zigzagmanner.
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Puneet K. Goel was born in New Delhi, India, in1966. He received the M.Tech. degree in power ap-paratus and system from the Indian Institute of Tech-nology (IIT) Delhi, New Delhi, India, the M.B.A.degree in finance from Indira Gandhi National OpenUniversity, Maidan Garhi, India, and the M.S. de-gree in electrical engineering from the University ofSouthern California, Los Angeles. He is currentlyworking toward the Ph.D. degree in IIT Delhi.
He has been in the Indian Administrative Servicesince 1991. Currently, he is working as Director with
the Ministry of Power, Government of India. His main research interests aredistributed generation, small hydro, wind energy and power generation frombiomass.
Bhim Singh (F’10) was born in Rahamapur, India,in 1956. He received the B.E. (electrical) degreefrom the University of Roorkee, Roorkee, India, in1977 and the M.Tech and Ph.D. degrees from theIndian Institute of Technology (IIT) Delhi, NewDelhi, India, in 1979 and 1983, respectively.
In 1983, he was with the Department of ElectricalEngineering, University of Roorkee, as a Lecturerand became a Reader in 1988. He has been withthe Department of Electrical Engineering, IIT Delhi,where he was an Assistant Professor in December
1990, became an Associate Professor in 1994, and has been a Professor since1997. His area of interest includes power electronics, electrical machines anddrives, active filters, FACTS, HVDC, and power quality.
Dr. Singh is a Fellow of the Indian National Academy of Engineering,the Institution of Engineers (India), and the Institution of Electronics andTelecommunication Engineers, and a life member of the Indian Society forTechnical Education, the System Society of India, and the National Institutionof Quality and Reliability.
GOEL et al.: ISOLATED WIND–HYDRO HYBRID SYSTEM USING CAGE GENERATORS AND BATTERY STORAGE 1153
S. S. Murthy (SM’99–LSM’04) was born inKarnataka, India, in 1946. He received the B.E. fromBangalore University, Bangalore, India, the M.Tech.degree from the Indian Institute of Technology (IIT)Bombay, Mumbai, India, and the Ph.D. degree fromIIT Delhi, New Delhi, India.
He has been with IIT Delhi since 1970 and was theChairman of the Department of Electrical Engineer-ing from 1998 to 2001. He has held assignment withthe University of Newcastle, Newcastle Upon Tyne,U.K., University of Calgary, Calgary, AB, Canada,
Electrical Research and Development Association, Baroda, India, and KirloskarElectric, Bangalore, India. He is the holder of four patents on the SEIG,microhydel applications, and a novel braking scheme. He has also transferredtechnology of self-excited and grid-connected induction generators to industryfor low-and medium-power generation under stand-alone or grid-connectedmode. He has completed several industry-sponsored research and constancyprojects dealing with electrical machines, drives, and energy systems. Recently,he has been instrumental in establishing start-of-the-art energy audit andenergy conservation facilities at IIT under World Bank funding. His areas ofinterest include electric machines, drives, special machines, power electronicapplications, renewable energy systems, energy efficiency, and conservation.
Dr. Murthy is a Fellow of IEE, Life Fellow of the Institution of Engineers(India), and Life Member of Indian Society for Technical Education (ISTE). Heis the recipient of many awards including the ISTE/Maharashtra GovernmentAward for outstanding research and the IETE/Bimal Bose Award for contribu-tion in Power Electronics. He has made significant contributions to professionalsocieties, including being the General Chair of the first IEEE InternationalConference on Power Electronics, Drive and Energy Systems (PEDES’ 96) heldin January 1996 in New Delhi.
Navin Kishore was born in Patna, Bihar, India, in1977. He received the B.Tech. degree in electricaland electronics engineering from the National Insti-tute of Technology, Calicut, India, in 1997, and theM.Tech. degree in power generation technology fromthe Indian Institute of Technology Delhi, New Delhi,India, in 2008.
He has been with the Central Power Utility (NTPCLtd.), New Delhi, India, since 1998, where he ispresently working in the Corporate Planning De-partment. His main research interests are Renewable
