Voltage and Frequency Controller for Three Phase Four Wire Isolated Double Wind Energy Conversion System using Cage Generators

Voltage and Frequency Controller for Three Phase Four Wire Isolated Double Wind Energy Conversion System using Cage Generators

Avinash D. Matre

PG Student, Electrical Department Walchand College of Engineering.

Sangli, India.

Roshan P. Haste

PG Student, Electrical Department Walchand College of Engineering.

Sangli, India.

Mrs. S. L. Shaikh

Asst. Prof., Electrical Department Walchand College of Engineering.

Sangli, India.

AbstractThis paper presents a new voltage and frequency Controller for three phase four wire isolated double wind energy conversion system using one squirrel cage induction generator (SCIG) driven by a variable-speed wind turbine and another SCIG driven by a constant power wind turbine feeding three phase four-wire local loads. The proposed system consist two back to backconnected pulse width modulation controlled insulated gate bipolar transistor based voltage source converters (VSCs) with a battery energy storage system at their dc link. The main purpose of the control algorithm for the VSCs is to achieve control of the magnitude and the frequency of the load voltage. The proposed system has a capability of bidirectionalactive and reactive power flow, by which it controls the magnitude and the frequency of the load voltage. In this system wind turbineand a voltage and frequency controller are modeled and simulated in MATLAB using Simulink and Sim Power System set toolboxes and various aspects of the proposed system are studied for different types ofloads, and under varying wind speed conditions. The performance of the proposed system is presented todemonstrate its capability of voltage and frequency control (VFC) and load balancing.

Keywords Battery energy storage system (BESS), Wind energy conversion system, Squirrel cage induction generator (SCIG).

I.INTRODUCTION

Due to souring prizes of fossil fuels and increase in emission of greenhouse gases, the reasonableattention given to the renewable energy sources.Renewable energy sources are the natural energyresources that are inexhaustible, for example, wind, solar,geothermal, biomass, and small hydrogeneration.

As considering wind turbines, they are consisting of two types of wind turbine generators fixed speed and variable speed turbinein which rotational speedvaries inaccordance with wind speed. Theenergy-conversion efficiency of fixed speed wind turbine is very lowfor widely varying wind speeds.In early years, windturbinetechnology has switched from fixed speed to variablespeed. The features ofvariable speed machines are they reducemechanical stresses, dynamically compensate for torque andPower pulsations, and improve power quality and systemefficiency [1].

When the renewable energy sources are connected to the grid, the total active power is fed to the grid. Forisolated

systems supplying local loads, if the extracted power ismore than the local loads (and losses), the excess power is supplied to a dump load orstored in the battery bank [1].

WhenSCIG is used for wind generation, its reactivepowerrequirement 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.,

In this paper, a new three-phase four-wire isolated wind energy conversion system is proposed for isolated locations, which cannot be connected to the grid. The proposed system utilizes variable speed

Wind-turbine-drivenSCIG (subscript vfor variable speed wind), and a constant-speed/constant-power wind-turbine-

drivenSCIG (subscript c for constant-speed wind). For the rest of this paper, the subscript v is used to denote the

parameters and variables ofthe variable speed wind-turbine generator, and the subscript c is used to denote the parameters and variables of the constant power wind-turbine generator.A battery energy storagesystem (BESS) is used in the dc link, which performs thefunction of load leveling in the wake of uncertainty in thewind speed and variable loads. In order to remove the ripplesfrom the battery current an inductor isconnected in series with the BESS.

A new control algorithm is proposed for the double wind energy conversion systemit has the capability of loadleveling, load balancing, along with voltage andfrequency control (VFC).

For the proposed system, there are three modes of operation. In the first mode, the required active power of the load is less than the power generated by theSCIG , and the

excess power generated by the SCIG is transferred to the

BESS through the load-side converter. Moreover, the power

generated by the SCIG is transferred to the BESS.Second mode, the required active power of the load is morethan the

power generated by theSCIG but less than the total

power generated by SCIG and SCIG . Thus, portion of thepower generated by SCIG is supplied to the load through theload-side converter and remaining power is stored in

BESS. Inthe third mode, therequired active power of the

load is morethan the total power generated by SCIG and

SCIG . Thus,the deficit power is supplied by the BESS, and

thepowergenerated by

SCIG

and the deficit met by BESS

= 1 + ( ( ) –

are suppliedto the load through the load-side converter.

1. PRINCIPLE OF OPERATION

   This system uses two back to- back-connected PWM- controlled IGBT-based VSCs. These VSCs are referred to

   as the machine (SCIG ) side converterand load-side

   ( 1))+ ( ) (8)

   ( )

   ()

   = (1) + ( ( )- ( 1))

   + ( ) (9)

   The reference three-phase SCIGC currents are thencompared with the sensed SCIGC currents ( , , and )

   converter. The objectives of the machine(SCIG ) side converter are to convert AC to DC, and the objectiveof the

   load-sideconverter is VFC at the load terminals

   tocompute the SCIGC

   current errors as

   =

   (10)

   bymaintaining active- and reactive-power balance..

   = (11)

   The load-side converter is controlled for theregulation

   =

   (12)

   ofload-voltage magnitude and loadfrequency. To maintain theload-frequency constant, it is essential that any surplus

   activepower in the system is diverted to the battery. Also, thebattery system should be able to supply any deficit in thegenerated power. Similarly, the magnitude of the load voltageis maintained constant in the system bybalancing the reactivepowerrequirement of the load through the load side converter.

2. CONTROL ALGORITHM

   1. Control of Machine Side Converter

      The main purpose of the load-side converter is to convert AC into DC. It is used as a rectifier.

   2. Control of Load-Side Converter

   The main purpose of the load-side converter is to maintain rated voltage and frequency at the load terminals irrespective of connected load.

   Generation of Reference Three-Phase Currents:

   The reference voltages ( , , and ) for the control of

   These current errors are amplified and theamplifiedsignals are compared with a fixedfrequency (10 kHz)triangular carrier wave of unity amplitude to generate gatingsignals for IGBTs of the load-side converter.

3. DESIGN OF SCIG-BASED DOUBLE WIND ENERGY CONERSION SYSTEM

   The system is designed for an isolated location with theload varying from 30 to 90 kW at a lagging pwer factor (PF)of

   0.8. The average load of thesystem is considered to be 60Kw.

   1. Selection of Rating of SCIGs

      The rating of the variable speed wind turbine is considered as 55Kwand that of constant speed wind turbineis taken as 35Kw.Both turbines arecoupled to SCIGs. Therating of the

      the load voltages at time tare given as

      SCIG is equal to the rating of the variable speedwind turbine,which is 55kW. The rating of the SCIG should be

      = 2 sin(2) (1)

      equal tothe rating of the constant speed wind turbine, which

      (2)

      is 35kW.

      = 2 sin(2 120)

      = 2 sin(2 + 120) (3)

      wheref is the nominal frequency, which is considered as 50

      Hz, and is the rms phase-to- neutral load voltage, which is considered as 240 V.

      The load voltages ( , , and ) aresensed andcompared with the reference voltages.The error voltages( ,

      and ) at the nthsampling instant arecalculated as

      Vanerr(n) ={V*an(n) Van(n)} (4)

      Vbnerr(n) ={V*bn(n) Vbn(n)} (5)

   2. Modeling of wind turbine

      The mechanical power captured by the wind turbine is

      = 0.5 2 3 (14)

      Where =coefficient of performance, r=radius of turbine, =wind speed, =density of air.

      C.Selection of voltage of dc link and battery design

      Vcnerr(n) ={V*cn(n) Vcn(n)} (6)

      For satisfactory PWM control, the dc busvoltage(

      ) must

      The reference three-phase SCIGC currents ( , , ) are

      be more than the peak of the linevoltage [8]

      generated by feeding the voltage error

      = 2 2 (15)

      = 1

      + (

      – 1 ) 3

      ( )

      ( )

      ( ) ( )

      where

      is the modulation index normally with a

      + ( ) (7)

      maximumvalue of one and is the rms value of the line

      voltage on theac side of the PWM converter. The maximum

      rms voltage atSCIG terminals as well as the rms value of the line voltage atthe load terminals is 415 V. Substitute the

      value ofac=415V,

      the value of Vdc should be obtained as 677.7 V. The voltage ofthe dc link and the battery bank is selected as 700 V. Battery is an energy storage unit, its energy isrepresented in kilowatt-hour, when a capacitor isused to modelthe battery

      90 kW at 0.8 lagging PF. The reactive power of the load is supplied by the load-side converter. Hence, the reactive power flow through load-side converter ( ) is equal to

      thereactive power demand of the load ( ). At a load of 90

      kW at0.8 lagging power factor,

      = = (90/0.8) × 0.6 = 67.5kvar.

      Therefore, the kVA rating of the load-side

      converter( ) is

      unit, the equivalentcapacitance

      is given as[7].

      =

      = 156.5

      = ×3600 ×1000

      0.5 2 2

      (18)

      Where

      3

      is the rms voltage on the ac side of theload side

      converter, which is 415 V.

      Where and are the minimum and maximumopen circuit voltage of the battery under fully

      discharged andcharged conditions. HereThevenins model is used fordescribing the battery in which the parallel combination ofcapacitance ( ) and resistance( ) in series

      with internalresistance( ) and an ideal voltage source of

      The maximum current through the switching devices in the load-side converter =1.25 × (11.1 + 221.3) = 290.5 A.

      The voltage rating of the switching devices is decided by the dc-link voltage, whose maximumvalue is 750 V. Taking a 25% margin, the voltagerating of the switching devices of the load-sideconverter should be more than 1.25×750 V,

      voltage 700VTaking the values of 680V, andkW · h = 600, the value of

      = 750V, =

      i.e., 937.5 V.

      The commercially available rating for switching device

      obtained is 43156F.

      1. Selection of Rating of Machine ( ) Side Converter

         The maximum active-power flow through the machine side

         converter = 55 kW, and the maximum reactive power flow provided from the machine-side converter ( ) is calculated as

         (IGBT) higher than 937.5 V and 290.5 A is 1200 V and 300

         A.

         F.Selection of rating of AC inductor and RC filter on ac side of load-side converter

         An inductor is used on the ac side of the load-side converter for boost function. For 5% ripple in the current through the

         inductive filter, inductance ( ) of the inductive filter can be

         2

         =

         2

         2

         = 18.4 (19)

         calculated as[8]

         Where

         is the maximum line voltage generated at the

         = { 3 /(6 ( ) )} (19)

         SCIG terminals, which is 415 V, at a frequency(f) of 50 Hz generated at a wind speed of 11.2 m/s.

         The V A rating ( ) of the machine-sideconverter is calculated as

         Where

         = switching frequency=10kHz

         = 0.76

         =(2 + 2 ) = (552 + 18.422) = 58kVA

         A high-pass first-order filter tuned at half theswitching frequency is used to filter out the noise from the voltage at

         and the maximum rms machine-side convertercurrent as

         the load terminals. The timeconstant of the filter should be very small compared with the fundamental time period (T), or RC <<T/10.When T = 20 ms, considering, C = 5F, R

         =

         3

         = 80.7 .

         can be chosen as 5.

         The voltage rating of the switching devices is decided by the dc-link voltage, whose maximum value is 750V. Taking a 25% margin,the voltage rating of the switching devices of the machine-side

         Converter should be more than 1.25 750 V, i.e., 937.5 V. The maximum current through the switching device is 1.25{0.05 (2) 80.7 + (2) 80.7}A =149.8 A.

         The ratings of the commercially available device (IGBT) higher than these values are 1200 V and 200A, and the same are selected for the design purpose.

      2. Selection of Rating of Load-Side Convertor

   The rating of the load-side converter is determined by the case when the connected load is at its maximum value, i.e.,

4. MATLAB BASED MODELING

   A simulation model is developed in MATLAB using Simulink and Sim Power System set toolboxes. The developed MATLAB model for the double wind-energy conversion system is shown in Fig.1.

   Fig.1. MATLAB simulation diagram of double wind energy conversion system.

   Fig.2. Performance of double wind energy conversion system with balanced linear load (60Kw) at wind speed of 11.2 m/s.

   Fig.3. Performance of double wind energy conversion system with balanced linear load(100Kw)at wind speed of 11.2 m/s.

   Fig.4. Performance of double wind energy conversion system with balanced linear load (20Kw)at wind speed of 8 m/s.

5. SIMULATION RESULTS

   The performance of the double wind energy conversionsystem with the proposed control algorithm is demonstrated under different dynamic conditions (various electrical conditions andmechanical conditions) as shown in Figs. 2-4. Moreover, performance of the double wind energy conversion system is studied with various electrical loads. Theperformance of the system is also studied under

   varying SCIG rotor speeds due to wind speed variations

   .The simulated transient waveforms of the three

   phaseSCIG stator current(Isv), SCIG stator current(Isc),SCIG stator power( ),SCIG stator power( ),load voltage( ),load frequency( )are shown for different operatingconditions. Thus it verifies the three

   modes of operation.

6. CONCLUSION

Among the renewable energy sources, wind energy conversion system is more reliable source of energy.A new three-phase four ire autonomous double wind energy conversion system, using one cage generator driven by variable speed wind turbine andanother cage generator driven by constantspeed/constant power wind turbine along withBESS has been modeled and simulated in MATLAB usingSimulink and sim power system. Theperformance of thedouble wind energy conversion system has beendemonstrated under differentelectrical andmechanical dynamic conditions. It has beendemonstrated that the proposed double wind energy conversion system performssatisfactorily under different dynamicconditions whilemaintaining constant voltage and frequency. Moreover, it hasshown capability of load balancing.

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BIOGRAPHY

Avinash D. Matrehas obtained B.E. (Electrical)from B.M.I.T,Solapur India in 2011. Currently he is pursuing

M.Tech in Electrical Power System, from Walchand College of Engg. Sangli, India under the guidance of Prof. Mrs. S. L. Shaikh

Roshan P. Haste has obtained B.E. (Electrical) fromGovt College of Engg,

ChandrapurIndia in 2010. Currently he is pursuing M.Tech in Electrical Power System, from Walchand College of Engg. Sangli, India under the guidance of Prof. Mrs. S. L. Shaikh

Prof. Mrs. S. L. Shaikh is an Assistant Professor in Department of Electrical Engineering at Walchand college of Engineering, Sangli. Her area of interest is Power system Stability.