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Abstract -- In this paper a local generation system (LGS)based on an induction generator (IG) is presented. The IGsupplies the active power to the system with the support of avoltage source converter (VSC) for power conditioning.With the LGS connected to the grid through couplinginductors the deficit or the excess of power production ofthe IG is imported or exported to the grid at unity powerfactor. In autonomous operation of the LGS a buckconverter, bi-directional boost converter and battery set areconnected at the DC terminals of the VSC to control the DCbus voltage and supplying power to the local load atconstant voltage frequency and amplitude. The LGSincludes an active islanding detection method, batterycharge function and a procedure to reconnect the LGS withthe grid after the voltage is reestablished.

Index Terms -- Local generation system, power flow control,islanding detection, and autonomous operation.

I. INTRODUCTION

In the references [1]-[2] a single to three-phaseconversion system was presented to connect a three-phase

IG to the single-phase grid. The system improves thelocal power quality for linear and non-linear loads,transfer power to the grid at unity power factor usingVSC operating in line-interactive form. In references [3]-[4] two low-power isolated induction generator systemswere presented, in which the dc voltage monitoring isused to control the system operation. In the first schemethe speed-governor is operated to maintain DC busvoltage, tracking the reference value in order to attain thesystems power balance. In the second system, the speedregulator is not included and the power is determinate bythe prime mover. Without control in the power generated,the regulation of the DC bus voltage is accomplished by

sending the excess of energy (non consumed by the acload) to the utility grid through a current inverterconnected to the single-phase line. A new scheme of localgeneration system (LGS) is proposed in this paper asshown in Fig. 1. In this system the induction generator isconnected to the grid. The VSC behaves as an on-linepower-conditioning device for any source connected to itsdc link terminals and as a line-interactive converter forthe induction generator connected to AC terminals. Thepower generated by the IG is determinate by the primermover, supplying partially or totally the power of thelocal load (linear or non-linear and unbalanced load aswell), exporting or importing active power to the grid at

unity power factor. In the DC bus circuit, a battery set is

This work was supported by FAPESP (proc. 05/54525-0).

connected to the VSC through the bi-directional boostconverter and buck converter unit. Both converters arecontrolled to manage the dc power flow when the LGShas to operate in autonomous form. The battery set isused as power back up. With the voltage grid in normalcondition, and with the LGS connected to the grid, theboost converter is controlled to keep the battery charged.With the LGS operating in autonomous form, the control

employs a procedure to reconnect with the grid when theAC voltage is reestablished. The control of the LGSincorporates voltage and frequency protection and activeislanding detection.

Sources

- Hydro- ICE

Utility

C

Local Load

Induction Generator

Filter

VoltageSource

Converter

Bi-directional Boost Converter

VCVT VSLs

S 1

S

is

Vdc

QsPs

BatterySet

PIG

PLoadQLoad

QIG

Qinv

Pinv

Buck Converter

iboost

izRz

iinviload

if vc

Fig. 1. Local Generation System (LGS)

The goals of this article are: 1) to present a strategy tokeep the DC bus voltage constant by controlling thephase angle of the local voltage, V C, when the grid isconnected or by the control of the bi-directional boostconverter and buck converter when the LGS operate inautonomous form; 2) A new strategy of islandingdetection is proposed, combining an active and passivemethods; 3) Non-linear and unbalanced load can besupplied by the LGS without injection of harmoniccurrent to the grid and without local voltage distortion.

II. C ONTROL STRATEGIES

A. Grid connected mode

In grid-connected mode the LGS is controlled to injector absorbs the excess or deficit of power production ofthe IG to keep the active power balance of the system,expressed by the equation (1).

Power Flow Control and Islanding Detectionof the Local Generation System with

Induction Generator

R. M. Martinez. *, J. A. Pomilio * and L. C. Pereira da Silva** School of Electrical and Computer Engineering FEEC State University of Campinas - UNICAMP, Postal Box 6101 13081 970Campinas SP Brazil

rodomore@dsce.fee.unicamp.br , antenor@dsce.fee.unicamp.br , lui@dsee.fee.unicamp.br

SPEEDAM 2008International Symposium on Power Electronics,Electrical Drives, Automation and Motion

958978-1-4244-1664-6/08/$25.00 2008 IEEE

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Load IGS PPP = (1)

sin X

VsVcP

LsS

= (2)

PS is the grid active power, P IG is the active powergenerated by the IG and P Load is the active powerconsumed by the local load. is the angle between V Cand V S. The power balance is obtained by the powerangle regulation that controls the active power flow tothe grid according to equation (2). V C is the voltage at thefilter capacitors terminals, V S is the voltage of the grid,XLs is the coupling reactance with the grid. The operationof the LGS at unity power factor can be obtained makingQs = 0 from equation (3), for the reactive power balance,and can be kept by adjusting the amplitude of the voltageVC according to equation (4).

)( IGinv Load S QQQQ = (3)

cos2

=

Ls

S C

Ls

C S X

V V X V

Q (4)

Qs is the reactive power of the grid, Q Load is thereactive power consumed by the local load, Q IG is thereactive power consumed by the IG and Q inv is thereactive power of the VSC. Fig. 2a and 2b show the blockdiagram of these controllers.

ref PI

V dc,ref

V dc

+

0

-

22

1

1

(a)

Q s

Q s,ref = 0PI

+

+

+

Qs,dist (t)

V ref cV ,

V c,nom

+

PIsincV

+

0

0

cV

- 1

1

2

2

(b)

dq

ref

P

dq S V P W M

ref cd V ,

ref cqV ,

ref cV ,

ref cV ,

e

e

cV cV

f i f i

VoltageController

PLL

ref cV ,

V T

. . . . . .

1

2

6

abccV ,

abc abc f i ,

abc

(c)Fig. 2. Control block diagrams of the LGS(a) of outer DC voltage

control loop, (b) outer power factor control loop. (c) VSC controller

These two reference signals are transformed to alfa-beta frame using the angular position of the voltage

grid as show Fig. 2c calculated by a PLL system [15],shown in Fig. 4. The voltage controller is that describedin reference [2] and shown in Fig. (3). The V_Reg standfor multiple P + Resonant controllers for selectivecompensation of low-order voltage harmonics [1][6].

+ +PI i

sC f G1

+ +

G2

-

+

+ref cV ,

ref cV ,

sC f G 1

cV

cV

V Reg.

G2

PI i +

ref f i ,

ref f i ,

f i

+

e

+

+ -

-

V Reg.

f i-

+ +

Fig. 3. Voltage control scheme of the VSC

The outputs of the voltage controller are the inputsignal to the space vector modulator to generate thecommand pulses for the VSC. Note that by imposingsinusoidal voltage on the load bus, the VSC automaticallycompensate the harmonic and the reactive power of theload.

wg

w0

PI +

+

+

vTa

vTb

vTc

() sinsin (120)

sin (+120)

T

T 01

21

0

f

s

1

+

uv

au

bu

cu

+

-

dp

0=

dp

dp

PI r sinc sincw

1

2

1

2

Fig. 4. PLL system

B. Autonomous operation mode

In the autonomous operation the LGS is not anymoreconnected to the grid and the VCS is controlled to keep

the amplitude and frequency of the local voltage constant.This is obtained by changing the position of the switch inthe figure 2a, 2b and 4 from position 1 to 2 [11]-[12].Without the grid voltage, the DC bus voltage is no longercontrolled by the power angle . Instead the buck andthe boost converters control the DC bus voltage.

When the IG power is lower than the power consumedby the load, the DC bus voltage decreases. If it reachesthe lower limit, current injection by the boost converter isturned on (battery discharged) as show in Fig. 5(a). Thecurrent increases the DC voltage; if it reaches the upper

limit, the current is turning off, to keep the DC voltageinside the voltage band. When the IG power is greaterthan the power load, the DC voltage increase, if it reaches

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the upper limit, the buck converter is turned on toconsume power in the resistance R Z and reduce the DCvoltage as shown in Fig. 5b. Fig. 6 shows the boost andbuck converters control scheme, including the batterycharge function.

i_ boost

t 1 t (s)T

V dc

V d c , m a x

V d c , m i n

I _ boost

V d c , n o m

(a)

t 1 t ( s )T

V d c

V d c , m a x

V d c , m i n

i z

V d c , n o m

V d c / R z

(b)Fig. 5. DC bus voltage control (a) When IG power is lower than power

load (b) When IG power is higher than the power load

Battery state

Vdc = 599

Vdc = 560

1

0

hi 1

low 0

+I_batt

discharge

Vdc = 580

Vdc = 550

1

0 X

++ PI i

i_batt

+

+-

-

i_batt,ref

Battery Charge

Battery Discharge

boost

Vbat= 295

Vbat = 280

V _batt

PI VV _batt,ref

0

1

0

1

0

i_batt_limit

lim

Vdc = 650

Vdc = 620

Buck control

1

0buck

Fig. 6. Block diagram for boost and buck converter control

When the LGS is connected to the grid the batterybank will be charged or kept charged by the outer voltagecontrol loop. The active and reactive power balance, inthe autonomous operation, are expressed by (5) and (6)

IGinv Load PPP = (5)

IGinv Load QQQ = (6)

When the DC voltage (V dc) is in discharge range,depending on battery state of charge the current injectionis regulated by the PI i current control. When V dc voltage

is in the battery charged range, the voltage control PI V isenabled. If the battery is deeply discharged, it will becharged at constant current. After the voltage reaches

recommended value, the charge process continues atconstant voltage.

III. I SLANDING DETECTION

The unbalance between the power of the IG and thelocal load powers can be used to detect the islandingoperation. After switch S opens (islanding) there is notpower flow to the grid and the power mismatch flows tothe DC bus. The DC voltage control changes the angle , trying to keep the DC voltage constant, to reestablishthe previous power balance. The consequence is thevariation of the frequency of the local voltages V C and V Tuntil be detected by the frequency protection. Noislanding detection happens when the power generated bythe IG is equal to the power consumed by the local load[8]-[11] in this case, there are not amplitude andfrequency variations of the voltage V T (V T is equal to V C)and DC neither in the bus voltage value.

rmsT V ,

BPF0

rmsT V ,PeakValue

S 1_status

(a)

rmsT V ,

V Limit

Monostable

t d Comparator

S 1_status

S1T 1

(b)Fig. 7. Islanding detection system (a) Detection disturb diagram block

(b) Islanding discriminator diagram block.

To overcome this problem, an active islandingdetection method is used, in which a reactive powerdisturbance dist sQ , is injected [9] on the power factor

control block (Fig. 2b) equation (7)

)2()(, dist dist s f SinQt Q = (7)

Q is the disturbance amplitude, f dist is the

disturbance frequency. Fig. 7 shows the islandingdetection system. A band pass filter (BPF) used to detectthe disturb (with central frequency at 15 Hz). When thepeak value is higher than the threshold, the output signalof the AND logical port is activate. A delay blockintroduces a delay time t d to discriminate the actual signalof others produced by voltage disturbances.

IV. R ECONNECTION TO THE GRID

After the grid voltage recovers, the reconnectioninitiated with the reduction of the phase and amplitudevoltage differences, if exist [14]. Fig. 2b shows the

voltage difference detector that adjusts the local voltage( cnomc V V , ) with a PI regulator to reduce de voltage

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difference ( sincV ). Fig. 4 shows the phase difference

detector that adjusts the frequency ( sincww 0 ) of the

LGS to reduce de phase difference ( sinc ). With thedifferences adequately reduced, a command signal is sendto close the switch S1, reconnecting the LGS with thegrid, with minimum transient.

V. S IMULATION RESULTS

The system of Fig. 1 is simulated (PSCAD/EMTDCenvironment) with a parallel RLC local load. Tables I andII show the parameters of the system.

TABLE ISYSTEM PARAMETERS

Dc-link voltage 600 V Dc-link capacitor 4200 uF Inverter filter inductor 2,5 mH Inverter filter capacitor 110 uF Coupling inductor of the LGS with the grid 1,5 mH

Battery voltage 300 V Boost inductor 1,0 mH Three phase RLC load (Q = 2.5) 2000 W

5000 Var-5000 Var

RMS line voltage of utility 220 V Frequency 60 HzZ-grid 0.144 ohm

TABLE IIINDUCTION GENERATOR PARAMETERS

Rate RMS phase voltage 220 V Frequency 60 HzPower 3 HPStator resistance 0,435 Stator leakage inductance 4,0 mH Rotor resistance 0,816 Rotor leakage inductance 2,0 mH Magnetizing inductance 69,31 mH Inertia 0,05 kg.m 2

No. of poles 4

The parameters of the disturbance signal are: Q =

10.0 var, f dist = 15.0 Hz. Fig. 8 shows the active power ofthe load, the induction generator (IG) and the grid. At t =1.0 s the load is connected with minimum powergenerated by the IG. Therefore, all the power of the localload is imported from the grid at unity power factor, asshow the Fig. 9 (active and reactive power of the grid)

and by the active and reactive power of the VSC as showin Fig. 10. At t = 1.5 s the power of IG is increased tocompensated the power of the local load totally andtherefore the power mismatch is equal to zero.

Time (s)

1.00 1.50 2.00 2.50 3.00 3.50-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

A c t

i v e

P o w e r

( K W )

Pload Pig Ps

Fig. 8. Active power of the load P load , grid P S and inductiongenerator P ig

1.00 1.50 2.00 2.50 3.00 3.50-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0 Ps Qs

Time (s) A c t

i v e ( K

W ) & R e a c t

i v e

( K V a r

) P o w e r

Fig. 9. Active P S and reactive power Q S of the grid

1.00 1.50 2.00 2.50 3.00 3.50-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0 Pinv Qinv

Time (s) A c t

i v e

( K W ) & R e a c t

i v e

( K V a r

) P o w e r

Fig. 10. Active P inv and reactive power Q inv of VSC

At t = 2 s islanding is simulated by opening the switchS (Fig. 1). A voltage ripple ( V = 0.14 V), of the samefrequency of the reactive disturbance, appears in thevoltage V T, as shown in Fig. 11. Fig. 12 shows the outputsignal of BPF and its peak value, used for islandingdetection. The islanding is confirmed after 300 ms Theswitch S 1, opens letting the LGS in autonomousoperation, supplying the local load with constantfrequency and voltage amplitude.

1.00 1.50 2.00 2.50 3.00 3.50126.2

126.4

126.6

126.8

127.0

127.2

127.4

127.6

V o l

t a g e

( V )

V_nom vt_ef

Time (s)

Fig. 11. RMS voltage value V T.

1.00 1.50 2.00 2.50 3.00 3.50- 0.30- 0.25

- 0.20- 0.15

- 0.10- 0.05 0.000.05

0.10 0.15 0.20

0.25

V o l

t a g e

( V )

Vt_p_max vt_per

Time (s)Fig. 12. Signal Perturbation and its peak value measured at V T

At t = 2.5 s a power unbalance is introduced, byreducing 50% the power generated by the IG (Fig. 8). DCbus voltage decreases until turning-on the boost converterto transfer power of the battery to the dc link capacitor,keeping the dc voltage within the hysteresis band (580V,550V), as show the Fig. 13. The IG and the battery feedthe local load. The current injected by the boost

converter, setting in 4.0 A, is shown in Fig. 14.

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1.00 1.50 2.00 2.50 3.00 3.50

-100

0.00

100

200

300

400

500

600

700

V o l

t a g e

( V )

Vdc Vdc_ref

Time (s)Fig. 13. DC bus voltage V dc

1.00 1.50 2.00 2.50 3.00 3.50-1.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

C u r r e n t

( A )

I_boost Iz

Time (s)

Fig. 14. Boost current.

At t = 2.5 s the switch S was closed to simulate thegrid voltage Vs recovering, with phase angle of 30degrees delayed of the voltage V T. Once detected the gridvoltage by the control scheme, the re-synchronizationprocess is started. The re-synchronization process reducesthe amplitude and phase differences between V T and V S.Fig. 15 shows the frequency command generated by thePLL system to reduce the phase difference. At t = 3.2 s,with minimum phase difference, the switch S 1 is closed,reconnecting the LGS with the grid. The transient processis shown in Fig. 16. With the LGS reconnected to the

grid, the DC bus voltage returns to the reference value(Fig. 13). The boost converter is turned-off and therefore,no more active power is injected by the battery or VSC asshow in Fig. 10 and 14.

Time (s)1.00 1.50 2.00 2.50 3.00 3.50

59.20

59.40

59.60

59.80

60.00

60.20

60.40

60.60

60.80

F r e q u e n c y

( H z )

fr2 fr_nom

Fig. 15. Frequency of V T

The system imports the power deficit from the gridautomatically, as show the Fig. 8 and 9, supplying thelocal load. Fig. 16 shows the line current and its transientbehavior at the reconnection.

1.00 1.50 2.00 2.50 3.00 3.50

-12.5

-10.0-7.5-5.0

-2.5

0.02.5

5.0

7.510.0

12.515.0

C u r r e n t

( A )

Isa

Time (s)Fig. 16. Current grid i s.

A single-phase non-linear local load, whoseparameters are shown in table III, was tested:

TABLE IIISINGLE -PHASE RECTIFIER

Active power 2000 WFilter capacitor 800 uF Load resistance 40.0

The non-linear load is connected at t = 0.9 s. At t = 1.3s, the induction generation inject 3.25 kW more than thepower consumed by the load. The power mismatch isexported to the grid at the unity power factor, as shown inFig. 17. At t = 1.8 s the islanding is simulated and at t =1.85 s it is detected by the frequency protection,disconnect the LGS of the grid, as show Fig. 18. Fig. 19shows the DC bus voltage. The power excess isdissipated in the resistance R Z of the buck converter,keeping the dc voltage within predetermined hysteresisband (650V, 620V). Fig. 20 shows the distorted loadcurrent that is compensated by the VSC, as show in Fig.21. Fig. 22 shows the grid current. At t = 2.2 s, thereconnection with the grid begin.

1.00 1.50 2.00 2.50 3.00 3.50-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0Ps Qs

Time (s) A c t

i v e

( K W ) & R e a c t

i v e

( K V a r

) P o w e r

Fig. 17. Active and reactive power of the grid

1.00 1.50 2.00 2.50 3.00 3.50

59.2059.3059.4059.5059.60

59.7059.8059.9060.0060.1060.2060.3060.4060.5060.6060.7060.80

fr2 fr_nom

Time (s)

F r e q u e n c y

( H z )

Fig. 18. Voltage frequency.

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1.00 1.50 2.00 2.50 3.00 3.50-100

0.00

100

200

300

400

500

600

700

800 Vdc Vdc_ref

V o l

t a g e

( V )

Time (s)

Fig. 19. DC bus voltage

2.120 2.140 2.160 2.180 2.200 2.220 2.240 2.260

-50.0

-40.0-30.0-20.0

-10.0

0.00 10.0

20.0 30.0 40.0 50.0 60.0

C u r r e n t

A

I_La

Time (s)Fig. 20. Load current.

1.600 1.610 1.620 1.630 1.640 1.650 1.660 1.670 1.680 1.690-600

-400

-200

0.0

200

400

600

V o l

t a g e

( V ) a n d

C u r r e n t x

1 0 ( A )

Iax10 Vc_an

Time (s)Fig. 21. Current i inv and voltage V C of the VSC.

1.600 1.620 1.640 1.660 1.680 1.700-200

-150-100

-50

0.0

50

100

150

200 Isax10 Ean

V o l

t a g e

( V ) a n d

C u r r e n t x

1 0 ( A )

Time (s)

Fig. 22. Current i s and voltage V s of the grid.

VI. C ONCLUSIONS

The simulation results show that the parallel operationof the VSC in the LGS with the IG can supply a localload, importing or exporting power to the grid at unitypower factor. In islanding operation, the LGS can keepthe continuity of the power to the local load, supported bythe IG and the battery set. The effectiveness of the

islanding detection method and the reconnectionprocedure of LGS with the grid were confirmed.

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