2013 13th Iranian Conference on Fuzzy Systems (IFSC)

978-1-4799-1228-5/13/$31.00 ©2013 IEEE.

Fault duration

1

0.95

0.8

Voltage (pu)

Time (sec.)

0 0.5 1 15

Low Voltage Ride Through Capability Enhancement

in Wind Farms by UPQC Based Fuzzy Controller

Majid Aryanezhad

Electrical Engineering Department

Shahid Chamran University

Ahvaz, Iran

M.Aryanezhad@gmail.com

Elahe Ostadaghaee

Electrical Engineering Department

Tabriz University

Tabriz, Iran

E.Ostadaghaee@gmail.com

Mahmood Joorabian

Electrical Engineering Department

Shahid Chamran University Ahvaz , Iran

mjoorabian@scu.ac.ir

Abstract—In this paper a novel control strategy presented to

grid integration of wind farms using squirrel-cage induction

generators (SCIG) by unified power quality conditioner (UPQC)

based on Fuzzy Logic Controller (FLC). The interaction between

wind generators and grid cause increasing short circuit current

level , instability, fault ride-through (FRT) or low voltage ride-

through (LVRT) capability problem during fault condition. A

new control strategy established for generation refrence signals

of series converter (SERC) and shunt converter (SHUC)

considering Spanish grid code. This control scheme can fulfill

compensate all types of voltage sags. Also the proposed controller

supplements the phase jump recovery. This control scheme deal

with a dual control of real and reactive power transaction

between wind energy conversion system based SCIG (WECS-

SCIG) and grid. As well as, FLC makes a fast dynamic response

to any changes in speed, voltage and other important parameters

of SCIG facing of short circuit and instability of wind farm.The

simulation results can verified the high efficiency of proposed

FLC-UPQC strategy to enhancement of Low Voltage Ride Through (LVRT) capability of wind farm.

Keywords—UPQC; Wind Farm; LVRT; Fuzzy Controller;

Grid Code

I. INTRODUCTION

In recent years, dramatically increasing the penetration of wind farms in electrical power system, caused the importance behavior of wind turbines under grid faults and low voltage conditions or other disturbances. Therefore comprehensive survey to recognize the interaction between wind farms and power grid is necessary [1]. Some literatures studies different impact of wind farms on power system under symmetrical and asymmetrical grid faults [2-5].

Faults or low voltage in the grid cause voltage sag at the point of common coupling (PCC). Voltage sag and other voltage disturbances will decrease the electrical torque of Squirrel-Cage Induction Generators (SCIG); consequently, the

active power of Wind Energy Conversion System based SCIG (WECS-SCIG) will be reduced. Whereas, the mismatch between mechanical and electrical power make increasing the speed of rotor of SCIG. By increasing of rotor speed, the wind farm absorb more reactive power that can cause more depression in voltage magnitude. After clearing the fault, if the rotor speed does not over its critical speed, SCIG can get the equilibrium point. Otherwise, if the wind generators unable to withstand against faults, it must be disconnected from the grid and it may cause a cascading voltage collapse and the breakdown of the rest of wind farm generators. In order to overcome of these mentioned drawbacks, it is necessary to restore the wind turbine normal operation by passing of Grid Code Integration (GCI) requirements.

In this paper the Spanish grid code is used to improve the stability of WECS-SCIG facing the fault conditions.

Fig. 1 defines the area of voltage sag at the PCC that should be borne by wind farms and Fig. 2 depicted the admissible reactive power absorption by wind farms during the voltage sag period [2].

Fig. 1. Spanish grid code- Area of voltage dip that should be borne by

wind farms [2]

In Fig.1, shaded area demonstrated that wind farm must be stay connected during the voltage sag under this requirements. Additionally, wind farm must injected maximum current for voltage sag recovery during the fault (see Fig. 2). For example, wind farm must be stay connected during a voltage sag (with 80% depth) at PCC for 500 msec.

Fig. 2. Spanish grid code- Admissible area of operation during voltage dip

that should be borne by wind farms [2]

Some methods to improve LVRT have been investigated in [6], [7]. Flexible AC Transmission System (FACTS) devices such as Static Var Compensator (SVC) [8], STATic synchronous COMpensator (STATCOM) [9], Dynamic Voltage Restorer (DVR) [10]-[11] are effective solution for LVRT capability enhancement of WECS-SCIG. These devices absorb or inject reactive and active power to the grid to overcome the fault problems. In the other hand, FACTS devices can control the transaction power between SCIG and grid to avoid disconnection of wind turbine from grid [4]-[5].

Unified power quality conditioner (UPQC) is one of the versatile FACTS devices that consists of two parts which are connected in series (SERC) and shunt (SHUC) Voltage Source Inverters (VSI) [12] [13]. Depending on the type of control system, the UPQC represents a practical solution to protect sensitive loads in the presence of grid disturbances, such as voltage sags, swell, unbalance and etc. Some methods have been published to control of UPQC such as UPQC-P [12], [14], UPQC-Q [14]. UPQC-P can compensate only the magnitude of voltage but it could not compensate the phase jump recovery. This control scheme compensate the voltage sag by minimum injected voltage. UPQC-Q injected a large voltage magnitude and this can increase the rating of UPQC. However, UPQC-Q injected the minimum energy to the grid [14]. Also the conventional Proportional Integral (PI) controllers that use in these control strategy, requires precise linear mathematical model. It is difficult to obtain its model under parameter variations, non-linearity, load disturbance etc. Recently, intelligence controllers such as fuzzy logic controller (FLC), artificial neural network etc, have been applied to achieve suitable performance of the controller [15], [16].

So, this paper presented a new control strategy for SERC and SHUC parts of UPQC with a fast response under any faults. This controller is based on FLC by substitution of PI controllers. Also, phase jump recovery is one of the other advantages of this control scheme.

This paper is organized as follows. Section II introduces the model of power system; Section III describes the control strategy of the UPQC in wind farm interconnected to the grid. Section IV introduces the UPQC based FLC; Section V illustrates simulation results and compares responses of the tested system with conventional controller and FLC during transient condition.

II. THE MODEL OF POWER SYSTEM

The power generated by wind turbine PW and speed ratio is presented as following equations:

),(C3

S2

1P Pw

(1)

/R (2) Where ρ is air density, S is the swept area, Ω is the rotor

speed, ν is the wind speed, and CP is the power coefficient, λ is the speed ratio and β is the pitch angle. CP values are taken from a widely used classical model [13].

Fig. 4 demonstrate the wind farm based 6 set of 250 Kw SCIG that interconnected to the grid via UPQC. Wind farm interconnected to the grid through transformator T2 (25/0.66 kV, Y/Δ). Distribution system export 1.5 MW power generated by wind farm into the grid through the two transmission lines with 20 miles length.

Fig. 3. The configuration of power system with UPQC

A. Wind Farm Stability Under LVRT Conditions

Fig 4. [8], as shown electrical torque-slip and reactive power-slip of SCIG under LVRT on stable conditions. At the point of A1, when V1 decrease to V2 , slip increased to S2 untill fault clear at C1. After, voltage can recovered by find of the initial equilibrium point at A1. During the fault, generator draw more reactive power by increasing the slip and it maybe cause the voltage of generator could not restore even after the fault is cleared. Fig. 5., shows the instability conditions. For obtaining the constraint and margine of stability of SCIG;

dt

dJTT

rem

(3)

Where; Tm and Te are mechanical torque and electrical torque respectively. Also, ωr is angular velocity of rotor. Perturbation gives:

dt

(dJTeT

)rm

(4)

For small perturbation, the torque-slip curve can be assumed to be straight lines. Thus;

0

Reactive current (p.u.)

Total current mmmm

PCC voltage

(p.u.) 0.5 0.85

1

0.9

fault and recovery normal operation

Reactive Power

Generation

Reactive

Power Consumption

I sh

IW

T3

Fault

F 1MW

T1

T.L. 1

T.L

.2

T.L

.3

G

5M

W

UPQC controller

T2 M K

RP

C

Wind Farm

1M

W

Ig PC

C

rr

mm

TT

& r

r

mm

TT

(5)

Substituting from eq. (5) into eq. (4) gives:

dt

)(dJ

TeT rr

rr

m

(6)

Fig. 4. Stable operation of WECS-SCIG [8]

Fig. 5. Instable operation of WECS-SCIG [8]

By solving of differential eq. (6), obtained eq. (7);

k]e)[(t

TeTm

J

1

0rrr

(7)

Where; k and Δω0 are constant parameters and obtained from initial conditions. Hence, the stability equilibrium point should have the below constraint:

r

e

r

m TT

(8)

As well by considering Fig.4 and Fig.5, the margin point of stability of WECS-SCIG is the pull-out point of electrical torque. If the fault continued insofar as electrical torque reach over the pull-out poin, SCIG will be instable and may lead to system collapse and so it must to disconnected from grid.

In other hand; according to to Fig.4,when the grid fault is cleared, PCC voltage will return to equilibrium point (A1 point). Because the rotor slip not able to change momentarily due to mechanical momentum inertia, a large amount of reactive power is absorbed by SCIG and the slope of electrical

torque-speed curve )T

(r

e

will be more than the slope of

mechanical torque-speed curve )T

(r

m

. It causes the rotor

acceleration to reduce. The deceleration of the SCIG and reduction of rotor slip lead to reduction of reactive power

absorbed by the SCIG and it causes voltage to increase. Therefore, according to Fig. 4, the reactive power and electrical torque come back to their steady state condition. According to Fig. 5, if the fault is not cleared at suitable time (before to reach the pull-out electrical torque), the rotor slip may increase extensively and the WECS-SCIG will be instable.

Fig. 6 and Fig. 7 shows the magnitude and phase of voltage, respectively (when fault occurs between t=3 sec. to t=3.5 sec.) without utilizing UPQC. The magnitude of the voltage has been decreased significantly and the phase of voltage has been changed. Changing in voltage phase may cause to tripping of sensitive loads and equipments and other power electronic based devices such as battery chargers and Uninterruptable Power Supply (UPS).

In these figures, voltage sag at the PCC is 0.8 per-unit (pu) and phase jump (Δφ) is approximately equal to -28

ᵒ.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.2

0.4

0.6

0.8

1

1.2

Time (sec.)

Vo

ltag

e o

f b

us

K (

pu

)

Fig. 6. Voltage magnitude of wind farm without UPQC

Fig. 7. Voltage phase angle jump of wind farm without UPQC

Fig. 8 indicates the rotor speed of SCIG without utilizing UPQC. This figure shows, when the fault is cleared at t=3.5, rotor speed inceased extremely.Thus, wind generator should be disconnected from the grid.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

500

1000

1500

2000

2500

3000

3500

Time (sec.)

Speed o

f ro

tor

(rpm

)

Fig. 8. Rotor speed of SCIG without UPQC

In Fig. 9, active power (p) and reactive power (q) are shown separately. Increasing the value of reactive power cause the voltage magnitude collapse. On the other side, when

the fault is cleared at t=3.5, the active power where generated by wind farm tending to null.

Thus, considering to Spanish grid code integration requirements, the wind farm performances should be to improve and make better results under low voltage or fault events.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-10

-8

-6

-4

-2

0

2

4x 10

6

Time (sec.)

Acti

ve p

ow

er

(p)

& R

eacti

ve p

ow

er

(q)

p q

Fig. 9. Active power (p) and reactive power (q) of SCIG without UPQC

III. THE CONTROL STRATEGY OF THE UPQC

The conventional control strategy of UPQC are based on PI controller that has been studied in the literatures [12- 14].

Fig. 11 and Fig. 12 show a novel method to control of SERC and SHUC part of the UPQC to overcome any voltage sag without using any Energy Storage Systems (ESS) for DC-link of UPQC. The SHUC part regulates the voltage of DC link and control of the reactive power via control of current. The SERC part, regulates the bus voltage through stored energy in DC link through SHUC part [17].

A. SERC part Control Strategy

Some control strategies investigated in [12-14]. UPQC-P and UPQC-Q are two effective schemes for voltage sag compensation. These methods cannot restore phase jump of voltage and only compensated the magnitude of voltage.

Voltage phase jump is one of the drawback of these schemes. Because, the internal flux of SCIG is out of phase with the voltage. So this can cause a large transient at the beginning and end of the voltage dip [18].

Fig. 10 represent two methods for voltage sag compensation (Fig. 10(a): UPQC-Q) and (Fig. 10(b): UPQC-P).

Fig. 10. Phasor presentation of (a) UPQC-Q , (b) UPQC-P methods

When a fault occur in the grid, the voltage of wind farm

(Vpre-sag) changed to Vsag with a phase angle jump (θ).

Fig.11, shows the detailed phasor representation for required series voltage injection estimation, where Vinj1 added to Vsag to compensate the magnitude of voltage. For voltage angle jump recovery the Vinj2 used. Additionally, Vinj is the total injected voltage to restore the magnitude and phase of pre-sag voltage.

Equations (9) and (10) give required magnitude and phase angle, respectively. Series voltage should be injected to achieve θ power angle lead to desire value of Vpre-sag.

Fig. 11. Series converter control strategy of UPQC

]2

)V)cos(*V(2

))sin(*V[(V sagsagpresagpreinj (9)

sagpre

sag

inj

V

V)cos(

)sin(1tan

(10)

Phase locked loop (PLL) track the phase of PCC voltage. The PCC voltage is transformed to Vd,V𝑞 and V0 based on Park‘s transform [19],[20]. For regulation of PCC voltage, [V‘d, V‘q , V‘0]

Tinj extracted from (9) and (10) and added to

[Vd, Vq, V0]T

PCC.

B. SHUC part Control Strategy

The Fig. 12 represented the phasor diagram of different current due to phase angle jump (θ) advancement.

In steady state stability,

shwg III (11)

During voltage sag, eq. (11) changed to eq.(12)

'I'I'I shwg (12)

If only inject a series voltage in such a way that it is not at quadrature of source current, it causes a phase angle difference between PCC voltage and wind farms voltage (termed as power angle) without changing the resultant wind farm voltage magnitude. In that case then a certain amount of reactive as well as active power would flow through series inverter.

Briefly, this control strategy for UPQC can be stated as to inject a voltage through series inverter, with proper magnitude and phase angle, such that both the shunt and series inverters will share and support the wind farm reactive power demand, without increase or decrease in the steady-state or transient voltage.

ᶿinj

𝜃𝑖𝑛𝑗1

Vinj

O

d-axis

Vinj1

q-axis

C

A B

Vpre-sag

Vinj2

ᶿ

Vsag

Iw

VCom.

ᶿ

VPre-sag

Vinj

Vsag

(a)

ᶿ

VPre-sag

Vsag Vinj

(b)

In order to recover the reactive power demand, it is necessary to equalize power at wind farm ,bus M (PM) to the value of pre-sag power.

Fig. 12. Shunt converter control strategy of UPQC

So;

MPCC PP (13)

)cos(cosI ))( (180*w*sagpre180*g*sag 'IV'V (14)

Hence;

)cos(*w*sag

sagpreg 'I)

V

V('I

(15)

)]cos()cos()V

V[('I'I

sag

sagpre*wshd

(16) )sin('I'I *wshq (17)

Consequently, by (18) the magnitude of shunt injected current extracted.

22]

V

V['I'I ))sin()cos()cos( (

sag

sagpre

*wsh

(18)

IV. UPQC BASED ON FUZZY LOGIC CONTROLLER

Recently, fuzzy logic controllers (FLC) have generated a great deal of interest in various applications and have been applied in the power electronics and FACTS devices. The advantages of fuzzy logic controllers over the classic (PI controller) controller are that they do not require an accurate mathematical model; they can work with imprecise inputs, can handle nonlinearity, and may be more robust than the classic controller [15].

The fuzzy controller configuration incorporates attractive features such as simplicity, fast dynamic response, and automation, while using a low cost hardware and software implementation.

Fig. 13 shows the control system of the UPQC based on FLC. By replacement of FLC to PI controllers in both SERC

and SHUC parts of UPQC improved the dynamic response of UPQC against LVRT.

In this case, a two-input, one-output fuzzy logic controller was considered. The input signals for the FLC are the DC-link voltage variations in the SHUC controller that obtained from active powers of SERC and SHUC (PSERC, PSHUC) . Also variations of dq-components of the PCC voltage with the deviations are FLC input signals in the SERC controller.

(a)

(b)

Ig

γ

A

B

O -d-axis

q-axis

Ish‘’‘

Iw’

Ig‘

Ig Ish1

’

Ish Iw

Ish‘

C D

dqsagV

)V

V(tan

d

q11

θ1

Voltage

regulator

_

+

Sag detection

Fuzzy

controller

Z-1

_ +

θ

VPCC

PLL

abc

to

dq

S&H

Eq. (9),(10)

ǀVsagǀ θ

2qsag

2dsagsag )V()V(V

,

1

1 )(tan

d

sag

q

sag

V

V

injinjnj VVi

dq

shI

w

w

P

Q1tan

dq

shI

sh

refdI ,

Eq.

(16),(17)

q

shrefI , QPCC

ref

QPCC

Current

Regulator

Vdc-1

Fuzzy

Controll

er

DC voltage

Reulation

Vdcref

+

_ PSHUC

PSERC

Z-1

+

_

Fig. 13. The control strategy of the UPQC; a) SERC, b) SHUC

It is consists of fuzzification part, fuzzy inference part and constant outputs. The inference part consists of membership functions and rule bases which are obtained from an understanding of the UPQC behavior and using of systematic procedure. They are modified and tuned by simulation performance. A triangular membership function has the advantages of simplicity and easier implementation and is chosen for this application [15].

There are seven linguistic variables for each input and seven linguistic variables for output variable, namely, Positive Big (PB), Positive Medium (PM), Positive Small (PS), Zero (ZE), Negative Small (NS), Negative Medium (NM), and Negative Big (NB) has been shown in Fig. 14. Generally, FLC generates the required variable signal to change amplitude modulation ratio and control the magnitude of the injected voltage based on these rules.

(a)

(b)

(c)

Fig. 14. The Takagi-Sugeno FLC membership function of; (a) error, (b)

deviation of error, c) output

Table 1 shows the utilizing 49 linguistic rules directly processes three phase supply voltages to improve the response time of UPQC.

Table 1. The rule table of fuzzy controller

V. SIMULATION RESULTS

Fig.15 to Fig. 18 indicate the comparison of PI-UPQC and FLC-UPQC. These figures show the fast dynamic responsible of fuzzy controller to improve the transient stability of wind farm.

Fig. 15 shows the more better dynamic response of FLC-UPQC than PI-UPQC. Rising time of FLC-UPQC is very small and oscillation of speed of rotor at fault time (t=3 to 3.5 sec.) is very low. Fig. 16 and Fig. 17 show the active power (p) and reactive power (q) of SCIG by PI-UPQC and FLC-UPQC, respectively. Fig. 18 indicate the voltage of SCIG by PI-UPQC and FLC-UPQC .

Fig. 15. Comparison the rotor speed of SCIG without UPQC, PI, Fuzzy

Time (sec.)

Act

ive

(p)

& R

eact

ive

po

wer

(q

)

Fig. 16. Active and reactive power of SCIG with PI-UPQC

0 0.5 1 1.5 2 2.5 3 3.5 4-10

-8

-6

-4

-2

0

2

4x 10

6

Time (sec.)

Acti

ve (

p)

& R

eacri

ve (

q)

p q

Fig. 17. Active and reactive power of SCIG with FLC-UPQC

(a)

e

ė

NB NM NS ZE PS PM PB

NB NB NB NB NB NM NS ZE

NM NB NB NB NM NS ZE PS

NS NB NB NM NS ZE PS PM

ZE NB NM NS ZE PS PM PB

PS NM NS ZE PS PM PB PB

PM NS ZE PS PM PB PB PB

PB ZE PS PM PB PB PB PB

2.5 3 3.5 4-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time (sec.)

Vo

ltag

e (

pu

)

2.99 2.995 3 3.005 3.01 3.015 3.02-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time (sec.)

Vo

ltag

e (

pu

)

3.48 3.485 3.49 3.495 3.5 3.505 3.51 3.515 3.52-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time (sec.)

Vo

ltag

e (

pu

)

(b)

(c)

Fig. 18. Voltage of SCIG; (a) without UPQC; (b) with PI-UPQC (c)with

FLC-UPQC

REFERENCES

[1 ] The European Wind Energy Association, EWEA Publications, 2005.

[Online]. Available: http://www.ewea.org/.

[2 ] Secretar´ıa General de Energ´ıa,―Procedimiento de Operaci´on 12.3.

Requisitos de respuesta frente a huecos de tensi´on de las instalaciones

e´olicas‖, Spain, BOE no. 254, 24th Oct. 2006, in Spanish.

[3 ] P. S. Flannery and G. Venkataramanan, ―Unbalanced voltage sag ride

through of a doubly fed induction generator wind turbine with series

grid-side converter,‖ IEEE Trans. Ind. Appl., vol. 45, no. 5, pp. 1879–

1887, Oct. 2009.

[4 ] D. Santos-Martin, J. Rodriguez-Amenedo, and S. Arnaltes, ―Providing

ride-through capability to a doubly fed induction generator under

unbalanced voltage dips,‖ IEEE Trans. Power Electron., vol. 24, no. 7,

pp. 1747–1757, Jul. 2009.

[5 ] S.M.Muyeen, R. Takahashi, T.Murata, and J. Tamura, ―A variable speed

wind turbine control strategy to meet wind farm grid code

requirements,‖ IEEE Trans. Power Syst., vol. 25, no. 1, pp. 331–340,

Feb. 2010.

[6 ] S.K. Salman, and A.L.J. Teo, ―Windmill modelling consideration and

factors influencing the stability of a grid-connected wind power based

embedded generator‖, IEEE Trans. Power Syst., vol. 18, no. 2, pp. 793–

802, 2003.

[7 ] P. S. Flannery and G. Venkataramanan, ―A fault tolerant doubly fed

induction generator wind turbine using a parallel grid side rectifier and

series grid side converter,‖ IEEE Trans. on Power Electronics, vol. 23,

no. 3, pp. 1126 - 1135, May 2008.

[8 ] Lie Xu, Senior Member, IEEE, Liangzhong Yao, and Christian Sasse,

―Comparison of Using SVC and STATCOM for Wind Farm

Integration‖, International Conference on Power System Technology,

2006.

[9 ] M. Molinas, J. A. Suul, and T. Undeland,‖ Low Voltage Ride Through

of Wind Farms With Cage Generators: STATCOM Versus SVC‖ , IEEE

Trans. on Power Elec., vol. 23, no. 3, pp 1104-1117, May 2008.

[10 ] A. E. Leon, M. F. Farias, P. E. Battaiotto, ―Control Strategy of a DVR to

Improve Stability in Wind Farms Using Squirrel-Cage Induction

Generators‖, IEEE Trans. on Power Sys., vol. 26, no. 3,pp. 1609-1617,

Aug. 2011.

[11 ] D. Ramirez, S. Martinez, C. A. Platero, F. Blazquez and R. M. de

Castro, ―Low-Voltage Ride-Through Capability for Wind Generators

Based on Dynamic Voltage Restorers‖, IEEE Trans. on Energy

Conversion., vol. 26, no. 1, pp. 195-203, Mar. 2011.

[12 ] N.G. Jayanti M. Basu M.F. Conlon K. Gaughan, ―Rating requirements

of the unified power quality conditioner to integrate the fixedspeed

induction generator-type wind generation to the grid‖ IET Renew.

Power Gener., Vol. 3, Iss. 2, pp. 133–143, 2009.

[13 ] M.F. Farias, P.E. Battaiotto, M.G. Cendoya, ―Investigation of UPQC for

Sag Compensation in Wind Farms to Weak Grid Connections‖ , Journal

of Electrical Engineering: Theory and Application,Vol .1, no. 3, pp.

174-181, 2011.

[14 ] M. Basu, S.P. Das, G.K Dubey. ‗Comparative evaluation of two models

of UPQC for suitable interface to enhance power quality‘, Electr. Power

Syst. Res., vol. 77, no. 7, pp. 821–830, 2007.

[15 ] T. J. Ross, ―Fuzzy Logic with Engineering Applications‖, John Wiley &

Sons Ltd, 2004.

[16 ] W. Qiao, R. G. Harley and G. V. Kumar, ―Fault-Tolerant Optimal Neuro

control for a Static Synchronous Series Compensator Connected to a

Power Network,‖ IEEE Trans. on Industry Applications, vol. 44, no. 1,

Jan. 2008.

[17 ] V. Khadkikar and A. Chandra, ―A New Control Philosophy for Unified

Power Quality Conditioner (UPQC) to Co-ordinate Load Reactive

Power Demand Between Shunt and Series Inverters‖. IEEE Trans. on

Power Delivery, vol. 23, no. 4, p. 2522-2534, 2008.

[18 ] M. H. J. Bollen, ―Understanding Power Quality Problems—Voltage

Sags and Interuptions‖, IEEE Press, 2000.

[19 ] P. Kundur, Power system stability and control, McGraw-Hill Inc., 1994.

[20 ] M. Aryanezhad, M. Joorabian and E. Ostadaghaee, ―Modeling and

Simulation of PHEV as a Virtual UPQC Based on Vehicle to Grid

Technology‖, International Review on Modelling and Simulations

(I.RE.MO.S.), vol. 5, no. 4, Aug. 2012.