FACTS-based reactive power compensation of wind energy conversion system N. Dizdarevic, M. Majstrovic, Member, IEEE, and G. Andersson, Fellow, IEEE Abstract - Voltage control and reactive power compensation in a distribution network with embedded wind energy conversion system (WECS) represent main concern of this paper. The WECS is of a fixed speed/constant frequency type that is equipped with an induction generator driven by an unregulated wind turbine. The problem is viewed from shortterm (10 seconds) and mid-term (10 minutes) time domain responses of the system to different wind speed changes. Being disturbed by a variable wind speed, the WECS injects variable active and reactive power into the distribution network exposing nearby consumers to excessive voltage changes. In the FACTS-based solution approach, the Unified Power Flow Controller (UPFC) is used at the point of the WECS network connection to help solve technical issues related to voltage support and series reactive power flow control. Index Terms - wind energy conversion system (WECS), FACTS, UPFC, voltage control, reactive power compensation

I. INTRODUCTION Recently, alternative solutions treating distributed generation of electrical energy have appeared as a consequence of strong ecological concerns with regard to almost all major industrial branches [1]-[3]. Moreover, initiatives of potential investors come along with liberalisation of electrical energy market. It results with an additional impact to a need for conducting a new kind of technical analysis [4]-[5]. Grid integration aspects of renewable sources have become increasingly important as incentives come in large numbers [6]-[7]. From distribution network viewpoint, connection of small power plants with dispersed generation of electricity calls for urgent attention. In case of increased power ratings, dispersed power plants could be integrated in a transmission network. Dispersed generation of electricity is often a subject of polarised discussions. At one side, experienced engineers motivated by wide knowledge of complex power system operation are concerned regarding fundamental realisation of massive introduction of unregulated and uncontrollable generators into a distribution network. At the other side, enthusiastic proponents of renewable sources believe that such generating units are a necessity in operation should domestic and international requirements for reduction of CO2 emission be fulfilled. Moreover, they are convinced that renewables decrease dependence on dominant energy fuels (gas, oil, coal…) in times of large international crisis. Increased penetration of renewables such as wind energy creates an uncontrollable component in electric power system. Based on weather forecasts it is possible to predict a mean wind speed in short-term time period, but not dynamic changes as well, smaller or larger, which take place around a _______________________________ N. Dizdarevic is with the Energy Institute “Hrvoje Pozar”, Zagreb, Croatia (e-mail: [email protected]; web: www.eihp.hr/~ndizdar). M. Majstrovic is with the Faculty of Electrical Engineering, University of Split, Croatia (e-mail: [email protected]). G. Andersson is with the Federal Institute of Technology (ETH), Zurich, Switzerland (e-mail: [email protected]).

base speed. Dynamic changes of wind speed make amount of power injected to a network highly variable. Depending on intensity and rate of changes, difficulties with frequency and voltage regulation could appear making a direct impact to quality level of delivered electrical energy. Conditions of economic justification set project requirements for wind power plant installations in areas with high wind utilisation. Such areas are often located in rural zones with relatively weak electrical networks. In order to establish a balance between polarised attitudes, it is necessary to provide answers concerning technical, economic, and security aspects related to grid integration of wind power plants. From that viewpoint, the objective of this paper is set as to create a countermeasure aimed to suppress excessive voltage changes to nearby consumers and to minimise reactive power exchange between wind power plant and distribution network. Without a countermeasure, it is possible that at some locations only a small number of windturbines could be connected due to weak voltage conditions and increased losses in the nearby network. That would not only leave assessed wind potential unused, but it could also prohibit installation of larger number of wind turbines jeopardising the economics of the whole project. In an attempt to overcome negative dynamic impacts caused by wind speed changes, the voltage regulation and reactive power compensation problem is approached here not only from a conventional aspect, but from a FACTSbased one as well. Wind power plant induction generator is viewed as a consumer of reactive power. Its reactive power consumption depends on active power production. Conventionally, shunt capacitor banks are connected at the generator terminals to compensate its reactive power consumption. In some schemes, shunt capacitor banks could be automatically switched on/off by using feedback signal from generator reactive power. The capacitor switching is triggered through an algorithm if a generator reactive power is outside an allowed dead-band for a specified time period. Further on, continuous voltage control and reactive power compensation at the point of the WECS network connection is provided by using FACTS-based device. Among FACTS devices, the Unified Power Flow Controller (UPFC) is chosen due to its versatile regulating capabilities [8]. The UPFC consists of shunt and series branches, which could be interchangeably used. Being located at the point of the WECS connection to the distribution network, it is made possible to simultaneously control the WECS bus voltage magnitude and/or series reactive power flow that WECS exchanges with the network. This countermeasure is expected to contribute in making assessed wind site viable for connecting larger number of windturbines. II. SYSTEM MODELLING Improvement of voltage control and reactive power compensation by using the UPFC at the point of connection of the small-sized WECS (7x800 kW) is set here as the main

objective. The WECS is connected to the 10 kV distribution network (78 buses, 77 branches) with no other generating units except the one at the main in-feed point representing a slack bus at 110 kV (Fig. 1). It is supposed that the WECS is of a fixed speed/constant frequency type that is equipped with an induction generator driven by an unregulated wind turbine [9]. If such a system is connected to a weak network, some fast and large changes around a mean wind speed may cause excessive voltage changes to nearby consumers due to fluctuations in injected power by the WECS. The UPFC is located at the WECS connection point to the distribution network. The distribution network is connected through an LTC transformer to a 110 kV transmission network. Basic parameters of the system are given in the Appendix. G mVE4

TS mVE4; 10 kV

TS mVE5; 10 kV

G mVE5

0.380 km G mVE3

G mVE6 0.610 km

0.470 km G mVE2

TS mVE6; 10 kV

TS mVE3; 10 kV 0.300 km

G mVE1

G mVE7 0.820 km

TS mVE2; 10 kV 0.400 km

TS mVE7; 10 kV

TS mVE1; 10 kV

bus i UPFC bus j

Kiršina 10 kV

load

0.820 km 1.100 km

radial feeder x1

radial feeder x2

Pagplastika 10 kV load

0.610 km

radial feeder x2

RS Pag 10 kV

INFINITE BUS

2.995 km

TS Pag 10 kV

TS Pag 110 kV

Fig. 1. Distribution network with embedded WECS and UPFC

III. MATHEMATICAL MODEL The impact of the UPFC to voltage control and reactive power compensation of the WECS is investigated by using in-house developed and programmed combined dynamic and static model [8]. In time domain, a set of differential and algebraic equations is established. Differential equations are used to simulate transient behaviour of windturbine induction generator and an infinite bus synchronous generator. Algebraic equations are indispensable for computation of bus voltage magnitudes and angles within load flow analysis. Solution of differential equations by using Runge-Kutta 4th order method is sequentially followed by solving algebraic equations using Newton and Gauss methods. Number of differential equations, which depends on the number of windturbines, is being defined by 5 equations per each induction generator. Number of algebraic equations depends on the number of network buses, which for the network with approximately 80 buses makes 160 equations (80 for voltage magnitudes and 80 for voltage angles). Besides load flow algebraic equations there is a large number of other algebraic variables and equations related for example to wind speed (linear, gust, noise) and windturbines (power/wind characteristics, two-mass rotational shaft with torsion elasticity) [9]. Basic differential equation describing dynamics of induction generator transient model is given by

[

,

]

, d E gen 1 , (1) = − jω 0 S s E gen − ' E gen + j ( X − X ')I gen , dt T0 where reactance X and time constant T0' are obtained from (2) X = X S + X mag ,

T0' =

X r + X mag . ω 0 S Rr

(3)

After d-q decomposition, eq. (1) becomes dEq' Eq' X − X ' Id , = (ω 0 S − ω m )Ed' − ' + dt T0 T0'

(4)

dEd' E' X − X ' . (5) Iq = −(ω 0 S − ω m )Eq' − d' − dt T0 T0' Having a two-mass rotational shaft that is composed of two rotors coupled by a gear-box [10], the electromechanical dynamics of the shaft is defined by ω dΘ c (6) = ωT − m , dt n Pw (Vw ) 1 D − cc Θ c − (Dc + DT )ω T + c ω m S n ω dω T T ngen , (7) = dt 2H T Dc + D g   cc D ω m − Te Θ c + c ω T −  D m + n n n2 dω m , (8)   = H dt  g  2 H m + 2  n   where Θc shaft torsion angle between windturbine and induction generator rotor (rade), ωT, ωm windturbine and induction generator rotor speeds (per unit values, in steady state ωT=ωm/n), 1:n transmission gear ratio between two speeds, Pw aerodynamic power (W), Vw wind speed (m/s), Sngen rated apparent power of induction generator (VA), cc torsion stiffness coefficient (pu/rade), Dc torsion damping coefficient (pu/pu), DT damping coefficient of windturbine (pu/pu), Dg damping coefficient of gear-box (pu/pu), Dm damping coefficient of induction generator (pu/pu), HT inertia constant of windturbine (s), Hg inertia constant of gear-box (s), and Hm inertia constant of induction generator (s). The electromagnetic torque Te is computed as it follows (9) Te = (E d' I d + E q' I q ) ω 0 S . Besides differential equations, the transient model of induction generator comprises four algebraic equations (10) − X ' E q' + RS E d' − (RS2 + X ' 2 )I d − RS Vd + X 'Vq = 0 , ' ' 2 2 (11) RS E q + X ' E d − (RS + X ' )I q − X ' Vd − RS Vq = 0 , Vd − Vn sin Θ n = 0 , Vq − Vn cos Θ n = 0 .

(12) (13)

Numerical analysis is carried out here by using 4-term composite wind speed expression (14) Vw = VwB + VwR + VwG + VwN , where VwB represents base component, VwR linear (ramp) component, VwG gust component and VwN noise component. Base component VwB of wind speed Vw is defined as (15) VwB = const . Linear component VwR of wind speed Vw is defined as 0 za t < T1R  , (16) VwR = Vramp za T1R ≤ t ≤ T1R + TR  t > T1R + TR MAXR *VwB za where Vramp should be computed from t − T1R . (17) Vramp = MAXR *VwB TR

Constant MAXR defines maximum coefficient of linear change with respect to base component VwB, t time, T1R starting time and TR total lasting time of linear change. Gust component VwG of wind speed Vw is defined as za t < T1G 0  , (18) VwG = Vsico za T1G ≤ t ≤ T1G + TG 0 za t > T1G + TG  where Vsico should be computed from

Functional structure of the UPFC results with appropriate electric circuit arrangement [11]. The series converter AC output voltage is injected in series with the line (Fig. 3). It exchanges only active power with shunt converter. Reactance xS is the one seen from terminals of the series transformer.

 t − T1G   t − T1G  . (19) 1 1 − cos 2π  Vsico = − MAXG *VwB sin 3π TG  TG  2  

Constant MAXG defines maximum coefficient of gust change with respect to base component VwB, t time, T1G starting time and TG total lasting time of gust change. Noise component VwN of wind speed Vw is here defined according to spectral density function N

VwN = 2∑ [SV (ω i )∆ω ] 2 cos(ω i t + φ i ) , 1

(20)

i =1

 

1 2

ω i =  i − ∆ω , SV (ω i ) =

(21) .

2K N F 2 ωi   Fω i π 1 +    Vwπ 2

  

2

  

4

(22)

Fig. 3. The UPFC electric circuit arrangement

The UPFC injection model is derived enabling three parameters to be simultaneously controlled [8]. They are the shunt reactive power Qconv1, and the magnitude r and angle γ of the injected series voltage V S . Besides constant series branch susceptance bS, included in the system bus admittance matrix, the bus power injections of the UPFC PSi, QSi, PSj, and QSj are embedded in the model (Fig. 4).

3

In (20-22), variable φi defines random number based on uniform distribution within interval [0:2π], SV(ωi) spectral density function, ∆ω speed (for N=50, ∆ω=0.5-2.0 rad/s), KN surface coefficient (KN=0.001-0.040), F scale of turbulence (F=600-700 m) and Vw wind speed at referent height (m/s). On the basis of defined components of wind speed it is made possible to compute time-domain responses of state and algebraic variables of the wind power plant. Also, the other set of differential equations is defined for control system of the UPFC injection model being described by proportional-integration characteristics. The UPFC injection model is included in the overall system model. The control system of the injection model is proposed and the benefits within the WECS voltage/reactive power problem are explored. The UPFC can provide simultaneous control of all basic power system parameters (voltage, impedance and phase angle) and dynamic system compensation. The controller can fulfil functions of reactive shunt compensation, series compensation and phase shifting meeting multiple control objectives. From a functional perspective, the objectives are met by applying boosting transformer injected voltage and exciting transformer reactive current (Fig. 2). The injected voltage is inserted by using series transformer. Its output value is added to the network bus voltage from the shunt side, and is controllable both in magnitude and angle. The reactive current is drawn or supplied by using shunt transformer.

Fig. 2. The UPFC device circuit arrangement

Fig. 4. The UPFC injection model with control system

If there is a control objective to be achieved, the bus power injections are modified through changes of parameters r, γ, and Qconv1. Control system of the injection model is proposed in de-coupled single-input single-output proportional-integral form. It governs the system to a predefined operating point by set-point changes. Selection of input/output signals depends on the predetermined control mode. The shunt side could be controlled only in the voltage mode, Vi↔Qconv1, emphasising that Qconv1 represents reactive power loading of the shunt converter. The series side could be controlled through the r⇔γ pair in different modes. Moreover, during time-domain simulations the extended Jacobian and state matrices are evaluated utilising linearised differential equations of induction generators as well as steady-state load flow equations. Combined model enables usage of different types of predictive indices. Indices based on singular value analysis do not only predict severity of the problem, but also open a way toward a general sensitivity analysis. A small-disturbance impact to the combined model results with a sensitivity of an operating point along time domain trajectory [12].

Conventional and FACTS-based devices are applied in order to flatten voltage profile, preserve stability, correct power factor, and decrease power and energy losses by minimising reactive power flow in the network. Within a scope of voltage control and reactive power compensation problem, power conditions are analysed as a part of the whole problem related to technical aspects of grid integration of the WECS. The WECS is of a fixed speed/constant frequency type equipped with an induction generator that is driven by unregulated wind turbine. Power conditions at network buses are dynamically analysed as functions of wind speed changes. Conditions of increased interaction between the WECS and the LTC distribution transformer are predicted in times of extremely turbulent winds. Transfer of the WECS from an infinite-bus operating mode to an isolated one is also analysed. A. Linear change of wind speed Linear change of wind speed Vw (Fig. 5) is applied in order to pass through the windturbine Pw(Vw) curve (Fig. 6) in a full-scale range of its operating values. This type of wind speed change enables this windturbine to inject active power into a network from minimum to maximum value in a manner slow enough not to induce unwanted oscillations. According to the Pw(Vw) curve, this studied interval of linear wind speed change is approximately related to the one between cut-in and cut-out speeds. All computations are conducted within continuous operation of the WECS, i.e. without starting-up and shutting-down regimes. By choosing this slower linear change, an interference with transients that are caused by capacitor switching (CS) or generator stator 30

stator windings switching (GSWS) is avoided. Each induction generator is equipped with a set of 8x50 kvar capacitor banks that are switched on/off if generator reactive power exceeds inactivity zone of ±30 kvar for longer than 15 s. Moreover, each generator is equipped with two sets of stator windings that belong to smaller/larger rating powers (g/G). Since this linear change is defined for a full-scale activity range of the windturbine, the switching of generator stator windings happens at the wind speed of 7.335 m/s. During linear change of wind speed, the injected active power Pegen becomes variable according to the Pw(Vw) curve, while causing change in reactive power that induction generator simultaneously draws from the network (Fig.7). Capacitive nature of the induction generator operation needs a countermeasure by a local compensation device. At the network connection point, the WECS should inject active power with minimum exchange of reactive one. Therefore, as a conventional countermeasure local capacitor banks are switched on/off in a generator bus to keep generator reactive power exchange within predefined boundaries of ±30 kvar (Fig. 8). The step-wise discrete behaviour of capacitor switching is transferred further to the network. Generator active and reactive power (MW&Mvar)

IV. NUMERICAL RESULTS

1 0.8 0.6

0.2

GSWS

0 -0.2

Qegen

-0.4 -0.6 0

200

400

600

800 Time (s)

1000

1200

1400

Fig. 7. Active and reactive power of induction generator

20

0.1 15

10

5

0 0

200

400

600

800 Time (s)

1000

1200

1400

Fig. 5. Linear change of wind speed 900000 800000

Reactive power in generator bus (Mvar)

Wind speed (m/s)

Pegen

0.4

25

0

1

7 Qegen-Qbat

-0.1

-0.2

CS (on) -0.3

GSWS CS (off)

-0.4

Qegen Qbat

-0.5 0

700000 Aerodynamic power (W) snaga vjetroturbine (W)

PW

200

400

600

800 Time (s)

7 1000

1 1200

1400

Fig. 8. Reactive power flow in generator buses (GEN1 and GEN7)

600000 500000 400000 300000 200000 100000 0 -100000 0 1 2 3 4 5 6 7 8 9 10111213141516171819202122232425 Wind speed (m/s) brzina vjetra (m/s)

Fig. 6. Windturbine Pw(Vw) curve

FACTS-based countermeasure is capable to neutralise discrete behaviour of the conventional one by enforcing continuous response of the WECS voltage and reactive power to the wind speed change at the point of the network connection. If the UPFC serves as a coupler between the WECS and the network, it is made possible to simultaneously control bus voltage magnitude (Fig. 9) which indirectly decreases generator voltage changes (Fig. 10), and to annul series reactive power flow (Fig. 11) which indirectly keeps power factor precisely at unity (Fig. 12).

w/o UPFC

1.07 1.06

CS (off)

1.05

CS (on)

1.04 1.03

with UPFC

1.02

GSWS

1.01 1 0

200

400

600

800 Time (s)

1000

1200

1400

Fig. 9. The UPFC bus i voltage magnitude Vi

Generator 1&7 bus voltage magnitude (pu)

1.08

30

w/o UPFC

1.07

B. Noise change of wind speed Noise change of wind speed Vw (Fig. 13) is applied here to illustrate capability of the FACTS-based countermeasure to simultaneously flatten voltage profile and minimise reactive power exchange between the WECS and the network. The noise change is chosen as ±10% random change around initial value of the wind speed (11 m/s). With the UPFC, the bus voltage magnitude at the point of the WECS connection to the network is kept at a constant value (Fig. 14) not allowing transients from the capacitor switching to pass on deeper to the network. Also, the UPFC voltage support decreases voltage changes of the induction generator (Fig. 15). Moreover, the UPFC series reactive power flow control enforces minimum reactive power exchange with the network (Fig. 16). It makes an advantage not only in keeping power factor at unity, but in decreased power loss on the WECS radial distribution feeder as well.

7

1.06

25

1 CS (off)

1.05

Wind speed (m/s)

UPFC bus i voltage magnitude (pu)

1.08

CS (on)

1.04 1.03

7

1.02

1

with UPFC

20

15

10

5

1.01

GSWS

1

0

0

200

400

600

800 Time (s)

1000

1200

1400

0

300

400

500

600

Fig. 13. Noise change of wind speed 1.065

1.6

GSWS

1.4 1.2 1 0.8 0.6

CS (off)

CS (on)

0.4

w/o UPFC

0.2 0

UPFC bus i voltage magnitude (pu)

UPFC series reactive power flow Q_j2 (Mvar)

200

Time (s)

Fig. 10. Induction generator 1 and 7 bus voltage magnitude

w/o UPFC

1.06

with UPFC CS (on)

1.055

CS (off)

1.05

CS (off)

1.045

CS (on)

w/o UPFC

1.04

with UPFC 1.035

-0.2 0

200

400

600

800 Time (s)

1000

1200

0

1400

100

200

300 Time (s)

400

500

600

Fig. 14. The UPFC bus i voltage magnitude Vi

Fig. 11. The UPFC bus j series reactive power flow Qj2 1.065

with UPFC

1

0.99

w/o UPFC

CS (off)

CS (on)

0.98

0.97

0.96

GSWS

Generator 1 bus voltage magnitude (pu)

1.01

Power factor

100

w/o UPFC with UPFC

1.06

CS (on)

1.055

CS (off)

1.05

1.045

CS (off)

CS (on)

1.04

w/o UPFC 1.035

0.95 0

200

400

600

800 Time (s)

1000

1200

Fig. 12. Power factor at the UPFC bus j

1400

0

100

200

300 Time (s)

400

500

Fig. 15. Induction generator 1 bus voltage magnitude

600

1.05

0.4

Generator 1 bus voltage magnitude (pu)

UPFC series reactive power flow Q_j2 (Mvar)

0.6

w/o UPFC

0.2

0

with UPFC

-0.2

-0.4

-0.6

w/o UPFC 1.045

1.04

1.035

with UPFC 1.03

0

100

200

300 Time (s)

400

500

600

0

C. Gust change of wind speed Gust change of wind speed Vw (Fig. 17) also emphasizes unique capability of the FACTS-based countermeasure to prevent the WECS voltage and reactive power from fast fluctuations induced by fast wind speed changes. The gust change is defined during 10.5 s as 40% maximum deviation from initial value of the wind speed equal to 8.5 m/s. With the UPFC, the bus voltage magnitude at the point of the WECS connection to the network exhibits much smaller changes than in the case without the UPFC (Fig. 18). The UPFC voltage support decreases voltage changes of the induction generator as well (Fig. 19). Due to time delay of 15 s in the capacitor switching criterion, this gust change does not initiate any of it. Simultaneously, control of the UPFC series reactive power flow significantly decreases reactive power exchange with the network (Fig. 20). 30

Wind speed (m/s)

25

20

15

10

5

0 0

5

10

15

20 Time (s)

25

30

35

15

20 Time (s)

25

30

35

40

40

Fig. 17. Gust change of wind speed

0.8

0.6

0.4

0.2

w/o UPFC 0

with UPFC

-0.2

-0.4 0

5

10

15

20 Time (s)

25

30

35

40

Fig. 20. The UPFC bus j series reactive power flow Qj2

D. Interaction between the WECS and the LTC transformer The WECS is through a radial feeder connected to the low-voltage side of the LTC main in-feed transformer. Interaction between the WECS and the LTC transformer could appear in case of large and repeating gust changes of the wind speed Vw (Fig. 21). The gust changes are defined with a 100 s period and a 100% maximum deviation from initial value of the wind speed equal to 7.5 m/s. They represent an extremely large input disturbance at the mechanical side of the windturbine. Large deviations of the WECS injected active and reactive power are followed further on by making network voltages significantly disturbed. If the LTC transformer voltage regulation deadband is set within a narrow range (±0.010 pu) and its total time delay before the activation is related to the period of the gust change, the bus voltage fluctuations could initialise

1.05

30

w/o UPFC

25

1.045

Wind speed (m/s)

UPFC bus i voltage magnitude (pu)

10

Fig. 19. Induction generator 1 bus voltage magnitude UPFC series reactive power flow Q_j2 (Mvar)

Fig. 16. The UPFC bus j series reactive power flow Qj2

5

1.04

20

15

10

1.035 5

with UPFC 1.03

0 0

5

10

15

20 Time (s)

25

30

Fig. 18. The UPFC bus i voltage magnitude Vi

35

40

0

50

100

150

200 Time (s)

250

300

350

Fig. 21. Repeating extreme gust change of wind speed

400

0.98

LTC transformer tap ratio (pu)

0.975 0.97 0.965 0.96

LTC (down)

0.955

LTC (up) 0.95

Generator 1 bus voltage magnitude (pu)

with LTC 1.05

w/o UPFC 1.04

w/o LTC

1.03

with UPFC

1.02 0

50

100

150

200 250 Time (s)

300

350

400

Fig. 25. Induction generator 1 bus voltage magnitude 1.5

1

w/o UPFC 0.5

0

with UPFC

0

0.94 0

50

100

150

200 250 Time (s)

300

350

400

Fig. 22. The LTC transformer tap ratio 0.02

LTC regulated bus voltage change (pu)

1.06

-0.5

0.945

with UPFC

0.015

CS (on)

0.01 0.005 0

LTC (down)

-0.005

CS (off)

-0.01

LTC (up)

-0.015 -0.02 0

50

100

150

200 250 Time (s)

300

350

400

Fig. 23. The LTC transformer regulated bus voltage change 1.07 UPFC bus i voltage magnitude (pu)

1.07

UPFC series reactive power flow Q_j2 (Mvar)

operation of the LTC regulation scheme (Figs. 22-23). Mechanical parts of the tap-changer could experience increased 'wear-and-tear' stresses making maintenance costs increased due to larger number of activations. The FACTSbased countermeasure acts to avoid voltage fluctuations in the network that are induced by large and repeating wind speed changes. With the UPFC operated, the LTC scheme is prevented from being activated by controlling the bus voltage magnitude at the point of the WECS connection to the network (Fig. 24). It blocks large deviations of voltage magnitude to penetrate through the radial feeder from the WECS up to the in-feed point of the distribution network where the LTC transformer is located. The UPFC voltage support decreases voltage changes of the induction generator as well (Fig. 25). Simultaneous control of the UPFC series reactive power flow significantly decreases reactive power exchange with the network (Fig. 26). The FACTS-based countermeasure is again successfully operated.

1.06

with LTC 1.05

w/o UPFC 1.04

w/o LTC 1.03

with UPFC 1.02 0

50

100

150

200 Time (s)

250

300

Fig. 24. The UPFC bus i voltage magnitude Vi

350

400

50

100

150

200 250 Time (s)

300

350

400

Fig. 26. The UPFC bus j series reactive power flow Qj2

E. Isolated operation of the WECS The WECS of unregulated type as the one presented here is generally not capable to stay in operation isolated from the stiff system if there are no other regulated units in the same isolated system. Isolated operation is generally not viable even if there is sufficient wind speed at the WECS location. Primary the problem is related to a lack of the WECS capability for frequency regulation and secondary to the voltage regulation. In a hybrid scheme, the WECS is coupled with a regulated unit which enables it to stay in stable isolated operation. The WECS uncontrollability during isolated operation mode is illustrated hereafter. Three cases are analysed depending on active power mismatch between consumption and generation in the distribution network after being isolated (∆P=0.1 MW, 1.1 MW and 2.2 MW). If the LTC transformer is outaged, the distribution network is isolated from the stiff system having generation power injected from the WECS solely. Since the WECS is not of a regulated type, the isolation causes appearance of the frequency or/and voltage problem. Within 1.5 s, the network bus voltage magnitudes experience collapse decreasing values down to zero (Fig. 27). Isolated system frequency experiences extremely large values within the first second after the disturbance (Fig. 28). In each case, the isolated system frequency is getting highly increased after collapse since the power consumption of the network loads falls down to zero value due to voltage dependency. With the UPFC operated in voltage mode from both sides, the voltage part of the regulation problem could be solved. If the bus voltage magnitude at the point of the WECS connection to the network is supported by the UPFC, the network voltages are

UPFC bus i&j voltage magnitude (pu)

stabilised and kept at a pre-disturbance level. However, resulting frequency deviations could be excessive not allowing continuous operation in a longer term. with UPFC

1

0.8

0.6

0.4

w/o UPFC

0.2

[6]

S. Heier, Grid integration of wind energy conversion systems, John Wiley & Sons, 1998 [7] CIGRÉ, Modelling new forms of generation and storage, WG 38.01, Nov. 2000 [8] N. Dizdarevic, Unified Power Flow Controller in alleviation of voltage stability problem, Ph.D. thesis, University of Zagreb, Croatia, Oct. 2001, [Online]. Available: www.eihp.hr/~ndizdar [9] N. Dizdarevic et al., Grid integration of wind energy conversion system, project study for Croatian Electric Company, Energy Institute HRVOJE POZAR, Zagreb, Croatia, Feb. 2003, [Online]. Available: www.eihp.hr/~ndizdar [10] R. Chedid et al., ''Adaptive fuzzy control for wind-diesel weak power systems'', IEEE Trans. Energy Conversion, vol. 15, No. 1, March 2000, pp. 71-78 [11] M. Noroozian et al., ''Use of UPFC for optimal power flow control,'' IEEE Trans. Power Delivery, vol. 17, no. 4, pp. 1629-1634, Oct. 1997 [12] N. Dizdarevic et al., "Composite load sensitivity in voltage stability problem solved by Unified Power Flow Controller," Power System Computation Conference, Seville, Spain, June 2002, Paper no. 38/4

0 0

1

2

3

4

VII. BIOGRAPHIES

5

Time (s)

Fig. 27. The UPFC bus i voltage magnitude Vi

Nijaz Dizdarevic received his B.S., M.S., and Ph.D. degrees from Univ. Zagreb, Croatia, in 1990, 1994, and 2001, respectively. From 1991 till 2001 he was with the Dept. Power Systems at the same Faculty. During 1996 and 1997 he was at the KTH Stockholm, Sweden. Since 2002 he is with the Energy Institute “Hrvoje Pozar”, Zagreb, working on power system control and stability.

80

w/o UPFC

70

Frequency (Hz)

60

Matislav Majstrovic received his B.S., M.S., and Ph.D. degrees from Univ. Split and Univ. Zagreb, Croatia, in 1973, 1979, and 1986, respectively. He is currently a senior researcher at the Energy Institute “Hrvoje Pozar” Zagreb and a full professor at the University of Split, Faculty of Electrical Engineering in the area of electricity transmission and distribution network analysis.

50

with UPFC 40 30 20 10 0 0

1

2

3

4

5

Time (s)

Fig. 28. Frequency after islanding

V. CONCLUSIONS Within this paper, conventional and FACTS-based aspects of voltage control and reactive power compensation are compared. Benefits of applying power electronics-based devices are clearly depicted within grid integration aspects of the wind energy conversion system. The FACTS-based solution prevents large deviations of bus voltage magnitude induced by variable WECS injected power to penetrate through the distribution network. With the UPFC operated, the WECS voltage control and reactive power compensation problems are alleviated by simultaneous regulation of the bus voltage magnitude and series reactive power flow at the point of the WECS connection to the network. It is expected that presented results would help find another increasingly interesting possibility of FACTS implementation within grid integration aspects of wind energy conversion systems. VI. REFERENCES [1] [2] [3] [4] [5]

N. Jenkins et al., Embedded generation, IEE Power and Energy Series 31, ISBN 0 85296 774 8, London, UK, 2000 CIGRÉ, Impact of increasing contribution of dispersed generation on the power system, WG 37.23, Feb. 1999 T. Ackermann et al., ''Distributed generation: a definition'', Electric Power Systems Research, vol. 57, 2001, pp. 195-204 N. Hatziargyriou, ''Distributed energy sources: Technical challenges'', IEEE 2002 Winter Meeting, NY, USA, Jan. 2002 J. Lopes, ''Integration of dispersed generation on distribution network – Impact studies'', IEEE 2002 Winter Meeting, NY, USA, Jan. 2002

Göran Andersson (M’86, SM’91, F’97) received his Civ.Ing. and Ph.D. degrees from Lund Institute of Technology, Sweden in 1975 and 1980, respectively. Since 1986 till 2000 he was a Professor at the KTH Stockholm, Sweden. Since 2000 he is a Full Professor at the ETH Zurich, Switzerland. He is a Member of the Royal Swedish Academy of Engineering Sciences and the Royal Swedish Academy of Sciences.

VIII. APPENDIX Table A.1 The UPFC basic rated values SCONV1n (MVA) SCONV2n (MVA) rmax (pu) Xk (pu)

4 4 0.05 0.05

Table A.2 The WECS rated values (G/g)

Pn (kW) Un (V) Sn (kVA) 1:n RS (Ω) XS (Ω) Rr (Ω) Xr (Ω) Xmag (Ω) Hm (s) Hg (s) HT (s) Θc (°) cc (pu torque/rade) Dc (pu torque/pu speed) Dm (pu torque/pu speed) Dg (pu torque/pu speed) DT (pu torque/pu speed)

7x(800/200) 690 V ± 10 % 909/232 1:63.6 0.0131/0.1165 0.24/0.72 0.014/0.073 0.16/0.97 5.94/22.2 0.234/0.410 0.008/0.014 5.644/9.787 3.6°/3.6° 884/821 1200/1200 0.008664/0.008031 1.168/1.083 147.15/136.73