LONGER TERM STABILISATION OF WIND POWER PLANT VOLTAGE/REACTIVE POWER FLUCTUATIONS BY FACTS SOLUTION

NIJAZ DIZDAREVIĆ, MATISLAV MAJSTROVIĆ Energy Institute HRVOJE POŽAR

Savska 163, HR-10000 Zagreb Croatia

www.eihp.hr/~ndizdar

GÖRAN ANDERSSON ETH Zürich

Physikstrasse 3, CH-8092 Zürich Switzerland

andersson@eeh.ee.ethz.ch ABSTRACT Power conditions in distribution network with integrated wind energy conversion system (WECS) are dynamically analysed in longer term time domain. Responses of characteristic variables are evaluated as functions of measured wind speed at the site and of measured active and reactive load power in the network during 48-hour period. The constant minimum, constant maximum and intermittent operating regimes of the WECS are superimposed to simultaneously variable bus load powers in order to estimate severity of network operation. The FACTS-based countermeasure to fluctuating voltage and reactive power is proposed in form of the Unified Power Flow Controller (UPFC). The UPFC is situated at the point of the WECS connection to the network to flatten voltage profile and minimise reactive power exchange. KEYWORDS Wind energy conversion system (WECS), FACTS, UPFC, voltage control, reactive power compensation 1. 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 [8]. 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, but not dynamic changes as well, smaller or larger, which take place around a base speed. Dynamic changes of wind speed in short-term (10 seconds) and mid-term (10

minutes) time intervals make amount of power injected to a network highly variable as it is shown by the same authors in [9]. The FACTS-based countermeasure to fluctuating voltage/reactive power is proposed there in form of the Unified Power Flow Controller (UPFC) due to its versatile regulating capabilities [10]. The UPFC consists of shunt and series branches, which could be interchangeably used. Being situated 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. It suppresses excessive voltage changes to nearby consumers and minimises reactive power exchange between the WECS and distribution network. From that viewpoint, this paper comes as a subsequence in continuing analysis of the WECS operation from short-term (10 seconds) and mid-term (10 minutes) time intervals to longer term period (48 hours). Power conditions in distribution network with integrated wind energy conversion system (WECS) are dynamically analysed by using own ‘in-house’ developed software. Responses of characteristic variables are evaluated as functions of measured wind speed at the site and of measured active and reactive load power in the network during period of 48 hours. The constant minimum, constant maximum and intermittent operating regimes of the WECS are superimposed to simultaneously variable bus load powers in order to estimate severity of network operation. 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 deviations. The impact of the FACTS-based continuous countermeasure to fluctuating voltage/reactive power is elaborated further on. 2. System Modelling Improvement of voltage control and reactive power compensation in longer term period (48-hour) by using the UPFC at the point of connection of the small-sized WECS (7x800 kW) to the network 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 [8]. 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 situated at the WECS connection point to the network. The distribution network is connected through a transformer to a 110 kV transmission network. Basic parameters of the system are given in the Appendix.

TS Pag 10 kV TS Pag 110 kVRS Pag 10 kV

Pagplastika 10 kV

Kiršina 10 kV

INFINITE

TS mVE1; 10 kV

TS mVE2; 10 kV

TS mVE3; 10 kV

TS mVE4; 10 kV TS mVE5; 10 kV

TS mVE6; 10 kV

TS mVE7; 10 kVG mVE1

G mVE2

G mVE3

G mVE4 G mVE5

G mVE6

G mVE7

load

load

feederx2

radial

x2feederradial

radialfeederx1

2.995 km0.610 km

1.100 km

0.820 km

0.400 km

0.300 km

0.470 km

0.380 km

0.610 km

0.820 km

BUS

UPFCbus i

bus j

Fig. 1. Distribution network with embedded WECS and UPFC

3. 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]-[10]. In time domain, a set of differential and algebraic equations is established. Differential equations are used to simulate transient behaviour of wind turbine 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. Basic well-known differential equation describing dynamics of induction generator transient model is

( )[ ]gengengenSgen IXXjE

TEsj

dtEd '1 ,

'0

,0

,

−+−−= ω , (1)

where reactance X and time constant T0' are obtained from

magS XXX += , (2)

rS

magr

RXX

T0

'0 ω

+= . (3)

After d-q decomposition, eq. (1) becomes

( ) dq

dmSq I

TXX

TE

Edt

dE'

0'

0

''

0

' '−+−−= ωω , (4)

( ) qd

qmSd I

TXX

TEE

dtdE

'0

'0

''

0

' '−−−−−= ωω . (5)

Having a two-mass rotational shaft that is composed of two rotors coupled by a gear-box [11], the electromechanical dynamics of the shaft is defined by

ndtd m

Tc ωω −=

Θ , (6)

( ) ( )

T

mc

TTcccngenT

ww

T

Hn

DDDcS

VP

dtd

2

1 ωωωω

++−Θ−

= , (7)

+

−

++−+Θ

=

2

2

2nH

H

Tn

DDD

nD

nc

dtd

gm

emgc

mTc

cc

m

ωωω , (8)

where is Θc shaft torsion angle between wind turbine and

induction generator rotor (rade), ωT,ωm wind turbine 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 wind turbine (pu/pu), Dg damping coefficient of gear-box (pu/pu), Dm damping coefficient of induction generator (pu/pu), HT inertia constant of wind turbine (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

( ) Sqqdde IEIET 0'' ω+= . (9)

Besides differential equations, the transient model of induction generator comprises four algebraic equations

( ) 0''' 22'' =+−+−+− qdSdSdSq VXVRIXREREX , (10)

( ) 0''' 22'' =−−+−+ qSdqSdqS VRVXIXREXER , (11)

0sin =Θ− nnd VV , (12)

0cos =Θ− nnq VV . (13) Numerical analysis is carried out here by using a 48-hour sample set of wind speed data Vw measured consecutively at the WECS installation site for two months. The data set is treated as a suitably representative one for the analysis of the WECS continuous operation in the longer term. On the basis of measured wind speed it is made possible to compute time-domain responses of state and algebraic variables of the wind power plant.

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 and reactive power problem are explored. The UPFC provides simultaneous control of 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 exchanged by shunt transformer.

Fig. 2. The UPFC device circuit arrangement

Functional structure of the UPFC results with appropriate electric circuit arrangement [12]. 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 the series transformer.

Fig. 3. The UPFC electric circuit arrangement

The UPFC injection model is derived enabling three parameters to be simultaneously controlled [10]. They are the shunt reactive power Qconv1, and the magnitude r and angle γ of the injected series voltage SV . 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). 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 pre-defined 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.

Fig. 4. The UPFC injection model with control system

4. Numerical Results Within a scope of voltage control and reactive power compensation problem, power conditions are analysed as 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 are dynamically analysed as functions of wind speed changes within 48-hour period. Responses of characteristic variables are evaluated as functions of measured wind speed at the site and of measured active and reactive load power in the network during 48-hour period. Total amount of the load powers are measured at the main 10 kV in-feed point of distribution network (Fig. 5). Within 10-minute intervals, the total amount is distributed over 10 kV network load buses in proportion to their maximum loadings. The load powers represent average values of each 10-minute interval during 48-hour period.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Tot

al lo

ad p

ower

s (M

W &

Mva

r)

Time (h) Fig. 5. Total active and reactive load power in 10 kV network

P

Q

The constant minimum, constant maximum and intermittent operating regimes of the WECS are superimposed to simultaneously variable bus load powers in order to estimate severity of network operation. Different operating regimes of the WECS are simulated by applying different wind speed patterns (Fig. 6). First, two fully compensated constant regimes are analysed that correspond to minimum (4 m/s) and maximum (16 m/s) WECS engagements. Then, the impact of continually measured intermittent wind speed to the WECS operation is simulated. The intermittent wind speed pattern is taken as an average value of each 10-minute interval between 4 m/s and 25 m/s. It represents the WECS operation without starting-up and shutting-down discontinuities.

02468

1012141618202224262830

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Win

d sp

eed

(m/s

)

Time (h) Fig. 6. Constant and intermittent wind speed patterns

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 deviations. Variable active power depends on the wind turbine Pw(Vw) curve (Fig. 7).

-100000

0

100000

200000

300000

400000

500000

600000

700000

800000

900000

0 1 2 3 4 5 6 7 8 9 10111213141516171819202122232425

snag

a vj

etro

turb

ine

(W)

brzina vjetra (m/s) Fig. 7. Wind turbine Pw(Vw) curve

Conventional (shunt capacitor banks) and FACTS-based (UPFC) 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. The impact of the FACTS-based continuous countermeasure to fluctuating voltage and reactive power is elaborated further on emphasizing clear distinction with respect to shunt capacitor banks.

Due to different bus load powers and wind speed patterns, the WECS injected active power becomes variable according to the Pw(Vw) curve (Fig. 8), while causing change in reactive power that induction generator simultaneously draws from the network (Fig. 9). Capacitive nature of the induction generator operation needs a countermeasure by a local compensation device. Each induction generator is equipped with a set of 8x50 kvar shunt capacitor banks that are switched on/off if generator reactive power exceeds inactivity zone of ±30 kvar for longer than 15 s. At the network connection point, the WECS should inject active power with minimum exchange of reactive one. It is seen that not only this conventional countermeasure has a step-wise discrete behaviour, but its switching sequences are transferred further to the network. If the UPFC is activated in voltage/reactive power mode, the impact to the WECS active power is negligible. But, its impact to the WECS reactive power is significant.

-1

0

1

2

3

4

5

6

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

WE

CS

act

ive

pow

er (

MW

)

Time (h) Fig. 8. The WECS active power injected to 10 kV network

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

WE

CS

rea

ctiv

e po

wer

(M

var)

Time (h) Fig. 9. The WECS reactive power exchange with 10 kV network

If the UPFC serves as a coupler between the WECS and the network, in addition to series reactive power flow annulling (Fig. 9) it is made possible to simultaneously control bus voltage magnitude (Fig. 10). 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.

16 m/s

4 m/s

Wind speed (m/s)

Aer

odyn

amic

pow

er (W

) with UPFC

w/o UPFC

1.01

1.02

1.03

1.04

1.05

1.06

1.07

1.08

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

UP

FC

bus

i vo

ltage

mag

nitu

de (

pu)

Time (h) Fig. 10. The UPFC bus i voltage magnitude Vi

Different operating regimes of the WECS and bus load powers cause variable active and reactive power exchanges at main in-feed point 110 kV/10 kV (Figs. 11-12). At Vw=4 m/s, distribution network active power is supplied from 110 kV network (positive values). At Vw=16 m/s, active power has opposite direction (negative values). The WECS injects a part of active power that is not consumed at 10 kV up to 110 kV network. Exchange of reactive power is kept at rather constant level defined by initial bus load powers. Conventional countermeasure causes short-term deviations due to step-wise changes of 10-minute intervals. FACTS continuous compensation decreases deviation of exchanged reactive power.

-5

-4

-3

-2

-1

0

1

2

3

4

5

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Act

ive

pow

er e

xcha

nge

at in

-fee

d po

int (

MW

)

Time (h) Fig. 11. Active power exchange at in-feed point 110 kV/10 kV

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Rea

ctiv

e po

wer

exc

hang

e at

in-f

eed

poin

t (M

var)

Time (h) Fig. 12. Reactive power exchange at in-feed point 110 kV/10 kV

Variable bus load powers and different regimes of the WECS operation introduce changes in active power and energy losses in the 10 kV network (Figs. 13-14). With the WECS operated between minimum and maximum conditions, active power loss becomes heavily disturbed. The WECS maximum operation makes nearly 20 times larger losses in comparison to the minimum one. Energy losses depend on the power losses. At the end of 48-hour period, for three different regimes (without/with UPFC) active energy losses are equal to 333/327 kWh (4 m/s), 4125/4023 kWh (intermittent operation) and 8607/8572 kWh (16 m/s). With the UPFC applied, active energy losses are slightly decreased due to neutralisation of reactive power flow only through a radial feeder which connects the WECS to the main in-feed point.

0

0.025

0.05

0.075

0.1

0.125

0.15

0.175

0.2

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Act

ive

pow

er lo

sses

in 1

0 kV

net

wor

k (M

W)

Time (h) Fig. 13. Active power losses in 10 kV network

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Act

ive

ener

gy lo

sses

in 1

0 kV

net

wor

k (k

Wh)

Time (h) Fig. 14. Active energy losses in 10 kV network

During 48-hour period, the WECS delivers electrical energy to 10 kV distribution network (Fig. 15). At the end of the period, for three different operating regimes delivered energy is equal to -1 MWh (4 m/s), 159 MWh (intermittent operation) and 264 MWh (16 m/s). The UPFC has negligible influence to delivered electrical energy. At 4 m/s, the energy has a negative sign since the WECS own consume is larger than minimum production of all wind turbines. At the end of the period, energy loss in 10 kV network is approximately equal to 2.5% of delivered electrical energy in intermittent operating regime, and 3.3% in maximum one (16 m/s). Extrapolation of both values to yearly level is simple.

16 m/s

4 m/s

16 m/s

4 m/s

16 m/s

4 m/s

16 m/s

4 m/s

w/o UPFC

with UPFC

with UPFC

w/o UPFC

intermittent operation

intermittent

w/o UPFC

with UPFC

-50

0

50

100

150

200

250

300

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Del

iver

ed e

lect

rical

ene

rgy

(MW

h)

Time (h) Fig. 15. Delivered electrical energy of the WECS

5. Conclusions Conventional and FACTS-based aspects of voltage control and reactive power compensation are compared with respect to dynamic behaviour of the wind energy conversion system. Benefits of applying power electronics-based devices are clearly depicted during 48-hour period with variable bus load powers and different wind speed patterns. 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. References [1] N. Jenkins et al., Embedded generation, IEE P&E Series 31, ISBN 0 85296 774 8, London, UK, 2000

[2] CIGRÉ, Impact of increasing contribution of dispersed generation on the power system, WG 37.23, February 1999

[3] T. Ackermann, L. Söder, G. Andersson, ''Distributed generation: a definition'', Electric Power Systems Research, vol. 57, 2001, pp. 195-204

[4] N. Hatziargyriou, ''Distributed energy sources: Technical challenges'', IEEE 2002 Winter Meeting, NY, USA, January 2002

[5] J. Lopes, ''Integration of dispersed generation on distribution network – Impact studies'', IEEE 2002 Winter Meeting, NY, USA, January 2002

[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, November 2000

[8] N. Dizdarevic, M. Majstrovic, S. Zutobradic, D. Bajs, Grid integration of wind energy conversion system, project study for Croatian Electric Company, Energy Institute HRVOJE POZAR, Zagreb, Croatia, February 2003, [Online]. Available: www.eihp.hr/~ndizdar

[9] N. Dizdarevic, M. Majstrovic, G. Andersson, ''FACTS-based reactive power compensation of wind energy conversion system'', accepted for publication at IEEE Bologna Power Tech, Bologna, Italy, June 2003, [Online]. Available: www.eihp.hr/~ndizdar

[10] N. Dizdarevic, Unified Power Flow Controller in alleviation of voltage stability problem, Ph.D. thesis, University of Zagreb, Croatia, October 2001, [Online]. Available: www.eihp.hr/~ndizdar

[11] 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

[12] M. Noroozian et al., ''Use of UPFC for optimal power flow control,'' IEEE Trans. Power Delivery, vol. 17, no. 4, pp. 1629-1634, October 1997

Appendix Table A.1 The UPFC basic rated values

SCONV1n (MVA) 4 SCONV2n (MVA) 4 rmax (pu) 0.05 Xk (pu) 0.05

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

Pn (kW) 7x(800/200) Un (V) 690 V ± 10 % Sn (kVA) 909/232 1:n 1:63.6 RS (Ω) 0.0131/0.1165 XS (Ω) 0.24/0.72 Rr (Ω) 0.014/0.073 Xr (Ω) 0.16/0.97 Xmag (Ω) 5.94/22.2 Hm (s) 0.234/0.410 Hg (s) 0.008/0.014 HT (s) 5.644/9.787 Θc (°) 3.6°/3.6° cc (pu torque/rade) 884/821 Dc (pu torque/pu speed) 1200/1200 Dm (pu torque/pu speed) 0.008664/0.008031Dg (pu torque/pu speed) 1.168/1.083 DT (pu torque/pu speed) 147.15/136.73

4 m/s

16 m/s

intermittent