f871112369241051

8/3/2019 f871112369241051

1/23

256 Int. J. Renewable Energy Technology, Vol. 1, No. 3, 2010

Copyright 2010 Inderscience Enterprises Ltd.

Long-term stabilisation of grid connected wind farms

with FACTS controllers

N. Senthil Kumar* and M. Abdullah Khan

Department of Electrical Engineering,

B.S.A. Crescent Engineering College,

Vandalur, Chennai-48, India

E-mail: [email protected]

*Corresponding author

Abstract: The increasing power demand has led to the growth of newtechnologies that play an integral role in shaping the future energy market.Keeping in view of the environmental constraints, grid connected wind turbinesare promising in increasing system reliability. This paper presents the impactof FACTS controllers on the long-term dynamic stability of power systemsconnected with wind farms based on doubly fed induction generator systems.The stability assessment is made first for a three-phase short circuit without theFACTS controllers in the power network and then with the FACTS controllers.The long-term dynamic simulation results yield information on:

1 the impact of faults and slow wind speed changes on the performance ofwind driven induction generators

2 the change in controllable parameters of the FACTS controllers followingthe faults and wind speed changes

3 transient reactive power ratings of FACTS controllers for enhancementof rotor speed stability of induction generators and angle stability ofsynchronous generators.

Keywords: wind energy conversion systems; flexible AC transmissionsystems; rotor angle stability; rotor speed stability; long-term dynamic

simulation.

Reference to this paper should be made as follows: Kumar, N.S. andKhan, M.A. (2010) Long-term stabilisation of grid connected wind farms withFACTS controllers, Int. J. Renewable Energy Technology, Vol. 1, No. 3,

pp.256278.

Biographical notes: N. Senthil Kumar is presently working as an AssistantProfessor (Senior Grade) in the Department of Electrical and ElectronicsEngineering at BSA Crescent Engineering College. He has 12 years of teachingand six years of research experience. He has undergone the EUROSTAGsoftware training programme at Tractable Engineering, Brussels, Belgium. Hisarea of research includes modelling of FACTS devices for power systemstudies and modelling of wind energy conversion systems for power systemstability analysis. He has three international journals to his credit.

M. Abdullah Khan has 40 years of teaching and research experience. His areaof research includes power system analysis and optimisation and power systemdynamics. He has published several papers in reputed international journals.

8/3/2019 f871112369241051

2/23

Long-term stabilisation of grid connected wind farms with FACTS controllers 257

1 Introduction

As power systems became interconnected, areas of generation were found to be prone

to electromechanical oscillations. These oscillations have been observed in many

power systems worldwide. As the level of power transmission rose, largely through

existing interconnections, which were becoming relatively weak and inadequate, load

characteristics added to the problem causing spontaneous oscillations. The oscillations

may be local to a single generator or generator plant (local oscillations, 1.02 Hz) or

they may involve a number of generators widely separated geographically (interarea

oscillations, 0.20.8 Hz). If not controlled, these oscillations may lead to total or partial

power interruption (Kundur, 1994).

Wind energy development is consumer and environment friendly, it requires shorter

construction time and is cost competitive. It becomes one of the most competitive sources

of renewable energy. However, wind power has some disadvantages. For example, wind

power is considered as an intermittent power supply because wind does not blow 100%

of the time. Besides, the superior wind sites are usually located in remote areas; therefore,it may require substantial infrastructure improvement to deliver the wind-generated

power to the load centre. There are four major types of wind generators, which are used

very widely:

1 squirrel cage induction generators

2 doubly fed induction generators

3 direct driven synchronous generator

4 permanent magnet synchronous generator.

For the present work, we have used doubly fed induction generator model for executing

dynamic simulations. The dynamic stability of a single wind turbine generator supplying

an infinite bus through a transmission line was studied by developing the linearised

model of the power system under different loading conditions (Abdel-Magid andEl-Amin, 1987).

The effect of wind turbines on the transient fault behaviour of the Nordic power

system was investigated for different faults in Jauch et al. (2007). A novel error driven

dynamic controller for the static synchronous compensator (STATCOM) FACTS device

was designed to stabilise both wind energy conversion stand-alone system as well

as hybrid scheme of wind turbine with hydro generators (Elmoursi and Sharaf, 2006). A

new definition on rotor speed stability of asynchronous generators is proposed in

Samuelsson and Lindahl (2005). A control structure for DFIG-based turbines under

unbalanced conditions is proposed in Xiu and Wang (2007). The application of voltage

source converter (VSC) based transmission controllers for wind energy conversion

systems is discussed in Varma and Sidhu (2006). Wind turbine model for power system

stability analysis was derived and discussed in Coughlan et al. (2007). The dynamic

behaviour of the power system is analysed with high wind power penetration in

Akhmatov (2003). The impact of FACTS controllers on the rotor speed/rotor anglestability of power systems connected with wind farms is discussed in Kumar and Khan

(2008).

The objective of the present work is to study the impact of FACTS controllers on

the long-term dynamic stability of power systems with wind farms based on doubly

8/3/2019 f871112369241051

3/23

258 N.S. Kumar and M.A. Khan

fed induction generators. The dynamic stability of the system is studied by running time

domain simulations without and with FACTS controllers. The following FACTS

controllers are taken up for the analysis:

1 static var compensator (SVC)

2 static synchronous compensator (STATCOM)

3 thyristor controlled series capacitor (TCSC)

4 unified power flow controller (UPFC).

The induction generator absorbs reactive power during its normal operating condition.

This may create low voltage and dynamic instability in the system (Chompoo-Inwai

et al., 2005).

This paper is organised as follows. Section 2 presents the modelling of power system

and wind turbine along with FACTS controllers. Section 3 presents the dynamic

simulation results obtained for a three-phase transient fault on the system with andwithout FACTS controllers. Section 4 presents the results and discussion on the

simulation results obtained. Section 5 presents the conclusions.

2 Modelling of the power system and induction generator

In general, for the purpose of dynamic analysis, power systems are modelled by a set of

differential and algebraic equations (DAE), that is:

x f (x, y, , p)

0 g(x, y, , p)

=

=

(1)

where

n

x R is a vector of state variables associated with the dynamic states ofgenerators, loads and other system controllers; my R is a vector of algebraic variables

associated with steady state variables resulting from neglecting fast dynamics (e.g., most

load voltage phasor magnitudes and angles);l

R is a set of uncontrollable parameters,

such as variations in active and reactive power of loads; andk

p R is a set of

controllable parameters such as transformer tap, AVR settings and FACTS controller

reference voltages (Mithulananthan et al., 2003).

2.1 Modelling of wind energy conversion system

Normally, a wind turbine creates mechanical torque on a rotating shaft, while an

electrical generator on the same rotating shaft is controlled to produce an opposing

electromagnetic torque. The power and torque equations for the wind turbine are as

follows. The doubly fed induction generator is the most commonly used machine for

wind power generation. The rotor terminals are fed with a symmetrical three-phase

voltage of variable frequency and amplitude. This voltage is supplied by a VSC usually

equipped with IGBT-based power electronics circuitry. The basic structure of the DFIG-

based wind energy conversion scheme is shown in Figure 1.

8/3/2019 f871112369241051

4/23


3p

1P C AV

2

= (2)

PT =

(3)

where

P output power of the turbine (W)

T mechanical torque (Nm)

rotor speed of wind turbine (rad/s)

density of air (= 1.22 kg/m3)

A swept area of the blade (m2)

Cp performance coefficient

V wind speed (m/s).

Figure 1 Wind energy conversion scheme and its associated controls

The wind farm is represented as an aggregated model of ten wind turbines of each 2 MW

(EUROSTAG Release 4.3 Package Documentation Parts 1, 2 and 3, 2004).

The main controller manages the overall control functions, whereas pitch and

power controller are subordinate units. The wind energy conversion scheme used for

8/3/2019 f871112369241051

5/23


simulation consists of a doubly fed induction generator (rotor circuit connected to the

grid through power electronic converter). The converter adjusts the frequency of the

rotor feeding current to enable variable speed operation. The power electronic converterconsists of two-VSCs connected through capacitor.

If shaft, turbine and generator damping are neglected, the two-mass model is

described by the following equations:

tt t s s

dT J K

dt

= + (4)

re g s s

dT J K

dt

= (5)

t r

d

dt

= (6)

where Tt is the mechanical torque referred to the generator side (Nm), Te is the

electromagnetic torque (Nm), Jt is the equivalent turbine-blade inertia referred to the

generator side (kg m2), t is the turbines rotational speed (rad/s), r is the generators

rotational speed (rad/s), Ks is the shaft stiffness (N.m/rad) and s is the angular

displacement between the ends of the shaft (rad). Figure 2 gives the two mass

representation of the wind turbine (Gaztanaga et al., 2007).

Figure 2 Two-mass representation of the wind turbine

2.2 Doubly fed induction generator model

Equations (7) to (11) represent the complete set of mathematical relationships that

describe the dynamic behaviour of the machine. The per unit system is adopted as a unit

of measurement for all quantities and the sign convention is chosen in such a way that

consumed inductive reactive powers are positive (note also that o is equal to 1.0 p.u.).

Voltage equations:

ss s s s

dr I j V

dt

= + (7)

r

r r r r r

d

r I j( ) Vdt

= + (8)

Flux linkages:

s s s m r l I l I = + (9)

8/3/2019 f871112369241051

6/23


r m s r r l I l I = + (10)

Equations of motion:

( )r sd sq sq sd ms

d 1i i t

dt

= +

(11)

where

Is, Ir stator and rotor currents

Vs, Vr stator and rotor terminal voltages

s, r stator and rotor flux linkages

lm mutual inductance (in per unit it is equal to Xm)

rs, rr stator and rotor resistances

r, rotor angular speed, synchronous speed

d, q-direct quadrature axis component

tm mechanical torque.

The DFIG model used is a third order model [equations (7), (8) and (11)], the state

variables being stator, rotor flux components and rotor speed. The variable frequency

rotor voltage permits the adjustment of rotor speed to match the optimum operating point

at any practical wind speed. In normal mode, the rotor side converter is used to control

the real and reactive power outputs of the machine. Independent control of real and

reactive power can be achieved through rotor current control. The rotor current controller

is modelled using the model editor menu of EUROSTAG.

From the basic equations of DFIG (seven to ten) setting all derivatives to zero (steady

state) and with stator resistance rs = 0, we get:

s m rr r r m r r

s

(v jx i )V r i js x x i

jx

= + +

(12)

Considering a coordinate system where the d-axis is located along Vs, it follows that:

( )mrd r rd s rq r s

xV r i s V i x

x

= +

(13)

rq r rq rd r V r i s i x= + (14)

where leakage coefficient2m

r s

x1

x x

=

is introduced.

s is the operating slip of the generator. The voltage drops over the rotor resistance in

(13) and (14) can be interpreted as auxiliary signals, which are outputs of the intended

rotor current controller. PI controllers are introduced to control the rotor voltages and

hence rotor currents.

8/3/2019 f871112369241051

7/23


( )rd I rd ref rd1

1V K 1 i i

pT

= +

(15)

and

( )rq I rq ref rq1

1V K 1 i i

pT

= +

(16)

The corresponding block diagram of the rotor current controller is shown in Figure 3. The

wind turbines use a simplified control scheme and are not LVRT equipped.

Figure 3 Rotor current controller (see online version for colours)

The line or grid side converter has to transmit the active power from the dc link to the

grid so that the dc link voltage is kept within limits. The corresponding controller and

converter model is shown in Figure 4. The output is the active current that is injected into

the grid node.

Figure 4 Block diagram of line side converter control

8/3/2019 f871112369241051

8/23


2.3 Synchronous generators

In power system dynamic studies, the synchronous generator is commonly represented by using models of varying degrees of complexity the simplest being the classical

(second order) model that assumes a constant voltage behind transient reactance. The

synchronous machine model used for this dynamic analysis is the two-axis model with

four state variables (Ed, Eq, , ).

2.4 Static var compensators

SVC is basically a shunt connected static var generator/absorber whose output is adjusted

to exchange capacitive or inductive current so as to maintain or control specific power

system variables; (Figure 5) typically, the controlled variable is the SVC bus voltage.

One of the major reasons for installing an SVC is to improve dynamic voltage control,

and thus, increase system loadability. The SVC bus is modelled as a PV bus in the load

flow programme. It is modelled as a variable susceptance controller as shown in Figure 6for the execution of the dynamic simulation programme.

Figure 5 Ideal SVC

Figure 6 Dynamic model of SVC

8/3/2019 f871112369241051

9/23


An additional stabilising signal and supplementary control superimposed on the voltage

control loop of an SVC can provide damping of system oscillations during transient faults

and disturbances.

2.5 Static synchronous compensator

Figure 7 Block diagram of STATCOM

Figure 8 Dynamic model of STATCOM

8/3/2019 f871112369241051

10/23


The STATCOM resembles in many respects a synchronous compensator, but without

inertia. The basic electronic block of a STATCOM is the VSC, (Figure 7) which in

general converts an input dc voltage into a three-phase output voltage at fundamentalfrequency, with rapidly controllable amplitude and phase. is the phase shift between the

controller VSC AC voltage and its bus voltage V. Vref is the reference voltage setting.

A phase control strategy is assumed for control of the STATCOM bus voltage and

additional control block and signals added for oscillation damping as shown in Figure 8.

2.6 Thyristor controlled series capacitors

TCSC schemes typically use a thyristor-controlled reactor in parallel with a capacitor

to vary the effective compensating reactance. In practice, several capacitor banks, each

with its own thyristor-controlled reactor, may be used to meet the specific application

requirements. For the purpose of analysis, the TCSC, regardless of its practical

implementation, can be considered simply as a continually variable capacitor whose

reactance is controllable in the range of 0 Xc Xcmax. The controllable series capacitivereactance cancels part of the reactive line reactance resulting in a reduced overalltransmission impedance and correspondingly increased transmittable power. The single

line diagram of the TCSC is shown in Figure 9.

Figure 9 Thyristor controlled series capacitor

Figure 10 Dynamic model of TCSC

8/3/2019 f871112369241051

11/23


The variable reactance model of TCSC is as shown in Figure 10. Vcap is the voltage

across the capacitor and Xc is the variable reactance inserted by the TCSC module into

the line. Xref is the steady state reactance of the fixed capacitor under normal conditionsto maintain the steady state power flow (Mathur and Varma, 2002).

2.7 Unified power flow controllers

The UPFC is the most versatile FACTS controller developed so far, with all

encompassing capabilities of voltage regulation, series compensation and phase shifting.

It comprises two VSCs coupled through a common dc link. The series converter (VSC2)

is controlled to inject a voltage phasor Vse, in series with the line, which can be varied

from zero to Vsemax and the phase angle of Vse can be independently varied from zero

to 360. The single line diagram is shown in Figure 11.

The UPFC is modelled in the power flow programme using the power injection

model with two real and reactive power injections at two nodes of the system. The power

injections at both the nodes are selected such that the base case power flow with

induction generator is maintained. The active and reactive power flow control loops of

the UPFC are shown in Figures 12 and 13.

Figure 11 Single line diagram of UPFC

Figure 12 Active power control loop

8/3/2019 f871112369241051

12/23


Figure 13 Reactive power control loop

The stabilising signal for the UPFC is derived from a power oscillation-damping block,

which uses active power flow as the input signal. Pflow ref is the reference value of active

power flow in the line on which UPFC is connected. The reference value of the active

power flow can be set depending on the operating condition of the power system. Pflow is

the actual active power flow in the line. Vseq is the component of series injected voltage in

quadrature with the line current.

Qref from the above figure is the reference value of reactive power flow in the UPFC

controller. Qflow is the actual reactive power flow in the line and Vsep is the component of

AC voltage injected in phase with the line current.

2.7.1 Parameter tuning

The gains of the FACTS controllers in the forward path of the transfer function are

tuned by using an optimisation algorithm which minimises the voltage oscillations of

the induction generator bus. The tuning is posed as an optimisation problem with the

objective as minimising the oscillations of point of common coupling (PCC) voltage from

the desired value and is given by:

2ref k

k

min max

Minimise: SSDI (V V )

K K K

=

(17)

where Vref = 1.0 per unit and SSDI is the sum squared deviation index of the PCC

voltage.

3 Long-term dynamic simulation results and stability investigation

The single line diagram of the test system with the wind turbine connected is shown

in Figure 14. The test system consists of a seven bus system with two synchronous

generators G1 and G2. The doubly fed induction generator (IG) is connected to the grid

through a three winding transformer. IG denotes the stator of the doubly fed induction

generator. At node ST, the stator of induction generator is connected, and at node RT, the

rotor of the doubly fed induction generator is connected denotes the rotor of the inductiongenerator.The dynamic performance of the FACTS controllers with wind turbines is

investigated following the disturbance sequence mentioned below.

1 at t = 2 seconds, a three-phase fault on line 23 in the second line near bus 3

8/3/2019 f871112369241051

13/23


2 at t = 2.125 seconds, the three-phase fault is cleared

3 at t = 20 seconds, the wind speed increases from 2 m/s to 7 m/s (a ramp increase)

4 at t = 50 seconds, the load at bus 5 is increased by 15 MW (load increase).

At bus 5, the load is represented as a combination of impedance and voltage frequency

dependant load in the dynamic simulation. The shunt connected FACTS controllers

(SVC and STATCOM) are located at bus 3 and the series connected FACTS controllers

are located in one of the lines in grid wind farm line (23). The total MW load on the two

load buses 5 and 4 of the test system is 500 MW and 5,000 MW respectively. The steady

state active power generated by generator G1 800 MW and that of G2 is 5,000 MW. The

wind genertor (IG) supplies 5.5 MW in the steady state.

This specific test system is chosen for the dynamic simulation study as this system

has only two synchronous machines which is good enough for conducting stability

investigation on a wind farm. The doubly fed induction generator is modelled as two

active power injections in the load flow programme of EUROSTAG at nodes ST and RT.

The FACTS controllers are modelled as power injections or current injections in theload flow programme. The SVC is modelled as a shunt reactive current in the load

flow programme. The TCSC and UPFC is modelled with two power injections between

buses 23.

Figure 14 Single line diagram of the power system with wind turbine stator connected to node STand rotor connected to node RT

BUS 3

IG

BUS 1 BUS 2

G1

ST

ROTOR

BUS 5

RT

G2

BUS 4

The dynamic simulation results will be presented in the following sequence:

1 rotor angle deviations of synchronous generators with wind farm without and with

FACTS controllers

2 rotor speed deviation of induction generator without FACTS controllers and with

FACTS controllers

8/3/2019 f871112369241051

14/23


3 active power injected by the wind turbine stator and rotor without and with FACTS

controllers

4 induction generator termianl (stator and rotor) voltages without and with FACTS

controllers

5 controllable outputs of the FACTS controllers following the disturbance and wind

speed changes.

3.1 Rotor angle deviations of synchronous generators with wind farm and withFACTS controllers

Figure 15 shows the rotor angle deviation of the synchronous generator G1 without

FACTS controllers in the network.

From Figure 15, it can be observed that after the fault the generator rotor angle of G1

oscillates around the steady state rotor angle value of 65.8. It can be observed that after

the load increase at time t = 50 seconds, the rotor angle of synchronous generatoraccelerates to a new equilibrium value. The rotor angle response of the generators

with SVC is stable with a settling time of ten seconds after the transient fault. The

controller parameters of the SVC/STATCOM are tuned to stabilise the oscillations. The

dynamic performance of STATCOM and TCSC are almost comparable and give a stable

response after the load increase at time t = 50 seconds. The peak overshoot following the

load shedding is more with STATCOM than SVC.

Figure 15 Rotor angle deviation of synchronous generator G1 without and with FACTScontrollers

8/3/2019 f871112369241051

15/23


From Figure 15, it can be observed that there are no sustained oscillations with UPFC in

the network even after the load increase at bus 5. Also, there are no oscillations following

load increase at bus 5 in 50 seconds. This is due the fact that the shunt branch UPFCcontributes reactive power to the network after the transient fault and subsequent changes

in the network conditions. Table 1 shows the comparative analysis of different FACTS

controllers in the network. The comparison is made on the basis of the settling time

obtained after the three-phase fault is cleared with FACTS controllers in the network.

Table 1 Comparison of SVC, STATCOM, TCSC and UPFC for enhancement of dynamicstability in a multimachine power system with DFIG after the transient fault

S. no.WithoutFACTS

SVC STATCOM TCSC UPFC

Settling time 20 sec 10 sec 1 sec 2 sec 1 sec

All the FACTS controllers are located in the electical midpoint of the transmission

line, so that intergenerator oscillations occuring between the two areas are damped out

effectively.

3.2 Rotor speed deviation of induction generator without FACTS controllersand with FACTS controllers

Figure 16 Rotor speed deviation of induction generator FACTS

8/3/2019 f871112369241051

16/23


Figure 16 demonstrates the rotor speed deviation of the induction generators without

FACTS controllers in the network. From Figure 16, it can be observed that the rotor

speed deviation without FACTS controllers in the network following wind speed increaseat time t = 20 seconds is more than 1.12 p.u. It can be observed that the DFIG speed

remains closer to the steady state operating speed (0.9 p.u.) with UPFC in the network.

This is due to the reactive power contribution from both the series and shunt branches of

UPFC. The DFIG speed with SVC is around 1.12 p.u. and with STATCOM it is near

1.06 p.u. This can be attributed due to the faster trasient response of the STATCOM than

the SVC.

3.3 Active power injected by the wind turbine stator and rotor without and withFACTS controllers

Figure 17 shows the active power injected by the wind farm into the grid following the

grid disturbance and wind speed increase. The stator protection system associated with

the induction generator disconnects the stator from the grid if the terminal voltage of theinduction generator is less than 0.75 p.u. for a period of 0.1 seconds, hence, the stator

active power delivered comes down to zero after the fault. The stator of the induction

generator is connected back to the grid after a period of 60 seconds by a reconnection

mechanism. Observe the solid line in Figure 17 which shows the active power injected

into the grid by DFIG without FACTS.

Figure 17 Active power injected by stator and rotor of induction generator without FACTS

8/3/2019 f871112369241051

17/23


The active power injected comes down to zero from its initial value of 5.5 MW specified

in the load flow, as the DFIG protection system disconnects the stator from the grid at

two seconds because of the transient fault. The fault is cleared at time t = 2.125 seconds.The induction generator terminal voltage increases and the machine is connected back to

the grid after a specified time delay at time t = 65 seconds without FACTS controllers in

the network.

With the FACTS controllers in the network, the voltage does not reduce below

0.75 p.u. Hence, the induction generator is not disconnected from the grid. At time

t = 20 seconds, the wind speed increases to 7 m/s. Hence, the real power injected by the

induction generator increases from its initial value of 5.5 MW to 18 MW.

The power calculation according to (2) is based on a single wind speed. However,

in reality, the wind speed may differ slightly in direction and intensity across the area

traversed by the blades. To consider this effect, the wind speed is supplied through a lag

block to the power conversion equation. This creates a slight change in the active power

delivered to the grid before the disturbance at one second (Figure 17).

It can be observed that the STATCOM has an improved transient response compared

to SVC. There are no sustained oscillations in active power delivered to the grid with

STATCOM (Figure 17). It can also be observed that the wind farm is not disconnected

from the power grid with shunt connected FACTS controllers in the network.

After the load increase at time t = 50 seconds, the stabilising loops of all the FACTS

controllers (Figures 6, 8, 10 and 12) are activated which modulates the line active power

flow and regulates the bus voltage. The active power injected reduces only by a fraction

of an MW with UPFC and STATCOM in the network.

3.4 Induction generator termianl voltages without and with FACTS controllers

The response of PCC terminal voltage following the transient fault and wind speed

increase without and with FACTS is shown in Figure 18.

Figure 18 Voltage at bus 5 (PCC voltage) without and with FACTS (see online versionfor colours)

8/3/2019 f871112369241051

18/23


From Figure 18, it is clear that after subsequent fault clearing the voltage at the stator

terminals of the induction generator lies at around 0.7 p.u. after the fault clearance. This

leads to disconnection of DFIG stator terminals from the network as the under voltagerelay is set with a voltage threshold of 0.75 p.u. for 0.1 seconds. However, with UPFC

and STATCOM in the network, the dip in the PCC terminal voltage is only 0.8 p.u. after

the transient fault. Hence, the induction generators are not disconnected from the grid.

3.5 Controllable outputs of the FACTS controllers following the disturbanceand wind speed changes

Figures 19 and 20 show the change in transient ratings of FACTS controllers following

the disturbance and wind speed changes with SVC and STATCOM controllers.

It can be observed from Figure 19 that there are transients in the reactive power

injected after the load shedding at time t = 50 seconds.

From Figures 19 and 20, it can be observed that the dynamic MVAr rating of

STATCOM is lesser than SVC for the transient fault and wind speed changes. Thereactive power exchanged with the system swings from (10 MVAr to 50 MVAr) with

SVC, whereas the swing in reactive power is only (0.08 MVAr to 0.06 MVAr)

following the disturbance and wind speed changes.

The corresponding change in reactive power at the TCSC and UPFC terminals is

shown in Figures 2122.

Figure 19 Dynamic reactive power support provided by SVC

8/3/2019 f871112369241051

19/23


Figure 20 Dynamic reactive power support provided by STATCOM

Figure 21 Change in reactive power at TCSC terminals

8/3/2019 f871112369241051

20/23


Figure 22 Change in reactive power at UPFC terminals

Figure 21 shows the change in TCSC reactive power at its terminals. The dynamic

reactive power support provided by the TCSC terminals swings from 350 MVAr to

+350 MVAr after the transient fault and wind speed changes. The corresponding change

in reactive power rating of UPFC is between (400 MVAr to +500 MVAr) after thedisturbance (Figure 22). There is no appreciable change in reactive power delivered to the

network following wind speed changes for both TCSC and UPFC (observe the graph

between 1050 seconds).

4 Results and discussion

From the results, we conclude that among shunt connected FACTS controllers, the

STATCOM provides better damping the oscillations compared to that of the SVC.

Also, the dynamic reactive compensation (transient rating) required by the STATCOM is

lesser than SVC. This is due to the fact that a STATCOM is basically voltage sourced

converter-based shunt controller and SVC is a shunt connected passive thyristor switched

capacitor/reactor. Among the series connected FACTS controllers, the UPFC damps

both rotor angle oscillations of synchronous generators and rotor speed oscillations

of induction generator very effectively when compared with TCSC. This is due to the

reactive support provided by the shunt branch of the UPFC following the disturbance.

However, the reactive power rating of UPFC is very high compared to that of the TCSC.

It is suggested that a STATCOM of suitable rating may be installed at the PCC with or

8/3/2019 f871112369241051

21/23


without a capacitor may be used for stabilising rotor speed oscillations associated with

doubly fed variable speed induction generators following transient faults and wind speed

increase.

5 Conclusions

The development of wind turbine and wind farm models is vital because as the level of

wind penetration increases, it poses dynamic in stability problems in the power system.

For the present work, we have taken a taken a doubly fed induction generator model and

illustrated the presence of sustained oscillations with wind farms. This paper has

described a comprehensive study of the application of both shunt and series connected

FACTS controllers to a wind farm for enhancement of power system damping. The

dynamic simulation study is carried out to take care of wind speed changes. All the

FACTS controllers modelled make use of a steady state loop (primary objective of

regulating bus voltage or maintaining power flow) and a stabilising loop. The transient

reactive power ratings of FACTS controllers are arrived after carrying out long-term

dynamic simulation study.

Acknowledgements

I sincerely thank my father Mr. S.K. Natarajan and wife Bhuvana for encouraging me at

times of pressure during the research work.

References

Abdel-Magid, Y.L. and El-Amin, I.M. (1987) Dynamic stability of wind-turbine generators underwidely varying load conditions, Electrical Power and Energy Systems, July, Vol. 9, No. 3,

pp.180188.Akhmatov, V. (2003) Analysis of dynamic behavior of electric power systems with large amount

of wind power, PhD thesis, Technical University of Denmark, April.

Chompoo-Inwai, C., Lee, W-J., Fuangfoo, P., Williams, M. and Liao, J.R. (2005) System impactstudy for the interconnection of wind generation and utility system, IEEE Transactions on

Industry Applications, Vol. 41, No. 1, pp.163168.

Coughlan, Y., Smith, P., Mullane, A. and Malley, M.O. (2007) Wind turbine modeling for powersystem stability analysis a system operator perspective, IEEE Transactions on PowerSystems, Vol. 22, No. 3, pp.929936.

Elmoursi, M.S. and Sharaf, A.M. (2006) Novel STATCOM controllers for voltage stabilization ofstand alone hybrid (wind/small hydro) schemes, International Journal of Emerging Electric

Power Systems , Vol. 7, No. 3, Art. 5, pp.127.

EUROSTAG Release 4.3 Package Documentation Parts 1, 2 and 3 (2004).

Gaztanaga, H., Etxeberria-Ottadui, I. and Ocnasu, D. (2007) Real time analysis of the transient

response improvement of wind farms by using a reduced scale STATCOM prototype, IEEETransactions on Power Systems, May, Vol. 22, No. 2, pp.658666.

Jauch, C., Sorensen, P., Norheim, I. and Rasmussen, C. (2007) Simulation of the impact of wind power on the transient fault behavior of the Nordic power system, Electric Power SystemResearch, Vol. 77, pp.135144.

8/3/2019 f871112369241051

22/23


Kumar, N.S. and Khan, M.A. (2008) Impact of FACTS controllers on the dynamic stability of power systems connected with wind farms, International Journal of Wind Engineering,

March, Vol. 32, No. 2, pp.115132.Kundur, P. (1994)Power System Stability and Control, McGraw Hill, New York.

Mathur, R.M. and Varma, R.K. (2002) Thyristor-based FACTS Controllers for ElectricalTransmission Systems, IEEE Press, Wiley and Sons Publications.

Mithulananthan, N., Canizares, C.A. and Rogers, G.J. (2003) Comparison of PSS, SVC andSTATCOM controllers for damping power system oscillations,IEEE Transactions on PowerSystems, Vol. 8, No. 2, pp.786792.

Samuelsson, O. and Lindahl, S. (2005) On speed stability,IEEE Transactions on Power Systems,Vol. 20, No. 2, pp.11791180.

Varma, R.K. and Sidhu, T.S. (2006) Bibliographic review of FACTS and HVDC applications inwind power systems, International Journal of Emerging Electric Power Systems, Vol. 7,

No. 3, Art. 7, pp.116.

Xiu, L. and Wang, Y. (2007) Dynamic modeling and control of DFIG based wind turbines underunbalanced network conditions, IEEE Transactions on Power Systems, February, Vol. 22,

No. 1, pp.314323.

Appendix

A Parameters

Base values for the per unit system conversion:

base power: 100 MVA

base voltage: 0.69 KV for low voltage bus bar, 150 KV for high voltage bus bar.

B Doubly fed induction generator

Rated apparent power MVA: 2 MVA

Rotor inertia: 3.527 MW s/MVA

Rs (p.u.) = 0.0693

Xs (p.u.) = 0.080823

Rr(p.u.) = 0.00906

Xr(p.u) = 0.09935

Xm (p.u) = 3.29

Rotor speed for synchronisation: 0.9 p.u.

Minimum rotor speed: 0.56 p.u.

Maximum rotor speed: 1.122 p.u.

8/3/2019 f871112369241051

23/23


C Transformers

Three winding transformer (150 KV: 0. 69 KV)

Primary rated apparent power = 25 MVA

Secondary rated apparent power = 25 MVA

Tertiary rated apparent power = 6 MVA.

f871112369241051

Documents